Fanuc spindle alarm 41 means the one-rotation signal (Z signal) of the αi position coder is generated at an incorrect position — the once-per-revolution reference pulse that anchors threading, rigid tapping and orientation is arriving where it shouldn't, or inconsistently. Its sibling AL-27 is the coder signal missing entirely; AL-41 is subtler — the signal exists but cannot be trusted. Untreated, this is the alarm behind mysterious thread-pitch errors and taps that break ‘randomly’. Fanuc lists five causes; notice that three of the five are the cable, which tells you where to look first.
Cause 1 — Sensor parameter setting incorrect
Wrong sensor-type settings make the drive misinterpret the coder's signal train. Check the sensor parameters against the AC Spindle Motor αi Parameter Manual (B-65280EN) — prime suspect immediately after amplifier swaps, restores or commissioning, per the usual forensic rule: settings do not change themselves.
Cause 2 — The position coder itself is faulty
Fanuc's test is direct: observe the Z signal of the position coder — if it is not generated once per rotation, replace the coder. With a check board or oscilloscope, watch the one-rotation output while turning the spindle slowly by hand: exactly one clean pulse per revolution, at a consistent angular position. Multiple pulses, missing pulses, or a wandering position = a failing coder (contaminated/damaged disc, dying electronics). Also check the mechanical drive to it — a slipping belt or loose coupling shifts the Z position randomly, mimicking a bad coder (same checks as AL-27).
Causes 3 & 4 — The cable: shielding and routing (the usual suspects)
- Faulty shielding: the coder cable must be shielded, with the shield connected to frame ground via a cable clamp — exactly as Fanuc's connection diagram shows. A shield cut, disconnected, or ‘terminated’ through a long pigtail lets noise inject phantom Z pulses. Verify shield continuity and its clamp against B-65282EN (‘Connection’).
- Bundled with servo power leads: the manual's explicit callout — if the sensor cable runs cable-tied alongside a servo motor power lead, separate them. PWM switching noise from a neighbouring power cable is a classic source of spurious one-rotation signals. The same ‘tidy loom’ trap as AL-24: separation beats neatness.
Cause 5 — The spindle amplifier
Last suspect: the coder input stage on the amplifier's control board misreads a healthy signal. Board-level repair or exchange — but only after parameters, coder and cable have been cleared, because a good amplifier condemned for a cable fault comes back with the same alarm.
Diagnosis checklist (in order)
- Recent work? → verify sensor parameters (B-65280EN).
- Hand-rotate and observe the Z signal: one clean pulse per rev at a stable position?
- Check the coder's belt/coupling for slip; then the coder itself.
- Verify cable shield continuity and frame-ground clamping; walk the route — separated from power leads?
- All clear → amplifier coder-input stage: bench test, board repair or exchange.
- After the fix: air-cut a test thread and verify pitch before production.
Related guides
- Fanuc spindle alarm 27 (position coder disconnected)
- Fanuc spindle alarm 21 (position sensor polarity)
- Fanuc spindle alarm 24 (serial communication noise)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained