Built a free G-code viewer with real cycle-time simulation. Try to break it.

I've built a free G-code viewer that runs in the browser. It uses real motion control algorithms the same kind as your CNC controller.

Set your machine's rapids and acceleration in the "Machine Dynamics" box to match your machine, or just leave the defaults.

Nothing is uploaded. The file is parsed in your browser and never leaves your machine, so you can paste anything.

What it handles: G0 to G3 arcs in all planes, canned cycles including peck drilling and rigid tapping, M98 / M97 / o-sub subprograms nested and expanded, dialect-aware G04 dwell (Fanuc milliseconds vs LinuxCNC seconds), G94/G95, G61/G64 blending in the time model, and optional S-curve jerk if you set it. Anything it can't fully model gets mentioned in the message panel instead of being quietly ignored.

The challenge. Throw your ugliest post-processor output at it and try to make it time wrong, draw wrong, fall over, or simulate something wrongly without telling you. A missing disclosure counts too. Find one and I'll fix it and credit you in the changelog.

The rules and the full supported-code list: https://ultranc.com/challenge/
The viewer, no signup: https://ultranc.com/gcode-viewer/

The screenshot below is the "UltraNC Sign.nc" demo, timed at 44:35. It's in the viewer's demo file menu, so load it and check the number for yourself, then bring your own worst file.

Free, no accounts, no catch. Tell me where it's wrong.