You command T5 and the machine loads a completely different tool — or alarms mid-change and now every pocket is off by one. An ATC tool number mismatch means the control's tool table no longer matches what is physically in the magazine. It usually happens after a power failure or E-stop during a tool change, a crashed arm, manual rotation of the magazine, or a battery/data loss event. Here is how the tool memory works and how to re-synchronize it on Fanuc, Mitsubishi and Haas machines.
How the control tracks tools
On most machining centers the control keeps a tool registration / pot table — a data table mapping magazine pockets to tool numbers, maintained by the PLC/PMC (not the NC part-program side). Random-access magazines also swap table entries at every change (the tool going back rarely returns to its original pocket). If a change is interrupted half-way, the table and reality diverge — and they stay diverged until you fix the table, not the mechanics.
Step 1 — Physically audit the magazine
- Note the tool currently in the spindle (read the tool itself, not the screen).
- Jog/index the magazine in manual mode and write down which tool is physically in each pocket.
- Compare against the control's tool table — the difference tells you whether one swap failed (two tools exchanged) or the whole table shifted (off-by-one after manual magazine rotation).
Step 2 — Correct the tool table
Fanuc machines: the pot table lives in PMC data tables (builder-specific D-addresses) and is usually exposed on a builder's TOOL LAYOUT / MAGAZINE screen. Edit the pocket-to-tool mapping there to match your audit, including the spindle-tool and standby-arm entries. If there is no builder screen, the table can be edited on the PMC DATA screen — addresses are in the builder's ladder documentation. (Protection may need the program-protect key or PWE — see our write-protection guide.)
Mitsubishi (M64–M80): the tool registration screen (Tool Regist / Magazine screen) lists pockets and tool numbers directly — correct the entries, including the spindle position. On many builds R-registers hold the live pot pointer; a builder ‘ATC recovery’ or ‘magazine reset’ operation re-zeros the magazine to pocket 1.
Haas: use the TOOL TABLE screen. For a side-mount changer, put the changer in a safe state, then use ATC RECOVERY (NGC: Recovery button) to step the arm back to home. If the table itself is wrong, ORIGIN → clear the tool table and re-enter tools pocket by pocket. Haas also stores ‘Tool in spindle’ — make sure it matches reality before the first change.
Step 3 — Re-home the changer and verify
- Complete any interrupted change cycle using the builder's ATC recovery mode (arm to home, magazine to reference pocket).
- Command a tool change to a known, short, light tool in MDI. Watch the full cycle.
- Step through 4–5 more changes covering both magazine directions, verifying the right tool arrives each time.
Why it keeps happening — root causes to fix
- Magazine position sensor / proximity switch failing — intermittent pocket-count errors shift the table over days. Replace the switch, don't keep re-syncing.
- Battery-backed memory lost the table — if mismatches appear after every power-off, the control or PLC battery is flat (see our battery guides for Fanuc, Mitsubishi and Haas).
- Operators rotating the magazine by hand during setup without using the manual index function — train it out; the control cannot see hand rotation on most geared magazines.
- Slipping magazine drive coupling or worn Geneva mechanism — the commanded index and actual pocket drift apart under load; a mechanical repair, not a data fix.