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Why haunch a m&t joint?

Why haunch a m&t joint? (post #162286)

umbach on Sun, 02/27/2011 - 10:28

Joinery

Is there an advantage to using a haunch m&t joint rather than just running the tennon to the top of the joint? I'm thinking of a top rail application (ie. The top rail of say an end table).

Mechanical Strength (post #162286, reply #1 of 5)

by AutumnWoods in reply to umbach [original] on Sun, 02/27/2011 - 13:08

Having a haunch keeps the mortise away from the end of the leg (or other structure) thus giving it mechanical strength. The end grain directly above the tennon is what will fail against racking forces. The haunch is designed to reinforce the joint against twisting stress, keeping it from blowing out the sides of the mortise, the second weakest part. Seems to me the design evolved when glues weren't all that great and mechanical strength through pinned M&T joints was what held a piece together. Even with our modern day long lasting glues I never consider any glued joint to be permanent. Until they invent a glue that re-aminates the wood fibers so they physically grow together I will never trust glue to last forever.

I understand what you're (post #162286, reply #2 of 5)

by umbach in reply to umbach [original] on Sun, 02/27/2011 - 15:44

I understand what you're saying but I'm still not sure I understand the difference. I guess there is more strength If you don't run the tenon all the way to the top Of the joint. Thanks.

umbach, In the case of a (post #162286, reply #3 of 5)

by joinerswork in reply to umbach [original] on Sun, 02/27/2011 - 19:33

umbach,

In the case of a top rail, say above a drawer, in a table, there is no advantage to haunching a tenon that is only as wide as the rail is thick- 3/4-7/8". Indeed, more strength would be lost (in the rail) in having the tenon only 3/8" tall (glue surface area and shear strength), as would be gained in the leg by leaving a 3/8" web above the mortise. Frequently, top rails of tables (and case pieces) are dovetailed into the legs ( or case ends) from the top, rather than mortised.

The forces that tend to break table joints are exerted when the table is dragged or pushed across the floor, setting up a leveraged prying effect at the top of the legs. On the aprons' joints, 3 or more inches wide, a haunch at the top of the joint adds considerable strength to prevent the top of the leg from splitting away from the tenon. Above the drawer opening, the flare of a dovetail joint offers mechanical resistance to a tenon being pulled out of its mortise by these forces.

Ray

Thanks Ray. That adds some (post #162286, reply #4 of 5)

by umbach in reply to joinerswork [original] on Sun, 02/27/2011 - 19:39

Thanks Ray. That adds some clarity to my confusion.

Absolutely (post #162286, reply #5 of 5)

by AutumnWoods in reply to joinerswork [original] on Tue, 03/01/2011 - 07:49

Thanks for the clarification. I was referring to large M&T joints like the ones found on dining room table legs and the like. The method you described is the one I also use for case pieces and the like. Good comment.