John,
Since the machine can not "see" if the bit touches, it relies on sensing more current needed to drive the Z motor.
I think what is happening is that there is something on the Z rails, a bad bearing or two, dust in the belt, or dust in the belt gear. Also check out the motor and that gear assembly.
Something is causing it to sense some resistance.
Ken,
V-1, 2, & 3
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