You send a drawing to a CNC factory. A few days later, a finished metal part arrives. Every cut is in the right place. Every dimension is correct.
What made that happen?
G code and M code. These are the two types of instructions that run every CNC machine. G code controls how the tool moves. M code controls everything else, like the spindle, the coolant, and the tool changer.
This guide explains both, with real examples, in plain English.
What Is G Code?
G code is the programming language used to control CNC machines. It defines tool movement, feed rate, and cutting path.
Think of a CNC machine like a worker who follows written instructions. It does not guess. It does not improvise. It reads each line of instructions and does exactly what it says.
G code is those instructions.
Every time a CNC machine moves a cutting tool, it is following a G code command. G code tells the machine three things: where to move, how fast to move, and what kind of movement to make.
The letter G comes from the word "geometric." That makes sense. G code is all about geometry: position, direction, distance, and speed.
What the Letters Mean
Each letter in a G code line has a specific meaning.
- G: The type of motion or operation, such as rapid move, straight cut, or arc.
- X: Position on the X axis.
- Y: Position on the Y axis.
- Z: Position on the Z axis.
- F: Feed rate, how fast the tool moves during cutting.
- S: Spindle speed in RPM.
- T: Tool number, selects which tool to use.
- M: Machine function, such as spindle on/off, coolant, or tool change.
A Simple Example
Here is one line of G code:
G01 X50 Y20 Z-5 F200
This is not as complicated as it looks. Each part has a simple meaning:
| Code | What it means |
|---|---|
| G01 | Move in a straight line while cutting |
| X50 | Go to the 50mm position on the X axis |
| Y20 | Go to the 20mm position on the Y axis |
| Z-5 | Go 5mm below the surface of the part |
| F200 | Move at a speed of 200mm per minute |
Common G Codes
You do not need to memorize every G code. But these are the ones that show up in almost every CNC program:
Movement codes:
| Code | What it does | Example |
|---|---|---|
| G00 | Rapid move, no cutting, maximum speed | G00 X0 Y0 Z50 |
| G01 | Straight line cut at set feed rate | G01 X100 F300 |
| G02 | Circular arc, clockwise | G02 X50 Y50 R20 F200 |
| G03 | Circular arc, counterclockwise | G03 X50 Y50 R20 F200 |
| G04 | Pause for a set amount of time | G04 P500 |
Coordinate and position codes:
| Code | What it does |
|---|---|
| G20 | Use inch units |
| G21 | Use millimeter units |
| G28 | Return to machine home position |
| G90 | Absolute positioning (coordinates are fixed from origin) |
| G91 | Incremental positioning (coordinates are measured from current position) |
| G54 to G59 | Work coordinate systems, used to set part origin |
Tool compensation codes:
| Code | What it does |
|---|---|
| G40 | Cancel cutter radius compensation |
| G41 | Cutter compensation, offset to the left |
| G42 | Cutter compensation, offset to the right |
| G43 | Apply tool length offset |
| G49 | Cancel tool length offset |
Canned cycles (for drilling and tapping):
| Code | What it does |
|---|---|
| G40 | Cancel cutter radius compensation |
| G41 | Cutter compensation, offset to the left |
| G42 | Cutter compensation, offset to the right |
| G43 | Apply tool length offset |
| G49 | Cancel tool length offset |
What Is M Code?
M code controls the machine functions that support the cutting process, such as spindle rotation, coolant flow, tool changes, and program stops.
G code moves the tool. M code handles everything else.
None of these involve moving the tool along a path, but all of them are needed to complete a real machining job.
Common M Codes
These are the M codes you will see in almost every CNC program:
| Code | What it does |
|---|---|
| M03 | Spindle on, turning clockwise |
| M04 | Spindle on, turning counterclockwise |
| M05 | Spindle stop |
| M06 | Change tool (used with T code, e.g. T02 M06) |
| M07 | Mist coolant on |
| M08 | Flood coolant on |
| M09 | Coolant off |
| M00 | Program stop (machine waits for operator) |
| M30 | End of program, reset to beginning |
How M Code Works in a Real Job
S2500 M03 (start spindle at 2500 RPM)
M08 (turn coolant on)
...cutting happens here...
M09 (turn coolant off)
M05 (stop spindle)
M30 (end program)
That sequence runs on every job, in roughly that order. The spindle starts first, then coolant, then cutting, then coolant off, spindle off, program done.
G Code vs M Code, Difference and Uses
G code tells the tool where to go. But it cannot make the spindle spin. M code can start the spindle. But it cannot move the tool along a cutting path.
A machine running only G code would move a tool through air with no spindle turning. A machine running only M code would spin a spindle and spray coolant with no movement at all. Neither produces a finished part.
In a real program, you need both. G code and M code sit in the same file and run in sequence, line by line.
A Practical Example
Here is a short sequence that shows how they take turns:
S2000 M03 (M code: start spindle at 2000 RPM)
G00 X0 Y0 (G code: rapid move to start position)
M08 (M code: coolant on)
G01 Z-5 F150 (G code: feed down into the part)
G01 X60 F300 (G code: cut along X axis)
G00 Z50 (G code: retract tool)
M09 (M code: coolant off)
M05 (M code: spindle stop)
Conclusion
G code moves the tool. M code controls the machine. Together they run every CNC job from start to finish.
You do not need to memorize every code. But understanding how they work makes it easier to read a program, catch errors, and communicate with the shop floor.
If you have a CNC project that needs precise programming and reliable results, get in touch with our team at Aria Manufacturing.
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