WOOD 



61 



boards would have curled up or warped, as shown in 

 Fig. 70. 



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 warp- 

 ing and shrinkage, remembering 

 that the shrinkage is only across 

 the grain. 



Let us consider the problem of constructing a draw- 

 ing 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 per- 

 fectly 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. 



FIG. 76. Showing Effect 

 of Warping 



