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Kerf, explained

· Fox Bright

The laser removes a sliver of material on every cut. Here is how to measure it on your machine and design joints that account for it.

Two pieces of laser-cut plywood with finger joints slotted together, showing small gaps at the joint.

Every laser cut removes material. The beam has a width, the material chars back from it, and the result is a slot that is a little wider than the line you drew. That missing width is the kerf. On our 3 mm plywood it is somewhere between 0.15 and 0.25 mm, which sounds like nothing until you try to slot two pieces together and they rattle.

If your finger joints are loose, this is almost always why.

Where the material goes

The laser vaporises a channel through the sheet. The channel is widest at the top surface, where the beam is focused, and tapers slightly toward the bottom. Two things follow from this:

  • A part comes out slightly smaller than drawn, by half a kerf on every edge.
  • A hole or slot comes out slightly larger than drawn, by half a kerf on every edge.

So a 10 mm tab going into a 10 mm slot is really a 9.8 mm tab in a 10.2 mm slot. That 0.4 mm gap is the rattle.

Try it

Drag the kerf slider and watch what happens to a finger joint drawn at exact size. Then turn compensation on to see the fix.

Measure it on your machine

Kerf changes with material, thickness, power, and speed, so measure it rather than trusting a number from the internet.

  1. Draw a rectangle 100 mm wide and cut ten parallel lines through it, 10 mm apart, so it falls into ten strips.
  2. Push the strips back together tightly and measure the total width with calipers.
  3. Subtract from 100 mm and divide by the number of cuts. If the strips measure 98.0 mm across, the kerf is 2.0 mm ÷ 10 = 0.20 mm.
Ten laser-cut plywood strips pushed together beside digital calipers
A kerf test: ten strips pushed back together and measured. The missing width divided by the number of cuts is the kerf.

Write the number on the machine. You will use it every time you design a joint.

Designing for it in Fusion

Do not shrink the slots by hand. Put the kerf in a user parameter and let every joint reference it:

kerf = 0.20 mm
tabWidth = 10 mm
slotWidth = tabWidth - kerf

Then sketch slots at slotWidth. When you move to a different sheet, change one number and the whole model updates. If you want a press fit, subtract a little more; a friction fit on plywood usually wants a slot about 0.1 mm narrower than the tab after kerf.