Install a lock miter bit in your router and mount the tool in a table. Attach a notched auxiliary fence (page 90) and screw an extension board to the miter gauge. Set the bit height so the uppermost cutter is centered on the board end with the work – piece flat on the table. Position the fence so the bit will dado the stock without shortening it. Holding the workpiece against
the fence and the miter gauge extension, feed the stock into the bit (above, left). To cut the mating piece, clamp a guide block to it to ride along the top of the fence. Then feed the board on end into the cutter, keeping it flush against the fence with one hand while pushing it and the guide block forward with the other hand (above, right).
Making the mortising jig
Assembled from %-inch plywood, the jig shown at right will help you rout mortises. Make the top and upright about 10 inches long and 6 inches wide. Cut an oval slot in the middle of the top, making it slightly longer than the mortise outline and wide enough to contain the outline and the template guide you will use with the bit. Screw the pieces together in a T shape, countersinking your fasteners; make the gap between the slot and the upright measure at least one-half the thickness of the thickest stock you plan to mortise. This will enable you to center the mortise outline under the slot with the board face flush against the upright; you can center thinner stock by placing shims between the workpiece and the upright.
2 Routing the mortise
Clamp the jig to the workpiece with the mortise outline centered under the slot; place shims between the workpiece and upright, if necessary. Secure the upright in a bench vise, setting a support board under the workpiece to hold it snug against the top. Install a %-inch mortising bit in a plunge router, attach a template guide to the sub-base, and adjust the cutting depth so you can rout the mortise in two or three successively deeper passes (right). Hold the router flat on the jig top with the bit centered over one end of the slot. Turn on the tool and plunge the bit into the stock. Then feed the tool to the other end of the slot to finish the cut, pressing the template guide against the inside edge of the slot throughout the procedure.
Miter gauge extension.
Cutting the tenon on a router table
Mount the router in a table, screw an extension board to the miter gauge, and mark the tenon shoulders on the work – piece, using the finished mortise as a guide. Adjust the cutting width so the gap between the fence and the bit equals the distance between the shoulder line and the end of the board. Set the cutting height to obtain a tenon slightly thicker than the mortise you cut in Step 2. Butting the rail against the fence
and the miter gauge extension, feed the workpiece face down into the cutter. Turn the board over and repeat to cut the other tenon cheek (above). Test-fit the thickness of the tenon in the mortise and make additional passes until the fit is snug. Then mark the top and bottom of the tenon on the cheek, adjust the bit height to cut to the line and feed the stock on edge, making a pass on each edge to complete the tenon.
4 Rounding over the tenons
One way to ensure that a square – edged tenon fits snugly in a router-made mortise—with its rounded corners—is to round over the edges of the tenon, using either your table saw or a shop knife. For the table saw method, cut a V-shaped wedge out of a wood block, creating a jig that will hold the workpiece at an angle to the saw blade. Position the jig on the table alongside the blade, slide the miter gauge up against the jig, and clamp the gauge in place. Screw the jig to the miter gauge. Adjust the cutting height so the blade will just trim the corner of the tenon. The rip fence should be positioned so the saw blade will not cut into the tenon shoulder. Then turn on the saw and, holding the face of the workpiece flush against one side of the V, slide the stock along the jig (as shown by the arrow in the illustration) until it touches the fence, trimming the corner of the tenon. Reposition the workpiece in the jig to trim the remaining tenon corners (above, left). For the shop knife method, simply round over the tenon by trimming the corners from the end of the board to the shoulder (above, right).
Mount your router in a table with a mortising bit in the tool. The bit’s diameter should be equal to the mortise width—typically, about one-third the stock thickness. Mark the beginning and end of the mortise on all four sides of the workpiece, then center an end of the board on the bit and butt an auxiliary fence against the stock. Clamp a featherboard to the table, bracing it with a support board clamped at a 90° angle to its edges; round over the top corner of the featherboard slightly to facilitate lowering the workpiece onto the bit. To help you determine the location of the cutter when it is hidden by the workpiece during this cut, mark the points on the fence where the bit starts and stops cutting. Next, align the mortise start line on the workpiece with the bit, butt a stop block against the bottom end of the stock and clamp it to the infeed side of the fence. Then align the mortise end line with the bit, butt a second stop block against the top end of the workpiece, and clamp it in place. Turn on the router and lower the workpiece onto the bit, keeping it flush against the fence and the infeed-side stop block (above). Feed the workpiece across the table, pressing the stock against the fence and flat on the table (right). Once the workpiece contacts the stop block on the outfeed side of the fence, turn off the router.
Routing a deep through mortise
If the desired depth of a mortise exceeds your router’s maximum depth of cut, use an electric drill to help cut the cavity. The illustration above shows the three steps necessary to cut a mortise through thick stock. Start by installing a mortising bit in the router and making as many passes as you can until you can go no deeper (A). Then use the drill with a bit that is larger than your router bit to bore a hole through the remaining waste (B). Install a piloted flush-trimming bit in the router and turn the workpiece over. Inserting the bit through the hole made by the drill, rout out the waste (СУ, keep the pilot bearing pressed against the walls of the mortise to complete the cut. Use a chisel to square the mortise corners, if desired.
Clamped in a bench vise, a commercial mortise-ami – tenon jig guides a router as it cuts a tenon. The jig template is turned end-for-end to rout the matching mortise.