You guys were a big help getting my spindle running the right rpm for tapping, thank you for that. Now, I still have an odd issue, my rpm changes from what is called and it's completely random.

Machine is a 1992 Partner 4 Knee mill, with a Centurion 5 "A" version control.

For example, I'm running a job with 5 tool changes, one larger drill I run at 2000 rpm, everything else runs 3000rpm. I would say about once per part, one of the tools will run the full 5000rpm that the machine is capable of, it's never the same tool every time, it varies, sometimes it's the slow drill, sometimes it's one of the other tools that's supposed to be running 3000rpm. It appears completely random, I've counted tool changes between full speed errors, I've check time, like every ten minutes or something.... and nothing seems to fit a pattern.

I thought maybe it was a bad voltage signal off the Acroloop card, and I had a newer "spare" X axis card, so during the tapping issue I swapped out the card, hoping my errant speed issues would go away. No such luck. Now that I have found out I had pots inside the control console to adjust for minimum and maximum voltage on the keyboard circuit board, could my current speed issue be coming from something in there? I have checked voltages when it's in the incorrect speed, and the voltage going to the VFd is 10V even when it should be say 4V (for 2000rpm). Spindle speed over ride pot will decrease the speed for me, even though the screen reads in error (it always reads the commanded speed).

Given I'm machining mostly aluminum at the moment, it hasn't been life or death if the machine runs 3000 or 5000, but when I have a large drill that should only be running 1500 or so, or tapping.... a run away speed is going to be disastrous. Any suggestions would be greatly appreciated.