G-Code Basics for Software Developers

What Is G-Code?

G-code is the programming language for CNC (Computer Numerical Control) machines — mills, lathes, routers, plasma cutters, and 3D printers. Each line of G-code is an instruction: move here, cut at this speed, turn the spindle on.

If you're building manufacturing software, you'll eventually need to generate, parse, or validate G-code. This guide covers the fundamentals.

Coordinate System

CNC machines use a Cartesian coordinate system:

• X, Y — horizontal plane (left-right, front-back)
• Z — vertical axis (up-down, or depth for a router)
• G90 — absolute positioning (coordinates relative to machine origin)
• G91 — incremental positioning (coordinates relative to current position)

Essential Commands

Motion Commands

• G0 X_ Y_ Z_ — Rapid move (fastest speed, no cutting). Used for positioning.
• G1 X_ Y_ Z_ F_ — Linear cut at feed rate F. The workhorse command.
• G2 X_ Y_ I_ J_ F_ — Clockwise arc. I,J are the arc center offset from the start point.
• G3 X_ Y_ I_ J_ F_ — Counter-clockwise arc.

Spindle & Tool

• M3 S12000 — Spindle on clockwise at 12000 RPM
• M5 — Spindle off
• T1 M6 — Select tool 1 and perform tool change

Program Control

• M0 — Program pause (operator must press cycle start to continue)
• M30 — Program end and rewind

A Simple Program

G90 G21          (Absolute mode, millimeters)
T1 M6            (Select tool 1)
M3 S10000        (Spindle on at 10000 RPM)
G0 X0 Y0 Z5      (Rapid to start position)
G1 Z-2 F200       (Plunge cut to -2mm at 200mm/min)
G1 X50 F800       (Cut along X to 50mm)
G1 Y30            (Cut along Y to 30mm)
G1 X0             (Cut back to X0)
G1 Y0             (Cut back to Y0)
G0 Z5             (Retract)
M5                (Spindle off)
M30               (Program end)

This cuts a 50mm x 30mm rectangle at 2mm depth.

Generating G-Code Programmatically

When building a G-code generator in C# or Python:

1. Define your toolpath as a sequence of points and arc segments
2. Apply feed rates based on material, tool, and operation type
3. Add safe retract moves (Z clearance) between operations
4. Format output — G-code is plain text, one command per line
5. Validate — check for rapid moves into the workpiece, out-of-bounds coordinates, and missing feed rates

Tips for Developers

• G-code is case-insensitive, but conventionally uppercase
• Comments use parentheses: (this is a comment)
• Feed rate (F) is modal — it stays active until changed
• Always start with G90 (absolute) and G21 (metric) or G20 (imperial) to establish the coordinate mode
• Test generated G-code in a simulator (e.g., NCViewer.com) before running on a real machine