G-code program structure in CNC Programming - Step-by-Step Execution

Concept Flow - G-code program structure

Start Program

↓

Initialize Settings

↓

Execute Commands

↓

Check for End

↓

Next Command

The G-code program starts, sets up machine parameters, runs commands step-by-step, and ends when the program finishes.

Execution Sample

CNC Programming

N10 G21 (Set units to mm)
N20 G90 (Absolute positioning)
N30 G01 X50 Y25 F100 (Move linearly to X50 Y25 at feed 100)
N40 M30 (End program)

This simple G-code program sets units, positioning mode, moves the tool, then ends the program.

Execution Table

Line Number	Command	Action	Machine State	Output
N10	G21	Set units to millimeters	Units=mm	Units set to mm
N20	G90	Set absolute positioning mode	Positioning=Absolute	Positioning set to absolute
N30	G01 X50 Y25 F100	Move tool linearly to X=50, Y=25 at feed 100	Position=(50,25), Feed=100	Tool moved to X50 Y25
N40	M30	End program	Program Ended	Program stopped

💡 Program ends at line N40 with M30 command

Variable Tracker

Variable	Start	After N10	After N20	After N30	After N40
Units	Default (inches)	mm	mm	mm	mm
Positioning	Default (Incremental)	Incremental	Absolute	Absolute	Absolute
Position	(0,0)	(0,0)	(0,0)	(50,25)	(50,25)
Feed	Default	Default	Default	100	100
Program State	Not started	Running	Running	Running	Ended

Key Moments - 3 Insights

Why does the tool move to X50 Y25 at line N30?

Because line N30 uses G01 (linear move) with absolute positioning set by G90 at N20, so the tool moves directly to coordinates X=50, Y=25.

What happens if M30 is missing at the end?

The program would not officially end, so the machine might wait or repeat commands depending on controller settings. See exit at N40 in execution_table.

Why is G21 important at the start?

G21 sets units to millimeters; without it, the machine might interpret distances in inches, causing wrong moves. This is shown in variable_tracker Units change after N10.

Visual Quiz - 3 Questions

Look at the execution table, what is the machine position after line N30?

A (50, 25)

B (0, 0)

C (100, 50)

D (25, 50)

💡 Hint

Check the Position column in the execution_table row for N30

At which line does the program officially end?

A N10

B N20

C N40

D N30

💡 Hint

Look at the Action and Output columns in execution_table for program end

If G90 at N20 was changed to G91, how would the position after N30 change?

A Position would be (50, 25)

B Position would be relative to current, so (50, 25) added to previous

C Position would reset to (0,0)

D Position would not change

💡 Hint

G91 means incremental positioning, so moves add to current position (see variable_tracker Position changes)

Concept Snapshot

G-code program structure:
- Start with program setup (units, positioning)
- Execute movement and machine commands line by line
- Use N numbers for line labels
- End program with M30
- Commands affect machine state stepwise

Full Transcript

This visual execution shows a simple G-code program structure. The program starts by setting units to millimeters with G21, then sets absolute positioning mode with G90. Next, it moves the tool linearly to coordinates X=50 and Y=25 at feed rate 100 using G01. Finally, the program ends with M30. The execution table traces each line's command, action, and machine state changes. The variable tracker shows how units, positioning mode, tool position, feed rate, and program state change after each command. Key moments clarify why absolute positioning affects moves, the importance of ending the program, and unit settings. The quiz tests understanding of position after moves, program end line, and effect of changing positioning mode. This step-by-step trace helps beginners see how G-code commands control CNC machine behavior in order.