On Wed, Aug 05, 2015 at 03:37:02PM +0100, Peter Fairbrother wrote:
On 05/08/15 00:47, Robert Watzlavick wrote:
I need to tap some #4-40 holes in an SS 316 valve body (Swagelok
SS-4P4T) for a bracket. I'd really like to avoid breaking the tap so
any tips? I usually use SS 304 and aluminum so I haven't worked much
with the harder stainless steels. The tap is HSS and I was just going
to use Tap Magic for the cutting fluid. If I have to, I can drill all
the way through the body and put a nut on the other side but I'd rather
use a threaded hole.
If you can find them, serial taps - these are a set of three taps, but
unlike the normal taper/plug or taper/second/bottoming sets, the first
and second taps are undersize: the first tap only cuts part of the
thread diameter, the second tap cuts a bit more, and the third tap cuts
the full diameter.
You have to do it three times, but each operation is three times easier.
They are reasonably available here in the UK for metric threads, but I
don't know about Imperial threads in the US.