I have a problem with my CNC Toyoda Grinder model # GL5P 32II. The spindle keeps slowing down below the minimum speed without showing an alarm. No one is turning the spindle off, an the manufactor has never heard about this. I have switch the relays for the invertor but still have the same problem. Does anyone have any more ideas???

how old are the belts?

Just changed two weeks ago when problem first started, We also had the service tech out for other grinder when it happened while he was here. It started out once a week now it is twwice a day.

Spindle speed
One easy check is amperage out of frequency drive. Get clip on amp meter, digital display. Then check each lead out of frequency drive. If no load amperage increases as spindle speed decreases there likely is problem with spindle binding or related interference. If amperage drops with decreasing speed, inverter issues likely exist.
Also switch inverter to freqency and amperage displays to see if agreement with clip on multimeter is present.