Kerf: the invisible line that decides whether your parts fit
Your laser doesn't cut on the line — it cuts the line away. That missing sliver of material is the kerf, and if you ignore it, press-fit joints come out loose and inlays rattle.
What kerf actually is
Kerf is the width of material vaporized by the beam. On a typical CO₂ laser it's 0.1–0.2 mm; on a diode laser it can be 0.15–0.3 mm. It changes with material, thickness, focus, speed, and power — so there is no universal number. You measure it on your machine, with your settings.
Measure it in 2 minutes
- Cut a 20 mm square out of your usual material with your usual settings.
- Measure the cut-out piece with calipers. Say it reads 19.84 mm.
- Kerf = (20 − 19.84) ÷ 2 = 0.08 mm per side.
Write that number on a sticky note on your machine. Re-measure when you change material or thickness.
Compensating in LightBurn
Select the layer, open Cut Settings → Kerf Offset, and enter half your measured kerf (0.04 mm in the example above). Choose outward for parts that must stay full-size (tenons, inlay pieces) and inward for holes and slots that receive them. LightBurn handles the rest.
The slot-fit rule of thumb
For 3D slot-together designs in 3 mm plywood: draw slots at material thickness + kerf for a snug fit that still assembles by hand, or exactly material thickness for a friction fit you'll tap together with a mallet. When in doubt, cut one test slot before committing a full sheet.
What this means for bought DXF files
Good DXF packs (like ours) draw slots at nominal size and document the intended material thickness, so you apply your kerf offset at cut time. Be suspicious of files with pre-widened slots — they were tuned for someone else's machine, not yours.