Tapping broken taps (a.k.a. I'm all tapped out)

[dlavery]

I was recently asked to remove a broken tap from a plate of aluminum, where it had broken off in the bottom of a hole. Not wanting to manufacture a whole new part, I wanted to remove the tap without damaging the piece.

Larger taps (1/4-20 and larger) can be removed relatively easily. Tap extractors are available from Small Parts and other sources that work by sticking little “fingers” down the flutes of the tap and allowing you to back out the broken piece. For very large taps, reverse screws can be inserted into holes drilled in the end of the broken piece, and used to back the piece out of the hole.

But sometimes the tap may be too small for these to work. If you break off a small tap in the bottom of a 3/4” hole, you can use an old machinists trick to remove it. Steel taps will dissolve in a solution of alum and water. If the part you are working on is aluminum or some other non-ferrous metal, you can dissolve out a broken tap. Here is how:

Step 1 – Place about 1 quart of hot water in an aluminum or glass bowl. Do not use a cast iron or steel pan! Remember – you are going to use a solution that dissolves steel!
Step 2 - Dissolve alum in hot water.
Step 3 – Drop part containing broken tap in dissolved alum and water.
Step 4 – Heat gently (just below simmering) – the solution will work at room temperature, but works better when heated
Step 5 – After 1-2 hours, remove from water and shake out remaining pieces of tap.

Alum is available at chemical supply houses if you are going to buy a whole lot (i.e. you need to dissolve an entire ’68 Chevy engine block that has been sitting in your living room since last summer). Alum is used to make pickles, and is also available at food supply wholesalers if you are willing to buy 10 pounds at a time (you just want to dissolve the steel parts from your little brother’s bicycle). You can also find it in small quantities (1-2 ounce jars) in the spices section of grocery stores, if you just need a little to repair one or two pieces. In general, the solution is contaminated once it is used. So if you break a second tap a day later, make up a new solution and start again.

-dave

p.s. Important Lesson learned: DO NOT use your spouse’s expensive aluminum Calphalon pans as the “heating vessel!” The alum solution will very effectively remove the anodizing from the pans and leave bare aluminum. The pan is then useless for preparing food. This will be made clear to you in no uncertain terms. You will then have to take your spouse out for dinner at an extremely expensive restaurant. The total cost of this dinner will be at least four times the cost of a new Calphalon pan. You will realize, too late, that if you had just bought a cheap new pan to use as your “tap removal pan” to begin with, you would have saved a lot of money. Stupid you.