Broken tap removal

Nitric, straight from the bottle

Roy, you could do the job with nitric acid, but good luck trying to buy some! There is a kit you can buy which will do this for you. The kit is called "Tap Out" and the only place I know for sure where to get it is at Swift Tool, a machinist supply house local to Seattle. I haven't had any luck finding it online, sorry.
Grant

Nitric won't dissolve aluminum, while other acids (H2SO4 or HCl) will. It will etch the alloying elements in the stuff, however, so if you have critical sizes, that may not be in your best interest.
I used to use nitric to clean 6061-T6 aluminum parts after buffing. It was the easiest way to remove the buffing compound. I found sizes changed by a tenth or two. My parts were placed in a dish of tech grade nitric and water, maybe 25% acid, 75% water, and heated to below boiling. You should be able to do the same thing with a broken tap, but allow plenty of time for the tap to dissolve. It may not be very fast.
Harold

It's actually better to include some water (hydronium ion).
Harold

According to RoyJ : Others have covered that part of your question, but I have to ask whether you have tried the broken tap extractors.
For No. 6 taps, they are made in 2-flute, 3-flute and 4-flute sizes, though you can use the four-flute one for 2-flute taps with care.
You need to grind the broken tap off close to flush with the surface of the workpiece (assuming that it projects outside the hole), or to grind it flat inside using a small mounted grinding point in a Dremel or equivalent so the fingers don't shear off. (Be sure to read the instructions which come with it.) I've got a built-up set which covers from #4 screws up through 1/2" at least. The set came from an eBay auction, which was filled out (there were a couple missing) and extended to the smaller sizes from MSC purchases.
I think that the brand is "Waldrom", but the set is downstairs, and the MSC web site is not happy at the moment. :-)
I did recently use the set to remove a four-flute 1/2-13 tap which broke off in the workpiece -- and I replaced the tap with a couple of 1/2-13 spiral point gun taps for better results.
Enjoy, DoN.

The part is question is a simple 'U' bracket with set screws on either side as part of a student project. It was the last hole, he got overconfident. My guess is the tap drill size was too small. If it doesn't come out with tossing it into a beaker of acid, I'll just have him remake the part.
D> According to RoyJ :

Go down to an old style drugstore and buy some alum. It'll be in the aisle with the boric acid and epsom salts.
Get a beaker with enough volume to hold your part. immerse your bracket in a saturated solution of alum in water and heat to around 150-180 deg f. You will soon see bubbles coming off your tap. Add enough water to counter evaporation. Continue til the tap dissolves.
It is cheap, safe, reasonably non toxic, and (best of all) you don't end up with yellow nitric acid stains on your fingers.
Paul K. Dickman

According to RoyJ : Always. :-) (Though actually, the 1/2-13 that I broke recently was the first of two holes -- and I had had the tap for a long time, so it may have gotten dull, too.
The 6-32 is probably the easiest tap to break in the common sizes. If *you* assigned the part, then you really wanted to test his threading. :-)
That depends. Some aluminum alloys gum up rather easily, and if you don't reverse (assuming a normal tap instead of a spiral-point "gun" tap) you do run the risk of breaking it more easily than when tapping a nice steel like 12L14.
O.K.
Enjoy, DoN.

way down to "nitric acid" (not alphabetical list). Many different concentrations. NOT cheap & subject to hazmat shipping charges.
Bob

Heh, you think I get to OK every detail with 18 teams working on prototype projects? Nope, just get the "what do I do now? part. It gets real hectic toward the end of the semester.
Now that I think about it, it was probably an 8-32. Didn't seem to be real relevant at the point where I was looking at it. And all I was looking at was the flush stub of the tap.
Oh well, back at it in the AM.
D> According to RoyJ :

"Paul K. Dickman" wrote in news: snipped-for-privacy@news3.newsguy.com:
I agree Paul!
I'm one of the posters from the past that used aluminium ammonium sulfate to remove small (2-56 and 4-40 taps and tap drills) from aluminum parts my students were machining. These were small and took a week (weekdays only) in a heated bath of water and alum on a stirred hot plate. I use lab grade aluminium ammonium sulfate (alum) from our chem stock room. It worked. I tried the same thing with alum from the grocery store's spice rack recently and it didn't seem as effective (I was also in a hurry too).
Ken

According to RoyJ : O.K. You have about as much choice as the students do. :-)
Hmm ... any of the break angling inside the hole, or is it all right at or above the surface. If the latter, the broken tap extractors really can do a good job -- if whoever uses them takes the time to read and *obey* the instruction sheet which comes with it.
Anyway -- tonight it is early enough so I've gone down to the shop to get my set and check the brand. It is "Walton", and my collection goes from #4 (both 2-flute and 3-flute) up through 5/8" 4-flute only in my collection). The #8 one also is specified as being for 4mm metric screws. The smallest size which *officially* fits in my set is the 5/16" 4-flute. All of the smaller sizes sort of rest between the larger ones in their fitted pockets. The set (not quite complete) appears to have cost me $25.00 at some form of flea market with four or five of the official six present. Filling out the set certainly cost me as much as the set if there were two missing -- not counting what I added in the smaller sizes.
Good Luck, DoN.