Fanuc spindle alarm C8 means that in spindle synchronization, the speed deviation exceeded the alarm detection level (parameter 4515) — the difference between the specified spindle-end speed (computed from position error and position gain) and the actual speed grew too large. Spindle synchronization is what lets a sub-spindle grab a part from the main spindle at speed — and C8 says the follower is not tracking. Fanuc gives two scenarios, and the first is a genuinely subtle sequence bug worth knowing.
Case 1 — Immediately after motor excitation (SFR/SRV) is cycled during synchronization
Fanuc's insight: if the ladder turns motor excitation off and back on while spindle synchronization is active, position error accumulates while the motor is dead — and at re-excitation the motor accelerates hard to eliminate the accumulated error, blowing through the speed-deviation limit. The fix is in the sequence: turn motor excitation off only after spindle synchronization is released. The same family of sequencing fault as AL-36's SFR/SRV case — suspect it when C8 fires at a consistent point around mode transitions, especially after ladder work.
Case 2 — During cutting
- Overload: the synchronized pair cannot hold speed under the cut — review the cutting conditions (and tooling; a dull tool at the part-transfer cut is a classic).
- No overload present: review whether the detection level in parameter 4515 is appropriate — against the builder's value, not upward until silence: a loosened threshold lets a genuinely mis-tracking synchronization pass, and the next symptom is a botched part hand-off.
Related guides
- Fanuc spindle alarm C9 (synchronization position error)
- Fanuc spindle alarm 36 (SFR/SRV sequencing)
- Fanuc spindle alarm 43 (master coder, differential control)