G-code program structure in CNC Programming

Introduction

G-code program structure shows how to organize instructions so a CNC machine can follow them step-by-step to make parts.

When you want to tell a CNC machine how to cut or shape a part.

When you need to start a new CNC machining job with clear steps.

When you want to make sure the machine runs safely and correctly.

When you want to repeat the same machining process multiple times.

When you want to add comments or notes inside your CNC program.

Syntax

CNC Programming

O1234 (Program number and start)
N10 G21 (Set units to millimeters)
N20 G90 (Set absolute positioning)
N30 G01 X10 Y10 F100 (Move in a straight line)
N40 M30 (End program)

O number is the program ID and usually appears at the start.

N numbers are line numbers to organize commands clearly.

Examples

A simple program that sets absolute mode, moves quickly to X0 Y0, then ends.

CNC Programming

O1000
N10 G90
N20 G00 X0 Y0
N30 M30

Program using millimeters, moves in a straight line at feed rate 200, then ends.

CNC Programming

O2000
N10 G21
N20 G01 X50 Y50 F200
N30 M30

Program with comments explaining each step.

CNC Programming

O3000
N10 (Start of program)
N20 G90 (Absolute positioning)
N30 G00 X0 Y0 (Rapid move to origin)
N40 M30 (Program end)

Sample Program

This program draws a square by moving the tool around the edges using straight line moves.

CNC Programming

O1234 (Sample G-code program)
N10 G21 (Set units to millimeters)
N20 G90 (Use absolute positioning)
N30 G00 X0 Y0 (Rapid move to start point)
N40 G01 X50 Y0 F150 (Cut straight line at feed 150)
N50 G01 X50 Y50 (Cut straight line up)
N60 G01 X0 Y50 (Cut straight line left)
N70 G01 X0 Y0 (Cut straight line down to start)
N80 M30 (End of program)

Important Notes

Always start with a program number (O code) to identify your program.

Use line numbers (N codes) to keep commands organized and easy to follow.

Include comments in parentheses to explain what each part does for easier reading.

Summary

G-code programs start with a program number and end with M30 to stop.

Line numbers help organize commands and make the program easier to read.

Commands include setting units, positioning mode, moving the tool, and ending the program.

Practice

1. What is the correct way to start a G-code program?

easy

A. Write G90 as the first line

B. Start with the command M30

C. Use G01 to start the program

D. Begin with a program number like O1000

Solution

1. Step 1: Identify the program start syntax

   G-code programs start with a program number, usually written as O1000 or similar.

2. Step 2: Understand the role of other commands

   M30 ends the program, G01 and G90 are motion and positioning commands, not start markers.

3. Final Answer:

   Begin with a program number like O1000 -> Option D

4. Quick Check:

   Program start = O1000 [OK]

Hint: Program always starts with O-number like O1000 [OK]

Common Mistakes:

• Confusing M30 as start instead of end
• Using motion commands as start
• Skipping program number line

2. Which of the following lines correctly ends a G-code program?

easy

A. G00 X0 Y0

B. O1000

C. M30

D. G90

Solution

1. Step 1: Identify the program end command

   The command M30 is used to stop and end a G-code program.

2. Step 2: Differentiate from other commands

   G00 moves the tool, G90 sets positioning mode, and O1000 is a program number, not an end command.

3. Final Answer:

   M30 -> Option C

4. Quick Check:

   Program end = M30 [OK]

Hint: Program ends with M30 command [OK]

Common Mistakes:

• Using motion commands as program end
• Confusing program number with end command
• Omitting M30 at program end

3. Given the following G-code snippet, what is the tool's final position?

O1000
G90
G00 X10 Y5
G01 X20 Y15 F100
M30

medium

A. X20 Y15

B. X10 Y5

C. X0 Y0

D. X100 Y100

Solution

1. Step 1: Trace the tool movements

   G00 X10 Y5 moves the tool rapidly to X=10, Y=5. Then G01 X20 Y15 F100 moves it linearly to X=20, Y=15.

2. Step 2: Identify final position before program end

   The last position command is G01 X20 Y15, so the tool ends at X=20, Y=15.

3. Final Answer:

   X20 Y15 -> Option A

4. Quick Check:

   Last move = X20 Y15 [OK]

Hint: Final position is last coordinate command before M30 [OK]

Common Mistakes:

• Confusing rapid move (G00) as final position
• Ignoring linear move (G01)
• Assuming start position is final

4. Identify the error in this G-code program snippet:

O2000
G01 X10 Y10
G90
M30

medium

A. G90 should come before G01 for positioning mode

B. M30 is used incorrectly

C. Missing program start number

D. No error, program is correct

Solution

1. Step 1: Check command order for positioning

   G90 sets absolute positioning and should be set before any move commands like G01.

2. Step 2: Analyze the given order

   The program moves with G01 before setting G90, which can cause unexpected positioning behavior.

3. Final Answer:

   G90 should come before G01 for positioning mode -> Option A

4. Quick Check:

   Set positioning mode before moves [OK]

Hint: Set G90 before moves to avoid positioning errors [OK]

Common Mistakes:

• Placing G90 after move commands
• Ignoring command order importance
• Assuming M30 ends program incorrectly

5. You want to write a G-code program that sets absolute positioning, moves the tool to X=50 Y=50 at feed rate 200, then ends the program. Which is the correct structure?

hard

A. O3000
G01 X50 Y50 F200
G90
M30

B. O3000
G90
G01 X50 Y50 F200
M30

C. G90
O3000
G01 X50 Y50 F200
M30

D. O3000
M30
G90
G01 X50 Y50 F200

Solution

1. Step 1: Confirm program start and positioning

   The program must start with a program number O3000 and set absolute positioning G90 before moves.

2. Step 2: Check move command and program end

   G01 X50 Y50 F200 moves the tool at feed rate 200, and M30 ends the program properly.

3. Final Answer:

   O3000 G90 G01 X50 Y50 F200 M30 -> Option B

4. Quick Check:

   Start with O-number, set G90, move, then M30 [OK]

Hint: Start with O-number, set G90, move, then M30 to end [OK]

Common Mistakes:

• Placing M30 before moves
• Setting G90 after moves
• Starting without program number