The corner half-lap joint is often used to assemble frames and doors. Adding dowels or screws to the joint provides an extra measure of strength. The joint can be cut on a table saw with a dado blade, but a router will do the job just as well. Do not try to make the cut freehand. This joint depends upon perfectly square shoulder cuts. Use a T-square like the one shown below to guide the router. If you are making many repeat cuts on boards that are the same size, take the time to build the jig shown on page 93.
Using a T-square jig
To rout half-laps with shoulders that are straight and square to the edges of the stock, use a T-square jig like the one shown at right. Make the jig from %-inch plywood so that each piece is about 4 inches wide; the fence should extend on either side of the edge guide by about the width of the router base plate. Assemble the jig by attaching the fence to the guide with countersunk screws, using a try square to make certain the two pieces are perpendicular to each other. Mark the shoulder of the halflap on your workpiece and set the stock on a work surface. Install a straight bit in the router, align the cutter with the shoulder line of the half-lap, and clamp the jig atop the workpiece so the edge guide is butted against the router base plate, and the edges of the fence and workpiece are flush against each other. Rout the half-lap with a series of passes that run across the end of the stock, as shown by the arrow in the illustration. Start at the end of the workpiece and continue until you make the last pass with the router riding along the edge guide.
3UILP IT YOURSELF
CORNER HALF-LAP JOINT JIG
If you have to make corner half-laps in several boards of the same size, it is worth taking the time to build the jig at right. Cut the two base pieces and the stop block from plywood that is the same thickness as your stock. The base pieces should be wide enough to support the router base plate as you cut the half-laps and mount the side and end guides. Use solid wood strips for the four edge guides.
To assemble the jig, mark the shoulder of the half-lap on one workpiece and set the board face-up on a work surface. Butt the base pieces against the edges of the board so the shoulder mark is near the middle of the Dase pieces. Install a straight bit in the router and align the cutter with the shoulder mark. Position one end guide across the base pieces and against the tool’s base plate. Without
moving the workpiece, repeat the procedure to position the opposite guide. Now align the bit with the edges of the workpiece and attach the side guides,
leaving a slight gap between the router base plate and each guide. (The first half-lap you make with the jig will rout reference grooves in the base pieces.) Slip the stop block under the end guide, butt it against the end of the workpiece, and screw it in place. Countersink all fasteners.
To use the jig, clamp it to the work surface and slide the workpiece between the base pieces until it butts against the stop block. Protecting the stock with a wood pad, clamp the workpiece in place. Adjust the router’s cutting depth to one-half the stock thickness. Then, with the router positioned inside the guides, grip the tool firmly, turn it on, and lower the bit into the workpiece. Guide the router in a clockwise direction to cut the outside edges of the half-lap, keeping the base plate flush against a guide at all times. Then rout out the remaining waste (left), feeding the tool against the direction of bit rotation.