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The Tap & Die Co is grateful to P Millerchip, Devon who has given a useful tip for removing a broken tap, drill or reamer :
"... have broken an 8BA tap in a brass casting and it is too small and difficult to drill out after all other means have failed (eg using grips to unscrew the broken bit, if protruding from the surface).
The casting can be saved with a little patience and household chemistry. - The remains of the tap can be easily dissolved away completely leaving the hole undamaged, using ALUM - Otherwise known as Potash of Alum or Potassium Ammonium Sulphate. This comes as a white crystalline powder in the purest form or a slightly greyish powder in its unrefined form. It is safe, inert and non-toxic. - Alum is used commercially in preserving foodstuff s, as drying agent , in treatment of athlete's foot and in cosmetics.
Alum can be bought from pharmacies and some food shops but this is not always the case these days - I purchased mine from a kitchen staore on E-Bay (about £3 for 300 grams).
METHOD: - Using a non-ferrous saucepan (aluminium or glass/Pyrex is best) - add boiling water from a kettle and stri in the Alum until no more will dissolve (a saturated solution). - Dont use a steel saucepan.
Immerse the casting in the solution so that the broken tap is covered and leave on a low heat (just below boiling point - this is essential). Top up with boiling water periodically so that the broken tap hole is always immersed in the solution.
A stream of 'fizzy' bubbles will emerge from the hole. - It may take 2 or 3 hours of simmering to completely dissolve the remnants of the tap, but eventually the hole will be completely clear and shiny new.
There is no nasty odour or chemical fumes. The ferrous material is just slowly dissolved away ! - That's it.
The length of time for the chemical to work depends on the size of the cross-section of the item to be dissolved.
NOTES: This method only works where the broken bit is lodged in non-ferrous materials - ie. where the broken tap is in brass, bronze, gun metal, aluminium.
The solution must be kept at just below boiling point - just a few bubbles rising and the surface of the solution gently seething.
It will remove broken taps, drill bits, reamers. The bigger the size, the longer it takes - be patient.
This method will only work on iron/steel based broken tool tips - not tungsten carbide etc
If you try using a stainless steel saucepan - the Alum will try dissolving that!
This method was commonly used 50 years ago - It now seems forgotten.
Powdered Alum can be difficult to source from pharmacies (years ago it was a common household item) - but is available on E-Bay"
