I'm cutting 1/2-13NC threads and was using 500 RPM. Threads were looking really bad. Torn/Ruff. Went up to 1000 RPM and finish improved but the "nut" would not go on, PD (pitch diameter) was correct with 3 wire measurement, insert was not broken and OD was good. This drove me nuts for over a hour until I found the pitch was off for the first thread. I moved the start point away in plus Z by 1/4" and the problem went away. Seems the machine just needs more time to sync up. Am I going too fast? Does the spindle need some type of speed calibration? No mention of max speed in the manuals I have. This is a 1995 machine with the Siemens 840C control. VER 4. I think I would like to even go faster, what's the limit?