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Fanuc Alarm 6 Overtravel — What It Means
Fanuc alarm 6 means hardware overtravel — a physical limit switch on one of the machine’s axes has been tripped. On Fanuc 0, 0i, 16/18/21 series controls, each linear axis has hardwired overtravel (OT) limit switches at both ends of travel. When the axis moves far enough to contact one of these switches, the control issues alarm 6 and immediately cuts servo drive power to that axis (and sometimes all axes depending on the machine builder’s configuration). The alarm identifies which axis and direction: “OT+X” is positive X overtravel, “OT-Z” is negative Z overtravel, and so on.
Common Causes
- Manual jog in wrong direction — The most common cause. An operator jogged an axis past its software limit or the machine reference position wasn’t established, and the axis ran into the hardware limit switch.
- Work offset or fixture error — A program with an incorrect work offset can command the axis into a hardware OT position during a machining cycle.
- Reference position not established — If the machine was powered on without performing a reference return (homing), the software travel limits aren’t enforced and a jog operation can reach the hardware limit.
- Limit switch failure or adjustment — A limit switch that has drifted out of position, been physically moved, or has failed closed can falsely trigger alarm 6.
Step-by-Step Fix
- Identify the overtravel direction from the alarm — Note whether the alarm shows OT+ or OT- and which axis. This tells you which direction to move the axis to clear the limit switch.
- Enter overtravel release mode — On Fanuc controls, hold the OT RELEASE key (or the equivalent machine builder key, often a dedicated key or function key on the operator panel) while jogging the axis in the direction that moves it away from the tripped limit switch. Move slowly in small increments.
- Move the axis off the limit switch — Jog the axis in the direction opposite to the OT direction until the limit switch clears. You will feel resistance come off. Typically only 1–2mm of movement is needed.
- Release the OT RELEASE key and perform reference return — Once off the limit, perform a manual reference return (REF/HOME) to re-establish the coordinate system.
- Reset the system — Press RESET on the control after the reference return completes. Alarm 6 should clear. Investigate and correct the root cause (offset error, operator procedure, limit switch position).
Parts Often Needed
| Part | Notes |
|---|---|
| Overtravel limit switch | OMRON or equivalent; match the NC (normally closed) contact rating |
| Limit switch mounting bracket | Replace if the switch was physically displaced |
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 alarm 6 appears during automatic cycle operation with correct work offsets and a verified reference position, the limit switch position may need adjustment by the machine tool builder or an authorized Fanuc service technician.