We use thread forming in 6061-t6 all the time where I work, and the
threads are quite strong (even when the hole was a little oversize).
What hasn't been mentioned is that as the hole becomes larger, the
forming tap creates a pair of threads, or a double start situation. The
second start is not purposely made by the tap, and is not as smooth, but
in some cases is just as likely to be the thread the screw starts in as
the intended thread. It is annoying, because the unwanted thread binds
up much more than the intended thread, but sometimes the intended thread
is harder to find, and in a production situation you don't have the time
to be trying to find the right one.
Usually even in the unwanted thread, if a 4-40 slot head screw grabs
3 turns, the 6061-t6 will hold better than the slot in the head of the
screw. I can crank it down very tightly.
On the issue of TOP/BOTTOM posting I must comment.
It seems to me that the subject line should give enough context for
people to know what the thread is about. If most people get enough
context from that, then quoting the relevant context from previous
messages below will help the one who tunes in without prior knowledge.
If context isn't needed, then I get to the relevant information first,
and I can move on. If I'm commenting on a specific issue, the whole
message is too much information. I just need to quote the two sentences
that apply to my message.
Here I'm only commenting on forming taps, so I don't need to quote
any of the previous information on cutting taps.
I understand the need for in-line posting when commenting on a number
of issues raised.
Bottom posting doesn't bother me, but it doesn't seem optimum. The
thing that does annoy me is when commenting on one small point one
quotes the whole 80 lines of text, when only 2 to 5 lines actually
provide the context for the comment.
Just my 2 cents.
>> >
>> > In all this fuss over how to machine tap no one has mentioned thread
>> > forming. The few times I've tried it by hand it has worked well.
>> >
>> > True, but one has to be very careful with threadforming if thread strength
> is important:
>
>
> http://machinedesign.com/article/internal-thread-strip-out-tests-yield-surprising-results-0113?page=0%2C0
>
> shows that thread strength is critically dependent on hole diameter when
> threadforming, much more so than for regular threading
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