The Technique of FURNITURE MAKING

Bending properties

Although plywood is normally used as a flat material, its natural resilience enables it to be bent to reasonably small radii of curvature without fracturing. The safe minimum of curvature will depend on the overall thickness, the individual thickness of the veneers, the species of timber employed and, to a lesser extent, the nature of the adhesive, while three - layer plywoods composed of equal veneers will bend more easily than plywoods composed of thick cores and thin outer veneers. It is always better, therefore, to make up thick curved panels out of several thin plies rather than one thick ply, as the following table shows:

Radius of curvature

Thickness Along grain Across grain

7/4in(6.3mm) 2ft0in(0.609m) lft3 in (0.381m)

3/8in(9.5mm) 4ft6in(1.371m) 3 ft Oin (0.914 m)

1/2 in (12.7 mm) 8ft Oin (2.438 m) 6 ft Oin (1.828 m) 5/8 in (15.8mm) 10 ft Oin (3.048 m) 8 ft Oin (2.438 m)

3/4in(19.0mm) 12 ft Oin (3.657 m) lOftOin (3.048m)

The above figures were prepared by the Forest Products Research Laboratory in connection with the use of Douglas fir plywood, but also offer a reliable guide as to the bending movements of most other hardwoods bent dry at ordinary room temperatures. Synthetic resin bonded plywoods can also be soaked or steamed, and will then bend to smaller radii dependent upon the wood species. As will be seen from the table above the bending movement is greater across the grain than along the grain, therefore cross-grained panels should be chosen (see below). Plywood can also be scarf jointed provided the same type of adhesive be used, and the inclination of the joint is not less than 1 in 10 for boards under 1/2 in (12.5 mm) thick and 1 in 8 for over 1/2 in (12.5 mm).