What causes FANUC alarm 039?

Chamfering (CHF) or corner rounding (CNR) command is programmed on the same block where G41 or G42 tool nose radius compensation is activated. CHF or CNR is programmed in the cancel block where G40 turns off tool nose radius compensation. A switch between G41 and G42 occurs in the same block that contains a chamfer or corner R command. Programmer placed chamfer or corner R in the startup or cancel move block instead of a separate intermediate block.

How do I fix FANUC alarm 039?

1. Search the program for all CHF and CNR commands and check if any of them appear in the same block as G41, G42, or G40. 2. Move the G41 or G42 activation to a separate approach block before the block containing the CHF or CNR command so compensation is already active when chamfering begins. 3. If canceling compensation with G40, make sure the CHF or CNR command is not in that same cancel block. Add a plain move block before the G40 cancel. 4. If switching between G41 and G42, insert a G40 cancel block first, then a plain move, then reactivate with the new compensation direction before any CHF or CNR block. 5. After editing, dry run the program in single block mode to verify compensation activates and cancels cleanly without any chamfer or corner R commands in those transition blocks.

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FANUC Controller

Program/G-code · Basic · 10-20 minutes

Alarm 039

Fanuc Alarm 039 - CHF/CNR NOT ALLOWED IN G41/G42

Alarm 039 occurs when a chamfer (CHF) or corner rounding (CNR) command is programmed during tool nose radius compensation (G41/G42) startup, cancel, or mode switching. The Fanuc control cannot calculate proper compensation geometry when CHF/CNR modifiers are active during these transition blocks, as the compensation offset requires at least two consecutive linear or circular moves to establish the offset vector.

FANUCProgram/G-codeBasic10-20 minutes

039

Fanuc Alarm 039 - CHF/CNR NOT ALLOWED IN G41/G42

FANUC ALARM 39

Root Cause Summary

Alarm 039 occurs when a chamfer (CHF) or corner rounding (CNR) command is programmed during tool nose radius compensation (G41/G42) startup, cancel, or mode switching. The Fanuc control cannot calculate proper compensation geometry when CHF/CNR modifiers are active during these transition blocks, as the compensation offset requires at least two consecutive linear or circular moves to establish the offset vector.

Causes and Fixes

1
Review the program block that triggered the alarm. Locate the G41 or G42 command and check if CHF or CNR appears in the same block or the immediately following block.
2
Remove the CHF or CNR command from the compensation startup block. Insert at least one full linear move (G01) after the G41/G42 command before using CHF/CNR modifiers.
3
Verify tool nose radius value is correctly set in the offset register (OFFSET SETTING screen > GEOMETRY for tool nose radius). Typical parameters are #5001-#5200 (geometry offsets) depending on control series.
4
Test the modified program in single block mode to confirm compensation vector establishes properly before any CHF/CNR commands execute.
Ensure at least two consecutive blocks exist after G41/G42 before using CHF/CNR to allow proper offset calculation.

DIY Feasiblehigh confidence

Call a technician if:

·Parameter 5004 changes required
·Persistent alarms after program corrections
·Complex multi-axis compensation issues

Prevention

Always separate G41/G42 activation from CHF/CNR by at least 2 blocks
Use G40 cancel before switching between G41 and G42
Program geometric modifications only during stable compensation states
Maintain minimum 2x tool radius approach distance for compensation startup

Common Mistakes

Combining G41/G42 with CHF/CNR in same block
Programming CHF/CNR immediately after compensation activation
Direct switching between G41 and G42 without G40 cancel
Insufficient approach distance for compensation vector calculation

Related Alarms

037 038 040 041

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