Kerf test

How to use it

Cut it with what you will cut afterwards. The same material, the same power, the same speed and the same focus. Kerf does not belong to the machine: it belongs to the beam in that wood at that power, and it changes if any of the four does. A 3 mm MDF and a 3 mm acrylic do not share one.

Try each key in the notch with the same number. The right one goes in with a firm push of the finger, no hammering, and does not drop out when you lift the strip. Smaller numbers go in loose or wobble; bigger numbers will not go in. That number is what goes in “Kerf” in every template.

If you land between two, use the one in the middle. And to fine tune, cut a second, smaller test around the one you got, in steps of 0.01. By default it runs from 0 to 0.2 in steps of 0.025: nine pairs, which cover what a CO2 or a diode laser cuts.

The number is not a matter of taste, it is the width of the cut. In the templates every notch is drawn half a kerf narrower on each side and every tab half a kerf wider. The laser, as it cuts, eats half its beam width on each side of the line. The two cancel out when the number entered equals the real width of the cut, and then tab and notch come out exact. So there is one right answer, and this test finds it with no calipers and no sums.

Both sides are compensated, like real joints. The key comes out one kerf wider and the notch one kerf narrower, just like a tab and its notch on a box. That doubles the sensitivity: from one pair to the next the fit changes by two steps, five hundredths of a millimetre with steps of 0.025, and you can feel it.

The second row checks the thickness. They are closed slots like the ones that take the shelves in the templates: the thickness minus the kerf. Push the key in edgewise into the slot with the number you got above. If it fits just as snug, the thickness you entered is right. If it is loose, your sheet is thinner than that: lower the thickness. If it will not go in, it is thicker: raise it. MDF sold as “3 mm” usually comes in at 2.7.

Some machines cut differently one way than the other. If the beam is not round, the kerf across the bed and along it differ. To find out, cut the test twice, the second time turned ninety degrees. If they differ, use the one in between.