What causes FANUC alarm 415?

Machine system or function indicated by the alarm name has detected a fault. Control parameter or configuration error related to the alarm condition. Hardware component fault on the CNC or machine. Part program error or illegal operation for the indicated function. Machine interlock or safety condition not satisfied.

How do I fix FANUC alarm 415?

1. Note the alarm number and check the Fanuc alarm screen for any sub-code or additional details. 2. Identify the specific machine system or function from the alarm name. 3. Power cycle the machine to clear transient faults. 4. Consult the Fanuc parameter or operator manual for alarm 415 specific remedy. 5. Contact Fanuc service if the alarm persists after basic troubleshooting.

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FANUC Controller

Servo/Axis · Intermediate · 20-45 minutes

Alarm 415

Fanuc Alarm 415 - Speed Command Overflow

Alarm 415 EXCESS SHIFT occurs when the commanded pulse rate exceeds 511875 pulses/second on the specified axis, typically caused by incorrect CMR (Command Multiply Ratio) parameter settings in relation to the detection unit and motor configuration. This alarm indicates a parameter mismatch between command pulse generation and servo amplifier scaling that causes the internal velocity command to overflow.

FANUCServo/AxisIntermediate20-45 minutes

415

Fanuc Alarm 415 - Speed Command Overflow

Excess Shift

Root Cause Summary

Alarm 415 EXCESS SHIFT occurs when the commanded pulse rate exceeds 511875 pulses/second on the specified axis, typically caused by incorrect CMR (Command Multiply Ratio) parameter settings in relation to the detection unit and motor configuration. This alarm indicates a parameter mismatch between command pulse generation and servo amplifier scaling that causes the internal velocity command to overflow.

Safety

Reduce speeds before testing

Causes and Fixes

1
Navigate to SYSTEM > PARAM and check Parameter 2169 (CMR numerator) and Parameter 2170 (CMR denominator) for the alarmed axis. Default is typically 1/1 but varies by amplifier and motor type.
2
Verify Parameter 1820 (detection unit per motor revolution) and Parameter 1815 (reference counter) are correctly set for the installed encoder type (serial or pulse-coder). Mismatch causes CMR calculation errors.
3
Calculate required CMR using formula: CMR = (Motor encoder pulses × gear ratio) / (Machine unit per revolution). Ensure the result keeps pulse rate below 511875 at maximum feedrate.
Incorrect CMR may cause position deviation or motor runaway. Record original parameters before changes.
4
After correcting CMR parameters, cycle power to the CNC and amplifiers (do not use soft reset). Test axis motion at low feedrate (10% override) before full speed operation.

DIY Feasiblehigh confidence

Call a technician if:

·Encoder hardware replacement needed
·Servo amplifier replacement required
·Multiple axes affected simultaneously

Prevention

Regularly backup and verify critical servo parameters
Implement feedrate limit checking in G-code post-processors
Schedule annual encoder cable inspection and cleaning

Common Mistakes

Changing CMR without calculating proper value
Ignoring encoder connection quality during troubleshooting
Testing at full speed immediately after parameter changes

Related Alarms

414 416 417 420

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Always verify with your machine manual or a certified technician before performing service procedures.