I don't like using high spiral taps because they are more prone to
breakage. For blind holes I like form taps better. So usually if the
material allows it I will use form taps. But I had a job last week
that I felt required high spiral taps. The parts were cast silicon
bronze discs. By the time I got them they were already worth $720.00.
I then put another $300.00 or so labor into each. The castings were of
dubious quality. I thought I might have some tap breakage if the tap
hit a piece of sand deep in the casting. And form taps are really hard
to remove if they break in the part. Each part gets 25 3/8-24 tapped
holes. I used the high spiral taps(they really should be called high
helix) and one tap did indeed break about halfway to programmed depth.
I was able to use a 3/16 carbide endmill to mill away the web of the
tap and the three flutes just fell into the hole. I retrieved the tap
pieces with a magnet and hand tapped the hole the rest of the way. It
only took me about twenty minutes to remove the tap and finish tapping
the hole. So sometimes high spiral taps are best.
Eric