A mortise and tenon jig is an apparatus used to guide a cutting tool when making a mortise and tenon joint. When cutting such a joint, the mortise and tenon jig is used to steady the tool being used, and to guide the tool so that a straight cut can be made in accordance with the cuts necessary to complete the joint. The type of mortise and tenon jig used depends on the cutting tool being used, but many modern versions of the mortise and tenon jig focus on power tools, such as the router.
The mortise and tenon joint has been used for centuries to secure pieces of wood or other materials being joined at a ninety degree angle or close to it. Many variations of the joint exist, but the basic concept is the same: one piece of wood is cut to leave a square or rectangle hole. This part of the joint is called the mortise. The second piece of wood, called the tenon, is cut to protrude into the hole. In order to make a strong mortise and tenon joint, these cuts must be made carefully so the pieces fit together relatively snugly.
To make a cut that will fit snugly, a mortise and tenon jig is used in conjunction with a router. A router is a power tool that has a spinning bit on the bottom of it. The router sits flat against the mortise and tenon jig, and the bit protrudes through the jig. The piece of wood to be cut is then secured beneath the surface of the jig. Once the wood is in place, the router can be moved to make the cut. The mortise and tenon jig has guides that prevent the router bit from straying from a set path that will create the correct cut.
The jig itself is adjustable to accommodate different sizes and shapes of cuts. Modern jigs can range in price, depending on features and adjustability, and they can be made from a variety of materials, including lightweight aluminum, steel, plastic, or even wood. Making a jig from scratch can be challenging, but it is possible; a few pieces of wood carefully measured and clamped together can guide the router in much the same way that a fully adjustable store-bought jig can do, but the amount of adjustability of a homemade version depends on how the jig is made and from what materials. If a router and jig setup is not being used, a woodworker may opt to use a mortiser, which is a machine specially designed to drill square holes.