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Spindle speed bogs down while cutting

I was working an Emag CNC machine the other day. The customer was having problems when they were boring into the part. The spindle would bog down during the cut. When viewing the Spindle actual RPM compared to the commanded spindle speed the actual would decrease to about 1200 from 3000rpm. To try to determine the problem I had a few things I checked first. The tool ins

erts were my first thought. I found that they were in fact bad and the new ones helped but didn't eliminate the problem completely. Next I checked the spindle motor insulation resistance with a megger. The reading was less then 20megohms. When checked with a regular meeter it checked above 30meg. Why the difference in the readings? The reason was that the megger outputs 500vdc and the ohm meter hardly nothing.

Next option was to send the motor out to get rebuilt. This particular machine had a coolant rotary union on the top that was recently replaced. My guess was that the coolant had seeped into the motor and caused the insulation resistance to drop.



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