"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote in message
. 3.70...
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.

??? Didn't you say you already basically ruined as a regular tap ???

I have made taps out of bolts before. They work good for chasing a thread,
but only fair for tapping a smooth hole. I have made taps out of acme
threaded rod before. They work awesome in plastic, and horrible in
aluminum. Ok. I have some threaded couplers made out of aluminum I tapped
with one, but it was a nightmare.

For tapping with the mill I have a small 3/16 right hand boring bar and a
3/16 left hand boring bar that have a nice 60 degree point. I use these
with Mach 3 NFS threading wizard, or if I am being anal I create a helix in
CAD, and then tool mops relative to it in CAM.

I've never used an actual tap on one of my mills. I do use a small tapping
head on one of the drill presses though. Nor have I deliberately broken a
tap... the truth of which is not necessarily born out by the number broken
taps I have thrown away...

If any of that helps... Yeah! I am a hero. If not, then I'll waive the
consulting fee.