I'm fighting a weird oscillation on Contouring and circles. See pictures. Tool is traveling around the contour but I'm getting a very repeatable oscillation on every pass. I'm doing a finishing pass taking at most 0.02" with a 1/4" carbide ball mill. The pocket is approx. 6" x 12" x 4" and shaped like an elongated tear drop. X runs left to right on the picture, y up and down.

My machine is very rigid. Structural steel and Cast plate mainly. I'm running 1HP DMM-tech AC servos and using their older smaller servo drives. They are tuned as good as I can get them but I suspect it could be the root cause of the oscillation. The X-axis is dual driving 2 racks and pinions 3:1 belt drive reduction from CNC router parts. The Y-axis is 2:1 reduction. Spindle is 2.2KW water cooled Chinese.

I'm running Linux CNC in open loop and treating the servo and controller just like a stepper. I'm driving them with 2000 steps per revolution on each axis.

I've got the acceleration and max velocity set the same on both axis. I've tried different parameters but no change in cut quality.

Speed does affect the size and amplitude of the oscillation. Going slower is better. It's not noticeable by ear but the oscillation shows in the cut. Straight cuts are perfect. It's when I'm moving both axis at the same time I have issues. It does seem like both the x and y are contributing to the issue.

The loads on the x is much higher than the y and the x has more friction in the linear bearings. My gantry weighs approx 250 - 300 lbs. Just the y and x is probably 150 lbs. All axis are running Hiwin square linear rails and bearings. X I'm using 1" size and the y is using smaller size.

I'm using Rhino and MADcam for the rest of the software. The tool paths in MADcam are perfect and don't show and oscillation.

I've been fighting the circle issue for a while now and can't seem to find a final solution.

My suspicion is I have a velocity oscillation created by bad tuning job on the servos but I'm getting no other indication that my tuning is bad. I get very repeatable positioning, no audible noise. I can drive them fast and hard. My acceleration and deceleration are short but not unreasonable. The dual drive isn't making tuning any less complicated on the x either so I may have them fighting each other as another possibility. Another issue might be that my x and y are so different in servo tuning due to the different loads that I'm getting some sort of beat frequency in acceleration/velocity between the 2. A third option might be that I'm getting some sort of interaction between the rack and pinion and the servo tuning. The period of the oscillation is approx. the same as my rack tpi. I thought it would help by going from 2:1 to 3:1 on the belt reduction on the x. It did help my servo tuning but I get the same results before and after the hardware change.

Gantry deflection can't be it because the oscillation shows up even when just engraving with a tiny bit and small cut loads all the way up to hogging AL with a 1/2" bit at 150 ipm.

I really need to solve it very soon. I have a customer waiting on these molds and I would like to get a good surface finish right off the machine.

Any suggestions on what to try would be greatly appreciated! Going slower is really not an option as these pieces takes hours even at 150ipm.