Mill Motor trips GFI outlet

I have a Leeson 1/4HP PM DC motor on my Taig mill and a treadmill speed control. I only have 5 to 10 hours on the motor, it was new when I acquired it. Yesterday, I was using the mill to cut the heads off some rivets in some bed frame angle iron. Out of the blue, my bench shut down. It turned out that it tripped the GFI outlet. I reset it and started the PC and brought up Mach3, turned on the stepper driver power and moved the mill away from the rivet and then started the motor. Just as it was reaching full speed, it tripped the GFI again. I reset the GFI and this time, only started the motor. I slowly increased speed and again, just as it reached full speed, the GFI tripped again.

I am guessing that one of the brush wires is shorting to ground, but it seems that would cause a blown fuse in the control. There is no moisture involved, so I'm not getting leakage there tripping the GFI. Any words of wisdom before I pull the motor apart?

Don