Red beam off centre on a fiber laser — what it tells you

The red guide beam is not the cutting beam. It is coaxial with it, which makes it the fastest way to check the whole optical path without cutting anything.

If it is visibly off centre, diverging, or irregular in shape, stop here. Check the optical path before anything else — before focus, before gas, before you open the parameter table.

Reading the mark on the tape

A clean test: punch a mark on tape at the nozzle and look at where it lands.

• Mark concentric with the nozzle bore — nozzle and beam path are aligned. Move on.
• Mark off centre — the head is out of alignment. This alone will produce a bevel, a one-sided cut face, or an unstable process.

This matters most after a rebuild. The single most common mix-up is a full-reflection mirror fitted where the deflecting mirror belongs. Everything looks assembled correctly and nothing cuts properly.

When alignment is fine and the alarm is still there

If the mark is concentric and you still get faults, the trigger is the process, not the mechanics. A useful split:

• Close the shutter and run the job with the beam off. If the alarm appears anyway, it is motion-induced — typically the capacitance alarm triggered by the machine's own movement rather than by cutting.
• If it only appears with the beam on, the trigger is in the cutting process itself, and the alignment work is genuinely done.

That one test separates two problems that otherwise look identical on the screen.

The lens and the focus

Two things worth ruling out before you blame the lens:

• A contaminated focusing lens combined with a beam that has drifted off axis produces a slower, worse version of a pure alignment fault. Clean or replace, then re-check the red beam.
• On some heads the focus drifts quickly. The usual cause is an unreasonable motorised-focus parameter rather than the optic at all — replacing a perfectly good lens will not change it.

The order that saves time

Red beam first, then lens, then focus, then parameters. It costs a few minutes and removes the two most common false trails — swapping hardware that was fine, and tuning a parameter table that was never the problem.

The same test on a cold machine, and again an hour later

Do the tape shot twice: once before the first shift starts, once after the machine has been running for an hour.

• Concentric both times — the optical path is stable. Look elsewhere.
• Concentric when cold, off centre when hot — something is moving with temperature. On most heads that is the focusing lens or its mount expanding unevenly, and on older machines it is a head that is not being cooled properly.
• Off centre both times — it was assembled like that. Start at the collimator and work down.

Crescent, blob, or a double dot

The shape matters as much as the position.

• A crescent means the beam is being clipped on one side. Something in the path is off axis, or the fibre end is damaged.
• A soft blob with no defined edge means the beam is not being focused at all at that distance. The guide beam is diverging, which usually means the collimating side, not the cutting lens.
• Two dots is the easiest of all: a reflection off a tilted window, or a protective glass installed the wrong way round.

Keep the photo

Photograph the mark on the tape with your phone, dated, and keep the last few. When a cut quality problem appears three weeks later, that photo is the baseline. Without it you are arguing with your own memory, and memory always says "it was fine last week".

After a rebuild or a head swap

The days after a rebuild are when these faults cluster, because everything was touched at once. Do the red beam before the first job, not after the first complaint. On heads that use separate collimating and focusing optics, check the collimator first: a cutting lens replaced three times will not fix a collimator that was never seated.