Finger joint
Also called: finger joint, comb joint, box joint, box corner joint
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A finger joint, also called a box joint or comb joint, joins two boards at a corner by cutting a row of straight, interlocking rectangular fingers into the end of each and gluing them together. Its strength comes from the many tight glue lines running down the side grain, the face of the wood that runs along the length of the fibres rather than across their ends.
How hard it is to cut
Accounts of the joint's difficulty differ. Frid (1979) states that no special skill is needed to lay out or cut a finger joint, while Wearing (2010) judges that hand-cutting a comb joint is about as hard as cutting a common dovetail, which looks better once finished. The joint mainly turns up in industrial furniture manufacture. A nonstructural finger joint has fingers much shorter than the two structural kinds; a scarf joint or a finger joint is used to approximate the tensile strength of clear, defect-free solid wood.
Cutting the joint
A shopmade template lets the joint be routed in whatever size or pattern is wanted, and an angled finger joint is started by crosscutting the board ends to the chosen angle on a table saw. Machining box joints on a table saw carries the same risk of tear-out, splintering pulled up from the wood rather than cut cleanly, as a router-cut edge lap, a lap joint whose notch runs across the grain; because a box-joint notch runs with the grain rather than across it, though, any tear-out is confined to the top edge of the notch rather than both sides. Jigs vary in how they handle spacing: a standard box-joint jig has its pin size fixed to a single dado-blade width, which limits it to one spacing and makes it finicky to set up, while an adjustable jig can be reused for any spacing and avoids the fiddly work of getting the spacing exact. One such jig design is described as generic, with nothing specialised about it.
Other uses
Turning a box joint into a working hinge takes little effort; on card tables and other occasional tables, swing legs rely on finger joints and knuckle joints, a related hinge-like joint, to pivot. Finger or wedge joints can also blend veneer grain almost seamlessly where two pieces of veneer meet end to end. A mock finger joint looks like a real one from the outside but has no alternating fingers locking the pieces together.
Japanese variants
In Japanese joinery, hako mechigai tsugi is a box joint in which the grain direction differs between the parts being joined, and inro tsugi is a small box joint built up from three or five stacked pieces.