This is a differential cyclodial gearbox which achieves a large ratio by a similar mechanism to the "split planetary" gearbox, but with far fewer and much less delicate parts.
With this large a gear reduction ratio, small & light "tin can" or "pancake" stepper motors, usually considered useless, may be useful for things like extruders. This one is designed to fit a PF-35T-48L4 motor I got from salvage. (Very similar one on sparkfun https://www.sparkfun.com/products/10551 )
Sources available on my personal site: http://burningsmell.org/3d/hypo35/
I did not want to put four little, fragile pins on the inside of the gear as pictured in most cycloidal solutions. It is designed so that the output is co-axial with the input, letting everything load-bearing easily sit on ball bearings.
It requires a few vitamins.
1 X 625 ball-bearing.
2 X 12mm M3 screws and 2x M3 nuts to attach the input motor.
1 X M5 hex-head screw to fit in the output gear.
4 X 25mm M3 screws and nuts to attach the output to anything willing to attach to a NEMA17
I used a center pin machined from metal instead of a printed one. The main difficulty with a printed one is fixing it solidly to a round shaft, something like krazy glue may be needed.
The offset pin, dark blue, is attached solidly to the motor shaft but is free to spin in the 625 bearing(violet). This forces the light blue gear to "roll" around the inside. Since it is only 1 tooth fewer than the outside, it rotates once per 12 rotations.
It fits into the output gear (not shown) with a different ratio, 1 in 11. Since the ratio differs, the output gear is forced to rotate to make up the difference. This speed is the difference between the ratios, so becomes very small.