May 29, 2025
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Whether you’re a CNC machinist, programmer, or manufacturer looking to optimize production, understanding M codes and G codes is non-negotiable. These two types of commands form the foundation of CNC programming, enabling precise control of machine operations like cutting, tool changes, coolant flow, and motion paths.
This article provides a comprehensive guide to common G and M codes, how they function, and where they're used—especially within FANUC-controlled CNC systems.
G Codes (Geometric Codes) define machine movement and tool paths.
M Codes (Miscellaneous Codes) handle machine functions such as turning on coolant, stopping the spindle, or tool changing.
Think of it this way:
G codes = How the machine moves.
M codes = What the machine does while moving (or before/after).
Both are essential for a successful CNC program.
| G Code | Function | Description |
|---|---|---|
| G00 | Rapid positioning | Moves tool quickly from point to point without cutting |
| G01 | Linear interpolation | Moves tool in a straight line at a set feed rate |
| G02 | Circular interpolation (clockwise) | Cuts arcs or circles in a clockwise direction |
| G03 | Circular interpolation (counter-clockwise) | Cuts arcs or circles counter-clockwise |
| G04 | Dwell | Pauses the machine for a set time |
| G17 | Select XY plane | Default plane for 2D machining |
| G18 | Select ZX plane | Useful in turning centers |
| G19 | Select YZ plane | For vertical or sidewall machining |
| G20 | Inch units | Sets machine units to inches |
| G21 | Metric units | Sets machine units to millimeters |
| G28 | Return to machine zero | Sends tool to home position |
| G40 | Cancel cutter compensation | Turns off tool radius offset |
| G41 | Cutter compensation left | Offsets tool to the left of path |
| G42 | Cutter compensation right | Offsets tool to the right of path |
| G43 | Tool length compensation + | Compensates for tool height |
| G49 | Cancel tool length compensation | Resets tool length offset |
| G54 – G59 | Work coordinate systems | Allows multiple part setups |
| G80 | Cancel canned cycle | Stops any active drilling cycle |
| G81 | Drilling cycle | Basic vertical drilling operation |
| G82 | Drilling with dwell | Pauses at bottom of drill before retract |
| G83 | Peck drilling cycle | Breaks chips while drilling deep holes |
| G90 | Absolute programming | Coordinates based on part origin |
| G91 | Incremental programming | Coordinates based on last position |
| G94 | Feed per minute | Feed rate in mm/min or in/min |
| G95 | Feed per revolution | Feed rate based on spindle RPM |
Note: G codes can vary slightly depending on the machine type (mill vs. lathe) and the controller (FANUC, Siemens, Haas, etc.)
| M Code | Function | Description |
|---|---|---|
| M00 | Program stop | Stops machine until manually restarted |
| M01 | Optional stop | Stops only if optional stop is enabled |
| M02 | Program end | Ends the program |
| M03 | Spindle on (CW) | Starts spindle clockwise |
| M04 | Spindle on (CCW) | Starts spindle counter-clockwise |
| M05 | Spindle stop | Stops the spindle rotation |
| M06 | Tool change | Automatically changes to next tool |
| M08 | Coolant on | Activates coolant flow |
| M09 | Coolant off | Stops coolant flow |
| M30 | Program end and rewind | Ends program and resets to the top |
| M98 | Call subprogram | Runs a separate subroutine |
|
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