In CNC machining, whether you’re milling, turning, drilling, or cutting complex 5-axis parts, there’s one fundamental skill every programmer and operator must understand: G and M codes.
These codes are the language CNC machines speak. They tell the machine what to do, how to move, and when to perform specific actions. A well-written CNC program that uses G and M codes correctly can mean the difference between high-quality parts and costly scrap.
Let’s break down what these codes are, how they differ, and why mastering them is essential for reliable machining.
What Are G and M Codes?
CNC machines are essentially automated tools guided by instructions. These instructions are written in a structured language made up of G codes and M codes:
G codes — control the geometry, movement, and toolpath of the machine.
M codes — control miscellaneous machine functions that support the machining process.
Both work together in a CNC program to produce accurate, repeatable results.
Understanding G Codes: Geometry and Motion
The “G” in G code is often thought of as geometric or general code. These codes tell the CNC machine how and where the tool should move.
What G Codes Do
G codes define:
Tool motion type — rapid, feed, circular, or interpolated motion
Movement axes — X, Y, Z and rotational axes (A, B, C)
Feed rate (F), spindle speed (S), and other motion-related settings
Even though the same G code can sometimes behave slightly differently across machine brands or controllers, the basics remain consistent.
Common G Codes You Need to Know
| G Code | Function |
|---|---|
| G00 | Rapid positioning (move quickly to a point) |
| G01 | Linear feed move (controlled feed rate) |
| G02 | Circular interpolation (clockwise) |
| G03 | Circular interpolation (counterclockwise) |
| G17 | Select XY plane |
| G18 | Select XZ plane |
| G19 | Select YZ plane |
| G20 | Programming in inches |
| G21 | Programming in millimeters |
| G90 | Absolute programming |
| G91 | Incremental programming |
These are just a subset — CNC machines can support dozens of G codes, especially when working with advanced axes or functions.
Understanding M Codes: Miscellaneous But Essential
While G codes control movement, M codes handle the support actions that make a machining program work in the real world. Think of them as machine control commands.
M codes manage functions such as:
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Starting/stopping the spindle
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Tool changes
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Turning coolant on/off
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Program stop or end
Unlike G codes, M codes are often machine-specific, meaning different CNC controls (Fanuc, Haas, Siemens, Mazak, etc.) may use slightly different M-code definitions.
Examples of M Codes in Use
Here are some common M codes and their purpose:
| M Code | Function |
|---|---|
| M00 | Program stop |
| M01 | Optional stop (only if enabled) |
| M02 | End of program |
| M03 | Spindle on, clockwise |
| M04 | Spindle on, counterclockwise |
| M05 | Spindle stop |
| M06 | Tool change |
| M08 | Coolant on |
| M09 | Coolant off |
| M30 | End program and reset |
For example, M03 will start the spindle turning clockwise before a cut, while M05 will stop it when a move is complete. Knowing when and where to insert these codes is critical for safe and accurate machining.
How G and M Codes Work Together
A CNC program consists of lines or blocks of text, each containing a code that tells the machine something to do. A single line might include:
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A line number (N)
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One or more G codes
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One or more M codes
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Movement coordinates (X, Y, Z)
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Optional parameters like feed (F) or speed (S)
Together, these instructions tell the machine move here, cut this way, start the spindle, stop the coolant — all in a step-by-step sequence.
For example:
This structured language gives CNC machines the precision and repeatability that makes them so valuable in modern manufacturing.
Why Learning G and M Codes Matters
With today’s advanced CAD/CAM software, many programmers never hand-write code. Yet understanding what the code represents is crucial for:
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Debugging and tweaking programs
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Reading and editing generated code
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Understanding machine behavior
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Improving cycle times and quality
Even if you rely on CAM tools to generate programs, knowing how G and M codes function makes you a more capable and versatile machinist.
Final Thoughts
G and M codes are the backbone of CNC programming — guiding the machine’s motion and operations one command at a time. While modern software can generate these codes automatically, mastering them gives you confidence, control, and deeper understanding of the machining process.
With a solid grasp of G and M codes, you can write and troubleshoot programs, optimize machining strategies, and take your CNC skills to the next level.