This is based on emmett's cube gears ( http://thingiverse.com/thing:6073 ), but with a lot of improvements to the script. Rendering still takes a long time at first (as much as 10 minutes for this version), but I've added caching so that you don't have to wait that long every time. The render statements I used to do this also stop the OpenCSG renderings from looking like something homicidal impressionist clown with dyslexia painted during an earthquake. I also made the animation work, though even with the caching it won't exceed a few fps. The most important thing, though, is that I added a module to the parameters section to define the shape to cut into gears. You can now make gears out of any STL just by dropping the "import_stl" into this module.

Instructions

The gears are pretty similar, but it will only become a smooth sphere if it is assembled just right, so you should number them as you finish printing them. While looking at the center with any four faces visible, mark one of the large faces with a 1, then 2-3 moving clockwise from there. Face 5 is adjacent to 1 and 4, and if you turn the center piece so that side 5 and the remaining three sides are visible, you can number them in a counter-clockwise direction. You can also place them by identifying them in the OpenCSG rendering; gears 1 and 2 are grey, gears 3 and 4 are red, gears 5 and 6 are green, and gears 7 and 8 are blue (even-numbered gears are small, odd-numbered gears are large).

You can assemble them by threading M3 screws into the plastic of the center block. Use 16-18mm screws for the large gears and 18-20mm screws for the small gears.