How to measure your laser’s kerf (and why hinge boxes need it)
A five-minute test cut that gives you your kerf to a hundredth of a millimetre, typical values for diode and CO₂ lasers, and what kerf compensation does to tabs, slots and hinges.
5 min read
Kerf is the width of wood the laser burns away. It is small, between a tenth and a quarter of a millimetre, and it decides whether the tabs of your box press into their slots or rattle in them. Five minutes with a test square gives you the number.
Why a hinge box cares
A finger-joint box can be glued tight whatever the kerf. A living-hinge box is different: the wall is one strip that wraps the base and locks with a few tabs, and the lid drops into a rim with a sliver of clearance. Every edge the laser cuts ends up half a kerf short of where the drawing says. Hingesmith compensates for that in the file: outer edges and tabs are drawn half a kerf larger, slots and holes half a kerf smaller, so what comes off the laser matches the design. If the kerf in the file is wrong, the tabs are loose (kerf set too low) or will not go in (kerf set too high).
The links of the hinge are affected too: a wider kerf leaves narrower bars between the slots, which twist more easily but are also weaker.
The test
- Draw a 20 × 20 mm square on the cut layer in your laser software.
- Cut it in the plywood you will use for the box, with the box's power, speed, focus and air assist.
- Measure the width of the square that fell out with digital calipers, in two places, and average.
- Kerf = 20 − measured width. A square that measures 19.84 mm means a 0.16 mm kerf.
Measure the square, not the hole: calipers close on a piece squarely and repeatably. For a sanity check, cut ten 10 mm strips side by side and measure how much shorter the row is than 100 mm; divided by ten, that is the kerf again.
Typical values
| Laser | Kerf in 3 mm plywood |
|---|---|
| Diode, 5–20 W (xTool D1, Atomstack, Ortur, Creality, Sculpfun) | 0.08–0.15 mm |
| Diode, 40 W class (xTool S1, Creality Falcon2 40W) | 0.10–0.18 mm |
| CO₂, 40–60 W (K40, Omtech, Monport, xTool P2, Glowforge) | 0.15–0.25 mm |
| CO₂, 80 W and up, or 6 mm stock | 0.20–0.30 mm |
The Studio's default is 0.15 mm, which lands inside the range of most diode lasers and many CO₂ machines. If your measured kerf is within 0.03 mm of that, the free export will fit. Further out, enter your value: kerf optimisation is part of Pro, and it is the single setting that makes tabs press-fit without sanding.
When to measure again
- A new lens, a cleaned lens, or a different focus height.
- A different thickness or a different supplier's plywood.
- A change of power or speed, including switching between one and two passes.
- Any time a box that used to fit stops fitting.
Write the number on the sheet with a pencil. Then cut the free bend test from the same sheet, and you know both of the things that make a hinge box fit before you cut one.
Questions
Can I measure the slot instead of the square?
Yes, but the square is easier to measure accurately: calipers close on it squarely. If you measure the hole it left, kerf = hole width − 20.
My kerf is different on the two axes.
Common on diode lasers, whose beam is a rectangle. Average the two. The difference is usually a few hundredths of a millimetre, which the tab fit absorbs.
Does kerf change with thickness?
A little: thicker stock is cut slower or with more power, and the beam widens below the focus point. Measure each thickness you cut, and re-measure after changing the lens or the focus height.
Try it on a real box
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