54 
BULLETIN" 1500, U. S. DEPARTMENT OF AGRICULTURE 
Corresponding plies should not only exert their forces in the same 
direction, but the forces should be of the same magnitude. The forces 
exerted by two otherwise similar plies are proportional to their 
thickness. There is a practical limit to the allowable thickness of 
the face plies, however, because thick veneers exert larger stress than 
thin plies so that even strong joints may fail when thick face-plies 
are used. This is especially true for the high- density woods. A 
3-ply panel with faces of unequal thickness but otherwise alike will 
become concave on the thicker side when dried. When moist, the 
panel of the same type construction will become convex on the thicker 
side. Unequal sanding of the face plies of a panel produces an effect 
similar to the use of faces of unequal thickness. 
In addition to being correctly spaced from the core, the wood used 
in corresponding plies should be of the same shrinkage and density 
to obtain a balanced effect. The shrinkage of wood varies with the 
species, with the method of cutting, and with the density. In some 
woods there is as large a difference in shrinkage between the quar- 
tered-grain wood and the rotary-cut flat-grained wood of the same 
stock as there is between woods of different species. Consequently, 
flat-grain and quartered material of the same wood may not balance 
each other as closely as is possible with different woods. For the best 
results, therefore, pieces from the same species should be used which 
are of similar density and which are cut in the same manner. 
Table 9 shows the average shrinkage and density of woods com- 
monly used for plywood and veneered panels. Shrinkage data for 
quartered (radial) and rotary-cut (tangential) stock are shown since 
some species are manufactured and used extensively in both forms. 
The table should enable one to select species which have about the 
same percentage of shrinkage and density. Differences in density 
between two wo§ds can theoretically be compensated for by varying 
the thicknesses of the plies in inverse proportion to their specific 
gravities. This method of compensation would result in using a pro- 
portionately thicker ply of the lighter species, which may be advan- 
tageous in some cases. The practice, however, is not common. 
Table 9. — Average shrinkage and density of woods commonly glued 1 
Species 
Mahogany (Swietenia sp.) 
Northern white pine 
Western yellow pine 
Chestnut 
Yellow poplar 
Black cherry 
Black walnut 
Red alder 
Sycamore 
Tupelo gum 
Douglas fir (west coast)... 
Red maple 
Shrinkage 2 
Den- 
Tan- 
gential 
Radial 
sity 3 
Per ct. 
Per ct. 
4.7 
3.4 
0.48 
5.9 
2.2 
.39 
6.4 
3.9 
.41 
6.7 
3.4 
.44 
6.9 
4.1 
.41 
7.1 
3.7 
.51 
7.1 
5.C 
.57 
7.3 
4.4 
.42 
7.6 
5.1 
.50 
7.6 
4.2 
.52 
7.9 
5.0 
.44 
8.1 
3.8 
.54 
Species 
Shortleaf pine 
Red oak 
White ash 
Yellow birch. 
White oak 
Cottonwood.. 
Sugar maple.. 
Bass wood 
American elm 
Red gum 
Beech 
Shrinkage 2 
Tan- 
gential 
Per ct. 
8.2 
8.3 
8.7 
9.0 
9.0 
9.2 
9.2 
9.3 
9.5 
9.9 
10.6 
Radial 
Per ct. 
5.1 
3.9 
5.3 
7.4 
5.3 
3.9 
4.8 
6.6 
4.2 
5.2 
4.8 
Den- 
sity 3 
0.54 
.63 
.64 
.63 
.69 
.43 
1 Data, except mahogany, from U. S. Dept. Agr. Bui. 556 (87). 
2 Shrinkage from green to oven-dry condition expressed in per cent of dimensions when green, 
3 Density expressed as specific gravity based on oven-dry weight and air-dry volume. 
