Dross on the bottom edge: the order I actually check things in

Dross on the bottom edge is the most common thing people send me. It is also the one people most often try to fix in the wrong order — usually by dragging cutting speed down until the whole sheet is slow and the edge is still rough.

The melt was not expelled. That is the whole problem. Everything below is just finding out why.

Step 1 — Is the gas actually doing its job?

On carbon steel, oxygen does the cutting, not just the blowing. If the pressure at the nozzle is low, the slag has nothing to push it out and it re-solidifies the moment the head moves on.

• Check the pressure at the nozzle, not at the regulator on the wall.
• Check the nozzle itself: a chipped or ovalised bore changes the flow far more than the gauge suggests.
• Check for a partially clogged line or a leaking joint downstream of the gauge.

If the pressure reads fine but the dross is still there, the gas is arriving, but not where it needs to be.

Step 2 — Is the material actually penetrated?

Cutting speed too high produces exactly the same symptom as low pressure, and the two are constantly mistaken for each other. The head moves on before the kerf is through, so the last of the melt is left behind.

The tell is where the dross sits. Speed-induced dross is usually even along the cut and worse on corners, where the machine is still accelerating. Pressure-induced dross tends to be heavier on one side of the part.

Step 3 — Before you touch the parameter table again

Two things look like process problems and are not:

• Rack-and-pinion backlash combined with aggressive acceleration — the axis is not where the controller thinks it is, so the effective speed varies along the cut.
• An acceleration parameter set too high for the moving mass — same effect, and it shows up as an unstable cut even at a constant commanded speed.

Both of these will follow you around no matter how you tune the gas or the focus.

One case where the answer is not a parameter at all

On rusty or film-covered material, no parameter change will save the cut. Reduce the cutting pressure, lower the focus, replace the lens and add a descaling pass. Clean metal cuts completely differently, and chasing parameters on a rusty sheet is time you do not get back.

What this looks like in practice

On a call I work outward from the gas, to the penetration, to the mechanics, and only then to the parameter table. Roughly four out of five cases are settled in the first two steps. The rest are mechanical or optical, and no amount of speed reduction will touch them.

If you have already been through all of this and the dross is still there, that is exactly the kind of case worth sending me — see the note at the bottom of this page.

If the dross comes with no penetration at all, the order changes — start with what I check first when a machine stops cutting before touching the gas.

FAQ

What is dross in laser cutting?

Dross is molten metal that was not blown clear before the cut solidified. It clings to the bottom edge as slag or a sharp burr. Low-pressure dross with sharp tails usually means speed or gas; sticky, rounded dross usually means focus or power.