NEMA 17 bowden extruder drive

Based on Wade's extruder, with the X carriage mounting holes removed since this will be mounted on the Huxley frame instead. Also includes some other changes to take the bowden tube (3mm OD) and work with a 1.75mm diameter filament, driven by an M4 toothed brass insert.

The gears are 9 and 53 tooth gears respectively, giving a ratio of nearly 5.9:1. I might try using a NEMA14 instead in future since the force required to push 1.75mm filament through the nozzle is a lot less than 3mm filament.