[Dick Linn]

In this case, they were only grinding the end of the allen key. Think of it as a broach with one edge. Similar to a mortising chisel attachment for a drill press.

A clever machinist/toolmaker can do amazing things with rather simple machinery. In this case, the idea is to make a real broach. Even if you were to taper it and make several steps, it's really only the final step that makes much difference. Whether it will work or not depends on the precision of fit required.