Tapping small stainless steel parts


I have been having a problem with machining the stop valve spindle for a model beam engine (Mary). This part is stainless steel and is turned down and threaded 12 BA on one end. I know that you shouldn't take too fine a cut on stainless for fear of work hardening so I cut it down to size in one pass - no problem. However tapping it was a different matter: Taps take lots of fine cuts by their nature and the thing seized and broke off in the die. 12 BA dies are expensive and I don't want to risk another one - any advice?

Thanks in advance, Russell.

Reply to
Russell Eberhardt
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If you can't take the advice " don't " :-)

Rockol cutting grease is the best lubricant I have found.

You must start the tap absolutely square. Starting by hand is seldom good enough in anything over a couple of thread diameters thick.

Stainless blunts taps, blunt taps break. I throw away my M4 HSS taps after tapping 20 holes on 8mm thick plate.....( 316 grade stainless ).

Tungsten taps are not good enough for even one deep hole.

I have not found coatings like Tin to be helpful.

Spiral points are useful in through holes.

If you start with a second to establish the thread then switch to a plug which has a smaller number of cutting edges this helps, but the plug needs lots of backing off if the swarf is not to build up.

I still think DON'T is the best advice.

-- Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

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Reply to
Jonathan Barnes

In addition to what Jonathan said I would suggest that you drill the tapping hole oversize and not try to cut a full thread. If its not a part that need to frequently removed/replaced then you could always use loctite as well.

Ian Phillips

Reply to

Jonathan said it right. I cut hundreds of threads in stainless, (although not as small as 12BA) and ROCOL RTD cutting compound is by far the best I've found. If you only need a small amount I'd be happy to mail you some for £postage.

Do you know what type of stainless you're trying to thread? The free-cutting A1 / 303 is almost as easy to machine as brass, whereas A2 and A4 (304 and 316) are pretty tough on drills and taps and are as hard as the Hobs of Hell, especially if not using a surgically sharp tool.

Best threading is done in the lathe, on as low a backgear speed as you can find. If you have a Coventry diehead, you could consider starting oversize with a roughing cut, then adjust the head or use a split die to take a finishing cut.

festina lente


Reply to
Mark Pinkney

I've never tried this, but in an old issue of MEW some chap swore by a formula he called "angel's breath" for very small threads in tough alloys. The recipe was;

1 part genuine turpentine 2 parts white spirits 3 parts olive oil

He did also warn not to smoke while using it, for obvious reasons!

Good luck


Reply to
Kevin Steele

Thanks for the offer, Mark. I have recently moved (complete with workshop) to the South of France and have not yet managed to find a ROCOL outlet. I had to leave all my paints, lubricants, etc behind as the removal company refused to carry them. I am gradually restocking. Some things are easier to find, would you beleive that acetone and hydrochloric acid are available at most supermarkets? I may take you up on your offer if I can't locate a local source of supply.

It was sold as 'free cutting' but who knows?

Yes, I should have said threading not tapping. I was threading a turned part in the lathe using a tailstock die holder (and a brand new die dammit!).

Bonnes fetes.


Reply to
Russell Eberhardt

Thanks, I'll try that if I can't find RTD compound here.


Reply to
Russell Eberhardt

Hi Russell,

If you send me your contact details to mark(dot)pinkney(at)petroplus(dot)co(dot)uk I'll whap a splodge in the post for you.


and festina lente


Reply to
Mark Pinkney

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