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Broken tap in expensive unit!

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Andrew Tinsley13/09/2017 16:58:02
679 forum posts

Just had a panic call from a friend. He has broken a 6mm HSS tap in a brass terminal which is connected to a VERY expensive capacitor! The hole is blind and tapped I think a BSF thread. He is a metric person so decided to tap the hole 6mm. Bang first tap broke. Then to show what an idiot he is, he does exactly the same on a second capacitor Aagh!!!!!!!!!!!!!!!!!!!!!

I seem to remember that hydrochloric acid can be used, sufficient to loosen the tap. I also remember that alum can be used. I have seen the acid treatment work , in fact done it myself years ago. But I can't remember if it was hydrochloric or some other acid?

Failing the above maybe a carbide drill might be the modern recommended way of doing things?

Anyway HELP he is up against some serious time constraints and needs the capacitors urgently!


Nige13/09/2017 17:11:32
228 forum posts
101 photos

For what it is worth I n the distant past I used a stick welder to splash the stick onto the broken end of the tap then used a pair of grips to gently twist it out. Required several 'dabs' and twists. Ok if tap hasn't broken off to deep,

Bob Rodgerson13/09/2017 17:18:22
544 forum posts
165 photos

A Carbide drill will drill out the tap but it will be difficult to keep it on centre, especially with the surrounding material being brass. Welding onto the tap may melt the brass too. Acid may de-zincify the brass and leave it in a mess, not sure what Alum will do to brass, probably turn it a lovely shade of green.

Spark erosion might be the way to go but not sure if the article has to be submerged in fluid whilst the tap is being eroded.

Andrew Johnston13/09/2017 17:20:10
3727 forum posts
452 photos

Hmmm, if he's managed to break two M6 taps in brass he should possibly consider a career change. wink 2

I've not used any of the chemical treatments, but I have used carbide cutters to remove broken HSS tools, several times. embarrassed I find that plain carbide slot drills are better than normal drills; they tend to wander less. However, that does imply the use of a milling machine and proper clamping of the work. I'd start with a 4mm cutter for M6 at a few hundred rpm and reasonable feed. Pecking is no problem, and I'd run without coolant. Probably not a good idea to use coolant with a capacitor anyway.


SillyOldDuffer13/09/2017 17:23:46
2666 forum posts
549 photos

Don't use Hydrochloric Acid because it attacks brass. Not tried it myself but a saturated solution of Alum is recommended in one of my old books. Be careful to get the right 'Alum' - various chemicals have been or are called 'Alum' at different times and in different countries. They are not the same. I believe the right one for dissolving steel drills and taps is Potassium Aluminium Sulphate.

Not high voltage vacuum or doorknob capacitors I hope; they are pricey!

Good luck


Clive Foster13/09/2017 18:59:16
1301 forum posts
29 photos

If the terminals are proud of the capacitor make some sort of clamp on external support brace before attempting to machine out the broken taps.

A 5 flute centre cutting E-Mill (perhaps by Nachi??) effectively shifted the remains of three 2.5 mm taps snapped off in s some slightly dubious aluminium alloy. Bridgeport 2J2 Varispeed head running flat out in the red zone of the dial. Threads appeared to survive just fine. Cutter was pretty much lunched by the end tho'. Managed one more rescue on a larger tap before finally dying.

Cutter was maybe £10 delivered via E-Bay. Gawd knows what the real commercial price was.


KWIL13/09/2017 19:16:25
2933 forum posts
55 photos

Alum without doubt, takes a little time but the tap reduced to a black mush. Surround hole with a plastacine bund, cover with cling film to keeo evaporation lower. Top up as it dries out.

Use a spiral flute tap that throws the swarf out backwards and does not clog the flutes and cause it to jamb.

John Olsen13/09/2017 19:31:00
877 forum posts
86 photos
1 articles

I just had to remove the remains of a spiral fluted tap which I had managed to bust off in a piece of quarter inch thick high tensile steel plate. It was bust off pretty well flush with both sides so no chance of turning it. I accomplished the task with a diamond burr in a small air die grinder. The burr was a long pointed taper which I was able to work into the flutes, cutting towards the centre. It came in a cheap set of tooling meant for the Dremel style of tool. I find that this sort of tooling generally works much better with the higher speed of the air die grinder. The diamond burrs are best worked wet on this sort of job, just dripping some soluble oil onto the job. Actually water with a little detergent would be perfectly satisfactory too. working gently withpatience is the best approach, pressing hard will just strip the diamond coating off the burr. It took me about an hour, but was accomplished without damage to the thread.

As for why I was using high tensile plate...well, mild would have done, but I had a piece given to me for free, didn't I.


Jeff Dayman13/09/2017 19:31:53
1167 forum posts
27 photos

If the units are needed asap/right now you should take them to a mouldmaker or a tooling shop and ask for the broken taps to be EDM'd out. Care will be needed with the grounding arrangements to avoid damaging the caps.

Just in case, I'd look into sourcing new caps, and plan to send the bill to Mr. Ham Phist who broke the two M6 taps in brass.

If there is any protruding metal of the terminals with broken taps, maybe a clamp on temporary terminal could be attached, then it could be used to connect the cap to the circuit . If no metal protrudes, I'd try EDM'ing the taps out.

I wouldn't let Mr. Phist near cutting tools again. Just my $0.02 worth.

larry Phelan13/09/2017 19:36:17
544 forum posts
17 photos

Why bother to change the thread in the first place ? If it aint broke,dont fix it.As they say "Let sleeping dogs lie"

Martin Newbold13/09/2017 19:49:02
397 forum posts
225 photos

Use a specialised drill with one that will cut a carbide tip if your going to drill out. You are foobar if you cant get it apart and mount ir rock solid in drill press.

Cheep taps are the problem . Dont buy Cheep taps. This rings in my own ears as have done this many times.

Tap removers are really not good . Have tried several and they bend.

If you can get some lubricant over several days into it try that first . Then try to get something small and like spring steel into the chanels on the tap and try and rotate the three you insert with strong accurate pliers . Try this first you might be lucky go gentle with it.


Neil Wyatt13/09/2017 19:52:26
13193 forum posts
571 photos
68 articles

Alum, dead cheap off eBay also known as Fatakdi as it's used in cooking.and for a bewildering number of other random purposes (e.g. deodorant).

Jeff Dayman13/09/2017 19:56:05
1167 forum posts
27 photos

Martin, these taps are reportedly broken off in electrical capacitors. Your ideas for tap removal will be effective if the taps are broken off in solid metal but any sort of high force maneuver on taps in a capacitor may damage it. They are usually very lightly made, they have little or no mechanical force on them normally, except while making connections.

SillyOldDuffer13/09/2017 20:11:45
2666 forum posts
549 photos
Posted by Neil Wyatt on 13/09/2017 19:52:26:

... for a bewildering number of other random purposes...


Andrew Tinsley13/09/2017 22:24:32
679 forum posts

Thanks guys. I have probably 3 or 4 days to sort out the problem. My Pal is a PLONKER, just why he did the tapping, I do not know. There is a shop within a 1/4 mile of him that sells BSF bolts. The capacitors are Maxwells, very high voltage and eye wateringly expensive. To do one broken tap in a totally misguided enterprise is very bad, but to do it twice, well I dare not type what I called him!

I am sure I can rectify the problem in the available time. This is for the dread Tesla machine mentioned in another thread. I have repaired the damage to the secondary and it is due to be demonstrated at Crossness (Sp?) pumping station a week on Saturday. The Maxwells have to be mounted within the machine and coupled up with bus bar copper after the taps have been removed!

Thanks everyone for their help. Looks as if alum and or a 4mm carbide slot drill or two may sort things out!


peak414/09/2017 00:00:17
492 forum posts
36 photos

Alternatively, as a possible temporary fix, is the blind hole on a stud or similar that sticks out from the main body of the device?

If so, make something out of brass which will clamp around the boss with a suitable threaded hole in it to attach the bus bar; leave the broken tap in place to be dealt with another day.


Neil Wyatt14/09/2017 07:53:41
13193 forum posts
571 photos
68 articles

Or admit partial defeat, use a razor saw and convert the tags into slotted ones and use nuts...

Ian S C14/09/2017 09:52:51
6738 forum posts
223 photos

Use the Alum, build a little dam around the hole, be prepared to wait a few days. Be generous with the tapping drill.

Ian S C

Antony Powell14/09/2017 10:03:36
146 forum posts
19 photos

I Have a small portable spark erroder for this sort of job, as above build a small dam around the job and fill with de-ionised water then away you go simple and effective.




ps where are you Andrew ?

Edited By Antony Powell on 14/09/2017 10:07:07

Andrew Tinsley14/09/2017 10:16:10
679 forum posts

Hello Anthony,

I am in Rutland and my idiot pal is in Stamford Lincs!


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