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  Joint design and wood movement... analysis and new thoughts  

Choosing the correct joint that will endure, not twist, and give the best structural support is an essential aspect of furniture construction.

Wood movement in joints is also critical to consider.  In joining 2 boards of substantial width, say 8" or more , where the grain of one is perpendicular to the other, there are sometimes problems.  Where we often like to think that "True" mortise and tenon is the best choice, loose tenon has definite advantages in my opinion...  I haven't read this in woodworking books, however looking at a free body diagram of a frame joint... my analysis seems reasonable.

Wood moves across the grain to a much greater extent than with the grain, and this fact is the source of the problem.  It is the same reason why we often re-saw our own veneer and glue it to both faces and both edges of a panel to be used as a cabinet door, or component.  We see this construction in Krenov's work and others... he calls it "Real Veneer" and suggests a suitable core.

When loose tenon joinery is used in a frame joint, the stability is similar to a real veneered panel and will eliminate most of the lateral stress common to joints in wider boards.  The core is the tenon and should be selected for stability... quarter sawn, vertical grain is the best choice.  When the mortise is cut the cross section becomes a panel similar to veneer and a suitable core. 

The tenon is then coated with glue on all surfaces and becomes inert and very stable and since the Tightbond II or epoxy is waterproof... t is unlikely the tenon will expand since the moisture content will not change very much.  The mortised board is reduced to almost a veneer in thickness...say 1/4" or so... not much material to expand and cause problems.

At the bottom of the mortise the wood is solid and the stress is, as usual, very high and will transition to very low at the point of contact with the other board.  This is exactly what is desired in a joint!  If the tenon is small or it is "true " mortise and tenon design you are back to normal stress and potential problems.

Another overlooked point is that where the loose tenon grain is running the same as the mortise... a much smaller embedment is required, since the long grain to long grain strength is occurring.  In the other board the penetration depends on surface area and glue strength... much more depth is necessary.

In designing furniture these simple ideas are helpful in joint selection... try to think what is going on, rather than just selecting a joint at random...

Mark Singer
Laguna Beach, CA
July 2006
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