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Fanuc Alarm 1 Overtravel — What It Means
Fanuc alarm 1 (OT0001: OVER TRAVEL: -X, or the corresponding axis) is a hardware overtravel alarm indicating that an axis has physically hit the minus-direction hardware overtravel limit switch. This is a positive limit detection — an actual mechanical switch mounted at the end of the axis travel has opened, telling the CNC that the axis has exceeded the physical travel boundary in the negative direction. The CNC removes servo power from the axis and requires an operator reset with the axis jogged away from the limit switch before normal operation can resume.
Common Causes
- Axis jogged or commanded past machine travel limits — An operator jogged the axis in the minus direction without watching position, or a part program commanded a position outside the machine’s soft and hard travel limits.
- Incorrect work offset or fixture offset — A G54–G59 work coordinate offset combined with a tool path puts the commanded position outside the physical travel range, driving the axis into the hardware limit.
- Soft limits not configured — If the CNC’s software travel limits (parameters 1320/1321 on Fanuc 0i/18i series) are not set, the machine has no software barrier before the hardware overtravel switch, making it easier to hit the physical limit.
- Overtravel switch mechanical fault — The limit switch bracket is loose, the switch was moved, or the switch contacts are damaged and producing a false overtravel signal at mid-travel.
Step-by-Step Fix
- Enter the overtravel release mode — On Fanuc CNC, hold the OT RELEASE key (or the machine-specific equivalent) while switching to JOG mode. The alarm must be in reset state (MDI key switch to appropriate position depending on machine builder).
- Jog the axis in the positive direction — While holding the OT release button, jog the affected axis in the + direction (away from the minus hardware limit). Move until the switch is no longer actuated — typically a few millimeters to a few centimeters.
- Release the OT release key and clear the alarm — Once the axis is clear of the switch, release the OT key, press RESET to clear the alarm, and confirm the axis is back in the valid travel range.
- Check work offsets and tool length offsets — Navigate to the offset page and review the G54–G59 work offsets and tool length offsets. Identify whether any combination of offsets plus commanded position drove the axis to the limit.
- Verify and set soft travel limits — In the parameter screen (PWE=1 required), check parameters 1320 and 1321 for each axis. Set the soft limits to leave a safe margin (typically 5–10mm) before the hardware overtravel switch.
Parts Often Needed
| Part | Notes |
|---|---|
| Hardware overtravel limit switch | Replace if switch is damaged or producing false signals |
| Switch mounting bracket | Replace if bracket is bent and mis-positioning the switch |
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
If the axis hits the overtravel switch during normal operation with correct offsets and the soft limits are set, the servo system may be following position errors that exceed the tolerance — a servo gain or mechanical problem requiring a Fanuc service engineer to diagnose.