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.
The huxley_gears.scad file uses Greg Frost's involute gear script http://www.thingiverse.com/thing:3575
I'm not totally happy with how this mounts onto the frame yet, but it's useable (with a few zip ties) and running on my eMAKER Huxley prototype machine.
You will need the following vitamins:
4x M3x10 Socket Head screw
4x M3 washer
1x M3x10 Grub screw
5x M4 nut
4x M4x40 Screw
1x M4x60 Screw
1x M4 toothed insert
Handful of M4 washers
1x NEMA17 hybrid stepper motor
Print all parts and assemble as per the photos, (similar to a standard Wade's extruder drive). I printed the gears with a 0.5mm nozzle on an UP printer and they came out really well.