How do you create a round mortise that’s larger at the bottom than at the top, and how do you fit a similarly shaped tenon in the mortise? Most importantly, why the heck would you do such an infernally confounded thing? To answer the last question, such a joint would be so strong, the wood would have to fail before the joint. To answer the how-to part of it, Rockler created the Tenon-Lok cutter.
Place the Tenon-Lok cutter into a pre-drilled mortise of the same size. Bottom out the cutter in the hole and spin it up. As you press down on the bit, the spring action blades flare out to create the reverse taper. When you’re finished, just pull up on the cutter — the blades retract, and the cutter comes out of the mortise as easily as it went in.
Fitting the tenon into the hole is even easier. Place a sharpened and tapered steel wedge ring in the bottom of the mortise and drive the matching tenon into place. As the tenon is driven into the ring, it flares to match the reverse taper of the mortise. You’re left with a joint that ain’t coming apart, at least not without destroying the project first.
Rockler sells the Tenon-Lok cutters in 1/2″, 5/8″, 3/4″, and 1″ sizes for about $20 apiece, give or take a few bucks. Twelve-packs of wedge rings run $5 to $8.
Tenon-Lok Cutters [Rockler]