Jerky Motion, Threading Under Load

[Magneto]

Folks,

We're consistently getting sub-par threads and are noticing a visibly jerky Z-axis motion when the threading insert gets deeper into the threads.

Motion while cutting the threads near the OD are fine. Also, threading speed doesn't seem to matter much.

System is an Oak, circa 2013. Heavy, 2-axis slant bed lathe. All other functions are fantastic.

Thanks.

[cncsnw]

When you post a report, we can look at:
1) What CNC11 or CNC12 software version you are using
2) Whether you are running in analog velocity mode or in pulse-train position mode
3) If in analog velocity mode, what your PID gains look like

In addition to posting a report, you will need to just tell us what make and model your servo drives and motors are. That information is generally not contained in the report files.

[Magneto]

Here is a report from the machine. In addition it has a SGDV-170A01A servo drive and a SGMGV-20A3A6C servo motor on the x axis and a SGDV-330A01A servo drive and a SGMGV-44A3A61 servo motor on the z axis.

Let us know if you need any other information.
Thanks

[cncsnw]

Quick response, since I am supposed to be working on another machine right now:
1) Your control uses OpticDirect boards to interface to the Yaskawa drives.
2) The analog sections of your OpticDirect boards are not populated, so unfortunately you cannot set up servo axis load meters.
3) You are running your axis servos in pulse-train position mode (what Centroid calls "precision mode").
4) Therefore, the drives' ability to follow the commanded motion profile under varying load conditions is determined entirely by the drive tuning.
5) You should test whether the issue is indeed related to cutter load, and not to toolpath geometry, by identifying a job that clearly exhibits the jerky motion, then dry-running that job with no stock to cut or with no tool in place.
6) If the jerky motion is only present when there is cutter load, then it would be worth re-running the Yaskawa drive auto-tuning procedure through the SigmaWin+ software.
7) If the jerky motion happens only on the deeper passes, but happens even if there is no cutter load, then it might be worth looking at Centroid Machine Parameters 240-242 to see if you can change the threading accel/decel ramps.