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David Billington

wrote:
On Sun, 22 Apr 2012 22:18:09 -0700, "anorton"
wrote:

wrote in message
...

To all you folks who are actually making parts from metal,
I use form taps, AKA roll taps, as much as possible. I use them for
not just the easy to tap materials such as 6061 and mild steel. 304
SS, 17-4 SS, 4340 and 4140 steel are metals I have had good luck with.
Die cast aluminum alloys too, though I don't know what specific
alloys. I have also tapped sand cast aluminum parts but only a few
times and I don't know how well the threads held up. But now I'm doing
a lot more sand cast aluminum parts that are various alloys, 356 being
one of them. I would like to form tap all the cast parts and I'm
looking for advice from anyone here who hasreal world experience with
sand cast aluminum parts that have had form tapped holes. I want to
give the customer the best threads and even though formed threads are
usually better I know that sometimes the formed material can become
brittle from the forming operation leading to weaker threads. I have
contacted a couple tap makers with this question but they have not
given me any information beyond saying that I need to determine what's
best.
Thanks,
Eric

If thread strength and durability in aluminum is really an important issue,
you might want to ask your customer to modify the design to use helicoils.

Many of the castings I machine do get thread inserts. In fact, that's
part of the reason I got some of the jobs. I convinced the customer
that I could install the hundreds of heli-coils faster and cheaper
than they were doing them. I use a tapping head to install them. But
even the parts with thread inserts would benefit from stronger
threads. So I'll be using Bob's idea and measuring the pull out
strength of cut versus formed threads for each particular alloy that
I'm threading.
Eric

Possibly you should look at the likes of Keenserts or Timeserts as I
believe those provide a stronger installed thread than a heli-coil. It
may have been one of those that a company I worked for used in a
magnesium alloy casting for a racing gearbox and as it would be
dismantled regularly a thread insert was a must, IIRC the inserts used
required a more complicated machining than heli-coil and they were
intended to be replaceable if they became damaged, also due to the wall
thickness the thread in the alloy had a greater diameter and so was
stronger.