What Is Kerf in Plasma Cutting?
Kerf is the width of the cut made by the plasma arc: the strip of metal that is melted and blown out as the torch passes. Every plasma-cut part is affected by it, because the torch removes metal on both sides of the line it follows. If the machine is not told about the kerf, outside shapes come out smaller and holes come out larger than the drawing.
How the kerf is formed
A plasma torch drives a jet of ionized gas through a small nozzle. An electric arc heats the gas to a very high temperature, the jet melts a narrow path through the plate, and the gas flow blows the molten metal out of the bottom of the cut. The width of that path is the kerf.
What sets the kerf width
Current and nozzle size
Higher current needs a larger nozzle orifice, which gives a wider arc and a wider kerf.
Cutting speed
Too slow widens the kerf at the top and leaves heavy dross; too fast narrows it but lags the arc and leaves dross underneath.
Torch height
A higher stand-off spreads the arc and widens the kerf. Automatic torch height control keeps it constant on warped plate.
Plate thickness and material
Thicker plate needs more current and gives a wider kerf. Stainless steel and aluminum behave differently from mild steel.
Consumable wear
A worn nozzle orifice is no longer round, so the kerf widens and becomes uneven.
Plasma system
High-definition plasma squeezes the arc harder and gives a narrower, squarer kerf than conventional air plasma.
Typical kerf widths
Kerf is given in the cut charts that come with every plasma source. As a guide for mild steel:
Swipe the table sideways to see all columns.
| Plasma current | Typical plate | Approximate kerf |
|---|---|---|
| 30–45 A | Sheet up to about 6 mm | 1.0–1.5 mm |
| 65–85 A | 6–16 mm | 1.5–2.0 mm |
| 105–130 A | 12–25 mm | 2.0–2.5 mm |
| 200 A | 20–40 mm | 2.5–3.5 mm |
| 300–400 A | 40 mm and thicker | 3.5–5.0 mm |
Approximate values. Use the cut chart of your plasma source; it lists kerf for each current, gas and thickness. A fiber laser, by comparison, has a kerf of only a few tenths of a millimetre.
Kerf compensation
CNC plasma controls and nesting software offset the torch path by half the kerf width so that the finished part matches the drawing. Outside contours are offset outward and holes inward. In the program this is the left or right compensation (G41 / G42) or an automatic offset set in the nesting software.
- Cut a 100 × 100 mm test square and a Ø50 mm hole at your normal settings
- Measure both with calipers
- If the square is 98.4 mm, the kerf is about 1.6 mm (100 − 98.4); enter that value in the kerf table
- Re-check whenever you change current, gas, nozzle or plate thickness
Design tips
- Leave at least one kerf width plus about 1 mm between parts in the nest
- Plan holes no smaller than the plate thickness on standard plasma; HD plasma can cut somewhat smaller
- Add lead-ins and lead-outs outside the part so the start and end marks do not land on the finished edge
- For tight-fit slots, cut a test piece first: plasma kerf is slightly wider at the top than at the bottom
Choosing a table and a plasma source is covered in how to choose a CNC plasma cutting table and the plasma cutting thickness chart.
Ask an engineer
Send a drawing and your plate thickness, and we will tell you which plasma source and settings give the kerf and accuracy you need.