A small usable 4:1 planetary gearbox. This was both an exercise to find the smallest printable unit and a tooth profile that would reliably print on my machine. This gearbox is the result that someone out there may be able to use. Printed on my Up! mini without support.
The planets work best if reamed out to 4.5mm. This can be done with an ordinary Jobber drill bit. Some sanding and filing can help to smooth the teeth and bearing surfaces. Both the input bearing and output bearing are standard 608Z skate bearings. Both input and output shafts also have 3mm hole to accept a M3 screw. A 10x5x4 bearing can be used to support the input shaft in the output as pictured. For best performance pack the unit with your choice of grease or oil such as all-purpose EP2.
The original drive was a multiplication gearbox for a sunsky brushless quadcopter motor. Friction is quite high without lube and at 50,000 RPM output it welded itself together. However it is usable as a reduction box with an input less than 20,000 RPM.