Is A6 really that difficult to machine?

I am having a devil of a time getting a good finish on A6 steel. I am making a custom tap. When I thread the rod, the thread finish is very rough. It seemed to turn to diameter OK (not great), so the problem seems to be in threading.

My 60-degree threading tool is HSS, sharp and stoned, zero rake. I was turning 70 RPM, the OD was 0.625-in. (I am making a 5/8-24 tap.)

Just to make sure I am not doing something grossly wrong, I switched to 12L14 steel, and it worked beautifully.

The lathe is a JET 1340, I used a live center in the tailstock. The tool stickout is as small as I can make it.

Any wisdom?


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Just guessing, 'cause I don't know...

Cutting oil?

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Yup. Two kinds tried: Tap Magic and a threading oil. Same difference.


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