3D-printed gearboxes are always an interesting design challenge, especially if you want to make them compact. ZeroBacklash created a little strain wave gearbox (harmonic drive) for when you want to trade speed for torque on NEMA 17 stepper motors.
Strain wave gears work by deforming a stationary flexible spline into an ellipse so the teeth engage the internal teeth of the output spline. Add a couple of extra teeth on the output side, and you get a high-reduction gearbox with fewer parts and reduced volume than equivalent spur gearing. Keeping the flexible spline stationery is achieved by making half of it engage with a stationary spline with the same number of teeth.
In this case, there are 60 teeth on the input side and 62 on the output, giving a gear ratio of 30:1. The flexible spline is deformed using a set of bearing balls and an elliptical plug on the shaft of the motor. It makes for a compact design that matches the frontal size of the stepper motor and is only about 27 mm long. ZeroBacklash has not released any design files, but the idea should be simple to replicate.
Read more: 3D Printed Strain Wave Gearbox