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Check the spindle speed command in the program (S code) and verify it does not exceed the rated motor speed.

Access parameters 3741 (maximum spindle speed) and 3742 (gear-specific limits) and verify they match motor specifications and do not have incorrect values.

Check spindle override setting on operator panel - ensure it is not set above 100% if speed is already at maximum.

Review parameter 3772 (speed arrival tolerance) to ensure proper velocity detection window is configured.

Check spindle drive diagnosis screen for actual speed feedback and compare to commanded speed - look for erratic readings or noise.

Ensure spindle is fully stopped before inspecting connections

Inspect encoder cable for damage, proper shielding, and secure connections at both motor and drive ends. Check for cable routing near high-voltage lines.

Monitor encoder signals A, B, and Z phases using drive diagnostics or oscilloscope for noise, dropouts, or incorrect voltage levels.

Use proper grounding and isolated test equipment when checking encoder signals

Check spindle motor encoder bearing condition - excessive runout or bearing wear causes signal instability. Manually rotate spindle and feel for rough spots.

Lock out machine power and ensure spindle cannot start during manual rotation

Review maintenance history for recent parameter changes, drive replacement, or motor replacement that may require re-tuning.

Check spindle velocity loop gain parameters (3730-3736 range) for incorrect values or recent modifications. Compare to baseline or OEM documentation.

Perform spindle velocity loop tuning procedure using Fanuc servo guide or drive tuning software to optimize gain and response.

Perform tuning in T-mode or with workpiece and tooling removed to prevent damage

Test spindle acceleration at progressively higher speeds to identify if overshoot occurs at specific speed ranges, indicating gain adjustment needs.