Replies

1. 
   AutumnWoods | Feb 27, 2011 03:08pm | #1

   Mechanical Strength

   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.

2. 
   umbach | Feb 27, 2011 05:44pm | #2

   I understand what you're
   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.

3. 
   joinerswork | Feb 27, 2011 09:33pm | #3

   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

1. 
   umbach | Feb 27, 2011 09:39pm | #4

   Thanks Ray. That adds some clarity to my confusion.

2. 
   AutumnWoods | Mar 01, 2011 09:49am | #5

   Absolutely

   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.