This is bowden extruder which can adjust tension of spring easily with M3 allen wrench.
It is for MK7 drive gear and 688ZZ(8x16x5) bearing.
It uses fitting from E3D v6 hotend as bowden tube insert fitting, easy to use and looks simple.
Filament is guided all the way through inside extruder, to prevent possible stuck inside.
As for the spring, I use 25mm length of OD 8.2/ID 6.2. You might need stronger spring to have more grip in high speed printing.
Four 30mm M3 bolts (or Three 30mm and One 25mm) are needed for assembly
It has 21mm thick and designed to mount on the wall, I mounted it on 5mm acrylic frame. If you want to mount thick wall or bracket, you would need longer bolts.
I have used this extruder more than 3 weeks without any problem.
Hopefully you enjoy it.
I found stronger spring is needed for higher speed (>100mm/s).
I uploaded solidworks source files.(extruder-MK7.zip)
Feel free modify as you want.
I am inspired by Ultimaker 2 feeder.
I wanted it looks simple and it's tension be adjustable