Re: ATM Plywood plys

[Aart Olsen <aolsen@prairienet.org>]

Plywood is always made symmetrical so that it won't warp.  This makes
the stiffness somewhat non-isotropic but warping is considered the more
serious problem.  If you need to double the thickness of plywood and
want to minimize warpage you should glue the pieces parallel to each
other.  To triple the thickness you can rotate the center piece 90
degrees from the (parallel) outer pieces.  I don't understand why a four
layer sandwich should have each layer 45 degrees from its neighbor; it
seems to me 60 degrees apart would be better.  Perhaps in boat building
they want more stiffness in length and can give up some in width.  Five
layers @ 45 degrees makes sense.  If you have other means of controlling
the tendency to warp, e.g. if edges or corners are mechanically
constrained really well, then you can probably alternate the pieces
sequentially instead of symmetrically, to maximize isotropic stiffness 

> Making plys 45 degrees apart (if you have four) from the last will make
> good plywood and is recommended in boat building. I have no experience
> on using it for making mirror cells.
> 
> If, on the other hand, you are contemplating gluing two "pieces" of plywood
> together then your statement has merit. But why would you want the new
> center
> "ply" to be two thicknesses when turning the second sheet 90 degrees would
> continue the
> alternating layers but make the piece twice as thick?
> 
> Frank Ward
> Atlanta, GA

-- 
Aart Olsen
aolsen@prairienet.org