Biscuit joint
Also called: biscuit joinery, plate joint, biscuit
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A biscuit joint holds two mating pieces of wood together with a compressed-wood plate, oval in shape and called a biscuit, glued into slots cut into both pieces where they meet.
How the joint is cut
A biscuit joiner, also called a plate joiner, is a dedicated power tool built for this one job. It cuts each slot with a small-diameter circular saw blade, known as a slotting blade, plunged briefly into the wood to leave a crescent-shaped kerf, the width of cut the blade leaves behind. It can cut slots in either the faces or the edges of a workpiece. Without a dedicated tool, a router fitted with a slot cutter of 5/32 inch width can cut edge-to-edge biscuit joints instead. Because it needs only this one tool to cut both slots, the joint goes together fast.
Sizing, marking and gluing
Standard plate joiners cut kerfs for three basic biscuit sizes; anything outside those, a different shape or an odd size, needs either a specific make of joiner or a router-cut slot instead. Biscuits are themselves sold in a range of thicknesses, chosen to match the thickness of the cutter being used. To lay the joint out, biscuit positions are marked on the joint line with a soft pencil while the parts are held together in their assembled position, and a test cut shows how far to move the work along so the slot lands at the right point for the length of biscuit in use. Once glued in, a compressed biscuit swells on contact with the glue, helping to lock it into its slot. Beyond gluing, a biscuit can also be used simply to register, that is align, two workpieces at their ends, in place of dowels or splines, thin strips let into matching grooves. In one glue-up of a workbench top, biscuits placed near both edges of a panel section helped keep it flat while the glue set.
Where the joint suits the work
Biscuit joints suit frame construction well, because their slots fall within long-grain material, the wood's surface running along the fibres, rather than the weaker end grain. They also suit quick work on shop and kitchen cabinets rather than the finest furniture work. Portable sanders can level a finished biscuit joint and smooth the surface afterward.