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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.

http://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.htmlhttp://g/mortise/strong.html

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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.

http://g/tenon/jig.htmlhttp://g/tenon/review.htmlhttp://g/table_saw/tenon_jig.htmlhttp://g/tenon/jig.htmlhttp://g/tenon/review.htmlhttp://g/table_saw/tenon_jig.html

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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

http://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/kitchen_chairs/index.htmlhttp://g/stool/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/kitchen_chairs/index.htmlhttp://g/stool/index.html

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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,

http://g/mortise/accuracy.htmlhttp://g/mortise/accuracy.htmlhttp://g/mortise/accuracy.htmlhttp://g/mortise/accuracy.html

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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

http://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/machine2.html#indexinghttp://g/horizontal_boring/machine2.html#indexinghttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/index.htmlhttp://g/horizontal_boring/machine2.html#indexinghttp://g/horizontal_boring/machine2.html#indexinghttp://g/horizontal_boring/machine2.html#indexing

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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

pocket

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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