On 2008-08-15, Terry Coombs wrote:
I've seen conversations about alum to dissolve broken taps , screws , etc in
aluminum ... how about in stainless ?
Personally, I don't think that you can trust that chemistry in
SS. Aluminum is special.
Last step in making an indexable
carbide end mill was to tap the screw hole to 6-32 . Got about a quarter
inch of tap broken off in the hole now , and really don't want to start all
over .
I'm sorry to tell you this *now*, but I would have earlier had
you asked. 6-32 is the most deadly thread for breaking taps of any
standard thread. The problem is that you have three sequential threads,
10-32, 8-32, and 6-32 with the same pitch --- and down at the #6, the
thread is too coarse, and thus the tap is weaker (as is the screw) and
very likely to fail if the task is difficult. And tapping SS can be very
difficult anyway, depending on the alloy.
And I would suggest something like 6-40 or finer for this kind
of task -- if you are stuck with #6 screws by the size of the hole in
the inserts.
I'm not sure just what alloy the SS is , it was a gift , but likely
an alloy used in the medical field , since it was that kind of shop it came
from .
I would expect medical stainless to be one of the 300 series
ones, and to be particularly likely to get work hardening and otherwise
be difficult to tap.
How many flutes is the tap? Two, three, or four? I would guess
three in that size and probably a standard tap instead of a gun tap
which would be the better bet for a through hole.
There is a tap remover which often can do what you need, but it
has to be bought not just for the size but for the number of flutes.
The 4-flute will work for 2-flute as well, by just leaving out two of
the four fingers.
I'm trying to remember the name -- but it starts with a 'W', and
I have a set which covers quite a few common sizes. Waldom? Wilson?
Look in the MSC catalog under "broken tap extractors". If you order the
size you need, order a few spare fingers, since you are likely to shear
some off learning to use it, especially with this small a tap.
Anyway -- for it to work properly, you want the broken part of
the tap to stick above the surface of the workpiece. Then take a small
stone in a Dremel type tool and grind it flat to the surface of the
workpiece. Slide the fingers into the flutes and as far down as
possible (hopefully breaking out some of the chips which are holding the
tap), then you slide a collar down to hold the fingers into the grooves
of the extractor, and with a tap wrench work the tap backwards and
forwards a little to break the chips free a bit more, and eventually (if
you are lucky) back the tap out. This works best with a HSS tap, not a
carbon steel tap.
Or -- if you are really unlucky, the tap was a carbon steel one
and it is broken into a lot of pieces inside the hole, each locking its
neighbor in place. If it were a larger size carbon steel tap, you could
break the tap up and remove the pieces one at a time.
Or -- if you can grind a flat on the back of the tap and then
start a center hole there, you can use a solid carbide circuit board
drill -- in a sufficiently rigid setup and a fast enough drill press.
(I've done it with a sensitive drill press, which would handle up to the
1/8" shank of the solid carbide circuit board drills which I pick up at
hamfests and other flea markets.) With this, you drill out the web of
the tap, allowing the broken parts which actually cut the threads to be
picked back into the hole and fished out. It is fiddly work, but
do-able. Be careful to not break off the carbide drill in the tap, or
you will have something even *more* difficult to remove. :-)
I do wonder why you selected a stainless steel for this project,
BTW. I would have gone for a hardenable tool steel instead. Or even a
free-machining mild steel like 12L14 would be better than stainless
(other than in appearance).
We are NOT amused ...
*That* I can understand.
Good Luck,
DoN.
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