Custom G-Code: A Complete Guide to CNC Programming

What Is Custom G-Code?

At its core, G-code is the universal programming language that tells computerized manufacturing machines—like 3D printers, CNC routers, and laser cutters—how to move, how fast to go, and when to heat up.

While modern slicing software automatically generates thousands of lines of G-code from your 3D models, custom G-code refers to the specific scripts you manually inject into certain parts of a print job. Whether you want your 3D printer to wipe its nozzle on a specialized silicon brush before every run, play a melody when finished, or pause mid-print to change filament colors, custom G-code is the key that unlocks total hardware control.

Why Use Custom G-Code?

Standard slicer settings cover ninety percent of everyday printing needs. However, custom G-code lets you push past standard limitations to achieve advanced behaviors:

• Automated Nozzle Priming: Instead of relying on random blobs or default purge lines, custom start scripts can lay down a smooth, clean priming line along the edge of the build plate to clear out any oxidized plastic.
• Precise Thermal Management: You can insert commands to wait for bed and nozzle temperatures in a specific sequence to prevent oozing, or ramp up cooling fans only after specific overhang layers are reached.
• Mid-Print Interventions: Inserting pause commands (M600 for filament changes or custom height triggers) allows for multi-color prints on single-extruder machines or embedding nuts and magnets directly inside parts.
• Smart End Sequences: Ensure your print head moves safely out of the way, turns off heaters, and retracts filament automatically to prevent heat creep.

Essential G-Code Commands to Know

Writing custom scripts doesn’t require learning an entire programming language. Most custom blocks rely on a handful of standard commands (known as “G” and “M” codes):

| Command | Function | Description |
| :— | :— | :— |
| G28 | Auto Home | Moves all axes (X, Y, Z) to their endstops to calibrate position. |
| G1 | Linear Move | Moves the toolhead to specific coordinates (X, Y, Z) at a given feedrate (F). |
| M104 / M109 | Extruder Temp | Sets hotend temperature (M104 continues immediately, M109 waits until target is reached). |
| M140 / M190 | Bed Temp | Sets heated bed temperature (M190 waits for stabilization). |
| M600 | Filament Change | Triggers a filament change routine, retracting current material and prompting the user. |

Where to Add Custom G-Code

Most advanced slicers (such as OrcaSlicer, PrusaSlicer, and Ultimaker Cura) feature dedicated text boxes under printer settings where you can inject custom scripts:

1. Start G-Code: Executes right before the print begins, handling homing, bed leveling mesh loading, and nozzle priming.
2. End G-Code: Runs after the final layer is complete, powering down heaters and parking the toolhead.
3. Before Layer Change / After Layer Change: Scripts that execute dynamically at every new layer height, useful for tweaking flow rates or z-offsets on the fly.

Safety Warning: Always test new custom G-code scripts dry or with your hand hovering over the power switch. Incorrect coordinate commands or missing homing sequences can cause the print head to crash into your bed or frame.