On 05/08/15 21:24, Peter Fairbrother wrote:
On 05/08/15 18:39, Norman Yarvin wrote:
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.
Even in a work-hardening material like 316 stainless, where taking
shallow cuts can be more difficult than taking deeper cuts? Even for
small screws where the cuts are pretty shallow in the first place?
If you are getting work hardened cut surfaces then yes, a deeper cut can
cut behind the hardened surface - but a better [1] solution is to avoid
work hardening in the first place.
With sharp tools, correct feeds and speeds, and proper cutting angles
this can be done, even with practice on smaller, less massive and less
stiff machines, though it is easier on larger heavier stiffer machines.
Tools for 316 do have to be sharp. Sharp sharp sharp.
Proper cutting angles are also important, you need a positive cutting
angle of about 10-15 degrees, sometimes more. That way the cut swarf
bends, but the remaining material - the new surface - of the workpiece
doesn't deform, and so doesn't work harden.
The other requirement for machining eg 316 without work hardening is for
a definite cutting action, to avoid surface skidding or side-to-side [2]
action deforming the material of the workpiece. For lathes and mills
this means heavy and stiff machinery makes the job much easier - but for
taps the cutting action is already definite, because the tap can't move
side-to-side, and this isn't an issue.
If serial taps are sharp, I don't find much difference in the force
needed for the different serial taps - it doesn't get harder after the
first tap. Which I think shows that work hardening isn't happening, or
at least not enough to be a problem.
I have only used serial taps (in M3 and M5 sizes) maybe a couple dozen
times in the year or two since I learned about them - they take longer
to use, so I don't use them regularly. I have never had a problem or
breakage though, and I save them for when problems or breakages are
likely, eg in stainless, monels, inconels and the like.
[1] usually better, as far as the smaller workshop is concerned. If you
are making a million pieces a day then scraping a work-hardened surface
away with a blunt tool may well be more economic on tooling, which can
be blunt and therefore last longer, and on machine time, which can be
lessened with the larger cuts. But this doesn't apply to the likes of
me, and probably you as well.
[2] on eg a normal lathe turning cut, there should be no forces acting
to press the tool against, or pull the tool away from, the cut surface,
in order to avoid deformation and work-hardening.
This does not necessarily mean that the there is no in-and out force on
the tool, it has to bend the swarf away from the piece, and the main
cutting action usually involves forces with components which are not at
right angles to this; but there should be just enough in-and-out force
on the tool to do those. Plus the downwards force, of course.
-- Peter Fairbrother