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Mitsubishi CNC Alarm 500 — What It Means
Alarm 500 on a Mitsubishi CNC (M70, M70V, M80, M800 series controls) indicates a servo error — the position deviation between the commanded position and the actual position (from the servo encoder) exceeded the allowable following error limit. This “excessive position deviation” alarm means the servo axis could not follow the commanded motion, either because of a mechanical problem, a servo drive fault, or incorrect servo parameters.
Common Causes
- Mechanical binding or obstruction — A tight ball screw, seized bearing, jammed way cover, or crash causes the axis to fall behind the commanded position, triggering the following error alarm.
- Servo drive fault — If the MDS servo amplifier trips internally (overcurrent, overtemperature), the motor produces no torque and the position deviation grows instantly to the alarm limit.
- Servo gain parameters too high or too low — Incorrectly tuned position loop gain causes axis hunting or sluggish response that leads to following error exceedances.
- Encoder cable damage — A broken or intermittently open feedback cable causes the drive to lose position data, allowing the commanded-vs-actual gap to grow unchecked.
Step-by-Step Fix
- Read alarm detail — On the M70/M80 display, open the Alarm screen and note the full alarm number and axis. Alarm 500 is usually followed by the axis letter (e.g., “500 X-axis servo error”) and may include the drive’s sub-fault code.
- Check for mechanical resistance — With power off and LOTO applied, manually move the faulted axis through its travel. Resistance or inability to move indicates a mechanical issue — inspect ball screw, bearings, and way covers.
- Read MDS amplifier fault code — The MDS servo amplifier has a 2-digit LED display showing its internal fault status. This sub-code identifies the specific drive-level fault (overcurrent, overtemperature, encoder fault).
- Verify encoder cable continuity — With power off, inspect the feedback cable from motor to drive for damage. Check both the power cable and the encoder cable for bent pins or damaged insulation.
- Power cycle and re-home — After correcting the fault, perform a full machine power cycle. On M70/M80 systems with absolute encoders, verify that axis positions are restored correctly after power-on.
Parts Often Needed
| Part | Notes |
|---|---|
| MDS servo amplifier | Mitsubishi MDS-D2/EX series; match axis kW rating |
| Servo motor (HF/HC series) | Mitsubishi OEM; match drive and machine specifications |
| Encoder feedback cable | Mitsubishi preassembled; use OEM for reliable shielding |
| Encoder backup battery | Replace every 3–5 years on absolute encoder systems |
Technician Tips
CNC alarms should always be diagnosed by reading the full alarm text, not just the number. Modern Fanuc, Haas, and Mazak controls provide detailed alarm descriptions — press the help key while viewing the alarm for diagnostic suggestions. The alarm history screen stores the last 50-100 alarms with timestamps; review this before clearing to identify patterns. A servo alarm (SV series on Fanuc) that appears only during rapid traverse points to a different cause than one that appears during cutting.
Prevention: Back up your CNC parameters monthly. Battery-backed memory faults are much easier to recover when you have a current parameter, ladder, offset, and macro backup. Replace control batteries on a planned maintenance schedule before voltage drops far enough to risk data loss.
Need Replacement Parts?
Before ordering, match the machine-control part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
Mitsubishi M70/M80 servo system repair and re-parameterization requires Mitsubishi-certified technicians. Alarm 500 following a crash should trigger a machine geometry verification — axis squareness and backlash compensation may need to be re-measured before returning the machine to production.