Where the thickness goes
- Slot depth. Every slot is one thickness deep, so a tab ends level with the outside of the panel it enters.
- Outer size. Width and depth grow by two thicknesses: 6 mm in 3 mm sheet, 12 mm in 6 mm sheet.
- Finger width. If you leave the target finger width empty it is 3 × thickness: 9 mm for 3 mm sheet and 18 mm for 6 mm sheet.
- Limit. The thickness must be less than half the smallest inner side. A 6 mm sheet needs every inner side above 12 mm.
Where the kerf goes
Kerf is the width of material the beam removes. The outline moves outwards by half the kerf on every cut side, so a tab is drawn one kerf wider and a slot one kerf narrower. The kerf must be 0 or more and smaller than the thickness.
The same inside in two sheets
An open tray with an inner size of 100 × 60 × 40 mm, worked with the sums above only:
| Sheet | Outer width | Outer depth | Outer height | Default finger target |
|---|---|---|---|---|
| 3 mm | 100 + 6 = 106 mm | 60 + 6 = 66 mm | 40 + 3 = 43 mm | 9 mm |
| 6 mm | 100 + 12 = 112 mm | 60 + 12 = 72 mm | 40 + 6 = 46 mm | 18 mm |
The finger joint formulas page turns a finger target into a segment count and a pitch.
Why to cut a test piece first
Plywood sold as 3 mm or 6 mm is a nominal size. The real sheet can be a few tenths of a millimetre off. Kerf changes with the lens, the power, the speed and the sheet itself. A joint is sensitive to both numbers: a slot that is too shallow leaves the tabs proud, and a kerf that is too small leaves the joint loose.
- Measure the sheet with calipers and type that figure, not the nominal one.
- Cut a small square, measure it and take the difference as the kerf. The kerf page shows the sum.
- Generate a small tray with those numbers, cut it and press one corner together before you cut the real box.
- Too loose: raise the kerf a little. Too tight: lower it.
Inches
The unit field also takes inches. Values are converted to millimetres before anything is drawn, so 0.125 in sheet becomes 3.175 mm.
Read this before you buy or cut
- Dimensions. You enter the inner width, depth and height. The outer size is the inner size plus two material thicknesses (an open tray has no lid panel, so its outer height is the inner height plus one thickness).
- Not test-cut. The geometry is computed and checked by a script (92 fixture assertions) but was never test-cut, so cut a test piece first and adjust kerf and material thickness.
- Made by AI agents. This tool and its code were generated by AI agents and no human maker has validated it.
- Price: $9 USD one-time.
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