Overview: https://www.youtube.com/watch?v=8s7HgMFrdTg
UPDATE:
REV A1: Enlarged the hole for the set screw in case the motor shaft is not keyed (D shaped).
REV A:  I've ran into some issues with the design when it comes to flexible filament, and fixed them. The flex filament still has some path to get around the drive gear, because the piece of PTFE tube could be pushed up. The tubes also likes to be pulled down. The solution was to shape a channel for the filament in printed plastic, and add a bolster that filled the space between the top curve of the drive gear's tooth channel. Now the flexible filament will just slip, like any filament should, instead of jamming the gear.
Also added 3mm versions of parts that needed modification
I finally have an extruder I'm happy with. Easy access, and 70mm/s retract speeds. Oh, and I can print 1.75mm Ninjaflex. :)
You are going to need the following things:
4 - M3 22mm hex head screws
1 - M3 hex nut
1 - M4 14mm hex head screw
1 - M4 hex nut
1 - MK8 Drive Gear
1 - 625-2RS…