Mechanical Engineering

Linear actuator

noreply@blogger.com (widi san) — Wed, 20 Aug 2008 06:58:00 +0000

From Wikipedia, the free encyclopedia
A linear actuator is a device that develops force and motion, from an available energy source, in a linear manner, as opposed to rotationally like an electric motor. There are various methods of achieving this linear motion. Several different examples are listed below.

Types of Linear Actuators

Mechanical actuators

Mechanical actuators typically convert rotary motion of a control knob or handle into linear displacement via screws and/or gears to which the knob or handle is attached. A jackscrew or car jack is a familiar mechanical actuator. Another family of actuators are based on the segmented spindle. Rotation of the jack handle is converted mechanically into the linear motion of the jack head. Mechanical actuators are also frequently used in the field of lasers and optics to manipulate the position of linear stages, rotary stages, mirror mounts, goniometers and other positioning instruments. For accurate and repeatable positioning, index marks may be used on control knobs. Some actuators even include an encoder and digital position readout.[1] These are similar to the adjustment knobs used on micrometers except that their purpose is position adjustment rather than position measurement.

Hydraulic actuators
Hydraulic actuators or hydraulic cylinders typically involve a hollow cylinder having a piston inserted in it. The two sides of the piston are alternately pressurized/de-pressurized to achieve controlled precise linear displacement of the piston and in turn the entity connected to the piston. The physical linear displacement is only along the axis of the piston/cylinder. This design is based on the principles of hydraulics. A familiar example of a manually operated hydraulic actuator is a hydraulic car jack. Typically though, the term "hydraulic actuator" refers to a device controlled by a hydraulic pump.

Piezoelectric actuators
The piezoelectric effect is a property of certain materials in which application of a voltage to the material causes it to expand. Very high voltages correspond to only tiny expansions. As a result, piezoelectric actuators can achieve extremely fine positioning resolution, but also have a very short range of motion. In addition, piezoelectric materials exhibit hysteresis which makes it difficult to control their expansion in a repeatable manner.

Electro-mechanical actuators

Electro-mechanical actuators are similar to mechanical actuators except that the control knob or handle is replaced with an electric motor. Rotary motion of the motor is converted to linear displacement of the actuator. There are many designs of modern linear actuators and every company that manufactures them tends to have their own proprietary method. The following is a generalized description of a very simple electro-mechanical linear actuator.

Simplified Design
Typically, a rotary driver (e.g. electric motor) is mechanically connected to a lead screw so that the rotation of the electric motor will make the lead screw rotate. A lead screw has a continuous helical thread machined on its circumference running along the length (similar to the thread on a bolt). Threaded onto the lead screw is a lead nut with corresponding helical threads. The nut is prevented from rotating with the lead screw (typically the nut interlocks with a non-rotating part of the actuator body). Therefore, when the lead screw is rotated, the nut will be driven along the threads. The direction of motion of the nut will depend on the direction of rotation of the lead screw. By connecting linkages to the nut, the motion can be converted to usable linear displacement. Most current actuators are built either for high speed, high force, or a compromise between the two. When considering an actuator for a particular application, the most important specifications are typically travel, speed, force, and lifetime.

Principles

In the majority of linear actuator designs, the basic principle of operation is that of an inclined plane. The threads of a lead screw act as a continuous ramp that allows a small rotational force to be used over a long distance to accomplish movement of a large load over a short distance.

Variations
Many variations on the basic design have been created. Most focus on providing general improvements such as a higher mechanical efficiency, speed, or load capacity. There is also a large engineering movement towards actuator miniaturization.

Most electro-mechanical designs incorporate a lead screw and lead nut. Some use a ball screw and ball nut. In either case the screw may be connected to a motor or manual control knob either directly or through a series of gears. Gears are typically used to allow a smaller (and weaker) motor spinning at a higher rpm to be geared down to provide the torque necessary to spin the screw under a heavier load than the motor would otherwise be capable of driving directly. Effectively this sacrifices actuator speed in favor of increased actuator thrust.

Some lead screws have multiple "starts". This means that they have multiple threads alternating on the same shaft. One way of visualizing this is in comparison to the multiple color stripes on a candy cane. This allows for more adjustment between thread pitch and nut/screw thread contact area, which determines the extension speed and load carrying capacity (of the threads), respectively.

Linear motors
A linear motor is essentially a rotary electric motor laid down on flat surface. Since the motor moves in a linear fashion to begin with, no lead screw is needed to convert rotary motion to linear. While high capacity is possible, the material and/or motor limitations on most designs are surpassed relatively quickly. Most linear motors have a relatively low load capacity compared to other types of linear actuators.

Wax motors
A wax motor typically uses an electric current to heat a block of wax causing it to expand. A plunger that bears on the wax is thus forced to move in a linear fashion.

Segmented spindles
KATAKA actuators consist of discrete chain elements which are interlinked to form a rod (the technology is known as the segmented spindle) thus making the actuator extremely compact (see www.kataka.dk).

History Of Welding

noreply@blogger.com (widi san) — Wed, 20 Aug 2008 06:05:00 +0000

The history of joining metals goes back several millennia, with the earliest examples of welding from the Bronze Age and the Iron Age in Europe and the Middle East. Welding was used in the construction of the Iron pillar in Delhi, India, erected about 310 and weighing 5.4 metric tons. The Middle Ages brought advances in forge welding, in which blacksmiths pounded heated metal repeatedly until bonding occurred. In 1540, Vannoccio Biringuccio published De la pirotechnia, which includes descriptions of the forging operation. Renaissance craftsmen were skilled in the process, and the industry continued to grow during the following centuries. Welding, however, was transformed during the 19th century—in 1800, Sir Humphry Davy discovered the electric arc, and advances in arc welding continued with the inventions of metal electrodes by a Russian, Nikolai Slavyanov, and an American, C. L. Coffin in the late 1800s, even as carbon arc welding, which used a carbon electrode, gained popularity. Around 1900, A. P. Strohmenger released a coated metal electrode in Britain, which gave a more stable arc, and in 1919, alternating current welding was invented by C. J. Holslag, but did not become popular for another decade.

Resistance welding was also developed during the final decades of the 19th century, with the first patents going to Elihu Thomson in 1885, who produced further advances over the next 15 years. Thermite welding was invented in 1893, and around that time, another process, oxyfuel welding, became well established. Acetylene was discovered in 1836 by Edmund Davy, but its use was not practical in welding until about 1900, when a suitable blowtorch was developed. At first, oxyfuel welding was one of the more popular welding methods due to its portability and relatively low cost. As the 20th century progressed, however, it fell out of favor for industrial applications. It was largely replaced with arc welding, as metal coverings (known as flux) for the electrode that stabilize the arc and shield the base material from impurities continued to be developed.

World War I caused a major surge in the use of welding processes, with the various military powers attempting to determine which of the several new welding processes would be best. The British primarily used arc welding, even constructing a ship, the Fulagar, with an entirely welded hull. Arc welding was first applied to aircraft during the war as well, as some German airplane fuselages were constructed using the process. Also noteworthy is the first welded road bridge in the world built across the river Słudwia Maurzyce near Łowicz, Poland) in 1929, but designed by Stefan Bryła of the Warsaw University of Technology in 1927.

During the 1920s, major advances were made in welding technology, including the introduction of automatic welding in 1920, in which electrode wire was fed continuously. Shielding gas became a subject receiving much attention, as scientists attempted to protect welds from the effects of oxygen and nitrogen in the atmosphere. Porosity and brittleness were the primary problems, and the solutions that developed included the use of hydrogen, argon, and helium as welding atmospheres. During the following decade, further advances allowed for the welding of reactive metals like aluminum and magnesium. This, in conjunction with developments in automatic welding, alternating current, and fluxes fed a major expansion of arc welding during the 1930s and then during World War II.

During the middle of the century, many new welding methods were invented. 1930 saw the release of stud welding, which soon became popular in shipbuilding and construction. Submerged arc welding was invented the same year, and continues to be popular today. Gas tungsten arc welding, after decades of development, was finally perfected in 1941, and gas metal arc welding followed in 1948, allowing for fast welding of non-ferrous materials but requiring expensive shielding gases. Shielded metal arc welding was developed during the 1950s, using a flux coated consumable electrode, and it quickly became the most popular metal arc welding process. In 1957, the flux-cored arc welding process debuted, in which the self-shielded wire electrode could be used with automatic equipment, resulting in greatly increased welding speeds, and that same year, plasma arc welding was invented. Electroslag welding was introduced in 1958, and it was followed by its cousin, electrogas welding, in 1961.

Other recent developments in welding include the 1958 breakthrough of electron beam welding, making deep and narrow welding possible through the concentrated heat source. Following the invention of the laser in 1960, laser beam welding debuted several decades later, and has proved to be especially useful in high-speed, automated welding. Both of these processes, however, continue to be quite expensive due the high cost of the necessary equipment, and this has limited their applications.

Processes

Arc welding

These processes use a welding power supply to create and maintain an electric arc between an electrode and the base material to melt metals at the welding point. They can use either direct (DC) or alternating (AC) current, and consumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and filler material is sometimes used as well.

Power supplies

To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power supplies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.

The type of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Nonconsumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. However, with direct current, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ignited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, making rapid zero crossings possible and minimizing the effects of the problem.

Processes

Shielded metal arc welding

One of the most common types of arc welding is shielded metal arc welding (SMAW), which is also known as manual metal arc welding (MMA) or stick welding. Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process. The electrode core itself acts as filler material, making a separate filler unnecessary.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work. An operator can become reasonably proficient with a modest amount of training and can achieve mastery with experience. Weld times are rather slow, since the consumable electrodes must be frequently replaced and because slag, the residue from the flux, must be chipped away after welding. Furthermore, the process is generally limited to welding ferrous materials, though special electrodes have made possible the welding of cast iron, nickel, aluminium, copper, and other metals. Inexperienced operators may find it difficult to make good out-of-position welds with this process.

Gas metal arc welding (GMAW), also known as metal inert gas or MIG welding, is a semi-automatic or automatic process that uses a continuous wire feed as an electrode and an inert or semi-inert gas mixture to protect the weld from contamination. As with SMAW, reasonable operator proficiency can be achieved with modest training. Since the electrode is continuous, welding speeds are greater for GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal arc welding process makes it easier to make out-of-position welds (e.g., overhead joints, as would be welded underneath a structure).

The equipment required to perform the GMAW process is more complex and expensive than that required for SMAW, and requires a more complex setup procedure. Therefore, GMAW is less portable and versatile, and due to the use of a separate shielding gas, is not particularly suitable for outdoor work. However, owing to the higher average rate at which welds can be completed, GMAW is well suited to production welding. The process can be applied to a wide variety of metals, both ferrous and non-ferrous.

A related process, flux-cored arc welding (FCAW), uses similar equipment but uses wire consisting of a steel electrode surrounding a powder fill material. This cored wire is more expensive than the standard solid wire and can generate fumes and/or slag, but it permits even higher welding speed and greater metal penetration.

Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding (also sometimes erroneously referred to as heliarc welding), is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material. Especially useful for welding thin materials, this method is characterized by a stable arc and high quality welds, but it requires significant operator skill and can only be accomplished at relatively low speeds.

GTAW can be used on nearly all weldable metals, though it is most often applied to stainless steel and light metals. It is often used when quality welds are extremely important, such as in bicycle, aircraft and naval applications. A related process, plasma arc welding, also uses a tungsten electrode but uses plasma gas to make the arc. The arc is more concentrated than the GTAW arc, making transverse control more critical and thus generally restricting the technique to a mechanized process. Because of its stable current, the method can be used on a wider range of material thicknesses than can the GTAW process, and furthermore, it is much faster. It can be applied to all of the same materials as GTAW except magnesium, and automated welding of stainless steel is one important application of the process. A variation of the process is plasma cutting, an efficient steel cutting process.

Submerged arc welding (SAW) is a high-productivity welding method in which the arc is struck beneath a covering layer of flux. This increases arc quality, since contaminants in the atmosphere are blocked by the flux. The slag that forms on the weld generally comes off by itself, and combined with the use of a continuous wire feed, the weld deposition rate is high. Working conditions are much improved over other arc welding processes, since the flux hides the arc and almost no smoke is produced. The process is commonly used in industry, especially for large products and in the manufacture of welded pressure vessels. Other arc welding processes include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas welding, and stud arc welding.
Gas welding a steel armature using the oxy-acetylene process.
Gas welding a steel armature using the oxy-acetylene process.

Gas

The most common gas welding process is oxyfuel welding, also known as oxyacetylene welding. It is one of the oldest and most versatile welding processes, but in recent years it has become less popular in industrial applications. It is still widely used for welding pipes and tubes, as well as repair work. It is also frequently well-suited, and favored, for fabricating some types of metal-based artwork. Oxyfuel equipment is versatile, lending itself not only to some sorts of iron or steel welding but also to brazing, braze-welding, metal heating (for bending and forming), and also oxyfuel cutting.

The equipment is relatively inexpensive and simple, generally employing the combustion of acetylene in oxygen to produce a welding flame temperature of about 3100 °C. The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.[23] Other gas welding methods, such as air acetylene welding, oxygen hydrogen welding, and pressure gas welding are quite similar, generally differing only in the type of gases used. A water torch is sometimes used for precision welding of small items such as jewelry. Gas welding is also used in plastic welding, though the heated substance is air, and the temperatures are much lower.

Resistance

Resistance welding involves the generation of heat by passing current through the resistance caused by the contact between two or more metal surfaces. Small pools of molten metal are formed at the weld area as high current (1000–100,000 A) is passed through the metal. In general, resistance welding methods are efficient and cause little pollution, but their applications are somewhat limited and the equipment cost can be high.

Spot welding is a popular resistance welding method used to join overlapping metal sheets of up to 3 mm thick. Two electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. Weld strength is significantly lower than with other welding methods, making the process suitable for only certain applications. It is used extensively in the automotive industry—ordinary cars can have several thousand spot welds made by industrial robots. A specialized process, called shot welding, can be used to spot weld stainless steel.

Like spot welding, seam welding relies on two electrodes to apply pressure and current to join metal sheets. However, instead of pointed electrodes, wheel-shaped electrodes roll along and often feed the workpiece, making it possible to make long continuous welds. In the past, this process was used in the manufacture of beverage cans, but now its uses are more limited. Other resistance welding methods include flash welding, projection welding, and upset welding.

Energy beam

Energy beam welding methods, namely laser beam welding and electron beam welding, are relatively new processes that have become quite popular in high production applications. The two processes are quite similar, differing most notably in their source of power. Laser beam welding employs a highly focused laser beam, while electron beam welding is done in a vacuum and uses an electron beam. Both have a very high energy density, making deep weld penetration possible and minimizing the size of the weld area. Both processes are extremely fast, and are easily automated, making them highly productive. The primary disadvantages are their very high equipment costs (though these are decreasing) and a susceptibility to thermal cracking. Developments in this area include laser-hybrid welding, which uses principles from both laser beam welding and arc welding for even better weld properties.

Solid-state

Like the first welding process, forge welding, some modern welding methods do not involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. The equipment and methods involved are similar to that of resistance welding, but instead of electric current, vibration provides energy input. Welding metals with this process does not involve melting the materials; instead, the weld is formed by introducing mechanical vibrations horizontally under pressure. When welding plastics, the materials should have similar melting temperatures, and the vibrations are introduced vertically. Ultrasonic welding is commonly used for making electrical connections out of aluminum or copper, and it is also a very common polymer welding process.

Another common process, explosion welding, involves the joining of materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated. The process is commonly used for welding dissimilar materials, such as the welding of aluminum with steel in ship hulls or compound plates. Other solid-state welding processes include co-extrusion welding, cold welding, diffusion welding, friction welding (including friction stir welding), high frequency welding, hot pressure welding, induction welding, and roll welding.

Geometry

Common welding joint types – (1) Square butt joint, (2) Single-V preparation joint, (3) Lap joint, (4) T-joint.
Common welding joint types – (1) Square butt joint, (2) Single-V preparation joint, (3) Lap joint, (4) T-joint.

Welds can be geometrically prepared in many different ways. The five basic types of weld joints are the butt joint, lap joint, corner joint, edge joint, and T-joint. Other variations exist as well—for example, double-V preparation joints are characterized by the two pieces of material each tapering to a single center point at one-half their height. Single-U and double-U preparation joints are also fairly common—instead of having straight edges like the single-V and double-V preparation joints, they are curved, forming the shape of a U. Lap joints are also commonly more than two pieces thick—depending on the process used and the thickness of the material, many pieces can be welded together in a lap joint geometry.

Often, particular joint designs are used exclusively or almost exclusively by certain welding processes. For example, resistance spot welding, laser beam welding, and electron beam welding are most frequently performed on lap joints. However, some welding methods, like shielded metal arc welding, are extremely versatile and can weld virtually any type of joint. Additionally, some processes can be used to make multipass welds, in which one weld is allowed to cool, and then another weld is performed on top of it. This allows for the welding of thick sections arranged in a single-V preparation joint, for example.
The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat-affected zone, and the lightest gray the base material.
The cross-section of a welded butt joint, with the darkest gray representing the weld or fusion zone, the medium gray the heat-affected zone, and the lightest gray the base material.

After welding, a number of distinct regions can be identified in the weld area. The weld itself is called the fusion zone—more specifically, it is where the filler metal was laid during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials. It is surrounded by the heat-affected zone, the area that had its microstructure and properties altered by the weld. These properties depend on the base material's behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.

Quality

Most often, the major metric used for judging the quality of a weld is its strength and the strength of the material around it. Many distinct factors influence this, including the welding method, the amount and concentration of energy input, the base material, the filler material, the flux material, the design of the joint, and the interactions between all these factors. To test the quality of a weld, either destructive or nondestructive testing methods are commonly used to verify that welds are defect-free, have acceptable levels of residual stresses and distortion, and have acceptable heat-affected zone (HAZ) properties. Welding codes and specifications exist to guide welders in proper welding technique and in how to judge the quality of welds.

Heat-affected zone
The blue area results from oxidation at a corresponding temperature of 600 °F (316 °C). This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ is the narrow area that immediately surrounds the welded base metal.
The blue area results from oxidation at a corresponding temperature of 600 °F (316 °C). This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ is the narrow area that immediately surrounds the welded base metal.

The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used, the HAZ can be of varying size and strength. The thermal diffusivity of the base material plays a large role—if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Conversely, a low diffusivity leads to slower cooling and a larger HAZ. The amount of heat injected by the welding process plays an important role as well, as processes like oxyacetylene welding have an unconcentrated heat input and increase the size of the HAZ. Processes like laser beam welding give a highly concentrated, limited amount of heat, resulting in a small HAZ. Arc welding falls between these two extremes, with the individual processes varying somewhat in heat input.[30][31] To calculate the heat input for arc welding procedures, the following formula can be used:

Q = \left(\frac{V \times I \times 60}{S \times 1000} \right) \times \mathit{Efficiency}

where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding speed (mm/min). The efficiency is dependent on the welding process used, with shielded metal arc welding having a value of 0.75, gas metal arc welding and submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.

Distortion and cracking
http://upload.wikimedia.org/wikipedia/commons/thumb/2/21/Pipe_root_weld_with_HAZ.jpg/200px-Pipe_root_weld_with_HAZ.jpg

Welding methods that involve the melting of metal at the site of the joint necessarily are prone to shrinkage as the heated metal cools. Shrinkage, in turn, can introduce residual stresses and both longitudinal and rotational distortion. Distortion can pose a major problem, since the final product is not the desired shape. To alleviate rotational distortion, the workpieces can be offset, so that the welding results in a correctly shaped piece. Other methods of limiting distortion, such as clamping the workpieces in place, cause the buildup of residual stress in the heat-affected zone of the base material. These stresses can reduce the strength of the base material, and can lead to catastrophic failure through cold cracking, as in the case of several of the Liberty ships. Cold cracking is limited to steels, and is associated with the formation of martensite as the weld cools. The cracking occurs in the heat-affected zone of the base material. To reduce the amount of distortion and residual stresses, the amount of heat input should be limited, and the welding sequence used should not be from one end directly to the other, but rather in segments. The other type of cracking, hot cracking or solidification cracking, can occur with all metals, and happens in the fusion zone of a weld. To diminish the probability of this type of cracking, excess material restraint should be avoided, and a proper filler material should be utilized.

Weldability

The quality of a weld is also dependent on the combination of materials used for the base material and the filler material. Not all metals are suitable for welding, and not all filler metals work well with acceptable base materials.

Steels

The weldability of steels is inversely proportional to a property known as the hardenability of the steel, which measures the probability of forming martensite during welding or heat treatment. The hardenability of steel depends on its chemical composition, with greater quantities of carbon and other alloying elements resulting in a higher hardenability and thus a lower weldability. In order to be able to judge alloys made up of many distinct materials, a measure known as the equivalent carbon content is used to compare the relative weldabilities of different alloys by comparing their properties to a plain carbon steel. The effect on weldability of elements like chromium and vanadium, while not as great as carbon, is more significant than that of copper and nickel, for example. As the equivalent carbon content rises, the weldability of the alloy decreases. The disadvantage to using plain carbon and low-alloy steels is their lower strength—there is a trade-off between material strength and weldability. High strength, low-alloy steels were developed especially for welding applications during the 1970s, and these generally easy to weld materials have good strength, making them ideal for many welding applications.

Stainless steels, because of their high chromium content, tend to behave differently with respect to weldability than other steels. Austenitic grades of stainless steels tend to be the most weldable, but they are especially susceptible to distortion due to their high coefficient of thermal expansion. Some alloys of this type are prone to cracking and reduced corrosion resistance as well. Hot cracking is possible if the amount of ferrite in the weld is not controlled—to alleviate the problem, an electrode is used that deposits a weld metal containing a small amount of ferrite. Other types of stainless steels, such as ferritic and martensitic stainless steels, are not as easily welded, and must often be preheated and welded with special electrodes.

Aluminum

The weldability of aluminum alloys varies significantly, depending on the chemical composition of the alloy used. Aluminum alloys are susceptible to hot cracking, and to combat the problem, welders increase the welding speed to lower the heat input. Preheating reduces the temperature gradient across the weld zone and thus helps reduce hot cracking, but it can reduce the mechanical properties of the base material and should not be used when the base material is restrained. The design of the joint can be changed as well, and a more compatible filler alloy can be selected to decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned prior to welding, with the goal of removing all oxides, oils, and loose particles from the surface to be welded. This is especially important because of an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen.

Unusual conditions

Underwater welding

While many welding applications are done in controlled environments such as factories and repair shops, some welding processes are commonly used in a wide variety of conditions, such as open air, underwater, and vacuums (such as space). In open-air applications, such as construction and outdoors repair, shielded metal arc welding is the most common process. Processes that employ inert gases to protect the weld cannot be readily used in such situations, because unpredictable atmospheric movements can result in a faulty weld. Shielded metal arc welding is also often used in underwater welding in the construction and repair of ships, offshore platforms, and pipelines, but others, such as flux cored arc welding and gas tungsten arc welding, are also common. Welding in space is also possible—it was first attempted in 1969 by Russian cosmonauts, when they performed experiments to test shielded metal arc welding, plasma arc welding, and electron beam welding in a depressurized environment. Further testing of these methods was done in the following decades, and today researchers continue to develop methods for using other welding processes in space, such as laser beam welding, resistance welding, and friction welding. Advances in these areas could prove indispensable for projects like the construction of the International Space Station, which will likely rely heavily on welding for joining in space the parts that were manufactured on Earth.

Safety issues

Arc welding with a welding helmet, gloves, and other protective clothing.
Welding, without the proper precautions, can be a dangerous and unhealthy practice. However, with the use of new technology and proper protection, risks of injury and death associated with welding can be greatly reduced. Because many common welding procedures involve an open electric arc or flame, the risk of burns is significant. To prevent them, welders wear personal protective equipment in the form of heavy leather gloves and protective long sleeve jackets to avoid exposure to extreme heat and flames. Additionally, the brightness of the weld area leads to a condition called arc eye in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Goggles and welding helmets with dark face plates are worn to prevent this exposure, and in recent years, new helmet models have been produced that feature a face plate that self-darkens upon exposure to high amounts of UV light. To protect bystanders, translucent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets.

Welders are also often exposed to dangerous gases and particulate matter. Processes like flux-cored arc welding and shielded metal arc welding produce smoke containing particles of various types of oxides, which in some cases can lead to medical conditions like metal fume fever. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, many processes produce fumes and various gases, most commonly carbon dioxide, ozone and heavy metals, that can prove dangerous without proper ventilation and training. Furthermore, because the use of compressed gases and flames in many welding processes poses an explosion and fire risk, some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace. Welding fume extractors are often used to remove the fume from the source and filter the fumes through a HEPA filter.

Costs and trends

As an industrial process, the cost of welding plays a crucial role in manufacturing decisions. Many different variables affect the total cost, including equipment cost, labor cost, material cost, and energy cost. Depending on the process, equipment cost can vary, from inexpensive for methods like shielded metal arc welding and oxyfuel welding, to extremely expensive for methods like laser beam welding and electron beam welding. Because of their high cost, they are only used in high production operations. Similarly, because automation and robots increase equipment costs, they are only implemented when high production is necessary. Labor cost depends on the deposition rate (the rate of welding), the hourly wage, and the total operation time, including both time welding and handling the part. The cost of materials includes the cost of the base and filler material, and the cost of shielding gases. Finally, energy cost depends on arc time and welding power demand.

For manual welding methods, labor costs generally make up the vast majority of the total cost. As a result, many cost-savings measures are focused on minimizing the operation time. To do this, welding procedures with high deposition rates can be selected, and weld parameters can be fine-tuned to increase welding speed. Mechanization and automatization are often implemented to reduce labor costs, but this frequently increases the cost of equipment and creates additional setup time. Material costs tend to increase when special properties are necessary, and energy costs normally do not amount to more than several percent of the total welding cost.

In recent years, in order to minimize labor costs in high production manufacturing, industrial welding has become increasingly more automated, most notably with the use of robots in resistance spot welding (especially in the automotive industry) and in arc welding. In robot welding, mechanized devices both hold the material and perform the weld, and at first, spot welding was its most common application. But robotic arc welding has been increasing in popularity as technology has advanced. Other key areas of research and development include the welding of dissimilar materials (such as steel and aluminum, for example) and new welding processes, such as friction stir, magnetic pulse, conductive heat seam, and laser-hybrid welding. Furthermore, progress is desired in making more specialized methods like laser beam welding practical for more applications, such as in the aerospace and automotive industries. Researchers also hope to better understand the often unpredictable properties of welds, especially microstructure, residual stresses, and a weld's tendency to crack or deform.

Welding

noreply@blogger.com (widi san) — Wed, 20 Aug 2008 06:01:00 +0000

From Wikipedia, the free encyclopedia

Welding is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material (the weld puddle) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in outer space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, eye damage, poisonous fumes, and overexposure to ultraviolet light.

Until the end of the 19th century, the only welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop late in the century, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding, flux-cored arc welding and electroslag welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.

Computer-Aided Design

noreply@blogger.com (widi san) — Wed, 20 Aug 2008 05:32:00 +0000

From Wikipedia, the free encyclopedia

"CADD" and "CAD" redirect here. For other uses, see CADD (disambiguation) and CAD (disambiguation).
"ECAD" redirects here. For other uses, see ECAD (disambiguation).

Computer-aided design CAD is the use of computer technology to aid in the design and especially the drafting (technical drawing and engineering drawing) of a part or product, including entire buildings. It is both a visual (or drawing) and symbol-based method of communication whose conventions are particular to a specific technical field.

Drafting can be done in two dimensions ("2D") and three dimensions ("3D").

Drafting is the integral communication of technical or engineering drawings and is the industrial arts sub-discipline that underlies all involved technical endeavors. In representing complex, three-dimensional objects in two-dimensional drawings, these objects have traditionally been represented by three projected views at right angles.

Current CAD software packages range from 2D vector-based drafting systems to 3D solid and surface modellers. Modern CAD packages can also frequently allow rotations in three dimensions, allowing viewing of a designed object from any desired angle, even from the inside looking out. Some CAD software is capable of dynamic mathematic modeling, in which case it may be marketed as CADD — computer-aided design and drafting.

CAD is used in the design of tools and machinery used in the manufacture of components, and in the drafting and design of all types of buildings, from small residential types (houses) to the largest commercial and industrial structures (hospitals and factories).

CAD is mainly used for detailed engineering of 3D models and/or 2D drawings of physical components, but it is also used throughout the engineering process from conceptual design and layout of products, through strength and dynamic analysis of assemblies to definition of manufacturing methods of components.

CAD has become an especially important technology within the scope of computer-aided technologies, with benefits such as lower product development costs and a greatly shortened design cycle. CAD enables designers to lay out and develop work on screen, print it out and save it for future editing, saving time on their drawings.

History
Designers have long used computers for their calculations. Initial developments were carried out in the 1960s within the aircraft and automotive industries in the area of 3D surface construction and NC programming, most of it independent of one another and often not publicly published until much later. Some of the mathematical description work on curves was developed in the early 1940s by Robert Issac Newton from Pawtucket, Rhode Island. Robert A. Heinlein in his 1957 novel The Door into Summer suggested the possibility of a robotic Drafting Dan. However, probably the most important work on polynomial curves and sculptured surface was done by Pierre Bezier (Renault), Paul de Casteljau (Citroen), Steven Anson Coons (MIT, Ford), James Ferguson (Boeing), Carl de Boor (GM), Birkhoff (GM) and Garibedian (GM) in the 1960s and W. Gordon (GM) and R. Riesenfeld in the 1970s.

It is argued that a turning point was the development of the SKETCHPAD system at MIT in 1963 by Ivan Sutherland (who later created a graphics technology company with Dr. David Evans). The distinctive feature of SKETCHPAD was that it allowed the designer to interact with his computer graphically: the design can be fed into the computer by drawing on a CRT monitor with a light pen. Effectively, it was a prototype of graphical user interface, an indispensable feature of modern CAD.

The first commercial applications of CAD were in large companies in the automotive and aerospace industries, as well as in electronics. Only large corporations could afford the computers capable of performing the calculations. Notable company projects were at GM (Dr. Patrick J.Hanratty) with DAC-1 (Design Augmented by Computer) 1964; Lockheed projects; Bell GRAPHIC 1 and at Renault (Bezier) – UNISURF 1971 car body design and tooling.

One of the most influential events in the development of CAD was the founding of MCS (Manufacturing and Consulting Services Inc.) in 1971 by Dr. P. J. Hanratty,[6] who wrote the system ADAM (Automated Drafting And Machining) but more importantly supplied code to companies such as McDonnell Douglas (Unigraphics), Computervision (CADDS), Calma, Gerber, Autotrol and Control Data.

As computers became more affordable, the application areas have gradually expanded. The development of CAD software for personal desktop computers was the impetus for almost universal application in all areas of construction.

Other key points in the 1960s and 1970s would be the foundation of CAD systems United Computing, Intergraph, IBM, Intergraph IGDS in 1974 (which led to Bentley Systems MicroStation in 1984)

CAD implementations have evolved dramatically since then. Initially, with 3D in the 1970s, it was typically limited to producing drawings similar to hand-drafted drawings. Advances in programming and computer hardware, notably solid modeling in the 1980s, have allowed more versatile applications of computers in design activities.

Key products for 1981 were the solid modelling packages -Romulus (ShapeData) and Uni-Solid (Unigraphics) based on PADL-2 and the release of the surface modeler CATIA (Dassault Systemes). Autodesk was founded 1982 by John Walker, which led to the 2D system AutoCAD. The next milestone was the release of Pro/ENGINEER in 1988, which heralded greater usage of feature-based modeling methods and parametric linking of the parameters of features. Also of importance to the development of CAD was the development of the B-rep solid modeling kernels (engines for manipulating geometrically and topologically consistent 3D objects) Parasolid (ShapeData) and ACIS (Spatial Technology Inc.) at the end of the 1980s and beginning of the 1990s, both inspired by the work of Ian Braid. This led to the release of mid-range packages such as SolidWorks in 1995, Solid Edge (Intergraph) in 1996, IronCAD in 1998, and Autodesk Inventor in 1999. Today CAD is one of the main tools used in designing products and architecture and also in the field of engineering because of cutting edge technologies that have come out in the last few years.

In the early days of CAD the computer was used as a replacement for the drawing board. Early systems could create simple 2D arcs, lines and curves. Over time, these evolved into 3D Modellers, but still only capable of representing objects as wireframes (collections of arcs, lines, and curves).

By the early 80s two new 3D technologies had appeared. Solid Modellers that created 3D objects by merging simple primitive shapes (blocks, cylinders, swept volumes etc.) and surface modellers that defined only the outer 'skin' of a part, but could cope with much more organic forms.

The closing stages of the 20th Century saw the arrival of Hybrid Modellers. These are capable of defining 3D models as a combination of both surfaces and solids. Designers typically use surfaces to define the outer form of a component and solids to create the mechanical details, such as ribs, clips, fixings and so on.

In parallel, systems working on point cloud data were also being developed. Point clouds are typically the result of scanning physical objects with devices that use a touch probe, white light or a laser. The resulting points are then triangulated into a mesh to represent the scanned object's surface. Imperfections and noise tend to make the definition of the object less precise than solid modelling. However, the detail can be used to advantage to represent texture and intricate decoration that is difficult to model with solids or surfaces.

So there are advantages to all three means of representing shape; solid models for geometric shapes, surfaces for organic forms, and triangle models for more complex, "artistic" parts.

Now, in the 21st century another new technology is appearing that combines surfaces and solids with triangles. These TRIBRID [1] Modellers let designers work with almost unlimited freedom. Mechanical parts are designed using solid modelling techniques, while complex, free-form surfaces can be employed to create eye-catching and pleasing shapes. For adding 3D textures, features such as engraved logos or textures, triangles are seamlessly combined to create CAD models that would otherwise be virtually impossible.

One of the first of this new generation of Tribrid Modellers is PowerSHAPE from UK CADCAM Developer Delcam International.

Capabilities
The capabilities of modern CAD systems include:

* Tribrid [3] modelling
* Wireframe geometry creation
* 3D parametric feature based modelling, Solid modelling
* Freeform surface modelling
* Automated design of assemblies, which are collections of parts and/or other assemblies
* Create engineering drawings from the solid models
* Reuse of design components

* Ease of modification of design of model and the production of multiple versions
* Automatic generation of standard components of the design
* Validation/verification of designs against specifications and design rules
* Simulation of designs without building a physical prototype
* Output of engineering documentation, such as manufacturing drawings, and Bills of Materials to reflect the BOM required to build the product

* Import/Export routines to exchange data with other software packages
* Output of design data directly to manufacturing facilities
* Output directly to a Rapid Prototyping or Rapid Manufacture Machine for industrial prototypes
* maintain libraries of parts and assemblies
* calculate mass properties of parts and assemblies
* aid visualization with shading, rotating, hidden line removal, etc.
* Bi-directional parametric association (modification of any feature is reflected in all information relying on that feature; drawings, mass properties, assemblies, etc.)
* kinematics, interference and clearance checking of assemblies
* sheet metal
* hose/cable routing
* electrical component packaging
* inclusion of programming code in a model to control and relate desired attributes of the model
* Programmable design studies and optimization
* Sophisticated visual analysis routines, for draft, curvature, curvature continuity.

Software technologies

Originally software for CAD systems were developed with computer language such as Fortran, but with the advancement of object-oriented programming methods this has radically changed. Typical modern parametric feature based modeler and freeform surface systems are built around a number of key C programming language modules with their own APIs. A CAD system can be seen as built up from the interaction of a graphical user interface (GUI) with NURBS geometry and/or boundary representation (B-rep) data via a geometric modeling kernel. A geometry constraint engine may also be employed to manage the associative relationships between geometry, such as wireframe geometry in a sketch or components in an assembly.

Unexpected capabilities of these associative relationships have led to a new form of prototyping called digital prototyping. In contrast to physical prototypes, which entail manufacturing time and material costs, digital prototypes allow for design verification and testing on screen, speeding time-to-market and decreasing costs. As technology evolves in this way, CAD has moved beyond a documentation tool (representing designs in graphical format) into a more robust designing tool that assists in the design process.

Hardware and OS technologies

Today most CAD computers are Windows based PCs. Some CAD systems also run on one of the Unix operating systems and with Linux. Some CAD systems such as QCad or NX provide multiplatform support including Windows, Linux, UNIX and Mac OS X.

Generally no special basic memory is required with the exception of a high end OpenGL based Graphics card. However for complex product design, machines with high speed (and possibly multiple) CPUs and large amounts of RAM are recommended. CAD was an application that benefited from the installation of a numeric coprocessor especially in early personal computers. The human-machine interface is generally via a computer mouse but can also be via a pen and digitizing graphics tablet. Manipulation of the view of the model on the screen is also sometimes done with the use of a spacemouse/SpaceBall. Some systems also support stereoscopic glasses for viewing the 3D model.

Using CAD
CAD is one of the many tools used by engineers and designers and is used in many ways depending on the profession of the user and the type of software in question. Each of the different types of CAD systems requires the operator to think differently about how he or she will use them and he or she must design their virtual components in a different manner for each.

There are many producers of the lower-end 2D systems, including a number of free and open source programs. These provide an approach to the drawing process without all the fuss over scale and placement on the drawing sheet that accompanied hand drafting, since these can be adjusted as required during the creation of the final draft.

3D wireframe is basically an extension of 2D drafting. Each line has to be manually inserted into the drawing. The final product has no mass properties associated with it and cannot have features directly added to it, such as holes. The operator approaches these in a similar fashion to the 2D systems, although many 3D systems allow using the wireframe model to make the final engineering drawing views.

3D "dumb" solids (programs incorporating this technology include AutoCAD and Cadkey 19) are created in a way analogous to manipulations of real word objects. Basic three-dimensional geometric forms (prisms, cylinders, spheres, and so on) have solid volumes added or subtracted from them, as if assembling or cutting real-world objects. Two-dimensional projected views can easily be generated from the models. Basic 3D solids don't usually include tools to easily allow motion of components, set limits to their motion, or identify interference between components.

3D parametric solid modeling (programs incorporating this technology include Alibre Design, TopSolid, T-FLEX CAD, SolidWorks, and Solid Edge) require the operator to use what is referred to as "design intent". The objects and features created are adjustable. Any future modifications will be simple, difficult, or nearly impossible, depending on how the original part was created. One must think of this as being a "perfect world" representation of the component. If a feature was intended to be located from the center of the part, the operator needs to locate it from the center of the model, not, perhaps, from a more convenient edge or an arbitrary point, as he could when using "dumb" solids. Parametric solids require the operator to consider the consequences of his actions carefully. What may be simplest today could be worst case tomorrow.

Some software packages provide the ability to edit parametric and non-parametric geometry without the need to understand or undo the design intent history of the geometry by use of direct modeling functionality. This ability may also include the additional ability to infer the correct relationships between selected geometry (e.g., tangency, concentricity) which makes the editing process less time and labor intensive while still freeing the engineer from the burden of understanding the model’s design intent history.

Draft views are able to be generated easily from the models. Assemblies usually incorporate tools to represent the motions of components, set their limits, and identify interference. The tool kits available for these systems are ever increasing, including 3D piping and injection mold designing packages.

Mid range software was integrating parametric solids more easily to the end user: integrating more intuitive functions (SketchUp), going to the best of both worlds with 3D dumb solids with parametric characteristics (VectorWorks) or making very real-view scenes in relative few steps (Cinema4D).

Top end systems offer the capabilities to incorporate more organic, aesthetics and ergonomic features into designs (Catia, GenerativeComponents). Freeform surface modelling is often combined with solids to allow the designer to create products that fit the human form and visual requirements as well as they interface with the machine.

The Effects of CAD

Starting in the late 1980s, the development of readily affordable CAD programs that could be run on personal computers began a trend of massive downsizing in drafting departments in many small to mid-size companies. As a general rule, one CAD operator could readily replace at least three to five drafters using traditional methods. Additionally, many engineers began to do their own drafting work, further eliminating the need for traditional drafting departments. This trend mirrored that of the elimination of many office jobs traditionally performed by a secretary as word processors, spreadsheets, databases, etc. became standard software packages that "everyone" was expected to learn.

Another consequence had been that since the latest advances were often quite expensive, small and even mid-size firms often could not compete against large firms who could use their computational edge for competitive purposes. Today, however, hardware and software costs have come down. Even high-end packages work on less expensive platforms and some even support multiple platforms. The costs associated with CAD implementation now are more heavily weighted to the costs of training in the use of these high level tools, the cost of integrating a CAD/CAM/CAE PLM using enterprise across multi-CAD and multi-platform environments and the costs of modifying design workflows to exploit the full advantage of CAD tools.

CAD vendors have been effective in providing tools to lower these training costs. These tools have operated in three CAD arenas:

1. Improved and simplified user interfaces. This includes the availability of “role” specific tailorable user interfaces through which commands are presented to users in a form appropriate to their function and expertise.
2. Enhancements to application software. One such example is improved design-in-context, through the ability to model/edit a design component from within the context of a large, even multi-CAD, active digital mockup.
3. User oriented modeling options. This includes the ability to free the user from the need to understand the design intent history of a complex intelligent model.

The adoption of CAD studio or "paper-less studio," as it is sometimes called, in architectural schools was not without resistance, however. Teachers were worried that sketching on a computer screen did not replicate the skills associated with age-old practice of sketching in a sketchbook. Furthermore, many teachers were worried that students would be hired for their computer skills rather than their design skill, as was indeed common in the 1990s. Today, however, (for better or worse, depending on the authority cited) education in CAD is now accepted across the board in schools of architecture. It should be noted, however, that not all architects have wanted to join the CAD revolution.

Fields of use

Architecture, engineering, and construction (AEC) industry

* Architecture
* Architectural engineering
* Interior design
* Interior architecture
* Building engineering
* Civil engineering and infrastructure

* Construction
* Roads and highways
* Railroads and tunnels
* Water supply and hydraulic engineering
* Storm drain, wastewater and sewer systems
* Mapping and surveying
* Chemical plant design
* Factory layout
* Heating, ventilation and air-conditioning (HVAC)

#Mechanical (MCAD) engineering

* Automotive - vehicles
* Aerospace
* Consumer goods
* Machinery
* Shipbuilding
* Bio-mechanical systems

# Electronic design automation (EDA)

* Electronic and electrical computer-aided design (ECAD)
* Digital circuit design

# Electrical engineering

* Power engineering or Power systems engineering
* Power systems CAD
* Power analytics
* RF microwave CAE CAD

# Manufacturing process planning
# Industrial design
# Software applications
# Apparel and textile design

* Fashion design

# Designing musical instruments
# Garden design
# Lighting design
# Medicine
# Production design

References
1. ^ "History of CAD/CAM". CADAZZ (2004).

2. ^ Pillers, Michelle (1998.03). "MCAD Renaissance of the 90's". Cadence Magazine.
3. ^ Bozdoc, Martian. "The History of CAD". iMB.
4. ^ Joneja, Ajay. "Some Important Events in the Development of Computer-Aided Design and Manufacturing". IELM.
5. ^ Carlson, Wayne (2003). "A Critical History of Computer Graphics and Animation". Ohio State University.
6. ^ "MCS Founder".

CNC MACHINES

noreply@blogger.com (widi san) — Wed, 20 Aug 2008 05:08:00 +0000

From Wikipedia, the free encyclopedia
(Redirected from Cnc)

The abbreviation CNC stands for computer numerical control, and refers specifically to a computer "controller" that reads G-code instructions and drives a machine tool, a powered mechanical device typically used to fabricate components by the selective removal of material. CNC does numerically directed interpolation of a cutting tool in the work envelope of a machine. The operating parameters of the CNC can be altered via a software load program.

Historical overview

CNC was preceded by NC (Numerically Controlled) machines, which were hard wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the MIT Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.

Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by RS232 cables, floppy disks, and now is commonly tied directly into plant networks. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their ISO certification requirements.

The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action have been dramatically reduced.

With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components.One new emerging technology currently being used in the metal industry is the CNC machine CNC stands for computer numerical control

Production environment

A series of CNC machines may be combined into one station, commonly called a "cell", to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by CAM software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator's mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.

Types of instruction

Main article: G-code

A line in a G-code file can instruct the machine tool to do one of several things.

Movements

Lately, some controllers have implemented the ability to follow an arbitrary curve (NURBS), but these efforts have been met with skepticism since, unlike circular arcs, their definitions are not natural and are too complicated to set up by hand, and CAM software can already generate any motion using many short linear segments.

Drilling

A tool can be used to drill holes by pecking to let the swarf out. Using an internal thread cutting tool and the ability to control the exact rotational position of the tool with the depth of cut, it can be used to cut screw threads.

A drilling cycle is used to repeat drilling or tapping operations on a workpiece. The drilling cycle accepts a list of parameters about the operation, such as depth and feed rate. To begin drilling any number of holes to the specifications configured in the cycle, the only input required is a set of coordinates for hole location. The cycle takes care of depth, feed rate, retraction, and other parameters that appear in more complex cycles. After the holes are completed, the machine is given another command to cancel the cycle, and resumes operation.

Parametric programming

A more recent advancement in CNC interpreters is support of logical commands, known as parametric programming. Parametric programs incorporate both G-code and these logical constructs to create a programming language and syntax similar to BASIC. Various manufacturers refer to parametric programming in brand-specific ways. For instance, Haas Automation refers to parametric programs as macros. GE Fanuc refers to it as Custom Macro A & B, while Okuma refers to it as User Task 2. The programmer can make if/then/else statements, loops, subprogram calls, perform various arithmetic, and manipulate variables to create a large degree of freedom within one program. An entire product line of different sizes can be programmed using logic and simple math to create and scale an entire range of parts, or create a stock part that can be scaled to any size a customer demands.

Parametric programming also enables custom machining cycles, such as fixture creation and bolt circles. If a user wishes to create additional fixture locations on a work holding device, the machine can be manually guided to the new location and the fixture subroutine called. The machine will then drill and form the patterns required to mount additional vises or clamps at that location. Parametric programs are also used to shorten long programs with incremental or stepped passes. A loop can be created with variables for step values and other parameters, and in doing so remove a large amount of repetition in the program body.

Because of these features, a parametric program is more efficient than using CAD/CAM software for large part runs. The brevity of the program allows the CNC programmer to rapidly make performance adjustments to looped commands, and tailor the program to the machine it is running on. Tool wear, breakage, and other system parameters can be accessed and changed directly in the program, allowing extensions and modifications to the functionality of a machine beyond what a manufacturer envisioned.

There are three types of variables used in CNC systems: local variable, common variable, and system variable. Local variable is used to hold data after machine off preset value. Common variable is used to hold data if machine switch off does not erase form data. The System variable this variable used system parameter this cannot use direct to convert the common variable for example tool radius, tool length, and tool height to be measured in millimeters or inches.

Typical logic to a parameter program is as follows;

First define variables to start your program.
-bolt circle radius
-how many holes
-centerpoint of bolt circle
Next build a subprogram that crunches the math.
When you are ready to drill or tap your holes, run the drill cycle
off of your math in subprogram.

Tool call,
spindle speed,and offset pickup,etc
G43 in some cases (tool length pickup)
G81(drill cycle)
call sub program
N50
G80
M30

Subprogram
N100 (this line here is used as a marker)
#100=15 (this line is your radius)
#105=((COS#104)*#100) (x location)
#106=((SIN#104)*#100) (y location)
x#105 y#106 (remember your G81 code is modal)
If #104 GT 360 goto N50
#104=(#104+(360/#101))
Goto 100

This is just a model to show the logic of programming.
As all languages have some differences, the logic is all similar.

Hex Cap Screw

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 13:15:00 +0000

Hanger Bolts

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 13:13:00 +0000

RB & W Frame Bolts Grade 8 Alloy

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 13:10:00 +0000

Elevator Bolts

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 13:08:00 +0000

Carriage Bolts

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 13:07:00 +0000

External Threads Class 2A

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 12:47:00 +0000

Table of pitch for ISO Metric Coarse Threads

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 12:36:00 +0000

Size (mm)	Pitch (mm)
1.6	0.35
2	0.4
2.5	0.45
3	0.5
3.5	0.6
4	0.7
5	0.8
6	1
8	1.25
10	1.5
12	1.75
14	2
16	2
20	2.5
22	2.5
24	3
27	3
30	3.5
36	4
42	4.5
48	5
56	5.5
64	6
72	6
80	6
90	6
100	6

Table of pitch for UNF (fine) threads

noreply@blogger.com (widi san) — Mon, 28 Jul 2008 12:30:00 +0000

Size	Major Diameter	Threads Per
Size	Inches	Inch
0	0.06	80
1*	0.073	72
2	0.086	64
3*	0.099	56
4	0.112	48
5	0.125	44
6	0.138	40
8	0.164	36
10	0.19	32
12*	0.216	28
1/4	0.25	28
5/16	0.3125	24
3/8	0.375	24
7/16	0.4375	20
1/2	0.5	20
9/16	0.5625	18
5/8	0.625	18
3/4	0.75	16
7/8	0.875	14
1	1	12
1 1/8	1.125	12
1 1/4	1.25	12
1 3/8	1.375	12
1 1/2	1.5	12

screw

noreply@blogger.com (widi san) — Thu, 10 Jul 2008 09:24:00 +0000

Screw Thread Forms

The most common screw thread form is the one with a symmetrical V-Profile. The included angle is 60 degrees. This form is prevalent in the Unified Screw Thread (UN, UNC, UNF, UNRC, UNRF) form as well as the ISO/Metric thread.

The advantage of symmetrical threads is that they are easier to manufacture and inspect compared to non-symmetrical threads. These are typically used in general purpose fasteners.

Other symmetrical threads are the Whitworth, and the Acme. The Acme thread form has a stronger thread which allows for use in translational applications such as those involving moving heavy machine loads as found on machine tools. Previously square threads with parallel sides were used for the same applications. The square thread form, while strong, is harder to manufacture. It also cannot be compensated for wear unlike an Acme thread.

Basic Size is the nominal size to which the tolerance is applied to determine the maximum and minimum material size.

Allowance is the difference between the design (maximum material condition, MMC)size and the basic size. See Unified Standard Series.

Thread Classes: The different thread classes have differing amounts of tolerance and allowance. Classes 1A, 2A, 3A apply t external threads; Classes 1B, 2B, 3B apply to internal threads. See Unified Standard Series.

Classes 2A and 2B The maximum diameter of uncoated (unplated) class 2A, (external) thread are less than the basic by the amount of allowance. When a coating is intended, the max diameter of class 2A may be exceeded by the amount of allowance. For an internal thread, the minimum diameters are basic whether or not coated (plated)--no allowance is available at the maximum metal limits. See Unified Standard Series.

Classes 3A and 3B are used for closer tolerances than those available from classes 2A and 2B. The maximum diameters of Class 3A (external) threads and the minimum diameters of Class 3B (internal) threads are basic, whether coated (plated) or not--thus no allowance or clearance is available for assembly of components at maximum material condition. See Unified Standard Series.

Classes 1A and 1B are the replacement threads for American National Class 1. They are intended for special applications involving replacement parts, for quick and easy assembly even when the threads are slightly damaged or dirty. See Unified Standard Series.

Coating (or plating) of threads The external threads should not be greater than basic size after plating, and the internal threads should not be less than basic size after plating. For electro-plated parts, Class 2A allowance is sufficient for the plating build-up. After plating the external threads should pass a basic Class 3A GO gage and a Class 2A NO-GO gage. Class 3A does not include an allowance--the class 2A allowance may be used as long as it is adequate for the plating thickness considered. Since only Class 2A external threads have an explicit allowance for coating, other classes both internal and external should have the size limits adjusted to allow for the desired coating thicknesses.

Machining

noreply@blogger.com (widi san) — Thu, 10 Jul 2008 08:18:00 +0000

Machining: An Introduction

In terms of annual dollars spent, machining is the most important of the manufacturing processes. Machining can be defined as the process of removing material from a workpiece in the form of chips. The term metal cutting is used when the material is metallic. Most machining has very low set-up cost compared to forming, molding, and casting processes. However, machining is much more expensive for high volumes. Machining is necessary where tight tolerances on dimensions and finishes are required.

The Machining section is divided into the following categories:

Turning Machine

noreply@blogger.com (widi san) — Wed, 09 Jul 2008 13:59:00 +0000

From : Michigan Technological University's Turning Information Center

INTRODUCTION

What is turning?

Turning is the machining operation that produces cylindrical parts. In its basic form, it can be defined as the machining of an external surface:

with the workpiece rotating,
with a single-point cutting tool, and
with the cutting tool feeding parallel to the axis of the workpiece and at a distance that will remove the outer surface of the work.

Taper turning is practically the same, except that the cutter path is at an angle to the work axis. Similarly, in contour turning, the distance of the cutter from the work axis is varied to produce the desired shape.

Even though a single-point tool is specified, this does not exclude multiple-tool setups, which are often employed in turning. In such setups, each tool operates independently as a single-point cutter.

View a typical turning operation. This movie is from the MIT-NMIS Machine Shop Tutorial.

Adjustable cutting factors in turning

The three primary factors in any basic turning operation are speed, feed, and depth of cut. Other factors such as kind of material and type of tool have a large influence, of course, but these three are the ones the operator can change by adjusting the controls, right at the machine.

Speed, always refers to the spindle and the workpiece. When it is stated in revolutions per minute(rpm) it tells their rotating speed. But the important figure for a particular turning operation is the surface speed, or the speed at which the workpeece material is moving past the cutting tool. It is simply the product of the rotating speed times the circumference (in feet) of the workpiece before the cut is started. It is expressed in surface feet per minute (sfpm), and it refers only to the workpiece. Every different diameter on a workpiece will have a different cutting speed, even though the rotating speed remains the same.

Feed, always refers to the cutting tool, and it is the rate at which the tool advances along its cutting path. On most power-fed lathes, the feed rate is directly related to the spindle speed and is expressed in inches (of tool advance) per revolution ( of the spindle), or ipr. The figure, by the way, is usually much less than an inch and is shown as decimal amount.

Depth of Cut, is practically self explanatory. It is the thickness of the layer being removed from the workpiece or the distance from the uncut surface of the work to the cut surface, expressed in inches. It is important to note, though, that the diameter of the workpiece is reduced by two times the depth of cut because this layer is being removed from both sides of the work.

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LATHE RELATED OPERATIONS

The lathe, of course, is the basic turning machine. Apart from turning, several other operations can also be performed on a lathe.

Boring. Boring always involves the enlarging of an existing hole, which may have been made by a drill or may be the result of a core in a casting. An equally important, and concurrent, purpose of boring may be to make the hole concentric with the axis of rotation of the workpiece and thus correct any eccentricity that may have resulted from the drill's having drifted off the center line. Concentricity is an important attribute of bored holes. When boring is done in a lathe, the work usually is held in a chuck or on a face plate. Holes may be bored straight, tapered, or to irregular contours. Boring is essentially internal turning while feeding the tool parallel to the rotation axis of the workpiece.

Facing. Facing is the producing of a flat surface as the result of a tool's being fed across the end of the rotating workpiece. Unless the work is held on a mandrel, if both ends of the work are to be faced, it must be turned end for end after the first end is completed and the facing operation repeated. The cutting speed should be determined from the largest diameter of the surface to be faced. Facing may be done either from the outside inward or from the center outward. In either case, the point of the tool must be set exactly at the height of the center of rotation. because the cutting force tends to push the tool away from the work, it is usually desirable to clamp the carriage to the lathe bed during each facing cut to prevent it from moving slightly and thus producing a surface that is not flat. In the facing of casting or other materials that have a hard surface, the depth of the first cut should be sufficient to penetrate the hard material to avoid excessive tool wear.

Parting. Parting is the operation by which one section of a workpiece is severed from the remainder by means of a cutoff tool. Because cutting tools are quite thin and must have considerable overhang, this process is less accurate and more difficult. The tool should be set exactly at the height of the axis of rotation, be kept sharp, have proper clearance angles, and be fed into the workpiece at a proper and uniform feed rate.

Threading. Lathe provided the first method for cutting threads by machines. Although most threads are now produced by other methods, lathes still provide the most versatile and fundamentally simple method. Consequently, they often are used for cutting threads on special workpieces where the configuration or nonstandard size does not permit them to be made by less costly methods. There are two basic requirements for thread cutting. An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece. The second by requirement is that the tool must move longitudinally in a specific relationship to the rotation of the workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of the carriage relative to the rotation of the spindle.

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CUTTING TOOLS FOR LATHES

Tool Geometry. For cutting tools, geometry depends mainly on the properties of the tool material and the work material. The standard terminology is shown in the following figure. For single point tools, the most important angles are the rake angles and the end and side relief angles.

The back rake angle affects the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces resulting in smaller deflections of the workpiece, tool holder, and machine. If the back rake angle is too large, the strength of the tool is reduced as well as its capacity to conduct heat. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. The higher the hardness, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range. There are two basic requirements for thread cutting. An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece. The second by requirement is that the tool must move longitudinally in a specific relationship to the rotation of the workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of the carriage relative to the rotation of the spindle.

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CUTTING TOOLS FOR LATHES

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CUTTING TOOLS FOR LATHES

Most lathe operations are done with relatively simple, single-point cutting tools. On right-hand and left-hand turning and facing tools, the cutting takes place on the side of the tool; therefore the side rake angle is of primary importance and deep cuts can be made. On the round-nose turning tools, cutoff tools, finishing tools, and some threading tools, cutting takes place on or near the end of the tool, and the back rake is therefore of importance. Such tools are used with relatively light depths of cut. Because tool materials are expensive, it is desirable to use as little as possible. It is essential, at the same, that the cutting tool be supported in a strong, rigid manner to minimize deflection and possible vibration. Consequently, lathe tools are supported in various types of heavy, forged steel tool holders, as shown in the figure.

The tool bit should be clamped in the tool holder with minimum overhang. Otherwise, tool chatter and a poor surface finish may result. In the use of carbide, ceramic, or coated carbides for mass production work, throwaway inserts are used; these can be purchased in great variety of shapes, geometrics (nose radius, tool angle, and groove geometry), and sizes.

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TURNING MACHINES

The turning machines are, of course, every kinds of lathes. Lathes used in manufacturing can be classified as engine, turret, automatics, and numerical control etc.

They are heavy duty machine tools and have power drive for all tool movements. They commonly range in size from 12 to 24 inches swing and from 24 to 48 inches center distance, but swings up to 50 inches and center distances up to 12 feet are not uncommon. Many engine lathes are equipped with chip pans and built-in coolant circulating system.

Turret Lathes. In a turret lathe, a longitudinally feedable, hexagon turret replaces the tailstock. The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be moved longitudinally, either annually or by power, to provide feed for the tools. When the turret assembly is backed away from the spindle by means of a capstan wheel, the turret indexes automatically at the end of its movement thus bringing each of the six tools into operating position. The square turret on the cross slide can be rotated manually about a vertical axis to bring each of the four tools into operating position. On most machines, the turret can be moved transversely, either manually or by power, by means of the cross slide, and longitudinally through power or manual operation of the carriage. In most cased, a fixed tool holder also is added to the back end of the cross slide; this often carries a parting tool.

Through these basic features of a turret lathe, a number of tools can be set on the machine and then quickly be brought successively into working position so that a complete part can be machined without the necessity for further adjusting, changing tools, or making measurements.

Single-Spindle Automatic Screw Machines. There are two common types of single-spindle screw machines, One, an American development and commonly called the turret type (Brown & Sharp), is shown in the following figure. The other is of Swiss origin and is referred to as the swiss type. The Brown & Sharp screw machine is essentially a small automatic turret lathe, designed for bar stock, with the main turret mounted on the cross slide. All motions of the turret, cross slide, spindle, chuck, and stock-feed mechanism are controlled by cams. The turret cam is essentially a program that defines the movement of the turret during a cycle. These machines usually are equipped with an automatic rod feeding magazine that feeds a new length of bar stock into the collect as soon as one rod is completely used.

CNC Machines. Nowadays, more and more Computer Numerical Controlled (CNC) machines are being used in every kinds of manufacturing processes. In a CNC machine, functions like program storage, tool offset and tool compensation, program-editing capability, various degree of computation, and the ability to send and receive data from a variety of sources, including remote locations can be easily realized through on board computer. The computer can store multiple-part programs, recalling them as needed for different parts. A CNC turret lathe in Michigan Technological University is shown in the following picture.

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TURNING RESEARCH AT MICHIGAN TECH

Here at Michigan Technological University, turning research is being conducted in three areas:

Non-Circular Turning

Vibration Abatement

Chatter Suppression

Our Equipment:

Our Actuator/Flexor system uses a magnetostrictive material known as Terfenol-D to provide nearly instantaneous elongation. The cutting tool is mounted in an aluminum flexor, which provides motion similar to a hinge, but without friction or backlash.

We have a pentium based pc that is used to control the elongation of the actuator, a Power Amplifier to provide the necessary current to the actuator, and a compliment of sensors, including: Load Cells, Acceleromenters, a displacement sensor, and other equipment. The total actuation system has a bandwidth of 1.8 kHz.

Here is a picture of the actuator in operation. The material being cut is an aluminum alloy. The actuator/flexor system is mounted to the tool holder turret of MTU's CNC lathe. We hope to have a video soon....better wear your safety glasses!!

More information can be found at Precision Machining

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Metric Nut Size Table

noreply@blogger.com (widi san) — Wed, 09 Jul 2008 13:57:00 +0000

Nut Size	Diameter*	Height
Nut Size	Diameter*	Hex Nut	Jam Nut	Nylock Nut
2	4	1.6	1.2	-
2.5	5	2	1.6	-
3	5.5	2.4	1.8	4
4	7	3.2	2.2	5
5	8	4	2.7	5
6	10	5	3.2	6
7	11	5.5	3.5	-
8	13	6.5	4	8
10	17	8	5	10
12	19	10	6	12
14	22	11	7	14
16	24	13	8	16
18	27	15	9	18.5
20	30	16	10	20
* This is the diameter across the flats. It is also the size of wrench to use.

bolt/screw

noreply@blogger.com (widi san) — Wed, 09 Jul 2008 13:25:00 +0000

Screw

From Wikipedia, the free encyclopedia

(Redirected from Screw/Bolt)

A screw is a shaft with a helical groove or thread formed on its surface and provision at one end to turn the screw. Its main uses are as a threaded fastener used to hold objects together, and as a simple machine used to translate torque into linear force. It can also be defined as an inclined plane wrapped around a shaft.

Screws and Bolts

A screw used as a threaded fastener consists of a cylindrical shaft, which in many cases tapers to a point at one end, and with a helical ridge or thread formed on it, and a head at the other end which can be rotated by some means. The thread is essentially an inclined plane wrapped around the shaft. The thread mates with a complementary helix in the material. The material may be manufactured with the mating helix using a tap, or the screw may create it when first driven in (a self-tapping screw). The head is specially shaped to allow a screwdriver or wrench (British English: spanner) to rotate the screw, driving it in or releasing it. The head is of larger diameter than the body of the screw and has no thread so that the screw can not be driven deeper than the length of the shaft, and to provide compression.

Screws can normally be removed and reinserted without reducing their effectiveness. They have greater holding power than nails and permit disassembly and reuse.

The vast majority of screws are tightened by clockwise rotation; we speak of a right-hand thread. Screws with left-hand threads are used in exceptional cases, when the screw is subject to anticlockwise forces that might undo a right-hand thread. Left-hand screws are used on rotating items such as the left-hand grinding wheel on a bench grinder or the left hand pedal on a bicycle hub nuts on the left side of some automobiles. (both looking towards the equipment) or

Threaded fasteners were made by a cutting action such as dies provide, but recent advances in tooling allow them to be made by rolling an unthreaded rod (the blank) between two specially machined dies which squeeze the blank into the shape of the required fastener, including the thread. This method has the advantages of work hardening the thread and saving material. A rolled thread can be distinguished from a thread formed by a die as the outside diameter of the thread is greater than the diameter of the unthreaded portion of the shaft. Bicycle spokes, which are just bolts with long thin unthreaded portions, always use rolled threads for strength.

Differentiation between bolt and screw

Carriage bolt with square nut.

Structural bolt DIN 6914 with DIN 6916 washer and UNI 5587 nut.

A universally accepted distinction between a screw and a bolt does not exist.

In common usage the term screw refers to smaller (less than 1/4 inch) threaded fasteners, especially threaded fasteners with tapered shafts and the term bolt refers to larger threaded fasteners that do not have tapered shafts. The term machine screw is commonly used to refer to smaller threaded fasteners that do not have a tapered shaft.

Various methods of distinguishing bolts and screws exist or have existed. These methods conflict at times and can be confusing. Old SAE and USS standards made a distinction between a bolt and a cap screw based on whether a portion of the shaft was un-threaded or not. Cap screws had shafts that were threaded up to the head and bolts had partially threaded shafts. Today a bolt that has a completely threaded shaft might be referred to as a tap bolt.

ASME B18.2.1 defines a bolt as "an externally threaded fastener designed for insertion through the holes in assembled parts, and is normally intended to be tightened or released by torquing a nut". Using this definition to determine whether a particular threaded fastener is a screw or a bolt requires that an assumption be made about the intended purpose of the threaded fastener and as a practical matter doesn't seem to be followed by most threaded fastener manufacturers. It also conflicts with common usage such as the term, "head bolt", which is a threaded fastener that mates with a tapped hole in an engine block and is not intended to mate with a nut.

It is possible to find other distinctions than those described above, but regardless of the particular distinction favored by an individual or standards body the use of the term screw or bolt varies. More specific terms for threaded fastener types that include the word screw or boltmachine screw or carriage bolt) have more consistent usage and are the common way to specify a particular kind of fastener. (such as

The US government made an effort to formalize the difference between a bolt and a screw, because different tariffs apply to each. The document seems to have no significant effect on common usage and does not eliminate the ambiguous nature of the distinction for some fasteners. It is available here.

Other fastening methods

Alternative fasteners to screws and bolts are nails, rivets, roll pins, pinned shafts, welding, soldering, brazing, gluing (including taping).

Another option is the threaded insert. Examples include Helical Inserts [1] and Keensert [2].

Materials and strength

Screws and bolts are made in a wide range of materials, with steel being perhaps the most common, in many varieties. Where great resistance to weather or corrosion is required, stainless steel, titanium, brass, bronze, monel or silicon bronze may be used, or a coating such as brass, zinc or chromium applied. Electrolytic action from dissimilar metals can be prevented with aluminium screws for double-glazing tracks, for example. Some types of plastic, such as nylon or Teflon, can be threaded and used for fastening requiring moderate strength and great resistance to corrosion or for the purpose of electrical insulation. Even porcelain and glass can have molded screw threads that are used successfully in applications such as electrical line insulators and canning jars.

The same type of screw or bolt can be made in many different grades of material. For critical high-tensile-strength applications, low-grade bolts may fail, resulting in damage or injury. On SAE-standard bolts, a distinctive pattern of marking is impressed on the heads to allow inspection and validation of the strength of the bolt. However, low-cost counterfeit fasteners may be found with actual strength far less than indicated by the markings. Such inferior fasteners are a danger to life and property when used in aircraft, automobiles, heavy trucks, and similar critical applications. Gradings are indicated as markings, while grade 0 is the lowest, grade 10 is the highest. Here is the sequence of bolt strength and markings, from least to most. Grade 0, 1 and 2 bolts have no markings, grade 3 has 2 radial lines, grade 5 has 3, grade 6 has 4, grade 7 has 5, grade 8 has 6, grade 9 has 7, grade 10 has 8.

In some applications joints are designed so that the screw or bolt will intentionally fail before more expensive components. In this case replacing an existing fastener with a higher strength fastener can result in equipment damage. Thus it is generally good practice to replace fasteners with the same grade originally installed.

Mechanical analysis

Rotating screw and fixed trough

A screw or bolt is a specialized application of the inclined plane. The inclined plane, called its thread, is helically disposed around a cylinder or shaft. That thread usually either fits into a corresponding (negative or female) helical thread in a nut, or forms a corresponding helical cut in surrounding softer material as it is inserted. A simple screw, such as for fastening, is typically pointed, and thereby is commonly distinguished (in informal terminology) from a bolt or machine screw. Common screws, and usually bolts, have a head which may be mechanically driven or rotated, which usually serves as a stop, and may have an unthreaded shoulder portion beneath the head.

The technical analysis (see also statics, dynamics) to determine the pitch, thread profile, coefficient of friction (static and dynamic), and holding power of a screw or bolt is very similar to that performed to predict wedge behavior. Wedges are discussed in the article on simple machines.

Critical applications of screws and bolts will specify a torque that must be applied when driving it. The main concept is to tension the bolt, and compress parts being held together, creating a spring-like assembly. The stress thus introduced to the bolt is called a preload. When external forces try to separate the parts, the bolt experiences no strain unless the preload force is exceeded.

As long as the preload is never exceeded, the bolt or nut will never come loose (assuming the full strength of the bolt is used). If the full strength of the bolt is not used (for example, a steel bolt threaded into aluminium, then a thread-locking adhesive or insert may be used.

If the preload is exceeded during normal use, the joint will eventually fail. The preload is calculated as a percentage of the bolt's yield tensile strength, or the strength of the threads it goes into, or the compressive strength of the clamped layers (plates, washers, gaskets), whichever is least.

Tensile strength

Rusty hexagonal bolt heads

Screws and bolts are usually in tension when properly fitted. In most applications they are not designed to bear large shear forces. For example, when two overlapping metal bars joined by a bolt are likely to be pulled apart longitudinally, the bolt must be tight enough so that the frictionfretting). For this type of application, high-strength steel bolts are used and should be tightened to a specified torque. between the two bars can overcome the longitudinal force. If the bars slip, then the bolt may be sheared in half, or friction between the bolt and slipping bars may erode and weaken the bolt (called

High-strength steel bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. The property classes most often used are 5.8, 8.8, and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

Tensile ultimate strength is the stress at which the bolt fails (breaks in half). Tensile yield strength is the stress at which the bolt will receive a permanent set (an elongation from which it will not recover when the force is removed) of 0.2 % offset strain. When elongating a fastener prior to reaching the yield point, the fastener is said to be operating in the elastic region; whereas elongation beyond the yield point is referred to as operating in the plastic region, since the fastener has suffered permanent plastic deformation.

Mild steel bolts have property class 4.6. High-strength steel bolts have property class 8.8 or above. An M10, property class 8.8 bolt can very safely hold a static tensile load of about 15 kN.

There is no method to measure the tension of a bolt already in place other than to tighten it and identify at which point the bolt starts moving. This is known as 're-torqueing'. An electronic torque wrench is used on the bolt under test, and the torque applied is constantly measured. When the bolt starts moving (tightening) the torque briefly drops sharply - this drop-off point is considered the measure of tension.

Types of screws and bolts

Threaded fasteners either have a tapered shaft or a non-tapered shaft. Fasteners with tapered shafts are designed to either be driven into a substrate directly or into a pilot hole in a substrate. Mating threads are formed in the substrate as these fasteners are driven in. Fasteners with a non-tapered shaft are designed to mate with a nut or to be driven into a tapped hole.

A phillips wood screw being driven into a board with a drill

Fasteners with a tapered shaft (tapping screws)

Screw - There is not a universally accepted definition of the word, screw. It generally refers to a smaller threaded fastener with a tapered shaft. See the Differentiation Between Bolt and Screw section above for a more detailed discussion.
Wood Screw – Generally has an un-threaded portion of the shaft below the head. It is designed to attach two pieces of wood together.
Lag Screw (Lag Bolt) – Similar to a wood screw except that it is generally much larger running to lengths up to 15 inches (381 mm) with diameters from a quarter inch to 1/2 inches (6.4 mm-12.25 mm) in commonly available (hardware store) sizes (not counting larger mining and civil engineering lags and lag bolts) and it generally has a hexagonal head drive head. Lag bolts are design for securely fastening heavy timbers (post and beams, timber railway trestles and bridges) to one another, or to fasten wood to masonry or concrete.
- Lag bolts are usually used with an expanding insert called a lag in masonry or concrete walls, the lag manufactured with a hard metal jacket that bites into the sides of the drilled hole, and the inner metal in the lag being a softer alloy of lead, or zinc amalgamated with soft iron. The coarse thread of a lag bolt and lag mesh and deform slightly making a secure near water tight anti-corroding mechanically strong fastening.
Sheet Metal Screw (Self-tapping Screw, thread cutting screws) - Has sharp threads that cut into a material such as sheet metal, plastic or wood. They are sometimes notched at the tip to aid in chip removal during thread cutting. The shaft is usually threaded up to the head. Sheet metal screws make excellent fasteners for attaching metal hardware to wood because the fully thread shaft provides good retention in wood.
Self-drilling screw (Teks^(R) screw) - Similar to a sheet metal screw, but it has a drill-shaped point to cut through the substrate to eliminate the need for drilling a pilot hole. Designed for use in soft steel or other metals.
Drywall screw - Specialized screw with a bugle head that is designed to attach drywall to wood or metal studs, however it is a versatile construction fastener with many uses. The diameter of drywall screw threads is larger than the shaft diameter.
Particle Board Screw (Chipboard Screw) - Similar to a drywall screw except that it has a thinner shaft and provides better holding power in particle board.
Deck Screw - Similar to drywall screw except that it is has improved corrosion resistance and is generally supplied in a larger gauge.
Double ended screw (Dowel screw) - Similar to a wood screw but with two pointed ends and no head, used for making hidden joints between two pieces of wood.
Screw Eye(Eye Screw) - Screw with a looped head. Larger ones are sometimes call lag eye screws. Designed to be used as attachment point, particularly for something that is hung from it.

Combination flanged-hex/Phillips-head screw used in computers

Fasteners with a non-tapered shaft

Bolt - There is no universally accepted definition of the word bolt. It generally refers to a larger threaded fastener with a non-tapered shaft. See the Differentiation Between Bolt and Screw section above for a more detailed discussion.
Cap Screw – In places the term is used interchangeably with bolt. In the past the term, cap screw was restricted to threaded fasteners with a shaft that is threaded all the way to the head, however this is now a non-standard usage.
Hex Cap Screw – Cap screw with a hexagonal head, designed to be driven by a wrench (spanner). An ASME B18.2.1 compliant cap screw has somewhat tighter tolerances than a hex bolt for the head height and the shaft length. The nature of the tolerance difference allows an ASME B18.2.1 hex cap screw to always fit where a hex bolt is installed but a hex bolt could be slightly too large to be used where a hex cap screw is designed in.
Hex Bolt - At times the term is used interchangeably with hex cap screw. An ASME B18.2.1 compliant hex bolt is built to different tolerances than a hex cap screw.
Socket Cap Screw – Also known as a socket head cap screw, socket screw or "Allen bolt," this is a type of cap screw with a hexagonal recessed drive. The most common types in use are fitted with a cylindrical head whose diameter is nominally 1.5 times (1960 series design) that of the screw shank (major) diameter. Other head designs include button head and flat head, the latter designed to be seated into countersunk holes. A hex key hex driver is required to tighten or loosen a socket screw. Socket screws are commonly used in assemblies that do not provide sufficient clearance for a conventional wrench or socket. (sometimes referred to as an "Allen wrench") or
Machine screw - Generally a smaller fastener (less than 1/4 inch in diameter) threaded the entire length of its shaft that usually has a recessed drive type (slotted, Phillips, etc.). Machine screws are also made with socket heads (see above), in which case they may be referred to as socket head machine screws.
Self Tapping Machine Screw – Similar to a machine screw except the lower part of the shaft is designed to cut threads as the screw is driven into an un-tapped hole. The advantage of this screw type over a self tapping screw is that if the screw is reinstalled new threads are not cut as the screw is driven.
Set screw (grub screw) - Generally a headless screw but can be any screw used to fix a rotating part to a shaft. The set screw is driven through a threaded hole in the rotating part until it is tight against the shaft. The most often used type is the socket set screw, which is tightened or loosened with a hex key or hex driver.
Tap Bolt - A bolt that is threaded all the way to the head. An ASME B18.2.1 compliant tap bolt has the same tolerances as an ASME B18.2.1 compliant hex cap screw.
Stud - similar to a bolt but without the head. Studs are threaded on both ends. In some cases the entire length of the stud is threaded, while in other cases there will be an un-threaded section in the middle. (See also: screw anchor, wedge anchor.)
Eye Bolt – A bolt with a looped head.
Toggle Bolt – A bolt with a special nut known as a wing. It is designed to be used where there is no access to side of the material where the nut is located. Usually the wing is spring loaded and expands after being inserted into the hole.
Carriage Bolt (Coach Bolt) - Has a domed or countersunk head, and the shaft is topped by a short square section under the head. The square section grips into the part being fixed (typically wood), preventing the bolt from turning when the nut is tightened. A rib neck carriage bolt has several longitudinal ribs instead of the square section, to grip into a metal part being fixed.
Stove Bolt - Similar to a carriage bolt, but usually used in metal. It requires a square hole in the metal being bolted to prevent the bolt from turning.
Shoulder Screw - Screw used for revolving joints in mechanisms and linkages. A shoulder screw consists of the shaft, which is ground to a precise diameter, and a threaded end, which is smaller in diameter than the shaft. Unlike other threaded fasteners, the size of a shoulder screw is defined by the shaft diameter, not the thread diameter. Shoulder screws are also called stripper bolts, as they are often used as guides for the stripper plate(s) in a die set.
Thumb Screw – A threaded fastener designed to be twisted into a tapped hole by hand without the use of tools.
Tension Control Bolt (TC Bolt) – Heavy duty bolt used in steel frame construction. The head is usually domed and is not designed to be driven. The end of the shaft has a spline on it which is engaged by a special power wrench which prevents the bolt from turning while the nut is tightened. When the appropriate torque is reached the spline shears off.

Other threaded fasteners

Thread rolling screws - have a lobed (usually triangular) cross section. They form threads by pushing outward during installation. They may have tapping threads or machine threads.
Superbolt, or Multi-Jackbolt Tensioner Alternative type of fastener that retrofits or replaces existing nuts, bolts, or studs. Tension in the bolt is developed by torquing individual jackbolts which are threaded through the body of the nut and push against a hardened washer. Installation and removal of any size tensioner is achieved with hand tools, which can be advantageous when dealing with large diameter bolting applications.
Hanger Screw – A headless fastener that has machine screw threads on one end and self tapping threads on the other designed to be driven into wood or another soft substrate. Often used for mounting legs on tables.

Shapes of screw head

(a) pan, (b) button, (c) round, (d) truss, (e) flat (countersunk), (f) oval

pan head: a low disc with chamfered outer edge.
button or dome head: cylindrical with a rounded top.
round head: dome-shaped, commonly used for machine screws.
truss head: lower-profile dome designed to prevent tampering.
flat head or countersunk: conical, with flat outer face and tapering inner face allowing it to sink into the material.
oval or raised head: countersunk with a rounded top.
bugle head: similar to countersunk, but there is a smooth progression from the shaft to the angle of the head, similar to the bell of a bugle.
cheese head: disc with cylindrical outer edge, height approximately half the head diameter.
fillister head: cylindrical, but with a slightly convex top surface.
socket head: cylindrical, relatively high, with different types of sockets (hex, square, torx, etc.).
mirror screw head: countersunk head with a tapped hole to receive a separate screw-in chrome-plated cover, used for attaching mirrors.
headless (set or grub screw): has either a socket or slot in one end for driving.

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and disassembly and also provides an easily-inspectable joint to guarantee proper assembly.

Types of screw drive

Part of the series on
Screw drive types
	Slotted
	Phillips ("Crosshead")
	Pozidriv (SupaDriv)
	Torx
	Hex (Allen)
	Robertson
	Tri-Wing
	Torq-Set
	Spanner Head
	Triple Square (XZN)
	Polydrive
	One-way
Others:
spline drive, double hex
This box: view • talk • edit

Phillips vs. Frearson

BNAE driver bit

Modern screws employ a wide variety of drive designs, each requiring a different kind of tool to drive in or extract them. The most common screw drives are the slotted and Phillips; hex, Robertson, and TORX are also common in some applications. Some types of drive are intended for automatic assembly in mass-production of such items as automobiles. More exotic screw drive types may be used in situations where tampering is undesirable, such as in electronic appliances that should not be serviced by the home repair person.

Slot head has a single slot, and is driven by a flat-bladed screwdriver. The slotted screw is common in woodworking applications, but is not often seen in applications where a power driver would be used, due to the tendency of a power driver to slip out of the head and potentially damage the surrounding material.
Cross-head, cross-point, or cruciform has a "+"-shaped slot and is driven by a cross-head screwdriver, designed originally for use with mechanical screwing machines. There are five types:
- The Phillips screw drive has slightly rounded corners in the tool recess, and was designed so the driver will slip out, or cam out, under high torque to prevent over-tightening. The Phillips Screw Company was founded in Oregon in 1933 by Henry F. Phillips, who bought the design from J. P. Thompson. Phillips was unable to manufacture the design, so he passed the patent to the American Screw Company, who was the first to manufacture it.
- A Reed & Prince or Frearson screw drive is similar to a Phillips but has a more pointed 75° V shape. Its advantage over the Phillips drive is that one driver or bit fits all screw sizes. It is found mainly in marine hardware and requires a special screw driver or bit to work properly. The tool recess is a perfect cross, unlike the Phillips head, which is designed to cam out. It was developed by an English inventor named Frearson in the 19th century and produced from the late 1930s to the mid-1970s by the former Reed & Prince Manufacturing Company of Worcester, Massachusetts, a company which traces its origins to Kingston, Massachusetts, in 1882, and was liquidated in 1990 with the sale of company assets. The company is now in business.
- A JIS (Japanese Industrial Standard) head, commonly found in Japanese equipment, looks like a Phillips screw, but is designed not to cam out and will, therefore, be damaged by a Phillips screwdriver if it is too tight. Heads are usually identifiable by a single dot to one side of the cross slot. The standard number is JIS B 1012:1985
- French Recess, also called BNAE NFL22-070 for Bureau de Normalisation de l'Aéronautique et de l'Espace, a French standards organization.
- Pozidriv is patented, similar to cross-head but designed not to slip, or cam out. It has four additional points of contact, and does not have the rounded corners that the Phillips screw drive has. Phillips screwdrivers will usually work in Pozidriv screws, but Pozidriv screwdrivers are likely to slip or tear out the screw head when used in Phillips screws. Heads are marked with single lines at 45 degrees to the cross recess, for identification. (note that two lines at 45 are a different recess: a very specialised Phillips screw). Pozidriv was jointly patented by the Phillips Screw Company and American Screw Company in the USA. Developed by GKN in the 1960s, the recess is licenced from Trifast PLC in the rest of the world.
- Supadriv is similar to Pozidriv.
TORX is a star-shaped "hexalobular" drive with six rounded points. It was designed to permit increased torque transfer from the driver to the bit compared to other drive systems. TORX is very popular in the automotive and electronics industries due to resistance to cam out and extended bit life, as well as reduced operator fatigue by minimizing the need to bear down on the drive tool to prevent cam out. TORX screws were found in early Apple Macintosh computers, to discourage home repairs^{[citation needed]}. TORX PLUS is an improved version of TORX which extends tool life even further and permits greater torque transfer compared to TORX. A tamper-resistant TORX head has a small pin inside the recess. The tamper-resistant TORX is also made in a 5 lobed variant. These "5-star" TORX configurations are commonly used in correctional facilities, public facilities and government schools, but can also be found in some electronic devices, such as Seagate's external drives.
TTAP is an improved "hexalobular" drive for without wobbling and stable stick-fit. TTAP is backward convertible with generic hexalobular drive.

Hex socket screws

Hexagonal (hex) socket head has a hexagonal hole and is driven by a Hex Wrench, sometimes called an Allen key or Hex key, or by a power tool with a hexagonal bit. Tamper-resistant versions with a pin in the recess are available. Hex sockets are increasingly used for modern bicycle parts because hex wrenches are very light and easily carried tools. They are also frequently used for self-assembled furniture.
Robertson head, invented in 1908 by P.L. Robertson, has a square hole and is driven by a special power-tool bit or screwdriver. The screw is designed to maximize torque transferred from the driver, and will not slip, or cam out. It is possible to hold a Robertson screw on a driver bit horizontally or even pendant, due to a slight wedge fit. Commonly found in Canada in carpentry and woodworking applications and in Canadian-manufactured electrical wiring items such as receptacles and switch boxes.
Square-drive head is an American clone of the Robertson that has a square hole without taper. Due to the lack of taper, the hole must be oversize relative to the screwdriver, and is much more likely to strip than the Robertson.
Tri-Wing head has a triangular slotted configuration. They were used by Nintendo on several consoles and accessories, including the Game Boy, Wii, and Wii Remote, and on some Nokia phones and chargers to discourage home repair.
Torq-Set or offset cruciform may be confused with Phillips; however, the four legs of the contact area are offset in this drive type. Is commonly used in the aerospace industry.
Spanner drive uses two round holes opposite each other, and is designed to prevent tampering. Commonly seen in elevators in the United States.
Clutch Type A or standard clutch head resembles a bow tie. These were common in GM automobiles, trucks and buses of the 1940s and 1950s, particularly for body panels.
Clutch Type G head resembles a butterfly. This type of screw head is commonly used in the manufacture of mobile homes and recreational vehicles.

Combination drives

Some screws have heads designed to accommodate more than one kind of driver, sometimes referred to as combo-head or combi-head. The most common of these is a combination of a slotted and Phillips head, often used in attaching knobs to furniture drawer fronts. Because of its prevalence, there are now drivers made specifically for this kind of screw head. Other combinations are a Phillips and Robertson, a Robertson and a slotted, a Torx and a slotted, and a triple-drive screw which can take a slotted, Phillips or a Robertson. The Recex drive system claims it offers the combined non-slip convenience of a Robertson drive during production assembly and Phillips for after market serviceability. Quadrex is another Phillips/Robertson drive. Phillips Screw Company offers both Phillips and Pozidriv(sic) combo heads with Robertson.

Tamper-resistant screws

Tamper-resistant external-TORX driver

One-way slotted screw

Many screw drives, including Phillips, TORX, and Hexagonal, are also manufactured in tamper-resistant form. These typically have a pin protruding in the center of the screw head, necessitating a special tool for extraction. In some variants the pin is placed slightly off-center, requiring a correspondingly shaped bit. However, the bits for many tamper-resistant screw heads are now readily available from hardware stores, tool suppliers and through the Internet. What is more, there are many commonly used techniques to extract tamper resistant screws without the correct driver — for example, the use of an alternative driver that can achieve enough grip to turn the screw, modifying the head to accept an alternative driver, forming ones own driver by melting an object into the head to mould a driver, or simply turning the screw using a pair of locking pliers. Thus, these special screws offer only modest security.

The slotted screw drive also comes in a tamper-resistant one-way design with sloped edges; the screw can be driven in, but the bit slips out in the reverse direction.

There are specialty fastener companies that make unusual, proprietary head designs, featuring matching drivers available only from them, and only supplied to registered owners^[1]. An example of this would be the attachment for the wheels and/or spare tires of some types of car; one of the nuts may require a specialized socket (provided with the car) to prevent theft.

The break away bolt is a high security fastener that is extremely difficult to remove. It consists of a counter-sunk flat head screw, with a thin shaft and hex head protruding from the flat head. The hex head is used to drive the bolt into the countersunk hole, then the wrench or hammer is used to knock the shaft and hex head off of the flat head, leaving only a smooth screw head exposed. Removal is facilitated by drilling a small hole part way into the outer part of the head and using a punch and hammer at a sharp angle in a counter-clockwise direction. This type of screw is used primarily in prison door locks.

Tools used

The hand tool used to drive in most screws is called a screwdriver. A power tool that does the same job is a power screwdriver; power drills may also be used with screw-driving attachments. Where the holding power of the screwed joint is critical, torque-measuring and torque-limiting screwdrivers are used to ensure sufficient but not excessive force is developed by the screw. The hand tool for driving cap screws and other types is called a spanner (UK usage) or wrench (US usage).

Mechanics of use

An electric driver screws a self tapping phillips head screw into wood

When driving in a screw, especially when the screw has been removed and is being placed again, the threads can become misaligned and damage, or strip, the threading of the hole. To avoid this, slight pressure is applied and the screw is driven in reverse, until the leading edges of the helices pass each other, at which point a slight click will be felt (and sometimes heard.) When this happens, the screw will often assume a more aligned position with respect to the hole.

Immediately after the 'click', the screw may be driven in without damage to the threading. This technique is useful for re-seating screws in wood and plastic, and for assuring the proper fit when screwing down plates and covers where alignment is difficult.

Thread standards

ISO metric thread	M1.6	M2	M2.5	M3	M4	M5	M6	M8	M10	M12	M16	M20	M24	M30	M36	M42	M48	M56	M64
wrench size (mm)	3.2	4	5	5.5	7	8	10	13	17	19	24	30	36	46	55	65	75	85	95

ISO metric thread	M14	M18	M22	M27	M33	M39	M45	M52	M60	M68
wrench size (mm)	22	27	32	41	50	60	70	80	90	100

History

Screw making machine, 1871

In antiquity, the screw was first used as part of the screw pump of Sennacherib, King of Assyria, for the water systems at the Hanging Gardens of Babylon and Nineveh in the 7th century BC.^[3]

The screw was later described by the Greek mathematician Archytas of Tarentum (428 – 350 BC). By the 1st century BC, wooden screws were commonly used throughout the Mediterranean oil and wine presses. Metal screws used as fasteners did not appear in Europe until the 1400s. world in devices such as

The metal screw did not become a common woodworking fastener until machine tools for mass production were developed at the end of the eighteenth century. In 1770, English instrument maker, Jesse Ramsden (1735-1800) invented the first satisfactory screw-cutting lathe. The British engineer Henry Maudslay (1771-1831) patented a screw-cutting lathe in 1797; a similar device was patented by David Wilkinson in the United States in 1798.

In 1908, square-drive screws were invented by Canadian P. L. Robertson, becoming a North American standard. In the early 1930s, the Phillips head screw was invented by Henry F. Phillips.

Standardization of screw thread forms accelerated during WWII so that interchangeable parts could be produced by any of the Allied countries.

Prior to the mid nineteenth century, cotter pins or pin bolts, and "clinch bolts" (now called rivets), were used in ship building.

In 1744, the flat-bladed bit for the carpenter's brace was invented, the precursor to the first simple screwdriver. Handheld screwdrivers first appeared after 1800.

Legal issues

In the United States a screw and a bolt have different import duties. The difference between them is therefore of keen interest to importers and customs authorities.

This was the subject of a court case Rocknel Fastener, inc v. United States: 34 page PDF. The position is outlined in a current US government document Distinguishing Bolts From Screws: 21 page PDF.

noreply@blogger.com (widi san) — Tue, 01 Jul 2008 14:38:00 +0000

engineering management

developmental overload

How does it happen? How can you manage it?

By Bradford L. Goldense and John R. Power

How many of us have been in the situation where there are just too many projects under way for the people available? How does it happen? Why do senior managers continue to add development projects so that they have a "rich" portfolio, until often the best product development people, engineers in particular, are assigned to four, five, or more projects concurrently?

As the number of projects increases, the coordinating, meeting, and relearning time goes up, and productive work time goes down. The assigned staff seems to spend most of its time running between projects. Overload exceeds real capacity in the first place, and then the process of juggling complexity reduces productive time spent on the projects, thereby decreasing real capacity.

Totaling all of the needs for all of the projects in a department, plus customer support and sustaining engineering, and comparing the result to the engineering headcount for a given time period will show how much the staff is overextended.

The ideal practice is to commit 85 percent of the hours available to planned projects so there is a constant and even flow of product development work and completion. This level of commitment allows for adjustments and capacity for rapid response to unexpected needs. For such a calculation, an estimate of the work to be done, in terms of staff effort, must exist. The more accurate the estimating system, the better the capacity management data.

Problems of overcommitment can arise from misestimates of time requirements, sometimes by as much as 700 percent. Such errors in estimating are not uncommon.

Certainly, R&D is different from manufacturing. There is intrinsically more intellectual and creative content and more variability of results. How might R&D officers manage capacity and predict future needs? The solution would appear to be in several parts.

Resources must be planned early. Rather than waiting until detailed development is initiated, at first sight of a potential development program a rough estimate will help frame the capacity impact that the proposed program may have on the organization.

Estimates must be made for each discipline or competency. Once serious development work is decided on, detailed estimates to the engineering specialty level are necessary. It's not just the number of engineers, but also the specific demands for specific skills that determine overall capacity.

Brent Arnold is director of product development at one of Goldense Group's clients, C-COR in State College, Pa. C-COR designs and markets systems and components for digital signal distribution, primarily to cable companies and telephone companies for broadband, a rapidly evolving industry.

"Managing capacity to this level"—that is, across all product development disciplines—"is especially important if multiple development facilities are involved in the same project," Arnold said. "The range of disciplines should also include cross-functional resource needs, especially for key functions such as product management and purchasing."

Estimating time periods must become more precise by breaking them down into shorter blocks. The more precise you are, the better—down to the week, or to the day, if possible. This approach provides for more accurate estimating and tracking. Too often, the rough estimate becomes adjusted, but the level of detail remains a mystery.

Tools must evolve to deal with the details of managing the creative and intellectual resources of an engineering staff. Research by Goldense Group shows that the simple spreadsheet is the tool of choice today, but much more is needed to track every project, forecast allocation of every engineering specialty, and balance the staff across the highest-priority projects to maximize results. Tools are now becoming available that integrate project portfolio management, resource allocation and simulation, and time-keeping systems.

Actual results must be tracked as work progresses. Each discipline must record and track its time. This will permit rational adjustments of staff allocations as work proceeds, as projects encounter difficulties, and as tasks are completed. This knowledge should be quantified and brought back to check the estimating system.

Time recording system limitations should be lifted. Many companies mandate an artificial 40-hour-a-week cap on time entered when, in reality, some people work 45 to 90 hours some weeks.

We do all these things in the manufacturing function. But the same principles apply in R&D as well. There is a tremendous opportunity for R&D managers to refine their estimating processes and plan allocation of resources for improved results—the completion of new products on schedule.

Bradford L. Goldense is president of Goldense Group Inc., a consulting and education firm in Needham, Mass. John R. (Dick) Power is the firm's director of executive education.

Return to Index

noreply@blogger.com (widi san) — Tue, 01 Jul 2008 14:35:00 +0000

engineering management

experiment ...
early and often

To create a truly innovative product, you have to be willing to tinker. But establishing a culture to support that can be surprisingly hard.

By Jean Thilmany

Wouldn't it be great if you knew at the very beginning of the design cycle that a final product would work perfectly? If you knew that it would instantly meet marketplace approval? There'd be no need to mess around with pesky design details or marketing plans. You'd just dash off a design, ferry it right over to production, and have this miracle product on store shelves—and flying off them—within the week.

Alas, life doesn't work that way. A niggling little detail called uncertainty hounds product development.

"Without uncertainty, you could go straight to making products. You'd know what to do," said Stefan Thomke, an associate professor of business administration at the Harvard Business School in Cambridge, Mass.

He argues, however, that engineering managers can rework their departments to lay a clear path for engineers to walk through uncertainty toward a new and innovative product. The means of getting from start to finish along that trail? Experimentation.

The willingness to experiment can lead companies to the best, most innovative products possible. But it's a method many designers and their managers are leery of undertaking, Thomke admits.

He literally wrote the book on the importance of experimentation. He's the author of Experimentation Matters (2003, Harvard Business School Publishing Corp.), which posits that every company's ability to innovate depends on a series of experiments—and failures—that help create new products and improve old ones.

The period between the earliest point in the design cycle and final product release should be filled with experimentation, failure, analysis, and yet another round of experimentation. Lather, rinse, and repeat until an innovative product is ready to be ushered out the door, Thomke said.

Yet that's just where many companies fall down, he maintains. They're afraid to experiment. Becoming friendly with the uncertainty inherent in product development and being willing to experiment early and often are among the more savvy moves a manager can make. He advises managers to test those experiments with analysis systems that can predict end results.

New technologies—like computer modeling and simulation software that let engineers ask and answer what-if questions—change the economics of experimentation, greatly reducing the cost of prototyping. But to take full advantage of analysis software, engineering managers have to embrace experimentation, to improve the way their departments innovate, and to transform the very structure of the organization itself.

"The best companies are willing to experiment," Thomke said.
But it's not easy to do. Even if engineering managers recognize the importance of experimentation and of the proper analysis software that lets engineers do just that, they're often stymied by the corporate culture or by engineers themselves, who are afraid to fail and afraid of change.

"Managers," Thomke said, "are terrified of failure."
In his book, Thomke describes six principles engineering managers can follow to unlock their departments' innovative potential.

INNOVATION INGREDIENTS

First, managers need to fully exploit analysis technology early in the design cycle, Thomke said.

"New technologies are most powerful when they're deployed to test what works and what doesn't work as early as possible—the front-loading effect," Thomke writes in his book. "These experiments aren't as complete or perfect as late-stage tests, but they're able to direct early attention and integrated problem-solving at potential downstream risks."

According to Thomke, engineering managers ultimately make decisions about how their engineers use technology, and many engineering managers are rooted by the way they've done jobs in the past, whether outdated or not. Their engineers follow suit. It's just human nature to keep doing things the way we're used to and it's also human nature to shy from change, no matter how small.

"Say a company wants to use engineering software tools earlier in the design process," Thomke said. "It's appealing to use them early because you can find problems earlier.

"But engineers at that company are used to doing things their own way," he added. "They're not going to like adjusting to using those tools earlier in the process."

In product development, design changes become costlier as you get closer to the end because the tooling, once made, will need to be reworked.

"Managers can end up devoting an enormous amount of their time to dealing with late-stage problems—to meet launch dates, reallocate resources, unsnarl schedules, and so on," Thomke writes. "Such fire fighting is taken for granted because most product development processes aren't set up, much less optimized, for early experimentation."

Boeing managers were proactive about using software earlier in the design cycle when developing its new aircraft, the 777. The managers wanted a process that encouraged experimentation and problem solving well before final assembly, Thomke writes.

They coupled a three-dimensional, computer-aided design system with in-house software that enabled engineers to assemble and test digital mock-ups for interference problems. With such a mock-up, an engineer can assemble any part of the plane virtually to check for fit and for interferences.

The new software did more than find interferences earlier. The automatic checks also changed the way people interacted with each other, Thomke said. Not only did designers modify their designs earlier than in the past, they also relied on others to track the modifications via the software. Because they received immediate feedback, the designers were emboldened to experiment even more.

Using software earlier also means the structure of the organization will likely need to change. Engineers can change. If managers can just hang on during the learning curve, they'll be rewarded with productive and experimental engineers.

EXPERIMENT—BUT NOT TOO OFTEN

Though Thomke supports early and frequent experimentation, he cautions managers not to overload their organizations. This is his second principle toward innovation.

"A good experimentation strategy balances the values of early information against the cost of repeated testing," he said.

He advises managers to combine both new and traditional technologies in order to fully realize a department's new-product development potential. This is his third principle.

Managers must be aware of human nature as they work toward bringing in new technology to supplement, and eventually replace, the old.

The true potential of new technologies often lies in a company's ability to reconfigure its processes and organization to use them in concert with traditional technologies, Thomke said.

Today, technologies change faster than engineers can get used to working with them. That's why it's important to embrace new software while keeping the old.

"If you work with something 10 or 20 years and you have expertise with that certain technology, you're not going to be willing to quickly adapt to something else," he said. "Across an organization, up to about 10,000 people are going to be used to doing something in a certain way. You introduce a new technology, and it's not like people will suddenly embrace that."

For example, one engineering company that Thomke profiled in his book spent big bucks on high-end digital simulation technology. Yet, even after implementing that digital program, the company made more prototypes than any organization Thomke studied. The making of many prototypes was the very thing the company hoped to avoid. Technically, to get the most from a simulation system, the organization should rely primarily on that system and should build prototypes only after validating and analyzing most design possibilities via the software.

A little sleuthing soon uncovered the reason behind the plethora of prototypes.

The chief engineer told Thomke that, while company executives had asked his department to bring in the simulation technology to cut prototyping costs, his engineers didn't trust the new technology. Yes, they grudgingly used it to run design simulations. But then they built a prototype for every simulation they ran just to double check results. In the end, they built many more prototypes than before they'd had the software program.

A NEW PARTNER

Engineering managers have to set up their departments to support rapid innovation, according to Thomke's fourth principle. But many managers don't bother. As Thomke puts it, organizational structures get in the way of innovation. For instance, mechanical and electrical engineers are usually grouped at work by their expertise or their area of specialization.

"The thought is, if you put a lot of people together that build prototypes, they'll get better and better at doing that and can make better prototypes cheaper," he said.

While the idea seems sound, especially because groups of engineers usually talk to each other about a mutual project, it sidesteps quick product iteration, Thomke maintains.

"Those interfaces between groups actually get in the way," he said. "The design group isn't responsible for testing, so they hand their design over to testing. And then testing hands it back for redesign. All that takes a lot of time.

"By the time engineers get the tests back from the analysts, those engineers have already moved on to another design," he added. "Engineers can move quickly, but as they come up with these new ideas they need them tested. They don't have much time to wait for feedback from analysts and others."

Disparate engineering departments are usually managed separately as well. According to Thomke, the departments are too independent and too separate, although they're expected to interact quickly and efficiently. When a company wants to iterate at lightning speed, walls between departments get in the way, he said.

Thomke outlines one example, at BMW, in his book. The German automaker wanted to simulate vehicle crashes earlier in the design cycle. Crash tests run at roughly the same time as automobile design let analysts see more quickly where a design would fail. Design engineers needn't waste their time working through an entire model if they already know it is flawed.

But the change necessitated that two groups—analysts and engineers—work together in a way they hadn't previously.

Managers are often stymied by the corporate culture or by engineers themselves, who are afraid to fail and afraid of change.

"The downstream group, the engineers, was used to the upstream group, the analysts, coming in later in the cycle," he said. "For crash simulation, analysts needed to get some rough parameters from the people designing the doors. But the door designers were reluctant to give that information out. They didn't want to share their information until they were fully done. They didn't want anyone to see information if it was flawed."

Only after convincing the designers that their early rough data sufficed did the crash simulation group get the required information, Thomke writes in his book. But, even so, that exchange took six months. In a new development process, a six-month delay could derail an entire program.

It was incumbent upon crash simulation and design engineers alike to appreciate and understand not only each other's activities, but also the power of the new technologies that could leverage them. This understanding had to be built patiently over time.

BMW had a world-class crash simulation group, but unless the processes were changed and people started working together in new ways, its technical leadership meant little for development performance, Thomke writes.

Engineers eventually learned to change. But they didn't embrace it from the get-go. Managers shouldn't expect that. BMW engineers needed to understand why the new business process was necessary.

FAILURE VS. MISTAKE

Managers shouldn't confuse failures with mistakes. Fail early and often, but avoid mistakes, Thomke states. This is his fifth principle for unlocking innovative potential.

Experiments that result in failures are not failed experiments. Think of them instead as a way to generate new information the engineer couldn't foresee, Thomke suggests. The faster the experimentation-failure cycle, the more feedback the engineer gathers and incorporates into new rounds of development.

Mistakes are a different animal entirely. Thomke defines them as wrong actions that result from poor judgment or inattention. They are failures because they produce little new or useful information.

A poorly planned experiment with ambiguous data, for example, is a mistake, as is repeating a prior failure or learning nothing from experience.

Managers have to promote failure, but also must weed out mistakes.

A BIG EXPERIMENT

As his sixth principle for innovation, thomke advocates that managers take a different way of thinking about experimentation. Projects themselves should be conceived of as experiments, he said. An entire organizational-reform project should be thought of as an experiment.

For optimal organizational reforms, senior managers should have a portfolio of experimental projects they can learn from.

When BMW sought to put its new development process into play, busy managers and engineers balked. A year passed without any significant progress.

To jump-start the process, executives decided to use the latest 7 Series platform car—already one year into development—to test the new development system. The platform project became an experiment in and of itself, Thomke said, and eventually met with success.

It takes a good manager to walk engineers through change, to implement new technology whether or not employees balk, and to understand that there'll be a learning curve as employees adjust to the new tools and change their work procedures.

To unlock innovation and get full use of research and development, managers need to understand the power of experimentation and new technologies. They also have to change their processes, organization, and the way they manage innovation.

It's a tall order, right? But the great products that get to market quickly after engineers experiment and innovate as much as possible make the efforts well worthwhile.

engineering management

noreply@blogger.com (widi san) — Tue, 01 Jul 2008 14:34:00 +0000

engineering management

improv engineering

A comedy producer imparts some idea-generating tips he's learned on the job.

By Jean Thilmany

Question: What do engineers and improvisational comedy have in common?

Answer: More than you might think.

Or so says one Minneapolis comedy entrepreneur.

Whether they're creating a new product or a joke based on the political snafu of the moment, engineers and comedians are creative professionals who use the same idea-creation process, according to John Sweeney, owner and executive producer at Brave New Workshop Comedy Theater.

Engineering managers would do well to study the method that improvisational comedians use when they take a suggestion from the audience and in seconds turn it into comedy, Sweeney said.

In 1997, Sweeney and his wife took over the Brave New Workshop in Minneapolis, where Louie Anderson and Al Franken cut their teeth.

Flying by the seat of your pants is actually a skill that can be taught, says an improvisational comic who shares his art form with engineers.

Sweeney's theater hosts classes and performances in improvisational comedy, and puts on shows made up of sketch comedy. An improv comic takes an audience member's suggestion (for instance, to name a type of doctor) and in an instant creates a gag around that suggestion (usually involving a proctologist). Sketch comics work from a script. Think Whose Line Is It Anyway versus Saturday Night Live.

Sweeney studied the process of creating comedy at a moment's notice close up during his eight years at the helm of the workshop. The workshop's idea-generation process, which Sweeney dubbed the funnel process, uses improvisational techniques to create sketches and is easily transcribed to product innovation, he said. The actors work out the sketches using a process akin to the way engineers come up with new product ideas.

He has presented his findings to engineers at Hewlett-Packard, 3M, Medtronic, and other companies. He also wrote and published a book, Innovation at the Speed of Laughter, which is coming out in April. It outlines the workshop's product development process and offers management principles gleaned from Sweeney's years in improvisational comedy.

"I've taken what we learned about how individuals create good improv scenes, how they communicate, what skills they have as idea generators, what cultural skills they have, and applied those skills to the workplace," Sweeney said. "What I found can help increase the number and the quality of ideas engineers come up with."

A QUICK SIX HUNDRED

Sweeney's seven-step development method—the funnel process—works for engineers just as easily as it does for comics. His troupe never wavers from it, he said.

This is how it breaks down.

At the first step, the top of the funnel, team members generate ideas. The more outrageous the idea, the better. Quantity is the goal here, not quality.

"We have a ratio for how many ideas it takes to get to a great idea," Sweeney said. "Our ratio is 24 to 1. So that works out to 600 one-sentence ideas to produce 25 actual products. Our shows have 25 different sketches and skits, which we think of as our products.

"We actually keep so strictly to our process that we don't allow the team to go on to the next step until we've hit 600 one-sentence ideas. We've found that to be the most consistent number," he added.

A lot of those first ideas don't see the light of day. But that's not the point, Sweeney said.

In presenting his thoughts to engineering companies, Sweeney has found that engineers often fall short in the number of ideas they generate at the top of the product-development funnel, he said.

Often engineers simply don't realize they need to get their thoughts, no matter how crazy, out on the table. Maybe another team member can take that outlandish idea in a completely new direction.

"When engineering managers are putting together the project-management model, they don't allow themselves to get the quantity of ideas they'll need to really get the mathematics going," Sweeney said.

"Six hundred is the number of ideas at the top of the funnel and 25 is the number of products at the bottom of ours," he said. "In between, we have a number of interactions. We have the potential of 600 thoughts interacting and blending and spawning new ideas."

Sweeney encourages engineers to just brainstorm ideas, even if they clearly can't be made or won't fit product specs.

"I ask them to just forget about what the payload should be and start generating ideas" he said.

"That's a tough one for them," he added. "But when they see the mathematical potential 600 ideas give them, they reconsider."

The second step in Sweeney's funnel process calls for refining those ideas.

Every team member chooses approximately 20 or 25 ideas from the original 600 and breaks them down into building blocks. The same thing can be done for potential products.

For a comedy sketch, the building blocks are the satirical point, the characters that might appear, and the action points of the scene. Obviously, for engineers the building blocks would look much different, although they'd also be broken into components, such as the materials and performance specifications.

Sketch-comedy writers work in collaborative teams and, of course, it's no different for engineers. That's step three in the funnel process: collaboration.

Because the players aren't married to their ideas at this point, it's time to let go of the ones that aren't working.

"We're very conscious at this point in the process that certain ideas can organically lose their ability to make us passionate about them," Sweeney states in his book.

Then it's time to engineer the product, which is step four in the funnel process. For the workshop team, that means writing the first draft of a script. For an engineer, that means the first design. Group members give feedback about that first product, making sure to use wording that separates the work from the person.

In live theater, putting the product before a focus group—the fifth step in the product-innovation process—happens when the show is performed for a test audience. At the workshop, the troupe puts on bits of shows in development for select audiences to gauge feedback. Engineers can get customer reaction to a potential product by putting it before more traditionally organized focus groups and getting feedback.

Now it's time for step six—to road test the product.

The entire show is readied from front to back and run before preview audiences.

The troupe then reworks the show as necessary, based on audience reaction—read "laughter." This part of the process is akin to prototyping, then retesting the re-engineered product.

"We've had to develop consistency in a manufacturing process filled with variables," Sweeney said. "When I talk about the funnel process with engineers, they tend to think 'Wow, innovation and improvisation don't have to be being silly or working in an advertising firm or wearing cool glasses. We look at product creation like a process, just like the theater person does."

According to Sweeney, the difference between an improvisational theater group and an engineering company isn't as vast as it may seem. Both are rife with creative potential and both are aimed at producing an innovative product.

The comedy team doesn't manufacture ideas so much as it discovers them through a quantifiable process. Sweeney hopes that, when it comes to generating ideas, engineering managers can take a page from the improv comic's manual.

What's My Line? Comedy Skills for the Engineer

You're an engineer called into the conference room for yet another team-building seminar. A sense of dread wells inside. Get ready for an afternoon of forced heartiness and cringe-inducing, meet-your-neighbor-type exercises.

Maybe another motivational speaker will take the pulpit, your coworker jokes. That'd be okay, he says, because he could use another 40 winks.

Janice Kelson, marketing manager at a large, Minneapolis-based engineering company, has been there and expected to suffer through it again, but at a recent session, she opened the conference-room door instead on clusters of coworkers laughing themselves sick.

Before the end of the day, she had freely called out to coworkers the first word that came to her mind when thrown an imaginary ball and had appeared in a made-up-on-the-spot skit as the dim housewife of an English lord. Audience members had suggested the setup for the skit, and Kelson and her band of players improvised it at a moment's notice.

The marketing group had turned into theater students for the day, studying improvisational comedy techniques under the guidance of improv comedians and immediately putting their new skills into practice.

Kim Thomassen (left) and Gust Alexander are known as The Stagebenders.

Give a tweak to the techniques actors use to break down barriers between self and audience, and they offer engineers tools useful in the workplace, said Kim Thomassen, one-half of the Los Angeles-based improv duo, The Stagebenders. He and comedy partner Gust Alexander regularly teach theater skills to groups of engineers.

In improvisational comedy—where actors make up sketches in response to audience suggestions—the action is fast. There's no time to think, which opens up all kinds of creative avenues. This style of comedy can be seen most notably in reruns of the popular Drew Carey-hosted television show Whose Line Is It Anyway?

The techniques, with names like "word ball" and "minister's cat," demand that people push past natural reticence, gain self-esteem, and work on their communication skills while having fun, Thomassen said. The Stagebenders regularly puts on seminars for engineering groups.

"Through the theater games, you build better teamwork, better communication, and get a better understanding of other people. And you get to know your coworkers a little better," Thomassen said.

Word ball is a simple, early improv warmup that can help team members get over their shyness and get them thinking creatively. In this exercise, team members stand in a circle, with a person holding an imaginary ball. He or she throws it to another person, simultaneously saying the first word that comes to mind.

The person who catches the ball has to throw it immediately, at the same time saying the first word they think of that's related to the word tossed to them.

One person might say "green" and toss the ball. Depending upon how far along the game is—with creativity running high—the catcher might yell out "iguana," then toss the ball to another, who might say "scales."

When John Sweeney, who owns a comedy theater, the Brave New Workshop, in Minneapolis, leads seminars for engineers, he has eight engineers-turned-improvisers-for-the-day stand in a semicircle and count to 10. The trick is that no one knows who will say the next number. In the beginning, two people often call out the next number together.

"But relatively quickly, the group organically begins to individually count to 10 without two people counting at the same time," Sweeney said.

Groups that successfully count to 10 with no one stepping on another's toes count higher. The record so far has been 148, hit by seven employees of a Bismarck, N.D., utility company.

"People think they can't work together until they have a plan in place," Sweeney said. "They think they need to know each other and all these things need to happen before a team can gel. But we're doing a simple 10-step process without sending 50 e-mails back and forth and setting up conferences to coordinate things."

Teamwork isn't about being an extrovert. It's a skill that can be taught.

A number of engineers move on to take improv classes at Sweeney's Workshop to improve their communication skills. Those engineers recognize that their newfound improv skills carry over into their work life. Oh, and some of them just might want to be discovered.

— Jean Thilmany

noreply@blogger.com (widi san) — Tue, 01 Jul 2008 14:26:00 +0000

making sense of change

In a world of rapidly advancing technology, someone has to keep things under control.

By Wade H. Shaw

The field of engineering management has developed rapidly in the last 50 years. That's not too surprising in light of the increasing role that engineering and technology have played in people's lives over that time.

As a formal study, engineering management dates to the early 1950s, when engineers began to work in teams to design the increasingly complex products sought by a public with an insatiable appetite for improved quality of life. Decades later, schools around the world continue to see growing enrollment in their advanced-degree engineering management programs.

It's no longer the 1950s. Society has changed, and the phenomenon of the computer has exploded and morphed to serve a vital role in industry and in everyday life. The products that industry makes and the tools that it uses—including the technology of product development itself—are evolving daily.

Engineering managers oversee the product development cycle, from design to manufacture.

Industry has a growing awareness of the need to manage the innovation process, but even so, practitioners and instructors still hear: What exactly does the field of engineering management include? The answer seems to depend on whom you ask.

Some engineers think the discipline is an extension of their own specialty, but with management training added on. Others see the field as a variation of basic business management, with the precepts tweaked to adjust for managing technical people. Still others in industry debate how engineering management is related to other disciplines like industrial engineering or technology management.

At its most basic, engineering management is the process of envisioning, designing, developing, and supporting new products and services. Engineering managers cleave to a set of product requirements. As they keep those requirements uppermost in mind, they must also make sure the product stays within budget and on a set schedule.

And they must take just the right level of risk. They have to ensure that the design cycle doesn't lag, for example, but also doesn't move so fast that engineers gloss over important details.

Managers For A New Age

But why are engineering managers necessary? What do they do? How do they do it?

In answering these questions, we gain insight into the educational requirements, skills, and competencies of engineering managers. The answers also give us a frame of reference that can be passed to other engineers and managers who could benefit from an understanding of what engineering management is all about.

First question: Why engineering management? The answer in a nutshell: to design and develop products and services.

It's hard to imagine an engineering manager who toils to manage a research and development process that doesn't result in a product or service. Even those in a pure laboratory environment face business-performance issues. And even a research and development laboratory needs to see results. Basically, the reason engineering managers exist is to ensure that engineers use solid engineering skills and tools to design and deliver high-quality products and services.

Second question: What do engineering managers do? For that answer, we must turn to a systems engineering model.

Systems engineers develop large and complex assemblies or combinations of related parts that work together. Systems engineering has emerged in recent years as a way to translate customer needs and requirements into complex, engineered solutions.

The modern view of project management also includes the role of a chief engineer—or systems engineer—who oversees the technical aspects of a design project.

Calling upon all types of design technologies is part of an engineering manager's duties.

I suggest that the activities a systems engineer performs are the same as engineering managers take on. The basic concept is the same: translating customer requirements into completed design, while considering the product lifecycle.

Hence, engineering managers are engineers with a management mandate to accomplish system integration.

Third question: How do engineering managers do their work? Again, we turn to a model, this time project management.

Engineering managers manage projects. This is different from other types of managers, who manage people. Teams of people typically carrying out product engineering and those teams are tied together by a common purpose, a set cost, a set schedule, and defined quality constraints. The process of managing that engineering activity is engineering management.

Delineating this who, what, and how of engineering management leaves us with a more focused idea of the field. To wit, engineering management uses a systems engineering approach to oversee the team development of products and services. The idea applies to all fields of engineering.

And the linkage to customers, projects, requirements, budgets, schedules, quality, and risk puts the engineering manager in a distinctly executive management light.

Future Challenges

The who, what, and why of engineering management isn't static. Each aspect adapts to changes in technology and the marketplace. With that in mind, we can do some conjecturing about how the practice and education of engineering managers will look in the future.

Expect to see new products and radical changes in old ones. Advances in materials, nanotechnology, computing, bioengineering, and energy systems will definitely affect the practice of engineering, making for ever more complex products.

The need to understand and manage product complexity will emerge as a distinct challenge to engineering managers.

Expect, too, to see engineering managers focus not only on overseeing the product lifecycle in the future, but also on managing the innovation process. (Stefan Thomke of the Harvard Business School has some ideas of his own on managing innovation. Engineering Management's editor, Jean Thilmany, reports on some of them in "Experiment—Early and Often" in this issue.)

Managers will consider manufacturing and related costs when choosing a product design, rather than focusing mainly on the design costs. You also can expect to see more systems engineering concepts used to develop new products and services in the future.

Standards are a good indicator of the maturity of a practice, and the systems engineering field will likely mature in the near future to include a set of standards. The emergence of standards for systems engineering will provide a framework for best practices.

Systems engineers develop complex assemblies, much like engineering managers.

Systems engineering will become part of the project management discipline, to distinguish engineering projects from other initiatives organized to accomplish business objectives.

Simulation technology will continue to emerge, too, as a design tool. Engineers will take advantage of the technology to understand relationships between design variables, to predict performance, and to anticipate the lifecycle of the project itself. Because future designs will be much more complex, engineers won't be able to build a physical mock-up or prototype for each design iteration.

Project management for engineering should mature to a more specific form that could be called project engineering. This maturity will come as generic project management skills are enlarged to include the management of customer requirements, of risk, and of cost, and are expanded also to include system engineering and lifecycle planning considerations.

There will also be more interdisciplinary product teams. Team members might come from industrial design or information sciences. These types of teams are effective organizational structures and the complexity of modern engineering will further promote their adoption in industry.

Managers must have people skills and be aware of what's going on.

The makeup of the team will become important. We'll learn that constructing a project team is a design problem, as is constructing the product itself. Leadership issues will move to the forefront of project management and the selection of a project manager will become a process that's conducted much more carefully.

The traditional emphasis areas of managing a product's budget, schedule, and quality will shift to managing its customer requirements and its risks. Cost, schedule, and quality will be project constraints; the real goal will be managing the customers' requirements to ensure that their needs are met with acceptable levels of risk. Requirements and risk management will be used to drive and justify cost, schedule, and quality.

We'll see the emergence of a project strategy, whose goal will be to connect the overall plan of a project with the strategic plan of business. Currently, this connection doesn't exist and projects often exist with little vision toward their ultimate strategic value within the business.

Current and Future Education

All of these future engineering managers need training, and universities in the United States and abroad haven't been shy about stepping in to offer it.

Schools of engineering around the world have adopted engineering management programs in an effort to respond to student interests and industry needs. Engineering management programs exist at bachelor's, master's, and Ph.D. levels, although the master's option is by far the most prevalent. Most programs offer a combination of technical courses, management courses, and specialized engineering management courses.

Yes, an engineering manager is still an engineer, but an engineer with a graduate technical education—an engineer with the classic management skills associated with an MBA. However, an engineering manager is more than simply an engineer who receives management training or an MBA. To be sure, training and education are quite beneficial. But it's important to remember that engineering managers should possess a specific set of competencies beyond those covered by a technical or management education. Managers also must have people skills along with an awareness of what's going on in society.

Engineering managers could be said to act as a beam that connects the science and engineering side of an organization with the classical management aspects of the business. Engineering management is definitely a bridge between these two.

Simply telling engineers about management skills is likely to produce the same result as explaining engineering skills to managers: You get a better awareness of the roles, but you don't get the bridge.

The key to building successful engineering managers is to give them hands-on experience in the defining areas of product development, systems engineering, and project management. This could be done through an internship with industry, which many educational programs offer today. It is practice that will give these managers insight into their art.

After five decades, engineering management is a still-evolving discipline. But then, it probably must always be that way. Consider, after all, the rapidly changing world that engineering managers will serve.

Wade H. Shaw, an IEEE Fellow, is professor of engineering systems and director of the Center for Teaching and Learning Excellence at the Florida Institute of Technology in Melbourne, and is editor of the Engineering Management Review.

© 2005 by The American Society of Mechanical Engineers

noreply@blogger.com (widi san) — Tue, 01 Jul 2008 13:52:00 +0000

Bearing (mechanical)

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A bearing is a device to permit constrained relative motion between two parts, typically rotation or linear movement. Bearings may be classified broadly according to the motions they allow and according to their principle of operation as well as by the directions of applied loads they can handle.

An example of a four-point contact ball bearing

Bearing friction

Low friction bearings are often important for efficiency, to reduce wear and to facilitate high speeds. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces.

By shape, gains advantage usually by using spheres or rollers.
By material, exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
By fluid, exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching.
By fields, exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.

Combinations of these can even be employed with the same bearing. An example of this is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.

Principles of operation

Animation of ball bearing

There are at least six common principles of operation:

sliding bearings, usually called "bushes", "bushings", "journal bearings", "sleeve bearings", "rifle bearings", or "plain bearings"
rolling-element bearings such as ball bearings and roller bearings
jewel bearings, in which the load is carried by rolling the axle slightly off-center
fluid bearings, in which the load is carried by a gas or liquid
magnetic bearings, in which the load is carried by a magnetic field
flexure bearings, in which the motion is supported by a load element which bends.

Motions

Common motions permitted by bearings are:

Axial rotation e.g. shaft rotation
Linear motion e.g. drawer
spherical rotation e.g. ball and socket joint
hinge motion e.g. door

Loads

Bearings vary greatly over the size and directions of forces that they can support.

Forces can be predominately radial, axial (thrust bearings) or moments perpendicular to the main axis.

Speeds

Bearings vary typically involving some degree of relative movement between surfaces, and different types have limits as to the maximum relative surface speeds they can handle, and this can be specified as a speed in ft/s or m/s.

For rotational bearings generally performance is defined in terms of the product 'DN' where D is the diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute.

Generally in terms of relative speed of the moving parts there is considerable overlap between capabilities, but plain bearings can generally handle the lowest speeds while rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which have no known upper speed limit.

Life

Fluid and magnetic bearings can potentially give indefinite life.

Rolling element bearing life is statistical, but is determined by load, temperature, maintenance, vibration, lubrication and other factors.

For plain bearings some materials give much longer life than others. Some of the John Harrison lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear. clocks still operate after hundreds of years because of the

Maintenance

Many bearings require periodic maintenance to prevent premature failure, although some such as fluid or magnetic bearings may require little maintenance.

Most bearings in high cycle operations need periodic lubrication and cleaning, and may require adjustment to minimise the effects of wear.

History and development

An early type of linear bearing was an arrangement of tree trunks laid down under sleds. This technology may date as far back as the construction of the Pyramids of Giza, though there is no definitive evidence. Modern linear bearings use a similar principle, sometimes with balls in place of rollers.

The first plain and rolling-element bearings were wood, but ceramic, sapphire or glass can be used, and steel, bronze, other metals, and plastic (e.g., nylon, polyoxymethylene, teflon, and UHMWPE) are all common today. Indeed, stone was even used in various forms. Think of the "jeweled pocket watch", which incorporated stones to reduce frictional loads, and allow a smoother running watch. Of course, with older, mechanical timepieces, the smoother the operating properties, then the higher the accuracy and value. Wooden bearings can still be seen today in old water mills where the water has implications for cooling and lubrication.

Rotary bearings are required for many applications, from heavy-duty use in vehicle axles and machine shafts, to precision clock parts. The simplest rotary bearing is the sleeve bearing, which is just a cylinder inserted between the wheel and its axle. This was followed by the roller bearing, in which the sleeve was replaced by a number of cylindrical rollers. Each roller behaves as an individual wheel. The first practical caged-roller bearing was invented in the mid-1740s by horologist John Harrison for his H3 marine timekeeper. This used the bearing for a very limited oscillating motion but Harrison also used a similar bearing in a truly rotary application in a contemporaneous regulator clock.

An early example of a wooden ball bearing (see rolling-element bearing), supporting a rotating table, was retrieved from the remains of the Roman Nemi ships in Lake Nemi, Italy. The wrecks were dated to 40 AD. Leonardo da Vinci is said to have described a type of ball bearing around the year 1500. One of the issues with ball bearings is that they can rub against each other, causing additional friction, but this can be prevented by enclosing the balls in a cage. The captured, or caged, ball bearing was originally described by Galileo in the 1600s. The mounting of bearings into a set was not accomplished for many years after that. The first patent for a ball race was by Philip Vaughan of Carmarthen in 1794.

Friedrich Fischer's idea from the year 1883 for milling and grinding balls of equal size and exact roundness by means of a suitable production machine formed the foundation for creation of an independent bearing industry.

Tapered steering head bearings for a motorcycle

The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF 1907. ball-bearing manufacturer in

Henry Timken, a 19th century visionary and innovator in carriage manufacturing, patented the tapered roller bearing, in 1898. The following year, he formed a company to produce his innovation. Through a century, the company grew to make bearings of all types, specialty steel and an array of related products and services.

The Timken Company (Sale $4,973.4M, 2006), The SKF company($6,195.1M, 2005), the Schaeffler Group (Private), the NSK company($5,344.5M, 2006), and the NTN Bearing company($3,697.8M, 2006) are now the largest bearing manufacturers in the world.

Types

There are many number of different types of bearings.

Type	Description	Stiffness*	Speed	Life	Notes
plain bearing	rubbing surfaces, with lubricant	Good, provided wear is low, but some slack is normally present	low/moderate (often requires cooling)	moderate (depends on lubrication)	The simplest type of bearing, widely used, relatively high friction
Rolling element bearing	Ball or rollers are used to prevent or minimise rubbing	Good, but some slack is usually present	moderate-high (often requires cooling)	moderate (depends on lubrication, often requires maintenance)	Used for higher loads than plain bearings with lower friction
jewel bearing	off-center bearing rolls in seating	Low due to flexing	low	adequate (needs cleaning and lubrication requires maintenance)	Mainly used in low-load, high precision work such as clocks
fluid bearing	Fluid is forced between two faces and held in by edge seal	Very high	Very high- speeds usually limited by seals	Virtually infinite in some applications, may wear at startup/shutdown in some cases	Can fail quickly due to grit or dust or other contaminants. Maintenance free in continuous use.
magnetic bearings	Faces of bearing are kept separate by magnets (electromagnets eddy currents) or	low	Infinite	Infinite	Often needs considerable power. Maintenance free.
Flexure bearing	Material flexes to give and constrain movement	low	Very high	Very high or low depending on materials and strain in application	Limited range of movement

- stiffness is the amount that the gap varies when the load on the bearing changes, it is distinct from the friction of the bearing

nuts info

noreply@blogger.com (widi san) — Sun, 29 Jun 2008 06:36:00 +0000

We have a web site dedicated to training, have a look at www.bolting.info - for additional information on bolting technology.

ACORN NUT: A nut (so-called because of its shape) that has a domed top so that it prevents contact with the external thread.
AEROTIGHT NUT: A torque prevailing nut of all metal construction. The nut is slotted in two places which, after the nut has been tapped, are bent slightly inwards and downwards. When the nut is screwed onto the bolt thread the two slotted parts are forced back to their original position. Their stiffness causes the nut threads to bind onto the bolt threads and thus provides a prevailing torque. Aerotight is a registered trade mark of The Premier Screw and Repitition Co. Ltd of Woodgate, Leicester, United Kingdom, LE3 5GJ.
ANTI-FRICTION COATING: AF coatings are dry lubricants consisting of suspensions of solid lubricants, such as graphite, PTFE or molydbenum disulphide of small particle size in a binder. Such coatings can be applied to fastener threads to replace metallic coatings such as zinc and cadmium and offer maintenance free permanent lubrication. By careful selection of the lubricants, AF coatings can be designed to meet specific applications. The coatings are permanently bonded to the metal surface and provide a lubricating film preventing direct metal to metal contact.
ANTI-SEIZE COMPOUND: An anti-seize compound is used on the threads of fasteners in some applications. The purpose of the compound depends upon the application. It can prevent galling of mating surfaces - such compounds are frequently used with stainless steel fasteners to prevent this effect from occurring. In some applications it is used to improve corrosion resistance to allow the parts to be subsequently dis-assembled Thirdly, it can provide a barrier to water penetration since the threads are sealed by use of the compound.
AUTOLOK NUT: A torque prevailing nut of an all metal construction. Covered by UK patent 1180842 the nut is marketed by GKN Screws and Fasteners Limited.
ALLOWANCE: An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it's maximum metal condition. Not all classes of fit have an allowance. For metric threads the allowance is called the fundamental deviation.
ANAEROBIC ADHESIVE: An adhesive which hardens in the absence of air, such adhesives are often used as a thread locking medium.
ANGLE CONTROLLED TIGHTENING: A tightening procedure in which a fastener is first tightened by a pre-selected torque (called the snug torque) so that the clamped surfaces are pulled together, and then is further tightened by giving the nut an additional measured rotation. Frequently bolts are tightened beyond their yield point by this method in order to ensure that a precise preload is achieved. Bolts of short length can be elongated too much by this method and the bolt material must be sufficiently ductile to cater for the plastic deformation involved. Because of the bolt being tightened beyond yield, its re-use is limited.
BASIC THREAD PROFILE: This is the theoretical profile of external and internal threads with no manufacturing tolerance applied.
BEARING STRESS: The surface pressure acting on a joint face directly as a result of the force applied by a fastener.
BIHEXAGON HEAD: A bolt or screw whose cross section of its head is in the shape of a 12 pointed star.
BLACK BOLTS AND NUTS: The word black refers to the comparatively wider tolerances employed and not necessarily to the colour of the surface finish of the fastener.
BOLT: A bolt is the term used for a threaded fastener, with a head, designed to be used in conjunction with a nut.
BREAKAWAY TORQUE: The torque necessary to put into reverse rotation a bolt that has not been tightened.
BREAKLOOSE TORQUE: The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.
BRITISH STANDARD BRASS: A specialist thread form based upon the Whitworth thread and consisting of 26 threads per inch whatever the thread diameter.
BSF: British Standard Fine. A thread form based upon the British Standard Whitworth form but with a finer thread (more threads per inch for a given diameter). This thread form was first introduced in 1908, the thread form is specified in BS 84: 1956.
BSW: British Standard Whitworth. A thread form developed by Sir Joseph Whitworth in 1841. The thread form has rounded roots and crests, the thread form is specified in BS 84: 1956. This thread form was superceded by the Unified thread in 1948 and then the metric thread form.
BUMP THREAD: A modified thread profile patented and trade mark of the Bosco Tool Inc. The thread form has a small projection at the pitch diameter that eliminates the clearance from the thread assembly on both flanks. By doing this it is claimed that resistance to vibration loosening is significantly improved.
CADMIUM ELECTROPLATING: Coating of threaded fasteners with cadmium can provide the parts with excellent corrosion resistance. The appearance of the coating is bright silver or yellow if subsequently passivated. The friction values associated with this coating are also comparatively low. A chromate conversion coating is frequently applied to the surface to improve corrosion resistance. Cadmium is not now frequently used because of the environmental and worker health problems associated with the coating process and should not be used in applications above 250C or when contact with food is possible.
CLAMPING FORCE: The compressive force which a fastener exerts on the joint.
CLASS OF FIT: The Class of Fit is a measure of the degree of fit between mating internal and external threads. Three main Classes of Fit are defined for metric screw threads :; FINE: This has a tolerance class of 5H for internal threads and 4h for external threads.; MEDIUM: This has a tolerance class of 6H for internal threads and 6g for external threads.; COARSE: This has a tolerance class of 7H for internal threads and 8g for external threads.; For Unified threads, a similar designation as for metric threads is used. The thread classes used are 1A, 2A and 3A for external threads and 1B, 2B and 3B for internal threads.
CLEVELOC NUT: A torque prevailing nut of all metal construction. The collar of the nut is elliptical in cross section and it is this that provides the flexible locking element. The nut is pre-lubricated to reduce the tightening torque. Cleveloc is a registered trade name of Forest Fasteners.
COEFFICIENT OF FRICTION: A dimensionless number representing the ratio of the friction force to normal force. Typically for threaded connections it is between 0.10 to 0,18 but can vary significantly depending upon the materials used and whether a lubricant has been used.
COMMINGLING: A term used to describe the undesirable practice of mixing fasteners from different batches that are the same size and grade in the same container.
CONE PROOF LOAD: This is an axial applied force applied to a nut when it is seated on a cone shaped washer which has an included angle of 120 degrees. Failure in this test is usually due to the nut splitting. The intention of the test is to introduce a nut dilation operation which will assess the potential detrimental effects of surface discontinuities. This type of test is sometimes applied to nuts which are intended for high temperature service.
CREEP: Creep is deformation with time when a part is subjected to constant stress. Metals creep can occur at elevated temperature however with gasket materials it can occur at normal ambient temperatures. Creep resistance is an important property of gasket materials. Gasket materials are designed to flow under stress to fill any irregularities in the flange surface. The amount of creep sustained tends to increase with temperature. . However once the tightening is completed it is important that no further flow occurs since such deformation will lead to a reduction in bolt extension and subsequently the stress acting on the gasket. If this stress is reduced to below a certain minimum, which depends upon the type and construction of the gasket and the operating temperature, a high rate of leakage can be anticipated to occur.


DECOMPRESSION POINT: The point at which there is zero pressure at the joint interface as a result of forces applied to the joint. If the applied force is increased beyond the decompression point, a gap will form at the interface. Analytically, a criteria of joint failure is often taken as when the applied force on the joint reaches the decompression point. This is because forces acting on the bolt(s) can dramatically increase at this point. Loading beyond this point can also result in fretting at the interface that will lead to bolt tension loss that will subsequently lower the decompression point. This process can continue until bolt failure does occur. The failure can be by fatigue or other mechanism but the underlying cause was loading of the joint beyond the decompression point. It is for this reason that it is frequently taken as a failure criteria in analysis work.
DACROMET: A high performance surface coating that can be applied to fasteners. The coating consists of passivated zinc flakes that are stoved onto the metal surface. The coating can be coloured and eliminates the risk of hydrogen embrittlement associated with electroplated metal. DACROMET is a registered trademark of Metal Coatings International, Inc. of Chardon Ohio
DESIGN FORM OF THREAD: The design form of an internal or external thread is the thread form in it's maximum metal condition. It is the same as the basic thread profile except that the thread roots are rounded. If either the internal or external thread form exceeds the design form of the thread profile then a potential interference exists.
DIRECT TENSION INDICATORS: Direct Tension Indicators (DTI's) is a term sometimes used to describe load indicating washers. Projections on the face of the washer (usually on the face abuting the bolt head or nut) that deform under loading as the bolt is tensioned. An indication of the tension in the bolt can be made by measuring the gap between the washer face and the nut or bolt head. The smaller the gap - the greater the tension in the bolt. Commonly used in civil rather than mechanical engineering applications.
DYNAMIC FRICTION: Resistance to relative movement of two bodies that are already in motion.
EFFECTIVE DIAMETER: This is the diameter of an imaginary cylinder coaxial with the thread, which has equal metal and space widths. It is often referred to as pitch diameter. Sometimes referred to as the simple effective diameter to differentiate from the virtual effective diameter.
EFFECTIVE NUT DIAMETER: Twice the effective nut radius.
EFFECTIVE NUT RADIUS: The radius from the centre of the nut to the point where the contact forces, generated when the nut is turned, can be considered to act.
ELECTROLESS NICKEL: A relatively thin, hard coating that can be applied to threads and deposited uniformly. Bright metallic in appearance this coating has excellent resistance to wear and corrosion.
EMBEDMENT: Localized plastic deformation which occurs in the vicinity of clamped fasteners or in the fastener threads. . Embedding is local plastic deformations that occur under the nut face, in the joint faces and in the threads as a result of plastic flattening of the surface roughness. This occurs even when the loading is below the yield point of the bolt or limiting surface pressure of the joint material and is the result of the real area of contact between surfaces being less than the apparent area.
ENVIRONMENTALLY ASSISTED CRACKING (EAC): A process that can occur with the use of high strength steel fasteners in which crack initiation and growth occurs in the fastener at a comparatively low stress level as a result of interactions that occur with the environment. Hydrogen is suspected of causing EAC in high strength steel fasteners, the hydrogen being produced as a result of chemical reactions (galvanic corrosion in a moist environment) or being present from a plating process that may have been applied to the fastener.
EXTERNAL FORCE OR LOAD: Forces exerted on a fastener as a result of an applied loading to the joint.
EXTERNAL THREAD: A screw thread which is formed on an external cylinder, such as on bolts, screws, studs etc.
FLOATING TYPE FLANGE JOINT: A conventional flanged joint in which a gasket is compressed by bolts - the gasket is not rigidly located. Calculation methods such as the ASME code in the USA and the EN1591 code in Europe.
FLUORO-CARBON THREAD COATING: A low friction coating applied to threads. This type of coating is frequently used to prevent thread fouling when an assembly containing threaded fasteners is painted. Unless masked in some way before painting, electro deposited primers can cover the threads. If this occurs assembly difficulties can result unless the expensive chore of cleaning the threads is completed. A fluoro-carbon thread coating eliminates the need for masking or cleaning since paint will not adhere to the coating. This type of coating can also prevent problems caused by weld splatter obstructing the threads of weld nuts during their placement. Such coatings also have the property of reducing the torque-tension scatter during tightening.
FRICTION: Mechanical resistance to the relative movement of two surfaces. There are two main types of friction; STATIC FRICTION and DYNAMIC FRICTION. Typically static friction is greater than dynamic friction.
FRICTION STABILIZERS: Coating materials used on fasteners with the intention of reducing the scatter in the thread and bearing surface friction coefficients.
FUNDAMENTAL DEVIATION: An intentional clearance between internal or external thread and the design form of the thread when the thread form is on it's maximum metal condition. For metric threads the fundamental deviation are designated by letters, capitals for internal threads and small letters for external threads. Some tolerance classes have a fundamental deviation of zero. For imperial threads the fundamental deviation is called the allowance.
FUNDAMENTAL TRIANGLE HEIGHT: The fundamental triangle height is normally designated with the letter H. This is the height of the thread when the profile is extended to a sharp vee form. For 60 degree thread forms such as metric and Unified thread series, H equals 0.866025 times the thread pitch.
GALLING: A severe form of adhesive wear which occurs during sliding contact of one surface relative to another. Clumps of one part stick to the mating part and break away from the surface. (Can frequently occur when both the nut and bolt are made from stainless or high alloy steels, titanium or zinc coated fasteners.)
GRIP LENGTH: Total distance between the underside of the nut to the bearing face of the bolt head; includes washer, gasket thickness etc.
HARD JOINT: A joint in which the plates and material between the nut and bolt bearing surfaces have a high stiffness when subjected to compression by the bolt load. A joint is usually defined as hard if the bolt is tightened to its full torque and it rotates through an angle of 30 degrees or less after it has been tightened to its snug condition.
HARDENED WASHERS: The force under the head of a bolt or nut can exceed, at high preloads, the compressive yield strength of the clamped material. If this occurs excessive embedding and deformation can result in bolt preload loss. To overcome this hardened washers under the bolt head can be used to distribute the force over a wider area into the clamped material. A more modern alternative is to use a flange headed nuts and bolts.
HEAT TIGHTENING: Heat tightening utilises the thermal expansion characteristics of the bolt. The bolt is heated and expands: the nut is indexed (using the angle of turn method) and the system allowed to cool. As the bolt attempts to contract it is constrained longitudinally by the clamped material and a preload results. Methods of heating include direct flame, sheathed heating coil and carbon resistance elements. The process is slow, especially if the strain in the bolt is to be measured, since the system must return to ambient temperature for each measurement. This is not a widely used method and is generally used only on very large bolts.
HELICAL SPRING WASHER: A split type of spring washer whose purpose is to prevent self loosening of the nut or the bolt. The idea or principle behind the helical spring washer is for one end of the tang of the washer to indent into the fastener (the nut or bolt head) and the other into the joint surface so that any loosening rotation is prevented. Junker in his paper in 1969 on the cause of self-loosening of fasteners (reference:Junker, G., New criteria for self-loosening of fasteners under vibration. SAE Paper 690055, 1969) concluded that this type of lock washer has no ability to lock. This type of washer is sometimes called a spring lock washer or sometimes a standard lock washer.


HIGH STRENGTH FRICTION GRIP BOLTS: Sometimes abbreviated to HSFG bolts. Bolts which are of high tensile strength used in conjunction with high strength nuts and hardened steel washers in structural steelwork. The bolts are tightened to a specified minimum shank tension so that transverse loads are transferred across the joint by friction between the plates rather than by shear across the bolt shank.
HOLD AND DRIVE BOLTS: Special bolts that have a tang at the threaded end of the shank. This tang is gripped by the tightening tool during assembly so that the reaction torque is absorbed whilst the nut is tightened from the same side. Such bolts allow what used to have to be done by two men to become a one-man task.
HOT BOLTING: This term is used for the completion of maintenance work on a bolted joint when the joint is under loading. This can involve the replacement of individual bolts. There are risks both to the joint itself and to health and safety associated with this technique.
HYDRAULIC TENSIONER: A hydraulic tool used to tighten a fastener by stretching it rather than applying a large torque to the nut. After the fastener has been stretched, the nut is run down the thread to snug up with the joint, the hydraulically applied load is then removed resulting in tension being induced into the fastener.
HYDROGEN EMBRITTLEMENT: Steel fasteners exposed to hydrogen can fail prematurely at a stress level well below the materials yield strength. Hydrogen embrittlement occurs in fasteners usually as a result of the part being exposed to hydrogen at some time during its manufacturing process but it can also occur through in-service corrosion. Electroplating is generally considered to be a major cause of hydrogen absorption in steel fasteners due to the release of hydrogen during this process. Higher strength steels are more susceptible to hydrogen embrittlement than lower strength steels, however it is considered that there is no lower strength limit. As a rule of thumb, steels below Rockwell C 35 are considered to be far less susceptible. Tests such as the incremental load hydrogen embrittlement test can be completed to assess if hydrogen embrittlement is present in a batch of fasteners.
IMPACT WRENCH: A wrench, usually powered by electricity or air, in which repeated blows from little hammers are used to generate torque to tighten fasteners. The torque applied to the fastener depends upon the time and the air pressure applied to the tool (for pneumatic wrenches). The torque applied by an impact wrench to a fastener is influenced by the joint stiffness.
INSTANTANEOUS CENTRE OF ROTATION: The point in space that an eccentrically shear loaded joint rotates about. The deformation and the load sustained by an individual bolt in a bolt group is dependent upon the distance that the bolt is from the instantaneous centre. The direction that the individual bolt force acts is perpendicular to a line joining that bolt to the instantaneous centre.
INTEGRAL FASTENER: A term used to describe types of fasteners which are highly resistant to vibration loosening and/or removal. Some types have special thread forms.
INTERNAL THREAD: A screw thread which is formed in holes, such as in nuts.
JAM NUTS: See LOCKNUT
JOINT CONTROL TIGHTENING: See YIELD CONTROLLED TIGHTENING
K FACTOR: The factor in the torque tightening equation: T=KDF where T is the fastener tightening torque in Newton metres, D is the fastener diameter in metres, F is the fasteners preload in Newtons and K is a factor whose value is often taken as 0.2. The formula gives the approximate tightening torque for standard fasteners used under normal conditions.
KEPS: A pre-assembled nut and washer assembly (the washer is attached to the nut so that it won't fall off)- a trademark of ITW Shakeproof. The origin of the word came from ShaKEProof. The s on the end being acquired due to them being purchased in quantities usually greater than one.
LEFTHAND THREAD: A screw thread that is screwed in by rotating counterclockwise.
LENGTH OF ENGAGEMENT: The axial distance over which an external thread is in contact with an internal thread.
LOCK NUT: There are two common usage's of this term:; 1. A nut which provides extra resistance to vibration loosening by either providing some form of prevailing torque, or, in free spinning nuts, by deforming and/or biting into mating parts when fully tightened.; 2. The term is sometimes used for thin (or jam) nuts used to lock a thicker nut. When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut. If placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain.
MAJOR DIAMETER: This is the diameter of an imaginary cylinder parallel with the crests of the thread; in other words it is the distance from crest to crest for an external thread, or root to root for an internal thread.
MEANSHIFT: The difference in tightening torque values produced by the same tightening tool on hard and soft joints. A hard joint typically gives a higher torque value than a soft joint. Generally speaking, the lower the meanshift of a tightening tool, the better it will be in achieving a specified torque value irrespective of the joint condition.
METAL TO METAL CONTACT FLANGE JOINT: A flanged joint in which a gasket is compressed by bolts - the gasket being located in a recess within the joint so that it is compressed by the bolt loads until metal to metal contact occurs. Unlike the FLOATING TYPE FLANGE JOINT, for metal to metal type joints there are no standardised gasket factor definitions, test procedures, nor generally acknowledged calculation procedures available.
MINOR DIAMETER: This is the diameter of an imaginary cylinder which just touches the roots of an external thread, or the crests of an internal thread.
MODEL ENGINEERS THREAD (M.E.): A thread based upon the Whitworth thread form that was established in 1912. A very fine thread (a 3/32 inch thread having 60 tpi for example).
MOLYBDENUM DISULPHIDE: A solid lubricant that acts as a high pressure resistant film. Can be used by itself as a dry lubricant as well as in with other solid lubricants and in oils and greases. Used in threads, such lubricants act as a separating film to prevent corrosion formation on the thread surface (even under adverse temperature and environmental conditions) ensuring the release of the threaded connection. Such films can also act as friction stabilisers.
NICKED THREADS: Nicks or indentations in threads can occur during the manufacturing process and during fastener transportation. In general, nicked thread problems tend to increase as the thread diameter increases and for fine pitches.

There are acceptance tests for nicked threads that involve measuring the maximum torque required to drive a GO gauge down the thread. Examples of acceptance tests are SAE J123 and the Ford Motor specification WA990 1993. Nicks and indentations in threads are sometimes referred to as gouges.
NOMINAL DIAMETER: The diameter equal to the external diameter of the threads.
NUT DILATION: Under load, the wedging action of the threads causes dilation of the nut resulting in an increase in the minor diameter of the nut, and reducing the effective shear areas of both the external and internal threads.
NUT RUNNER: A torque control fastener tightening tool that is usually powered by compressed air. The design of the tool is such that attempts are made to ensure that the applied torque is independent of joint stiffness.
NYLOC NUT: A torque prevailing nut that uses a nylon patented insert to provide a locking feature. The nylon insert, it is claimed, helps to seal the bolt thread against seepage of water, oil, petrol, paraffin and other liquids. The nut is covered by UK patent 8028437 and European patent 81303450-1. Nyloc is a registered trade name of Forest Fasteners.
OCTAGON HEAD: A bolt or screw whose head cross section is a regular polygon with 8 sides.
OVERTAPPING: Tapping of a thread following a plating operation so that the thread tolerances comply within specification allowing the internal and external threads to assemble. It is normal practice to overtap the internal rather than the external thread.
PILES: Term used in structural engineering for the joint plates.
PITCH: The nominal distance between two adjacent thread roots or crests.
PLY: A single thickness of steel forming part of a structural joint.
POOCHING: Pooching is a term sometimes used to describe the effect of the area immediately surrounding a tapped hole being raised up as a result of the tension from the stud. Tapped holes are often bored out for the first couple of threads to eliminate this problem.
PRELOAD: The tension created in a fastener when first tightened. Reduces after a period of time due to embedding and other factors.
PREVAILING TORQUE: The torque required to run a nut down a thread on certain types of nuts designed to resist vibration loosening. The resistance can be provided by a plastic insert or a noncircular head.
PREVAILING TORQUE NUT: A type of lock nut which has a prevailing torque to assist in preventing self loosening. There are two main categories of prevailing torque nuts, all metal and nylon insert. All metal torque prevailing nuts generally gain a prevailing torque by distorting the threads at the top of the nut by some means. Nylon insert torque prevailing nuts ultilise a nylon (or other polymer) insert to achieve a prevailing torque.
PROOF LOAD: The proof load of a nut is the axially applied load the nut must withstand without thread stripping or rupture. The proof load of a bolt, screw or stud is the specified load the product must withstand without permanent set.
PROPERTY CLASS: A designation system which defines the strength of a bolt or nut. For metric fasteners, property classes are designated by numbers where increasing numbers generally represent increasing tensile strengths. The designation symbol for bolts consists of two parts:; 1. The first numeral of a two digit symbol or the first two numerals of a three digit symbol approximates 1/100 of the minimum tensile strength in MPa.; 2. The last numeral approximates 1/10 of the ratio expressed as a percentage between minimum yield stress and minimum tensile stress.; Hence a fastener with a property class of 8.8 has a minimum tensile strength of 800 MPa and a yield stress of 0.8x800=640 MPa.; The designation system for metric nuts is a single or double digit symbol. The numerals approximate 1/100 of the minimum tensile strength in MPa. For example a nut of property class 8 has a minimum tensile strength of 800 MPa. A bolt or screw of a particular property class should be assembled with the equivalent or higher property class of nut to ensure that thread stripping does not occur.
PRYING: The amplification of an external force acting on a bolt by a lever action which can occur when that force is an eccentric tensile load.
REDUCED SHANK BOLT: A bolt whose shank diameter is smaller than the nominal diameter of the bolt (normally the shank diameter of such a bolt is approximately equal to the effective diameter of the thread).
RELAXATION: The loss of clamping force in a bolt that occurs typically without any nut rotation occurring. Commonly occurs as a result of embedment but can also be due to gasket creep, metal creep (at elevated temperatures), differential thermal expansion and stress relaxation.
RIGHTHAND THREAD: A screw thread that is screwed in by rotating clockwise. The majority of screw threads are right handed.
ROLLED THREAD: A thread formed by plastically deforming a blank rather than by cutting. The majority of standard fasteners have their threads formed by rolling. Most threads are rolled before any heat treatment operation. Significant improvements in fatigue life can be achieved by rolling the thread after heat treatment, this improvement is due to compressive stresses being induced in the roots of the thread. However, because of the increased hardness of the bolt blank, the die life can be significantly reduced. Rolling the thread also generally improves the surface finish which can have a beneficial effect on fatigue life.
ROOT DIAMETER: Identical to MINOR DIAMETER
SCREW: A headed threaded fastener that is designed to be used in conjunction with a pre formed internal thread or alternatively forming its own thread. Historically, it was a threaded fastener with the thread running up to the head of the fastener that has no plain shank. However this definition has largely been superseded to avoid confusion over the difference between a bolt and a screw.
SCREW THREAD: A ridge of constant section which is manufactured so that a helix is developed on the internal or external surface of a cylinder.
SELF LOOSENING: Threaded fasteners can come loose on occasions without human intervention. This loosening can be due to creep, embedding, stress relaxation or the fastener self-rotating (which is often called vibration loosening). Creep, embedding and stress relaxation will generally not completely loosen a fastener, these loosening mechanisms occur without the nut rotating relative to the bolt. The term self loosening is sometimes used for the nut rotating relative to the bolt without human intervention. It is know that the fastener can self rotate under the action of transverse joint movement that can completely loosen a tightened fastener such that the nut will become detached from the bolt.
SEMS: A screw and washer assembly. A screw or bolt which has a captive washer. The washer is frequently loose on the plain shank of the fastener, the shank diameter being equal to the effective diameter of the thread; the thread being rolled from this diameter. The origin of the word is a frequent question. In the 1930's E. C. Crowther was a representative for a company that sold both shakeproof washers and screws. He came up with the idea of placing the washer on the screw before it was thread rolled. The major diameter of the screw being larger than the washer hole prevents it from coming off. The Illinois Tool Works made machines that produced these patented pre-asSEMbled washers and screws. The s at the end of SEMs is thought to have been subsequently picked up because they are not usually purchased individually. In spite of the original patents and trademarks the word SEMS is generally recognised as a generic term applicable to screw and washer assemblies.
SET SCREW: A set screw is a threaded fastener that is typically used to hold a sleeve, collar or gear on a shaft to prevent relative motion. It is a threaded member that normally does not have a head. Unlike most other threaded fasteners it is basically a compression device normally used to generate axial thrust. Various socket types are provided to allow the set screw to be rotated. These types include hexagon socket, fluted socket, screwdriver slot and square head. Various point designs are available (the part of the set screw that rotates against the shaft being secured) and include:

Cup - Hollowed end, is the most commonly used point style. Used when the digging in of the point is not undesirable.

Cone - Pointed end, this type generates the highest torsional holding power and is typically used for a permanent connection.

Oval - Rounded end that is typically used when frequent adjustment is required. The oval end prevents/reduces indentation.

Flat - Cause little damage to the shaft and are used when frequent adjustment is required.

Dog - Flat end with the threads stopping short of the end with the end fitting into a hole.
SHANK: That portion of a bolt between the head and the threaded portion.
SHOULDER SCREWS: A threaded fastener with a plain, precision machined, shank that is used for location purposes. They are typically used for pulleys and linkages.
SKIDMORE BOLT TENSION CALIBRATOR: The Skidmore-Wilhelm bolt tension calibrator is a hydraulic load cell used to determine the tension in a bolt or other threaded fastener. The tension in the bolt compresses fluid in a hydraulic cylinder, a pressure gauge connected to the cylinder is then calibrated to read in terms of force rather than pressure.
SNUG TORQUE: The torque required to pull plates together so that direct contact occurs; often used in angle control tightening. The snug torque ensures that metal to metal contact occurs at all the interfaces within the joint. It is only at this point that the required angle of rotation start in order that the bolt is tightened sufficiently. The snug torque is usually determined experimentally on the actual joint.
SNUGGING: The process of pulling parts of a joint together, most of the input turn during this process is absorbed in the joint with little tension being given to the bolt.
SOCKET HEAD CAP SCREW: A screw with a round head, usually with a hexagon indentation in the head for tightening purposes. Used on machine parts and is typically made from high strength steel (grade 12.9 in metric).
SOFT JOINT: A joint in which the plates and material between the nut and bolt bearing surfaces have a low stiffness when subjected to compression by the bolt load. In such a joint, the bolt (or nut) typically has to be tightened by two or more complete turns, after it has been torqued to the snug condition, before the full tightening torque is achieved. Often the placement of a gasket in a joint results in a soft joint.


SOFT TORQUE: An alternative name, used by some manufacturers, for snug torque.
SPIRAL WOUND GASKET: A type of gasket that is made by winding V-section metal strip and a softer filler material together. Support or retaining rings, inside and/or outside the spiral, improve the gasket's handling and fitting. The filler material used is typically graphite or PTFE. The metal strip and retaining rings being typically made from stainless steel.
STATIC FRICTION: Friction at rest; a force is required to initiate relative movement between two bodies - static friction is the force that resists such relative movement. Sometimes referred to as stiction.
STEP-LOCK BOLT (SLB): The Step-Lock Bolt (SLB) is a thread form that has been modified to resist vibration loosening. The thread has several horizontal portions (i.e. no lead angle) whose purpose is to prevent torsion being developed in the bolt as a result of the loosening purpose. It is these horizontal portions that are known as steps. Published literature indicates that the thread form performs well when tested on a transverse vibration test machine. However manufacturing difficulties may prevent its widespread adoption.
STIFFNUT: A term used to describe a lock nut which has a prevailing torque.
STRENGTH GRADE: See PROPERTY CLASS
STRESS AREA: The effective cross sectional area of a thread when subjected to a tensile force. It is based upon a diameter which is the mean of the pitch (or effective) and the minor (or root) diameters of the thread. The use of this diameter stems from the work of E. M. Slaughter in the 1930's. He completed carefully controlled tests using various sizes of standard threads and compared their strength with machined bars made from the same bar of material. He found that this mean diameter gave results that agreed with the tensile test results to within about 3%. The error on the minor and pitch diameters was about 15%. Tests completed subsequent to these by other investigators have also shown that the stress diameter is a reasonable approximation to a thread's tensile strength. (Referance: 'Tests on Thread Sections Show Exact Strengthening Effect of Threads.' by E. M. Slaughter, Metal Progress, vol 23, March 1933 pp. 18-20)
STRESS RELAXATION: A significant problem with bolting at high temperatures is a phenomenon known as stress relaxation. Creep occurs when a material is subjected to high temperature and a constant load. Stress relaxation occurs when a high stress is present that is relieved over time; the stress is relaxed with a subsequent reduction in the bolt’s preload. The only way to minimise the effects of stress relaxation is to use materials that have an adequate resistance to it at the product’s operating temperature. The effect of bolt stress relaxation is to reduce the clamp force provided by the bolts; this phenomena alone will not fully loosen a joint.


STRUCTURAL BOLT: A structural bolt is a heavy hexagon head bolt having a controlled thread length intended for use in structural connections and assembly of such structures as buildings and bridges. The controlled thread length is to enable the thread to stop before the joint ply interface to improve the fastener's direct shear performance.This term is used in civil and structural engineering but is not frequently used in mechanical engineering.
STUD: A fastener which is threaded at both ends with an unthreaded shank in between. One end (which often has a thread tolerance which results in more thread interference) is secured into a tapped hole, the other is used with a nut.
SYMMETRICAL THREAD: A symmetrical thread is one which has both flanks of the thread profile inclined at the same angle.
TAYLOR-FORGE METHOD: A method developed by four engineers of the Taylor-Forge Company in Chicago in the 1930's that subsequently formed the basis of the ASME code for flanged joint design. The assumptions made by the method are now generally regarded as too simplistic. This method gives rise to the m and y gasket factors.
TENSION WASHERS: A general name given to spring washers, curved washers, Belleville washers and disc springs. This type of washer provides a relatively low stiffness (compared to the joint stiffness) and can be used to act as a spring take-up with a bolt to prevent movement between parts.
THREAD CREST: The top part of the thread. For external threads, the crest is the region of the thread which is on it's outer surface, for internal threads it is the region which forms the inner diameter.
THREAD FLANK: The thread flanks join the thread roots to the crest.
THREAD HEIGHT: This is the distance between the minor and major diameters of the thread measured radially.
THREAD LENGTH: Length the portion of the fastener with threads.
THREAD ROOT: The thread root is the bottom of the thread, on external threads the roots are usually rounded so that fatigue performance is improved.
THREAD RUNOUT: The portion at the end of a threaded shank which is not cut or rolled to full depth, but which provides a transition between full depth threads and the fastener shank or head.
THREADLOCKER: Can be a term used for a number of vibration resistant products but is now usually reserved for threadlocking adhesives. Specifically, a liquid anaerobic adhesive applied to nut or bolt thread, once hardened it fills the inner spaces between the threads to produce a solid plastic of a known shear strength.
TIN/ZINC ALLOY ELECTROPLATING: Tin/zinc alloy coatings (typically 70% tin and 30% zinc) are applied to threaded fasteners to provide a corrosion resistant coating. One of the advantages of such coatings is that bimetallic corrosion will not occur when placed into contact with such metals as aluminium or steel.
TOLERANCE CLASS: A combination of tolerance grade and a fundamental deviation which is given to an internal or external thread. A tolerance class for an internal thread when combined with the tolerance class for an external thread gives the class of fit for the mating threads.
TOLERANCE GRADE: The difference between maximum and minimum metal conditions for a tolerance applied to a screw thread. For metric threads the tolerance grade is given a number.
TORQUE: A rotational moment; it is a measure of how much twisting is applied to a fastener. The units used to measure torque are in the form of force times length. Usually measured in newton-metres (Nm) if metric units are used or pounds feet (lb-ft) when imperial units are used.
TORQUE MULTIPLIER: A gearbox used to increase the torque produced by a small hand wrench.
TORQUE WRENCH: A manual wrench which incorporates a gauge or other method to indicate the amount of torque transferred to the nut or bolt.
TURN OF THE NUT METHOD: See ANGLE CONTROLLED TIGHTENING
U BOLT: A U shaped fastener threaded at both ends used primarily in suspension and related areas of vehicles.
ULTRASONIC EXTENSOMETER: An instrument which can measure the change in length of a fastener ultrasonically as the fastener is tightened or measure the length before and after it is tightened).
UNC: Unified National Coarse (UNC) is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a larger thread pitch than an equivalent diameter UNF thread. The unified thread is based on inch sizes and was first standardised in 1948 unifying the Whitworth and American standard thread forms.
UNEF: Unified National Extra Fine (UNEF) is a Unified thread form with a very fine (small) pitch that are typically used on instruments and parts requiring a fine adjustment.
UNF: Unified National Fine (UNF) is a thread form with a 60 degree flank angle rounded roots and flat crests. For a given diameter it has a smaller thread pitch than an equivalent diameter UNC thread.
UNR: Unified National (UN) thread form with a rounded root contour, applies only to external threads. (The UN thread form has a flat, or optionally, a rounded root contour.) The majority of fasteners with a Unified thread form have a rounded root contour i.e. are UNR threads.


VIRTUAL EFFECTIVE DIAMETER: The effective diameter of a thread but allowing for errors in pitch and flank angles.
WAISTED SHANK BOLT: A bolt whose diameter is less than the minor diameter of the thread. Frequently the shank of the bolt is 0.9 times the root diameter.
WIRE THREAD INSERT: A threaded insert that is typically used for tapped hole repair or to improve the thread stripping strength of softer metals such as zinc and aluminium. The inserts are assembled into a previously tapped hole using a special driving tool. A thread locking compound is frequently used to secure the insert if the assembly is subject to vibration.
YIELD CONTROLLED TIGHTENING: A fastener tightening method which allows a fastener to be tightened to yield. The angle of rotation of the fastener is measured relative to the applied torque, yield being assessed when the slope of the relationship changes to below a certain value. Sometimes called joint controlled tightening.
ZINC ELECTROPLATING: Zinc electroplating is a common way to protect threaded fasteners from the effects of corrosion. Zinc electroplating can be completed in acid chloride, alkaline or cyanide baths. Supplemental coatings are frequently applied to zinc electroplating. These coatings, such as zinc phosphate or chromate conversion, provide a protective passivation layer on the zinc which assists in reducing the corrosion rate.
ZINC/COBALT ALLOY ELECTROPLATING: This coating is similar to zinc electroplating completed in an acid chloride bath - a small amount of cobalt (typically about 1%) is added to increase the plating speed.
ZINC PHOSPHATE CONVERSION COATING: A zinc phosphate conversion coating is frequently added to zinc electroplated parts, such as bolt threads, to improve corrosion resistance. This type of chemical conversion coating provides a protective passivation layer on the zinc improving its corrosion resistance.

How to Read a Screw Thread Callout

noreply@blogger.com (widi san) — Sun, 29 Jun 2008 05:50:00 +0000

A thread gauge, for measuring thread pitch.

You have a loose screw --the threaded fastener sort--and you walk into the hardware store to obtain a replacement. There, you encounter an entire aisle of screws, nuts, washers, and other small hardware. Which one do you need? If you know a little bit about how screw sizes work, the process of finding the right part will be a lot easier.

Steps

Many different sizes.

Read the numbers. They will look something like one of these:
- #4-40 x .5
- 1/4-20 x 5/8
- M3-50 x 10
The major diameter for the threaded portion of the screw.

Interpret the first number. The first number gives the major, or largest, diameter.
- In Unified threads (measured in inches) there are numbered diameters #0 through #10, with 0 the smallest and 10 the largest. (Diameters #12 and #14 may also be found, but are usually on older equipment and needed for repairs or restorations. #14 is close to, but not exactly the same as, 1/4-inch.) The major diameter in Unified threads = 0.060" + 0.013"*(numbered diameter). So #2 has a major diameter of 0.086". The odd numbers exist, but the even numbers are in far more common use.
- For screws larger than a #10, the diameters are listed in fractional inches. For instance, a 1/4-20 screw has a 1/4-inch major diameter.
- For metric threads, e.g. M3.5, the number following the M is the major diameter of the external thread in millimeters.
The distance between adjacent threads, or thread pitch.

Interpret the second number. It has to do with the distance between adjacent threads. It may be given as the number of threads per unit length; or it may be given as the distance between threads, also called the thread pitch.
- For Unified threads, the number given is threads per inch. For instance, a 1/4-20 screw has 20 threads per inch.
- For metric threads, the thread pitch is given in millimeters per thread. Thus, a M2 x 0.4 screw has threads every 0.4mm. Although most metric fasteners have two or more standard pitches (fine & coarse threads), the pitch is often omitted from a thread callout, it is always helpful to carry a sample with you to the hardware store.
  - There are two major metric "industrial standards": DIN Deutsches Institut für Normung (German) and the JIS Japanese Industrial Standards. Although these standards are closely related and often identical, there will be cases where say a JIS M8 bolt may not have the same pitch as a DIN M8 bolt.
The length of most screws is measured from the bottom of the head.

Read the length, which is generally given after the "x". The length of most screws is measured from the bottom of the head, as shown. Note, however, that a flathead screw, designed to sit flush in a countersunk material, is measured to the top of the head.
- For unified threads the length is given in inches. A 1/4-20 x 3/4 screw is .75 inches long. The length may be given in fractional inches or the fractional equivalent.
- For metric threads, the length is given in millimeters.
Understand some other nomenclature that sometimes goes with screw threads.
- Thread classes refer to fit, how loosely or tightly the screw fits in the nut. The most common thread classes are 2A or 2B. A indicates an external thread, such as on a screw or bolt. B indicates an internal thread, such as on a nut. The 2 (or, far less commonly, 1 or 3) describes the tightness of the fit.
- You may see the abbreviations UNC and UNF. These stand for unified coarse and unified fine, respectively, and they refer to standard series of thread pitch. Each series assigns a pitch to diameter. For instance, a #10 UNC screw has 24 threads per inch, whereas a #10 UNF screw has 32 threads per inch. If a thread is specified by its series, look for the pitch in a table.
- Minor diameter is the smallest diameter of the thread, the innermost diameter. Major diameter is the largest diameter of the thread, the outermost diameter. The diameter given is typically the nominal major diameter of an external, or male, thread.
For a complete description of the screw, include the head and drive style. The Wikipedia article in the external links section includes diagrams of different heads and drives.

Tips

If you have an unknown fastener, a thread gauge or screw checker can be a big help in determining its size. If no such instrument is available, try screwing your fastener into a known, mating thread. Stop immediately if you feel undue resistance, to avoid stripping threads.
These screws are the same.

One way to check whether two screws are the same is to set them side by side facing opposite directions. If their threads mesh, they have the same thread pitch. This is also a quick way to check length.
To read aloud, say these callouts as follows:
- #4-40 x .5 -- Say "Four-forty by point five" or "Number four-forty by a half."
- 1/4-20 x 5/8 -- Say "Quarter-twenty by five eighths."
- M3-50 x 10 -- Say "Em three fifty by ten."
This guide is for machine screws. Other threads, such as wood screws, follow slightly different guidelines. Other thread series, such as the PG series and British Whitworth also exist, but they are relatively rare.
The majority of machine screws are right-handed threads, meaning that the screw will turn clockwise to insert and counterclockwise to remove. Remember, "righty, tighty; lefty loosie". One common exception is the thread holding the left pedal on a bicycle to the crank arm.
Screws typically come in certain round-numbered lengths, so a 1/4 inch screw may be far easier to find than a 5/32 inch screw.
Consult a size chart for more information.
A good rule of thumb with machine screws is that a minimum of three full threads should engage the mating thread. If there are not at least three threads engaged in a thin material, use a nut or other reinforcement.

Warnings

When specifying a fastener, make sure that the fastener is adequate for the job and compatible with the materials and environment.
Tapered pipe threads follow entirely different rules.
Metric standards are often very close to English standards and can often be confused. Sometimes context can be helpful (e.g. If the hardware is from a non-American car, it's probably metric).

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Table of pitch for ISO Metric Coarse Threads

Table of pitch for UNF (fine) threads

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LATHE RELATED OPERATIONS

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TURNING RESEARCH AT MICHIGAN TECH

Here at Michigan Technological University, turning research is being conducted in three areas:

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From Wikipedia, the free encyclopedia

Contents

Screws and Bolts

Differentiation between bolt and screw

Other fastening methods

Materials and strength

Mechanical analysis

Tensile strength

Types of screws and bolts

Fasteners with a tapered shaft (tapping screws)

Fasteners with a non-tapered shaft

Other threaded fasteners

Shapes of screw head

Types of screw drive

Combination drives

Tamper-resistant screws

Tools used

Mechanics of use

Thread standards

See also: Screw thread

[edit] ISO metric screw thread

Whitworth

British Association screw threads (BA)

Unified Thread Standard

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From Wikipedia, the free encyclopedia

Contents

Bearing friction

Principles of operation

Motions

Loads

Speeds

Life

Maintenance

History and development

Types

nuts info

How to Read a Screw Thread Callout

Steps

Tips

Warnings