How to make a log joint

What you need to know to prep logs for cottage projects

By Ryan ShervillRyan Shervill

Finished joint

Photo by Jacques Perrault

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Winter isn’t kind to small trees. Ice, heavy snow, strong winds, and frigid temperatures affect the little guys more than the mature ones, and come springtime there are always a few that haven’t survived. You should leave some as is for wildlife habitat, of course, but you can also harvest a few for cottage projects, including log furniture, sturdy log railings, or even complete log gazebos.

The key to truly strong log projects is the joinery. The best choice is the round mortise-and-tenon joint. It’s strong and beautiful, but can be tricky to make. Traditionally, woodworkers bored a hole for the mortise and then shaved the mating piece down with a drawknife to form a round tenon. These days, a tenon cutter, available from woodworking-tool suppliers, is the faster  and easier way: Attached to your drill, the cutter acts like a giant pencil sharpener and quickly reduces rough log ends to perfectly sized tenons. To make the mortise, simply drill a matching flat-bottomed hole with a Forstner or auger bit. A little polyurethane glue in the mortise and a few taps from a mallet form a solid, strong connection that will stand up to a lot of abuse.

Making a blind fox-wedged tenon

For a project that needs an even stronger joint, such as a chair, I combine a traditional joinery technique with the machine-cut tenon. Called a blind fox-wedged tenon, this joint is ingenious: A wedge in the tenon causes it to expand when it is driven into a specially shaped, tapered mortise. The result is a locked joint that can never separate because the end of the tenon is larger than the neck of the mortise. Imagine inserting your finger in a bottle and having it swell up and get stuck. Forever.

Step 1 for log joints

After drilling the mortise, carve the sides with a Forstner bit to flare the hole slightly.

 

1. Fashion your mortise and tenon with the tenon cutter and drill, as above, and test-fit your joint. When you’re satisfied with the fit, you’ll need to modify both parts.

2. Go back into your mortise with the Forstner bit, slightly angling it as you drill down, to shave one side of the mortise wall. Repeat on the opposite side of the mortise. The goal here is to make the mortise slightly wider at the base than at the neck, sort of like an inverted cone. Don’t overdo it; a very slight taper is all you need.

Making slot

Saw a slot in the centre of the tenon, stopping just shy of the depth of the mortise.

 

3. The tenon needs a blind (hidden) wedge and a slot for that wedge. Make a saw-cut up the centre of the tenon, stopping just short of the depth of the mortise.

4. Now make a hardwood wedge that matches the tenon saw-cut in length and width, but is slightly thicker at the base than the saw kerf. To assemble the joint, fit the wedge partway into the kerf, then insert this wedge-and-tenon assembly into the mortise. As you use your mallet to drive the tenon home, the wedge spreads the tenon’s base to match the cone-shaped profile of the mortise, locking the joint. Make sure the tenon is oriented correctly so it can expand into the tapered sides of the mortise and lock the joi; otherwise, it won’t insert fully as the sides are forced apart.

Final step

Drive the tenon onto a blind (hidden) wedge to lock the assembled joint.

This article was originally published on April 14, 2010


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