I work with Nakamura WTW-150's. We have 4. On 1 of them (the oldest 1), left spindle literally "shifts" .0035" toward the back of the machine between 3200 and 3500 rpm. It runs true as far as I can tell, just moves centerline over .0035". Put 1 indicator on the chuck (spindle nose) and another on the clamping cylinder (spindle rear) and they both just "move" the same amount. No crazy noise or vibration, surface finish is not affected and spindle taper does not change (turn test at 2500 and at 3500 1.25" rnd aluminum yield same results).

The problem comes with small parts. All our programs set G50 at 3500. Finish turn runs at 500 sfpm (hits G50), followed by a groove tool (on the same turret) at 2000 rpm. The groove tool won't blend right because it's .00175" deeper on one side. This also tells me that all my ID work is not concentric unless tool runs over 3200.
We can band-aid the problem by dropping G50 from 3500 to 3200 but I really want to correct the problem and not have to change all our programs.

I figure if spindle bearings were the problem, we would have finish issues...no?
Integrated spindle so no belts.
I've checked for all the mechanical problems that I can think of but like the title says, I'm out of ideas.

Any suggestions??

Could this be a servo issue? Do they get out of "tune" over time?