Cause table
| Symptom | Common cause | How to check | What to do |
|---|---|---|---|
| Not cutting through, dross bridging underneath | Wrong focus, or speed too high | Cut a 50mm square at three focus settings | Adjust focus; reduce speed 10% at a time |
| Cut fine yesterday, not today | Protective lens dirty or damaged | Remove and inspect under light; see the lens article | Replace the protective lens |
| Hard burr on the underside, carbon steel | Oxygen pressure too high or too low; wrong speed | Test ±0.2 bar and ±10% speed, one variable at a time | Fine-tune pressure and speed for the thickness |
| Soft, peelable burr, stainless | Nitrogen pressure too low or nozzle too small | Measure actual pressure at the head, not at the bottle gauge | Raise the pressure; go one nozzle size up |
| Yellow-brown edge on stainless | Cutting stainless with oxygen, or contaminated nitrogen | Check gas type and purity | Use nitrogen for stainless; check the line for leaks |
| Sharp corners burn, straight runs are clean | No power reduction entering corners | Check corner settings in the software | Enable power modulation with speed at corners |
| Striations and dross on thick plate | Worn nozzle, or nozzle not concentric with the beam | Check concentricity with burn paper | Replace the nozzle; re-centre it |
The order to check
Always in this order, and only one variable at a time:
- Optics — is the protective lens clean. This is the number-one cause of "it cut fine yesterday."
- Nozzle — worn, correct size, concentric with the beam.
- Focus — cut a square at three focus settings and compare the edges.
- Gas — right type, enough pressure, measured at the cutting head, not at the bottle gauge.
- Speed and power — last, in 10% steps.
The most commonly misjudged point
When a machine that has been cutting well suddenly stops cutting through, it is almost never the parameters.
Nobody changes parameters overnight. What changed overnight is a slightly dirtier protective lens, a slightly more worn nozzle, or a gas bottle running low. Go straight to those three before you touch the parameter table.
The reverse also holds: if the machine has *never* cut that thickness, then it is a parameter problem — or a power problem.
Safety
Do not open the cutting enclosure while the machine is running. Do not look into the cutting zone without eyewear rated for the wavelength. Fibre laser at 1064nm is invisible — there is no bright flash to warn you. When changing a nozzle or lens, power down the laser source, not just the program.