Key Takeaways

  • G-code vs M-code: G-code and M-code are two sets of commands in the same CNC language. G-code controls the geometry and movement of the machine, while M-code controls the miscellaneous functions and actions of the machine.
  • G-code examples: G-code commands start with the letter G, followed by a number from 0 to 99. For example, G0 means rapid positioning, G1 means linear interpolation, and G2 means circular interpolation clockwise.
  • M-code examples: M-code commands start with the letter M, followed by a number from 0 to 99. For example, M3 means spindle on clockwise, M5 means spindle off, and M8 means coolant on.
  • CNC software: CNC software can help design and generate the CNC code for machining. CAD-CAM software can create both G-code and M-code based on the CAD design and the desired settings. CNC software can also be used to inspect and modify the code if needed.

G and M codes are two sets of commands integral to CNC, as well as other manufacturing methods like 3D printing. 

Most people may only have heard of G-code, but M-codes are equally as important. Both G codes and M codes affect the CNC machining process, but control different aspects.

What Are G Code and M Code?

G-code refers to “geometry” code which instructs movement across the CNC machine’s axes. M-code means “miscellaneous” code which controls various other functions, like coolant, the direction the spindle spins in, or the opening of the door.

In CNC programming, we use a computer file that instructs the CNC what to do. This is usually a .NC file type, containing code that the CNC machine will understand.

The CNC language in an NC file consists of “words” that start with letters, and contains G and M-codes. 

For example, below are two lines of code in a CNC code file. The first contains a G-code, the second contains M-code.

  • N80 G0 Z5
  • N81 M5

For the first example using G-code in the bullet point above, this means that on line 80 of the code (N80) move at high speed (G0) on the Z axis to coordinate 5 (Z5).

For the second example, it means on line 81 of the code (N81), turn off the spindle (M5).

Each line of the CNC code represents a proverbial “sentence.” Every line (each sentence) has a command. The two words G0 and M5 are commanding words, telling the machine what to do.

So, to program a CNC, we use G words and M words to command the machine to do various tasks – or as some call them, G-codes and M-codes

But, it’s important to remember that G-codes and M-codes are not two different codes in two different languages. They are different commands, within the same CNC language.

It’s just that because G-code is the most common command in CNC programming, that G-code has become a popular and common way to refer to the entire CNC code. 

The Other Letters In CNC Code (Beyond G and M)

These four letters represent the important CNC commands:

  • G: Geometry commands
  • M: Miscellaneous commands
  • F: set feed rate (command)
  • S: set spindle speed (command)

Other letters are not commands, but specify something. For example, below are some other letters in CNC code:

  • X: corresponds to the x-axis
  • Y: corresponds to the y-axis
  • Z: corresponds to the z-axis
  • A: corresponds to the A axis

We also have a whole article on the letters I, J, K, and R in CNC programming.

Now, let’s discuss in more detail the most important commands in CNC code: G codes and M codes.

G-code

The “G” in “G-code” refers to geometry, and these instructions control the co-ordinates where the machine should move, to accurately form the part. These instructions tell the machine where to move to fabricate the part.

Instructions written in G-code include where and how to move, where to start, where to stop, and how quickly to move the spindle.

G codes comprise a number of other letters, and while the G part of the code tells the machine when to do (command part of the code), other letters like X, Y and Z tell where to move within each of these three cartesian dimensions.

There are about 100 G code commands, from G0 to G99. However, not all CNC machines are compatible with all of these G codes.

These are some of the most important G-codes that I’ve picked as examples:

G CodeCommand Meaning
G0Move at rapid speed
G1Move at feed rate speed
G2Move on circle arc clockwise
G3Move on circle arc counterclockwise
G4Dwell – pause
G40Cancel cutter compensation
G41Compensate cutter diameter to the left of path
G42Compensate cutter diameter to the right of path
G43Compensate cutter length
gcode vs mcode example of g code letters
An example G-code with instructions visualized.

M-code

The “M” references miscellaneous or machine code, and covers all the major instructions not covered by the G-code. Rather than geometric alphanumeric movements, M-codes instruct the CNC machine to start and stop certain actions or programs outside the G-code’s domain.

These instructions include when to use coolant, when to open the machine doors, when to change the direction the spindle spins in, or to change tools. 

Here are some important M codes:

M CodeCommand Meaning
M3Turn on spindle clockwise
M4Turn on spindle counter clockwise
M5Turn off spindle
M7Turn on coolant in mist mode
M8Turn on coolant in flood mode
M9Turn off colant
M98Call and execute subroutine program
M99Return from subroutine to main program

CAD-CAM software and generating G-code and M-code

Rather than programming CNC software code by hand, CAD-CAM software can both design and prepare your eventual part for production and produce these CNC files. 

CAD-CAM software takes your CAD design, and your desired settings, and uses this to generate your G-codes for machining.

Some experts may want to inspect and modify codes if they believe this could lead to a better quality part, but if you’re a beginner, you may prefer to just go with the code that the software generates.

For more information, we also have a guide to the best CNC software and become familiar with the technical jargon, do read our comprehensive guide to CNC terminology.