Mortising Jig

Rout end and edge slots quickly and easily

Loose tenon joinery, like that used in the Easy Chair on page 50, is a great alternative to traditional mortise-and-tenon joinery. The latter requires cutting an integral tenon in one of the parts to fit in a mating mortise cut in the other part. But with loose tenon joinery, the tenon is a separate element that spans the joint to fit into a mortise cut in each part. Because this “loose” tenon is an easily machined strip of wood, making the joint tends to be more efficient than taking the traditional approach. However, loose tenon joinery typically requires the ability to cut mortises into the ends of workpieces as well as their edges. That’s where this jig comes in. Used in conjunction with a plunge router outfitted with an edge guide, you can efficiently and accurately cut perfectly matched mortises in both the ends and edges of workpieces. In use, toggle clamps secure the work to the jig, and the end stops control mortise length.

Take care to build it square

The jig’s dimensions aren’t critical, so feel free to size it to suit your particular work. What’s important is that the top’s rear edge is parallel to the face, and that the face and top surfaces are square to each other. To ensure this, first run the glued-up assembly facedown over your jointer to bring the parts into one plane. Then run the jig upside down with the front face against the jointer fence to square the top surface to the face. Finally, run the whole jig upside down through your table saw to ensure that the rear edge of the top is parallel to the jig’s face.

Set up the jig. Outfit your plunge router with an edge guide and an upcut spiral bit, adjusting the router’s depth stop for the depth of your mortise. Clamp your workpiece to the jig with the marked “show” face oriented outward. Adjust the router’s edge guide to position the bit at the mortise location, and set the jig’s end stops to control the mortise length.
Using the vertical fence. Be sure that your workpiece is secure and flush with the top of the jig. Rout each mortise in a series of subsequently deeper passes, beginning with the router registered against the right-hand stop, and moving to the left until contacting that stop. Fully retract the bit at the end of each pass before beginning the next pass. For proper part alignment, mount all mating pieces with their “show” faces outward.

Using the horizontal fence. Install the jig’s horizontal fence, with toggle clamps attached. After setting up for the first cut using a marked-out mortise, pencil mortise-extent lines across the top of the jig for setting up subsequent cuts. When routing, bear down with your left hand to prevent router tipping, and make sure to keep the edge guide in contact with the jig.

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