Drive cooling fan
Common heat-related VFD and servo-drive failure point.
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. 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
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.