removing broken taps from attempting to thread SS with M8?

vas

vas

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keeping it short,

got a 16mm dia SS 304 bar, drilled a 6.2mm (I think!) hole in order to add M8X1.25 threads on it.
Two attempts, two different holes, two broken taps in the holes :-(
I know...

is there any clever way of removing the taps, or just cut the piece of rod, drill a new hole and try to get a new thread on?
threads had almost gone through (both) so removing the taps would leave me with two usable threaded holes.

cheers

V.
 
I'd start again...
as mentioned, 7mm would be a better hole size, done quite a few lately. Taps for stainless are blimmin pricey as well, come in sets of 3. hope you didn't break 2 of them ?
 
oops, sorry, just got my specs on and checked what I was using and unfortunately it was 6.8mm, will probably get a new cobalt bit at 7mm tomorrow with new high quality taps (if I can find any locally!) and try again.

I guess there's no way to remove without destroying the thread the broken tap, right?

V.
 
vas said:
no way to remove without destroying the thread the broken tap, right?
Click to expand...
Hit and miss, sometimes you can just grab enough of an end with some mole grips to get it out, or tap it round with a centre punch, but if it's jammed so tight to break the tap then if it doesn't budge within a few minutes fiddling maybe bite the bullet and start again.
 
vas said:
oops, sorry, just got my specs on and checked what I was using and unfortunately it was 6.8mm, will probably get a new cobalt bit at 7mm tomorrow with new high quality taps (if I can find any locally!) and try again.

I guess there's no way to remove without destroying the thread the broken tap, right?

V.
Click to expand...
There are ways, eg: spark erosion; dissolving the tap with acid; breaking it to bits with a punch; special 3-pronged extractor; etc, but it might be easier to start again with a fresh steel bar.
 
yeah, tried the mole grips trick, just managed to break some bits protruding, even tried (OK I knew it was pointless!) to drill a 3mm hole in the centre of the tap, nada.
 
Poignard said:
There are ways, eg: spark erosion; dissolving the tap with acid; breaking it to bits with a punch; special 3-pronged extractor; etc, but it might be easier to start again with a fresh steel bar.
Click to expand...
tig weld something on the tap and undo it? what chances do I have I wonder!
 
yes have a suitable cutting compound bought years ago, still half full. Was going 20-30deg, then backing off a turn, clean on it again, then obvs got bored and tried a 60deg in one go, bang!
anyway, will report tomorrow on my new attempts with new taps and 7mm hole!

cheers

V.
 
I’ve removed many broken taps and found that an EDM is the only effective way, but in your case probably not cost effective. All other methods seem to cause unwanted collateral damage. I would suggest starting afresh.
As others here have said. 6.8 or 7 mm dia. drill. When drilling, apply a lot of pressure with a very slow rotational speed. Keep the bit flooded with lubricant. The important thing is to keep the drill bit cutting. If it rubs it will quickly take the edge off the drill bit and also harden the stainless at the interface. Run a first taper tap through the prepared hole and be very, very gentle. Back off the tap frequently to break off chips as they form. With practice the tap will give you feedback and you’ll know when to back it off, but otherwise I would suggest every quarter turn. Run the first tap all the way through and then follow up with a number two, which apart from a short lead-in taper has a constant minor and major diameter along its axis.

Mike
 
20euro lighter with a single pass black german high tensile/whatever tap and a 7mm cobalt drill.
Success!
does take time though and lots of lubricant... More than 5mins for one tap (ok it's on a 16mm 304 solid rod)
one down, five to go!

cheers

V.

PS mikegunn, out of curiosity what's EDM, google is only suggesting Electronic Dance Music and no matter how awful it can be I very much doubt it would force the dead broken tap out of the hole :rolleyes:
 
EDM, electric discharge machining or spark erosion. Rather specialist op. Alum solution is sometimes recommended to dissolve a tap if the workpiece material is suitable. Slow business though.
 
vas said:
20euro lighter with a single pass black german high tensile/whatever tap and a 7mm cobalt drill.
Success!
does take time though and lots of lubricant... More than 5mins for one tap (ok it's on a 16mm 304 solid rod)
one down, five to go!

cheers

V.

PS mikegunn, out of curiosity what's EDM, google is only suggesting Electronic Dance Music and no matter how awful it can be I very much doubt it would force the dead broken tap out of the hole :rolleyes:
Click to expand...
As Earlybird wrote, Electro Discharge Machine, sometimes called a Spark Eroding Machine. The cutting tool, usually a copper tungsten alloy forms an anode and the workpiece the cathode. The area to be machined is immersed in an electrolyte and a current passed through from workpiece to anode via the electrolyte. This will gradually remove material from the workpiece, where the spark gap is narrowest. It’s a slow process but not necessarily expensive, because once set in motion the machine can be left unattended to perform per instructions. It imparts very little stress in the workpiece other than leaving the machined surfaces with a cast structure which may or may not have to be taken into consideration.

Glad you’ve managed to get you holes tapped without too much drama.

Mike
 
I've successfully removed broken taps using a diamond grinding tool in a dremel. You have to be patient as it takes a long time and you will probably need several tools.
 
vas said:
20euro lighter with a single pass black german high tensile/whatever tap and a 7mm cobalt drill.
Success!
does take time though and lots of lubricant... More than 5mins for one tap (ok it's on a 16mm 304 solid rod)
one down, five to go!

cheers

V.

PS mikegunn, out of curiosity what's EDM, google is only suggesting Electronic Dance Music and no matter how awful it can be I very much doubt it would force the dead broken tap out of the hole :rolleyes:
Click to expand...
Are you tapping all the way? You only need 8mm of tapped hole for a strong threaded fitting. Look at an 8mm nut, they are only abiout 6mm thick, and strong enough. I would drill in 8mm about 4mm deep into the bar each side and only tap the middle 8mm.
 
I would go for a 7.1 drill If you are threading to some depth. Then you do not need a lot of thread engagement in a hard material. If you do not want to do that then you only need to thread to a depth the same as the depth of a nut plus a tiny bit of play. Perhaps use a second tap so the thread tightens up on the last couple of turns. That way you will not be straining the tap. Depends why you are tapping the hole. ie to fit a stud or to clamp something with a bolt. In any event try to keep the threading to a minimum. You could drill 7.4 mm for some depth then finish off with a smaller drill bit if you have a long item to screw in the hole. You do not always need a long thread.
Edit -- overlapped with Norman E whilst typing-- sorry
 
pyrojames said:
The chap who taught me on the shop floor, welded an 8mm washer round the tap, and unscrewed it with mole grips. He was a very experienced old welder/fitter!
Click to expand...
Welding is very good because of the amount of heat involved. However, it does need a good amount of broken tap to be standing proud of the substrate or you'll be welding that as well.

Richard
 
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