I have battled for a long time to get an extruder drive that works reliably, delivering a steady speed especially when extruding HDPE. The problems were that the DC motor would slow under load and was often variable in speed as the load varied. Also the screw drive did not positivley grip the plastic rod
I have come up with a solution based on all the other pinchwheel extruders but made from easily accesible parts (Thanks Adrian Zak and everyone else).
From the photographs you should be able to see a 90 degree angle bracket (from B and Q) which I have fastened a stepper motor to. I have machined a pulley and given it a rough surface using the technique described in another blog (sorry I can't rember who) where you put a tap into the lathe and the work piece into a bracket so it rotates effectively cutting a bevel gear. This is better than knurling as 1. I don't have a knurler and 2. it gives a groove for the plastic to sit in so keeps it in the correct place.
Initially the stepper did not have enough torque to drive the plasitc consitantly but I rewired the stepper so the coils were bipolar parallel and it worked fine.
I have also made it so that the stainless tube comes as close to the pinch wheels as possible to prevent any buckling.
I just now need to work out how programme the arduino so that when it gets a stop signal the stepper reverses for a few steps then stops to stop oozing.