Nothing beats the clean look of mitered corners on a box. And while I like the look, the mitered corners won’t stand up to a lot of stress. So if the box is going to be handled much, I like to reinforce the corners by adding thin strips of wood called splines. Although you can hide these splines in grooves cut in the faces of the miters, I prefer to use exposed splines. It’s a great way to add a unique look.
The trickiest part of the process is creating the kerfs in the corners of a box. To cut across the joint, the box has to be held at 45° to the saw’s table. To do this safely, I built a cradle-style jig that runs along the rip fence.
It starts out as two sides cut from 3/4" plywood. To hold the box, there are a couple of supports that fit into angled dadoes cut in the sides. Before screwing the sides to the supports, cut a small “window” in one of the sides. This makes it a lot easier to set the height of the blade and to position the rip fence later.
With the jig complete, you’re ready to cut the kerfs. For this, you want to use a rip blade so the bottom of the kerf is perfectly flat. And in most cases, you want to cut the kerf as deep as possible without cutting through the inside corner of the box where the spline would show. An easy way to set the depth of cut is to look into the window and raise the blade until it’s just below (1/16" to 1/8") the inside corner.
The next step is to position the fence to locate the kerf. After removing the box, just slide a ruler into the window so it rests against the inside face of the cradle. Then adjust the rip fence until the measurement matches the desired location of the kerf. Cutting the kerfs is just a matter of pushing the cradle over the saw blade. The important thing is to make sure the box stays tight against the inside face of the cradle as you press the cradle against the rip fence.
Note: If you experience any chipout, you can slip a piece of hardboard beneath the box to back up the cut. Just be sure to reset your depth of cut.
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