This is a from-scratch redesign of the D-Bot extruder. I'd had issues with high filament pressure when printing very quickly causing the filament to bow out and either break or wrap around the hobbed gear, plus I want the option to use softer filaments with the D-bot.
This extruder design keeps PTFE support tubes in very close proximity to the bearing and hobbed wheel, so there's no possibility of filament bending out of the extrusion path.
The hardware needed to assemble it is pretty basic:
MK8 extruder gear (from the stock extruder)
625ZZ bearing (from stock extruder)
3x M3 8mm bolts (from stock extruder)
6 M3 washers
3x M3 30mm bolts
2x M3 nuts
Small length of 4mm OD/2mm ID PTFE tubing
2x M3-sized tensioner springs. (There are good options in the set of springs from Home Depot that is called out in the D-Bot parts list -- generally you want a somewhat stronger, small spring about 5mm in diameter, 20mm long, so it can compress down to 12-13mm)
Bowden connector (from the stock extruder)
1x M5 10mm bolt (same as all the small bolts in the D-bot)
Assembly should be pretty straightford. After printing the parts, insert the two captive M3 nuts first. The holes are small, you may need to work on them a bit with a hobby knife to get the nuts in depending on the tolerances of your printer. Its meant to be tight, and it took some fiddling, but I got them all in.
Cut some small lengths of the PTFE tubing and insert them from the outside inwards in the filament channel. The one that goes under the Bowden connector needs to be flush to the plastic inside the opening, so its pretty short. I was able to seat it using a hex bolt head to push it in place. Twisting can help. If its too tight, run an M4 screw through the holes and use it to bore them out to a true 4mm. Once the tubing is seated, cut the inner edges flush with the two curved surfaces with a fine hobby knife.This is important, its the proximity of the tubing to the moving parts that keeps the filament in alignment.
After that, screw the bowden connector in, and then bolt the stepper to the bracket using the three 8mm M3 bolts. The screw closest to the bowden connector (which has no inset) is left out.
Attach the bearing to the tension arm with the M5 10mm bolt. Use one of the 30mm M3 bolts, with a washer on each side, to run through the tension arm and screw into the stepper and main bracket.
The tension is maintained with the other two M3 30mm bolts -- assemble each with a washer, the spring, and another washer, and then screw into the holes on the tension arm. Adjust the screws for the amount of tension you want, I've found 12-13mm seems "about right".
My printer is dual extrusion, with one extruder on the left and one on the right, so there's versions for either side uploaded. You only need to print the two parts for the side you want.
Most of the parts on my D-bot were printed with PETG, but these don't have a lot of strain on them, nor are they near any heat, so I printed them with PLA because the prints end up more precise and the holes are pretty precise. I'm sure you could print it with ABS or PETG, as well.
You'll need supports because of the inset for the stepper motor on the rear, and the bowden connector. I strongly recommend printing with supports only on the bed -- the filament path and other holes shouldn't need supports and it'll be a real pain to dig the supports out if you print them with them.