I've seen conversations about alum to dissolve broken taps , screws , etc in
aluminum ... how about in stainless ? 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 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
We are NOT amused ...
I used alum to dissolve a hss drill bit stuck in 304 stainless steel;
worked great. "Medical" stainless is most likely 316 or 304 so should
work. Keep it warm and stir/shake often.
Carl Ijames carl dott ijames aat verizon dott net
(remove nospm or make the obvious changes before replying)
Personally, I don't think that you can trust that chemistry in
SS. Aluminum is special.
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
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).
*That* I can understand.
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An update ...
I remembered I had purchased a small assortment of solid carbide drills /
mills , HF specials , and all very small . I managed to face the end of the
tap square with a small carbide mill , and only broke three bits center
drilling the tap (all in the .030-.039 range) . Got the tap broken up and
removed , reground the end of the broken one , and promptly broke it again .
I've never had good luck tapping stainless ... and that piece is now back in
the material bin .
Okay , this has been a learning experience for me , and the next attempt
will be with a different material . I have on hand some O1 drill rod , some
4140 , and some 1018 crs .
I'm thinkin' the 1018 for my next try ...
wannabe machinist and tap breaker extraordinaire
Stainless is a bugger to tap. It's gummy; it produces a lot of friction
against a tool; and, because of its poor conductivity, the hole actually can
shrink in diameter as you tap it (or drill it, for that matter), depending
on how much mass surrounds the hole and constrains expansion, and thus
whether local heating at the cut interface expands the hole or shrinks it.
I hate working with it. But there are some experts around who know exactly
how to handle it.
I've heard of roll form tapping in stainless steel.
I'm scared, and I'd guess others in here are, as well.
Please share some details.
I've done it in sheet steel, used to production tap electrical boxes.
Thousands of them.
I've heard it is great in Aluminum, but I've never tried it.
SS chips are a huge problem when drilling, milling turning or tapping. They
are work hardened and "sticky". They do the same thing to a cutting tool
that they do to human flesh - slice it up. Ever notice how easily Stainless
chips cut your hands? It's because the edges are work hardened and don't
round off from the heat the way tool steels will.
300 series Stainless (Invar as well) is extremly ductile and form taps are
very strong by their very nature. The flutes are very shallow.
The coating on the tap is important.
ALTiN - C20 is a coating specifically developed for stainless.
It's different than the C-30 grade and if you buy OSG coated form taps, for
example, be sure and tell your tool vendor that you are going to be tapping
SS or A286 with them and that you want them coated appropriately.The C-20/30
desognation is brand specific and I don't know the chemistry.
Hole preparation is critical.
First, carbide drills in Stainless really aren't that useful without being
properly coated. They both gall and chip and the edge also isn't really
sharp. Cobalt is best unless you want to buy something like the MA Ford
drills I linked to and the drill geometry I've had the best luck with is a
142 degree split point.Titex and Guerhing make similar products and they are
terrific. They last a long long time and make a nice clean hole. They are
also pretty expensive. A tap drill for a 1/4-28 form tap from Ford in their
XT series in 80 bucks or so. Cobalt is about $15.00, maybe less.
The hole should be to the proper size, depth, and be STRAIGHT.
You want the hole to be on location so that when you tap, you are entering
with a balanced load. Otherwise you rub one side of the hole hard and things
go down hill from there - the tap will push towards the soft side of the
hole and bind.
I'll normally spot drill - not center drill - with a 90 degree tool to a
diameter that's ten percent larger than the OD of the tap. That is important
also. Don't try and tap a hole in any material without chamfering the hole
The only reason to use a form or roll tap in Aluminum is chip control and
that's a very good reason.
Cut taps, except for taps like the OSG chip extracting type, force the chip
in front of them and pack the hole.
You have to go slow enough to allow for proper chip formation and evacuation
and you must also be certain not to approach the bottom of the hole to
closely. The problem with the chip extracting tap geometry is that it is
weak dur to the deep fluting required to force the chips up the tap. I use
them a lot in aluminum because I perfer cut threads to formed but my real
preference is to thread mill unles there is either a requirement or
specification precluding it.
Ther - metalworking. Bet you thought I couldn't do it!
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