Four Ways to Make Mitre Joints
Mitre joints are used for picture-frames and larger jobs such as bookcases. The mitre angle of 45° must be cut and trimmed accurately for success [1 and 2]. It must also be strengthened in one of several ways.
The simplest strengthening method isand pinning . Glue both surfaces, then clamp the corner firmly in a vice between padding. This stops the pin hammer from knocking the joint out of shape. Tap the pins home in a dovetail shape and fill the holes.
A stronger method is to saw slots dovetail-fashion across the outer edge of the corner, with both pieces held together in the vice. Insert pieces ofor 1.5 mm ply, and trim when the dries .
Loose-tongue mitres are stronger still. Avoid damaging the edge of the mitre by making a 45° block the same width as the work  . Clamp the block and the work together in a vice and plough the groove, using the block as a guide. Cut the tongue from short-grain ply.
Dowelling is also effective, but the holes need careful drilling. Locate them exactly by tapping pins at the dowel positions in one mitre face . Snip the pins off and press them into the other face. Remove the pins and drill both sets of holes at right angles to the mitre.
Set the holes nearer the inner face of the work to allow a reasonable length of dowel.