I like building decorative boxes and trays, and have used various techniques for the joinery; lock miter bit, hand cut dovetails, box joints, and miter joints reinforced with horizontal visible splines or dowels with a contrasting wood. I have come to appreciate the clean look of miters without any visible joinery or reinforcement, but that produces a weaker joint. A common technique is to cut a vertical slot for a spline in the miter using the table saw, but since the slot goes the full length of the miter it is visible on the top and bottom of the joint. I wanted a way to reinforce the miter with a hidden spline, and this is how I solved the problem. Like many of you, problem solving is a part of woodworking I really enjoy.
I built a quick jig to use on the router table, which holds the work piece at a 45 degree angle and routs a perfect slot. Stop blocks on the router fence define the length of the slot so it isn’t visible once the joint is assembled. Since the edge of the mitered joint is not a reliable surface to contact the fence, the 2 side supports on the jig extend beyond the workpiece for consistent contact and positioning of the slot. I used a 3/16” router bit, but other sizes would work for different widths of wood. The splines were cut on the table saw and fine tuned for fit with a few swipes on sandpaper. An added bonus came when gluing up the box, because the splines keep the joints in perfect alignment without sliding.
On a related note, I cut the miters using a recently built small single runner sled dedicated to cut edge miters when the runner is in the right miter slot on the table saw. When I move the sled to the left miter slot on the saw, the blade is centered and used with my box joint blade. I had expected to fine tune the miters with my shooting board, but by testing and adjusting the blade angle in small increments using thicker stock, I was able to achieve perfect miters that didn’t require any fine tuning.
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