In woodworking terminology, a butt joint is regarded as the most basic form of joinery, where two pieces of wood are simply butted together without any interlocking elements. It is an easy joint to make, but it is not very attractive since the end grain of one board is usually visible—especially when the butt joints form the corner of a workpiece. But one variation of the butt joint, a miter joint, is a more attractive option since the eye doesn't see any of the end grain from either piece of wood.
How Miter Joints Are Made
Miter joints are made by joining two pieces of wood with the ends cut at an angle. When a workpiece is square or rectangular, the two mating pieces are cut at 45 degrees on the ends, so that when butted together they form a perfect 90-degree angle. However, with an irregular shaped workpiece that has a non-rectangular shape, the miters can be cut at different angles. For example, an eight-sided picture frame will have eight angles of 45 degrees each, with each segment of the frame mitered at 22 1/2 degrees. A five-sided picture frame, on the other hand, will have five angles of 72 degrees each, with each segment of the frame mitered at 36 degrees.
A butt joint, in general, are not a very strong method of joinery, and mitered joints are no exception, since they have no interlocking elements and often rely purely on wood glue to hold them. The most important element of a good mitered butt joint is cutting very precise angles, so that there is good contact along the cut faces of both pieces of wood. For making these angled cuts, the best tool is a quality miter saw with a sharp, fine-toothed blade designed for precision woodworking.
As in a basic butt joint, wood glue will provide most of the strength for holding a miter joint. However, because both sides of the glue joint will be on porous end grain, you will likely need to use more woodworking glue than used when gluing on side grain.
Reinforce With Mechanical Fasteners
As with a basic butt joint, a mitered joint doesn't have a lot of strength when it is held with glue alone. It can work well enough for very lightweight joints, such as joining the ends of decorative trim that is secured with finish nails elsewhere along its length, but glue alone isn't sufficient if the joint will experience any kind of load or stress.
It's a good idea to strengthen mitered joints by using finish nails, brads, or screws to give lateral strength. To use a finish nail or brad to secure the joint, apply glue to the joint and secure it with a woodworking clamp. Then, using a pneumatic finish nailer or brad nailer, drive a fastener through the face of one board into the end of the adjoining board. Follow up by driving another nail into the joint from the opposite direction. If the mitered pieces are hardwood, remember to drill pilot holes before driving nails or screws, which will prevent splitting.
Various types of other metal connectors can be used to secure mitered joints. For example, a bookcase where the side and top surfaces are mitered can be reinforced with L- brackets along the backside where they can't be seen, or with corner braces attached inside the bookcase. On picture frames, various reinforcement plates can be used to secure the joints on the backside of the frame.