How to Write G Code for CNC Lathe: Basic Syntax and Example

To write G-code for a CNC lathe, you create a sequence of commands that control the tool movement, spindle speed, and coolant. Each line typically starts with a G or M code followed by coordinates or parameters. For example, G01 X10 Z-5 F0.2 moves the tool linearly to position X=10, Z=-5 at feed rate 0.2.

Syntax

G-code commands for CNC lathe usually include:

• G-codes: Define motion types (e.g., G00 for rapid move, G01 for linear cut).
• M-codes: Control machine functions (e.g., M03 to start spindle clockwise).
• Coordinates: Positions on axes, typically X (diameter) and Z (length).
• Feed rate: Speed of tool movement, given by F.

Each line is a command executed in order.

gcode

N10 G00 X0 Z0
N20 M03 S1000
N30 G01 X10 Z-5 F0.2
N40 G00 X100 Z100
N50 M05
N60 M30

Example

This example shows a simple CNC lathe program that starts the spindle, moves the tool to cut a straight line, then stops the spindle and ends the program.

gcode

N10 G00 X0 Z0       ; Move tool to start position
N20 M03 S1200       ; Start spindle clockwise at 1200 RPM
N30 G01 X20 Z-10 F0.15 ; Cut line to X=20, Z=-10 at feed 0.15
N40 G00 X100 Z100   ; Rapid move away from part
N50 M05             ; Stop spindle
N60 M30             ; End program

Common Pitfalls

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

• Forgetting to start the spindle with M03 before cutting.
• Using wrong coordinate signs or mixing up X and Z axes.
• Not setting feed rate F, causing tool to move too fast or too slow.
• Skipping safety moves like rapid retract G00 after cutting.

Always verify your code with a dry run or simulation before actual machining.

gcode

Wrong:
N10 G01 X20 Z-10 F0.15  ; Cutting without spindle on

Right:
N10 M03 S1000           ; Start spindle
N20 G01 X20 Z-10 F0.15 ; Then cut

Quick Reference

Code	Meaning	Example
G00	Rapid positioning (move fast)	G00 X0 Z0
G01	Linear interpolation (cutting move)	G01 X10 Z-5 F0.2
M03	Spindle on clockwise	M03 S1200
M05	Spindle stop	M05
M30	Program end and rewind	M30
F	Feed rate (speed of cut)	F0.15

Key Takeaways

Start your CNC lathe program by setting spindle speed with M03 before cutting.

Use G00 for fast moves and G01 for cutting moves with feed rate F.

Always specify coordinates clearly for X (diameter) and Z (length) axes.

Include safety moves like retracting tool with G00 after cutting.

Test your G-code with simulation or dry run to avoid errors.