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Q & A: Mortise and Tenon Dimensions


Q & A: Mortise and Tenon Dimensions


I would like to use mortise and tenon joinery on a project. Are there any rules of thumb for sizing mortise and tenon joints?


Here are a few good rules of thumb for designing a mortise and tenon joint:

For strength, a tenon should be at least twice as long as it is thick. Thus, a 1⁄4-in. thick tenon should be at least 1⁄2-in. long. The thickness of the tenon is dependent upon how wide the mortise can be made.

In frame construction where the stile and rail thickness is the same, the tenon should be no more than one-third of the total thickness of the rail. Keeping the sides of the mortise at least as thick as the tenon produces the strongest possible joinery (see photo above).

If the piece into which the mortise is cut is wide enough to allow a set back, as when a table leg is joined to an apron, then the tenon width can be increased to as much as half the thickness of the piece from which it is cut (see photo above).

The width of the tenon is largely a function of the width of the piece it is cut from. Try to maximize the width for the glue surface while leaving enough of a shoulder for mechanical strength. If the tenon is more than 3-in. wide it is best to cut multiple tenons to better accommodate the expansion and contraction of the wood across the grain (see photo above).

This story originally appeared in American Woodworker October 1999, issue #75.

October 1999, issue #75

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