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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]


8 Responses to Rock Solid Joinery With Tenon-Lok

  1. Geoff K. says:

    Looks cool, but please help me understand how the tenon is inserted into the flared mortise. I don’t understand how the wedge ring allows the tenon that is bigger then the opening to the mortise to pass through the top of the mortise and then sit all the way to the back of the mortise.

  2. It took me a while to grasp to concept, and I didn’t explain that part really well. The lead in paragraph is a little misleading. The tenon actually starts out the same size as the top of the hole.

    Think of a wooden ax or hammer handle. The handle is inserted into the metal head and a wedge is driven into the handle to expand it, stopping the head from flying off when you swing it.

    Same principle here. You make a mortise and tenon joint like you normally would with the mortise and tenon diameter the same. Then you use the Tenon-Lok bit to hog out the bottom of the mortise, but the top of the mortise and the tenon stay the same size. When you drive the tenon into the mortise and into the wedge ring, the ring forces the tenon to expand.

    Sorry for the confusion.

  3. Daniel says:

    Geoff, the tenon is the same size as the entry hole. As the tenon is driven into the mortise, the wedge ring splits the end of the tenon, making it flare out, filling the flared mortise.

    It’s basically a fancy version of a blind fox tenon.

  4. Geoff K. says:

    So, the wedge ring is sacrificial, it goes into the mortise and remains in the joint after completion, is this right? From this description, the ring sits at the bottom of the mortise, and a blow with a mallet splits the end of the tenon, flaring it into the bottom of the mortise. It’s essentially a round wedge, flaring the end of the tenon, as Benjamen explained. Thanks for the clarification.

  5. Rob says:

    Call me crazy but wouldn’t you just want to expand in the direction of the grain, not across the grain? In this instance you’re expanding both ways. I guess if the hole is a perfect fit for the expanded tenon, it’s not a problem.

  6. Jeff says:

    Only problem is if the rung ever breaks or the chair needs repair in the future. I’m always complaining about pulling nails out of joints just to get them apart to reglue. Now the whole chair will have to be remade, won’t be worth fixing. At lease the joints will never come apart…

  7. Jeff says:

    On second thought, looks like wood in the tenon could still be drilled and carefully chiseled out and then just remove the metal band….

  8. Grey Doffin says:

    I think this is a foolish solution to a problem that doesn’t exist. This is not stronger (actually weaker I think) than a well made (meaning tight) joint properly glued and clamped, but when the day comes when the glue gives out, it will be impossible to reglue the joint. If the problem is a loose existing joint, the answer is to increase the diameter of the tenon by gluing veneer around. This metal wedge is likely to break the wood at the outside of the tenon on an older chair (with more brittle wood), leaving a joint that is doomed to fail.

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