--
'
"Guess who" wrote in message
...
On Thu, 3 Mar 2005 14:33:26 -0800, "J" wrote:


What about the cross section of two dowels in the same space a biscuit
would
occupy?

Do the math.
Cross section of dowel = PI x r^2
Cross section of biscuit = length x thickness (#20 is 2.375 x 0.147)
So it would take three 3/8" diameter dowels to match the shear strength
of
a #20 biscuit or two 1/2" dowels.
Assuming the same strength of materials.

Do more math: The dowel has the same cross-section throughout the
penetration into both pieces of wood, and the biscuit cross-section
lessens away from the joint.

Think about this a minute. You know how a pair of scissors (shears) cut
paper? One side goes one way the other goes the other way... that is what
shear is. Now you are telling us that because the dowel is longer it is more
resistant to shear? If that were true cutting a 1" wide strip of paper
across the width would be more difficult as the length of the paper gets
longer. I'm sure that you can prove to yourself that this is false.

There is no shear force at the base of the biscuit or the root of the dowel
unless the wood it is embedded in is split.


And, with today's glues the joint will
be the last to go. That makes the dowels the winners.

Actually no.

Also there is
no comparison with strength of materials; a solid hardwood dowel wins
hands down over pressed paper.

Biscuits (at least all that I've ever seen) are compressed hardwood. I
wouldn't use a paper biscuit.

We can theorise all we want until there is solid scientific
[experimental results] evidence one way or another to back up "common
sense".

Actually theory is quite useful in predicting the results. This is how the
entire profession of engineering works.
Calculation of shear is something that is well defined and done everyday.
There is no doubt that a #20 biscuit has more resistance to shear than a
3/8" dowel.

The point is that if you drop a ton weight on it, it won't
matter either way, and barring that, either is "strong enough", and
the important factor is ease and accuracy of assembly and cost, not
strength.

That may be your point. My point was that it is not a good idea to give
false information. What you have stated is false. Biscuits are not made of
paper. Resistance to shear is proportional to cross sectional area and a #20
biscuit has about the same cross sectional area as a 2/3" dowel.

-j