This is a 4.36:1 planetary gearbox meant to be inserted between your NEMA 17 extruder motor along with your extruder mechanism. This permits you to use your existing Airtripper or comparable extruder and your MK7 drive gear with a 5mm bore, yet still insert a gear reduction so your motor doesn't overheat.
This layout is based on my habit gearhead extruder, but I have extracted only the gear reduction mechanism and given it a face mounting plate which should permit you to mount it to your current extruder. I also have redesigned it to utilize common metric bearings and bolts in place of the eclectic mixture of parts from my junk bin that the initial design used.
Note - I have not yet actually assembled and tested this layout, these documents are unproven. The design it's based on has been running smoothly for hours, but I have not tested these modifications yet.
This layout is unproven, use at your own risk. Feedback is welcome should you print and assemble it.
You will need the following hardware:
8x 625 bearings
3x 25mm long M5 screw
1x 40mm long M5 hex head bolt
4 M5 hex nuts
4x 50mm long M3 screws
8x 20mm long M3 screws
4x 10mm long M3 screws
3x M3 set screws
15x M3 hex nuts
Your Current motor and extruder
You will need to print three of the 'Planet' part, and one of each of the rest of the parts.
Eliminate your motor from your extruder. Save the mounting hardware.
Insert two 625 bearings into every Planet gear, one in each side.
Push the 40mm M5 bolt to the Carrier piece from the bottom. The hex head of the bolt should fit snugly into the pit at the bottom of the carrier. Thread a M5 hex nut on the bolt from the other side and turn it until it's tight against the plastic. This will hold the bolt in place to the carrier. After installing the M5 bolt, press one 625 bearing into the pit at the bottom of the carrier.
Push 1 M5 hex nut into each of the three hexagonal pockets in the bottom of the Carrier part. Thread a 25mm M5 screw in through the top of the Carrier and through the nut. Tighten well, you don't need these coming loose.
Push three M3 hex nuts to the three cavities on the Sun gear. Make sure the holes onto the hex nuts are lined up with the holes in the plastic. Turn three M3 put screws into the holes and thread them into the nuts. Push the Sun gear on the output shaft of your motor, with the end of the sun gear with the set screws closer to the motor face. Don't tighten the screws yet, you'll want to wait before assembling the gearbox to tighten down that.
Attach the 'Rear Plate' part to your own motor with four 10mm M3 screws. The side of the plate that is contrary to the print bed goes against the motor face.
Put the 'Ring Back' piece against the 'Rear Plate' part. The flatter side of the ring should face away from the motor. Now insert the three planet gears between the ring and the sun gear. Notice that the orientation of the planet gears matters, they just go in one way. You will want to get them equally spaced 120 degrees around the sun gear. The 'carrier' piece can be used for this, since it will lock the 3 planets in place. You can also tighten the three set screws on the Sun gear at this point.
Once you've the planets and the carrier in place, you can install the 'Ring front' part. This only goes on one way, there are alignment arrows on both halves of the ring gear which should line up. Use the 8 20mm long M3 screws through the 8 holes in the ring gear halves to screw them together closely, you don't want them to have the ability to move relative to each other.
Put four M3 hex nuts to the four little hex cavities in the 'Front Plate' piece, these are for mounting it to your extruder. Push one 625 bearing into the circular cavity in the bottom of the 'Front Plate' piece. Now slide the front plate over the M5 bolt, and line up the 4 mounting holes on the plates and the 4 extended posts on the ring gear assembly. Use the 50mm long M3 screws thru the mounting holes to clamp everything together. Tweak the alignment until you can push the carrier around by hand. You might need to run the motor for an hour or so until the gearbox works smoothy.
You need to be able to use the equipment assembly in place of your current motor, attaching it to your extruder from the four M3 hex nuts on the front, and attaching your drive equipment to the M5 bolt. The M5 bolt isn't as well supported as a motor output shaft would be, so you'll want to ensure that your extruder includes a second 625 bearing on the output shaft to help support it.
ETA: I have added the source files for this project. Just the gears are scad, the rest of the parts are hand-drawn in Autocad. The Autocad files are also all scaled in inches, so you'll want to rescale them prior to exporting to metric stl files.