Fanuc Alarm 51: MISSING MOVE AFTER CHF/CNR

Fanuc Alarm 0: MISSING MOVE AFTER CHF/CNR

Fanuc Control Model: Fanuc 0i, 16i / 18i / 21i, 30i / 31i / 32i

Fanuc Alarm 51 (PS0051) is a Program/Software alarm that occurs during Chamfering (CHF) or Corner Rounding (CNR) operations. It triggers when the controller encounters a chamfering/corner rounding block that is not immediately followed by a valid axis movement block.

Official Alarm Description

T series
Improper movement or the move distance was specified in the block next to the chamfering or corner R block.Modify the program.

M series
Improper movement or the move distance was specified in the block next to the optional chamfering or corner R block. Modify the program.

Common Causes

This alarm typically happens due to programming logic errors in G-code:

1. Missing Subsequent Block: Chamfering (C) or Corner Rounding (R) was specified at the end of a line, but the next block contains no movement command (X, Z, or Y).
2. Invalid Non-Movement Block: A non-modal block like M-code (M08, M03), G-code dwell (G04), or a comment was placed immediately after the chamfering block without motion data.
3. Improper Block Structure: The direction of the move following the chamfering/rounding vector is logically impossible or zero in length.

Step-by-Step Troubleshooting & Solution

1. Switch to EDIT Mode: Access your CNC controller and switch to EDIT mode.
2. Identify the Faulty Block: Locate the line where the alarm triggered. Look for address C (Chamfering) or R (Corner Rounding) in Lathe/Mill programming.
3. Check the Following Line: Look at the block immediately after the C or R command. Ensure it contains a valid axis positioning or cutting movement command.
4. Remove Non-Motion Commands: Move any M-codes, auxiliary functions, or comments away from the line between the chamfer/corner rounding command and its following move block.
5. Reset and Test: Press the RESET button on the MDI panel, perform a dry run or single-block execution, and verify the path.

G-Code Example (Correct vs Incorrect)

Incorrect Code

G-Code

G01 X50.0 Z-20.0 C2.0 (Chamfer specified)
M08 (Error: Non-motion block placed immediately after C)
G01 Z-40.0

Correct Code (Fixed):

G-Code

G01 X50.0 Z-20.0 C2.0 M08 (M-code combined or moved)
G01 Z-40.0 (Valid axis move block follows C)

Frequently Asked Questions (FAQs)

Q1: Can I use M-codes on the same line as Chamfering (C) or Corner Rounding (R)?

Yes, you can place M-codes in the same block as the chamfer command, but the block immediately following must contain an axis movement command.

Q2: Will pressing RESET clear Fanuc Alarm 51?

Yes. Since Alarm 51 is a software programming alarm, correcting the G-code sequence and pressing RESET will clear the error immediately without requiring a machine restart.

Have you faced Fanuc Alarm 51 MISSING MOVE AFTER CHF/CNR on your machine? Share your experience or troubleshooting tips in the comments below to help other CNC operators and programmers clear this error faster!