Forgot to mention: spiral point or helical taps break less often than
straight taps.
Also maybe look at thread-forming taps. Steep learning curve, plus
another learning curve for each type of tapped hole, but don't break easily.
-- Peter F
On 05/08/15 15:37, 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.
Usually all three taps are taper form (as well), but you can get sets
with all three taps in plug form.
You could perhaps make them by taking a normal tap and grinding the
outsides down a bit.
They are reasonably available here in the UK for metric threads, but I
don't know about Imperial threads in the US.
General advice, maybe teaching granny to suck eggs:
Like all taps, if you hold them straight they will cut better. The best
way is to clamp the piece, drill in a mill/drill press, then, without
moving the mill or piece, use a piece of thin tube in the chuck to keep
the tap straight. The tube mates with the point on the top of the tap -
if it has a center hole, use a point instead. Keep light downward
pressure on the chuck when tapping, the idea is to
For thin taps, HSS is both good and necessary - carbon steel and even
carbide are too brittle for hand work.
Back the tap off a turn every two or three turns, and don't be reluctant
to take it out and clean the tap and hole partway through - trapped
shavings are probably the second most common reason for tap breakage.
(going in squint is probably first.. or maybe to small a hole.. or no
lube.. or using carbon or "tungsten steel" taps for small holes.. or ..).
-- Peter Fairbrother