trepanning small holes

I need to remove an HSS 8BA tap from a bronze fitting in the Bankhead of a 5" gauge loco boiler. A week of "alum" treatment hasn't done much, but its been done cold as I can't see a way of getting the boiler hot and the alum solution around the broken tap without the use of plastercine to make a wall around the tapped hole.

Spark erosion is out of the question as I don't have any contacts for that and professional help too expensive. The precision required for a 8BA tap removal dictated that I wouldn't build a home made setup. (I suppose precision isn't important if I was prepared to go along with plugging the hole and re-tapping !!)

Next idea is to make a trepanning tool that would leave enough metal to be able to tap the finished hole 2BA, remove the broken tap, make a 2BA plug and then re-tap 8BA. The trepanning tool would be 0.140" diameter with an internal diameter of 0.086" - giving 0.027" wall thickness. The depth of the trepanning needs to be 0.25". The tool would be made of hardened silver steel.

The boiler is too big to get under the head of my milling machine, but could possibly mount onto the saddle of the Boxford lathe. A guide plate could be mounted onto the existing fitting to guide the trepanning tool.

Anyone had experience of very small trepanning tools ??

As an alternative, I could remove the fitting altogether by boring it out on the lathe, but that would not be an easy option either. Removing by heat would mean that the complete backhead would have to be brought up to silver solder melting temp. If mechanically removed, a lower temp silver solder could be used to re-set a new fitting.

The first time I did a removal like this, I made a tubular cutter from a ‘slotted-spring’ pin.

... any suitably hard steel tube should do.

But, bronze being tough stuff ... You may do better with a little diamond coated core-bit.

MichaelG.

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For example: https://www.eternaltools.com/small-diamond-core-drills

Edited By Michael Gilligan on 23/02/2020 08:07:51

A pic? Alum will not do muchn in a month of Sundays, unless as concentrated a solution as possible and close to boiling point. Works a treat in those conditions.

If you can position the boiler so the backhead is horizontal then soft solder a short length of tube around it you can fill this with the alum and heat it with a small bow torch. Keep renewing the alum and keep applying a bit of heat. It will take about a week but it will come out eventually.

Thanks Michael, Just reminds me, I do have some small diamond burrs - perhaps try a bit of dental work ?

Ah ! Success, the diamond burrs 0.8mm diameter in the 'Dremel' have done the trick, the remaining hole will have to be made round, tapped and 'soldered' into place with some CuPSol before re-tapping 8BA. Fortunately, the tapped hole does not enter the wet side of the backhead and is in the flange of the fitting.

Thanks to others who have contributed.

Can't help with miniature trepanning, but have you got the right Alum? The name is used for several different chemicals making it dead easy to buy the wrong one! And if you have the wrong Alum, it either works at tired snail pace or not at all.

The 'Alum' needed for removing taps is Potassium Aluminium Sulphate. Accept no substitutes!

Dave

That's right, you can get the right alum on eBay . Heat is definitely needed to get it to work.

The Alum treatment sounds like a useful technique. However, I'm not familiar with it. Can somebody explain the technique, or point me towards a good source of reference.

Thank you.

Technique? I dissolved s 1/4” tap, from a hole in a brass feed screw nut, by submersion in a virtually saturated solution and maintained close to 100 Celsius for some time.

Potash alum is very soluble, solubility increasing considerably with increasing temperature. Dissolving the steel was very slow until the temperature was maintained at near boiling point with an excess of alum available. Then it dissolved the broken tap at what I considered a reasonable rate.

I think that if there is a ‘next time’, I might well dig out the old aluminium pressure cooker and press that into service to raise the temperature even further (I would expect the alum solution would boil at somewhat in excess of 100 Celsius). I would not anticipate any problem with the slow hydrogen generation, from the reaction, on the scale of my pressure cooker operation!