Fanuc 21iT Linear Scale feedback problem

[lethal375]

I have a Clausing lathe with a linear scale on the X axis.
Owned this lathe for 3 years and its always cut great, accurate, repeatable, etc.
Recently I had an issue with backlash while cutting a long slight tapper, x axis ball screw was moving but it would actually move after about .002 in ball screw movement. Kind of like a stick slip.
I found that the drive pully on the ball screw was loose enough to allow linear movement but not really any radial.
I removed the pully and found the nut on the thrust bearings could be turned by hand.
Pulled the ball screw and it had no preload, but not loose so I had it rebuilt.
Someone had been in there and did a hack job. Looked like they used a hammer and chisel to work on it. Pully threads and shaft all beat up.
While waiting for the ball screw I checked the gib adjustment and it was perfect. It slid up and down freely.
Pulled gib and checked all the Turcite. All lube lines clear and oil flowing
Now with the ball screw is back in, the required backlash setting is almost nothing.
With the lathe sitting idle the X axis will begin to hum. It will increase from nothing to a noticeable harmonic sound over about 10 sec. You can definitely see the belt vibrating. If I squeeze the belt from both sides, it will settle down. I can't see any actual movement in the X with an indicator.
What I am I wondering is how the scale and motor work together and not fight each other?
Is there an adjustment, parameter, or a way to tune the system?
Maybe the scale is not registering properly?
Should the backlash be set to zero?
I was told to turn off the scale then set the backlash. Turn scale back on.
Tried adjusting the backlash up and down to see if it would help but no change.
Even with this issue the lathe is noticeably better.

Any thoughts
Thanks

 

[lethal375]

Update
X axis got worse began overloading the servo and faulting out occasionally.
I took it back apart and found an issue with the spacers/washers above and below the brake.
As I mentioned in original post someone had been in there and I did not have a good breakdown of the parts and their exact locations.
Nothing stood out as a problem except for the loose pulley, loose nut on thrust bearings, and ball screw with no preload.
These three items seemed to add up to the problems I was experiencing.
I made sure all the parts went back in the same order as they came out.
I had removed the brake assembly complete with back mounting plate.
The spacers and washers were not originals and when the brake assembly went back on there was another issue I was unaware of.
Brake is spring applied and electrically released
When fully assembled you cant turn the ball screw unless the power is on and then with the motor on it will fight you.
With all the correct parts and in the correct places it works properly, no humming or oscillation in the belt.
Seems the previous owners had left the nut loose on the thrust bearings to make it operate.
Sure glad I got a good deal on this lathe to offset the stupid problems
Thanks