CadPlasma cutting kerf & speed chart for mild steel
Kerf is the width of material the arc removes. Get it wrong and every hole comes out small and every outside edge comes out small by the same amount. The chart below gives typical starting values for air plasma (roughly 45–65 A class machines) on mild steel. Use it to set up a first cut, then dial in your own machine with the test described further down.
| Thickness | Kerf (in) | Kerf (mm) | Cut speed (in/min) | Cut speed (mm/min) | Pierce delay |
|---|---|---|---|---|---|
| 1/16" (1.6 mm) | 0.045 | 1.14 | 300 | 7,620 | 0.1 s |
| 1/8" (3.2 mm) | 0.055 | 1.40 | 200 | 5,080 | 0.2 s |
| 3/16" (4.8 mm) | 0.060 | 1.52 | 130 | 3,300 | 0.3 s |
| 1/4" (6.4 mm) | 0.065 | 1.65 | 90 | 2,290 | 0.4 s |
| 5/16" (7.9 mm) | 0.070 | 1.78 | 70 | 1,780 | 0.5 s |
| 3/8" (9.5 mm) | 0.075 | 1.91 | 55 | 1,400 | 0.6 s |
| 1/2" (12.7 mm) | 0.085 | 2.16 | 38 | 965 | 0.8 s |
| 5/8" (15.9 mm) | 0.095 | 2.41 | 28 | 710 | 1.0 s |
| 3/4" (19.1 mm) | 0.110 | 2.79 | 20 | 510 | 1.2 s |
These are starting points, not gospel. Kerf changes with amperage, consumable size and wear, torch height, gas pressure and speed. Your plasma manufacturer’s cut chart for your exact consumables always wins.
How to measure your own kerf in five minutes
- Draw a 2.000" square part with a 1.000" round hole in the middle.
- Cut it with kerf compensation turned off in your CAM (so the torch follows the drawn line).
- Measure the outside with calipers. Kerf ≈ 2.000 − measured size. (Each edge loses half a kerf.)
- Check with the hole: kerf ≈ measured hole − 1.000.
- Enter that number as the kerf in FastCAM / your CAM (or in CadWercs, if you let it compensate the DXF).
Minimum hole size for plasma
A common rule for air plasma: don’t cut holes smaller than the material thickness, and aim for 1.5× thickness if the hole matters. Smaller holes come out tapered, with dross, and often out of round, because the pierce and lead-in take up most of the hole. For 1/4" plate that means 1/4" minimum, 3/8"+ preferred. For anything tighter, cut an undersized pilot or a centre mark and drill it.
Other things that ruin accuracy
- Islands: the middle of an O, A or D falls out unless it’s bridged to the rest of the part.
- Cut order: cut holes and inside features before the outline, or the part can shift once it’s free.
- Cut direction: with a standard swirl, cut outlines clockwise and holes counter-clockwise so the square side of the kerf is on the part.
- Tiny line segments: hundreds of little segments make the torch stutter. True arcs cut smoother.
CadWercs does all of this for you. Pick a plate thickness and it fills in kerf, speed and pierce from this chart, flags holes that are too small, bridges islands, cuts inside-first in the right direction, and exports true arcs. Try it free for 7 days →