I suppose you are thinking of the glue joint bit as a tongue and
groove type connection. Many times glue joint shapes are used on
opposing (edge-to-face) joints like a drawer corner and T&G (which is
usually deeper) is used for edge-to-edge but the glue joint bit is
mostly the same.
Biscuits are mostly about alignment. The glue joint or T&G adds much
more gluing surface area plus alignment.
If you are doing a panel glue up a simple edge butt joint will do fine
if milled and clamped correctly, biscuits will help with alignment and
a T&G type joint will be much stronger.
Another option is using a spline and you can use a stopped spline and
get the advantage of a long added glueing surface area and hid it from
the ends of the panel if they'll be exposed.
Are you sure about that?
It's accepted fact that a properly executed glue joint is stronger than
the surrounding wood. Given that any failure will already be in the
wood rather than the joint, how can a T&G type joint be any stronger at all?
But the increased surface area doesn't buy you anything. In either case
the failure will be in the wood, not the glue joint. There is the same
amount of wood in either case, therefore the strength will be identical.
As for the "failure in several areas" argument, it doesn't really make
sense. Any failure will be in the wood, not the glueline. What
difference does it make if the wood fails beside a simple edge joint or
beside a T&G joint?
T&G could make alignment simpler because the boards can't slip relative
to each other. But that's the only advantage that it has. There's no
way that it can possibly be any stronger than a well-made edge joint in
a panel glue-up, because the edge joint is already as strong as the wood
Its always interesting to see what statements will cause controversy.
I have not done any purely scientific tests but I did once have the
opportunity to do karate kicks of glued up panels some of which were
edge butted and some were T&G. The T&G's seemed about twice as hard to
Yes, everyone has bought into the concept that a well executed butt
glue joint with modern wood glues is stronger than the wood.
Interestingly in the case of PVa types (like Titebond II) there are
really only generally accepted therories about how it actually bonds
but no real provable science at the molecular level, just via physical
testing. However, break apart any butt glued joint, or any joint for
that matter and you'll see the wood fracturing in some places and the
glue joint breaking in others.
Things to consider in the non-perfect world we live in.
1. Strenght of the glue joint: Imperfections in the glue joint may
cause weak spots so some mechanical advantage could add strength.
2. Strength of the wood: Even if you just consider the wood strength,
involving the tongue of one piece inside the groove of another can
compensate for any weak areas in the grain of either. Where one might
split from direct downward force, the other might be strong enough to
3. Strength of the glue joint (again): With a much expanded surface to
surface connection between the mating parts, at least double I assume,
you have greatly increased the odds of good contact all along the
joint. You have greatly lowered the possibility that the two pieces
are joined at a coincedient weak point.
Adding a spline with an opposing grain orientation will add even more
strength than the tongue concept and still includes the benefit of
added glue surface area.
I have found that if you glue nothing but the biscuits you get a strong
joint. That suggests that gluing both the buiscuit and the edge will give a
stronger joint than if you didn't use biscuits. But I have never tested
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