Re: Getting warp out of plywood?


"Tom Nakashima" <tom@xxxxxxxxxxxxxxxxx> wrote in message
news:droe53$p3i$1@xxxxxxxxxxxxxxxxxxxx
>

> More Data:
> http://home.comcast.net/~tomnak/plywood_002.jpg
> -tom

Hm.....

O.k., so it looks like 5 foot squares, and there doesn't appear to be any
intrinsic reason to be concerned about a couple extra inches of height. I
think torsion box construction is probably your best bet.

Scrap the old cover and start over. Design to avoid piecing plywood
together edge to edge....it's inherently weak. Build whatever is most
convenient and/or conservative of materials and then connect the individual
boxes (if necessary) in whatever way makes them easiest to handle.

A torsion box is simply two sheets of plywood spaced apart from one another
and held together by a set of rails, one at each of the edges, and others at
regular intervals along the greatest dimension between the outside pair.
Since this is just a cover, strength is not much of an issue. All you need
to do is ensure that it won't warp, and this type of construction is ideally
suited to prevent that. You can use 1/4 plywood, or even lighter if you can
find it. As a matter of fact, lighter is better. Constraining thicker
wood....whether solid or laminated....is harder to do. To build, first cut
all your pieces to size (and carefully check for fit) and then lay out the
positions for the rails on the inner surfaces of both sheets of
plywood.....on the top surface of the bottom piece and vice versa. Lay a
liberal glue line (neatness doesn't count.....it all ends up on the inside)
where each of the rails goes (I'd make them about 3/4 inch thick, 3 inches
high, and space them about 10 inches apart for something like this) on one
of the plywood pieces. Put the rails down, clamp the ends, and turn the
whole mess over. Drive light gauge finish nails through the skin at
intervals of about 4 inches. Turn it over again. Lay down glue on the
exposed edges of the rails, lay on the other skin, and repeat nailing. Nail
heads can be set and puttied if desired.

The beauty of torsion box construction is that it is fast, easy, cheap, and
amazingly strong. Any stresses applied to the box (including a tendency to
warp) are converted to shear stress along the glue lines.....which also
happens to be where their greatest strength lies.

Wolfgang


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