What causes FANUC alarm 742?

Spindle encoder feedback signal is faulty, damaged, or has loose wiring causing incorrect position data. Rigid tapping parameters (such as gear ratio or encoder pulse count) are incorrectly set, causing the error counter to overflow. Spindle acceleration or deceleration is too aggressive for the rigid tap cycle, overwhelming the LSI counter. Spindle motor or amplifier issue causing the spindle to lag too far behind the commanded position. Mismatch between programmed spindle speed and the actual capability of the machine during rigid tapping.

How do I fix FANUC alarm 742?

1. Check the spindle encoder cable and connector for loose pins, damage, or contamination. Reseat the connector at both the encoder and amplifier ends. 2. Verify rigid tapping parameters in the CNC, specifically the spindle gear ratio parameters (typically P5241-P5243 or similar) and encoder pulse parameters match the machine specification. 3. Reduce the programmed spindle speed for the rigid tap cycle and test again. If the alarm clears, the spindle cannot accelerate fast enough and the speed or ramp time needs adjustment. 4. Check the spindle drive amplifier for any separate fault codes or warning LEDs that may indicate a drive or motor issue contributing to position lag. 5. Inspect the spindle encoder itself for contamination, physical damage, or excessive runout. Replace the encoder if signal quality is suspect. 6. If parameters were recently changed or a control backup was restored, compare all rigid tapping related parameters against the original machine setup documentation.

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FANUC Controller

Spindle · Intermediate · 45-90 minutes

Alarm 742

Fanuc Alarm 742 - RIGID TAP ALARM: LSI OVERFLOW

Fanuc Alarm 742 RIGID TAP ALARM: LSI OVERFLOW occurs when the spindle position error counter in the spindle amplifier LSI exceeds its limit during rigid tapping (G84/G74). This indicates synchronization loss between spindle encoder feedback and commanded position, typically due to encoder issues, incorrect rigid tap parameters, or spindle drive problems.

FANUCSpindleIntermediate45-90 minutes

742

Fanuc Alarm 742 - RIGID TAP ALARM: LSI OVERFLOW

SPINDLE ALARM 742

Root Cause Summary

Fanuc Alarm 742 RIGID TAP ALARM: LSI OVERFLOW occurs when the spindle position error counter in the spindle amplifier LSI exceeds its limit during rigid tapping (G84/G74). This indicates synchronization loss between spindle encoder feedback and commanded position, typically due to encoder issues, incorrect rigid tap parameters, or spindle drive problems.

Causes and Fixes

1
Check spindle encoder signals on DIAGNOSIS screen > SPINDLE > SP-FB (spindle feedback). Verify encoder position counts are stable and incrementing smoothly during manual spindle rotation. Check parameters 4020-4023 (serial spindle) or 4001-4008 (analog spindle) for encoder type settings.
2
Inspect spindle encoder cable connections at both encoder and amplifier. Check connector pins for damage, corrosion, or bent contacts. Verify cable routing away from power lines and proper shielding/grounding.
Power off CNC and amplifier before inspecting connections to prevent electric shock
3
Check spindle amplifier LEDs. READY should be solid green, ALM should be off. If encoder fault LED is illuminated, replace encoder cable first, then encoder if problem persists. Check alarm history (ALARM > HISTORY) for related spindle alarms (400-series).
4
Monitor DIAGNOSIS > SPINDLE > POSITION ERROR during manual rigid tap test at low RPM (100-200 RPM). Position error should remain within ±10 pulses. If error continuously grows or spikes, encoder or amplifier LSI is defective.

DIY Feasiblemedium confidence

Call a technician if:

·Spindle encoder replacement required
·Amplifier replacement needed
·Mechanical spindle rebuild required
·Multiple parameter sets need optimization

Prevention

Regular spindle encoder cleaning and inspection
Monitor rigid tapping parameters during preventive maintenance
Check spindle mechanical condition quarterly
Maintain proper electrical cabinet cooling

Common Mistakes

Ignoring encoder contamination during routine cleaning
Setting rigid tapping gains too high for quick cycle times
Not checking spindle runout after tool holder changes
Overlooking Z-axis mechanical issues when alarm points to spindle

Related Alarms

740 741 743 744 414 415

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