HAAS MDC-500 Z-Axis Servo Oscillation Issues (motor tuning)

Does anyone have experience tuning HAAS servos?

I have a 2007 HAAS machine here that's just had the Z-axis ballscrew and support bearings replaced. The locknut on the bearing block had come loose and the bearings had gone off kilter and started wearing into the ballscrew shaft until it seized.

The machine moves now, but the Z axis oscillates subtly when commanded to hold position if there is a disturbance. I don't have any instruments that can record frequency, but it's an audible hum and a bearing vibration meter on the head gives me 0.3mm/sec. Disturbances include, machining something, running at high RPM, or a bump from actuating the tool clamp/unclamp. Stops oscillating if you change the set point by even one micron.

Too me this sounds like a control issue, but I'm not sure where to start without any documentation. Could be that the servo performance has changed due to pushing through high load. Could be that the coupling has worn and allowing a bit of slop. Or could be that the servos had been tuned to suit the higher friction of the previous bearings and ballscrew.

Before throwing money at a new a servo motor that may not fix the problem I'd like to try tweaking the motor tunings. I'm inclined to try increasing D Gain, lowering the torque preload or I Gain slightly, or adding a bit of deadzone.

I could use some advice from anyone who has attempted tuning a HAAS machine previously.

Current Parameters	X-Axis	Y-Axis	Z-Axis
P Gain	32	32	32
I Gain	48	48	64
D Gain	6000	6000	4000
D2 Gain	1000	1000	1000
Deadzone	0	0	0
Acc/Dec T constant	500	500	350
Torque Preload	0	0	600

Seems to have been solved by leaving gains alone but enabling LowPass+2X on parameter 29. Need to put covers back on and test to be sure.