Looking for advice on how to tap some stainless.

I have an assembly I’m making and one of the parts are these small studs of stainless steel. They have an M8 through the center, three M6 blind holes from the back and an M5 from the side.

It’s 303 I think and I’ve not had too much trouble on the lathe. The mill was tricky but I got it down in the end but tapping it is another story.

Feels to me like its binding or something, after a few turns it tightens up quickly, then makes a loud creaking, then breaks the tap.

The force it takes to twist the M8 tap, to cut a few degrees of thread is just under how much force it takes to break the tap so I got an M8 done with lots of oil gong very very slowly.

The M6’s I’m not having so much luck with, broke three taps already and scraped one part. After a certain point the force it takes to cut the thread is just more than the tap can take no matter how slow I go.

I’ve tried everything I could think of today, someone once advised me to mix some anti-seiz and tapping oil for lubrication with stainless so I’ll try that tomorrow. I have a pot of the nickel stuff knocking about somewhere but I think I’ve already tried machining with that concoction and as I recall I didn't notice much difference.

I could really do with some advice.

Hi Neil
Stainless can be a bit tough but is easy enough if you just take it softly, check your finished hole dia to make sure it is in spec (Zeus machinist handbook) and only rotate the tap clockwise(assuming right hand thread) about a 90deg then back off to break the chips continue until hole finished. as you have described the metal creaking it is obvious that your lube isn't up to the job, i use a product called trefolex it's a paste and will stick to the tap and i have not had any problems tapping stainless.
softly softly catch'y monkey.
Regards
Mike

I will add to that:
1) Use the correct tap for the job - you can get special taps for stainless and other troublesome steels, the ones I was recommended have a green ring on them (from Europa tool IIRC), and I've yet to break one.
2) Whilst I would agree with Mekanik's advice if you're using straight flute taps, an alternative approach is to use a spiral point tap for through holes and just keep going slow and steady. The rationale behind this is that stainless work hardens rapidly and that the only way to keep on cutting into soft metal is to always cut into fresh metal. I did a number of M6 holes in a 10mm slab of 304 (I think!) this way with no problems at all.
Logic would say that for the blind holes a spiral flute tap (again green ring) would be best, but I've not tried that and wonder if they would be strong enough. I guess if you're good at contortion you can also put the work piece upside down and tap from below if you want to be super-careful with swarf removal!

http://www.europatool.co.uk/pdf-down...ation-taps.pdf

As has been said, hole size is critical.
If you don't need maximum strength, then you can make the hole slightly over size, which will make tapping far easier.

Spiral point/flute are good options for speed, but another option is a set of serial taps, which come as a set of 3 taps and you run them through the hole in order. Arc Euro sell them.

If you've not got anything like Trefolex, I'd prefer to use some general purpose grease rather than oil.

Does anyone know if thread milling would work in 303 ?

Does it need multiple passes therefore may work harden?

At least if the tap broke the work is recoverable

303 taps nicely with Spiral Point and Spiral Flute taps of the right type and with the correct drill size, which will never be nominal OD less pitch BTW.
You will only get hardening if you fail to take a decent DOC, I've never had 303 harden on me.

Quote:

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Originally Posted by routercnc

Does anyone know if thread milling would work in 303 ?

Does it need multiple passes therefore may work harden?

At least if the tap broke the work is recoverable

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yes thread milling will work in 303. if I had to thread mill them with the small sizes of thread I would do them in one cut with a micro thread mill. personally though I would roll tap the threads as it will create no swarf to get stuck in the tap and 303 forms very well

If your having problems with tapping like previous posts have mentioned a slightly oversized drill or threadmilling theres a supplier selling threadmills on a promo
www.smarttoolsuk.com starting at £32.50 had good feedback on them from the floor 2-3 passes i reckon on 303