Fig. 75.
After a tree has been cut down the cut end at first looks like [Fig. 72]. If it is allowed to lie for some time exposed to the weather, its appearance changes to [Fig. 74]. This is due to the evaporation of the sap, and as there is more sap toward the outside, the shrinkage is greatest there and becomes less toward the center where the heartwood is comparatively dry. This is an important fact to know, because if we had cut the log, while it was still green, into planks, as shown in [Fig. 75], the boards would have curled up or warped, as shown in [Fig. 76].
Fig. 76. Showing Effect of Warping
Besides warping, the evaporation of the sap causes the whole tree to shrink in diameter, and consequently our planks will tend to become narrower. This is called shrinkage, and in some woods amounts to a quarter of an inch to the foot, which means that a plank sawed twelve inches wide will, after a few months, measure only eleven and three quarter inches.
When we construct anything in wood we must always consider how the object will be affected by warping and shrinkage, remembering that the shrinkage is only across the grain.
Let us consider the problem of constructing a drawing board to see how warping and shrinkage may be overcome.
If we make it of one piece, like A ([Fig. 77]), the board will soon change its shape to that shown in B, which would make it useless for mechanical drawing, as a perfectly flat surface is necessary. We can overcome the warping by screwing heavy cleats on one side across the grain, as shown at C. The cleats would need to be heavy or the warping force would bend them.