"Dave D" wrote in message
...
I brought this up once before and got a lot of good advice. The
original problem was tapping 6-32 holes in side of 1/4" aluminum
round. Each piece has one hole. I use to do this by drilling the holes
on the press, then tapping with a power drill. Sounds primitive, but
it worked... On hundreds of them, no problem. Then it stopped working
and I never did figure out why. I can't get through 15 without
breaking a tap now and I've tried everything. The first time the
consensus seemed to be that it would be best to use a combo drill/tap
bit on a tapping head. I agree, except, I never got around to it. The
need for these parts hadn't happen since the last time... but I
decided I would do it that way when it comes up again. Well, it came
up again... Except this time the rounds already had the #36 holes
drilled in them... Hundreds of them. So I have no choice but to tap
them as is. Long story longer, I went through about $50 worth of taps
today just trying to get through 40 of them. And even just that took a
ridiculous effort.

So now what? I haven't tried it yet, but I'm thinking maybe opening
the holes to 7/64" would help? Or would that weaken the thread too
much? I don't know, I'm pretty much desperate for a solution at this
point. I've tried every type of tap, tap fluid, technique... I'm out
of ideas.

Did the problem start in the middle of a successful run, after changing a
tap?

This happened to me, rigid tapping in a VMC. Everything was fine, except
after someone broke the tap, and the new tap tore up the threads, no matter
what we did.

The consensus was that the first tap had a coating or finish conducive to
clean threads in 6061 Al.
With the new tap, the coolant in the vmc was not adequate, and the solution
was manually coating the tap between cycles with tap magic. There were
other tapping fluids people really liked, forgot what they were -- Moly D
might have been one.

You need to develop a sleuthing strategy -- mebbe try tapping other sized
holes -- 5-40, 8-32, see if you have similar problems, which would indicate
a material problem.
You should get a feel for manually tapping a hole, to try to assess if the
hole is "right", by how hard the hand process feels. Mebbe the material
changed unbeknownst to you, got gummy/****ty.

What's nice about tapping heads is that you can peck, with the safety of a
clutch.
Speaking of which, mebbe a cordless drill with a decent clutch should help
your breakage problem, in the meantime.

But a tapping head would hedge a few bets at once.
--
EA