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.