Hi Al, that toast is me.
I had always thought the DC common was to be kept seperate from the chassis ground so I made no effort in that regard.
I have a hard time understanding how the HF noise can interfere with a low impedence, relatively low freq dc signal (even though it is a square wave) , but I can't think of what else it could be unless the Galil board itself is losing count somehow.
I had a scope on the step signal for a while, don't remember seeing a significant amount of noise but I can't remember if I had the VFD running.
I have resigned myself to moving the VFD and spindle motor wiring outside the cabinet on the back of the mill. It is just so nice and neat the way it is.