Well, it seems to be my day for replying to posts!
I recall seeing a manufacturer that made a gear that could engage with the threads on a bolt. If you imaging 1/4-20 threads cut on the circumference of a wheel and a 1/4-20 bolt engaging the threads... I think you will get the idea. You could actually make something like this if you had a knee mill and an indexer. It might take a bit of trial-and-error, but it should be possible.
If you want to beat your brains out a bit:
http://www.mitcalc.com/doc/gear4/help/en/gear4.htmHere is the posting of a guy who did it this way:
http://www.dragonworks.info/Metalworkin ... Making.htmI know there is a company who actually sells this sort of thing, but I cannot recall it offhand. Perhaps these comments will jog someone's mind!