I've been "practicing" with mill threading with a tap. At least for now,
I really can't see dropping a couple of hundred bucks on a 'proper'
threading mill bit, so I've been fooling with a modified tap. I do this
about once in a blue moon. But I have a project that requires a lot of
1.5" holes to be tapped 1" deep at 10tpi.

I started with a 3/4"x 10tpi tap, and ground off all but one flute.

It works - ehh... - OK, but the surface finish sucks. Partly, that's
likely due to having to mill conventional, since the lead of the threads
is in that direction.

I'm thinking it might also be partly due to the rest of the teeth on that
flute sort of "rubbing" the work, and maybe also due to the thickness of
the flute (so the teeth are slightly over-cutting the top and bottom
shoulders of the vee on entry and exit from the profile.

So I'm thinking of removing all but four or five teeth, and thinning the
flute -- but this is my only 3/4x10 tap/thread mill to work with, and I'd
rather not ruin it just trying out an idea.

Any of you folks fooled around with doing thread milling by modifying a
tap; and does it do decent work?

Thanks,
Lloyd