On Mon, 07 May 2012 11:48:58 -0500, Jon Elson
wrote:

Karl Townsend wrote:

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.
The threadmill needs to safely clear the minor thread diameter so
it won't gouge the opposite wall while milling the thread, and also
can be removed from the completed bore. For large bores like your
3/4", it can probably be quite a bit smaller. All it needs is to have
enough tooth extending from the shaft to cut the thread.

A single-row threadmill is orbited around the bore as many times as there
are partial and complete threads in the bore depth.

A full threadmill is orbited only one turn plus a little bit.
They are generally plunged through the hole at the XY center and
then orbited counterclockwise up the hole to make the cut in the
climb-mill direction.

Jon

OK, thanks everybody. Sounds like I'm guilty of over thinking
something. I'm going to add single point thread milling to my list of
skills.

Karl