So I got tired of grinding filament with the Gregs/Wades/etc extruder so I endeavored to create my own. The goal was to make a geared extruder with a larger diameter filament driver. The work of MoonCactus with his rollerstruder inspired this. I found later on that instead of the notches he cut into the head just cutting a groove and hobbing it seemed to work really well and was easier than trying to get the notches spaced properly.
This is designed for 3mm filament and I got good results when I used it but have gone and switched to 1.75 with everyone else... for my bowden setups it is really the way to go...
Also the mounting holes are spaced to fit on the end of a Prusa though this results in the filament pointing forward like the reprappro.
For a direct drive extruder check out my NEMA23 one at: https://www.youmagine.com/designs/nema23-direct-drive-extruder
Non-printed hardware (vitamens):
3 - 608 ball bearings... you probably have a bunch of these right?
1 - NEMA17 stepper
1 - M8 hex head bolt 45mm or longer
1 - M5 threaded rod or similar diameter in SAE
1 - M5 knut ^^
1 - ~2mm x 20mm bolt
1 - 0.25" tubing to 1/8 NPT Pneumatic fitting
1 - Stiff spring like from that other extruder you are replacing
1 - M8 lock nut
1 - M8 nut
So the main part to machine is the 'hobbed bolt'. I have a small lathe so I turned the bolt head down with it but a file and drill would probably work. Once the hex becomes a circle cut a notch in it that will work for hobbing it.... and hob it. If the bolt is too hard you can anneal the metal by heating it to red hot with a torch and let it cool slowly in the air.
For the tensioner carefully cut a small hole through the end of the rod for the smaller bolt to go through and put the cam on it.
The Pneumatic fitting is meant to hold the tubing with the tubing going all the way through it so the hole on the threaded side needs to be drilled through.
When adding the large gear you probably want to add some washers to get it spaced right. Use the locknut on the large gear and add the other nut outside it to make sure the gear doesn't start un-threading itself.