The mortise-and-tenon joint has been used by woodworkers for centuries because of its combination of superior strength, simplicity, and the elegance of its appearance. It is very commonly used to join rails to the legs of tables or chairs, or fixed shelves to the side walls of cabinets or bookcases. The essence of a mortise-and-tenon is that a peg or pin (the tenon) cut into one piece of wood fits tightly into a slot or hole (the mortise) cut into the adjoining piece of wood. The resulting joint looks like a butt joint but requires no screws or nails, and has remarkable strength and durability. Today, most woodworkers use glue to secure the tenon inside the mortise, but in years gone by, woodworkers typically fashioned the tenons so that they protruded through the mortised stock and were secured by a wedge or dowel. Modern woodworkers seeking this vintage look sometimes duplicate this technique.
Mortise-and-tenon joints are typically used when one piece of stock is joined to the other at a 90-degree angle, but they may be used at a slightly lesser angle in certain circumstances. Bear in mind that the joint is strongest when the two pieces of stock are at right angles to one another.
Mortise-and-tenon joints depend on great precision when outlining and cutting the tenon on one piece of the stock and the mortise on the adjoining piece. Careful measurement and precise marking and cutting are crucial. Even the smallest of errors can spoil the joint or mar the symmetry of the project.
Forming the Tenon
Typically, the tenon is little more than a rectangular pin cut from the end of the stock. While tenons can be cut by hand, modern woodworkers will often employ a band saw or a tenoning jig on a table saw to safely form the tenon. When cutting a tenon, be careful not to remove too much material as a thinner tenon means a weaker joint. Care must be taken to cut the tenon precisely, with smooth, flat sides, since this joint depends on a tight fit for its strength.
Cutting the Mortise
Traditionally, mortises were cut into the receiving piece of stock using a chisel and mallet. Today, many woodworkers use a dedicated mortiser, which employs a drill bit encased inside a four-sided chisel. Many drill press manufacturers offer optional mortising attachments, making the drill press a much more versatile machine. There are also mortising attachments that can be used with a plunge router.
To cut a mortise using a mortiser, simply mark the position of the area to be cut, and then sink the bit into the material, taking small bites at a time. Set the depth stop to drill deep enough to encompass the entire length of the tenon, but no deeper than absolutely necessary (unless you are creating a through-tenon). When finished, use a sharp chisel to clean up any rough spots that remain on the walls of the mortise.
Once the mortise and tenon have both been completed, dry-fit the tenon into the mortise. The fit should be snug but not excessively tight. Once all joints have been formed and its time for assembly, apply glue to both the tenon and inside walls of the mortise. Coat all surfaces evenly with glue using a small brush. Assemble the pieces, tapping them together with a wooden mallet, if necessary. Let the glue dry completely before continuing assembly. Excess glue seeping out of the joint is best left to dry, then scraped off with a sharp chisel.
A good rule of thumb when creating mortise and tenon joints is to cut the mortise first, then the tenon. Leave the tenon a little bit fat for the first test fitting. It's always better to shave down a tenon that is too large than to cut it too narrow and find that you have a sloppy fit.