The jig shown allows you to rout a mortise in stock of any thickness. Its adjustable jaws ensure that the mortise will be positioned properly, normally centered in the edge of the board.
Cut the jig top from 3/4-inch plywood; make the piece about 15 inches long and wide enough to accept the thickest stock you expect to mortise. Cut the two jaws from 2-by-4- inch stock, sawing the pieces to the same length as the top. To prepare the top, mark a line down its center, then cut a notch along the line at one end using a router. The notch should be as wide as the template guide you will use with your router bit. (If you are using a top-piloted bit, rather than a non-piloted straight bit with a template guide, size the notch to accommodate the bearing.) The notch should be long enough to accommodate the longest mortise you expect to cut. Next, rout two adjustment slots perpendicular to the centerline. Finally, bore a viewing hole between the two slots. To assemble the jig, screw hanger bolts into the jaws, then fasten the top to the jaws with washers and wing nuts.
To use the jig, outline the mortise on the workpiece and mark a line down its center. Loosen the wing nuts and secure the stock between the jaws so the centerline is aligned with the line on the jig top; make sure the top edge of the workpiece is butted up against the top. Tighten the wing nuts. Align the bit with one end of the outline, then mark reference lines on the jig top along the edge of the router base plate. Repeat to mark lines at the other end of the outline. Rout the mortise, starting the cut with the base plate aligned with the first set of reference lines and stopping it when the plate reaches the second set.
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