Y AXIS OVERSHOOT SMALL MOVES

[MACHINE GOAT]

Have Monarch mill with Fanuc 16-m. While flowline 3d surfacing - most of the motion is in x z axis there are very small moves (+/- .0001 in y axis) within the program. The machine is overshooting these minute y moves leaving dings in the surface finish. looks really bad. The old machine has axis scales. The back lash parameter for y axis is set at 5 whatever that translates to not sure but seems really low for age.
any one know what's going on?

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[underthetire]

Check and report back parameter 1852

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[MACHINE GOAT]

PAR 1800 RBK is @ 1 so uses both feed and rapid parameter sets

FEED PAR 1851 @ x5 y5 z0 a2

RAPID PAR 1852 @ x1 y1 z1 a10

[generaldisarray]

I have tried surfacing on a Monarch "A" type years ago and had similar issues. Fanuc 15a or b. I attributed it to the quill type construction.

[MACHINE GOAT]

Overshoot was happening in area where the ball screw should have the most wear.
We set 1851 Y parameter to zero and now no more overshoot nor undershoot.
I am wondering why have backlash parameters at all if machine has scales?
Why were they set in the first place?
puzzled...