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 substituted. 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 patternmaker builds the model from a drawing made by a designer or draftsman.
* The student should review Chapters XI, XII, and XIII at this point.