Diamond grinding a broken tap ?

[picclock]

I've managed to break an M3 tap into mild steel part. Its a large part with a lot of hours so I'd rather attempt to remove the tap than remake. Its in a blind hole around 7mm deep. I have a diamond bit which came with my dremel tool which is 2.35mm diameter and about 9mm long. I figure that If I clamp it up on the mill and proceed carefully I should be able to grind the broken tap out.

So what's the best lubricant for this and what speed should I use ? Any other tips or removal suggestions much appreciated.

Best Regards

picclock

 

[Blogwitch]

P,

For diamond tooling, just use plain water to keep things cool, and if possible, flush the debris out as often as possible. It is the debris that clogs up the diamond.

As fast as your mill will go.


John

 

[picclock]

Wow that was fast - thanks John. I thought about building a wall with blu tack and filling it with water. What sort of pressure are we talking about here ? I've heard that diamonds can burn if the temperature gets too high - so I'm guessing just a very light touch.

Best Regards

picclock

 

[rhitee93]

A machinist at work used diamond cutters to "Mill" some pockets in thin glass plates a few years ago. He used putty like you mentioned to create a wall around the pocket and filled it with a coolant. I can't swear to it, but I thought he used isopropyl alcohol.

Worked well, but this was a mini-mill and he probably had it turning 10k rpm.

 

[pete]

It also may be far faster once you get a good flat area ground on the end of the broken tap to then go in with a solid carbide drill and then just drill it out. But if you breaK the carbide drill? Your next stop will be at a company that has a EDM.

Pete

 

[Holt]

If you know someone working with EDM wirecutters, they usually have a EDM machine to make start holes with brass tubes, they removes such a broken tap in under 5 min.

Holt

 

[Blogwitch]

Your main problem P will be to get the burr going down the centre of the tap.

If you can get it down through it, then you need to gently smash the tap apart, into the hole you have drilled and take it out bit by bit, not a pretty job at any time.

If you haven't already mashed it up trying to get it out, I would tap some panel pins (or old small drill shanks) down each flute and try to twist it out using mole grips. If you don't succeed at all, it is no big deal to put a larger hole in there, tap it out, insert a steel bolt or a bit of threaded rod and machine it off flush. Then re-tap the hole.


John

 

[doc1955]

I have had very good luck with carbide drills to drill out the web of taps. A good carbide drill called a 240 series and clamp the part in the mill so no movement and no coolant just an air blast and take it slow and easy.

 

[picclock]

@ John
The size of diamond tool I have is 2.35mm dia cylinder. The tapping size for M3 is 2.5. I'm pretty sure that if I can get the tool through the tap the body will be completely ground away leaving just the cutting edges to remove.

Largest size I could slip down the flutes was a pin (with some light tapping) and despite the demise of several pins no movement. It was quite a good quality HSS tap, made by Presto. I have several other sizes and they seem to perform very well.

@doc1955
Reluctant to use carbide as it is very brittle and may well compound the problem. I suspect I am a major user of 1/8 carbide endmills as they just seem to randomly break, often with no obvious reason.

@Holt
No one round here that I know of possess such a machine, but thanks for the thought. I have often thought of building one to tinker with but find it difficult to justify the time with more pressing projects.

Will have a go with diamond tool this morning.

Best Regards

Picclock

 

[picclock]

Two diamond tools later and all I have to show for it is a 0.6mm cut depth. The tools I used had just a thin coating of abrasive on the end and once that's gone your sunk. Have ordered some diamond coated drill bits from Richontools

http://richontools.com/index.php?main_page=index&cPath=1_4

and will give them a try when they arrive. I guess they are probably for drilling holes in stones for jewellery, so should make short work of HSS.

Many Thanks

Best Regards

Picclock

 

[Florian]

HI Picclock

The problem is, that if you use diamond tools on steel, the Iron will destroy them quite soon as it can take carbonate into its structure. It then is being converted to fe3c and the steel also gets harder because of this. (though a tap is already quite hard)

The Thing with the carbide endmills may be that you do not use the appropriate grade!
carbide is also very sensitive if there are vibrations which means your machine should be enough rigid!

And finally its also a question of what you pay for it. If you "only" buy cheap (chinese?) endmills, they will not last as log as a quality endmill does.

With carbide drills and patience you will get your tap out of your workpiece.

By the way: Some use modified (sharpened) masonry drills.

Cheers Florian

 

[Maryak]

If the part is not too big.................

Heat it to a bright cherry red and then bury it in lime with say a minimum of 2" lime cover all over.

Leave in the lime for 12 or more hours then remove it and the broken tap will have been annealed.

Dill it out with a normal drill and then use an easy-out or drill to tapping drill size and re-tap.

Hope this helps

Best Regards
Bob

 

[Blogwitch]

I have mentioned these before, but it doesn't hurt mentioning them again.

If you are UK based, hard plate drills, which are basically masonry drills, with a different grade of tungsten tip and ground to normal metal drilling angles can be obtained very reasonably from here. I have made up a set of them, and they have got me out of sticky situations a few times. Especially when a bit of material has work hardened on me when drilling a hole, these break through it like it wasn't there.

http://www.leofixings.com/fixings-products_Drill+Bits_Hardplate+TCT+Tipped+Drill+Bits-10-132.html


John