WOOD 



61 



-^^ul 



boards would have curled up or warped, as shown in 

 Fig. 76. 



Besides warping, the evaporation of the sap causes 

 the whole tree to shrink in diameter, aud consequently 

 our planks will tend to becijme narrower. This is 

 called shrmkage, and in some woods amounts to a 

 ciuaiier 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 l^e affected by warp- 

 ins; and shrinkage, rememberino- 

 that the shrinkage is only across 

 the grain. 



Let us consider the problem of constructing a draw- 

 ing board to see how warping aud shrinkage may be 

 overcome. 



If we make it of one piece, like A (Fig. 77 1, the board 

 will soon chai;ige 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 hj 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. 



Fk; 



76. Showing Effect 

 of Warping 



