Rob McDonald writes:

I hope somebody has the patience to read through this :-) I am trying to
include all the details because I don't know what I am doing wrong.


I recently tried to cut a 3/8-40 thread on a piece of 1018 HRS. First
pass was smooth. With later passes the surface seemed to get rough,

then before reaching final depth small sections of the thread tore out...

I had a similar problem turning threads on similar material- I gave up
on trying to get a smooth finish on the lathe, and switched to
"burnishing" on a wire wheel. Probably rank heresy but it did clean
up the burr pretty well. I was doing 1/4-20, that 40tpi might not
burnish so well...

Gregm