Kerf, slots and the fit that isn't luck.

What the beam takes

Laser cutting doesn't draw a line of zero width — the beam burns away material on both sides of every cut path. Each edge of a cut loses K/2, where K is the kerf. That's the whole theory of fit.

Measuring kerf: ten squares

The app's method is a ruler, not a manual: cut N squares of side S, push them end to end in a line, and measure the true total length M. Then:

K = S − M / N

The default: 10 squares of 20 mm — 200 mm drawn. Measured 198.4 mm together → K = 20 − 19.84 = 0.16 mm. Run this same math in the tools page.

The app's own test checks exactly this: its worked example gives 0.16 mm from a 198.4 mm measure.

Using it

With the offset off, work the kerf into your drawings yourself: slots come out drawn as T + c − K; tabs as the design width + K. With an offset entered — LightBurn's kerf offset — you give it K/2, and paths move outward on outer shapes and inward on holes, which is exactly what the physical cut removes.

The fit gauge

"What clearance should I design?" — stop asking, print it: the app lays out 10 slots from T − 0.25 to T + 0.20 mm in 0.05 mm steps, cut with the offset off, teeth 4 mm on a score layer, digits in seven-segment strokes. Press each slot until one fits. The gauge's teeth were cut with the same beam, so the fit tells you the real clearance directly:

c = the slot that fit + K − T

There is NO default clearance in the app — none invented, none shipped. Zero is a press fit, and the gauge measures everything else.

"3 mm" is a mood, not a size

Sheet sold as 3 mm varies by a few tenths of a millimetre — makers note 3 ± 0.2 mm tolerance on their plywood, and "3 mm" Baltic birch in the US is often sold at 1/8" (3.18 mm). The gauge's own demo says the same thing: caliper the sheet, then cut the gauge for that sheet.