Most people picture a plasma arc as a straight column coming out of the nozzle. A beam.
It isn't. It's shaped like a football.
Narrow where it leaves the nozzle, widest through the middle, narrowing again at the far end. And once you know that, a whole class of cut-quality problems stops being mysterious.
Cut with the flat part
Because the arc has a shape, it matters which part of it is passing through your plate.
The middle of that football — the long, parallel-sided section through the belly of it — is the part you want doing the cutting. That's where the arc is closest to a straight column, and a straight column is what gives you a square edge.
Get the torch height right and your material sits in that middle section. The sides of the kerf come out close to parallel, and the cut face comes off square.
Get the height wrong and you're cutting with one of the tapered ends instead. The arc is converging or diverging as it passes through the plate, and that angle transfers straight onto your cut face.
That's all bevel is. It's the shape of the arc, printed into the steel.
Which bevel you get tells you which way to move
This is the useful part, and it's why the bevel is worth reading rather than just cursing.
Torch too high → positive bevel. The kerf comes out wider at the top than the bottom, and the cut face slopes inward as it goes down. You're cutting with the diverging upper portion of the arc.
Torch too low → negative bevel. The kerf is wider at the bottom than the top. You're down into the lower end of the football, where the arc is converging.
So you don't have to guess at the correction. Look at the cut face and the direction of the lean tells you whether to come up or go down.
That's a better diagnostic than most people have, because the instinct when a cut goes off is to start adjusting everything at once. The bevel is specifically a height signal.
Why this matters more on a mechanized table
On a hand torch with a drag shield, the shield is doing this job for you. Those little feet on the end hold your standoff while you drag the torch on the plate — that's most of what a drag shield is for.
Mechanized is different. Nothing is resting on the material. Your height is whatever the height control says it is, and if that's wrong, it's wrong for the whole program.
Which means on a CNC table, bevel is usually telling you about one of three things:
Your height setting. The simplest case and the first thing to check.
Your height control, not keeping up. If it's hunting, lagging, or losing the plate, the height is varying while you cut and the bevel will vary with it.
Your plate, not where the machine thinks it is. Warped material, or slats that have been chewed up and are no longer holding the plate flat. The height control can be doing everything right and still be referencing the wrong surface. That's worth checking before you adjust anything.
And the arc shape changes as the nozzle wears
Here's the part that connects this back to parts.
That football shape is produced by the orifice in your nozzle. The nozzle constricts the arc into that form — that's its entire job.
So as the orifice goes out of round, the shape of the arc goes with it. You can have your height set perfectly and still get a bevel, or get a bevel that's worse on one side than the other, because the arc coming out of that nozzle isn't the shape it's supposed to be any more.
Which means if your bevel has appeared gradually, over a run, with nothing changed on the machine — check the nozzle before you start moving the height. Here's what a worn orifice looks like.
The short version
| What you see | What it's telling you |
|---|---|
| Square edge | you're in the middle of the arc — leave it alone |
| Wider at the top | torch too high |
| Wider at the bottom | torch too low |
| Bevel that appeared gradually, machine unchanged | check the nozzle before the height |
| Bevel that varies along the cut | height control, or the plate isn't flat |
Cut with the flat part of the football. Everything else is just a consequence of not doing that.
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