Problem: A DMG Mori CMX 1100V with default 8901=0 produced erratic thread pitch on M6x1.0 holes deeper than 20mm. Pitch error varied by 0.05mm.
Diagnosis: Servo lag in X axis (non-moving during tapping) was being unnecessarily referenced. The sync loop included X, causing phase shift at high spindle speeds.
Solution: Changed 8901 from 0 → 4. No hardware change.
Result: Thread pitch variation dropped to 0.005mm. Cycle time unchanged. Tapping speed increased from 1500 to 4000 rpm without breakage.
Once you set 8901 = 1, consider adjusting two companion parameters to complete the upgrade:
These three parameters working together create a holistic, predictable offset management system.
Setting 8901=2 is just the first step. To make it truly better, you must adjust the companion parameters. If you set 8901 to 2 but leave the tolerance loose, you won't see a difference.
Here is the "better" configuration checklist:
Setting Parameter 8901 to 2 activates AICC Mode II (or Advanced Preview Control). Here is why this is objectively better for 90% of milling operations:
Better is not a universal number. It is: fanuc parameter 8901 better
Always test with an air cut and a thread go/no-go gauge after changing. And remember: Parameter 8901 is write-protected (requires PWE = 1) and often requires a power cycle to take effect.
If you truly want the best behavior, combine 8901 with Parameter 4000#2 = 1 (high-speed rigid tapping mode) on 31i-B5 controls.
Final note: “Better” without diagnostics is guesswork. Use FANUC’s Servo Guide or look at Diagnosis 300 (synchronous error) before and after changing 8901. The goal is minimum following error difference between spindle and axis—that is the true measure of “better.”
In FANUC CNC systems, Parameter 8901 is primarily used to control how the system monitors and reacts to fan motor errors. It is most commonly referenced during troubleshooting for a 701 Fan Motor Overheat Alarm. Key Functionality
Bit 0 (FAN): This bit determines whether the CNC monitors the cooling fans within the control unit.
Set to 0: Monitoring is active. If a fan motor fails or runs too slowly, the system triggers an alarm (like Alarm 701) and often stops the machine to prevent overheating.
Set to 1: Monitoring is disabled. This suppresses the fan error alarm, allowing the machine to run even if a fan fault is detected. When to Adjust Parameter 8901
While changing this parameter can clear an immediate alarm, it should be done with caution:
Temporary Troubleshooting: If you receive a 701 alarm, setting 8901 bit 0 to "1" can help confirm if the control unit is still operational or allow you to finish a critical cycle. Fanuc Parameter 8901 — Explanation and Best Practices
Safety Warning: Disabling this check is not a permanent fix. Running the CNC without functional fans can lead to critical overheating and expensive damage to printed circuit boards or other hardware components.
Recommended Action: Instead of permanently disabling the alarm, it is safer to replace the faulty fan motors immediately. Related Alarms and Context
Alarm 701: Specifically indicates that a fan in the CNC control unit (not the spindle or external cabinet) has failed.
Maintenance: On modern systems like the Series 30i/31i/32i, these fans are considered "maintenance parts" that should be periodically inspected and replaced as they age. FANUC Series 30i/31i/32i-MODEL B MAINTENANCE MANUAL
Fanuc Parameter 8901 is primarily used to manage fan alarms (such as Alarm 701) on various CNC control systems, including the Series 0i. Modifying this parameter is typically a temporary troubleshooting step to bypass an alarm while waiting for a replacement part. Key Function: Alarm Suppression
On systems like the Fanuc 0iC or 0i-MODEL F, Parameter 8901#0 (the first bit) controls whether a fan speed drop or failure triggers a system alarm.
Setting 8901#0 = 1: Temporarily shields or releases the ALM 701 (control unit upper fan overheat/failure).
Setting 8901#0 = 0: Standard setting; the system will trigger an alarm if the fan fails. Why You Might Change It
The term "better" in your query likely refers to optimizing machine uptime. If you encounter a 701 alarm and a replacement fan is not immediately available, setting this parameter to 1 allows you to continue production—provided you apply forced air cooling to the electrical cabinet to prevent actual overheating. Parameter 5002 (bit 6): Controls how wear offsets
Warning: Incorrectly setting parameters can cause unpredictable machine movement or damage. Ensure you understand the function before modifying. Always return the parameter to 0 once a new fan is installed. How to Modify Fanuc Parameters
To change Parameter 8901, you must first enable "Parameter Write":
Select MDI Mode: Put the machine in MDI or Emergency Stop state.
Enable PWE: Press the SETTING key and change PARAMETER WRITE (PWE) from 0 to 1.
Locate Parameter: Press the SYSTEM or PARAM key, type 8901, and press No. SRH.
Edit Bit: Navigate to bit 0 and change it to the desired value. Disable PWE: Change Parameter Write back to 0. Other Potential Meanings
In some specific AMK-motion or specialized Fanuc-compatible configurations, P8901 may refer to the actual block number of the currently running program, though this is less common than the fan alarm function in standard Fanuc environments.
Are you currently troubleshooting a specific alarm code, or are you looking to optimize high-speed machining settings? PARAMETER MANUAL