Bad Ball Screw?

[Katoom]

We have a Makino FNC74.The Z axis is giving us problems.When machining
apart,the "Z" will go .012 deeper than it should.The read out reads the currect number,but the part is out of spec.
We used a dial indicator to check for this problem.What we found was at
a slow feed rate the "Z" was dead on everytime.But when we increased the
feed rate,the issue with the .012 returned.The readout reads the same every single time.and it does not change as the "Z" comes down.So I know that the Ball screw is not turning.
I was told that this could be caused by a bad Ball Nut.That at a higher feed rate,the balls move slightly.but then relax causing the "Z" to come down.
Is this true,or am I over looking something?And is there a way to verify that the Ball Nut is bad? Thanks

[RICHARD ZASTROW]

Is the ball nut mounted securely? Have the mounting screws loosened? Just a thought.

Dick Z