Broken Tap Removal by chemical methods
I managed to break off a 6-32 tap in a 304 SS block last week, and searched for several hours trying to find an easy method to reclaim the part. The tap was in over 1/2 inch deep when it snapped and it was stuck solid! I know, 6-32 is a weak tap, and the hole was far too deep, but that is what the customer wanted. At least it was a through hole.
I finally resorted to chemical methods (Nitric Acid) in this case, and the part was saved without damaging anything other than my wallet.
I attached 4 files file with the details of the chemical method I used, plus some other methods that are possible. PLEASE read the "Read Me First" file and be careful if you try any of these chemical methods, since serious injury or death is possible if proper procedures are not followed. I also recommend downloading and following the MSDS procedures for safe handling and disposal of any chemical used. These can be found easily with a simple internet search (Google Nitric acid MSDS for example)
Last edited by Bob Butcher; 02-09-2010 at 05:51 AM.
Reason: added attachment
What, no pictures?;-) I'm sure you were more concerned about removing the tap. Very nice writeup.
Possibly combine into a single PDF?
For once, at least it was a post *about* how to remove a broken tap rather than *asking* how. No doubt it is just a matter of time before every one breaks off a tap as has to do something to save the work.
single pdf of chemical methods
I certainly could do just that, but decided for now to leave it separate since each method uses slightly different chemicals and techniques. What a prospective member might choose could depend on what chemicals they have access to.
There used to be a product called TAP OUT that would erode a tap without hurting the part. It worked well in aluminum and stainless, but if you used it in 4130 or 1018 it would also erode the part.
You comment that it should work in aluminium as well... It does, very well. I first used it when I snapped the bleed nipple off a brake caliper. Got a small quantity of nitric acid from the chemistry lab at work. Since then I've always kept a litre or two along with the other workshop chemicals.
Originally Posted by Bob Butcher
An alternative when you have a tap or drill broken in a none ferrous work pieces is alum (potassium aluminium sulphate or ammonium aluminium sulphate).
alum is relatively inoccuous if you get it on you.
Where does one get these chemicals?
Sounds like a great method for saving $$$. Where would one go to to purchase these chemicals? Are they that expensive?
cost of chemicals and sources
Note: I recently saw a list of incompatible chemicals, and Ammonium Persulfate is not supposed to come into contact with Nitric Acid. If anyone uses the Tap Remover.pdf formula be sure to follow the recipe carefully and dilute the acid after adding the copper, but before adding the Ammonium Persulfate. Always store these two chemicals where they cannot come into contact. If mixed in a concentrated form, the results could be spectacular, although not desirable.
I also located a patent online using a similar mixture for electrolytic etching of ferrous materials. The full patent can be downloaded from:
Electrolytic etching method - Patent 3905883
I attached an updated file with this information included.
I purchased the Nitric Acid from
Colorado Scientific Company
95 Lincoln St.
This is a local (well, 50 miles each way) company. The acid was $42.95 for one liter of Analytic grade. Commercial or industrial grade if available should be cheaper, and several sources can be found online. The shipping costs can be significant due to Hazardous material surcharges. It is rumored that local radiator shops may be a source, but I have not confirmed this. Nitric acid is available in several concentrations and sold under several names. What you want is approx 50-70% concentration to start and then you can dilute it with about equal parts water. It will still dissolve the tap if weaker, but it will take longer.
I ordered Ammonium Persulfate on E-bay for $5 per pound plus shipping of $7.50. The recipe (Tap Remover.pdf) using Ammonium Persulfate seems to be the best if you can get the chemicals. I am not a chemist, but I suspect the Nitric acid alone tends to passivate the broken tap, which explains why I had to repeatedly "tickle" it with the copper wire. I believe the Ammonium Persulfate tends to prevent or maybe remove passivation, causing the process to continue without so much fuss. If anyone can explain the process to me, I would be grateful.
The other chemicals mentioned in the replys to this post, including Alum can also be found on E-bay for reasonable cost.
Liquid Nitrogen is available from local industrial gas suppliers, but you need a cryogenic dewar to transport it, and there may be minimum charges, plus it will boil off fairly quickly. This option is better for a workplace or college lab where the Liquid Nitrogen is use for some other purpose.
Last edited by Bob Butcher; 02-10-2010 at 07:53 AM.
Reason: updated information, add attachment
I am not sure, but the Tap Remover.pdf formula may be similar to Tap Out. The gentleman who told me about this formula years ago claimed it needed to be mixed and used within a few days, but I have not verified this. I have never tried to store it for long times since I usually don't break taps all that often. I have never used Tap Out, but have seen kits available for about $50.
I don't have access to liquid nitrogen but I have used wart remover for freezing things like that. It is sold in the drug store and it comes in a spray can along with little applicators that look like cigarette filters. You are supposed to spray it on an applicator and then hold it on the wart for fifteen seconds or so. It is basically the same principal that a dermatologist uses to freeze off warts with liquid nitrogen in the office. Cheap and very, very cold. It stores in the can indefinitely too unlike liquid nitrogen. Also works great for shrink fits where heat is not appropriate. I have never tried to use it to shatter a broken tap because I have never broken a tap before. Yeah. Right.
Re: Broken taps in SS
While working at the university, in the Oceanography department, I did a lot of work with stainless steel parts of moorings and instrument hangers. The trick that one of the old timers told me that worked like a champ was to submerge the part in "Ospho," that green primer that is used on rusty iron. It is a mild mix of phosphoric and oxyalic acids. A #4 or #6 broken tap would turn completely to black goo in a day and a half, leaving nothing but shiny SS. It also worked really well to passivate stainless bolts and nuts, to keep trace amounts of iron from the cutting tools from starting rusty spots on the surface of the stainless. Try it and you will see that I am not loco.
How would this work in 17-4 PH 1025 stainless? I have a broken 6-32 tap in a part 3" deep in a counter bore and the largest diameter tool I can get to it with is 7/16" I haven't found a collet extension or anything small enough that meets the length needed to get to the counterbore face.
How long ??
How long does it take for the acid to erode a small tap ??
Is it a couple of hours, days ??
An old watchmakers trick to getting out, screws, taps, that were broken, is to use alum.
1. make up enough saturated solution to cover the part.
2. put solution and part in a glass container
3. apply heat to bring it to steaming hot, but not boiling.
You know it is working when you see small bubbles coming up from the tap.
This method works on non ferrous material, do not use on anything containing iron, it will eat it.
If taking out, for example, a 4-40 tap, it might take about 6-8 hours.