Laser cuts then stops: why a cut dies halfway through

A cut that runs clean for the first hundred millimetres and then dies halfway through the plate is a different fault from a cut that never penetrates at all. When it never starts, you look at energy and gas from the beginning. When it starts and then stops, something is changing during the cut — and there are only a handful of things that change.

That distinction matters because the mid-cut stop has a short list of causes, and most of them show their evidence in the first ten minutes if you know where to look.

What actually changes during a cut

Four things drift while the head is moving:

• Heat. The lens, the protective window and the nozzle warm up as the cut progresses. A slightly dirty lens heats unevenly, its focal length shifts upward, and the focus climbs out of the plate. The cut fades from the middle of the contour outward. This is the classic pattern: perfect start, weak finish, and worse after lunch than first thing in the morning.
• Gas. Pressure that holds at standby can sag under flow. A regulator that loses 3 or 4 bar between pierce and full speed, a half-closed valve, a kinked line or an empty manifold all produce the same symptom: the cut dies on long contours but survives short ones.
• The nozzle. Soot building up on the tip during cutting changes the jet. If the cut quality degrades progressively and the stop point moves around, look at the tip after a failed cut — not before.
• The program. Long contours pass through corners, slow down for small features, and cross previous kerfs. A cut that dies at the same coordinate every time is not thermal — it is geometric. Mark the stop point on the sheet and check what the head is doing there.

The order I check things in

1. Same place every time? Re-run the part. A stop at a repeatable coordinate points to the program or a machine geometry issue, not optics. A stop that wanders points to heat or gas. 2. Look at the lens and window after the failure. Soot on the underside of the lens, or a halo on the protective window, means contamination — and contamination heats up progressively. Clean, re-cut, compare. 3. Watch the gas gauge during the cut, not before it. Standby pressure proves nothing. If pressure sags while the head is moving, you found it. 4. Feel the nozzle. A tip that is hot enough to spit when touched, or visibly sooted after one part, is leaking energy backwards. 5. Cut the same part in the morning and after four hours of production. If the afternoon copy fails and the morning one does not, the chiller or the optics are drifting with temperature. Log coolant temperature at both times.

Two cases that look the same and are not

Focus drift. The cut gets progressively shallower along the contour and the bottom edge closes up. Cause: dirty or heat-soaked optics shifting the focal point. Fix: clean or replace, then re-verify focus.

Gas sag. The cut starts well, then the bottom edge develops heavy dross and finally stops penetrating, often on the longest side of the contour. Cause: supply pressure dropping under flow. Fix: check regulator sizing and line restrictions — a gauge that reads fine at standby lies under load.

The treatment is opposite — one needs clean glass, the other needs bigger plumbing — and mixing them up costs a full day.

What to record so the next stop is faster

Note three things for every mid-cut stop: the coordinates where it stopped, the time into the job, and the coolant temperature. After three failures the pattern names itself: same coordinate means program or geometry, same elapsed time means heat, random means gas or material.

FAQ

Why is my laser cutting halfway through then stopping?

Usually one of two things: the focus is drifting up as the optics heat (dirty lens or window), or the assist gas pressure is sagging under flow. A stop at the same coordinate every run points to the program or machine geometry instead. Clean the optics, watch the gauge during the cut, and re-run the part — the pattern will tell you which one it is within two attempts.

Why does my laser cut fine in the morning but not in the afternoon?

Heat. Either the chiller is drifting with ambient temperature and the source is derating, or the optics are sooting progressively and losing transmission as they warm up. Log coolant temperature and inspect the protective window after the afternoon failure — one of the two will show it.

Can a dirty nozzle cause a cut to stop?

Yes, indirectly. Soot inside the tip narrows the jet and destabilises the flow, which first shows as edge dross and then as lost penetration on long contours. If the tip is discoloured or the cut quality degrades part after part, replace it before touching anything else.