Choosing the correct joint that will endure, not twist, and give
the best structural support is an essential aspect of furniture
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
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.
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.
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...
Laguna Beach, CA
"All great work starts with love .... then it is no longer work"