Spline jig for mitered joints
Miter joints are hard to clamp and aren't very strong, especially on large or heavy picture frames. I like to add inlaid splines to reinforce these joints and align them during assembly. This jig works with my trim router and a 1/4" straight bit to rout the grooves for the splines.
The jig consists of a plywood base with a pair of angled fences that position the workpieces. A 1/2" spacer between the fences creates a gap between the workpieces to provide clearance for starting and stopping the router bit. A wedge block holds the workpieces in position. I made the router guide from 1/4" MDF with a window cut out to view and set the position of the bit. A long fence guides the router while stop blocks, secured with double-faced tape, limit thelength of the groove.
Before using the jig, set the bit's cutting depth to about 1/4" (taking into account the router guide's thickness) and make test cuts to position the stop blocks and router bit. To use the jig, place each workpiece against the base fences with the tip of the miter against the spacer. Clamp the wedge block against the workpieces. Align the router guide then clamp it to the base and make the cut.
Make the splines from 1/4"-thick stock and round over the edges. While gluing up the frame, glue the splines in place. When the glue dries, flush-cut them and finish-sand.
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