Wedged Mortise & Tenon
The joint will never loosen!
by Tom Caspar
Tap, tap, tap. The wedges go home, the glue squeezes out and a big smile lights up your face. “This joint isn’t coming apart for a hundred years,” you say. “It’s as solid as a rock!”
Making a wedged mortise-and-tenon joint is richly rewarding. Once you understand how it works (see photo, below), you can’t help but admire the joint’s elegant simplicity. It also sends a message. A wedged joint says to one and all, “This was made by a skilled woodworker.”
How the Joint Works
Here’s a cutaway view of a wedged mortise-and-tenon joint. Driving in the wedges forces the tenon to flare into a fan or dovetail shape. The mortise is tapered to match the angle of each wedge. Like a dovetail, this joint can’t pull apart after the wedges go home.
This tenon has two unusual features: saw kerfs that create flexible strips and holes that disperse the strain that the wedges create. The wedges cause the strips to bend; the holes prevent the bend from splitting the rail.
Where could you use a wedged joint? It’s a candidate for any joint that receives a lot of stress. A table base is a good example (top photo). Pushing or leaning on this table might slowly force a standard joint apart, but wedges keep this joint locked together.
The wedged mortise-and-tenon joint isn’t difficult to make, but you should have some experience making standard mortise-and-tenon joints before tackling it.
Tools Required
To make this joint, you’ll need a tablesaw, drill press, plunge router, chisel and a bandsaw. If your mortise’s width is 5/8 in. or more, like the mortise I made, you’ll need a 1/2-in.-dia. top-bearing flush-trim bit ($19). If the mortise is more than 3/4 in. deep, you’ll need a bottom-bearing flush-trim bit ($20) (see Source, below). For a mortise less than 5/8 in. wide, you’ll need a straight router bit and a fence or jig for your plunge router.
Rout the Mortise
Before you begin your project, make a prototype joint (see photo, below).
Designing Your Wedged Joint
Each part of a wedged joint must often be tailored to fit the joint’s size, intended strength and type of wood. Make a prototype following these steps:
1. Substitute a notch made with a dado set for the mortise (see “How the Joint Works,” page 45). Taper both of the notch’s sides by angling the miter gauge.
2. Make a full-size tenon. Observe how well the flexible strips bend. You may be able to use smaller strain-relief holes or no holes at all.
3. Experiment with the notch’s angle. The wider the taper, the stronger the joint. My taper is 3 degrees, but you can increase it up to 8 degrees.
4. Test the bend. My flexible strips are only 1/8 in. thick opposite the strain-relief hole, so they bend easily.
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