The Old One - Broken Tap Removal?

Workpiece: a simple stainless-steel cutter holder for a TC Grinder.

It is a short length of steel rod reamed from each end with two sizes meeting half-way; and a cross-drilling close to each end, tapped for M4 grub-screws.

I selected the metal because it was some I had in stock, to within a thou of the drawing's 0.5" diameter. The drawings specified mild-steel.

Problem: on the very last of 8 holes (2 holders, one inch sizes, the other metric), I inadvertently put the tap into the opposite hole and snapped it.

It may have been a slightly blunt tap, and that would not have helped. I'd already scrapped two part-made holders the same way!

The broken piece is within the holder, the fracture just clear of the hole it had cut, and held by perhaps one turn in the opposite hole, with the point just peering out of the surface.

I tried leaving it overnight in hydrochloric acid, but that didn't do much beyond re moving remnants of tapping-compound and turning the acid slightly green. However, after very thorough washing I found the fragment of tap is very slightly loose; but there is insufficient room to back it off so it falls out, even if I could gain any purchase on it.

SO....

Before resigning myself to making yet another holder, can anyone please suggest a viable and readily-available "solvent" that will corrode the tool-steel tap fragment away without harming the stainless-steel?

I don't know if the tap is carbon-steel or HSS, or if that would make any difference. Time is not of the essence though , so it would not matter if the corrosive took several days to act.

An alternative might be to try to break the piece between a steel rod inserted from one end of the holder, and pin-punch from the other. I realise I would probably have to regrind the punch end afterwards. It also risks damaging the holder bores, but that's academic if I can't remove the bit of tap...

[How will I know which is metric and which imperial, of the two sets of 4 holders in each set? By a shallow groove round the metric ones, similarly to the groove designating the metric Clarkson 'Autolock' collets.]

Have a read-up on using alum to dissolve broken taps.
A warm/hot solution of potassium aluminium sulphate.
I've not tried it myself, but I gather it works well on aluminium alloys with a broken tap.
(I have some in stock ready for my next cack handed moment. along with my old slow cooker to keep the solution hot)

I understand that it also works for a tap in stainless, as it doesn't dissolve the workpiece, but I'd check on a spare bit first, as it might depend on the grade.

Bill

Edited By peak4 on 01/12/2020 00:34:04

IMO there is no prospect of success with selective dissolving of tap.

Go to shop with EDM facilities. They may help if sympathetic to your plea, time permitting.

If I understand it right, tap with a small hammer from the point end, turn and repeat the other side a few times and then see if you can winkle it out.

The alum does work but it takes a long time and needs to be quite warm to speed up the reaction. I broke a 3 ba tap in a mild steel workpiece the other day and managed to break most of it out. I got the rest out with my dremel and a small diamond bit. The best bit to use was about 3mm diameter and square ended. It wasn't anything special , just one of the cheap ones that come in sets of bits.

The alum does work but it takes a long time and needs to be quite warm to speed up the reaction. I broke a 3 ba tap in a mild steel workpiece the other day and managed to break most of it out. I got the rest out with my dremel and a small diamond bit. The best bit to use was about 3mm diameter and square ended. It wasn't anything special , just one of the cheap ones that come in sets of bits.

A photo would help to visualise the problem (which sounds like a job for the Walton tap remover as previously discussed here).

I recently broke an 8BA tap in a work piece. Why is it always the last hole?- in this case the last of 6. As I don't use BA sizes very much, I knew that the tap I had bought was carbon steel. ( does that make me prudent or a cheapskate?). So chucked the whole thing in the fire and retrieved it from the ashes the next day. The tap was nicely annealed so I drilled it out easily. I did drill oversize, fitted a plug of steel and the re-drilled and tapped. Job done.

Edited By Anthony Knights on 01/12/2020 10:45:51

I can't answer that one for you, but I do know that my late brother, who had his own commercial model-making business, used carbon steel taps for preference (down to 10 BA) because they kept their edge longer. That was the voice of long experience speaking.

George B.

You will be able to worry it out as it already has some movement. It takes time and patience, the axial tapping and attempts at turning it will work.

Thank you for the various suggestions.

The alum is on order!

Since I had already broken 2 other taps in the same material I will test the method on that before trying it on the work-piece itself. If it works on them I may continue to use them , making further holder with different bores, especially 3mm & 1/8-inch in place of the original design for a sort of split collet.

I have been trying the tapping, harder tapping and wiggling but without success. I also tried breaking the bit of tap up by resting the inner side of it on the end of a rod held in the vice, and driving a pin-punch down on the outer side. I abandoned that as it was only damaging the punch.

Sorry Ega - I have no way of photographing the thing. it is too small. The body is only 1/2-inch dia, the hole down the middle only 5mm. It's basically a thick-walled tube, cross-drilled and tapped in both walls. The broken bit of tap is in its far, or exit, wall; its point is just flush with the outer surface, the broken surface is just slightly inside the tube on the entry side.

The mistakes I now realise I'd made were firstly cross-drilling and tapping the stock as a first operation, and by the BSI standard tapping-size rather than fractionally larger (as this is in stainless steel), so I was overloading the poor taps. Also it meant having to drill and turn the main features across the grub-screw holes.

This advice is too late to be helpful in this particular case, but anyway... I have built and maintained three sailing dinghies over the years, and currently have a steam launch. Naturally this has lead to using a fair bit of stainless over the years. So my general advice is to avoid the use of stainless unless the job absolutely needs it and you know what grade is right. Some grades are easier to work with than others, but it is nasty stuff to work with, and does not always give the corrosion resistance you might be expecting. In particular, stainless threads love to seize up. If this is for use in the workshop, does it really need to be stainless?

John

Don't forget that with a suitable carbide slot drill you can drill a tap out. I have done this and was left with the 3 toothed sections of the tap.

David.

If the point is just contactable in the exit hole is there any chance that you could heat the workpiece whilst trying to grip the remains and screw it back the way that it came>

The idea is that warming the workpiece will expand it and decrease its grip on what remains of the Tap. Hopefully, once once the Tap /work grip is reduced, it will rotate, and can be sent back whence it came .

One way of "gripping" the Tap might be to simulate a Tap Remover. Insert as large a piece of wire as possible , for a sliding fit, into each flute, and grip the wires to rotate the tap.

HTH

Howard

Nigel Graham 2:

Thank you for the further explanation.

Howard Lewis:

Good idea - a "poor man's Walton"

Supposed you own a grinder, it should be easy to test the remnant of your tap if it is carbon steel or HSS?

The sparks are very different looking...

I would mill it out with a carbide cutter,

Regards,
Hans

I can't unwind it. The broken piece is mainly within the bore of a small but thick-walled-tube. There is insufficient room for any sort of tap-remover, and it would be impossible to engage it with the clear section of thread to wind it back out.

Hans - The fragment is inside the work-piece so not accessible to a grinder in the normal way. The point is just about flush with the outer surface, but the rest is all inside the item. However I did consider trying to grind the bit that is accessible a few mm down the 5mm diameter bore; using some old dental burrs... if I still have them and can find them. Anything larger risks damaging the work-piece. If that worked the very tip, in the tube wall, would probably fall back out with a bit of help.

John -

No it didn't really need to be stainless-steel, although the piece here is a free-cutting grade. I chose it for its accurate diameter. It's a tool-holder for a tool & cutter grinder, so all indoor use. The drawing specifies mild-steel.

It is too bad the broken tap fragment is loose in the hole. If it were tight I would suggest a 3mm diamond core bit from Banggood The sets of core drills are dirt cheap so it would be worthwhile to keep a couple on hand. The diamonds are plated on so they will not take much abuse.

Thank you for that information, John.

The thing is now dangling in a jar of hot alum solution, and after it had spent a time in the oven at Gas Mark 1, tapping the jar released a little stream of fine bubbles, and the tap has turned black, so something is happening.

I know it will be slow, especially as to save gas I'll just leave it in there to be heated up when I do any cooking! (I don't think it will release anything more than water-vapour and hydrogen, and only in very small amounts.)

'

The wretched project has give me a new headache! The smallest "collet" is a two-pronged affair and after cutting the slots in the two I made (for 3mm and 1/8-inch tools) I found the slitting-saw arbor has jammed so solidly that trying to unscrew the countersunk screw that holds its end-cap, merely twisted the Allen key.

.

That’s a familiar story except that mine was retained by a slotted pan-head screw, which I had [wrongly] assumed would be easy to undo.

I eventually resorted to drilling the head off ... it released with a resounding ‘crack’ and the remains of the screw thread were then simply unscrewed by hand.

Hope it goes as easily for you !

MichaelG.