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Broken drill bit in hole

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Garry Coles10/12/2019 17:37:53
52 forum posts

Hi, angry I have just broken a 1.6 mm drill bit in hole that was to be one of the two threaded holes for the fire door hinge in my copper boiler. The first hole was all good. But the second hole was going alright, then snap. no I was taking it very slowly as I know that copper can be a bit grabby.

Can anyone give me some ideas please on how to get round this problem. frown I've snapped the odd drill or tap over the years, but never this small. I've googled the problem, but they all seem to be about much larger bits.



Brian H10/12/2019 17:43:32
1338 forum posts
100 photos

Try soaking in an Alum solution. This will not dissolve the steel quickly by it will reduce the diameter to the point where it will fall out.

The solution works best with moderate heat and will not affect the copper.


Garry Coles10/12/2019 17:47:11
52 forum posts

Thanks for the reply Brian.

I saw someone using Alum, but that was on a steel part. I was not sure about using it on copper.


Brian H10/12/2019 17:58:21
1338 forum posts
100 photos

The alum will only work on steel or cast iron and will not affect the copper.


not done it yet10/12/2019 20:30:50
3774 forum posts
15 photos

My experience. I recently dissolved a 5mm or 6mm tap from what was to be a repaired brass feed screw nut. The old nut had been bored out and plugged with a solid brass plug before boring and threading and I was going to securely pin the parts together with a brass threaded screw as well as loctiting the assembly.

The tap was about 20mm, perhaps more, through the hole before breaking, and total thickness was around 25mm. Took some time to dissolve, needed as concentrated a solution as practical and required to be kept very close or at boiling point for any decent reaction rate. Afterwards I wished I had stuffed the reaction vessel in an old pressure cooker, to raise the temperature a lot more!

At one point I experimented a bit and used some small pieces of pure copper - to ensure as free a flow as possible of solution past the broken tap - to support the nut away from the vessel. Solution turned decidedly blue, so the copper chips were removed. Eventually the tap was dissolved virtually completely from the brass . Slow but effective.

So, it definitely works for dissolving steel from brass items. I would use it again, so am keeping an eye out for a sensibly priced alum supply. Small quantities on epay are expensive (used by hobbyist dyers, in particular). It is not aluminium sulphate (as reported on one thread). It is the double salt potassium aluminium sulphate (usually) - with an awful lot of water of crystallisation.

Neil Wyatt10/12/2019 20:44:58
16939 forum posts
690 photos
76 articles

You can get alum from middle eastern food suppliers really cheaply in 500g bags.


Dave Halford10/12/2019 20:55:45
521 forum posts
4 photos

Are there not two different Alums? One being useless


Ah NDIT got there first !

Edited By Dave Halford on 10/12/2019 20:57:17

Tomfilery11/12/2019 11:17:48
119 forum posts
4 photos


As you are making a boiler, and therefore have pickle for cleaning it up, just put your boiler in the pickle and leave it overnight. You will likely find your drill will have disintegrated, or can readily be removed the next morning.

I first tried this with a similarly sized broken drill in brass and, much to my surprise, found the drill was broken up the next day and basically dropped out of the hole. I used clean citric acid (i.e. which had not had copper in it) as I didn't want a layer of copper depositing on my brass.

Regards Tom

SillyOldDuffer11/12/2019 12:00:02
5002 forum posts
1061 photos
Posted by Dave Halford on 10/12/2019 20:55:45:

Are there not two different Alums? One being useless

Ah NDIT got there first !

Edited By Dave Halford on 10/12/2019 20:57:17

There are many 'Alums'! In chemistry Alums are a family of related compounds. They have different chemical properties, as unlike as cast-iron is from mild-steel, tool-steel, or stainless.

The word is also used loosely by various historic trades. The 'Alum' in tanning, may not be the 'Alum' used in Dyeing, nor is it the 'Alum' used in cookery, or as a fertilizer. Or perhaps they are! Alum might be the most misleading name in common use, only slightly more precise than 'thingy'.

The Alum needed for removing broken taps and drills is NDIY's Potassium Aluminium Sulphate. Although other Alum's are likely to have some corrosive effect on steel, so does water given long enough. My advice: accept no substitutes! Potassium Aluminium Sulphate is widely available, for example from Amazon.


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