What causes FANUC alarm OT0508?

Part program commands movement beyond the configured B+ axis software travel limit. Work coordinate offset (G54-G59) or tool length compensation error placing the programmed position outside the travel envelope. Axis homing not completed — software travel limits are not enforced until the axis returns to its reference position. Axis travel limit parameter (parameters 1320/1321 for stored stroke check 1, 1322/1323 for check 2) incorrectly set. Manual jog in JOG or HANDLE mode moved the axis past the software limit.

How do I fix FANUC alarm OT0508?

1. Jog the axis in the opposite direction (away from the limit) to clear the overtravel and allow the alarm to be reset. 2. After clearing the alarm, verify the part program coordinates and active work offsets are within the machine travel envelope. 3. Check tool length compensation values — an incorrect H-code value can place the tool path outside travel limits. 4. Verify the machine has completed its reference return — software stroke limits are inactive until homing is done. 5. Check the stroke limit parameters (1320/1321 series) against the machine's actual mechanical travel range.

axismd.ai

FANUC Controller

Overtravel/Limit · Basic · 10-20 minutes

Alarm OT0508

Fanuc Alarm OT0508 - B-Axis Positive Overtravel

FANUC OT0508 indicates the B-axis rotary table has exceeded its positive travel limit, triggered either by hardware overtravel limit switches (OT+) or software stored travel limit parameters. This alarm halts machine motion and requires identifying whether the violation is hardware-based (physical switch activation) or software-based (parameter limit exceeded), then clearing the condition and correcting axis positioning.

FANUCOvertravel/LimitBasic10-20 minutes

OT0508

Fanuc Alarm OT0508 - B-Axis Positive Overtravel

Overtravel +B

Root Cause Summary

FANUC OT0508 indicates the B-axis rotary table has exceeded its positive travel limit, triggered either by hardware overtravel limit switches (OT+) or software stored travel limit parameters. This alarm halts machine motion and requires identifying whether the violation is hardware-based (physical switch activation) or software-based (parameter limit exceeded), then clearing the condition and correcting axis positioning.

Safety

5-axis head may collide

Causes and Fixes

1
Check DIAGNOSIS > OT SIGNAL screen (softkey path: SYSTEM > DIAGNOSIS > select page with OT signals). Verify if 'B OT+' signal shows '1' (active). This confirms hardware limit switch is triggered.
2
Physically inspect B-axis positive limit switch, wiring harness, and mounting bracket for damage, misalignment, or contamination. Verify switch is not stuck closed or damaged. Check X71 connector on axis control board for loose connections.
Do not bypass or disable overtravel switches except for recovery procedures. This removes critical safety protection.
3
Use manual pulse generator (MPG) with OT release function: Hold RESET + direction key (negative B-axis direction) simultaneously while jogging axis away from limit switch at 1% rapid override. Monitor DIAGNOSIS screen until 'B OT+' signal clears to '0'.
Ensure adequate clearance exists before jogging. Incorrect direction will cause immediate alarm re-trigger.
4
After clearing alarm and moving axis to safe position, verify software travel limits in parameters 1320-1327 (stored travel limits). Set Parameter 1320 (B-axis positive limit) to appropriate machine coordinate value. Execute RETURN TO REFERENCE (home) to re-establish machine zero.

Service Recommended confidence

Seeing Multiple Alarms?

Seeing another alarm with this one? Enter the code.

Get machine-aware diagnosis

AxisMD logs alarms against your actual machines, tracks patterns, and gives AI context your manual doesn't have.

Start free

Was this guide helpful?

Always verify with your machine manual or a certified technician before performing service procedures.