The post you've all been waiting for!

It's a minimug!

I've been completely stymied by the extruder ever since the last post. Everything else has been working fine, but the extruder would jam after less than one layer had been printed. Nothing I did seemed to help.

After many attempts, I gave up on the original extruder and started figuring out how to build a stepper extruder without being able to print it. I spent a couple of days building a version of the Mendel extruder multiple times in various 3d apps, finally trying out OpenSCAD and loving it. It's far better to code the relationship between objects once and then tweak variables to make changes than it is to keep doing the math by hand in a traditional 3D CAD app. I'll be using it from now on - and if you're a programmer, you should too!

Of course, a 3D model is only useful if it can be instantiated, and for this I dug out my McWire mill. Sure, it's never actually milled anything yet, but what better time to try it out? Unfortunately, I hit a problem in converting the model into a g-code toolpath for running the mill. It's a solved problem, so long as you're running windows and have paid for a commercial app... the free linux tools left much to be desired. I can get close to a decent toolpath using Skeinforge (which is supposedly capable of milling as well as printing use) but not close enough to convince me to run the mill with it yet.

Therefore, I put together a version of the crudestruder out of various crap I had lying about. Unfortunately it looks like the small NEMA 17 motors we have lying about the space aren't nearly strong enough to drive the filament without being geared down; so this attempt was a bust too.

I was planning on borrowing the geared mendel extruder parts from Buzz that we received from Adrian a couple of weeks ago, except they require the new smaller bearings I have none of. I raced out at 4pm today to go to Hobbyparts.com.au, which is the best local supplier - they close at 5pm and aren't open on the weekend, so it was a real rush... only to discover the huge traffic jam that already occupied the Pacific Motorway. No bearings (and hence no mendel extruder) for me this week!

I was forced to return to the original extruder - but I had learned a couple of things about it and the hot end this week. Buzz brought in a thermocouple which I was able to stick down into the molten PLA in the heater barrel, and it read around 165 degrees when the barrel thermistor read 190. That makes the PLA far more viscous and hard to extrude. Therefore I increased the temperature of the barrel to '210', making the PLA much more fluid.

I also found a significant amount of friction in the extruder where the filament first enters it - there's a long section that the filament has to pass through before it gets to the drive screw, and it's perfectly straight. Unlike the filament, which has a significant curve to it. Disassembly (aaargh), lots of filing (aaargh) and some oil (eh) helped prevent the filament from jamming in that area. The oil also made the filament path far more transparent, improving visibility in the entire extruder.

Finally, the gearing at the top of the extruder wasn't correct - this being the first version of the rapman extruder, it placed the motor too far away from the drive screw gear, so that the gears would regularly skip teeth when the required force went up. I filed out the holes for the motor, tilted it, and drilled a new bolt hole for the motor at the gear end, which keeps the motor gear much closer to the drive gear. Now it no longer skips, regardless of the drive resistance.

And suddenly the extruder came good!

Now I need to go back into Skeinforge and start tweaking. It placed a ridiculous amount of inter-layer cooling time into the minimug gcode, the cooling movements on the bottom ten layers or so also significantly damaged the outer perimeter of those layers. Fixing that should speed up printing by about 5 times or so. Also, I don't think the minimug completely finished; it got somewhere close and then stopped, oozing a large blob onto the mug and causing the gcode uploader to spazz out a bit. I've heard about spurious serial corruption from other people, so that may be the culprit. Reducing the amount of useless gcode movements, and using one of the newer firmwares with checksumming should help. Finally, the aspect ratio of the minimug is off, as I have yet to properly calibrate the steps/mm values for the three axes.