After being inspired by a recent Hackaday article on hypercycloidal gears I looked into harmonic drives / strain wave gears. Other people have made their own and even 3D printed their parts. However, the flexible splined "cup" looked like the hardest part to make so I wondered if it could be made by constraining a closed timing belt against a flat disk. Seems like it works in principle!

I know there are limitations as to what desktop FDM printing can do but given the proliferation of 3D printable small robot arms, with the right bearings this might be a nice little mechanism to open up the range of motors that can power these hobby robot arms. There may be other uses such as a high resolution panning mechanism (photography, 3D scanning...).

If there's any interest I'll put the files on Thingiverse but I'm sure others would do a better job of turning this into a load-bearing design. An openscad or other parametric part generator would be good if it got to the point of being usable design.