How to Write Your First G-Code Program

Writing your first G-code program can feel intimidating, but it’s actually simpler than you think. G-code is just a set of instructions that tell your CNC machine what to do, step by step. Whether you’re making a simple cut or creating something more intricate, understanding the basics will set you on the right path. Let’s break it down so you can write your first G-code program with confidence.

What You Need Before You Start

Before diving into G-code, make sure you have the following:

CNC Machine: Ensure it’s set up and calibrated.
CAD/CAM Software: While this guide focuses on manual G-code, software like Fusion 360 or Easel can generate code automatically for reference.
Material: Use a simple material like wood or plastic for your first project.
Tooling: A basic end mill or drill bit will work fine.

Understanding the Basics of G-Code

G-code is made up of commands that tell your CNC machine how to move, cut, and perform various functions. Each line of G-code is called a block, and each block contains:

Commands: Like G01 (linear cutting) or G00 (rapid movement).
Coordinates: Specify where the machine should go (e.g., X10 Y20).
Parameters: Additional instructions like feed rate (F100) or spindle speed (S1200).

Here’s an example of a simple G-code line:

G01 X10 Y20 Z-1 F100

This tells the machine to cut in a straight line to the position X=10, Y=20, Z=-1 at a feed rate of 100 units per minute.

Step-by-Step Guide to Writing Your First Program

Step 1: Define the Setup

Start with basic setup commands to initialize the machine.

G21  ; Set units to millimeters
G90  ; Set absolute positioning
G17  ; Select the XY plane for cutting

G21 ensures the machine uses millimeters as the unit.
G90 sets the machine to absolute positioning mode.
G17 tells the machine to cut in the XY plane.

Step 2: Move to a Safe Starting Position

Use rapid positioning (G00) to move the tool to a safe location above the material.

G00 X0 Y0 Z5  ; Move to the starting position

This moves the tool to X=0, Y=0, and Z=5 (above the material).

Step 3: Start the Spindle

Turn on the spindle and set the speed.

M03 S1000  ; Start spindle clockwise at 1000 RPM

M03 starts the spindle in a clockwise direction.
S1000 sets the spindle speed to 1000 RPM.

Step 4: Make Your First Cut

Lower the tool and cut a straight line.

G01 Z-1 F100  ; Lower the tool to Z=-1 at a feed rate of 100
G01 X50 Y0    ; Cut a straight line to X=50, Y=0

The first line lowers the tool into the material.
The second line moves the tool in a straight line along the X-axis.

Step 5: Add Additional Movements

Continue with more movements to complete your design.

G01 X50 Y50   ; Move to X=50, Y=50
G01 X0 Y50    ; Move to X=0, Y=50
G01 X0 Y0     ; Return to the starting position

These lines create a square by connecting the corners.

Step 6: End the Program

Wrap up the program by turning off the spindle and returning to a safe position.

M05           ; Stop the spindle
G00 Z5        ; Raise the tool to a safe height
G28           ; Return to the machine’s home position
M30           ; End the program

M05 stops the spindle.
G00 Z5 lifts the tool above the material.
G28 sends the machine back to its home position.
M30 marks the end of the program.

Complete G-Code Program Example

Here’s the full program for cutting a square:

G21           ; Set units to millimeters
G90           ; Set absolute positioning
G17           ; Select the XY plane
G00 X0 Y0 Z5  ; Move to starting position
M03 S1000     ; Start spindle clockwise at 1000 RPM
G01 Z-1 F100  ; Lower the tool into the material
G01 X50 Y0    ; Cut to X=50, Y=0
G01 X50 Y50   ; Cut to X=50, Y=50
G01 X0 Y50    ; Cut to X=0, Y=50
G01 X0 Y0     ; Return to starting position
M05           ; Stop the spindle
G00 Z5        ; Raise the tool to a safe height
G28           ; Return to home position
M30           ; End program

360 Key Takeaways

Start with basic setup commands like G21 (units) and G90 (positioning).
Use G00 for rapid movement and G01 for precise cutting.
Always include safety steps like moving the tool to a safe height and turning off the spindle.
Practice simple shapes like squares or circles to build confidence.

Conclusion

Writing your first G-code program is all about understanding the basics and practicing simple designs. Don’t worry about making mistakes—trial and error is part of the learning process. Once you’re comfortable, you can start creating more complex programs and bringing your ideas to life with precision. So, fire up your CNC machine and give it a go!