Thursday, 22 October 2009

Extruder Part 2: the Pinch Wheel Drive

For some reason NEMA17 stepper motors are much harder to get hold of than NEMA23s. I got a great deal on a set of four 23s, but I couldn't find 17s at less than list price, which works out at about £25 each. Then I remembered some gear motors that I've had in my junk box for about 25 years and never used:

They're on a NEMA17 frame, rated 12V 0.7W, and geared to 8.2rpm. A quick test shows that they can lift 1Kg using a 2in pulley without straining, which works out at 0.25Nm so plenty of torque. Ideal!

Next job: make the drive block. Here is my first attempt:

As you can see, it is based on the standard RP part, but made out of oak. I've started writing my extruder PID software, and I couldn't get it to work at all, so I thought I'd see how the drive behaved under constant power. The results were terrible. Here's a typical run:

The blue line is the PWM power output to the motor, and the green line is the measured speed of the filament. Note the enormous spikes when the filament starts moving: it's obviously sticking, then springing free. No wonder my feedback loop didn't work!

Peering at the pinch wheel mechanism, I could see what was wrong. The ball bearing is held by an M3 cap screw, bolted to a fairly narrow bit of wood. The force needed to pinch the filament hard enough pushes the bearing sideways, and the filament doesn't line up with the hole it goes through. That's why it sticks, and also why it's so hard to load the filament.

I decided to make a new block which holds the bearing on both sides, so it can't be pushed sideways. The bearing slides in through a slot at the side, which is a bit fiddly, but I've assembled it and disassembled it several times, and it's not too bad. I even managed to get a washer in on each side of the bearing. Here it is:

You can see the slot at the side where the bearing goes in, the small hole for the bearing axle screw, and the large hole for the motor spindle. Assembled, it looks like this:

It loads the filament without any trouble, it happily lifts 2Kg, and the spikes on the graph are gone. Success! Now back to programming.


  1. All a bit academic now, but I got my NEMA 17s for ~ a tenner each from earlier this year.

  2. Wow, that is good. I think I looked at them at the time, but they were out of stock. Their current price (once you've added vat & shipping) is rather more than that, though still better than most. I got my 23s from Astrosyn's surplus stock for £47 for four, including vat & p&p.