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...
On Sun, 19 Mar 2017 18:38:43 -0400, Bob Engelhardt
wrote:

I was rummaging through my tap collection and came across a couple
of
forming taps. I have no idea how they got there, but there they
were.
Now, the question is: where are they better used than cutting taps?
Since they are more particular about the hole size, I won't use them
unless they have some advantage.

Thanks,
Bob
Greetings Bob,
Form taps make a stronger thread in materials that are fairly
ductile.
So for example, wrought aluminum aloys, mild and stainless steels,
and
some copper alloys. It's a little scary but I have form tapped
thousands of 10-32 holes in 304 SS. You need to pay attention to how
many holes have been tapped so that the tap can be changed before
the
torque gets too high and the tap breaks. Free machining brass is not
a
good material to form tap because it is kind of brittle, but silicon
bronze form taps well. When tapping blind holes form taps are nice
because there are no chips to worry about. They are especially good
when the max length thread is needed in a blind hole. I use form
taps
as much as possible in my shop.
Eric

I got a 6061 aluminum part back that the shop had form tapped in holes
that were apparently too large, so the tap left a second false thread
groove between the real ones that a screw could engage. They were
small screws, #3 or #4 IIRC, and the torque difference wasn't all that
noticeable at first.
-jsw