In our latest design, we’ve been trying to hammer out the details of our fan configuration, and along the way, decided to ditch fans altogether.  Initially, we assumed we would be using two fans, pointed towards each other, and just below the print head.  We purchased 3 different sized of fans to experiment with – 20mm by 20mm, 40mm by 40mm, and 60mm by 60mm.  Each pair had a different location within the printer – the 20mm fans were about an inch away from the print head, the 40mm fans were about 2.5 inches away, and the largest fans were about 11 inches away.

Here is a picture of the 40mm fans attached to the print head:

These were the fans we used:

Although we used a variety of test objects, we settled on a tall (70mm) three sided pyramid as our primary test object.  The objective was to print a tall, tapering object and see where it began to slump – in other words, at what point was the plastic being extruded too quickly, such that it could not cool enough to maintain its structural integrity.  Our assumption was that the fans would promote more rapid cooling, and thus the object would more structurally sound.

To our surprise, the increase in airflow gave us only marginal gains in the structural integrity of the test objects.  We additionally attempted to increase the airflow by supplying the 40mm fans with more voltage.  While this improved things, the results were not what we were looking for.  As can be seen in the picture below, the four different fan configurations we tried gave only small improvements in the final print’s quality.

Unsatisfied by the results we were getting, we began looking for other options.  Skeinforge has a plugin called “Cool”, which turned out to be just what we needed.  Particularly, one of the settings in the Cool plugin allows you to define a ‘Minimum Layer Time’.  What this does is let you specify the minimum amount of time it takes to build each layer.  Another setting, “Cool Type”, tells Skeinforge how to deal with layers when they would otherwise take less than the specified minimum time – we use the ‘Slow Down’ option.   Essentially, how this works is you specify a minimum time (in our case, 10 seconds seemed to work well), and then for any layer small enough, the print head moved correspondingly slower.

Here is a resulting print.  Note that both parts below were printed without the use of fans, the only difference being the use of the Cool plugin.

Our conclusion is that, while fans are of limited usefulness, Skeinforge’s Cool plugin is particularly useful for small parts that are prone to becoming too hot.  I might add that the Cool plugin is much more easy to implement too.

Check out our Flickr page for more photos.

Like our previous design, it is constructed primarily of T-slot aluminum extrusions (and associated components)  and flat aluminum parts.  Here’s a CAD rendering of the basic design:

It has a number of improvements over our previous model.  For one, we are now using linear ball bearings on all axes. We found that bushings can work well, but they are very finicky and can bind at the slightest provocation.   By contrast, we found linear bearings much easier to use. They are smooth running, almost self-aligning,  and will support huge side loads without complaining. They actually seem to become smoother the more load you put on them.

Moving to linear bearings allowed us to make the X and Y axes more compact.  Here’s a detail shot of the X & Y axes (and extruder) from below:

Using linear bearings also enabled us to use a simple, cantilevered Z stage.  The stage is very stiff, with no noticeable play.

We also designed the frame to allow acrylic panels to be mounted on all sides and added a door in front.  With an enclosed system like this, you can add some dryer vent tube and a bathroom fan to create  a nice fume extraction system.  The enclosure isn’t airtight, but the fan creates negative pressure within the enclosure which effectively contains the fumes. So far it has worked well and we haven’t noticed any smells while printing.

Here’s what the printer looks like when fully enclosed:

The frame also incorporates a bottom compartment for power supplies and electronic boards. Compared to our previous design which required a separate controller box, this change eliminated a ton of exposed wiring and greatly reduced the number of connectors. The end result is very clean and easy to assemble.

Here’s Pat assembling the prototype:

The T-slot parts are really nice to work with.  It took Pat two leisurely days to completely assemble the mechanical parts (with no instructions), and another day to complete the wiring.  The end result:

It is difficult to see in the photo, but the entire printer is enclosed in clear acrylic.

The design for this printer is almost complete, and I’ll be posting more info soon.  Our goal is to release the plans under an open license, and if the interest is there, to offer complete kits.

Did I mention it is BIG — at least by our standards.  Maybe 60% full size?  Pat found the model on Google 3D warehouse and sliced it into two parts (top and bottom) for easier printing. The print was made using a 1 mm nozzle.

Here’s a video showing our first small version, and then the final large version:

UPDATE:  If you’d like to try printing this, I’ve uploaded the files to Thingiverse.

We’ve found this design to be extremely reliable, powerful, and smooth running. And it uses the same NEMA 17 stepper motor that we use for all other machine axes.

The only improvement I’d like to see is the addition of a spring loaded pressure wheel, rather than the fixed position setup we use.

I’m currently assembling a new version that has a similar design, but is more compact. More pics to come.

UPDATE: I should mention that this was inspired by NopHead’s worm drive extruder

These are the y bearing, bed height spacer and x/z bearing plate. We’re pretty happy with them. We are aiming to build the highest quality parts possible, particularly in relation to build speed and total build time. These were all done at a base feed rate of 25mm/s.

One of the components to better build quality is use a stepper motor to drive the filament into the extruder, and we’ve tuned the motor to do very specific retractions to prevent ooze. But as you can see, we still get tiny blobs at the ends of lines (as seen in the small bumps on the edges of parts). So we are still working on the ‘perfect’ extruder. We build our own extruder heads and heater blocks, which is another important part – retractions only work with certain geometries.

Here is a short video of a bed height spacer being printed. We particularly enjoy the printer’s quiet operation.

All in all, printing Mendel parts has been a great learning tool.

We’ve been printing with it for the last few days and are very happy with the operation. I’ll try to post some video later this week.

I’ve incorporated a few things I’ve learned from building my first t-slot RepStrap.  But I’ve also approached the problem with different set of goals.

From the start, I wanted a machine that could be fully enclosed.   I like the idea of a clean and tidy machine, similar to commercial units. But I also want to experiment with a more controlled build environment, possibly with space heating.

I’ve also recently realized that a heated build surface & extruder is a bit of a hazard to curious onlookers and operators alike, so a partition to separate them from the mechanicals is probably a good thing.

The enclosure could also be useful for fume extraction.

I also wanted to reduce the overall size of the machine.  The result is a very compact 14″x14″x14″, with a build envelope of 6″x6.25″x3.25″.  But in reality, these sizes are arbitrary — the machine can be made whatever size you like by changing a few extrusion and guide lengths.

One thing I learned from building my first T-Rep is that the t-slot extrusions are considerably heavier than the thin-walled tubular extrusions that I’ve used in the past.  As a result, stacking the X-Y axes in a table configuration creates a significant mismatch between the masses of each axis slide.  This isn’t necessarily bad, but as I’d like to use the same motors and gearing for both axes, it isn’t an ideal configuration.  This new design separates the X and Y axes, with one carrying the table, and the other carrying the extruder — a much more symmetrical configuration.

I was also late in appreciating the benefits of wrapping the z axis around the build area, rather than placing the entire z-axis mechanism above the build area.   This configuration (also used by the Darwin and Mendel) greatly reduces the overall height of the machine, at the expense of slightly increased complexity.

I’d love to get some feedback from others about this design.  I’m hoping to finalize the design sometime this week,  and then send it out for manufacture.

I attached the heater element to the bottom of my build surface, and mounted the thermocouple (not shown).

The PID control is very easy to use. It has an auto-tune mode that quickly dialed in the appropriate PID parameters. Upping the set point to 120C, it took about 5 minutes for the temperature to reach that value and stabilize. The heater duty cycle appeared to be somewhere around 60%.

I’m really happy with how easily the heater system came together, and the performance seems more than adequate for my needs.

My concern now is for that 120C plate. This plate is really HOT! In my previous machine with no heater, or with the 60C heater, I’m constantly touching the plate. Now, it is a real hazard for the unwary or someone as forgetful as myself.