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post #1 of 33 (permalink) Old 01-16-2015, 05:54 PM Thread Starter
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Default Plywood movement/shrinkage

Question for the timber brainiacs. How come plywood is so stable? With solid timber we take a lot of care to account for shrinkage and movement when making tables, particularly when the grain for two pieces runs at right angles to each other. Plywood sheets are larger than most table tops and the individual plys are intentionally glued at right angles to each other, yet plywood is stable. How come the movement between the different layers doesn't break the glue?

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post #2 of 33 (permalink) Old 01-16-2015, 06:21 PM
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Plywood is constructed of multiple layers of thin veneers glued up with alternating grain direction. This process results in an extremely stable material that does not experience the same drastic changes in dimension as solid lumber, which in turn, allows for extremely stable assemblies that will last for years.

This would have been the week that I'd have finished chewing thru the restraints...
If only new layers hadn't been added....

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post #3 of 33 (permalink) Old 01-16-2015, 06:47 PM Thread Starter
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Plywood is constructed of multiple layers of thin veneers glued up with alternating grain direction. This process results in an extremely stable material that does not experience the same drastic changes in dimension as solid lumber, which in turn, allows for extremely stable assemblies that will last for years.
Yes, Stick, I know it does, but my question is why the alternating layers don't work against each other as they do in other joinery?

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post #4 of 33 (permalink) Old 01-16-2015, 06:56 PM
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Perhaps one reason is that the veneer is taken off the log rotationally, ie shaved off along the circumference(?). Like string it has strength in tension along its length, but not perpendicular to the fibres.
Just thinkin' out loud here...
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post #5 of 33 (permalink) Old 01-16-2015, 07:04 PM
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Also it is compressed with a lot of glue so it is not as affected by moisture like solid wood is.
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post #6 of 33 (permalink) Old 01-16-2015, 07:09 PM
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Yes, Stick, I know it does, but my question is why the alternating layers don't work against each other as they do in other joinery?
The alternating layers work with each other to stabilize.you mention joinery,think of a lap joint,two opposite grains ,nothing but glue but a extremely strong joint and the glue does not break
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post #7 of 33 (permalink) Old 01-16-2015, 08:27 PM
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Possibly because the layers are so thin the wood is able to stretch a little bit before it fractures? Also, the individual plies are very dry when glued up. They are unlikely to shrink but might swell a bit.

Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.
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post #8 of 33 (permalink) Old 01-16-2015, 10:08 PM
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Yes, Stick, I know it does, but my question is why the alternating layers don't work against each other as they do in other joinery?
between the glue and the thin layers lacking the mass to put up much of fight...

think...
the more layers (thinner veneer) the more stable...
any possible movement of a layer is canceled or kept in check by an opposing layer...
tug of war???

This would have been the week that I'd have finished chewing thru the restraints...
If only new layers hadn't been added....

Stick....
Forget the primal scream, just ROAR!!!
"SNORK Mountain Congressional Library and Taxidermy”
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post #9 of 33 (permalink) Old 01-16-2015, 11:42 PM
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Anybody ever look at a section of wood with and cross grain through a microscope? It looks like you have a handful of straws. Them babies suck up water even as condensation. Seal them off with a finish and you still have off gassing with nowhere to go when the temp rises.

Most plywood except for CDX has a snazzy veneer but the insides are often pressed (often unevenly) shavings flooded with glue. Underlayment is without voids in the layers to prevent high heels from puncturing it and severely denting or ruining vinyl flooring. The tube thingies swell with fluid, overcoming the surface tension of the water molecule. The plys thin 1/16" crossed layers cancel each others expansion out.

Ever notice the rotted bottoms of older sidewall shingles that run up a cheek? Move them a 1/4" above the roof shingles and you can add a decade or more to the shingle.

Through a scope

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post #10 of 33 (permalink) Old 01-17-2015, 01:07 AM Thread Starter
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between the glue and the thin layers lacking the mass to put up much of fight...

think...
the more layers (thinner veneer) the more stable...
any possible movement of a layer is canceled or kept in check by an opposing layer...
tug of war???
Thanks. So the thinner the layers the more stable.
Darryl

Providing Web and IT services through my own business (Darbeth) by day, and by night , saving my customer's sanity.

Escaping to the workshop to create things out of slabs of wood by day or by night , to save my own sanity.
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