Eliminate CNC crashes and waste with expert G-code debugging tips and free simulation tools like CAMotics, NC Viewer, Fusion 360 Simulator. Includes real-world examples, error codes, and productivity tips.
How to Debug and Simulate G-Code Like a Pro (2025)
Whether you’re running a simple GRBL setup or a full industrial Fanuc CNC machine, G-code debugging and simulation is a non-negotiable step before cutting any material. This comprehensive guide covers essential simulation tools, error types, and expert tips for optimizing your CNC workflow.
🔧 Why G-Code Debugging Matters
Without proper debugging, G-code errors can lead to:
- Broken tools
- Material waste
- Damaged machine components
- Extended production downtime
Simulating your program helps identify potential problems **before** they become expensive mistakes.
🧪 Top Free G-Code Simulators
| Tool | Platform | Features | Website |
|---|---|---|---|
| CAMotics | Windows/Linux/Mac | 3D Toolpath, Material Removal Simulation | camotics.org |
| NC Viewer | Web-based | 2D Toolpath Preview, Code Step-Through | ncviewer.com |
| Fusion 360 Simulator | Cloud + Desktop | CAM Toolpath + Simulation + Post-Processor | Fusion 360 |
| OpenBuilds CAM | Web-based | Simple G-code viewer + GRBL output | cam.openbuilds.com |
🚨 Common G-Code Errors and How to Fix Them
| Error | Meaning | Fix |
|---|---|---|
| “Undefined G-code” | Unsupported or mistyped command | Check spelling and controller limits |
| “Out of range” | Movement exceeds soft limits | Verify work offsets and zero points |
| “Feedrate not set” | No F value for motion | Add F100 or proper feed |
| “Spindle not started” | G1/G2/G3 used before M03 | Insert M03 before feed commands |
| “Alarm: Hard Limit” | Machine hit physical boundary | Check homing and workspace |
🔄 How to Simulate a G-Code File (CAMotics Example)
- Download and install CAMotics from camotics.org
- Create a new project
- Load your G-code file (*.nc, *.gcode, *.tap)
- Set machine size, tool parameters, and material stock
- Click “Simulate” and observe toolpath + material removal
Pro Tip:
Use slow playback to catch Z-axis plunges, collisions, and toolpath overlaps.
🧰 G-Code Debugging Techniques
- Line-by-line testing: Run G-code in single block mode on machine
- Simulate tool changes: Use
M06and verify tool length offsets - Check for modal conflicts: Use
G80to cancel canned cycles - Use comments generously: Add (notes) to avoid confusion
- Use offline editors: CNCjs, Notepad++, Fusion 360 Post Editor
📄 Sample G-code with Debug Notes
“`gcode
(G-Code Debug Example)
G21 G90 G17 ; Set mm, absolute, XY plane
G0 Z5 ; Safe retract
G0 X0 Y0 ; Move to start
M03 S1200 ; Spindle ON
G1 Z-2 F100 ; Feed down
G1 X50 Y0 F200 ; Cut
G1 X50 Y50 ; Cut
G1 X0 Y50 ; Cut
G1 X0 Y0 ; Cut
G0 Z5 ; Retract
M05 ; Spindle OFF
M30 ; End
📌 Best Practices for Safe and Accurate G-Code
- Always simulate complex operations like helical cuts, arcs (G02/G03)
- Use G54–G59 offsets and verify WCS vs. MCS
- Apply slow feedrates in first test runs
- Back up machine parameters and firmware settings
🔗 More Resources
- CAMotics Simulator (Free)
- NC Viewer (Online)
- CNC CookBook – G-code Debug Tips
- Fusion 360 CAM & Simulation
Conclusion
Mastering G-code simulation and debugging techniques gives you total confidence before hitting the START button. Use the tools and examples in this guide to avoid crashes, improve efficiency, and become a professional CNC programmer. Bookmark cnccode.com for more advanced CNC programming tutorials and downloadable resources.
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