What does kerf mean in fabrication?

The term is older than laser cutting: woodworkers use it for the slot a saw blade leaves behind. In a profile cutting shop it means the narrow channel of metal the laser beam, plasma arc or waterjet removes along every cut.

Because that channel has width, a cutting head that followed the drawn line exactly would remove half the kerf from each side of the line. Outside edges would come out slightly small and holes slightly big. Cut a 100 mm (about 4 in) square with a 0.2 mm kerf and no allowance, and the part measures 99.8 mm.

Kerf width depends on the process. As a rough guide:

• Fibre laser: about 0.1 to 0.5 mm (0.004 to 0.02 in), at the narrow end on thin sheet
• Abrasive waterjet: about 0.6 to 1 mm (0.025 to 0.04 in)
• Plasma: about 1.5 to 3.5 mm (0.06 to 0.14 in), depending on amperage and torch

On any one machine, kerf also changes with material thickness, nozzle size, focus position and assist gas, and it widens as the nozzle wears.

How is kerf used?

Most of the time kerf is handled in the background. The cutting parameters for each material and thickness include a kerf value, and the CAM software uses it to offset the toolpath. People notice kerf when parts stop measuring right.

Day to day, kerf comes up when:

• a new material or thickness is set up and the kerf value is measured from a test cut
• first-off parts are checked against the drawing and the kerf setting is nudged by a few hundredths of a millimetre
• parts drift out of size partway through a run, often because the nozzle has worn or the focus has shifted
• tab-and-slot parts fit too tight or rattle
• parts are nested with common-line cutting, where the spacing has to match the kerf

Why does kerf matter?

Kerf decides whether parts come out at the size on the drawing. If the compensation is wrong, every part in the batch is off by the same amount, and a tenth of a millimetre is enough to stop press-fit holes, tab-and-slot assemblies and bolt patterns lining up with their mating parts.

It also limits what can be cut. A slot can't be narrower than the kerf, and fine details close to kerf width tend to burn away. That's one reason every thickness has a minimum hole size.

The most common kerf mistake starts on the drawing. A customer or drafter offsets the geometry to "allow for the laser", then the shop's software compensates again. Outside edges come out oversize and holes come out undersize. Ask for files drawn at finished size and let the machine handle the rest.

Kerf compensation and common-line cutting

Kerf compensation (also called kerf offset or beam compensation) is the offset applied to the toolpath. The cut is moved half a kerf width away from the finished part: outward on outside profiles, inward on holes. The kerf value sits in the cutting parameters, so a shop running mild steel, stainless and aluminium in several thicknesses keeps a separate value for each. Depending on the setup, the offset is applied by the CAM software or by the machine's CNC controller.

Common-line cutting is where kerf needs extra care. Two parts are nested side by side so one cut forms the edge of both, which saves cut time and material. Only one kerf is removed between them, so the spacing between the parts has to equal the kerf. If the setting is off, both parts come out slightly wrong along the shared edge, so shops usually measure the first parts before running the rest of the sheet.

Kerf and quoting

Kerf rarely appears as a line on a quote. The metal it removes is tiny next to the gap left between parts and the skeleton left after cutting, so material cost comes down to nesting. Cut time comes down to cut length and pierce count.

Kerf affects the price through the work around it:

• Parts with close-fitting holes or slots may need test cuts and first-off checks before the run. That's setup time, and it belongs in the quote.
• Plasma's wider kerf and edge taper make holes less accurate. A part that needs precise holes may need a laser, or plasma followed by drilling, and each option prices differently.
• A drawing with dimensions just over round numbers, such as a 100.2 mm square, may already have been offset for kerf. Check with the customer before quoting so the parts aren't cut oversize.

Tempus Tools quotes from the geometry in the customer's file and treats it as the finished size, which is how most shops and CAM systems expect files to arrive. Kerf compensation stays with the cutting parameters on the machine side.