Thread: Mortise vs Dowel Joints for Chair

Every chair I've had to repair has been because of failure of dowel joints, usually the chair-side rail joint. I would never use anything but a tenon or floating tenon in a chair.
I would usually make the mortise depth about 2/3 the depth of the leg that it's going into, and about 1/3-1/2 the width of the leg. Depending on how deep the rail is, you want to make the tenon as deep top to bottom as possible, but you don't want to go too close to the end of the leg in case it pops out under racking load. Try to have at least 25mm between the end of the leg and the mortise. When you're cutting the mortises, leave some extra on the end of the legs until after the glue up so it doesn't accidentally pop out.
You shouldn't use any of the wood with splits, but if it's absolutely unavoidable, try to get some epoxy into the splits, and clamp it up so that it's virtually solid. You can tint the epoxy to match the timber.

For interest, many chairmakers will only use hide glue on chairs, because unless they're well designed, they will rack at some time in their life when someone leans back in them, and hide glue was the only glue that would stick to itself. I think epoxy will also do so.

It's a bit hard to tell from the picture but if you are talking rail to leg, then traditionally it would be M&T or some type of bridle joint. Dowels would only be used in situations where the shape of the member made a tenon impossible. They wouldn't usually be found in that leg joint though. Not in a chair that's been made to last anyway.

For the timber sizes you mention, which seem a bit large for a chair but that's fine, I would make the tenon about 50mm or so long, 10 or 15mm thick and say about 80mm deep to make sure there's plenty of meat at the top of the leg.

The tenon thickness can be worked off whatever you're using to cut the mortice. If you have 1/2" mortice chisel or router bit to cut it with, then the tenon can be sized to match. As long as it's at least 1/3" the width of the leg. I'm assuming the leg and rail are both 100x30.

Edit: I was typing while Alex was replying. He is worth listening to

Thanks, I'll have start practising this type of joint. Just so I have it straight, the tenon on a 100x30 would be 80mm long by 10mm wide and 20mm high.

Cheers

The 80 x 10 sounds OK, but you could probably reduce the 80 as it is probably more than strong enough and you may want to keep it away from the leg with the tenon. (I'm thinking here about a rail morticed into a leg.) The 20mm seems a bit short - if the leg is thick enough I'd try to go further.

Have a look at a good reference book like Joyce's Encyclopaedia of Furniture Making to see the various types of M&T joints. There are many, to suit various circumstances.

Thanks, I'll check that out.