OT0509

Navigate to POSITION screen and verify current B-axis absolute position (ABSOLUTE display). Note the exact position value where alarm triggered.

Access SYSTEM > PARAM and check parameter 1321 (B-axis, 5th axis negative software limit). Compare trigger position to this limit value. Also verify parameter 1320 (positive limit) for proper range configuration.

Check parameter 1300 bit 2 (stored stroke check). If set to 1, software limits are active. Review part program G-code for B-axis commands (G00, G01) that exceed the -1321 limit, especially after G54-G59 work coordinate shifts or G92 offsets.

Do not modify parameter 1321 without verifying mechanical travel capability and collision zones with machine builder documentation

If program is correct, perform manual return: Press RESET, switch to JOG mode, disable software limits temporarily via parameter 1300 bit 2=0, manually jog B-axis positive direction to safe position, then re-enable parameter 1300 bit 2=1.

Disabling software limits removes critical safety protection - use extreme caution and verify clearances

Access DIAGNOSIS > I/O > DGN#300-399 and check diagnostic 300 (axis control status). Verify bit 7 shows DOT- (negative overtravel) signal is active for B-axis. Also check PMC ladder I/O status for X2.7 input.

Physically inspect B-axis rotary table for the negative limit switch (usually dog-and-switch type). Verify switch is not mechanically damaged, dog is properly positioned, and wiring connectors are secure. Check switch continuity with multimeter if accessible.

Ensure machine power is locked out before accessing limit switch hardware

Review parameter 1300 bit 3 (OT check). If set to 0, hardware limits are disabled which may have allowed overtravel. Set bit 3=1 to enable. Check PMC ladder logic (G-code LADDER screen) for custom OT signal handling that may be inhibiting limit switch signals.

To recover: Press RESET, switch to HANDLE/JOG mode, hold the +B direction button. Most Fanuc controls allow jogging away from an overtravel condition at reduced feedrate (parameter 1401 OT release speed, typically 2-5% rapid). Monitor diagnostic 300 until DOT- clears.

Move slowly in positive direction only - verify no mechanical obstructions before jogging

Check DIAGNOSIS > SERVO > DGN#300 for B-axis servo alarms. Look for encoder error bits or position deviation alarms (SV0410-series). Verify amplifier LED on B-axis servo drive shows normal green status, not red/flashing error codes.

Access SYSTEM > PARAM and verify parameter 1815 (B-axis reference point position) matches mechanical zero position. Check parameter 1320 bit 5 (reference point return complete flag). If flag is 0, reference point is lost and requires re-establishment.

Check absolute position encoder battery voltage: DIAGNOSIS > STATUS > check battery voltage display (should be >3.0V for absolute encoders). Low battery (SV0441 alarm history) causes position loss. Replace CNC and encoder batteries if voltage is low.

After battery replacement, absolute reference point must be re-established per machine builder procedure