Engineering Asked on January 6, 2022
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.
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.
Answered by A Gomez on January 6, 2022
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.
Answered by Greg on January 6, 2022
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.
Answered by Corey on January 6, 2022
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