ENCYCLOPEDIA OF WOOD

PLYWOOD

lthough it may not be as glamorous or as steeped in woodworking tra­dition as solid lumber, plywood offers several advantages to the cabinetmaker. First, it comes in a wide range of stan­dard thicknesses and sizes. Second, it is dimensionally stable and is unlikely to warp or show signs of checking or split­ting. Third, it is available with just about any commonly available veneer on its faces. And fourth, it is easy to cut. Indeed, plywood is a good choice for almost any design that does not involve intricate joinery such as dovetails.

Although veneer has a venerable history, and plywood is a relatively mod­ern development—first produced com­mercially in the mid-1800s—the two are closely related. Plywood, after all, is a layered wood material made from thin sheets, or plies, of veneer. Decorative plywood is often faced with matched
veneers made from high-grade hard­woods such as cherry or walnut. The veneer used in construction-grade plywood is peeled on a rotary lathe from eight-foot-long logs of poplar, pine or Douglas-fir.

As shown opposite, both decorative and construction-grade plywood are manufactured with an odd number of plies, giving the sheet a balanced con­struction. Three plies are usually the minimum number. Beneath the face and back veneers of a typical sheet are layers known as crossbands. The grain of each crossband runs at right angles to that of adjacent plies to counter wood move­ment. The result is a warp-resistant board that is equally strong across both dimensions. Some plywoods are also available with reinforced cores.

As with solid lumber, plywood is available in both hardwood and soft­
wood varieties, although the terms refer strictly to the face and back veneers. Hardwood plywood is a stable and cost - effective alternative to solid wood, and is used in woodworking applications where appearance matters, such as for cabinets, drawer fronts and furniture. Softwood plywood is generally used for carcase construction, bookcases and shelving.

Not all plywoods are created alike. More than 70 wood species are used in its manufacture. Plywoods are grouped according to strength and durability; both softwood and hardwood varieties are available in four groups or categories that are usually stamped on the sheet. Group 1 (softwood) and Category A (hardwood) species are the strongest and most durable; Group 4 and Category D are the poorest grades. Refer to the chart (below) for the species that make­up the various groupings.

COMMON WOODS USED IN PLYWOOD CONSTRUCTION

• Western Larch

• Sugar Maple

• Longleaf Pine

• Shortleaf Pine

• Southern Pine

• Tanoak

• Lauan

• Black Maple

• Virginia Pine

• Yellow Poplar

• Black Spruce

• Sitka Spruce

TYPES OF PLYWOOD

The basic design of all plywoods is the same: a core covered on both sides by layers of crossbanding and a face veneer. The most common type has a veneer core. All softwood plywoods are made this way, and they are stable, warp-resistant and inexpensive. Hardwood plywoods can also be made with solid lumber or particleboard cores. The middle ply of lumber-core
plywood consists of several narrow strips of solid wood—usu­ally mahogany, poplar or basswood—edge-glued together. Particleboard-core plywood has a solid core of particleboard or medium-density fiberboard. Lumber-core plywood holds nails and screws best and is preferable when additional strength and flatness are required.