52 BULLETIN" 1500, U. S. DEPARTMENT OF AGRICULTURE 
The schedules recommended in Table 8 apply where maximum 
strength of joints is of chief importance and may be in some cases 
stricter than required. 
In preparing the above schedules no attempt was made to list all 
the combinations of conditions which give good results or to cover 
all the conditions which exist in factory operations. For example, 
the temperature of the wood may in summer be as high as 80° or 85° 
F., and at the same time the character of the gluing operation may 
require an assembly time as short as one-half minute. In gluing 
such a species as birch with animal glue, other conditions shown in 
Schedule A3 must then be changed. In this case, a glue, which is 
one grade higher in jelly strength, mixed 2 parts water to 1 part glue 
(footnotes 1 and 4, Table 7), and a pressure of 100 pounds per square 
inch will give satisfactory results. 
Necessary prerequisites in all the schedules recommended are 
properly dried and machined wood, glue spreaders which spread the 
glue evenly, and presses that apply pressure uniformly over the joint. 
PRINCIPLES OF GLUED-WOOD CONSTRUCTION 
CROSS-BANDED CONSTRUCTION 
Cross-banded construction comprises a large class of products con- 
sisting of two or more layers of wood glued up with the grain of one 
or more layers at an angle (usually 90°) with the others. Plywood 
is a term generally used to designate a cross-banded construction 
where all plies are thin. The term veneered panels generally refers 
to a cross-banded construction with a core of lumber and with one 
or more layers of veneer on each face. Several types of plywood and 
veneered panels are shown in Figure 3. 
Plywood and veneered panels are usually three or five ply. In a 
3-ply construction the two outside plies are called faces and are 
usually laid at right angles to the grain of the center ply or core. 
(Fig. 3, A and B.) In 5-ply panels the outer plies are again called 
faces, or face and back, the second and fourth plies are the cross- 
bands, and the center ply is the core. (Fig. 3, C and D.) In a 5-ply 
construction the grain of the cross bands is usually at right angles to 
the grain of the face, back, and core. Plywood construction other 
than three or five ply may be used, but an odd number of plies is 
generally emploj^ed. (Fig. 3, E and F.) The core, over which the 
face and cross-banding veneers are laid, may be of veneer, of lumber, 
or of various combinations of veneer or lumber as shown in Figure 3, 
A, B, I, and J. Panels may range in total thickness from less than 
one-sixteenth of an inch to more than 3 inches. They may vary as 
to shape, number, and thickness of the different plies, and as to the 
kinds and the combinations of woods used. 
As compared with solid wood, the chief advantages of cross-banded 
construction are marked resistance to checking and splitting ; greater 
uniformity in strength properties with the width and length of the 
panels; less change in dimensions under changes of moisture content; 
and, in the case of properly constructed panels, less liability to 
warp (18). 
A piece of lumber or veneer will withstand a greater stress along 
the grain than across the grain. The strength along the grain is 
