Finger joints with the odd-fingers rule, and two lid styles whose play carries a source.
The finger-jointed box is the laser cutter's first real project — and the finger-box guide gets the details right where most generators hand-wave. The three details worth understanding: how the fingers get counted, where the clearance goes, and where the lid's play comes from.
The app picks the odd n ≥ 3 that brings the edge's width over the finger count closest to your target finger width, and on ties it takes more fingers. The guide's worked example: a 100 mm edge at a 10 mm target gives 11 fingers of 9.09 mm. Why odd: both ends of the edge land on the same panel, which keeps the corners unambiguous — even counts put the two ends in disagreement. The default finger width runs 10 mm, editable; boxes.py's convention for comparison is 2× the thickness.
| rule | value |
|---|---|
| the clearance's application | each finger boundary moves c/4, so a finger sits c narrower than its gap |
| outside vs inside | outside = inside + 2T on any axis with walls at both ends |
| baking the kerf in | every outline grows K/2, holes shrink K/2 |
| dividers | cross-halved: slots T + c wide, half the strip deep |
| the sheet layout | parts 3 mm apart |
A sliding or hinged lid needs play to move, and the app's number carries provenance: your clearance, or 0.05 × T at minimum — boxes.py's own default play for its slide-on lid. The sliding geometry is tested in 3D (the lid slides out clear, holds square, stops, sits down): the walls rise by the slot (T + play), a rail 2T tall, the notch at the left end, the tongue on the lid's far end. The hinged style turns a square pin on a circle — its corners swing at T√2/2, so the pin holes run T√2 + play wide, the pivot sits 1.5 T from the back face, and the app's test turns the lid to 90° without a touch.
And the honesty card every laser builder should frame: none of these lids has been cut on a real laser yet — the geometry is tested in 3D. Cut your first test box before you trust the fit with expensive sheet — the same discipline the test-grid post carries for every setting.
Count the fingers odd, leave the play in, cut the test box first. — the bench crew
