A coworker converted his Fuji track bike to belt drive. Since the belt is endless, he needed to cut the rear dropout to insert it. We designed a keyed removable stainless dropout to fill the slot and lock the two parts together, and then I machined the dropout and cut a pocket in the frame to match.
Cutting the stainless insert was pretty straightforward, but cutting the frame was one of the more interesting setups I've done on my little mill:
Close-up of the setup:
Because the dropouts aren't parallel, I've fixtured the frame via the upper dropout. The lower dropout is floating under my inverted machinist's vise. The rod is threaded into a T-nut in the mill table.
The bottom bracket supported on blocks. A collar threaded into the bottom allows the frame to be leveled:
Cleaning up the rough-cut slot:
Milling the pocket:
Close-up of the finished assembly. The insert is a precision fit in the frame. (Phillips-head screws were countersunk after this picture was taken.)