The call was for a Fanuc 0i-MF throwing SV0410 (excess error at stop) on the Z axis. Only on nights. Days ran clean. Two other engineers had been out and reset the parameters, no change.
What the machine was telling us
The alarm log said the fault fired within 30 minutes of shift change every night, then never recurred until the following night. Both engineers had chased the servo pack. Nothing there was wrong.
What clinched it: we asked the shift lead to walk the floor at 2am and note anything that felt different. His answer was that the compressor started cycling harder around then because a neighbouring unit went offline overnight. Voltage sag on the incoming supply was pulling the servo bus low just long enough to trip the safety threshold.
Fix
- Installed a line reactor on the incoming three-phase.
- Rebalanced loads with the site sparky so the CNC cell is not the last thing on the ring.
- Left the alarm history filtered for SV0410 so we would see it come back. Six weeks in, nothing.
Moral: not every alarm on a Fanuc is a Fanuc problem. Half of intermittent faults are power quality, and you will not find them by staring at the servo tune.
