# Dissolving a Broken Tap in Aluminum or Brass



## rake60

I had read several notes on this subject and decided to try it for myself.
A broken tap really can be dissolved from a non ferrous part.
Its a simple but time consuming method of simmering the part in a 
saturated solution of alum in water. My test piece was a scrapped aluminum 
flywheel with a 2-56 tap broken off in an angled hole through the hub that was
intended to accept the set screw.  After simmering in the alum solution 
for about an hour and a half the outer edges of the tap turned to a rusty sludge 
and it pushed out of the hole without any damage to the part.

For the size of the stock and time it took to make my test piece this process 
would have been more or less a waste of time, but I had to see for myself
if it would actually work.  It does quite nicely.


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## Hilmar

Hi Rake,the tap was it HSS or Carbon. Will it make a difference what it is?
I heard it works with carbon only
Hilmar


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## rake60

I honestly don't remember what that tap was.
It was broken off some time ago.  I did find another piece of 
aluminum that I know for sure has a HSS tap in it.
I'll try it when I get a chance.

Rick


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## Bogstandard

Hey Rick,
You got a supplier of bits with broken taps in them?
If so, can you give us the web address so I can get some, and try out the process.
(Just jokingly rubbing it in a bit).
BTW, what the hell is ALUM, you have some funny names for things in the US, there must be a UK equivalent name.

John


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## rake60

Alum is "aluminum potassium sulfate" 
Here its commonly used as a pickling agent in, well pickles.
This post from a UK clock site, suggests using it to remove broken screws
from brass. Removing Broken Screws
It says that in the UK Alum is used to treat bed-sores. 

My chemist son was home from college for the weekend and I was 
discussing this with him.  His explanation was it causes a rapid oxidation
reaction in ferrous metals.  My reply to that was, "Oh I thought it made 
it rust away real fast" 
That drew his usual _"I give up"_ grin.....  :lol:

As for my source of broken taps John,  of course they are purchased.
Why Ive never broken a tap in my life!





What No Lightning Strike???  LOL

Rick


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## Bogstandard

Thanks for the info lads.
It seems like you use it for pickling vegetables, we use it for pickling people. That explains our government then, they must have all got bed sores on their heads.
Just got to get some bed sores now so the doctor will give me a prescription to get some. 
I get free medication, so why pay for it.

John


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## sidecar_jon

I've got it from a normal high-street chemist before now, small packets of white crystals. I was using it to grow bigger crystals for my own amusement.


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## J. Tranter

Can you tell me wat the mix ratio for alum to water is. I broke a tap in a part that I have allready made three times and I really dont want to make it again.


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## rake60

It has to be a saturated solution.
Boil the water and add alum until you can't get any more to dissolve.
You only need enough water to keep the part submerged.
You do have to keep an eye on it and add water as it simmers off.

Rick


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## ChooChooMike

Daaaaaaaannnnnnnnnnggggggggg :'( Wish I'd know about this tip 5 mos ago. I busted off a tap in a brass block. My fledgling attempts at EDM'ing it out gave less then stellar results. Hole came out too big (copper probe I had made was too wide) and not deep enough. 

I wound up trashing the part after some other dismally-related tries and pathetic fixes.

I could have used this technique and saved the day !!! So-to-speak ... 

Mike


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## kellswaterri

Hello all,
check out my post on ...

http://modeleng.proboards20.com/

to my knowledge this process works with any ferrous material embedded in non ferrous material, broken taps, drills, screws, e/t/c...quite why, I have not been able to find out...
      John.
(SEARCH under post...REMOVING BROKEN TAP...reply no 7..2/21/06)


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## Swede

Interesting and useful tip. One thing you DON'T want to do is to try a DC salt/acid type of setup. I don't remember where I read about it, but essentially it involved using DC power and some sort of acid (I believe diluted nitric) in an attempt to dissolve/erode a 5-40 HSS tap from a precious aluminum crankcase that had already seen perhaps 40 hours of work, and was near completion. The process eroded the tap (slooowly) but it also egged and eroded the aluminum.

I finally got rid of the tap this way:  I chucked a new 1/4" carbide center-cutting end mill. Centered over the hole, I then moved the table slightly less than 0.125" to present the flutes, rather than the center of the cutter, to the broken tap hole. I plunged that end mill straight down and was amazed when that carbide end mill cut through the remnants of a HIGH SPEED STEEL tap like butter, with little chatter or fuss. I ended up with a .250" hole which I then plugged with a press-fit aluminum slug in a press, and redrilled and tapped the hole. The repair was invisible.

One way I've found to get through noxious materials like stainless, some aluminums, inconel, etc, is to alternate a taper and plug tap until you are through. Start with the taper, and when the forces get borderline scary, back it out, and then use the plug tap. Go back to the taper tap, and repeat. The two profiles complement each other, and while tedious, it beats a broken tap anyday.

Now all I need to do is find some alum for a rainy day!


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## Steve J

tried it today and it worked fine. 2 oz of alum in a 6 x 6 pyrex pan and about an inch of water. three different sessions of about an hour and a half each time and kept it at just a simmer. you could see the bubbles coming out of the hole as it simmered. little sludge in the pan and the tap (4-40) is gone! saved me about 3 hours to make a new one. the part is an aluminum connecting rod for a panther pup 4 cyl. thanks guys.steve


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## gilessim

Swede, thanks for that tip about the stainless!, I'm just encountering that one today, just off to replace one broken tap that I managed to get out with a centrepunch!.

Giles


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## alan2525

I'm feeling more at home here by the minute!

It's good to know that there are others here with draws full of odd shaped pieces of brass festooned with the remnants of broken off taps sticking out like a hedgehog!

I'm always under the impression that if you muck something up it's because it's a little difficult and if it was too easy every joe would be doing it. 

If you run out of space in the draws, for smaller components you can always push them into a potato and throw them over your neighbours fence!


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## kvom

Today I went to the drugstore to get "alum" for dissolving a broken tap in a brass piece. What I got was ammonium alum. Will this work the same as potassium alum?


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## Holescreek

Thanks for the tips guys! I just sent a link to a friend of mine with a freshly broken 6-32 tap in a chunk of aluminum. I have never tried the chemical route before. Last time I took a tap out of alminum I did it with an arc welder. I wrapped tape around the sides of the rod and stuck it down into the hole and let it attach to the tap. The initial blast of heat loosens the tap then I unscrewed it with the welding rod. I have also done this with a mig welder. -Mike


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## Holescreek

I waited a bit to see if my friend would post about trying the alum but I guess not. He went to the local grocery store and picked up a box of alum in the spice aisle, mixed it it water to saturation and immersed the aluminum part with the broken off 6-32 tap in it. Next day, nothing had happened. He went back to the store and found another box marked "pure alum", same process, nothing.

Any one else tried to use the store stuff yet? -Mike


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## rake60

This is what I buy:






It has to be a saturated solution left on a simmer over low heat.
It usually takes 10 to 12 hours.
I do that on an old electric hot plate on the back porch
since my wife isn't real happy about her kitchen smelling like
alum.

Rick


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## shred

I've used that same stuff from the spice aisle on a #50 drill that snapped off in some brass. It took a couple days to get it loose enough to pick out, but I didn't heat it beyond the initial over-saturation part (loose alum crystals hanging around in the liquid doesn't seem to hurt anything and that way you know it's saturated)


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## Tin Falcon

alum is also used as water treatment in swimming pools
http://www.lesliespool.com/Home/Pool-Chemicals/Water-Clarifiers/14070.html






a lifetime supply for under $20
Tin


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## Ken I

I have access to a couple of toolrooms with EDM that do commercial work for me and generally remove broken taps for free.

The electrode only needs to be as big as the "core" diameter of the tap to break the tap up. It is a mistake to use a "tap size" electrode.

You can spark on "thermoneuclear" settings since you are not concerned about finish so it is very quick - just a couple of minutes.

If I break a tap in a part at funny compound angles (I always tap in the machine for tap alignment) which might be arkward to reset - then I remove the part c/w vice and take it down to my friendly EDM shop - it saves on set-up time.

Regards,
      Ken


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## kendo

Hi Guys
      Recently i broke a 6-32 tap in a crank case, and really didn't fancy
      having to make it all over again, as a lot of work and hours went into it.

      So i took the advise of the members here, and used Alum which i 
      purchased from a local taxidermist.

      The result was amazing, the carbide tap just dissolved leaving the thread 
      in tact, and i was really quite surprised that it only took about three
      hours.
      OK it left the ally a little discolored but nothing that wont polish out.

      So thanks guys for a great bit of advise, and i have no doubt i will 
      to used this method again some time in the future.


         My Very Best Regards
                 Ken


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## Swede

I was all set to reply to this thread, then found that I already had, years ago, as a "guest"!  :big:

I STILL haven't tried alum, fortunately haven't broken a tap in years, because I am gun-shy with taps and go very carefully.

One thing I've found that really, really helps is the natural tendency to use a tap drill size that is designed to deliver a 75% + thread. 98 times out of 100, you can go with a 60% to 65% thread (a slightly larger tap drill) with total safety and better success. There are tap drill tables out there that list more than one tap drill for a given tap, with the appropriate thread % next to each drill size. Very handy to print and have on hand.


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## itowbig

rake60  said:
			
		

> This is what I buy:
> 
> 
> 
> 
> 
> 
> It has to be a saturated solution left on a simmer over low heat.
> It usually takes 10 to 12 hours.
> I do that on an old electric hot plate on the back porch
> since my wife isn't real happy about her kitchen smelling like
> alum.
> 
> Rick



ha ha i just tried this today . i had a cylinder that took me forever it seemed like to get right then bam i broke my only 2/56 tap off. so off to the store to buy this very same alum and i put it in a tuna can with water about half of the bottle and simmered it for three hrs. let tell u it was all gone. just to make sure i drilled the hole out again nothing in there man was i a happy camper. IM a beleiver now for sure .
THANK YOU


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## mhh

Does this only work on small taps? would it take to long to dissolve.. say a 10mm tap??!


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## Lew Hartswick

itowbig  said:
			
		

> i put it in a tuna can


You got to be careful about the container, it's steel also and you just discovered what
the alum does to steel. 
  ...lew...


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## AndyB

Brilliant! Thank you very much for this. :bow:

For the chap who bought his at the grocer's; check the spelling.

ALLIUM is the family of onion and garlic. I don't think garlic salt will quite cut it ;D

Andy


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## Swede

Something else that's worth mentioning, although it may apply only with imperial tooling...

You can use 4-40/5-40, 6-32/8-32, and 8-32/10-32 taps together. For example, let's say you have an 8-32 hole that is scary; deep, full thread, or bad material. Go in first with a 6-32 tap. Being undersized, it'll cut a tiny bit, but it certainly helps when the 8-32 tap goes in next.

In other words, drill for an 8-32 tap, cut it first with a 6-32, then finish with the 8-32. Same deal with 10-32, start with an 8-32 first. Obviously, you must have the same pitch between the two sizes. I don't know if this will work with metric, but it might.


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## Ken I

Swede, I read a book on threads once that stated those overlapping SAE series threads were conceived with the idea that you could replace / retap a thread by going up one size.

The larger sizes are sometimes refered to as "boiler threads".

Ken


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## Lew Hartswick

Swede  said:
			
		

> Something else that's worth mentioning, although it may apply only with imperial tooling...
> 
> You can use 4-40/5-40, 6-32/8-32, and 8-32/10-32 taps together. For example, let's say you have an 8-32 hole that is scary; deep, full thread, or bad material. Go in first with a 6-32 tap. Being undersized, it'll cut a tiny bit, but it certainly helps when the 8-32 tap goes in next.
> 
> In other words, drill for an 8-32 tap, cut it first with a 6-32, then finish with the 8-32. Same deal with 10-32, start with an 8-32 first. Obviously, you must have the same pitch between the two sizes. I don't know if this will work with metric, but it might.


That sounds like a slick trick especially for the 4 to 5 but I wonder (haven't looked yet) how much the jump of 6 to 8 or 8 to 10 actually takes off. I somehow doubt 
it more than just barely touches the bore. 
 Good thinking though.
  ...lew...


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## Mainer

Well, a #8 screw is (theoretically) 0.1653" in diameter and uses a #29 tap drill 0.1360" in diameter.
A #10 screw is 0.190" in diameter and uses a #21 tap drill 0.1570" in diameter.

So, theoretically, if you run an 8-32 tap through a #21 taping hole, it's oversize by 
0.1653" - 0.1570" = 0.0083".

In practice, it will probably be something else by a couple thou.


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## Lew Hartswick

Mainer  said:
			
		

> So, theoretically, if you run an 8-32 tap through a #21 taping hole, it's oversize by
> 0.1653" - 0.1570" = 0.0083".


That is diameter so the depth of cut will be on the order of .004 inches.  I doubt
that is even enough to "guide" the next tap. 
  ...lew...


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## Swede

It doesn't do much cutting when using that trick, but it can make the difference once in a while. More importantly, maybe, it allows for straighter and easier tapping when you do move up in size.



> Swede, I read a book on threads once that stated those overlapping SAE series threads were conceived with the idea that you could replace / retap a thread by going up one size.



That wouldn't surprise me one bit!

I read somewhere else (maybe here) that of all the common Imperial threadforms, 6-32 is a bad design in that the diameter of the screw, vs the size of the "V", makes for a weak fastener. In other words, a 6-38 or 40 would be more proportional and stronger. 8-32 is fine.

Ever since I read that, I look at 6-32 fasteners with suspicion, even though I've never had one fail in a critical area!  ;D

I've often wondered why 5-40 isn't more popular. I have taps, dies, and such for 5-40, but fasteners are not common.

When the smoke clears, we must be honest and acknowledge that the metric thread system is infinitely superior.


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## AndyB

Hi all,

I have read in several places that the metric system is very definitely inferior from the point of view of the amount of bearing face on the thread, as well as a load of technical stuff that I did not understand very well so will not try to define.

It would seem that the metric system was devised for planetary measurement (maps) and did not take into account engineering principles (being as they were in their infancy in the early 18th century.

A certain little Corsican corporal decreed that metric would be the way of his world and so it was.
Being as most of Europe was dominated by this person, most of Europe followed his decrees. This was mainly to standardise lengths of measurement for transporting troops...the Swedish mile is about 10 English miles (yes, I know, he didn't get to Sweden, but it shows the principle) :

Another little corporal, this time from Austria, also dominated most of Europe for a time, and he too was interested in moving large numbers of troops, so was keen to keep the system of measurement. :

Those who applied the science of measurement to engineering kept to the imperial measurements, and did not happen to get invaded at the beginning of (19thC), and rebirth of (early and middle 20thC) the industrial revolutions that were fed by the neccessities of war.

People have always developed a thread pattern for particular uses, check your Machinery's Handbook for the Loewenherz Thread which uses a 53 degree 8 minutes angle, used for measuring instruments in Germany. It is otherwise based on the metric system which, in its standard form, obviously didn't perform to requirements.
The point here is that, in my opinion, no one system is superior to any other, but you use which you want for the purpose that you are putting it to. :big:

I am not a scientist by any stretch of the imagination but I am interested in figuring out why certain systems use certain configurations. 
If anyone can tell me why Whitworth uses a 55 degree angle and BSF and Unified (and metric) use 60 degrees I would be very grateful. It would appear on the face of it that it is just more meat on the thread but it may well be more than that. ???

Another thing, and I can't find it anywhere because it must too universally known...what do you mean when you say 6-32 etc? I realise that 32 is the TPI but what does the first number relate to please? I am building an engine to American plans using BA threads. The first number is about the same as the BA size but not the thread rate... ??? I can see that the measurement is a decimal fraction of an inch but is the number just a standard or is there a reason? Also, what happened to 7, 9 and 11 ??? ??? ???

The only reason that I don't mix thread systems on a build is the pain of trying to find the right spanners when putting it together or taking it apart. 

Andy


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## kustomkb

> 6-32 etc? I realise that 32 is the TPI but what does the first number relate to please?



6 is the diameter. (6×0.013)+0.060=0.138 nominal OD. 0 is 0.060 diameter, IE 0-80 and more zero's are then subtracted from 0.060. IE 00-120 is 0.060-.013= 0.047 nominal OD.


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## AndyB

Thanks Kevin,

Oh bugger...that means I have got to learn to count more than taking my shoes and socks off... :-\

I can manage fractions of an inch because that makes sense to me, I can picture 1/64's in my mind.

I can see why some people want to stick with metric :big: but I can't picture the increments in my head...it's the way I was raised I suppose :

Andy


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## Swede

Interesting post, Andy. I guess when I say metric is superior, it is in a sense of understanding what it all means. A 6mm x 1.0 screw is self-explanatory. And metric socket head screws also tend to be more intuitive when reaching for a fitting hex wrench. But from an engineering standpoint, they all differ depending upon the application.

Your question re: the imperial system is very valid. Theoretically, you can have #'s 0, 1, 2, 3, 4 etc hardware, but the system tends to skip numbers for simplicity's sake. I've got boxes full of imperial taps and dies that date back a hundred years, and there are examples of #3, 5, 12, and other sizes that are very uncommon, including interesting fractional sizes like 3/16". I do use a lot of #5 hardware because it is simply a nice size for model work, but the fasteners aren't common, or cheap. #12 also used to be more common, but today, in a typical hardware store, you'll find only 4, 6, 8, 10, and from there it jumps to 1/4" and up; we normally use fractional sizes above #10.

When converting threads in plans, perhaps the easiest way is to have examples on hand for comparison. I do a lot of metric plans, but I have imperial hardware, so I simply substitute with the closest fastener, always keeping in mind the strength, and also the available space if going up in size. A very typical substitution is 1/4" - 28 for a 6mm fastener.

With no examples on hand, you can simply print out charts of common imperial, BA, and metric threads, and it should be pretty easy to do.


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## AndyB

Hi Swede,

Thank you very much for the information.

I have a printout on the wall that I worked out (I had to take my wife's shoes and socks off as well to count that high :big

I have uploaded it to the file section, called Simple Converter Chart.

I spend most every other minute checking it! :-[

Andy


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