OK I have my licensed copy of mach on a very tight lathe (no unauthorized movement in any axis) I have my tool ( a new, sharp, 60 degree threading tool) perpendicular to, and in the center of, the 1" diameter 6061 tubing .25" thick workpiece riding 3" from the chuck. I run the program with 2 cleanup passes and taking .005 each pass (just to make sure) up to .050 depth at 86 rpm. My .05 pitch threads are all U shaped and the tops are cut off.
Any guesses?