This jig is relatively simple to make. The only hard part is making the comb that guides the router bit. Ironically, the easiest way to make the comb is to use another type of box joint jig. A simple box joint jig will only take you half an hour or so to make, and will work well enough to create the comb.
You can make the jig any size you want. The base of mine is 22 inches because I happen to have quite a bit of 22 inch-wide stock on hand. The comb, which is the dark board at the top, is eighteen inches long and about six and three-quarters inches wide. The finished jig can join boards that are nearly thirteen inches wide, which is more than enough capacity for my shop. Except for the comb, which is made of 3/4" MDF, the rest of the jig body is made from 3/4" Baltic birch plywood.
Begin by laminating two piece of 3/4" plywood. Your final block should be about an inch shorter and two inches narrower than your comb. This is your top block. Screw this top block to your base, centered eight-to-left, and exactly flush to the front edge. Now you're going to create another block, except that this one should be the same length as your comb (or perhaps a little longer) and about two-and-a-half inches wide. Screw this to the bottom of your base, centered right-to-left and flush to the front edge. If you have not made your comb, you need to do so at this point.
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