PLYWOOD

Plywood is constructed by gluing together a number of layers (plies) of wood with the grain direction turned at right angles in each successive layer. This design feature makes plywood highly resistant to splitting. It is one of the strongest building materials available to Seabees. An odd number (3, 5, 7) of plies is used so that they will be balanced on either side of a center core and so that the grain of the outside layers runs in the same direction. The outer plies are called faces or face and back. The next layers under these are called crossbands, and the other inside layer or layers are called the core (figure 3-36). A plywood panel made of three layers would consist of two faces and a core.

Figure 3-33.-Stressed skin panel.

Figure 3-34: Scarf joints.

Figure 3-35.-Laminated beam.

Figure 3-36.-Grain direction in a sheet of plywood.

There are two basic types of plywood: exterior and interior. Exterior plywood is bonded with waterproof glues. It can be used for siding, concrete forms, and other constructions where it will be exposed to the weather or excessive moisture. Interior plywood is bonded with glues that are not waterproof. It is used for cabinets and other inside construction where the moisture content of the panels will not exceed 20 percent.

Plywood is made in thicknesses of 1/8 inch to more than 1 inch, with the common sizes being 1/4, 3/8, 1/2, 5/8, and 3/4 inch. A standard panel size is 4-feet wide by 8-feet long. Smaller size panels are available in the hardwoods.

Table 3-7.-Plywood Veneer Grades

Table 3-8.-Classification of Softwood Plywood Rates Species for Strength and Stiffness

Plywood can be worked quickly and easily with common carpentry tools. It holds nails well and normally does not split when nails are driven close to the edges. Finishing plywood presents no unusual problems; it can be sanded or texture coated with a permanent finish or left to weather naturally.

There is probably no other building material as versatile as plywood. It is used for concrete forms, wall and roof sheathing, flooring, box beams, soffits, stressed-skin panels, paneling, shelving, doors, furniture, cabinets, crates, signs, and many other items. Softwood Plywood Grades

All plywood panels are quality graded based on products standards (currently PS 1/74). The grade of each type of plywood is determined by the kind of veneer (N, A, B, C, or D) used for the face and back of the panel and by the type of glue used in construction. The plywood veneer grades are shown in table 3-7.

Many species of softwood are used in making plywood. There are five separate plywood groups based on stiffness and strength. Group 1 includes the stiffest and strongest; group 5 includes the weakest woods. A listing of groupings and associated woods is shown in table 3-8.

Figure 3-37.-Standard plywood identification symbols.

GRADE/TRADEMARK STAMP.- Construction and industrial plywood panels are marked with different stamps.

Construction Panels.- Grading identification stamps (such as those shown in figure 3-37) indicate the kind and type of plywood. The stamps are placed on the back and sometimes on the edges of each sheet of plywood.

For example, a sheet of plywood having the designation "A-C" would have A-grade veneer on the face and C-grade veneer on the back. Grading is also based on the number of defects, such as knotholes, pitch pockets, splits, discolorations, and patches in the face of each panel. Each panel or sheet of plywood has a stamp on the back that gives all the information you need. Table 3-9 lists some uses for constructiongrade plywood.

Industrial Panels.- Structural and sheeting panels have a stamp found on the back. A typical example for an industrial panel grade of plywood is shown in figure 3-38.

The span rating shows a pair of numbers separated by a slash mark (/). The number on the left indicates the maximum recommended span in inches when the plywood is used as roof decking (sheeting). The right-hand number applies to span when the plywood is used as subflooring. The rating applies only when the sheet is placed the long dimension across three or more supports. Generally, the larger the span rating, the greater the stiffness of the panel.

Figure 3-39 lists some typical engineered grades of plywood. Included are descriptions and most common uses.

Figure 3-38.-Structural stamp.

Table 3-9.-Plywood Uses

Figure 3-39.-List of engineered grade of softwood plywood.

Exposure Ratings.- The grade/trademark stamp lists the exposure durability classification for plywood. There are two basic types or ratings: exterior and interim. The exterior type has a 100-percent waterproof glue line, and the interior type has a highly moisture-resistant glue line. However, panels can be manufactured in three exposure durability classifications: Exterior, Exposure 1, and Exposure 2.

Panels marked "Exterior" can be used where there is continual exposure to weather and moisture. Panels marked "Exposure 1" can withstand moisture during extended periods, but they should be used only indoors. Panels marked "Exposure 2" can be used in protected locations. They may be subjected to some water leakage or high humidity but generally should be protected from weather.

Most plywood is made with waterproof exterior glue. However, interior panels may be made with intermediate or interior glue.

Hardwood Plywood Grades

Hardwood plywood panels are primarily used for door skins, cabinets, and wall paneling. The Hardwood Plywood Manufacturers' Association has established a grading system with the following grades: premium (A), good grade (1), sound grade (2), utility grade (3), and backing grade (4). For example, an A-3 grade hardwood plywood would have a premium face and a utility back. A 1-1 grade would have a good face and a good back.

Figure 3-40.-Planing and squaring to dimensions.

Figure 3-41:90 plain butt Joints.

WOODWORKING METHODS LEARNING OBJECTIVE: Upon completing this section, you should be able to identify the various methods and joints associated with woodworking.

In the following section, we will cover some of the methods used by Builders in joining wood.

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