I got an interesting question from a
member of this group. He noticed that in Woodsmith magazine, we
use yellow woodworkers glue almost exclusively, and rarely use dowels or
biscuits to strengthen a joint. And he was wondering why.
The short answer is that done properly, a "glue-only" joint is
stronger than one with dowels or biscuits. Maybe the short answer isn't really
enough...
Certain types of joinery can benefit (and be strengthened) by adding mechanical
reinforcements. For example, end-grain joints require mechanical aid to provide
a good glue surface.
But with edge to edge joints, biscuits, dowels, or splines may actually weaken
the joint if they are not mounted with great care. (However, it's worth noting
that when I'm working alone I often use biscuits to help align, not strengthen,
the mating surfaces of a panel during glue-up.)
How yellow glue works: A lot of people think yellow glue gets its strength from
"mechanical adhesion" -- the molecules of the glue penetrate the
pores of the wood and form little "hooks" that hold the wood together
like a strip of Velcro.
True, some of the glue does penetrate the wood fibers. But yellow glue gets
most of its strength from "specific adhesion." This creates a bond
between the glue and wood molecules -- a thin layer on one piece of wood sticks
to a thin layer on the mating piece. (To get an idea of what's going on, put a
drop of water between two flat pieces of glass -- then try to get them apart.)
Preparation of the edges is the key. If the edges of the wood have been
properly prepared, the result will be a bond that's actually stronger than the
wood. When a properly glued-up edge joint breaks, it breaks the wood, not the
glue line.
So what's the secret to a strong glue joint? Three things. First, the mating
surfaces must be as flat as possible (almost like glass). Second, the glue must
be applied consistently and evenly. Any interruption to the glue line will
weaken the bond. And third, when clamping the joint, there should be even
clamping pressure along the joint line -- enough pressure to cause the glue to
flow into a dense film inside the joint. And enough so you see small beads (not
gobs) of glue squeezing out along the joint line.
Go to Tip #41 |
