I cut miters on my radial-arm saw with the jigs shown in the drawing. In the plan view, the path of the sawblade should bisect the miter fences — a 45° cut on both sides. But as long as you are cutting stock of the same width using both fences for each joint, and the fences are exactly 90° apart, the miter will fit perfectly even if the saw is slightly off. To make sure of the angle, I hold my framing square between the two arms when I clamp them down.
The arms can be plywood or clear stock — just something that will remain true. I screwed and glued 3/4-in. by 2-in. fences to the arms after I made sure that the saw motor would clear them. The pivots are short sections of 1/2-in. dowels, glued into the arms.
James Baldwin, Chester, NJ