How to Write G Code for CNC Milling: Basic Guide

To write G-code for CNC milling, you create a sequence of commands that control the machine's movements and operations using codes like G00 for rapid moves and G01 for controlled cutting. Each line typically includes a command, coordinates, and feed rates to guide the milling tool precisely.

Syntax

G-code commands control the CNC milling machine. Each line usually has:

• G-code command (e.g., G00, G01) to specify the action.
• Coordinates (X, Y, Z) to move the tool to a position.
• Feed rate (F) to control speed during cutting.
• Optional codes for spindle speed (S) and tool changes (M).

Commands are written line by line and executed in order.

gcode

G00 X0 Y0 Z5    ; Rapid move to X0 Y0 Z5
M03 S1200        ; Start spindle at 1200 RPM
G01 Z-1 F100     ; Move down to Z-1 at feed rate 100
G01 X50 Y0 F200  ; Cut in X direction at feed rate 200
M05              ; Stop spindle

Example

This example shows a simple milling operation that moves the tool to start, lowers it to cut, moves in a straight line, and then stops the spindle.

gcode

G21             ; Set units to millimeters
G90             ; Use absolute positioning
M06 T1          ; Tool change to tool 1
M03 S1500       ; Start spindle at 1500 RPM
G00 X0 Y0 Z5    ; Rapid move to start position above workpiece
G01 Z-2 F100    ; Lower tool into material at feed rate 100
G01 X50 Y0 F200 ; Mill straight line in X direction
G01 X50 Y50     ; Mill straight line in Y direction
G00 Z5          ; Retract tool to safe height
M05             ; Stop spindle
M30             ; End program

Common Pitfalls

Common mistakes when writing G-code for CNC milling include:

• Forgetting to set units with G20 (inches) or G21 (millimeters).
• Not using G90 (absolute) or G91 (incremental) positioning correctly.
• Omitting spindle start (M03) or stop (M05) commands.
• Failing to set feed rates (F) causing tool damage or poor cuts.
• Not retracting the tool safely before rapid moves (G00).

Example of wrong and right usage:

gcode

; Wrong: No feed rate and no spindle start
G01 X10 Y10

; Right: Feed rate and spindle start included
M03 S1200
G01 X10 Y10 F150

Quick Reference

G-code	Meaning
G00	Rapid positioning (fast move without cutting)
G01	Linear interpolation (controlled cutting move)
G02	Clockwise arc move
G03	Counterclockwise arc move
G20	Set units to inches
G21	Set units to millimeters
G90	Absolute positioning
G91	Incremental positioning
M03	Spindle on (clockwise)
M05	Spindle stop
M06	Tool change
F	Feed rate (speed of cutting)
S	Spindle speed (RPM)

Key Takeaways

Start every G-code program by setting units and positioning mode with G20/G21 and G90/G91.

Use G00 for fast moves and G01 for cutting moves with proper feed rates.

Always start and stop the spindle with M03 and M05 commands.

Include safe tool retraction before rapid moves to avoid crashes.

Test your code on simulation software before running on the actual CNC machine.