Everyone likes to talk about how a good glue joint is stronger than the
wood itself. I think this may be true in a very technical sense, but
absolutely false in practice.
Take for example, a panel glue up with two 5" wide oak boards glued
edge to edge (no biscuits, etc). There is no way in the world that this
panel is stronger than a single 10" wide piece of oak.
If you whack the glued-up panel with a sledgehammer hard enough, it
will split along the glue line. It may take a little wood from either
side of the joint, but is will definitely break right along the joint.
Conversely, the solid 10" wide piece of oak will certainly withstand a
harder blow without breaking than would the glue-up panel.
What am I missing in this 'glue is stronger than the wood itself'
How about if you glued two 2.5" oak boards to either side of a 5" oak
board and wacked it with a sledgehammer? Would it still break along
one or both of the glue lines or right in the middle of the glued up
http://www.newwoodworker.com/useglue.html for a non-chemical discussion of
glue joints. Fact of the matter is that even in your example, while not
absolute, a properly done glue joint is likely to be stronger than the wood
itself. Even oak. (Although I'd suspect that a hardwood like oak would
certainly test the limits of the theory.)
Does seem strange, doesn't it?
I think Chuck just propped up a tar baby, but I'll take a poke at it
anyway. If "it takes a little wood from either side" doesn't that
mean that the glue is stronger than at least the part that it was
That's what I meant by being technically stronger, but not stronger in
practice. If the glued-up panel breaks with less force than the solid
wood panel, then the glued up panel is weaker. Whether it breaks clean
on the glue line, or takes a little wood with it is irrelevant.
No it's not. It's the key, as I already suggested in another post.
If it takes wood with it, then the break is wood-wood. If the
glue-wood bond isn't stronger, it will break away from the wood. That
is the glue and wood will separate, or the glue itself will separate,
with some still adhering to both bits of wood. Try using a *much*
lighter glue for example, or bashing itbefroe the glue has completely
set, but is adhering. The parts will separate easily at the joint.
In fact, this is the reason for failure of some joints.
NOW you are changing the whole story. In your title you indicate that glue
is not stronger than wood. NO ONE has said that glue makes a panel
stronger. Most everyone knows that a glued up panel will not break and
leave exposed glue, wood is always exposed. Hence the glue is stronger than
Consider the real-world mechanics of gluing two long strips of wood, long
edge to long end. Even the most minor of variations in thickness, or very
minor twisting will make the glue joint very hard to get perfect flat, even
with clamps. Both dowels and biscuits will allow aligning the flat, wide
surfaces, much better.
Does it? Have you tried? I haven't but I pretty much doubt it, never
had a glue joint break.
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No. If the joint is well-prepared and a good quality glue is properly
applied and cured (even PVA will work), the joint will rarely be what
actually fails. A wide panel breaks more easily because there's a
larger moment arm...actually, the appearance of "easier" is an illusion
as the stress before the breaking point will be (within the variability
of the wood itself) the same.
Have you actually done an experiment or just spouting?
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