A biscuit router is a very specialized hand-held power tool used in woodworking for a single purpose: cutting small slots in wood for thin wooden oval-shaped pieces of wood called biscuits. It employs a circular blade to cut a crescent-shaped slot, called the "mouth," into the edges of two pieces of wood that are to be joined together. Biscuits are usually made of beech that's been highly compressed and dried, and they fit into the mouths. The term "biscuit router" is rarely used; the tool is more commonly known as a biscuit joiner, plate joiner, or biscuit cutter. A biscuit joint, properly constructed, can be stronger than the original wood.
A biscuit joint is most commonly used when gluing up boards to make a tabletop or counter top, connecting sheet goods like plywood or fiberboard, or connecting planks or boards edge-to-edge to make a larger surface. Biscuit joints can also be used to connect two pieces of stock at right angles to each other. When using a biscuit router, biscuit joints are the fastest and easiest of all woodworking joints to construct.
A biscuit router has no other use in a woodworking shop but cutting biscuits; it cannot be adapted to any other task. No other tool approaches it in its efficiency in cutting biscuit slots, however, because no other tool is so constructed as to ensure that the cut will always be parallel to the work surface and precisely the same distance from that surface. For example, while it's possible to use a regular router to cut a biscuit slot, any variance at all in cutting the matching slot on the adjoining board will result in a very obviously uneven surface. In addition, using a router this way is inherently dangerous because of the exposed router bit; by contrast, the blade in the biscuit router is always inside the joiner housing, except when actually cutting the biscuit slot.
There are basically three sizes of biscuits, ranging from 1.75 inches (4.45 cm) long and 0.625 inches (1.6 cm) wide to 2.375 inches (6.03 cm) long and 1 inch (2.54 cm) wide. They're all approximately 0.071 inches thick (1.85 mm). For the strongest joint, the largest biscuit should be used for which a slot can be cut, but thinner stock will require smaller biscuits. There's no need to change the blade in the biscuit router: adjusting the depth of the cut permits the same blade to cut slots for all sizes of biscuits.
To operate a biscuit router, the two edges to be joined must be perfectly flat so that they will fit together without any gaps. The boards are placed together and marked to indicate the location of the biscuit slots. Each board, in turn, is secured and the biscuit router is butted up against its edge and turned on, forcing the spinning blade from inside the router body into the wood. The spinning blade is plunged into the stock to a depth set by adjusting a control on the tool, and the cut is completed in a matter of seconds.
Once the all the slots are cut on both boards, the edges and slots are glued up, biscuits inserted onto the slots on one board, and then the boards are lined up and clamped together. Once the biscuits come into contact with the glue, the process has to be completed quickly, because the biscuits will absorb the moisture from the glue and expand in the slots, making a very tightly-fitting joint.