316 isn't necessarily harder than 304 - I've often seen 304 grades that are
harder than 316 grades and vise versa so if you can tap into 304, the chances
are you can tap into 316.
If the valve body is a casting, the chances are it's actually not 316 but a
close variant with similar properties that can be identified as 316, but could
be a harder material than your true 316 stock material.
Tapping stainless in general: I generally drill the initial bore out wider than
normal to avoid tool breakage. When hand tapping, I'll rotate the tap a
revolution or so and then reverse half a rev or so to remove some of the
frictional stress on the tap and follow that process all the way to completion.
Use a good grade of tapping grease if possible or at least cutting grease. Make
sure your tap is aligned straight with the hole - preferably start the tapping
process in the same drill press position as the drilling process if you used a
drill press or fixed machine to drill the hole.
If machine tapping - tool life can be increased significantly by following
the suggested speeds and feeds that are practically difficult to achieve by
hand tapping.
Tapping small fine threads in hard stainless materials is hard especially by
hand.
Troy
-----Original Message-----
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On
Behalf Of Robert Watzlavick
Sent: Wednesday, 5 August 2015 9:48 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Tapping holes in SS 316
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.
-Bob