CHAPTER XXX 

 IRON AND IRON MOLDING 



410. The Value of the Metal Trades. Before designing 

 and building a machine, it is necessary to know what the 

 machine is expected to do, and the strain under which the 

 whole machine and each part will be expected to operate. 

 Given this information, the designer is able to draw, and the 

 machinist to build, a machine that will not break. Machines 

 are made of different kinds and grades of metal. The kind of 

 metal selected depends upon its strength, ease of manufacture, 

 its price, and its resistance to the rubbing of other metals. 

 Cast iron is suitable for large, intricate shapes. Brass and 

 bronze are used for small castings, because they are easier to 

 mold. 



As cast iron is exceedingly brittle, it cannot be used in some 

 cases; therefore parts of wrought iron and steel are substi- 

 tuted. Shape is given to these two metals in the machine- 

 shop by hammering and pressing them when hot, and the 

 parts thus shaped are called forgings. It is necessary, then, 

 to know the properties of the different metals* and how they 

 are made and finished before a machine can be designed. 

 Before the parts of the machine or contrivance are cast, a 

 pattern is usually prepared of the same shape. The pattern- 

 maker builds the model from a drawing made by a designer 

 or draftsman. 



* The student should review Chapters XI, XII, and XIII at this 

 point. 



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