Hi all,

I hope someone can come up with any ideas as to why I am getting oval holes on the diagonal (10.30 and 4.30 o'clock). I have managed to minimize it but still getting a 0.5mm error either side in holes 20mm in diameter. I have rack and pinion on x and y with negligible backlash.
Things I have tried include:
  • Lowering feedrate,
  • increased acceleration
  • Exact stop mode
  • Smaller tolerance settings
  • Lower kernel frequency and increased buffers

I read this in another forum:

"Backlash affects you at MOTION X DIRECTION CHANGES (12, 3, 6, 9 o'clock) - the monkey motion problems at 45 deg are happening while the motors are attempting to do the most/fastest relative changes in velocity - one is speeding up, the other axis doing the opposite SIMULTANEOUSLY... A simple look and see at the motors will show/prove this
To make the motors "walk and chew gum" at the same time, they have to be timed EXACTLy the right speed relative to each other thought the motion cycle. We chased this all over hell and back until we FINALLY figured out how to properly tune for "follower error". Cutting ROUND circles is THE definitive test to assure you that you have servo follower error tuned properly.

Trust, me, it isn't back lash and it ish't backlash comp. It is a problem in the instantaneous RATE of feed change that the axis are making that is causing the 45 deg out of round problems. If the controller is NOT changing the RATE at which the step pulses are instantaneously being fed to the steppers, they won't/can't keep the poper relative speed to each other and they thus CAN'T cut round holes."

"It depends where it is out of round and by how much. Errors at the direction change points (12, 3, 6, 9 o'clock) are typically caused by backlash and take up errors in your drive'ball screws. Out of roundness at 45 degrees off the vertical or horizontal axis are due to servo tuning errors. At this point, both servor are affecting a high rate or change in opposite directions. Unless the tuning is perfect, you;ll start getting follower error and out of roundness."

I have ordered an extension so I can connect to the com port on the drivers (LCD A86Hs). Is there anyone familiar with these settings or had issues such as this before? Jazz, I know you use Lichuans, have you ever had to tune the drivers for such errors?

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