CHAPTER XXXI 

 PROBLEMS IN PATTERN-MAKING 



426. Allowances for Shrinkage. All molten metal shrinks 

 when solidifying. The amount of shrinkage varies in different 

 metals and also in different castings of the same metal. A 

 cylinder, for example, will shrink more in length than in dia- 

 meter, largely because of the resistance of the central core. 

 Therefore, patterns must be made larger than the required 

 castings. The usual allowances for different metals are as 

 follows : 



Cast iron -$% in. per foot Aluminum 3% in. per foot 



Steel A in. " " Zinc 3% in. " " 



Brass i\ in. " " Lead gV in. " " 



Yellow brass uV in. " " Tin A in. " " 



Bronze .-yV in. " " 



When the molten metal cools to the point at which it is 

 about to become solid, it suddenly flashes, expands, and then 

 immediately contracts. The flash is called a " higher heat. " 

 This sudden expansion makes it possible to get impressions 

 of small, fine lines on the casting. 



427. Graduated Shrink Rule. A graduated rule, the 

 purpose of which is to allow for shrinkage, is known as a 

 shrink rule. Such rules are on sale in almost all graduations, 

 and in one, two, and three shrinkages. 



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