SINUMERIK 840D sl alarm 25040: axis moves at standstill, what to check?
I'm getting a SINUMERIK 840D sl alarm 25040 on one of my machine's axes. It happens after the axis has stopped, and it seems to drift slightly. What should I check first?
Alarm 25040 on a SINUMERIK 840D sl indicates that an axis, which should be stationary, has moved beyond its allowed position tolerance. This monitoring activates after a delay once interpolation has finished. Common causes include external forces pushing the axis, insufficient clamping, or issues with the servo control.
Likely causes and checks
1Machining force pushing a stopped axis
- Points to it
- External machining forces can push an axis that is supposed to be holding position, especially if it's not clamped or the force is significant.
- Check
- Observe if the alarm occurs while another axis is cutting or when the tool is in contact. Consider reducing the machining force by adjusting feed rates or speeds.
2Low position-loop gain
- Points to it
- If the position controller gain is too low, the axis may not be able to hold its commanded position effectively under load, leading to drift.
- Check
- Check if the axis drifts under load even with the servo enabled. If there have been recent tuning changes, the position controller (Kv factor in MD32200 $MA_POSCTRL_GAIN) may need optimization.
3Insufficient clamping
- Points to it
- If the axis is equipped with a clamp, and it's not holding effectively, the axis can move when subjected to forces.
- Check
- Verify the operation of the clamp and its pressure, especially if the alarm occurs when the clamp is applied or under load. Increasing clamping pressure might be necessary.
4Axis mechanically jammed with servo enable off
- Points to it
- If the servo enable is off and the axis is under load, a mechanical jam could prevent it from settling properly.
- Check
- Confirm that the servo enable (DB31, … DBX2.1) is 0 when the alarm occurs. Look for any signs of a mechanical jam and check the NC/PLC interface signal.
Next check: First, observe the machine's behavior when the alarm triggers. Does it happen during cutting, or when the axis is simply holding position? This will help narrow down whether external forces or servo control issues are more likely.
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