Every laser removes material. The width of that cut is the kerf, and if you ignore it your parts come out slightly wrong in a very specific way: holes too large, tabs too loose, and joints that need glue instead of friction.
Measure kerf in three cuts
- Draw a rectangle exactly 20 mm wide and cut it out.
- Measure the resulting part with calipers. It will be smaller than 20 mm.
- Kerf = 20 mm − measured width.
Repeat at the material and power setting you actually use. In 3 mm plywood a 60 W CO₂ laser typically lands between 0.10 and 0.20 mm, but the number changes with focus, speed, lens and material batch. Measure, do not assume.
Compensating holes and slots
Cutters remove half the kerf on each side of the path. That means a drawn hole of Ø 5 mm becomes Ø 5 mm + kerf, and a drawn tab becomes its nominal size minus kerf.
For a finger joint in 3 mm material with 0.15 mm kerf, the slot that receives the tab should be drawn at roughly 2.85 mm for a press fit. Add clearance for a looser joint:
- Press fit — no clearance. Permanent, needs a mallet.
- Snug fit — around 0.1 mm. Holds by friction, disassembles.
- Slip fit — 0.2 mm or more. Easy assembly, needs glue.
Do the maths once, reuse it
Once you know your kerf and your preferred clearance, keep them in a note next to the machine. Our free kerf and finger joint calculator returns the slot width, compensation value and a sensible tab count for the edge you are joining — so you are not re-deriving it for every project.
Common mistakes
- Measuring kerf on a different material than the one you are cutting.
- Applying kerf compensation twice — once in CAD and again in the laser software.
- Assuming all machines in the workshop have the same kerf.
Measure, compensate in one place only, and document the value per material. That single habit removes most assembly problems.