Fanuc spindle alarm 72 occurs only with Dual Check Safety, and means the spindle amplifier's speed check result does not match the CNC's speed check result. This is Dual Check Safety doing exactly what its name says: two independent processors — one in the amplifier, one in the CNC — each monitor spindle speed and continuously compare answers. When the two disagree, the system cannot know which one is right, so it stops. The spindle may be behaving perfectly; the disagreement itself is the fault.
What Fanuc says — and what to check first anyway
The manual's remedy goes straight to hardware: replace the CPU card in the CNC or the spindle amplifier's control PCB — one of the two monitors is computing wrongly. Before shipping boards, three free checks:
- One clean power cycle — a single transient disagreement after a power dip is not a trend; a repeating AL-72 is.
- Alarm history: companion codes name the sick side — amplifier self-test codes (AL-13, AL-74/75, AL-16) → amplifier board; CNC-side memory/system alarms → CPU card.
- Recent work: a swapped board with mismatched software series/edition between CNC and amplifier can make the two safety channels compute differently — verify compatibility before condemning hardware (the recurring lesson of AL-14).
The repair
With history and compatibility checked, split the pair: swap-test the amplifier (or its control board) against a known-good unit if available — the fault follows the guilty board. Then board-level repair or exact-spec exchange of the failed side, completed by a verified safety-function test: on a Dual Check Safety machine, a speed-monitoring repair is only finished when both channels demonstrably agree through the speed range, guard open and closed.
Related guides
- Fanuc spindle alarm 69 (safety speed with guard open)
- Fanuc spindle alarm 77 (axis number check mismatch)
- Fanuc αi drive alarms — PSM, SVM & SPM codes