A kerf is the width of material removed when you cut through metal. If you’ve ever noticed that a saw blade doesn’t just split wood apart but actually chews away a thin strip of it, that’s exactly what happens during thermal cutting processes used in welding preparation. The kerf is that narrow gap left behind after the cutting tool passes through.
While the term comes up most often in woodworking, it’s just as important in metal fabrication, where plasma torches, lasers, and oxy-fuel torches all remove a measurable strip of material as they cut. Understanding kerf matters because that missing material affects the final dimensions of every part you cut.
How a Kerf Forms in Metal Cutting
In thermal cutting, the kerf forms because the cutting tool doesn’t slice through metal the way a knife goes through paper. Instead, it melts, vaporizes, and blows away a thin band of material. A plasma cutter, for example, sends a superheated jet of ionized gas into the workpiece. That jet melts the metal directly in its path while high-pressure gas blows the molten material out of the joint. As the torch moves across the plate, the kerf gradually appears as a continuous slot.
Laser cutting works on the same principle but with a focused beam of light doing the heating. Oxy-fuel cutting uses a chemical reaction between pure oxygen and heated steel to burn through the metal. In every case, material is physically removed from the plate, not just pushed aside. That removal is what creates the kerf.
Typical Kerf Widths by Cutting Method
Different cutting tools produce very different kerf widths, and knowing the typical range for your process helps you plan cuts accurately.
- Laser cutting produces the narrowest kerf, typically between 0.1 mm and 0.5 mm depending on the material and laser power. Fiber lasers on thin sheet metal can cut even finer than that.
- Plasma cutting leaves a wider kerf, generally ranging from about 1.5 mm to 5 mm or more. Higher-amperage torches cutting thicker plate tend to produce wider kerfs.
- Oxy-fuel cutting creates the widest kerf of the three thermal methods, often 2 mm to 4 mm on standard plate thicknesses, and wider on very thick material.
Mechanical cutting methods like sawing and waterjet cutting also produce a kerf. A bandsaw blade, for instance, has a set width that determines how much metal it removes. Waterjet kerf falls somewhere between laser and plasma, typically around 0.5 mm to 1.5 mm.
What Affects Kerf Width
The kerf you get isn’t fixed for any given cutting method. Several variables change how wide the cut ends up being.
For laser cutting, the focal length of the lens and the pressure of the assist gas (compressed air or nitrogen blown into the cut zone) both influence how much material gets removed. Kerf widths can even vary on the same sheet of material depending on whether the laser is cutting a straight line or a curve, and whether it’s moving in the X or Y direction of the cutting table. The manufacturing tolerance of the material itself plays a role too, since slight variations in thickness or composition change how the metal responds to the heat source.
For plasma cutting, the main factors are amperage, cutting speed, torch height, and nozzle condition. A worn nozzle produces a wider, less focused arc, which increases the kerf. Cutting too slowly allows more material to melt away on either side of the cut. Cutting too fast can cause the arc to lag behind the torch, angling the kerf and creating a bevel on the cut edge.
Material type matters across all methods. Metals with higher thermal conductivity, like aluminum and copper, spread heat away from the cut zone more quickly. This can change both the kerf width and the quality of the cut edge. Aluminum’s oxide layer also complicates things because the oxide has much lower thermal conductivity (around 30 W/mK) than the base metal, creating uneven heat transfer during cutting.
Why Kerf Matters for Fabrication Accuracy
If you’re cutting a 100 mm wide part from a steel plate using a plasma cutter with a 3 mm kerf, and you cut right on your layout lines, the finished part will be roughly 1.5 mm too narrow. That’s because the kerf eats into the part from each side of the cut. On a single cut this might seem minor, but when you’re nesting multiple parts on one sheet, those millimeters add up fast. Ignoring kerf can mean parts that don’t fit together during welding, gaps that are too wide for a clean weld joint, or assemblies that end up out of tolerance.
This is especially critical when preparing parts for precise weld joints. If your cut parts are undersized because you didn’t account for kerf, your root gaps will be wider than planned. That changes how much filler metal you need, affects penetration, and can compromise the strength of the finished weld.
Kerf Compensation in CNC Cutting
CNC cutting machines handle the kerf problem through a setting called kerf compensation (sometimes called cutter compensation). The software offsets the cutting path by half the kerf width so the finished part comes out at the correct dimension.
For external cuts, where you’re cutting around the outside of a part, the machine shifts the tool path outward so the kerf falls in the scrap material. For internal cuts, like holes or slots, the path shifts inward for the same reason. Most CAM software includes default kerf compensation values, commonly 0.05 mm, 0.10 mm, and 0.15 mm for laser, with the option to enter custom values for wider-kerf processes like plasma.
Getting this setting right requires you to know your actual kerf width, not just the theoretical one. Experienced fabricators will often make test cuts on the same material and thickness they plan to use, measure the kerf with calipers, and enter that value into their software. The kerf can drift over time as consumables wear, so periodic checks keep parts accurate.
Kerf and Weld Joint Preparation
In welding work, the kerf plays a direct role in joint preparation. Many weld joints require beveled edges, and thermal cutting is one of the most common ways to create those bevels. The kerf characteristics of your cutting process determine the surface quality of the bevel face, the angle accuracy, and whether the joint fits up cleanly for welding.
Plasma-cut edges, for instance, tend to have a slight bevel built in because of how the arc behaves. One side of the kerf is typically squarer than the other. Skilled operators position the “good side” on the part and let the beveled side fall on the scrap. Laser-cut edges are generally squarer and smoother, requiring less cleanup before welding. Oxy-fuel cuts on thick plate can leave a rougher surface with an oxide layer that needs to be ground off before welding to avoid contamination in the joint.
For tight-tolerance work, some shops will cut parts slightly oversized and then machine the edges to final dimension. But for the vast majority of structural and general fabrication welding, understanding your kerf width and compensating for it during cutting is enough to produce parts that fit together properly and weld cleanly.