Kerf is the width of material the beam burns away. It is small, typically 0.1 to 0.2 mm on diode and CO2 lasers in 3 mm wood, but it is removed from both sides of every joint. A tab comes out narrower and its slot wider than drawn, and a box that looked perfect on screen rattles when you assemble it.
What compensation does
Box Maker takes the full kerf in its Kerf field and widens every tab by half of it on each side, narrowing every gap to match, so the parts meet at the drawn line after the beam has taken its share. The polygon box uses the same rule for its tabs and notches. The field accepts 0 to 0.5 mm.
Measure it instead of guessing
The free kerf test card cuts five small blocks, each with a slot cut down from the top edge. Each slot is the measured thickness minus a kerf value, and the five values step 0.025 mm apart, spanning 0.05 mm either side of an estimate. The estimate grows with the laser's spot size (a fiber spot is smallest, then diode, then CO2), with thickness and with material density, so 6 mm stock starts wider than 3 mm. Push a scrap of the same sheet into each slot: the number under the snug one is your kerf.
Thickness matters as much as kerf
Every slot is drawn as thickness minus kerf, so a thickness typed from the label moves every slot by the same error. Nominal 3 mm plywood often measures 2.7 to 3.2 mm. Measure with calipers in two or three places across the sheet and use the average, in Box Maker and on the test card.
Cut the card like the job
Kerf depends on focus, power, speed and the number of passes. Cut the card with exactly the settings you will use for the box, on the same sheet. A card cut faster or at a different focus measures a different kerf.
Do not compensate twice
LightBurn's cut settings include a kerf offset that moves cut paths by itself. If the design already carries compensation from Box Maker, leave that offset at zero for the layer, or the joints will be compensated twice and come out tight. Pick one place to apply kerf and stick with it.
When the fit is still wrong
If the joints are loose, raise the kerf a little; if they will not close, lower it. On thick stock the beam cuts a slightly tapered edge, so the kerf at the top and bottom of the cut differ and any single value is a compromise: dry-fit one corner before cutting the whole sheet.
Frequently asked questions: Fabrication
What kerf value should I type into Box Maker?
The number under the slot on the kerf test card that your scrap fits snugly. That is the full kerf: Box Maker splits it, widening each tab by half the kerf on each side and narrowing each gap to match. Typical values on diode and CO2 lasers in 3 mm wood are 0.1 to 0.2 mm.
Why are my finger joints still loose after compensation?
Usually one of three things: the thickness was typed from the label instead of measured, the test card was cut at different settings from the box, or the kerf was applied twice, once in the design and again with a kerf offset in the laser software. Fix the thickness first; it shifts every slot by the same error.
Does a living hinge need kerf compensation?
No. A living hinge has no joint to fit, so the living hinge generator asks for the material thickness instead and flags line spacing under half of it, where the strips are likely to char or snap. Test the bend on a strip from the same sheet.