I just add the cnc4pc interface card to take the 0-5v logic signal and turn it into 0-10v for the VFD. I also added the index pulse card. This all works great and was easy to get running. In open loop, the spindle speed is within +/- 10rpm. My problems arise when i try to close the loop. No matter what i do with the pid settings, i cannot get a good spindle response. The spindle calibration feature is helpful to look at but not for actually tunning the spindle. Really all i need is a good response down low for tapping. Up high, it doesn't matter if it takes 2-4seconds to stabalise the rpm. I did some searching on here but haven't found anything positive and useful. Is there something i'm missing here? Are any of you running closed loop?

Well I 'm having exactly the same problem. I did managed to change the PID rates and get a very stable RPM, now they only fluctuate somewhat 1 or 2 high and low! I'm stunt! it was a battle up hill but my threading still cutting in the most horrible way!