Laser kerf: how much it takes away and how to compensate
A laser does not cut a line: it burns a groove. That groove has a width, called the kerf, usually between one and five tenths of a millimetre. Half of it is taken from one side of the cut and half from the other, so every part comes out smaller than drawn and every hole comes out larger.
On a single part you will not notice. On a joint you will: two tenths less on each side is enough to make a slot that looked perfect on screen wobble in your hands.
How to measure it, once
Half an hour and a scrap of material.
1. Draw a 50 mm square and cut it on your machine, with the power and speed you normally use, on the material you normally use. 2. Measure the cut piece with a caliper, on both sides. 3. The difference is the kerf: if the square measures 49.8 mm, you lost 0.2 mm in total, that is 0.1 mm per side. The kerf is 0.2 mm.
Do it again when you change material or thickness: the kerf is not a property of the machine, it is a property of machine + material + settings together.
Typical values, as a starting point only:
| Material | Thickness | Indicative kerf |
|---|---|---|
| Poplar plywood | 3 mm | 0.15 – 0.25 mm |
| Birch plywood | 4 mm | 0.20 – 0.30 mm |
| MDF | 3 mm | 0.15 – 0.20 mm |
| Acrylic | 3 mm | 0.10 – 0.20 mm |
| Greyboard | 2 mm | 0.10 – 0.15 mm |
How to set it
The “Material” panel has a Laser kerf field. Type the measured value and the program widens the outlines by half the kerf per side before exporting: the cut part comes out at the size you drew.
Nothing to correct by hand, nothing to redraw: the compensation happens only at export time, and the model stays clean.
Kerf and clearance are not the same thing
They get confused often, and the result is a joint that will not go together.
- The kerf corrects a machine error: without it, the part comes out smaller than drawn.
- The clearance is a design choice: it deliberately widens the slots, because two parts at the exact size never go together, not even with a perfect cut.
You need both, and they add up. With the right kerf and zero clearance, the joint stays tight.
Try it now
Open the app, put your kerf in the “Material” panel and look at the cutting layout preview: the outlines shift by that half tenth that makes the difference between a joint that holds and one that rattles.
Plyjoin works out the joints for you and gives you a DXF ready for the machine. It runs in your browser, with nothing to install.
Try it now