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Busted tap & brute force

----how I removed a broken tap.

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Rik Shaw03/01/2019 13:25:09
1294 forum posts
350 photos

I am in the (slow) process of building a Double Tangye stationary steam engine using a set of second hand castings. They have been part machined – and not particularly well either!

A lot of the re-work involves opening out poorly positioned holes, re-drilling oversize and filling with loctited studs which can then be re-drilled in the correct position.

All was going well till this morning when I realised that one of my bearing standard castings had a busted 6BA tap stuck in the ‘ole.

Years ago I would have given it to my ‘oppo who would have spark eroded it out for me but these days I’m on my own – “so sort this one out mister” I thought!

First attempt to remove the tap was with a 3mm dia mounted point grinding wheel with nil result. Then I tried a similarly sized cheap diamond version, again, nil result. Both attempts were made using an old mini drill I had bought in a field on a Sunday morn earlier this summer. These days my old mitts are not steady enough to hold a dremel and I do not own a stand else I would have tried that method first. (I’ve put a stand on the shopping list).


I read on the forum recently that somebody (I cannot remember who, so apologies to that bod) had reminded me that carbide was quite capable of cutting HSS. It so happens that I had a part pack of ten cheap carbide engraving cutters earmarked for use for the odd engraving job on my small CNC mill.


Once again and with the drill still set at top speed of 8,500 rpm I bought the engraving cutter to bear on the broken tap and gave it some welly. It certainly made some progress before the tool bit shattered but “nil carborundum” as they say. I fitted a new cutter and had another go. The second cutter was starting to break up when it suddenly broke through and the busted tap was out. It was brutal, it was noisy but it got the dirty job done. I would not subject a nice new posh Proxxon to this sort of abuse but my three quid car boot special I consider fair game.


It just remains to open out the hole to 5mm and tap 6mm for yet another stud and my casting is saved.

I am confident that the high speed was the key to success – it must have been the closest I had been to producing plasma whilst “fettling”. smile d


Andrew Johnston03/01/2019 13:36:25
4692 forum posts
527 photos
Posted by Rik Shaw on 03/01/2019 13:25:09:

I am confident that the high speed was the key to success

The idea is to get the work red to orange hot at the shear zone where it starts to soften. The carbide will withstand the heat, at least in the short term. Having said that it is possible to machine HSS toolbits with carbide cutters at slower speeds - carbide is harder than HSS so will cut even without the softening.


Ian Hewson03/01/2019 15:17:20
259 forum posts
24 photos

Bought a few cheap carbide drills at one of the regular stalls at the Midlands Show, tried them on an old hss tool using slowish speeds and feed on the Fobco. They cut without any problems, wish I had bought a few more and had some years ago for jobs such as the above.

Edited By Ian Hewson on 03/01/2019 15:17:57

Edited By Ian Hewson on 03/01/2019 15:18:35

IanT03/01/2019 15:24:22
1266 forum posts
128 photos

I've got a large (part-machined) casting with a broken tap in it (no, not by me this time - found it after purchase). Someone has made a mess of trying to get it out and it's well buried - so I was thinking of building a (simple) spark eroder (like one seen at Guildford MEX a year or so ago).

HOWEVER - last week I saw a YT of a similar problem resolved by cutting out the tap with a diamond tipped holesaw


Toolstation sell the JCB holesaws - and the 6.5mm one is available at £15.84, which sounds easier/cheaper than building the eroder?


So - whilst I've not tried this myself yet, I think it looks a promising approach. I'll then make a threaded metal insert, drill & tap it to the original tapped hole size and then screw in (& Loctite) the insert and make good cosmetically... Sound like a plan?



IanT03/01/2019 15:31:05
1266 forum posts
128 photos

​P.S. The same guy has another YT of removing broken studs by drilling a bolt centrally - and then using this as a drill guide to get a hole started/drilled without spoiling the existing thread - and then using a screw extractor - but I liked the idea generally and I'm sure it has a few uses....




mechman4803/01/2019 19:18:22
2391 forum posts
374 photos

I have drilled broken taps out using carbide tipped glass drills using IanT's guide method above.


Samsaranda03/01/2019 20:07:57
722 forum posts
5 photos

Diamond hole saws/drills are sold by Arceurotrade at very reasonable prices, I used one to open out the mounting hole in a hardened steel QCTP, had to open it out from 11.2 mm to 12 mm, doesn’t sound much but by crickey the steel was hard but the diamond managed it whereas cobalt drills and carbide drills just bounced of it.

Dave W

IanT03/01/2019 23:31:51
1266 forum posts
128 photos

Well spotted Dave - quite a lot cheaper!



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