Fiber laser stopped cutting through the sheet — what is wrong?
Updated:
Short answer
When a machine that used to cut through stops penetrating, the beam is losing energy on its way to the sheet or landing in the wrong place. In order of likelihood: dirty or cracked protective glass, worn nozzle, focus drift, gas pressure loss, and only then the source itself.
Why penetration disappears
| Symptom | Likely cause | First action |
|---|---|---|
| Gets worse through the shift; piercing takes longer; sometimes a visible burn spot on the glass. | Contaminated protective glass | Inspect under a light at an angle. Replace — cleaning a burned coating does not restore it. |
| Cut starts fine and fails on thicker parts; gas jet audibly uneven. | Worn nozzle or wrong diameter | Replace the nozzle and match the diameter to thickness; an enlarged orifice drops the pressure at the kerf. |
| Fails uniformly on all thicknesses; kerf looks wide and rounded. | Focus drift after a collision or lens change | Re-run the focus calibration; check that the head sensor zero matches the actual standoff. |
| Cutting fails on nitrogen but oxygen still works, or fails at the end of a cylinder. | Gas pressure or purity loss | Measure pressure under flow at the head; check filters, hoses and cylinder change procedure. |
| Only this batch fails: rusty, mill-scaled, painted or a different grade or thickness tolerance. | Material differs from the program | Measure the sheet, check the certificate; hot-rolled with scale needs different parameters than cold-rolled. |
| Everything above is ruled out; power meter reads low; problem is stable across materials. | Real source power loss | Log the power measurement and call service — do not compensate by slowing the feed indefinitely. |
Send a photo of the cut and get the cause ranked — inside the app. Open SBM Laser Help →
Or take it with you:
Fast triage at the machine
- Cut the same test square in thin material: if thin still cuts cleanly, the beam is present but weakened or misplaced.
- Pull and inspect the protective glass; replace if any burn, spatter or haze.
- Fit a new nozzle of the correct diameter and re-centre.
- Re-run focus calibration and confirm standoff with a feeler gauge.
- Watch the gas pressure gauge while cutting — pressure that sags under flow means a supply restriction.
- If penetration is still short, measure output power before touching parameters further.
Related questions
- Should I just increase power to push through?
- Raising power masks the real cause and burns consumables faster. If a machine needed 4 kW yesterday and needs 5.5 kW today for the same sheet, something in the path degraded — find it.
- Can compressed air replace nitrogen when penetration drops?
- Air cuts thin mild steel and aluminium acceptably, but it is roughly 78% nitrogen with oxygen and moisture, so the edge oxidises and burr increases. It is a fallback, not a fix for lost penetration.
- Why does it cut through in the middle of the sheet but not at the edges?
- That is usually sheet flatness or support slat wear: the standoff changes across the table. Check the slats and clamping before the optics.