a panel has its edges fastened in place rather than being simply confined, it
may show no ill effects during the humid/swelling phase of the cycle but
may crack open when redried to its original moisture condition.
The classic experiment shown diagrammatically in Figure 12
demonstrates the typical extreme consequences of restrained swelling and
compression shrinkage in panels.Wa r p
Although dimensional change alone may be a serious consequence of
moisture variation, even minor amounts of uneven shrinkage or swelling
can cause warp, defined broadly as the distortion of a piece from its
desired or intended shape. Various forms of warp include cup(deviation
from flatness across the width of a board), bow(deviation from lengthwise
flatness of a board), crook(departure from end-to-end straightness along
the edge of a board), and twist(in which four corners of a flat face do not
lie in the same plane).
Inpainting panels, cupping is perhaps the most commonly
encountered form of warp and can result from a variety of causes, singly
or in combination. Uneven moisture change in opposite faces of a panel
may cause a slight, and usually temporary, cupping concave to the drier
face, which may disappear as moisture equalizes through the thickness of
the panel. Growth-ring placement within a board is an important factor in
the determination of cupping potential. Quarter-sawn (radially cut) boards
tend to remain flat as MC changes. Flat-sawn (tangentially cut) boards,
however, routinely cup, or attempt to cup, as they season (Fig. 13).
Cupping results from the different components oftangential and radial
gr ain orientation across opposite faces of the boards. Panels fashioned
from flat-sawn boards will tend to cup additionally as MC varies. IfC P P W 19A B C A B C A B C6 % 1 8% 6 %Figure 12
Classic experimental demonstration ofthe
effects ofrestrained swelling and compression
shrinkage on elements ofwood representing
panels. At an initially low MC, three matched
specimens are machined to equal tangential
dimension. Element A is restrained and fas-
tened to rigid restraining surfaces at both
edges, element B is restrained but fastened
only at one edge, and element C is unre-
strained and fastened only at one edge. All ele-
ments are slowly conditioned to a high MC
and then brought back to the original lowMC.
The restrained specimens, A and B, show typi-
cal consequences ofcompression shrinkage.
Figure 13
Warp in flat-sawn (tangentially cut) boards
during seasoning. The severity and distribu-
tion of cupping is related to the location in
the log and the resulting curvature of growth
rings, as well as to the tangential/radial ratio
of shrinkage percentages. Note that flat-sawn
boards located closest to the pith have the
most severe cup, concentrated near the center.
Panels that are held flat may crack if the nor-
mal cupping is prevented.