A simple way to cut box joints using a special jig and your router table.
A sliding router table jig, such as the Rockler Router Table Box Joint Jig works with any router table with a miter slot (Photo 1). The jig’s sliding fence holds the workpiece vertically and guides it over the router bit. The jig works much the same way as a sliding table saw box joint jig (as described above), except that instead of having a pin attached to its sliding fence, the Rockler jig has a guide rail that’s used to evenly space notches along the length of the joint. By switching rails and router bits, you can use the jig to cut 1/4-in., 3/8-in. or 1/2-in. box joints.
The first step before cutting a joint is to fit the jig’s sliding fence with a sacrificial face, cut from a scrap of 1/2 MDF or plywood that’s notched to fit over the rail. The distance between the bit and the rail has to be the same as the diameter of the bit; in this example, that’s 1/4-in. (Photo 2).
Make the setting by adjusting the jig’s side-to-side position in the miter slot: the jig has two expanding strips that lock into the slot. Now, adjust the height of the bit so that it’s just a hair higher than the thickness of your workpiece (in the example shown, the workpiece is 1/2-in. thick, so the bit’s cutting height above the jig is 1/2-in. plus 1/64-in.) (Photo 3).
Now butt one edge of the workpiece against the rail and hold it firmly against the fence as you slide it through the spinning bit. Reposition the stock to the left so that the notch you just cut fits over the guide rail, and repeat the process of routing notches along the length of the joint. To cut the mating workpiece, offset the first cut by the width of a pin, then continue routing as before. You can adjust joint tightness by shifting the jig relative to the bit: less space between the bit and guide rail produces a looser fit; more space yields a tighter fit (Photo 4).