FAQ Group: Tool Tips

I recently suffered the embarrassment of breaking a tap in one of the backhead bushes for the firehole door supports on my Compbyne boiler.
I tried dissolving it out with concentrated sulphuric acid, which is available as a drain cleaner from hardware shops, but this time without success. 
(I live in Ham-fisted City).

I had heard that a hot alum solution could do the job but did not know anybody who had tried it.

The technique is to dissolve the alum in very hot water till no further crystals will dissolve, (i.e. make a saturated solution), reheat to boiling, add the engineering mishap and “boil for about an hour”.

During this time the steel will be etched away giving you the opportunity to start again. THE PAN SHOULD BE ALUMINIUM for obvious reasons. I do not know about non-stick surfaces.

There are different forms of alum with different chemical names. I used Potassium Aluminium Sulphate obtained by Googling one of the numerous
suppliers on the web.

Looking on various forums and talking to Brian, this seemed to be a wonder tool using standard ¼” HSS tool bits, which are far cheaper than
the current vogue for indexable carbide tools.

Eccentric Engineering sell a commercial item but at around £78 (plus £4 P&P) seemed a bit expensive, plus I wanted two – one for each of my lathes. 

The whole process to make the two holders plus a grinding jig, took about 3 hours.

Apart from the angles shown, none of the dimensions are critical.

It is possible with a little care to make the holder with the right clearances such that the tool holder does not require to be moved from a
traverse cut to facing.

The cutter works perfectly well on all materials – steel, brass, aluminium, plastics etc.

The holder shown is for a standard L/H tool; but a right hand tool can easily be made using the dimensions on the drawing making a mirror item
of the tool holder part.

I have used M5 threads as the cap screws are cheaply available, but use whatever you have.

It will quite happily take a 3/16” cut with no undue load. On lighter cuts the finish is good & can be improved by rounding off the cutting corner
of the tool with an oil stone.

All in, the time spent in making the tool has been worthwhile and is now left on both lathes as the tool of choice. Apart from form and parting
off tools, this is all you need.

I am told that this is not a new idea and goes back to the late 1800’s and was shown in volume 1 of the Model Engineer.

Peter Newby

17/01/2015

Peter has produced 2 drawings to show dimensions for this very useful tool, they are available from the secretary (lindanic@sky.com) for a small donation towards the new clubhouse fund of £3 (plus p&p). As well as the drawings the pack includes a hard copy of the photographs & description above.