How to Program CNC Milling from Drawing: Step-by-Step Guide

To program CNC milling from a drawing, first interpret the drawing dimensions and features, then write G-code commands that control the tool path, speeds, and operations. Use G01 for linear moves and G02/G03 for arcs, matching the drawing's shapes and sizes precisely.

Syntax

CNC milling programs use G-code commands to control the machine. Key parts include:

• G00: Rapid move to position
• G01: Linear cutting move at set feed rate
• G02 / G03: Circular interpolation (clockwise/counterclockwise arcs)
• X, Y, Z: Coordinates for tool position
• F: Feed rate (speed of cutting)
• M03: Spindle on clockwise
• M05: Spindle stop

Each line is a command telling the machine what to do next.

gcode

N10 G21         ; Set units to millimeters
N20 G90         ; Use absolute positioning
N30 M03 S1000   ; Spindle on clockwise at 1000 RPM
N40 G00 X0 Y0   ; Rapid move to start point
N50 G01 X50 Y0 F200 ; Cut in a straight line to X50 Y0 at 200 mm/min
N60 G02 X50 Y50 I0 J50 ; Cut clockwise arc to X50 Y50 with center offset
N70 M05         ; Stop spindle
N80 M30         ; End program

Example

This example programs a simple square pocket 50mm by 50mm from a drawing. It shows how to move the tool along the edges and control spindle and feed.

gcode

N10 G21         ; Set units to millimeters
N20 G90         ; Absolute positioning
N30 M03 S1200   ; Spindle on clockwise at 1200 RPM
N40 G00 X0 Y0   ; Rapid move to start corner
N50 G01 Z-5 F100 ; Lower tool 5mm into material
N60 G01 X50 Y0 F300 ; Cut bottom edge
N70 G01 X50 Y50 ; Cut right edge
N80 G01 X0 Y50  ; Cut top edge
N90 G01 X0 Y0   ; Cut left edge
N100 G00 Z10    ; Raise tool
N110 M05       ; Stop spindle
N120 M30       ; End program

Common Pitfalls

Common mistakes when programming CNC milling from drawings include:

• Misreading dimensions or units (mm vs inches)
• Forgetting to set absolute (G90) or incremental (G91) mode correctly
• Not setting spindle speed or feed rate properly, causing tool damage or poor finish
• Incorrect arc center offsets (I and J) leading to wrong curves
• Skipping safety moves like tool retraction before rapid moves

Always verify the program with a dry run or simulation before actual milling.

gcode

Wrong:
N10 G21
N20 G91
N30 M03 S1500
N40 G00 X0 Y0
N50 G01 X50 Y0 F500
N60 G02 X50 Y50 I50 J0 ; Incorrect arc center

Right:
N10 G21
N20 G90
N30 M03 S1500
N40 G00 X0 Y0
N50 G01 X50 Y0 F500
N60 G02 X50 Y50 I0 J50 ; Correct arc center

Quick Reference

Command	Description
G00	Rapid positioning (non-cutting move)
G01	Linear interpolation (cutting move)
G02	Clockwise arc interpolation
G03	Counterclockwise arc interpolation
G90	Absolute positioning mode
G91	Incremental positioning mode
M03	Spindle on clockwise
M05	Spindle stop
F	Feed rate (cutting speed)
S	Spindle speed (RPM)

Key Takeaways

Always interpret the drawing carefully to extract exact dimensions and shapes.

Use G-code commands like G01 for straight cuts and G02/G03 for arcs matching the drawing.

Set units, positioning mode, spindle speed, and feed rate correctly before cutting.

Verify your program with a dry run or simulation to avoid errors and tool damage.

Remember to include safety moves like tool retraction before rapid moves.