Miters are not the strongest joints, but we use them all the time for joining picture frames together. One method for strengthening this connection is to add a spline, a thin sliver of wood into a slot where the two miters join together. A common way to make these is to cut a slot about halfway through the corner after gluing the frame together and slide in the spline.
A better method for making splines
In this method, the picture frame and the splines are glued up all at the same time. These splines are stronger because they run all the way through the width of the miter and the grain is positioned to run perpendicular to the miter.
Making the slot-cutting jig is simple. It’s basically the opposite of my regular spline jig.
Making the slot-cutting jig
Glue together two scraps of plywood. One a little taller than your rip fence, and the other piece square. I added a couple screws so I wouldn’t have to wait for the glue to dry.
Next, square this assembly up by cutting off the bottom triangle of the square, shaving off a little of the rectangle at the same time.
It should stand square to the table top and fence.\
Using the jig
After cutting your picture frame pieces with their 45 degree miters. Line up one end with the triangle so that the end is flush with the table. To keep this from slipping, clamp the board to the upper part of the jig and cut out a slot.
To make the slot on the opposite end, keep the same side toward the fence and cut the slot using the other side of the triangle.
Making the splines
The grain direction of these splines if what helps give these miters their strength, You want the grain to be running perpendicular to the miters, not the same direction. To make these, select a board that is wider than the length of the miter joint.
Then resaw a board to the thickness of your slots.
Then crosscut this thin board into the widths of your miter slots. I used a thicker board to hold down the thin splines and keep them from flying away when I cut them
The nice thing about these splines is that they help with the corner alignment of the picture frame and prevent them from sliding apart. Glue them into the slots and glue the miters together all at one. One the glue is dry, cut off the excess and sane the frames smooth.