Box Joints: Square Fingers, Spacing and Fit

Every finger has two long-grain cheeks that glue to the cheeks of its neighbours, so a box joint has a great deal of glue area for its size. What it does not have is a mechanical lock: the fingers are square, so the boards can be pulled apart the way they went together, and the joint relies on glue where a dovetail relies on the flare of its tails. In return it is quicker to cut, because every finger and slot is the same width and one jig setting repeats them all.

• Two meanings of 'finger joint'. Woodworkers use it for the corner joint on this page. The Wood Handbook uses it for a different joint — fingers cut into the ends of two boards to join them end to end, which it calls scarf joints folded over to reduce waste. Search results mix the two.
• Box joint or dovetail? Both join boards at a corner. The box joint is square, regular and glue-dependent; the dovetail is flared, locks one way and can be laid out with uneven spacing. See choosing a wood joint.

A box joint jig is a fence on the miter gauge with a key — a strip the width of the slot — fixed a set distance from the cutter. Freud's instructions set that distance at one pin width: the first board is butted against the key and cut, then each new slot is dropped over the key for the next cut, so the spacing repeats exactly. Because fingers and slots are one width, the number of widths across the board decides how the edges end: an odd number gives a full finger at both edges of one board and a full slot at both edges of the other. Plan the board's width as a multiple of the finger width before cutting.

• Height: Freud sets the cutter 1/64 in higher than the stock is thick, so the fingers stand slightly proud and are sanded flush after glue-up.
• Fit: the two boards should go together easily but with slight friction. Freud says a loose fit with visible gaps means the space between cutter and key is too narrow; an over-tight joint means it is too wide. Adjust and recut test pieces in stock of the finished size.
• Glue: Freud spreads glue on the inside faces of the pins of one board, assembles and clamps — see clamps.
• Safety: Freud's warnings are to use both blades, never work freehand, check the cutter clears the throat plate before power is connected, disconnect power before each re-clamp, keep hands clear and read the saw's own manual first. Saw compatibility and dado setup are in dado blades; general rules in table saw safety.