On Tue, 24 Apr 2012 10:28:26 -0400, Ed Huntress
wrote:

On Tue, 24 Apr 2012 07:13:43 -0700, wrote:

On Mon, 23 Apr 2012 10:04:46 -0400, Ed Huntress
wrote:

On Mon, 23 Apr 2012 06:53:23 -0700, wrote:

On Sun, 22 Apr 2012 20:20:11 -0700, "Bob La Londe"
wrote:

wrote in message
om...
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


Makes some parts, tap them with a couple different processes and clearances,
then put a bolt in them and twist with a torque wrench to see when the
threads strip. Use the best compromise between machinability and real world
strength requirements.


I think that's a great idea. I'll ask the customer today for some of
the pieces cut from the castings and test them.
Thanks,
Eric

Because you wanted someone with experience tapping 356, I didn't
respond earlier, but the other issue you mentioned is that there are
some different alloys involved.

I haven't personally tapped aluminum casting alloys but I know you're
going to find some big differences in their ductility and, thus, their
response to form-tapping.

356 is moderately ductile for a casting alloy; A-356 is much better.
Neither of them will be as good in that department as wrought 6061.
The high-silicon casting alloys will be problematic.

In other words, if you're going to run some tests, and you're going to
be dealing with a variety of alloys, they could respond very
differently to form-tapping.
Greetings Ed,
I spoke with the customer yesterday about test pieces. I will be
getting test pieces made from the various alloys I will be tapping.
Some bronze alloys along with the aluminum. I'm pretty sure that the
356 alloy will be better off cut rather than formed but I've been
surprised before. Bob's good idea of actual pull out torque tests will
be put to the test.
Eris

That kind of practical testing is always a good idea, and there are
enough variables involved that predicting results would be tough in
any case.

It sounds like you have the right plan. BTW, the engineers at Emuge
have been helpful to me in the past (10 years ago) in writing
articles, and they know their stuff. Maybe you'd want to call them to
get some perspective: The testing you're planning sounds like the best
idea, however.

http://www.emuge.com/taps/
Thanks Ed, I'll give them a try. I've used Emuge taps plenty of times
and like them. They do make good taps. Even though I will be relying
on my real world tests I'm sure that getting info from the engineers
at Emuge will help.
Eric