And my first post here! Hello Everyone! I've been doing machining and fabrication (bridgeport, manual lathe, waterjet, and tig welding) for about 15 years now (started when i was 15) have owned my own shop for the last 5 or so years and just bought my first CNC machine, a late 80's VMC40 with a CNC88 controller.

I just got it powered up after cleaning it, oiling the ways, etc, have only jogged the servos so far, haven't turned on the spindle yet. My issue is with the z-axis, it has a shudder at regular inch intervals, i should say the shudder goes away at regular intervals (every 0.200") and gets worse as you approach halfway between those intervals, it will just sit there and vibrate shaking the whole machine, I did not leave it there very long, maybe only 15-20 seconds it didn't go away so i moved it back to where it stops, it does it the whole way up the travel in 0.200" increments. Now this is my first foray into a CNC mill, but i am somewhat familiar servos and controls. It seems to me this is maybe an encoder/tach adjustment issue? like the board doesn't think the servo has moved so it tries again, maybe overshoots, or is in hysteresis on the feedback, and then gets into an oscillation?

A little background on the machine, it has been sitting in mothballs for 10 years probably, so this is it's first powerup in a long time, i have not done any backlash adjustment in the controller, just turned it on and lifted the Z off the wood parking block, and cycled the table x/y.

What should be the first thing i start with here?