Plywood, in one form or another, has been around for millennia. No, not that crap you buy at the big box store, but a real shop-made product that’s (relatively) dimensionally stable and improves your woodworking and extends the life of your projects.
You may be asking, “Why is that stuff from the big box store so horrible?” And the answer is, much of the mass-produced plywood produced today has core (and often face-veneer) problems. The 20th century has been hard on veneer. Although we’re talking more about its reputation, the introduction of central heat is the primary cause of its glorious decline. Because veneer is an integral part of plywood, its reputation too has suffered. It all has to do with wood movement.
Most woodworkers are aware that wood is made up of tubular shaped fibers. These fibers are added to the tree each spring and summer in groupings called annual rings. Wood is hygroscopic in nature, which means it absorbs and releases moisture depending on the humidity of its environment. When it absorbs moisture it expands, and when it releases it, it shrinks – pretty basic stuff. (Fig. 1)
All the expansion and contraction occurs around the circumference of the rings, which is why flat-sawn lumber expands and contracts across its width. The the fibers don’t expand or contract over their length nearly as much as in diameter. This is why boards don’t tend to change dimensionally over their length.
So, what does all this have to do with plywood? Pretty much everything we do in woodworking revolves around taking what we know about the weaknesses of wood and putting it to work to our advantage. A plane (hand or power) takes advantage of tendency for the fibers to remain intact over their length by splitting them apart in a controlled manner to create a flat, smooth surface.