Fanuc Alarm 437 — What It Means
Alarm 437 on a Fanuc CNC system is a 4th axis (typically the A or B rotary axis) servo following error — the difference between the commanded position and the actual encoder position has exceeded the allowable following error tolerance. Fanuc generates following error alarms in a series: alarm 400 series indicates servo following errors by axis (e.g., 435 = X axis, 436 = Y axis, 437 = 4th axis). The CNC immediately stops all motion and requires a manual reset. Alarm 437 is most common on horizontal machining centers with a rotary pallet table or on 5-axis machines with a rotary A-axis.
Common Causes
- Mechanical binding or overload on the 4th axis — A workpiece that is too heavy, a fixture that has shifted, or a collision between the table and a fixture creates mechanical resistance that exceeds the servo’s torque capability, causing the following error to spike.
- Scale or encoder fault — A dirty, damaged, or failed encoder/resolver on the 4th axis servo motor provides erroneous position feedback, causing the CNC to calculate an ever-growing following error even when the axis is actually stationary.
- Servo drive alarm on the 4th axis — If the 4th axis servo amplifier has an independent fault (overcurrent, overheat, feedback error), the motor output drops and the axis cannot track the command, generating alarm 437.
- Following error tolerance set too tight — Parameters 1828 (In-position tolerance) and 1838 (Following error tolerance) for the 4th axis may be set tighter than the mechanical system can reliably achieve.
Step-by-Step Fix
- E-stop and power cycle — The alarm halts all motion. Do not attempt to manually jog the 4th axis until the mechanical situation is assessed.
- Inspect the 4th axis for mechanical binding — Remove the workpiece and manually jog the 4th axis slowly at low feedrate. It should rotate smoothly with no hesitation or grinding. Any resistance indicates a mechanical problem (bearing failure, lubrication issue, or debris in the rotary table).
- Check the servo amplifier for the 4th axis — Look at the servo amplifier module associated with the 4th axis on the control cabinet. Note any fault LEDs or sub-alarm codes. A secondary fault on the amplifier will cause alarm 437 on the CNC.
- Inspect the encoder/resolver — Check the feedback cable from the 4th axis motor back to the servo amplifier for damage, loose connectors, or contamination. Clean the encoder if accessible.
- Check Fanuc parameters 1838 (following error tolerance) for axis 4 — If the tolerance was recently changed or the axis is new, verify the parameter is appropriate for the axis type and speed.
- Reset and test — After addressing the root cause, press the RESET key, home the 4th axis, and run a low-speed test program before returning to production.
Parts Often Needed
| Part | Notes |
|---|---|
| 4th axis encoder/resolver | Match Fanuc servo motor model and encoder type (αi, βi) |
| Servo amplifier module | Match axis rating and Fanuc CNC model series |
| Rotary table bearing | Replace if manual rotation shows binding or roughness |
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. Parameter loss from dead batteries is the #2 cause of unexpected CNC downtime (after mechanical crashes). Replace control batteries annually before they fail.
Need Replacement Parts?
Finding the right replacement parts for your equipment is critical to getting it running again. Check current prices on Amazon — prices and availability change frequently.
When to Call a Pro
Alarm 437 on a 5-axis machine or a large rotary pallet table requires a Fanuc-certified CNC technician to perform servo tuning and parameter verification. Incorrect following error tolerance or servo gain settings can cause poor part quality or repeated crashes.