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A mortise bit is a specialized drill bit used with a mortising machine or a drill press equipped with a mortising attachment. Its purpose is to cut corners into mortises, the slots cut into wood stock to form one-half of a mortise and tenon joint, one of the mainstays of woodworking joinery. A mortise bit allows these slots to be cut cleanly and evenly.
Most good woodworking does not use nails or screws to join pieces of wood together; instead, joints are made, whereby two pieces of wood are glued together. One of the strongest woodworking joints possible is the mortise and tenon joint. In this joint, a slot, the mortise, is cut into one piece of wood and a tongue, the tenon, is cut into end of the second, often with another specialized tool called a tenon saw. Once both are cut, the tenon fits securely into the mortise. The two pieces are then glued and clamped until the glue sets.
Much of a woodworker's work involves shaping wood and making joints. To enhance the accuracy of this work and reduce the time involved, most furniture shops and production-oriented woodworkers will use power tools for as much of their work as possible, employing a wide variety of special drill and router bits, saw blades, and "jigs" — custom-made accessories designed to help craft particular joints and shapes. A mortise and tenon cutter, for example, is a special jig which employs a router to cut both a mortise and a perfectly matching tenon without changing the settings.
The problem with a mortise-and-tenon joint is that it's faster and easier to make a tenon with square corners, but it's also faster and easier to drill a round mortise. Shaping a tenon to fit securely into a round mortise is more time-consuming than squaring off the corners of a mortise, which can be done with a hammer and chisel. There are two problems associated with using a chisel, however: it's time-consuming, and the work can be ruined with a single misplaced stroke.
A mortise bit, on the other hand, combines the drilling out of the wood stock with chisels that square off the corners of a mortise. The circular bit itself is enclosed in a square steel tube, called a mortising chisel, the ends of which are cut to chisel sharpness. When activated, the drill bit spins, but the mortising chisel remains stationary.
The bulk of the stock is removed from the mortise when the spinning drill bit is plunged into the wood, and only a small portion of stock remains to be carved off by the mortising chisel. This chisel slices into the wood, creating a sharp, perfectly-defined corner. Mortise bits are generally small in diameter, because they're employed primarily to cut the mortise's corners; the bulk of a large mortise can be created with a regular bit in a router or drill press, using the mortise bit just to shape the corners.
A good mortise bit, with carbide-tipped blades, can be relatively costly. It should last for a long time, however, when properly used and maintained. It can produce perfect mortise cuts in a fraction of the time required when using a hammer and chisel.