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I'm finding myself doing quite a lot of thread cutting in stainless steel lately, particularly 304 and 316, often these are moderately deep threads ~ 25 mm or so and at M12 it is becoming a bit of a chore. I'm using a decent tap wench and cutting oil with carbon steel taps (taper, second cut and finishing/plug) but I was wondering if anybody has any experience or knowledge of any ways to make this a bit easier eg by a different tap material or coating.

I'm not doing enough volume to justify any major investment in tooling as this is all prototype work but it's that difficult area of a dozen or so at a time.

Also it is specifically stainless which is the problem and these are stressed welded components so free machining stainless isn't really an option (as far as I know) and I do appreciate that this is a bit of a 'how do I have my cake and eat it' type question.

The main issue is the taps getting progressively tighter, I haven't broken any yet but they often feel like they are getting to the point where they might. I generally start the taps off in a lathe (not under power) with a sliding chuck in the tail-stock so I'm fairly confident that they are going in pretty square.

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    $\begingroup$ If no one can give you an answer here, maybe you can ask at this site: practicalmachinist.com $\endgroup$
    – J. Ari
    Apr 5, 2017 at 20:13
  • $\begingroup$ Sounds like a blind hole, can you confirm? Also, do you have access to a drill press/mill? Have you considered power tapping? What's your main issue? Are taps breaking? Taking too long? Too much labor? Tools wearing/breaking down? Ditch the carbon steel for HSS taps. Also, remember that stainless steel work hardens. Awaiting your response... $\endgroup$
    – GisMofx
    Apr 6, 2017 at 1:46
  • $\begingroup$ Mostly through holes. I do have a drill press and I have seen power tapping attachments for sale but I'm not sure whether they would solve the underlying problem ie getting progressively tighter in the last 10% or so. I'll certainly give HSS taps a go though. $\endgroup$
    – Chris Johns
    Apr 15, 2017 at 20:36
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    $\begingroup$ Not the "correct" way to do things, but my solution for copper and some tough Al alloys is to simply oversize the hole by a few thou. just enough to ease the pressure at the cutting edge of the tap. For example a 1/4-20 thread would normally be drilled 13/64" or #7 - but a #4 drill will make the tapping job much easier without significant loss of performance - at least in larger threads. Oh, and try the best smelling tapping fluid for hand tapping stainless - Johnson's Baby Oil. $\endgroup$
    – Donald Gibson
    Apr 17, 2017 at 1:26

3 Answers 3

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I always cheat the thread engagement percentage in tough materials. When allowable of course, going from say 75% to 65% will make it much easier to tap. So for a M12, chart says letter "Y" drill at .404" will give you 75% engagement. I'd try going up to letter "Z" drill at .413". ONLY if you can! Otherwise, it sounds like your problem is chip extraction, the chip formed from cutting the threads packing in the bottom of the hole, making it hard to get that last bit of thread. A lighter weight cutting oil and a tap that aids in chip removal will help. You can also get a tap with a coating that aids in chip removal. Check Travers tool and Mcmaster, they have decent descriptions of types of taps and coatings and what there good for.

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  • $\begingroup$ a very practical suggestion! Next time I have to cut threads in a piece of stainless, I'll do this. $\endgroup$
    – niels nielsen
    Sep 4, 2018 at 21:29
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Do a google search for HSSE VAP (high speed steel with cobalt and vaporised surface) threading taps. They have a 1400nm/mm2 cutting capacity. Speacialy made for stainless steel. I’ve used them and they are great.

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Thread percentage my friend. Drill dia.=thread dia. -(1.229XPITCHX.075%) 0.75 = Percentage of thread. Can substitute with 0.60 or 0.65% for harder tough to tap materials such as C276 or Duplex.

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