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Wood joint strength testing
I had
previously
tested amortise
and tenon
joint
against a
dowel joint
for
strength.
In that test
the
mortiseand tenon
joint won.
But
questions
remained
- how
much
stronger is one joint vs. the other, and is it possible to improve on the dowel
joint?
So I set out to do some more thorough experiments, this time using more
samples and actually measuring the breaking force of the joints.
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Building the test stand
I needed some sort of sturdy test fixture that would allow me to slowly apply a
measured amount of force to the joint. At left, you can see the test stand I came
up with. It consists of an L-shaped frame that the joint under test is clamped to, abathroom scale, and a hydraulic jack.
Like just
about
everything
else, I
made thistest stand
out of
wood.
With the
frame
made out
of wood, I
had to
make sure
that it
would be
stronger
than any
joint I
might try
to break with it. Nevertheless, I still used spruce, seeing that it's so much
cheaper. I compensated for the softer wood by joining it with a really massive
triple tenon joint.
The triple tenon joint is about 6 cm deep. This meant that I couldn't use my screw
advance box joint jig to cut the tenons. I also couldn't use my quick-set tenon jig,because that jig doesn't move far enough side to side. My crummy delta tenon jig
has even less range of motion. So I ended up pulling out my old tenon jig which I
built more than 10 years ago. This jig, placed on the right side of the saw had just
barely enough stroke to cut this massive joint with. The adjustment screw on it is
a 10 turn per inch acme screw. That came in handy for spacing the 1/2" wide
tenons at exactly one per inch.
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I cut the
triplemortises
with an
endmill in
my
horizontal
boring
machine.
The
endmill
cutsdeeper
than most
router bits,
but it
works
better at
lower speeds. So I just used my horizontal boring machine as a slot mortiser to
cut the mortises, and then chiselled the ends of the mortises square. Each
mortise is 1/2" (13 mm) wide, 6 cm deep, and 8 cm long. The pencil gives a bit of
a sense of scale.
After gluing it together, I glued another piece to the
backside of the joint where it pushes together to give
it an extra support.
Making joints to test
I decided to standardize the size of my joints to be a 6 x 1.7 cm rail attached to apost 3.5 x 3.5 cm in size. This is close to the size of joint one might use putting
together a chair or a stool. It's that sort of joint that gets stressed a lot - so much
so that most store-bought wooden chairs will come apart at the joints eventually.
I used yellow carpenters glue for all of my tests. For the wood, I used spruce and
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then retested with maple.
My
favourite
is of
course the
mortise
and tenon
joint. I
made all
my tenons
3/8" (10mm) thick,
27 mm
deep, and
55 mm
wide.
I made a
number of mortise and tenon joints, ranging from tight enough that I had to drive
it in with a hammer, to some being loose by a few thou, so that the dry fitted joint
would still have a tiny bit of play. More about mortise and tenon joint fit and
accuracy
I bought a
bag of
these pre-
cut
dowels, 2"
long, and3/8" in
diameter
(10 x 51
mm).
They have
ridges,
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which I figure should help with gluing. They are also supposed to be pre-
compressed, and re-expand when glue is applied, for a better fit. I figured these
dowels should really give the dowel joints a better chance.
I also
went for
three
dowels,
instead of
two. The
spacing of
thesedowels is
at 3/4" (18
mm). I
used my
horizontal
boring
machine,
with its
indexing
function to
exactly space my holes.
In terms of
strict
wood
joints
joining a
rail to apost, I
couldn't
think of
anything
else other
than
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maybe to use biscuit joints. But I don't have a biscuit jointer. Besides, if you cared
about strength, you wouldn't be using biscuit joint.
I also decided to test several types of screw joints. I rigged up an inclined plane
on my horizontal boring machine, which would allow me to mount the work pieceat an angle for drilling pocket holes.
Here's
one of my
hole
joints.
This one I
fastened
with 2"
drywall
screws.
I also
tested a
joint with
the
screws
through
the post
and into
the
endgrainof the rail.
I used
three 3.6"
(80 mm)
#8 wood
screws, screwed through the post, and straight into the end grain of the spruce. I
drilled the pilot holes quite small, so that the thread could really bite into the
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