RepRap: Blog

Inspired by Zach's lasercut pinch-wheel extruder and a comment from Ian ("I bet you could drive that direct with a stepper motor.") I decided to do just that and to create an RP design that RepRap could make.

Unfortunately, my RepRap was in bits having a couple of new toothed-belt cogs fitted, so I had to make this one in the Strat.

So. Here is a prototype. It works...


What's more, it only has two RP parts: the clamp that holds the heated barrel and attaches the device to the RepRap machine, and the pinch device itself. Here the whole thing is being driven by Zach's new stepper driver.


Here it is in bits. As you can see I put a brass collar on the stepper shaft. The collar has a knurled section to drive the plastic filament. (Improvement: I suspect it'll be possible to knurl the stepper motor's shaft itself and to get rid of that brass bit.) The filament is squashed against a ball race. (Improvement: use a smaller ball race and cut the size down.) Neither the ball race nor the collar need any sort of groove to guide the plastic - that is achieved by the hole down the block through which it runs.

I adjust the gap by sliding the whole motor in slots, rather than the more obvious method of having a slot for the ball race. This allows the latter to be very firmly mounted, and means that there are four sliding screws achieving the adjustment, which again makes things very sturdy when it's all tightened up. To get the gap, I just put a 2.5mm drill down it, push the motor against that, and tighten the screws. That compresses the 3mm filament into the knurl-ridges by 0.5 mm, which seems fine for ABS.

I'm driving it by a modified version of the GCode firmware running on a Sanguino. I need to get the hang of the timer interrupts as I don't yet have the resolution to make fine speed adjustments. That is one point: the stepper is running very slowly - about 4 steps per second. The elasticity in the system seems to smooth out the flow, though - you can see a very slight jerkiness as it comes out of the nozzle, but I don't think that will cause too much trouble.

Finally, I tried a new way of mounting the PTFE in the clamp:

I put the PTFE in a Black and Decker and used a triangle file to run grooves in the top of it. (The little blob of Blu Tack is to prevent contamination.) I made a hole with three support webs in into which it just fits.


Then I set it up like this, put more Blu Tack round underneath to seal it, and poured PU resin into the gap alongside the support webs. It set, and made a very solid join. I suspect that any thermoset like Araldite or JB Weld would work just as well as PU.

This was the prototype (for example it uses a very non-standard ball race that I just happened to have in a drawer). I'll do a proper design and stick it in the repository ASAP.

Labels: direct drive, extruder, pinch-wheel, stepper motor