Replacement head tilt gear for Rockwell 21-100 mill

Replaces the broken head tilt gear in my Rockwell mill from 1973.

I printed it in nylon, since it's an area that the manual calls for oiling. It obviously won't hold up as well as the aluminum piece it's replacing, but if you're relying on this for strength instead of the head lock bolts, then I'm not sure you're doing the right thing.

I made this with the OpenSCAD involute gear library (the old version from years ago). It would probably be better done with an ACME thread library, but this is squarely in "good enough" territory, as the gear will wear in, and there's more slop in the thrust fixing for the worm that drives it than there is between the gear and the worm itself.