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 from . 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.
-- Regards, 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 the inserts.
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).
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 ...
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.
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 first.
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! LOL
One thing to watch out for with SS is work hardening. Depending on how you drill the hole, you can end up with a nice hard surface inside, which your tap will not appreciate.
Well , if I had some ... my supplier doesn't stock any leaded steel , dunno why . Of the three I have on hand (4140 , O1 , or 1018), which would you choose ? The article/plans I'm following in the HSM magazine used mild steel , no other designation that I can recall . I'd check the article , but I'm 165 miles from the house right now , coming to you from an internet cafe in Mountain View Arkansas .
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