mechanical engineer topics

air conditioning

2014-04-26T19:07:00.000-07:00

AIR:

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation)

Structure of the atmosphere

Principal layers

In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions (see the temperature section, below). Because the general pattern of the temperature/altitude profile is constant and recognizable through means such as balloon soundings, the temperature behavior provides a useful metric to distinguish between atmospheric layers. In this way, Earth's atmosphere can be divided (called atmospheric stratification) into five main layers. From highest to lowest, these layers are:

Exosphere: >700 km (>440 miles)
Thermosphere: 80 to 700 km (50 to 440 miles)
Mesosphere: 50 to 80 km (31 to 50 miles)
Stratosphere: 12 to 50 km (7 to 31 miles)
Troposphere: 0 to 12 km (0 to 7 miles)

AIR CONDITIONING:

Air conditioning (often referred to as aircon, AC or A/C) is the process of altering the properties of air(primarily temperature and humidity) to more favourable conditions, typically with the aim of distributing the conditioned air to an occupied space to improve comfort. More generally, air conditioning can refer to any form of technology, heating, cooling, de-humidification, humidification, cleaning, ventilation, or air movement, that modifies the condition of air

powerplant

2013-03-08T05:31:00.003-08:00

Energy is required for everything that we do, and it is the next important thing apart from the food upon which the lives of nations depend. Lack of power could cause economies to cripple. The flourishing power generation industry is considered to be a sign of prosperity for any nation. Learn more about the types of power plants used to generate this energy.

Introduction

Energy comes in various forms but electrical energy is the most convenient form of energy since it can be transported with ease, generated in a number of different ways, and can be converted into mechanical work or heat energy as and when required. In this article we will learn about a few of the most commonly used methods of generating electrical energy.
The Power Plant

Power or energy (let me remind you at this juncture that though the words are used in the synonymous sense here, technically they have somewhat differently meanings) is generated in a power plant which is the place where power is generated from a given source. Actually the term “generated” in the previous sentence is a misnomer since energy cannot be created or destroyed but merely changed from one form to the other. More correctly, a power plant can be said to be a place where electrical energy is obtained by converting some other form of energy. The type of energy converted depends on what type of power plant is being considered.

In the industrial use of the word, the term power plant also refers to any arrangement where power is generated. For example the main engine of a ship or an aeroplane for that matter. But in the context of this articles (and other articles on this topic), just remember that power plant basically refers to electrical energy generation facility. This leads us to the next question that how many types of power plants are used commonly for electrical energy generation?
Types of Power Plants

There are several different types of power plants used across the world today. Two will be discussed here very briefly since it is not possible to elaborate on different types of power plants in one article, but they shall be taken one by one, in the series. Each of these plants has their own set of advantages and drawbacks from various perspectives and various factors govern which type of power plant is best suited for a particular region or situation.
- Thermal Power Plants – as the name suggests, these power plants convert heat energy into electrical energy. The working fluid of these plants is mostly steam and they work on the Rankine cycle. A steam power plant consists of a boiler which is used to generate the steam from water, a prime mover like a steam turbine to convert the enthalpy of the steam into rotary motion of the turbine which is linked to the alternator to produce electricity. The steam is again condensed in the condenser and fed to the boiler again.
- Hydro Power Plants – these plants use the kinetic energy of flowing water to rotate the turbine blades, hence converting kinetic energy into electrical energy. These types of power plants are very good for peak loads. Their main disadvantage lies in the fact that their location depends on a number of factors which are beyond the control of human beings such as the hydrological cycle of the region and so forth. If there is shortage of water it could lead to shut down of these plants. For this reason alternative arrangements such as thermal power plants need to be made to ensure uninterrupted generation of power.
Apart from these main two types there are plants which use nuclear energy, solar energy and even wind energy to generate power. We will discuss more about these in later articles.

DRILLING MACHINES

2012-12-18T06:31:00.003-08:00

A drilling machine comes in many shapes and
sizes, from small hand-held power drills to bench mounted
and finally floor-mounted models. They can perform
operations other than drilling, such as countersinking,
counterboring, reaming, and tapping large or small holes.
Because the drilling machines can perform all of these
operations, this chapter will also cover the types of drill bits,
took, and shop formulas for setting up each operation.
Safety plays a critical part in any operation involving
power equipment. This chapter will cover procedures for
servicing, maintaining, and setting up the work, proper
methods of selecting tools, and work holding devices to get
the job done safely without causing damage to the equipment,

yourself, or someone nearby.

Injection Molding Process

2012-11-21T16:54:00.000-08:00

mold or mould :

Mold is the American spelling for all senses of the word meaning, among other things,
(1) a frame for shaping something,
(2) to shape in a mold, and
(3) any of various fungi that commonly grow on organic matter and are often associated with decay.

Mould is the British spelling. American English has no mould, and British English has no mold.
Australian and Canadian English favour the British spelling, though mold is fairly common in Canadian publications.
The distinction extends to all derivative words, including molding (American) and moulding (British).

Casting is a basic molding process as it requires the least amount of complex technology. Plastic is simply heated so it turns into a fluid, and then transferred into a mold. It is left to cool and the mold is removed. This process can be used for intricate shapes and performed under a low pressure. However, it is a common process used for making plastic sheeting, starting from 0.5 inches thick and greater.

Injection molding is used for creating high-quality three-dimensional objects, that can be commercially reproduced. The molding process begins by melting plastic in a hopper. Then the plastic is injected into a tightly closed, chilled mold. The plastic quickly takes the shape of the surrounding mold. Once it has completely set, the mold is opened and the plastic object is released. Yogurt pots, butter tubs, toys and bottle caps are made using this process.

Blow molding is a process used for making piping and milk bottles. Plastic is heated until molten. Then it is injected into a cold mold. The mold has a tube set within it, which has a particular shape when inflated. So, while the plastic is molten, air is blown into the tube and the plastic is formed around the tubing. It is then left to cool and removed from the mold.

The most labor-intensive type of molding process is compression molding. Therefore, it is only used for large-scale production purposes, and not for mass production. For example, boat hulls and car tires are made using this method. Molten plastic is poured into a mold. Then a second mold is pressed into it. This squeezes the plastic into the desired shape before being left to cool and removed from the mold.

Toys, shipping drums, storage tanks and items of consumer furniture are made using rotational molding. Each object is made by coating a mold from the inside. A mold is held in place between two mechanical arms. Then, the arms rotate the mold constantly at the same level, while molten plastic is placed inside. As it turns, the plastic coats the inside of the mold to create a new hollow, plastic object.

Injection Molding:

http://www.bpf.co.uk/Data/Image/InjectionMoulding.swf

Injection molding has been one of the most important fabrication tools for the plastics industry since the reciprocating screw machine was patented in 1956. Today, it's almost impossible to do anything without using injection molded parts. They are used in automotive interior parts, electronic housings, housewares, medical equipment, compact discs, and even doghouses. Injection molding is used to fabricate pallets, toys, crates, and pails, thin-wall food containers, promotional drink cups, lids, and milk bottle caps.

The injection molding process involves melting the plastic in an extruder and using the extruder screw to inject the plastic into a mold, where it is cooled. Speed and consistency are vital keys to running a successful injection molding operation, since profit margins are normally below 10 percent.

Speed:

A molder will maximize output by minimizing cycle time which is the amount of time that is taken to melt the plastic, inject it into the mold, cool, and eject a finished part.
Using larger molds that produce more than one part each time the machine performs a cycle can also increase output. These molds are known as multiple cavity molds.

Consistency:

Consistency, or elimination of scrap and downtime, is just as important as output in a successful molding operation. The most consistent processing results from careful control of plastic temperature, plastic pressure as it fills the mold, the rate at which the plastic fills the mold, and the cooling conditions. These four primary molding variables are interdependent and can often be used to understand process changes and solve problems. While the variables apply to almost all injection molding processes, the process will be slightly different in each shop, depending on the application, the plastic being used, and the molder's preferences.

Fill rate:

In thin wall applications, the material must be injected into the mold as quickly as possible to prevent the plastic from freezing before the part has been completely filled. The newest resin and machine technologies in this area almost always focus on faster, easier fills. In addition to minimizing cycle time through better filling ability, the molder could realize resin cost savings through the ability to fill thinner molds or achieve higher outputs by using larger, higher cavity molds.
Thin wall molding is accomplished using machines that inject the material in less than one second and are big enough to support large, multiple cavity molds. Thin wall lids and containers tend to be small, so molds may be used to fabricate over 100 small lids at a time.

Defects in injection molding:

Black Spots, Brown streaks:

Description

Black spots and brown streaks appear as dark spots or streaks in the molded part and are usually caused by thermal damage to the melt.

Possible Solutions

Check the material for contamination.
Decrease the melt temperature.
Decrease the overall cycle time.
Purge and/or clean the screw and barrel.
Decrease the screw speed. High screw speeds may cause the material to degrade.
Material may have too much regrind content.
Material may be overdried. Decrease drying time/temperature. Refer to drying instructions provided by the material supplier.
Material may be prone to thermal degradation. It may be necessary to use a more thermally stable material.
Dead spots may be occurring, ensure that the alignment between the machine nozzle and mold sprue is correct.
Residence time may be too long, or the shot size may be too small for the machine. It may be necessary to move the mold to a machine with less injection capacity.

Blisters (Air Entrapment):

Description

Blisters are hollows created on or in the molded part. In contrast to a void (vacuum) this entrapped gas can also appear near the walls.

Possible Solutions

Decrease melt temperature.
Decrease screw speed.
Dry material.
Increase back pressure.
Increase mold temperature.
Ensure regrind is not too coarse.
Provide additional mold vents.
Relocate gate.

Brittleness:

Description

Brittleness is a condition where the part cracks or breaks at a much lower stress level than would normally be expected based on the virgin material properties.

Possible Solutions

Check for material contamination.
Decrease amount of regrind use.
Decrease back pressure.
Decrease injection pressure.
Decrease screw speed.
Increase melt temperature.
Dry material. Refer to the drying instructions provided by the material supplier.

Bubbles:

Description

Bubbles are similar to blisters in that there is air entrapped in the molded part.

Possible Solutions

Decrease injection speed.
Decrease injection temperature.
Dry material further.
Increase injection pressure.
Increase number and/or size of vents.
Increase shot size.

Burn Marks, Dieseling:

Description

Burn Marks or Dieseling show up on the finish molded parts as charred or dark plastic caused by trapped gas and is usually accompanied by a distinctive burnt smell.
Note: If this problem is allowed to continue without fixing the root cause it will very quickly cause damage to the molding surface.

Possible Solutions

Alter gate position and/or increase gate size.
Check for heater malfunction.
Decrease booster time.
Decrease injection pressure.
Decrease injection speed.
Decrease melt and/or mold temperature.
Improve mold cavity venting. Vents may become smaller over time due to wear and they will need to be brought back to their original depth.
Reduce clamp force to improve venting. Vents may become smaller because they are being crushed by the clamping force. If it is possible to reduce the clamping force without causing flash then this should be done. Note: This is always good practice to minimize wear on the mold and machine.
Improve venting at the burn location. Burn marks often occur on deep ribs that have no venting. If possible it may be helpful to put an ejector pin or sleeve at the burnt area to allow the trapped gas to escape to atmosphere.

Cracking, Crazing:

Description

Cracking or Crazing is caused by high internal molded in stress or by an external force imposed upon the part. They can also be caused by an incompatible external chemical being applied to the finished parts The cracks often don't appear until days or weeks after the parts have been molded.

Possible Solutions

Decrease injection pressure.
Dry material.
Increase cylinder temperature.
Increase mold temperature.
Increase nozzle temperature.
Modify injection speed.
If the material is partially crystalline then it may help to reduce the mold and/or melt temperature.
If the material is amorphous then it may help to increase the mold and/or melt temperature.

Delamination:

Description

Delamination occurs when single surface layers start flaking off the molded part.

Possible Solutions

Adjust injection speed.
Check for material contamination. Incompatible resins or colorants may have been accidently mixed causing this condition to be seen.
Dry material.
Increase melt temperature.
Increase mold temperature.
Insufficient Blending. Check melt homogeneity and plasticizing performance.

Discoloration:

Description

Discoloration is similar to burn marks or brown streaks but generally not as dark or severe. It may cause the part to be a darker shade than the virgin pellets and is often found nearest the gate area, however it can also appear as
dark streaks throughout the part.

Possible Solutions

Check hopper and feed zone for contamination.
Decrease back pressure.
Decrease melt temperature.
Decrease nozzle temperature.
Move mold to smaller shot-size press.
Provide additional vents in mold.
Purge heating cylinder.
Shorten overall cycle.

Excessive Flash:

Description

Excessive Flash is often seen near sealing faces, out of vent grooves, or down ejector pins. It appears as thin or sometimes thick sections of plastic where it would not be on a normal part.
Note: Flash can very quickly (within a few cycles) damage the parting line surfaces.

Possible Solutions

Decrease back pressure.
Decrease cylinder temperature.
Decrease injection hold time.
Decrease injection pressure.
Decrease injection speed.
Decrease mold temperature.
Increase clamp pressure.
Check mold venting. Vents may have been ground too deep for the material being used.
Check sealing surfaces to ensure that they seal off properly by "blueing" them in under clamp tonnage.
Check ejector pin bore diameter to pin diameter tolerances. The tolerances may be too large allowing plastic to flash down the opening. The tolerances may be too large for the material being used and can occur due to wear over time.

Flow, Halo, Blush Marks:

Description

Flow, Halo, Blush Marks are marks seen on the part due to flow of the molten plastic across the molding surface.

Possible Solutions

Decrease injection speed.
Increase cold slug area in size or number.
Increase injection pressure.
Increase melt temperature.
Increase mold temperature.
Increase nozzle temperature.
Increase size of sprue/runner/gate.

Gate Stringing, Drooling:

Description

The part does not break cleanly from the gate area.

Possible Solutions

Insufficient cooling time during the cycle.
Excessive heat in the gate area. Check thermocouple in the nozzle or decrease the temperature of the hot runner manifold and nozzle.
Increase cooling at the gate area.Ensure that you have controllable turbulent flow in the gate area.

Gels:

Description

Gels are bubbles, or blisters seen on or in the part due to poor melt quality.

Possible Solutions

Change screw speed.
Increase back pressure.
Increase cylinder temperature.
Increase overall cycle time.
Increase plasticating capacity of machine or use machine with large plasticating capacity.

Jetting:

Description

Jetting is caused by an undeveloped frontal flow of melt in the cavity. The uninterrupted plastic flows or "snakes" into the cavity and cools off enough so that it does not fuse homogeneously with the material that follows.

Possible Solutions

Decrease injection speed.
Change the melt temperature, up or down.
Use higher compression screw.
Increase the gate diameter.
Move the gate so that when the plastic first enters the cavity it hits an obstruction such as a rib or wall.

Material Leakage:

Description

Material Leakage is usually caused by material forces overcoming the structural strength of the mold.
NOTE: One sign that indicates that material has leaked is that the manifold reaches processing temperature very slowly.

Possible Solutions

Manifold locator is oversize.
Processing temperature may be too low causing increased pressure in the manifold.
Manifold locator may be hobbed into the mold. Decrease the force applied to the nozzle pad by the machine then repair the damaged area, then check and if necessary replace the locator.
Insufficient number of mold assembly screws. Ensure that the quantity, type of screw, and the location of the screws correspond to the general assembly drawing.
Nozzle may have overheated causing damage to the seal or gate. Check/replace the thermocouple in the nozzle, then check and if necessary repair the nozzle well area.
Manifold may have overheated. Check and replace if necessary the following components; nozzle well area, thermocouple, valve disks, sprue disks, or pressure disks.

Oversized Part:

Description

Part is too large when compared to the drawing specifications.

Possible Solutions

Decrease booster time.
Decrease cylinder temperature.
Decrease holding pressure.
Decrease injection pressure.
Decrease injection speed.
Decrease overall cycle time.
Increase gate size and/or change gate location.
Increase mold temperature.

Part Sticking:

Description

Part is getting not pulling out of the cavity and in rarer circumstances cannot be ejected off the core.

Possible Solutions

Check mold for undercuts and/or insufficient draft.
Decrease booster time.
Decrease cylinder and nozzle temperature.
Decrease injection pressure.
Decrease injection-hold.
Decrease mold cavity temperature.
Increase clamp pressure.
Increase mold-close time.
Texturing on part is too deep. The parts may stick in the cavity if a new texture or a retexturing has been performed on the cavity half of the mold.
If possible add undercuts to the core to allow the part to pull out of the cavity.

Short Shot (Incomplete Filled Parts):

Description

Short Shots occur when the part does not completely fill.

Possible Solutions:

Increase back pressure.
Increase injection pressure.
Increase injection speed.
Increase melt temperature.
Increase mold temperature.
Increase nozzle temperature. Ensure that the manifold and nozzles have reached the set temperature.
Increase shot size and confirm cushion.
Make sure mold is vented correctly and vents are clear.
Confirm that the non-return valve used is not leaking excessively.
Increase the switch over pressure, distance, or time (whichever method is being used) point from fill to hold so the fill stage is used longer.
Change part design. Thin areas of the mold may not fill completely, especially if there is a thick to thin transition, or there is a long rib that cannot be vented very well. If the part design allows it, change in these areas can
improve the situation.

Sink Marks:

Description

Sink Marks occur during the cooling process if certain areas of the part are not cooled sufficiently causing them to contract.

Possible Solutions:

Decrease amount of regrind use.
Decrease back pressure.
Confirm that the non-return valve being used is not leaking excessively.
Decrease melt temperature. Do this if the sink marks are near the gate or thick walled areas.
Decrease mold temperature. Do this if the sink marks are near the gate or thick walled areas.
Decrease injection rate. Do this if the sink marks are near the gate or thick walled areas.
Dry material.
Increase injection pressure. Do this if the sink marks are away from the gate or in thin walled areas.
Increase injection speed. Do this if the sink marks are away from the gate or in thin walled areas.
Increase mold temperature. Do this if the sink marks are away from the gate or in thin walled areas.
Increase injection-hold.
Increase shot size and confirm that the a cushion is being maintained.
Increase size of sprue and/or runners and/or gates.
Relocate gates on or as near as possible to thick sections.
Increase cooling time.
If possible change the mold design to maintain an even wall thickness throughout the part.

Splay Marks, Silver Streaks:

Description

Splay Marks, Silver Streaks are usually caused by water vapor blisters at the flow front burst and freeze on the wall of the molding surface.

Possible Solutions:

Check for contamination.
Decrease melt temperature.
Decrease nozzle temperature.
Dry resin pellets before use. As per the manufacturers recommendations.
Incorrect storage of pellets. Moisture on the pellets could be transferred into the melt, especially if the resin is not normally pre-dried.
Raise mold temperature. This will prevent condensation on the mold walls from being carried into the melt.
Ensure the mold is not leaking water onto the cores or cavities. Again this will prevent condensation on the mold walls from being carried into the melt.
Relocate gates on or as near as possible to thick sections.
Shorten overall cycle.

Sprue Sticking:

Description

Sprue Sticking generally occurs in a cold runner mold when the sprue is staying in the mold.

Possible Solutions:

Check mold for undercuts and/or insufficient draft.
Decrease booster time.
Decrease injection pressure.
Decrease injection speed.
Decrease injection-hold.
Decrease mold close time.
Decrease nozzle temperature.
Increase core temperature.
Open the gates.
Ensure that the correct design of nozzle tip for the material is being used.

Surface Finish (Low Gloss):

Description

Surface Finish (Low Gloss). Gloss is the appearance of the surface of the molded part when light is reflected off of it. Molds that are textured or resins that are filled have an inherently reduced level of gloss when compared to highly polished mold surfaces.

Possible Solutions:

Clean mold surface.
If the part design allows increase the polish of the molding surface.
Increase cylinder temperature. This applies to molds that have a polished surface.
Increase injection pressure. This applies to molds that have a polished surface.
Increase injection speed. This applies to molds that have a polished surface.
Increase mold temperature. This applies to molds that have a polished surface.
Decrease cylinder temperature. This applies to molds that have a textured surface.
Decrease injection pressure. This applies to molds that have a textured surface.
Decrease injection speed. This applies to molds that have a textured surface.
Decrease mold temperature. This applies to molds that have a textured surface.
Increase melt temperature.
Make sure venting is adequate.

Surface Finish (Scars, Wrinkles):

Description
Surface Finish (Scars, Wrinkles). Is the appearance of the ripples or wrinkles on the surface of the molded part.

Possible Solutions:

Decrease back pressure.
Decrease nozzle temperature.
Increase booster time.
Increase the melt temperature.
Increase injection pressure.
Increase injection speed.
Increase overall cycle time.
Increase shot size.
Inspect mold for surface defects.

Undersized Part:

Description

Part is too small when compared to the drawing specifications.

Possible Solutions:

Decrease mold temperature.
Increase booster time.
Increase cylinder temperature.
Increase hold-time.
Increase holding pressure.
Increase injection pressure.
Increase injection speed.
Inspect mold for surface defects.

Valve Pin Does Not Close:

Description

Valve pin does not close properly. This will leave the gate protruding from the part. This may also occur if the valve pin is too hot, the material may stick to the valve pin.

Possible Solutions:

Valve pin is too short. Check and replace if necessary.
Valve pin fit. Ensure that the valve pin is lapped to the gate steel when appropriate.
Damaged gate. Check if valve pin is too long, rework if necessary. Also check to ensure that the valve pin is concentric with the gate, if not replace it.
Hydraulic / Pneumatic seals may be worn. Replace as necessary.
Insufficient pin/land area in the gate area of the mold. Increase the gate area cooling, or increase the valve pin land contact.
Insufficient hydraulic or air pressure. Increase the pressure up to but not beyond the maximum rating of the unit being used.
Excessive hold time. Decrease the hold time.

Voids:

Description

Voids are hollows created in the part. They are normally found in thick sectioned parts caused by material being pulled away from the hot center section towards cold mold walls leaving a void in the center.

Possible Solutions:

Clean vents.
Decrease injection speed.
Decrease melt temperature.
Dry material.
Increase injection pressure.
Increase injection-hold.
Increase mold temperature.
Increase shot length.
Increase size of gate.
Increase size of sprue and/or runners and/or gates.

Warping, Part Distortion:

Description

Warping, Part Distortion is shows up as parts being bowed, warped, bent or twisted beyond the normal specification outlined on the drawing.

Possible Solutions:

Adjust melt Temperature (increase to relieve molded-in stress, decrease to avoid overpacking). stress, decrease to avoid over packing). stress, decrease to avoid over packing).
Check gates for proper location and adequate size.
Check mold knockout mechanism for proper design and operation.
Equalize/balance mold temperature of both halves.
Increase injection-hold.
Increase mold cooling time.
Relocate gates on or as near as possible to thick sections.
Try increasing or decreasing injection pressure.

Weld Lines:

Description

Weld Lines are created when two or more melt flow fronts meet possibly causing a cosmetically visible line. It can also create a weakened area in the finished molded part especially with filled resins.

Possible Solutions:

Increase injection pressure.
Increase injection speed.
Increase injection hold.
Increase melt temperature.
Increase mold temperature.
Make sure part contains no sharp variation in cross-sections.
Vent cavity in the weld area.

LATHES AND LATHE MACHINING OPERATIONS

2012-11-10T00:25:00.000-08:00

The engine lathe, its use, and its principal parts and their uses are knowledges and skills expected of an EN2. Although machine shop work is generally done by personnel in the Machinery Repairman (MR) rating, there may be times that you will find the lathe essential to complete a repair job. This chapter will help you to identify the engine lathe’s attachments, accessories, and their uses. Also, it will identify and explain different machining operations and the factors related to machining operations. Of course, you will be expected to know and to follow the safety precautions associated with machining operations.

There are a number of different types of lathes installed in the machine shops in various Navy ships. These include the engine lathe, the horizontal turret lathe, and several variations of the basic engine lathe, such as bench, toolroom, and gap lathes. All lathes, except the vertical turret type, have one thing in common. For all usual machining operations, the workpiece is held and rotated about a horizontal axis, while being formed to size and shape by a cutting tool. In the vertical turret lathe, the workpiece is rotated about a vertical axis. Of the various types of lathes, the type you are most likely to use is the engine lathe. Therefore, this chapter deals only with engine lathes and the machining operations you may have to perform.

NOTE: Before you attempt to operate any lathe, make sure you know how to operate it. Read all operating instructions supplied with the machine. Learn the locations of the various controls and how to operate them.

ENGINE LATHE

An engine lathe is found in every machine shop. It is used mostly for turning, boring, facing, and thread cutting. But it may also be used for drilling, reaming, knurling, grinding, spinning, and spring winding. Since you will primarily be concerned with turning, boring, facing, and thread cutting, we will deal primarily with those operations in this chapter.

The work held in the engine lathe can be revolved at any one of a number of different speeds, and the cutting tool can be accurately controlled by hand or power for longitudinal feed and crossfeed. (Longitudinal feed is the movement of the cutting tool parallel to the axis of the lathe; crossfeed is the movement of the cutting tool perpendicular to the axis of the lathe.)

Lathe size is determined by two measurements:
(1) the diameter of work it will swing (turn) over the ways and
(2) the length of the bed.

For example, a 14-inch by 6-foot lathe will swing work up to 14 inches in diameter and has a bed that is 6 feet long.

Engine lathes vary in size from small bench lathes that have a swing of 9 inches to very large lathes for turning large diameter work such as low-pressure turbine rotors. The 16-inch lathe is the average size for general purposes and is the size usually installed in ships that have only one lathe.

PRINCIPAL PARTS

To learn the operation of the lathe, you must be familiar with the names and functions of the principal parts. Lathes from different manufacturers differ somewhat in construction, but all are built to perform the same general functions. As you read the description of each part, find its location on the lathe in figure 9-1 and the figures that follow. (For specific details of features of construction and operating techniques, refer to the manufacturer’s technical manual for your machine.)

Bed and Ways

The bed is the base or foundation of the parts of the lathe. The main feature of the bed is the ways, which are formed on the bed’s upper surface and run the full length of the bed. The ways keep the tailstock and the carriage, which slide on them, in alignment with the headstock.

Headstock

The headstock contains the headstock spindle and the mechanism for driving it. In the belt-driven type, shown in figure 9-2, the driving mechanism consists of a motor-driven cone pulley that drives the spindle cone pulley through a drive belt. The spindle can be rotated either directly or through back gears.

When the headstock is set up for direct drive, a bull-gear pin, located under a cover to the right of the spindle pulley, connects the pulley to the spindle. This connection causes the spindle to turn at the same speed as the spindle pulley.

When the headstock is set up for gear drive, the bull-gear pin is pulled out, disconnecting the spindle pulley from the spindle. This allows the spindle to turn freely inside the spindle pulley. The back-gear lever, on the left end of the headstock, is moved to engage the back-gear set with a gear on the end of the spindle and a gear on the end of the spindle pulley. In this drive mode, the drive belt turns the spindle pulley, which turns the back-gear set, which turns the spindle. Each drive mode provides four spindle speeds, for a total of eight. The back-gear drive speeds are less slower than the direct-drive speeds.

Tailstock

The primary purpose of the tailstock is to hold the dead center to support one end of the work being machined. However, the tailstock can also be used to hold tapered shank drills, reamers, and drill chucks. It can be moved on the ways along the length of the bed and can be clamped in the desired position by tightening the tailstock clamping nut. This movement allows for the turning of different lengths of work. The tailstock can be adjusted laterally (front to back) to cut a taper by loosening the clamping screws at the bottom of the tailstock.

Before you insert a dead center, drill, or reamer, carefully clean the tapered shank and wipe out the tapered hole of the tailstock spindle. When you hold drills or reamers in the tapered hole of the spindle, be sure they are tight enough so they will not revolve. If you allow them to revolve, they will score the tapered hole and destroy its accuracy.

Carriage

The carriage is the movable support for the crossfeed slide and the compound rest. The compound rest carries the cutting tool in the tool post. Figure 9-3 shows how the carriage travels along the bed over which it slides on the outboard ways.

The carriage has T-slots or tapped holes to use for clamping work for boring or milling. When the carriage is used for boring and milling operations, carriage movement feeds the work to the cutting tool, which is rotated by the headstock spindle.

You can lock the carriage in any position on the bed by tightening the carriage clamp screw. But you do this only when you do such work as facing or parting-off, for which longitudinal feed is not required. Normally the carriage clamp is kept in the released position. Always move the carriage by hand to be sure it is free before you engage its automatic feed.

Apron

The apron is attached to the front of the carriage and contains the mechanism that controls the movement of the carriage and the crossslide.

Feed Rod

The feed rod transmits power to the apron to drive the longitudinal feed and crossfeed mechanisms. The feed rod is driven by the spindle through a train of gears. The ratio of feed rod speed to spindle speed can be varied by using change gears to produce various rates of feed.

The rotating feed rod drives gears in the apron; these gears in turn drive the longitudinal feed and crossfeed mechanisms through friction clutches.

Some lathes do not have a separate feed rod, but use a spline in the lead screw for the same purpose.

Lead Screw

The lead screw is used for thread cutting. It has accurately cut Acme threads along its length that engage the threads of half-nuts in the apron when the half-nuts are clamped over it. The lead screw is driven by the spindle through a gear train. Therefore, the rotation of the lead screw bears a direct relation to the rotation of the spindle. When the half-nuts are engaged, the longitudinal movement of the carriage is controlled directly by the spindle rotation. Consequently, the cutting tool is moved a definite distance along the work for each revolution that the spindle makes.

Crossfeed Slide

The crossfeed slide is mounted to the top of the carriage in a dovetail and moves on the carriage at a right angle to the axis of the lathe. A crossfeed screw allows the slide to be moved toward or away from the work in accurate increments.

Compound Rest

The compound rest mounted on the compound slide, provides a rigid adjustable mounting for the cutting tool. The compound rest assembly has the following principal parts:

1. The compound rest SWIVEL, which can be swung around to any desired angle and clamped in position. It is graduated over an arc of 90° on each side of its center position for easier setting to the angle selected. This feature is used for machining short, steep tapers, such as the angle on bevel gears, valve disks, and lathe centers.

2. The compound rest, or TOP SLIDE, which is mounted on the swivel section on a dovetailed slide. It is moved by the compound rest feed screw.

This arrangement permits feeding the tool to the work at any angle (determined by the angular setting of the swivel section). The graduated collars on the crossfeed and compound rest feed screws read in thousandths of an inch for fine adjustment in regulating the depth of cut.

Accessories and Attachments

Accessories are the tools and equipment used in routine lathe machining operations. Attachments are special fixtures that may be mounted on the lathe to expand the use of the lathe to include taper cutting, milling, and grinding. Some of the common accessories and attachments are described in the following paragraphs.

TOOL POST:

The sole purpose of the tool post is to provide a rigid support for the tool. It is mounted in the T-slot of the compound rest. A forged tool or a toolholder is inserted in the slot in the tool post. By tightening a setscrew, you will firmly clamp the whole unit in place with the tool in the desired position.

TOOLHOLDERS:

Notice the angles at which the tool bits are set in the various holders. These angles must be considered with respect to the angles ground on the tools and the angle that the toolholder is set with respect to the axis of the work.

Two types of toolholders that differ slightly from the common toolholders are those used for threading and knurling.

The threading toolholder has a formed cutter which needs to be ground only on the top surface for sharpening. Since the thread form is accurately shaped over a large arc of the tool, as the surface is worn away by grinding, the cutter can be rotated to the correct position and secured by the setscrew.

A knurling toolholder carries two knurled rollers which impress their patterns on the work as it revolves. The purpose of the knurling tool is to provide a roughened surface on round metal parts, such as knobs, to give a better grip in handling. The knurled rollers come in a variety of patterns.

ENGINE LATHE TOOLS:

Left-Hand Turning Tool:

This tool is ground for machining work when it is fed from left to right. The cutting edge is on the right side of the tool, and the top of the tool slopes down away from the cutting edge.

Round-Nosed Turning Tool:

This tool is for general-purpose machine work and is used for taking light roughing cuts and finishing cuts. Usually, the top of the cutter bit is ground with side rake so the tool may be fed from right to left. Sometimes this cutter bit is ground flat on top so the tool may be fed in either direction.

Right-Hand Turning Tool:

This is just the opposite of the left-hand turning tool and is designed to cut when it is fed from right to left. The cutting edge is on the left side. This is an ideal tool for taking roughing cuts and for all-around machine work.

Left-Hand Facing Tool:

This tool is intended for facing on the left-hand side of the work. The direction of feed is away from the lathe center. The cutting edge is on the right-hand side of the tool, and the point of the tool is sharp to permit machining a square corner.

Threading Tool:

The point of the threading tool is ground to a 60-degree included angle for machining V-form screw threads. Usually, the top of the tool is ground flat, and there is clearance on both sides of the tool so it will cut on both sides.

Right-Hand Facing Tool:

This tool is just the opposite of the left-hand facing tool and is intended for facing the right end of the work and for machining the right side of a shoulder.

Square-Nosed Parting (Cutoff) Tool:

The principal cutting edge of this tool is on the front. Both sides of the tool must have sufficient clearance to prevent binding and should be ground slightly narrower at the back than at the cutting edge. This tool is convenient for machining necks and grooves and for squaring comers and cutting off.

Boring Tool:

The boring tool is usually ground the same shape as the left-hand turning tool so that the cutting edge is on the right side of the cutter bit and may be fed in toward the headstock.

Inside-Threading Tool:

The inside-threading tool has the same shape as the threading tool but it is usually much smaller. Boring and inside-threading tools may require larger relief angles when used in small diameter holes.

LATHE CHUCKS:

The lathe chuck is a device for holding lathe work. It is mounted on the nose of the spindle. The work is held by jaws which can be moved in radial slots toward the center of the chuck to clamp down on the sides of the work. These jaws are moved in and out by screws turned by a special chuck wrench.

The four-jaw independent lathe chuck is the most practical chuck for general work The four jaws are adjusted one at a time, making it possible to hold work of various shapes and to adjust the center of the work to coincide with the axis of the spindle. The jaws are reversible.

The three-jaw universal or scroll chuck can be used only for holding round or hexagonal work All three jaws move in and out together in one operation and bring the work on center automatically. This chuck is easier to operate than the four-jaw type, but, when its parts become worn, its accuracy in centering cannot be relied upon. Proper lubrication and constant care are necessary to ensure reliability.

The draw-in collet chuck is used to hold small work for machining in the lathe. It is the most accurate type of chuck made and is intended for precision work. The collet, which holds the work, is a split-cylinder with an outside taper that fits into the tapered closing sleeve and screws into the threaded end of the hollow drawbar. As the handwheel is turned clockwise, the drawbar is moved toward the handwheel. This tightening up on the drawbar pulls the collet back into the tapered sleeve, thereby closing it firmly over the work and centering the work accurately and quickly. The size of the hole in the collet determines the diameter of the work the chuck can handle.

Faceplates:

The faceplate is used for holding work that, because of its shape and dimensions, cannot be swung between centers or in a chuck. The T-slots and other openings on its surface provide convenient anchors for bolts and clamps used in securing the work to it. The faceplate is mounted on the nose of the spindle.

The driving plate is similar to a small faceplate and is used mainly for driving work that is held between centers. The primary difference between a faceplate and a driving plate is that a faceplate has a machined face for precision mounting, while the face of a driving plate is left rough. When a driving plate is used, the bent tail of a dog clamped to the work is inserted into a slot in the faceplate. This transmits rotary motion to the work.

Lathe Centers:

The 60-degree lathe centers provide a way to hold the work so it can be turned accurately on its axis. The headstock spindle center is called the LIVE CENTER because it revolves with the work. The tailstock center is called the DEAD CENTER because it does not turn. Both live and dead centers have shanks turned to a Morse taper to fit the tapered holes in the spindles; both have points finished to an angle of 60°. They differ only in that the dead center is hardened and tempered to resist the wearing effect of the work revolving on it. The live center revolves with the work and is usually left soft. The dead center and live center must NEVER be interchanged. (There is a groove around the hardened dead center to distinguish it from the live center.)

The centers fit snugly in the tapered holes of the headstock and tailstock spindles. If chips, dirt, or burrs prevent a perfect fit in the spindles, the centers will not run true.

To remove the headstock center, insert a brass rod through the spindle hole and tap the center to jar it loose; then pull it out with your hand. To remove the tailstock center, run the spindle back as far as it will go by turning the handwheel to the left. When the end of the tailstock screw bumps the back of the center, it will force the center out of the tapered hole.

Lathe Dogs:

Lathe dogs are used with a driving plate or faceplate to drive work being machined on centers; the frictional contact alone between the live center and the work is not sufficient to drive the work

The common lathe dog is used for round work or work having a regular section (square, hexagon, octagon). The piece to be turned is held firmly in the hole (A) by the setscrew (B). The bent tail (C) projects through a slot or hole in the driving plate or faceplate so that when the tail revolves with the spindle it turns the work with it. The clamp dogmay be used for rectangular or irregularly shaped work. Such work is clamped between the jaws,

Center Rest:

The center rest, also called the steady rest, is used for the following purposes:

1. To provide an intermediate support for long slender bars or shafts being machined between centers. The center rest prevents them from springing, or sagging, as a result of their otherwise unsupported weight.

2. To support and provide a center bearing for one end of the work, such as a shaft, being bored or drilled from the end when it is too long to be supported by a chuck alone. The center rest is clamped in the desired position on the bed and is kept aligned by the ways. The jaws (A) must be carefully adjusted to allow the work (B) to turn freely and at the same time remain accurately centered on the axis of the lathe. The top half of the frame is a hinged section (C) for easier positioning without having to remove the work from the centers or to change the position of the jaws.

Follower Rest:

The follower rest is used to back up small diameter work to keep it from springing under the cutting pressure. It can be set to either precede or follow the cutting action. It is attached directly to the saddle by bolts (B). The adjustable jaws bear directly on the part of the work opposite the cutting tool.

Taper Attachment:

The taper attachment is used for turning and boring tapers. It is bolted to the back of the carriage. In operation, it is connected to the cross slide so that it moves the cross slide traversely as the carriage moves longitudinally, thereby causing the cutting tool to move at an angle to the axis of the work to produce a taper.

The desired angle of taper is set on the guide bar of the attachment. The guide bar support is clamped to the lathe bed Since the cross slide is connected to a shoe that slides on this guide bar, the tool follows along a line parallel to the guide bar and at an angle to the work axis corresponding to the desired taper.

The operation of the taper attachment will be further explained under the subject of taper work

Thread Dial Indicator:

The thread dial indicator, shown in figure 9-16, eliminates the need to reverse the lathe to return the carriage to the starting point each time a successive threading cut is taken. The dial, which is geared to the lead screw, indicates when to clamp the half-nuts on the lead screw for the next cut.

The threading dial consists of a worm wheel which is attached to the lower end of a shaft and meshed with the lead screw. On the upper end of the shaft is the dial. As the lead screw revolves, the dial is turned and the graduations on the dial indicate points at which the half-nuts may be engaged.

Carriage Stop:

The carriage stop can be attached to the bed at any point where the carriage should stop. It is used primarily for turning, facing, or boring duplicate parts, as it eliminates taking repeated measurements of the same dimension. In operation, the stop is set at the point where the feed should stop. To use the stop, just before the carriage reaches the stopping point, shut off the automatic feed and manually run the carriage up against the stop. Carriage stops are provided with or without micrometer adjustment. Figure 9-17 shows a micrometer carriage stop. Clamp it on the ways in the approximate position required, and then adjust it to the exact setting by using the micrometer adjustment. (Do not confuse this stop with the automatic carriage stop that automatically stops the carriage by disengaging the feed or stopping the lathe.)

MAINTENANCE:

Every lathe must be maintained strictly according to requirements of the Maintenance and Material Management (3-M) Systems. The first requirement of maintenance to your lathe is proper lubrication. Make it a point to oil your lathe daily where oil holes are provided. Oil the ways daily-not only for lubrication but to protect their scraped surfaces. Oil the lead screw often while it is in use; this is necessary to preserve its accuracy, for a worn lead screw lacks precision in thread cutting. Make sure the headstock is filled to the proper oil level; drain the oil out and replace it when it becomes dirty or gummy. If your lathe is equipped with an automatic oiling system for some parts, make sure all those parts are getting oil. Make it a habit to CHECK frequently to see that all moving parts are being lubricated.

Before engaging the longitudinal ‘feed, be certain that the carriage clamp screw is loose and that the carriage can be moved by hand. Avoid running the carriage against the headstock or tailstock while it is under the power feed; running the carriage against the headstock or tailstock puts an unnecessary strain on the lathe and may jam the gears.

Do not neglect the motor just because it may be out of sight; check its lubrication. If it does not run properly, notify the Electrician’s Mate who is responsible for caring for it. He or she will cooperate with you to keep it in good condition. On lathes with a belt driven from the motor, avoid getting oil or grease on the belt when you oil the lathe or motor.

Keep your lathe clean. A clean and orderly machine is an indication of a good mechanic. Dirt and chips on the ways, on the lead screw, and on the crossfeed screws will cause serious wear and impair the accuracy of the machine.

NEVER put wrenches, files, or other tools on the ways. If you must keep tools on the bed, use a board to protect the finished surfaces of the ways.

NEVER use the bed or carriage as an anvil. Remember, the lathe is a precision machine, and nothing must be allowed to destroy its accuracy.

BASIC SETUP

A knowledge of the basic setup is required if you are to become proficient in performing machine work with a lathe. Some of these setups are considered in the following sections.

Cutting Speeds and Feeds:

Cutting speed is the rate at which the surface of the work passes the point of the cutting tool. It is expressed in feet per minute (fpm).

Feed is the amount the tool advances for each revolution of the work. It is usually expressed in thousandths of an inch per revolution of the spindle. Cutting speeds and tool feeds are determined by various considerations: the hardness and toughness of the metal being cut; the quality, shape, and sharpness of the cutting tool; the depth of the cut; the tendency of the work to spring away from the tool; and the strength and power of the lathe. Since conditions vary, it is good practice to find out what the tool and work will stand and then select the most practical and efficient speed and feed for the finish desired.

When ROUGHING parts down to size, use the greatest depth of cut and feed per revolution that the work, the machine, and the tool will stand at the highest practical speed. On many pieces where tool failure is the limiting factor in the size of the roughing cut, you may be able to reduce the speed slightly and increase the feed to remove more metal. This will prolong tool life. Consider an example where the depth of cut is 1/4 inch, the feed 0.020 inch per revolution, and the speed 80 fpm. If the tool will not permit additional feed at this speed, you can drop the speed to 60 fpm and increase the feed to about 0.040 inch per revolution without having tool trouble. The speed is therefore reduced 25 percent, but the feed is increased 100 percent. Thus the actual time required to complete the work is less with the second setup.

For the FINISH TURNING OPERATION, take a very light cut, since you removed most of the stock during the roughing cut. Use a fine feed to run at a high surface speed. Try a 50 percent increase in speed over the roughing speed. In some cases, the finishing speed may be twice the roughing speed. In any event, run the work as fast as the tool will withstand to obtain the maximum speed during this operation. Be sure to use a sharp tool when you are finish turning.

COOLANTS:

A cutting lubricant serves two main purposes: (1) It cools the tool by absorbing a portion of the heat and reducing the friction between the tool and the metal being cut. (2) It also keeps the cutting edge of the tool flushed clean.

The best lubricants to use for cutting metal must often be determined by experiment. Water-soluble oil is acceptable for most common metals. Special cutting compounds containing such ingredients as tallow, graphite, and lard, marketed under various names, are also used. But these are expensive and used mainly in manufacturing where high cutting speeds are the rule.

Some common materials and their cutting lubricants are as follows:

A lubricant is more important for threading than for straight turning. Mineral lard oil is recommended for threading the majority of metals that are used by the Navy.

CHATTER:

Chatter is vibration in either the tool or the work The finished work surface appears to have a grooved or lined finish instead of a smooth surface. The vibration is set up by a weakness in the work, work support, tool, or tool support and is probably the most elusive thing you will find in the entire field of machine work As a general rule, strengthening the various parts of the tool support train will help. It is also advisable to support the work by a center rest or follower rest.

The fault may be in the machine adjustments. Gibs may be too loose; hearings may, after a long period of heavy service, be worn; the tool may be sharpened improperly, and so on. If the machine is in excellent condition, the fault may be in the tool or tool setup. Grind the tool with a point or as near a point as the finish specified will permit; avoid a wide, round leading edge on the tool. Reduce the overhang of the tool as much as possible. Be sure all the gib and bearing adjustments are properly made. See that the work receives proper support for the cut and, above all, do not try to turn at a surface speed that is too high. Excessive speed is probably the greatest cause of chatter. The first thing you should do when chatter occurs is reduce the speed.

Direction of Feed:

Regardless of how the work is held in the lathe, the tool should feed toward the headstock. This causes most of the pressure of the cut to bear on the work-holding device and the spindle thrust bearings. When you must feed the cutting tool toward the tailstock, take lighter cuts at reduced feeds. In facing, the general practice is to feed the tool from the center of the workpiece outward.

PRELIMINARY PROCEDURES:

Before starting a lathe machining operation, always ensure that the machine is set up properly. If the work is mounted between centers, check the alignment of the dead center and the live center and make any necessary changes. Ensure that the toolholder and cutting tool are set at the proper height and angle. Check the work-holding accessory to ensure that the workpiece is held securely. Use the center rest or follower rest to support long workpieces.

PREPARING THE CENTERS:

The first step in preparing the centers is to see that they are accurately mounted in the headstock and tailstock spindles. The centers and the tapered holes in which they are fitted must be perfectly clean. Chips and dirt left on the contact surfaces prevent the bearing surfaces from fitting perfectly. This will decrease the accuracy of your work. Make sure that there are no burrs in the spindle hole. If you find burrs, remove them by carefully scraping and reaming the hole with a Morse taper reamer. Burrs will produce the same inaccuracies as chips or dirt.

A center’s point must be finished accurately to an angle of 60°. Figure 9-18 shows the method of checking this angle with a center gauge. The large notch of the center gauge is intended for this purpose. If this test shows that the point is not perfect, you must true it in the lathe by taking a cut over the point with the compound rest set at 30°. You must anneal the hardened tail center before it can be machined in this manner, or you can grind it if a grinding attachment is available.

CHECKING ALIGNMENT:

To turn a shaft straight and true between centers, be sure the centers are aligned in a plane parallel to the ways of the lathe. You can check the approximate alignment of the centers by moving the tailstockup until the centers almost touch and observing their relative positions.

To test center alignment for very accurate work, take a light cut over at each end with a micrometer and, if readings are found to differ, adjust the tailstock accordingly. Repeat the procedure until alignment is obtained.

SETTING THE TOOLHOLDER AND THE CUTTING TOOL:

The first requirement for setting the tool is to have it rigidly mounted on the tool post holder. Be sure the tool sets squarely in the tool post and that the setscrew is tight. Reduce overhang as much as possible to prevent the tool bit from springing during cutting. If the tool has too much spring, the point of the tool will catch in the work, causing chatter and damaging both the tool and the work

The point of the tool must be correctly positioned on the work Place the cutting edge. slightly above the center for straight turning of steel and cast iron and exactly on the center for all other work To set the tool at the height desired, raise or lower the point of the tool by moving the wedge in or out of the tool post ring. By placing the point opposite the tailstock center point, you can adjust the setting accurately.

HOLDING THE WORK:

You cannot perform accurate work if the workpiece is improperly mounted. The requirements for proper mounting are as follows:

1. The work center line must be accurately centered along the axis of the lathe spindle.

2. The work must be held rigidly while being turned.

3. The work must NOT be sprung out of shape by the holding device.

4. The work must be adequately supported against any sagging caused by its own weight and against springing caused by the action of the cutting tool.

There are four general methods of holding work in the lathe: (1) between centers, (2) on a mandrel, (3) in a chuck, and (4) on a faceplate. Work may also be clamped to the carriage for boring and milling, in which case the boring bar or milling cutter is held and driven by the headstock spindle.

Other methods of holding work to suit special conditions are (1) one end on the live center or in a chuck and the other end supported in a center rest, and (2) one end in a chuck and the other end on the dead center.

Holding Work Between Centers:

To machine a workpiece between centers, drill center holes in each end to receive the lathe centers. Secure a lathe dog to the workpiece. Then mount the work between the live and dead centers of the lathe.

CENTERING THE WORK:

To center round stock where the ends are to be turned and must be concentric with the unturned body, mount the work on the head spindle in a universal chuck or a draw-in collet chuck If the work is long and too large to pass through the spindle, use a center rest to support one end. Mount a center drill in a drill chuck in the tailstock spindle and feed it to the work by turning the tailstock handwheel.

For center drilling a workpiece, the combined drill and countersink is the most practical tool. These combined drills and countersinks vary in size and the drill points also vary. Sometimes a drill point on one end will be 1/8 inch in diameter, and the drill point on the opposite end will be 3/16 inch in diameter. The angle of the center drill must always be 60° so that the countersunk hole will fit the angle of the lathe center point. If a center drill is not available, center the work with a small twist drill. Let the drill enter the work a sufficient distance on each end; then follow with a 60° countersink.

In center drilling, use a drop or two of oil on the drill. Feed the drill slowly and carefully to prevent breaking the tip. Take extreme care when the work is heavy, because you will be less able to "feel" the proper feed of the work on the center drill.

If the center drill breaks during countersinking and part of the broken drill remains in the work, you must remove this part. Sometimes you can drive the broken piece out by a chisel or by jarring it loose, but it may stick so hard that you cannot remove it this way. Then you must anneal the broken part of the drill and drill it out.

We cannot overemphasize the importance of proper center holes in the work and a correct angle on the point of the lathe centers. To do an accurate job between centers on the lathe, you must ensure that the center-drilled holes are the proper size and depth and that the points of the lathe centers are true and accurate.

Holding Work on a Mandrel:

Many parts, such as bushings, gears, collars, and pulleys, require all the finished external surfaces to run true with their center hole, or bore.

General practice is to finish the bore to a standard size within the limit of the accuracy desired. Thus a 3/4-inch standard bore would have a finished diameter of from 0.7495 to 0.7505 inch This variation is due to a tolerance of 0.0005 inch below and above the true standard of exactly 0.750 inch. First drill the hole to within a few thousandths of an inch of the finished size; then remove the remainder of the material with a machine reamer, following with a hand reamer if the limits are extremely close.

Then press the piece on a mandrel tightly enough so the work will not slip while being machined Clamp a dog on the mandrel, which is mounted between centers. Since the mandrel surface runs true with respect to the lathe axis, the turned surfaces of the work on the mandrel will be true with respect to the bore of the piece. A mandrel is simply a round piece of steel of convenient length which has been center drilled and ground true with the center holes. Commercial mandrels are made of tool steel, hardened and ground with a slight taper (usually 0.0005 inch per inch). This taper allows the standard hole in the work to vary according to the usual shop practice and still provides a drive to the work when the mandrel is pressed into the hole. The taper is not great enough to distort the hole in the work The center-drilled centers of the mandrel are lapped for accuracy. The ends are turned smaller than the body of the mandrel and provided with flats, which give a driving surface for the lathe dog.

Holding Work in Chucks:

The independent chuck and universal chuck are used more often than other work-holding devices in lathe operations. The universal chuck is used for holding relatively true cylindrical work when the time required to do the job is more important than the concentricity of the machined surface and the holding power of the chuck When the work is irregular in shape, must be accurately centered, or must be held securely for heavy feeds and depth of cuts, an independent chuck is used. FOUR- JAW INDEPENDENT CHUCK.-Figure 9-23 shows a rough cylindrical casting mounted in a four-jaw independent lathe chuck on the spindle of the lathe. Before truing the work, determine which part you wish to have turned true. To mount this casting in the chuck, proceed as follows:

1. Adjust the chuck jaws to receive the casting. The same point on each jaw should touch the same ring on the face of the chuck If there are no rings, put each jaw the same distance from the outside edge of the body of the chuck.

2. Fasten the work in the chuck by turning the adjusting screw on jaw 1 and then on jaw 3, a pair of jaws which are opposite each other. Next, tighten jaws 2 and 4.

3. At this stage the work should be held in the jaws just tightly enough so it will not fall out of the chuck while you turn it.

4. Revolve the spindle slowly by hand and, with a piece of chalk, mark the high spot (A in fig. 9-23) on the work while it is revolving. Steady your hand on the tool post while holding the chalk.

5. Stop the spindle. Locate the high spot on the work and move the high spot toward the center of the chuck by releasing the jaw opposite the chalk mark and tightening the one nearest the mark

6. Sometimes the high spot on the work will be located between adjacent jaws. In that case, loosen the two opposite jaws and tighten the jaws adjacent to the high spot.

THREE-JAW UNIVERSAL CHUCK:

The three-jaw universal or scroll chuck is made so that all jaws move at the same time. A universal chuck will center almost exactly at the first clamping, but after a long period of use may develop inaccuracies of up to 0.010 inch in centering the work. You can usually correct the inaccuracy by inserting a piece of paper or thin shim stock between the jaw and the work on the high side.

When you chuck thin sections, be careful not to clamp the work too tightly because the work will distort. If you machine distorted work, the finished work will have as many high spots as there are jaws, and the turned surface will not be true.

Care of Chucks:

To preserve the accuracy of a chuck, handle it carefully and keep it clean and free from grit.

NEVER force a chuck jaw by using a pipe as an extension on the chuck wrench.

Before mounting a chuck, remove the live center and fill the hole with a rag to prevent chips and dirt from getting into the tapered hole of the spindle. Clean and oil the threads of the chuck and the spindle nose. Dirt or chips on the threads will not allow the chuck to run true when it is screwed up to the shoulder. Screw the chuck on carefully, tightening it just enough to make it difficult to remove. Never use mechanical power to install a chuck.

To remove a chuck, place a spanner wrench on the collar of the chuck and strike a smart blow on the handle of the wrench with your hand. When you mount or remove a heavy chuck, lay a board across the bed ways to protect them; the board will support the chuck as you put it on or take it off.

The comments on mounting and removing chucks also apply to faceplates.

Holding Work on a Faceplate:

A faceplate is used for mounting work that cannot be chucked or turned between centers because of its size or shape.

Work is secured to the faceplate by bolts, clamps, or any suitable clamping means. The holes and slots in the faceplate are used for anchoring the holding bolts. Angle plates may be used to position the work at the desired angle. Note the counterweight added for balance.

For work to be mounted accurately on a faceplate, the surface of the work in contact with the faceplate must be accurately faced. It is good practice to place a piece of paper between the work and the faceplate to prevent slipping.

Before you clamp the work securely, move it about on the surface of the faceplate until the point to be machined is centered accurately with the axis of the lathe. Suppose you wish to bore a hole, the center of which has been laid out and marked with a prick punch. First, clamp the work to the approximate position on the faceplate. Slide the tailstock up until the dead center just touches the work. (NOTE: The dead center should have a sharp, true point.) Now revolve the work slowly; if the work is off center, the point will scribe a circle on the work. If the work is on center, the point of the dead center will coincide with the prick punch mark.

Using the Center Rest and Follower Rest:

Place the center rest on the ways where it will give the greatest support to the workpiece. This is usually at about the middle of its length.

Ensure that the jaws of the center rest are adjusted to support the work while allowing it to turn freely.

The follower rest differs from the center rest in that it moves with the carriage and provides support against the forces of the cut only. Set the tool to the diameter selected, and turn a "spot" about 5/8 to 3/4 inch wide. Then adjust the follower rest jaws to the finished diameter to follow the tool along the entire length to be turned.

Use a thick oil on the center rest and follower rest to prevent "seizing" and scoring of the workpiece. Check the jaws frequently to see that they do not become hot. The jaws may expand slightly if they get hot, pushing the work out of alignment (when using the follower rest) or binding (when using the center rest).

Holding Work in a Draw-In Collet Chuck:

The draw-in collet chuck is used for very fine, accurate work of small diameter. Long work can be passed through the hollow drawbar. Short work can be placed directly into the collet from the front. The collet is tightened on the work by rotating the drawbar to the right; this draws the collet into the tapered closing sleeve. The opposite operation releases the collet. Accurate results are obtained when the diameter of the work is exactly the same size as the dimension stamped on the collet. In some cases, the diameter may vary as much as 0.002 inch; that is, the work may be 0.001 inch smaller or larger than the collet size. If the work diameter varies more than this, it will impair the accuracy and efficiency of the collet. That is why a separate collet should be used for each small variation or work diameter, especially if precision is desired.

MACHINING OPERATIONS:

Up to this point, you have studied the preliminary steps leading to the performance of machine work in the lathe. You have learned how to mount the work and the tool and which tools are used for various purposes. Now, you need to consider how to use the proper tools in combination with the lathe to perform various machining operations.

FACING:

Facing is the machining of the end surfaces and shoulders of a workpiece. In addition to squaring the ends of the work, facing provides a way to cut work to length accurately. Generally, only light cuts are required since the work will have been cut to approximate length or rough machined to the shoulder.

Figure 9-26 shows the facing of a cylindrical piece. The work is placed between centers and driven by a dog. A right-hand side tool is used as shown. Take a light cut on the end of the work, feeding the tool (by hand crossfeed) from the center toward the outside. Take one or two light cuts to remove enough stock to true the work Then reverse the workpiece, install the dog on the just finished end, and face the other end to make the work the proper length. To provide an accurate base from which to measure, hold another rule or straightedge on the end you faced first. Be sure there is no burr on the edge to keep the straightedge from bearing accurately on the finished end. Use a sharp scribe to mark off the dimension desired. Figure 9-27 shows the use of a turning tool in finishing a shouldered job having a fillet corner. Take a finish cut on the small diameter. Machine the fillet with a light cut. Then use the tool to face the work from the fillet to the outside of the work.

In facing large surfaces, lock the carriage in position, since only crossfeed is required to traverse the tool across the work. With the compound rest set at 90° (parallel to the axis of the lathe), you can use the micrometer collar to feed the tool to the proper depth of cut.

TURNING:

Turning is the machining of excess stock from the periphery of the workpiece to reduce the diameter. In most lathe machining operations requiring removal of large amounts of stock, a series of roughing cuts is taken to remove most of the excess stock Then a finishing cut is taken to accurately "size" the workpiece.

Rough Turning

When a great deal of stock is to be removed, you should take heavy cuts to complete the job in the least possible time. This is called rough turning. Select the proper tool for taking a heavy chip. The speed of the work and the amount of feed of the tool should be as great as the tool will stand.

When you take a roughing cut on steel, cast iron, or any other metal that has a scale on its surface, be sure to set the tool deep enough to get under the scale in the first cut. Unless you do, the scale on the metal will dull or break the point of the tool.

Rough machine the work to almost the finished size; then take careful measurements.

Bear in mind that the diameter of the work being turned is reduced by an amount equal to twice the depth of the cuts; thus, if you desire to reduce the diameter of a piece by 1/4 inch, you must remove 1/8 inch of metal from the surface.

Figure 9-28 shows the position of the tool for taking a heavy cut on large work. Set the tool so that if anything occurs during machining to change the position of the tool, it will not dig into the work, but rather will move in the direction of the arrow-away from the work

Finish Turning:

When you have rough turned the work to within about 1/32 inch of the finished size, take a finishing cut. A fine feed, the proper lubricant, and, above all, a keen-edged tool are necessary to produce a smooth finish. Measure carefully to be sure you are machining the work to the proper dimension. Stop the lathe when you take measurements.

If you must finish the work to close tolerances, be sure the work is not hot when you take the finish cut. If you turn the workpiece to exact size when it is hot, it will be undersize when it has cooled.

Perhaps the most difficult operation for a beginner in machine work is to make accurate measurements. So much depends on the accuracy of the work that you should make every effort to become proficient in the use of measuring instruments. You will develop a certain "feel" in the application of micrometers through experience alone; do not be discouraged if your first efforts do not produce perfect results. Practice taking micrometer measurements on pieces of known dimensions. You will acquire skill if you are persistent.

Turning to a Shoulder:

Machining to a shoulder is often done by locating the shoulder with a parting tool. Insert the parting tool about 1/32 inch from the shoulder line toward the small diameter end of the work Cut to a depth 1/32 inch larger than the small diameter of the work. Then machine the stock by taking heavy chips up to the shoulder. This procedure eliminates detailed measuring and speeds up production.

Figure 9-29 illustrates this method of shouldering. A parting tool has been used at P and the turning tool is taking a chip. It will be unnecessary to waste any time in taking measurements. You can devote your time to rough machining until the necessary stock is removed. Then you can take a finishing cut to accurate measurement.

Boring:

Boring is the machining of holes or any interior cylindrical surface. The piece to be bored must have a drilled or cored hole, and the hole must be large enough to insert the tool. The boring process merely enlarges the hole to the desired size or shape. The advantage of boring is that a true round hole is obtained, and two or more holes of the same or different diameters may be bored at one setting, thus ensuring absolute alignment of the axis of the holes.

Work to be bored may be held in a chuck, bolted to the faceplate, or bolted to the carriage. Long pieces must be supported at the free end in a center rest. When the boring tool is fed into the hole of work being rotated on a chuck or faceplate, the process is called single point boring. It is the same as turning except that the cutting chip is taken from the inside. The cutting edge of the boring tool resembles that of a turning tool. Boring tools may be the solid forged type or the inserted cutter bit type.

When the work to be bored is clamped to the top of the carriage, a boring bar is held between centers and driven by a dog. The work is fed to the tool by the automatic longitudinal feed of the carriage. Three types of boring bars are shown in figure 9-30. Note the center holes at the ends to fit the lathe centers.

TAPERS

Although you will probably have little need to machine tapers, we have provided the following explanation for your basic knowledge.

A taper is the gradual decrease in the diameter of a piece of work toward one end. The amount of taper in any given length of work is found by subtracting the size of the small end from the size of the large end. Taper is usually expressed as the amount of taper per foot of length or taper per inch of length. We will take two examples.

Example l.–Find the taper per foot of a piece of work 2 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches.

The amount of taper is 2 inches minus 1 inch, which equals 1 inch. The length of the taper is given as 2 inches. Therefore, the taper is 1 inch in 2 inches of length. In 12 inches of length the taper is 6 inches. (See fig. 9-31.) Example 2.–Find the taper per foot of a piece 6 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches. The amount of taper is the same as in example 1, that is, 1 inch. However, the length of this taper is 6 inches; hence the taper per foot is 1 inch times 12/6, which equals 2 inches per foot

SAFETY PRECAUTIONS:

In machining operations, always keep safety in mind, no matter how important the job is or how well you know the machine you are operating.

Listed here are some safety precautions that you MUST follow:

1. Before starting any lathe operations, always prepare yourself by rolling up your shirt sleeves and removing your watch, rings, and other jewelry that might become caught while you operate the machine.

2. Wear goggles or an approved face shield at all times whenever you operate a lathe or when you are near a lathe that is being operated.

3. Be sure the work area is clear of obstructions that you might fall or trip over.

4. Keep the deck area around your machine clear of oil or grease to prevent the possibility of slipping or falling into the machine.

5. Always use assistance when handling large workpieces or large chucks.

6. NEVER remove chips with your bare hands. Use a stick or brush, and always stop the machine.

7. Always secure power to the machine when you take measurements or make adjustments to the chuck.

8. Be attentive, not only to the operation of your machine, but also to events going on around it. NEVER permit skylarking in the area.

9. Should it become necessary to operate the lathe while the ship is underway, be especially safety conscious. (Machines should be operated ONLY in relatively calm seas.)

10. Be alert to the location of the cutting tool while you take measurements or make adjustments.

11. Always observe the specific safety precautions posted for the machine you are operating.

video link for operation of lathe machine

http://youtu.be/YSlFHav2fFg

catalytic converter

2012-11-04T19:30:00.002-08:00

A catalytic converter is a device used to reduce the harmful emissions from an internal combustion engine (used in most modern day automobiles and vehicles). There is not enough oxygen available to oxidize the carbon fuel used in these engines completely into carbon dioxide and water, thus toxic by-products are produced. Catalytic converters are used in exhaust systems to provide a place for the oxidation and reduction of toxic by-products(like nitrogen oxides, carbon monoxides, and hydrocarbons) of fuel into less hazardous substances for the environment such as carbon dioxide, water vapor, and nitrogen gas.

Introduction

Catalytic converters were first widely introduced in American production cars in 1975 due to EPA regulations on toxic reductions. The United States Clean Air Act required a 75% decrease in emissions in all new model vehicles after 1975. This decrease was to be carried out with the use of catalytic converters. Without catalytic converters vehicles would release hydrocarbons, carbon monoxide, and nitrogen oxide. These gases are the largest source of ground level ozone, which causes smog and is harmful to plant life. Catalytic converters can also be found in generators, buses, trucks, and trains— almost everything with an internal combustion engine will have some sort of catalytic converter attached to its exhaust system.

How does it work?

A catalytic converter is a very simple device using the basic redox reactions in chemistry to help reduce the pollutants a car makes. It converts around 98% of the harmful fumes produced by a car engine into less harmful gases. It is composed of a metal housing that has a ceramic honeycomb-type interior with insulating layers. This honeycomb interior has thin wall channels that are coated with a washcoat of aluminum oxide. This is very porous and increases the surface area, which allows for more reactions to take place. This is where the precious metals are located. These metals include platinum, rhodium, and palladium. No more than 4-9 grams of these precious metals are used in a single converter. The converter utilizes simple oxidation and reduction reactions to convert toxic fumes into gases that are not nearly as harmful to the environment. Recall that oxidation is the loss of electrons and that reduction is the gaining of electrons. These precious metals listed earlier promote the transfer of electrons and in turn the conversion of toxic fumes.

The last part of the converter is a control system that controls the fuel-injection system. This system is aided by an oxygen sensor that monitors how much oxygen is in the exhaust stream. This sensor in turn tells the engine computer to adjust the air-to-fuel ratio, thus keeping the catalytic converter running at the stoichiometric point and near 100% efficiency.

Functions

A three-way catalytic converter has three simultaneous functions: Reduction of nitrogen oxides into elemental nitrogen and oxygen. (NO_x → N_x+ O_x)
Oxidation of carbon monoxide to carbon dioxide. (CO + O₂ → CO₂)
Oxidation of hydrocarbons into carbon dioxide and water. (C_xH_4x + 2xO₂ → xCO₂ + 2_xH₂O)

There are two types of "systems" that run in the catalytic converter- "lean" and "rich". When the system is running "lean", that means there is more oxygen than required, thus the reactions favor the oxidation of carbon monoxide and hydrocarbons (at the expense of the reduction of nitrogen oxides). On the contrary, when the system is running "rich", meaning there is more fuel than needed, the reactions favor the reduction of nitrogen oxides into elemental nitrogen and oxygen (at the expense of the two oxidation reactions). With a constant imbalance of the reactions, the system is never running at 100% efficiency. Note: converters can store "extra" oxygen in the exhaust stream for later use. This storage usually occurs when the system is running lean; the gas is released when there is not enough oxygen in the exhaust stream. The released oxygen compensates for the lack of oxygen derived from the reduction of NO_x, or when there is hard acceleration and the air-to-fuel ratio system runs rich suddenly, faster than the catalytic converter can adapt to it. Also, the release of the stored oxygen helps in the oxidation process of CO and HC.

Dangers of pollutants

Without the redox process to filter and change the nitrogen oxides, carbon monoxides, and hydrocarbons into less harmful chemicals, the air quality (especially in large cities) would reach a harmful level to the human being.

Nitrogen oxides- these compounds are in the same family as nitrogen dioxide, nitric acid, nitrous oxide, nitrates, and nitric oxide. When NO_x is released into the air, it reacts with organic compounds in the air and sunlight, the result is smog. Smog is a pollu tant and has adverse effects on children's lungs. NO_x reacting with sulfur dioxide produces acid rain, highly destructive to everything it lands on. Acid rain deteriorates cars, plants, buildings, national monuments and pollutes lakes and streams to such an acidity that is unsuitable for fish. NO_xcan also bind with ozone to create biological mutations, and reduce the transmission of light (like smog).

Carbon monoxide- this form of CO₂is a harmful variant of a naturally occurring gas. Odorless and colorless, this gas does not have many useful functions in everyday processes.

Hydrocarbons- inhaling hydrocarbons from gasoline, household cleaners, propellants, kerosene and other fuels can cause death in children. Further complications can be central nervous system impairments and cardiovascular problems.

Catalytic inhibition and destruction

The catalytic converter is a sensitive device with precious metals coating the inside. Without these metals, the redox reactions would not occur and the whole module would be useless. There are several substances and chemicals that inhibit the catalytic converter from doing its job.

Lead- most (if not all) vehicles run on unleaded gasoline, meaning all the lead has been taken out of the fuel. However, if lead is to somehow get inside the fuel and is burned, it leaves a residue that coats the catalytic metals (Pt, Rh, Pd, and now even Au) and prevents them from contacting the exhaust fumes and performing the necessary redox reactions.
Manganese and silicon- manganese is primarily found in the organometallic compound MMT (methylcyclopentadienyl manganese tricarbonyl). MMT is a compound used in in the 1990's to up the fuel's octane rating (a higher octane rating means the gas is less likely to combust, thus causing the engine to explode. This is important since higher performing engines have a high compression ratio, which would need a higher octane gas to complement the amount of compression in the engine), and has now been banned from commercial sale due to the EPA's regulations. Silicon can be leaked from the combustion chamber into the exhaust stream from the coolant inside the engine.

These contaminants prevent the catalytic converter from doing its job. However, this process could be reversed by running the engine at a constantly high temperature to increase the hot exhaust flow through the converter, thus melting or liquefying some of the contaminants and out of the exhaust pipe. This process does not work if the metal is coated with lead, because lead has a high boiling point. If lead poisoning is severe enough, the whole converter is rendered useless and a new one is needed.

Thermodynamics of catalytic converters

Recall that thermodynamics tells us whether or not a reaction or process is spontaneous under certain conditions, but NOT how fast or at the rate of which the process is going. The redox reactions below occur rather slowly without the catalyst, so even if the processes are thermodynamically favorable, they cannot occur without proper energy. This energy refers to the activation energy needed to overcome the initial barrier preventing the process from happening. A catalyst aids in the thermodynamic process by lowering the activation energy required for the reaction to start. However, the catalyst itself does NOT produce a product, but it does affect the AMOUNT and the speed at which the products are formed.

Reduction of nitrogen oxides into elemental nitrogen and oxygen. (NO_x → N_x+ O_x)
Oxidation of carbon monoxide to carbon dioxide. (CO + O₂ → CO₂)
Oxidation of hydrocarbons into carbon dioxide and water. (C_xH_4x + 2xO₂ → xCO₂ + 2_xH₂O)

Catalytic converter theft

Due to the use of precious metals in the coating of the inner ceramic structure, many catalytic converters have been targeted for theft. The converter is for the most part easy to get to, because it lies on the outside and under the car. A thief could easily slide under the car, saw the connecting tubes on each end, and make off with the catalytic converter. Depending on the type and amount of precious metals inside, a catalytic converter can be easily sold for $200 dollars a piece.

Global Warming

Though the catalytic converter has helped reduce toxic emissions from car engines, it has also done its part in harming the environment. During the conversion of hydrocarbons and carbon monoxide, carbon dioxide is produced. Carbon dioxide is one of the most common greenhouse gases and contributes immensely to global warming. Along with carbon dioxide, the converters sometimes rearrange the nitrogen-oxygen compounds to form nitrous oxide. This is the same stuff used for laughing gas and in cars as a speed boost. Nitrous oxide is much more potent than carbon dioxide as a greenhouse gas. It is 300 times more potent and thus contributes to global warming that much more.

References

Timberlake, Karen C. Chemistry : An Introduction to General, Organic, and Biological Chemistry. 10th ed. Upper Saddle River: Prentice Hall Higher Education, 2008.
Petrucci, Ralph H., William S. Harwood, and Geoff E. Herring. General Chemistry : Principles and Modern Applications. 9th ed. Upper Saddle River: Prentice Hall, 2006.

Outside Links:

http://en.wikipedia.org/wiki/Catalytic_converter http://www.howstuffworks.com/catalytic-converter.htm http://www.absorblearning.com/media/item.action?quick=ve http://www.youtube.com/watch?v=rmtFp-SV0tY

welding process

2012-11-01T20:25:00.000-07:00

Welding is a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone, and with or without the use of filler material.

Welding is used for making permanent joints. It is used in the manufacture of automobile bodies, aircraft frames, railway wagons, machine frames, structural works, tanks, furniture, boilers, general repair work and ship building.

TYPES

Plastic Welding or Pressure Welding

The piece of metal to be joined are heated to a plastic state and forced together by external pressure

(Ex) Resistance welding

Fusion Welding or Non-Pressure Welding

The material at the joint is heated to a molten state and allowed to solidify

(Ex) Gas welding, Arc welding

Classification of welding processes:

(i). Arc welding

1. Carbon arc

2. Metal arc

3. Metal inert gas

4. Tungsten inert gas

5. Plasma arc

6. Submerged arc

7. Electro-slag

(ii). Gas Welding

1. Oxy-acetylene

2. Air-acetylene

3. Oxy-hydrogen

(iii). Resistance Welding

1. Butt

2. Spot

3. Seam

4. Projection

5. Percussion

(iv) Thermit Welding

(v) Solid State Welding

1. Friction

2. Ultrasonic

3. Diffusion

4. Explosive

(vi) Newer Welding

1. Electron-beam

2. Laser

(vii) Related Process

1. Oxy-acetylene cutting

2. Arc cutting

3. Hard facing

4. Brazing

5. Soldering

Arc welding methods

1. Metal arc welding

It is a process of joining two metal pieces by melting the edges by an electric arc. The electric arc is produced between two conductors. The electrode is one conductor and the work piece is another conductor. The electrode and the work piece are brought nearer with small air gap. (3mm app.)

When current is passed an electric arc is produced between the electrode and the work piece. The work piece and the electrode are melted by the arc. Both molten piece of metal become one. Temperature of arc is about 4000°c Electrodes used in arc welding are coated with a flux. This flux produces a gaseous shield around the molten metal. It prevents the reaction of the molten metal with oxygen and nitrogen in the atmosphere. The flux removes the impurities from the molten metal and form a slag. This slag gets deposited over the weld metal. This protects the weld seam from rapid cooling

Equipments:

A welding generator (D.C.) or Transformer (A.C.)
Two cables- one for work and one for electrode
Electrode holder
Electrode
Protective shield
Gloves
Wire brush
Chipping hammer
Goggles

Advantages

Ø Most efficient way to join metals

Ø Lowest-cost joining method

Ø Affords lighter weight through better utilization of materials

Ø Joins all commercial metals

Ø Provides design flexibility

Limitations

Manually applied, therefore high labor cost.
Need high energy causing danger
Not convenient for disassembly.
Defects are hard to detect at joints.

2. Carbon arc welding

In carbon arc welding, the intense of heat of an electric arc between a carbon electrode and work piece metal is used for welding. DC power supply is used. The carbon electrode is connected to negative terminal and work piece is connected to positive terminal, because positive terminal is hotter (4000°c) than the negative terminal (3000°c) when an arc is produced. So carbon from the electrode will not fuse and mix up with the metal weld. If carbon mixes with the weld, the weld will become weak and brittle. To protect the molten metal from the atmosphere the welding is done with a long arc. In this case, a carbon monoxide gas is produced, which surrounds the molten metal and protects it.

Carbon arc welding is used to weld both ferrous and non ferrous metals. Sheets of steel, copper alloys, brass and aluminium can be welded in this method

Comparison of A.C. and D.C. arc welding

	Alternating Current (from Transformer)	Direct Current (from Generator)
1	More efficiency	Less efficiency
2	Power consumption less	Power consumption more
3	Cost of equipment is less	Cost of equipment is more
4	Higher voltage – hence not safe	Low voltage – safer operation
5	Not suitable for welding non ferrous metals	suitable for both ferrous non ferrous metals
6	Not preferred for welding thin sections	preferred for welding thin sections
7	Any terminal can be connected to the work or electrode	Positive terminal connected to the work Negative terminal connected to the electrode

GAS WELDING

Oxy-Acetylene welding

In gas welding, a gas flame is used to melt the edges of metals to be joined. The flame is produced at the tip of welding torch. Oxygen and Acetylene are the gases used to produce the welding flame. The flame will only melt the metal. A flux is used during welting to prevent oxidations and to remove impurities. Metals 2mm to 50mm thick are welded by gas welding. The temperature of oxyacetylene flame is about 3200°c.

Gas Welding Equipment

1. Gas Cylinders

Pressure

Oxygen – 125 kg/cm²

Acetylene – 16 kg/cm²

2. Regulators

Working pressure of oxygen 1 kg/cm²

Working pressure of acetylene 0.15 kg/cm²

Working pressure varies depends upon the thickness of the work pieces welded.

3. Pressure Gauges

4. Hoses

5. Welding torch

6. Check valve

7. Non return valve

TYPES OF FLAMES

• When acetylene is burned in air, it produces a yellow sooty flame, which is not enough for welding applications

• Oxygen is turned on, flame immediately changes into a long white inner area (Feather) surrounded by a transparent blue envelope is called Carburizing flame (30000c)

• This flames are used for hardening the surfaces

• Addition of little more oxygen give a bright whitish cone surrounded by the transparent blue envelope is called Neutral flame (It has a balance of fuel gas and oxygen)

• Most commonly used flame because it has temperature about 32000c

• Used for welding steels, aluminium, copper and cast iron

• If more oxygen is added, the cone becomes darker and more pointed, while the envelope becomes shorter and more fierce is called Oxidizing flame

• Has the highest temperature about 34000c

• Used for welding brass and brazing operation

Advantages

Equipment has versatile
Same equipment can be used for oxy acetylene cutting and brazing by varying the torch size
Heat can controlled easily

Disadvantages

Slower process
Risk is involved in handling gas cylinders

GAS CUTTING

• Ferrous metal is heated in to red hot condition and a jet of pure oxygen is projected onto the surface, which rapidly oxidizes

• Oxides having lower melting point than the metal, melt and are blown away by the force of the jet, to make a cut

• Fast and efficient method of cutting steel to a high degree of accuracy

• Torch is different from welding

• Cutting torch has preheat orifice and one central orifice for oxygen jet

• PIERCING and GOUGING are two important operations

• Piercing, used to cut a hole at the centre of the plate or away from the edge of the plate

• Gouging, to cut a groove into the steel surface

Weld joint

There are 5 basic joint types in welding

• Butt joint: Two materials are in the same plane, joined from the edges.

• Corner joint:The corners of two materials form a right angle and joined.

• Lap joint: Two parts overlaps.

• Tee joint: One part is perpendicular to the other, making a T shape.

• Edge joint: Edges of the two materials joined.

Types of weld

Fillet weld: Used in T joints,corner joints, lap joints.
Groove weld:Used in butt joints.
Plug weld: Used in lap joints.
Slot weld: Used in lap joints.
Spot weld: Used in lap joints.
Seam weld: Used in lap joints.
Flange weld:Used in edge joints.
Surfacing weld:Not a joining process, it is used to increase the thickness of the plate, or provide a protective coating on the surface

Weldability is the ease of a material or a combination of materials to be welded under fabrication conditions into a specific, suitably designed structure, and to perform satisfactorily in the intended service

Brazing and Soldering

Brazing

It is a low temperature joining process. It is performed at temperatures above 840º F and it generally affords strengths comparable to those of the metal which it joins. It is low temperature in that it is done below the melting point of the base metal. It is achieved by diffusion without fusion (melting) of the base

Depending upon the method of heating, brazing can be classified as

1. Torch brazing

2. Dip brazing

3. Furnace brazing

4. Induction brazing

Advantages

Ø Dissimilar metals which canot be welded can be joined by brazing

Ø Very thin metals can be joined

Ø Metals with different thickness can be joined easily

Ø In brazing thermal stresses are not produced in the work piece. Hence there is no distortion

Ø Using this process, carbides tips are brazed on the steel tool holders

Disadvantages

Ø Brazed joints have lesser strength compared to welding

Ø Joint preparation cost is more

Ø Can be used for thin sheet metal sections

Soldering

It is a low temperature joining process. It is performed at temperatures below 840ºF for joining.

Soldering is used for,

• Sealing, as in automotive radiators or tin cans

• Electrical Connections

• Joining thermally sensitive components

• Joining dissimilar metals

forging process

2012-10-30T01:01:00.003-07:00

Forging is the process by which metal is heated and is shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press.

Forging refines the grain structure and improves physical properties of the metal. With proper design, the grain flow can be oriented in the direction of principal stresses encountered in actual use. Grain flow is the direction of the pattern that the crystals take during plastic deformation. Physical properties (such as strength, ductility and toughness) are much better in a forging than in the base metal, which has, crystals randomly oriented.

Forgings are consistent from piece to piece, without any of the porosity, voids, inclusions and other defects. Thus, finishing operations such as machining do not expose voids, because there aren't any. Also coating operations such as plating or painting are straightforward due to a good surface, which needs very little preparation.

Forgings yield parts that have high strength to weight ratio-thus are often used in the design of aircraft frame members.

A Forged metal can result in the following

•

Increase length, decrease cross-section, called drawing out the metal.

•

Decrease length, increase cross-section, called upsetting the metal.

•

Change length, change cross-section, by squeezing in closed impression dies. This results in favorable grain flow for strong parts

Common Forging Processes

The metal can be forged hot (above recrystallization temperatures) or cold.

Open Die Forgings / Hand Forgings:
Open die forgings or hand forgings are made with repeated blows in an open die, where the operator manipulates the workpiece in the die. The finished product is a rough approximation of the die. This is what a traditional blacksmith does, and is an old manufacturing process.

Impression Die Forgings / Precision Forgings:
Impression die forgings and precision forgings are further refinements of the blocker forgings. The finished part more closely resembles the die impression.

Basic mechanical thermodynamics concept

2012-10-22T08:25:00.004-07:00

The field of thermodynamics deals with systems that are able to transfer thermal energy into at least one other form of energy (mechanical, electrical, etc.) or into work. The laws of thermodynamics were developed over the years as some of the most fundamental rules which are followed when a thermodynamic system goes through some sort of energy change.

What is Thermodynamics?:
Thermodynamics is the field of physics that deals with the relationship between heat and other properties (such as pressure, density, temperature, etc.) in a substance. Specifically, thermodynamics focuses largely on how a heat transfer is related to various energy changes within a physical system undergoing a thermodynamic process. Such processes usually result in work being done by the system and are guided by the laws of thermodynamics.

Laws of Thermodynamics
Heat transfer is guided by some basic principles which have become known as the laws of thermodynamics, which define how heat transfer relates to work done by a system and place some limitations on what it is possible for a system to achieve.

Thermodynamic Processes:
A system undergoes a thermodynamic process when there is some sort of energetic change within the system, generally associated with changes in pressure, volume, internal energy (i.e. temperature), or any sort of heat transfer.

There are several specific types of thermodynamic processes that have special properties:Adiabatic process - a process with no heat transfer into or out of the system.Isochoric process - a process with no change in volume, in which case the system does no work.Isobaric process - a process with no change in pressure.Isothermal process - a process with no change in temperature.

States of Matter:
The 5 states of matter
gas
liquid
solidplasma
superfluid (such as a Bose-Einstein Condensate)

Phase Transitions
condensation - gas to liquid
freezing - liquid to solid
melting - solid to liquid
sublimation - solid to gas
vaporization - liquid or solid to gas

Heat Capacity:
The heat capacity, C, of an object is the ratio of change in heat (energy change - denoted by delta-Q) to change in temperature (delta-T).
C = delta-Q / delta-T
The heat capacity of a substance indicates the ease with which a substance heats up. A good thermal conductor would have a low heat capacity, indicating that a small amount of energy causes a large temperature change. A good thermal insulator would have a large heat capacity, indicating that much energy transfer is needed for a temperature change.

Ideal Gas Equations:
There are various ideal gas equations which relate temperature (T1), pressure (P1), and volume (V1). These values after a thermodynamic change is indicated by (T2), (P2), and (V2). For a given amount of a substance, n (measured in moles), the following relationships hold:
Boyle's Law (T is constant):P1V1 = P2V2
Charles/Gay-Lussac Law (P is constant):V1/T1 = V2/T2
Ideal Gas Law:P1V1/T1 = P2V2/T2 = nRR is the ideal gas constant, R = 8.3145 J/mol*K. For a given amount of matter, therefore, nR is constant, which gives the Ideal Gas Law.

Laws of Thermodynamics
:Zeroeth Law of Thermodynamics - Two systems each in thermal equilibrium with a third system are in thermal equilibrium to each other.
First Law of Thermodynamics - The change in the energy of a system is the amount of energy added to the system minus the energy spent doing work.
Second Law of Thermodynamics - It is impossible for a process to have as its sole result the transfer of heat from a cooler body to a hotter one.
Third Law of Thermodynamics - It is impossible to reduce any system to absolute zero in a finite series of operations. This means that a perfectly efficient heat engine cannot be created.

The Second Law & Entropy:
The Second Law of Thermodynamics can be restated to talk about entropy, which is a quantitative measurement of the disorder in a system. The change in heat divided by the absolute temperature is the entropy change of the process.
Defined this way, the Second Law can be restated as:
In any closed system, the entropy of the system will either remain constant or increase.
By "closed system" it means that every part of the process is included when calculating the entropy of the system.

thermodynamics

2012-10-22T08:20:00.000-07:00

thermodynamics

Thermodynamic Systems and Processes Summary• A thermodynamic system is a collection of matter and space with its boundariesdefined in such a way that the energy transfer across the boundaries can be bestunderstood.• Surroundings are everything not in the system being studied.• Systems are classified into one of three groups:Isolated system - neither mass nor energy can cross theboundariesClosed system - only energy can cross the boundariesOpen system - both mass and energy can cross theboundaries• A control volume is a fixed region of space that is studied as a thermodynamicsystem.• Steady state refers to a condition where the properties at any given point within thesystem are constant over time. Neither mass nor energy are accumulating within thesystem.• A thermodynamic process is the succession of states that a system passes through.Processes can be described by any of the following terms:Cyclic process - a series of processes that results in the systemreturning to its original stateReversible process - a process that can be reversed resulting in no changein the system or surroundingsIrreversible process - a process that, if reversed, would result in a change tothe system or surroundingsAdiabatic process - a process in which there is no heat transfer across thesystem boundariesIsentropic process - a process in which the entropy of the system remainsunchangedPolytropic process - the plot of Log P vs. Log V is a straight line, PVn =constantThrottling process - a process in which enthalpy is constant h1 = h2, work= 0, and which is adiabatic, Q=0.

Top 10 Reasons – Why should one be a Mechanical Engineer ?

2012-10-18T17:43:00.000-07:00

Top 10 Reasons – Why should one be a Mechanical Engineer ?

1.You get the opportunity to create something tangible and useful.Ur creations will be used by others.It gives u the greatest joy.
2.Its the broadest branch of engineering…so your career options are open even after u graduate:-
Defence,Civil services,High end R&D,Manufacturing,Design,Energy sector,Management,Entrepreneurship,Masters(ME/MS)
3.Variety to be learnt- u learn how to design and make things ranging from a Safety Pin to a Spacecraft.
4.Easy to imagine and visualize whatever u learn
5.Develop a range of skills – u learn the work of a machine operator (machinist), a smith, a foundryman, a mechanic,a plant manager,a researcher and a policy maker.
6.U work with massive machines (majestic in nature) to tiny precision instruments,micro and nano devices.
u’ll be savviest engineer.
7.Importance of ur work.U form the human resource that is required for the survival of any industry and forms the backbone of modern human life.u r the person who may generate power/energy from natural resources,make equipments and processes to mine minerals,make cars, bikes ,buses, trucks, planes,ships(transportation can be compared to human blood that transports nutrients), make machines that manufacture products ranging from food to surgical instruments to weapons,mange factories and businesses.
8.Get paid handsomely(after gaining a few years experience even if not as a fresher).
9.Not much of girls hanging around(they usually don’t prefer to opt for this course,its thought to be a manly course).U dont have to worry about getting dressed perfectly for class or for girls giggling at u for some silly or not so silly but serious reason.Ur in a man’s world.But there are a few out of the ordinary and brainy girls who do take up this course and luv it.
10.It sounds and feels nice to be called a Mechanical Engineer.

Automatic Air Suspension

2012-10-16T19:02:00.002-07:00

Automatic Air Suspension

Air ride suspension carries the load on each axle with a pressurized air bag much like a high pressure balloon. Air ride suspension systems have been in common use for over forty years and have proven to provide the smoothest and most shock-free ride of any known vehicle suspension system. Modern air bags are constructed using the same methods as a tire by using high strength cords which are then encapsulated in rubber. These units are very durable in service and have a proven life of many years.

In addition to providing extremely smooth ride quality, air ride suspension also provides other important features. First, the system automatically adjusts air pressure in the air bag so that the trailer always rides at the same height, whether lightly loaded or heavily loaded. This allows the suspension system to always provide the maximum usable wheel travel independent of trailer load. In addition, the higher air bag pressure associated with higher trailer loads, automatically provides a stiffer suspension which is exactly what is required for a smooth ride. The lower air bag pressure for lightly loaded conditions, automatically provides for a softer suspension, thus providing the same ride quality for all trailer loading conditions. Since each axle is independently supported by its own air bag, the air ride suspension is a truly, fully independent suspension system.

The automatic control of the air bag pressure is accomplished by a solid state electronic control system specifically designed and packaged for vehicle use. This system continuously monitors the "ride height" of the trailer suspension and increases air pressure if the ride height is too low, by turning on an on-board air compressor. The air compressor stops automatically when the proper ride height is reached. If the ride height is too high, an automatic vent valve vents excess air pressure and stops venting when the proper ride height is reached. All required electrical power is provided by a 12 volt battery contained in the trailer equipment compartment

Introduction of Suspension system:

Suspension is the term given to the system of springs , shock absorbers and linkages that connects a vehicle to its wheels . Suspension systems serve a dual purpose — contributing to the car's road holding handling and braking for good active safety and driving pleasure, and keeping vehicle occupants comfortable and reasonably well isolated from road noise, bumps, and vibrations, etc. These goals are generally at odds, so the tuning of suspensions involves finding the right compromise. It is important for the suspension to keep the road wheel in contact with the road surface as much as possible, because all the forces acting on the vehicle do so through the contact patches of the tires. The suspension also protects the vehicle itself and any cargo or luggage from damage and wear. The design of front and rear suspension of a car may be different.

Figure 1: Locating Suspension Units

A suspension system comprises springs, shock absorbers and linkages. This suspension connects an automobile to its wheels. The suspension systems not only help in the proper functioning of the car's handling and braking, but also keep vehicle occupants comfortable and make your drive smooth and pleasant. It also protects the vehicle from wear and tear. To know about the suspension system, one needs to know about the spring rate or suspension rate. Various spring types are used for different vehicles. In case of heavier suspension loads, the spring rate is higher and vice versa. Spring rate is measured as a ratio used to measure how resistant a spring is to being compressed or expanded during the spring's deflection Besides spring rate, one needs to take in account the wheel rate. Wheel rate is the effective spring rate when measured at the wheel. It is generally equal to or considerably less than the spring rate.

There are two types of suspension systems- dependent and independent. A dependent suspension comprises a beam that holds wheels parallel to each other and perpendicular to the axle. An independent suspension helps in the rising and falling movement of the wheels. There is also a semi-dependent suspension where the motion of one wheel affects the position of the other but they are not rigidly attached to each other.

The dependent suspension includes Trailing arms, Satchell link, Panhard rod, Watt's linkage, WOBLink and Mumford linkage. The independent suspensions includes Swing axle, Sliding pillar, MacPherson strut/Chapman strut, Upper and lower A-arm (double wishbone), multi-link suspension, semi-trailing arm suspension, swinging arm and leaf springs.

Air Spring:

Air springs combine spring and shock absorbing action in one unit and were often used without metal springs. The first one was developed by Cowey Motor Works of Great Britain in 1909. It was a cylinder that could be filled with air from a bicycle pump through a valve in the upper part of the housing. The lower half of the cylinder contained a diaphragm made of rubber and cord which, because it was surrounded by air, acted like a pneumatic tire. Its main problem was that it often lost air.

The newest air spring, developed by Goodyear, is found on some late-model Lincolns. Like the ones that have preceded them, these ride-on-air units are more costly than conventional springs and hydraulic shock absorbers.
The air suspension system is an air-operated, microprocessor controlled suspension system. This system replaces the conventional coil spring suspension and provides automatic front and rear load leveling. The 4 air springs, made of rubber and plastic, support the vehicle load at the front and rear wheels'.

An air suspension supports the vehicle on the axles with an arrangement of air bags instead of some type of steel spring, leaf or coil, or some type of torsion spring arrangement. The air bags are sometimes referred to as air springs or bellows. Suspensions that have steel or torsion springs that are supplemented by the use of air bags are not considered air suspensions. There are combination systems that have both air and steel springs. Usually the air suspension components are used on the rear of the vehicle.

Figure 2: Advanced Air Suspension

Depending on the situation, this type of air suspension will probably have to be dealt with for leveling purposes. Normally, the air suspension is just one part of the air system on the vehicle. Most (but not all) vehicle with an air suspension also have air brakes along with other equipment that may be operated with air. Any of these other systems can cause problems with the air suspension. Other air systems including the brake systems in general, will not be discussed in this school. It is important to understand that on vehicles with air systems, especially with air brakes, manufacturers must follow specific regulations when designing their air systems. The brake system will always be the main concern for the air system. There will be safety features installed in the system that make the brake system the main priority for the air system.

Advantages of air suspenson system :

Excellent Suspension & Comfort to passengers

Fully Automatic Level Control

Good handling

Shields the vehicle from damage

Keeps the tires pressed firmly to ground

Easy To Service

Lifting & Lowering Function

Increases life of vehicle

Four-Wheel Steering System

2012-10-16T18:35:00.000-07:00

Four-Wheel Steering System

This paper deals with the details of four wheel steering (4WS) system. With the help of this system all the four wheels can be turned to any direction using the steering. Thus the vehicle can be controlled more effectively especially during cornering and parking. Also the speed of the vehicle can be increased or decreased. `

There are three types of production of four-wheel ¬steering systems:
1. Mechanical 4WS
2. Hydraulic 4WS
3. Electro/hydraulic 4WS

The mechanical 4WS uses two separate steering gears to control the front and rear wheels. The hydraulic 4WS uses a two-way hydraulic cylinder to turn both the wheels in the same direction. It is not possible to turn them in the opposite direction. The electro/hydraulic 4WS combine computer electronic controls with hydraulics to make the system sensitive to both steering angle and road speeds.

This system finds application in off-highway vehicles such as fork lifts, agricultural and construction equipment and mining machinery. It is also useful in passenger cars, mainly SUVs. Four-wheel steering, 4WS, also called rear-wheel steering or all-wheel steering, provides a means to actively steer the rear wheels during turning maneuvers. It should not be confused with four-wheel drive in which all four wheels of a vehicle are powered. It improves handling and help the vehicle make tighter turns.
Production-built cars tend to understeer or, in few instances, oversteer. If a car could automatically compensate for an understeer/oversteer problem, the driver would enjoy nearly neutral steering under varying conditions. 4WS is a serious effort on the part of automotive design engineers to provide near-neutral steering.The front wheels do most of the steering. Rear wheel turning is generally limited to 50-60 during an opposite direction turn. During a same direction turn, rear wheel steering is limited to about 10-1.50.

When both the front and rear wheels steer toward the same direction, they are said to be in-phase and this produces a kind of sideways movement of the car at low speeds. When the front and rear wheels are steered in opposite direction, this is called anti-phase, counter-phase or opposite-phase and it produces a sharper, tighter turn

Why Four-Wheel Steering System?:

To understand the advantages of four-wheel steering, it is wise to review the dynamics of typical steering maneuvers with a conventional front -steered vehicle. The tires are subject to the forces of grip, momentum, and steering input when making a movement other than straight-ahead driving. These forces compete with each other during steering maneuvers. With a front-steered vehicle, the rear end is always trying to catch up to the di¬rectional changes of the front wheels. This causes the vehicle to sway. As a normal part of operating a vehicle, the driver learns to adjust to these forces without think¬ing about them.

When turning, the driver is putting into motion a com¬plex series of forces. Each of these must be balanced against the others. The tires are subjected to road grip and slip angle. Grip holds the car's wheels to the road, and mo¬mentum moves the car straight ahead. Steering input causes the front wheels to turn. The car momentarily resists the turning motion, causing a tire slip angle to form. Once the vehicle begins to respond to the steering input, cornering forces are generated. The vehicle sways as the rear wheels attempt to keep up with the cornering forces already generated by the front tires. This is referred to as rear-end lag, because there is a time delay between steering input and vehicle reaction. When the front wheels are turned back to a straight -ahead position, the vehicle must again try to adjust by reversing the same forces developed by the turn. As the steering is turned, the vehicle body sways as the rear wheels again try to keep up with the cornering forces generated by the front wheels.

The idea behind four-wheel steering is that a vehicle requires less driver input for any steering maneuver if all four wheels are steering the vehicle. As with two-wheel steer vehicles, tire grip holds the four wheels on the road. However, when the driver turns the wheel slightly, all four wheels react to the steering input, causing slip angles to form at all four wheels. The entire vehicle moves in one direction rather than the rear half attempting to catch up to the front. There is also less sway when the wheels are turned back to a straight-ahead position. The vehicle responds more quickly to steering input because rear wheel lag is eliminated.

Mechanical 4WS

In a straight-mechanical type of 4WS, two steering gears are used-one for the front and the other for the rear wheels. A steel shaft connects the two steering gearbox¬es and terminates at an eccentric shaft that is fitted with an offset pin. This pin engages a second offset pin that fits into a planetary gear.

he planetary gear meshes with the matching teeth of an internal gear that is secured in a fixed position to the gearbox housing. This means that the planetary gear can rotate but the internal gear cannot. The eccentric pin of the planetary gear fits into a hole in a slider for the steer¬ing gear.

A 120-degree turn of the steering wheel rotates the planetary gear to move the slider in the same direction that the front wheels are headed. Proportionately, the rear wheels turn the steering wheel about 1.5 to 10 degrees. Further rotation of the steering wheel, past the 120¬degree point, causes the rear wheels to start straighten¬ing out due to the double-crank action (two eccentric pins) and rotation of the planetary gear. Turning the steer¬ing wheel to a greater angle, about 230 degrees, finds the rear wheels in a neutral position regarding the front wheels. Further rotation of the steering wheel results in the rear wheels going counter phase with regard to the front wheels. About 5.3 degrees maximum counter phase rear steering is possible.

Mechanical 4WS is steering angle sensitive. It is not sensitive to vehicle road speed.

Hydraulic 4WS:

The hydraulically operated four-wheel-steering system is a simple design, both in com¬ponents and operation. The rear wheels turn only in the same direction as the front wheels. They also turn no more than 11/2 degrees. The system only activates at speeds above 30 mph (50 km/h) and does not operate when the vehicle moves in reverse.

A two-way hydraulic cylinder mounted on the rear stub frame turn the wheels. Fluid for this cylinder is sup¬plied by a rear steering pump that is driven by the differ¬ential. The pump only operates when the front wheels are turning. A tank in the engine compartment supplies the rear steering pump with fluid.

When the steering wheel is turned, the front steering pump sends fluid under pressure to the rotary valve in the front rack and pinion unit. This forces fluid into the front power cylinder, and the front wheels turn in the direction steered. The fluid pressure varies with the turning of the steering wheel. The faster and farther the steering wheel is turned, the greater the fluid pressure.

The fluid is also fed under the same pressure to the control valve where it opens a spool valve in the control valve housing. As the spool valve moves, it allows fluid from the rear steering pump to move through and operate the rear power cylinder. The higher the pressure on the spool, the farther it moves. The farther it moves, the more fluid it allows through to move the rear wheels. As mentioned earlier, this system limits rear wheel movement to 11/2 degrees in either the left or right direction.

mechanical engineer

2012-09-28T02:18:00.000-07:00