Damn broken tap

Alum

Harold

IIRC, nitric acid dissolves steel and not aluminum. Test it.

I worked at a place making electronic chassis's and installing parts on them. We had a broken (tool steel!) tap in an aluminum chassis, and sent it to an old machinist we use (by "old " read 75 yrs and still working, salty, and excellent") to get the tap removed. He called us later that day, and said it was fixed. I asked him what he used to get it out, and he told me that he aligned his bridgeport very carefully with the hole and drilled it out with a carbide bit. It was a 4-40 tap!

That's what I use. It works extremely well. I haven't tried Harold's alum idea, but obviously that would be much easier to get. DON'T try hydrochloric acid Tim.

Mark Rand RTFM

hydrochloric

I haven't used the alum trick either, but remember reading about it some time ago.

Nitric doesn't attack aluminum, but it does attack copper and zinc, so there is usually a minor change in physical characteristics in aluminum when exposing it to nitric, depending on the grade, of course.

Years ago I used to make hammer housings for daisy wheel printers for Univac (does that tell you how long ago?). They required a polished (buffed) face, which always left them greasy and grungy and not easily cleaned because they had been tumble deburred for a uniform appearance. Nothing I had available would clean them without a ton of work, so I tried placing them in an evaporating dish with dilute nitric and slowly heated it to see what would come of it. Surprisingly, they came out beautifully, totally free of grease and discoloration, but I had to be careful to not leave them in too long because of a .001" tolerance dimension that was easy to drive undersized, thanks to the minor dissolution of some of the metal.

I fully endorse the idea of not using HCL, or H2SO4, either, both of which react violently with aluminum.

Harold

I've used it twice to remove broken taps. Once in a copper block and once in an aluminum block. It takes time, but worked great.

I submerged the metal blocks in a beaker of hot water and alum on a hotplate. This was a stirring hotplate so the solution kept circulating. I don't know the optimum ratio of alum to water. I think I used about 1 tablespoon of alum to 250 cc of water. It took several days. All the time you could see a stream of bubbles coming from the broken tap. After several day I removed the block, tapped it on the table, and out fell what was left of the tap. The tapped hole was fine.

hydrochloric

Hehe, I already know it burns up pretty quick. And doesn't do much to steel anyways!

I'll try the alum. They'd have it under spices at the store, eh?

And speaking of home remedies, vinegar+salt worked real nice on a rusty chunk of angle iron. (I'm finally building some sort of brake, the angle is for the hold-down/folds-against part, and the subject of this thread, I was tapping the ends of the bending arm (for the hinge leaf), which I cast 3/4x1" solid aluminum, extrusion and plate alloy for anyone interested.)

Tim

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