My understanding (U.K.) has been that ANY plane surface to plane surface
joint is a butt.. i.e. where one abutts the other, with no mechanical
I think that "butt" includes nails, screws and/or glue - All strength comes
from the glue or fasteners without which all you have is an "arrangement" not
That's whether end or long grain. It can be a _reinforced_ butt_ by adding
battens or fillets.
I think a biscuit is a sort of a loose tenon of some sort,(Thank you,
Queeeekdraw) but is not "fitted" precisely - it ONLY becomes a joint when
wetted / swollen with glue so, unlike a pure loose tenon, it is still a
(reinforced) butt joint if you follow my logic.
Now a pocket hole....
Generally speaking edge grain to edge grain with proper surface prep
and clamping using any modern glue of the sorts commonly used in
woodworking will be stronger against intermittent loads than the
original wood, i.e. the break will be other than in the glue line.
End grain to anything in general the break will be between the glue
and the end grain and will occur at a much lower level of stress than
for edge grain to edge grain.
This is why a distinction is made between the two kinds of joint.
Now, for edge grain, biscuits bring two things to the party. The
first is that if the slots are accurately cut then they serve as
positioning aids. The second is that under steady shear load many
glues will creep and fail at a stress level much lower than would be
the case for a load applied for a short time--the biscuit serves in
that case to provide a mechanical obstacle to creep.
For end grain they function as small loose tenons and can add
significantly to the strength of a joint that needs all the help it
Try gluing a joint with only glue on the biscuits, that will show you the
strength of the biscuit. The biscuit does indeed add strength to the joint
however most anything strong enough to break a joint with out a biscuit will
break the joint with a biscuit. Biscuits are more helpful for end grain
gluing and miter joints. They also some what help with alignment providing
the cutter produces a consistent width slot and the biscuits are of uniform
thickness. The biggest misconception of the strength of the biscuit is that
it is often used for the wrong application.
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