By request of Syvwlch ( http://www.thingiverse.com/syvwlch ) and WilliamAAdams ( http://www.thingiverse.com/WilliamAAdams ), here is a stand-alone public-domain OpenSCAD cycloidal speed reducer. As with the Wankel Engine and Roots Blower I recently posted, this is intended more as an example of an interesting mechanism than as a practical device. If you want a practical printable speed reducer, you might consider one of the other alternatives like

the worm drives on this Tank http://www.thingiverse.com/thing:8080 or
differential planetary gears http://www.thingiverse.com/thing:7390 or
cascaded spur gears http://www.thingiverse.com/thing:7379 or
this planetary gear reducer http://www.thingiverse.com/thing:8460

There are several cycloidal-type mechanisms already on Thingiverse, such as
http://www.thingiverse.com/thing:3617 and http://www.thingiverse.com/thing:3736

There are also several interesting external sites like:
http://www.zincland.com/hypocycloid/
http://fabricationsofthemind.com/2010/07/09/extruder-design-1-printable-1001-hypocycloidal-gearbox/
https://github.com/triffid/Differential_Hypocycloid
http://reprap.org/wiki/Differential_Hypocycloid
http://en.wikipedia.org/wiki/Gerotor
http://en.wikipedia.org/wiki/Gear_pump

and many many interesting youtube videos such as
http://www.youtube.com/watch?v=bRn1K2XeWVE
http://www.youtube.com/watch?v=3WvPF6uGCq4
http://www.youtube.com/watch?v=CG2sPuqEXBg
http://www.youtube.com/watch?v=AMtyFwMDL7w
http://www.youtube.com/watch?v=h236SP86nnQ

This present script is based on a design by M.F. Hill described in his 1928 patent "Internal Rotor", number 1,682,563:

http://www.google.com/patents/about?id=mdF5AAAAEBAJ&dq=1682563

Note that this design is based on an offset hypocycloid, similar to Figure I in Hill's patent. Most of the contemporary designs appear to be based on an offset epicycloid, more closely resembling Figure V in the patent.

The motivated student can modify the code so it generates epicycloidal-based profiles. Hint: start by making a module ``epitrochoidBandFast(n, r, thickness, r_off)". The motivated student could also probably clean up my train-wreck of code and/or figure out how to do arrays in OpenSCAD.

Note also that these rotors can be used for pumps - see the gifs in the comments for an example.

OpenSCAD for animate!
STLs for print!
DXF for cut!

The minimum clearance around shafts is about 0.6mm. If printed, there might be a little interference at the tips of the internal rotor lobes. The DXFs have the same profile as the STLs, so the beam kerf should provide ample clearance if anyone actually lasercuts it. There are some extra holes in the DXF profiles so pieces can be aligned and bolted/screwed together.

Yet-another-Note: The DXF files were generated directly from OpenSCAD. I am not sure why they preview as having dotted lines in some places, but when viewed with say, Open Office Draw, the lines appear solid.