In my recent topic on threading a 3/4 x 10 hole, threadmills were
suggested. many brought up using a hand made unit.

My question is how much smaller the thread mill needs to be than the
bore. Picture the worst case, if the thread mill is the same size as
the bore, it will just bore out the hole with no threads at all. Looks
to me as if a thread mill 90% of the bore size would leave a very flat
thread form. the math on this is way above my pay grade.

I'm looking to my winter project. making a custom RPD receiver. I
bought a 15mm x 1.5mm thread mill to make a 24 x 1.5 barrel thread.

Karl