A joint isn't always the answer

“Should I use a dovetail joint or a finger joint for attaching the bottom of a box to the sides?” I teach woodworking and carpentry and I get asked this question a lot. And for someone who has just learnt about finger, box and dovetail; this is a fair question. My answer is simply, think! You see a dovetail joint or a finger joint on side grain is simply a recipe for disaster. The tails or fingers have their bases in side grain, which means they would simply shear off.

If you have side to side grain, count your blessings and slather on the glue. If there is load to be taken then a groove on the sides will do the trick.

Which brings me to an interesting study in mortise and tenon joints. Here are 4 images. 2 of them are correct, and 2 are not. Try checking if you know the answers before reading ahead

The trick here is to examine the grain orientations. Remember that we are always trying to increase the side grain to side grain surface area. Hence, in the traditional method of aligning an MT, the broader dimension of the tenon member is always laid out along the grain of the mortised member. This insures that the maximum side grain is exposed to the cheeks of the tenon. See figure C

In figure A, a fairly familiar situation has arisen, The design itself calls for the narrower side of the tenon member to be laid out in the direction of the grain, “I’ve seen mortises cut in this manner.” But remember that very little side grain of the mortising member is taking part in strengthening this joint. The maximum surface area is of end grain, which counts for something, no doubt, but not much. The answer to this is Fig B, A double tenon, adds more side grain surface to the mortise while cutting down the end grain exposure, resulting in a far stronger joint.

But watch out for decisions like Fig D, you might think that this has increased surface area, but really what this has done is increase end grain surface area while also increasing the chance of the bridge breaking off since it is in with the grain. Infact the bridge will probably shear off while digging the mortise itself.

So, here’s the final solution.

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