In Machine Drawing, Part I, working shop drawings have been analyzed in detail, systematic processes for making them have been outlined, and numerous illustrations given and thoroughly discussed. The student as yet, however, has not been shown how to originate the theoretical outlines of the surfaces controlling motion in machines; and it is the purpose of Part II to accomplish this.
The theoretical shape of the working surfaces of a machine can be studied and developed to best advantage without any consideration as to their strength or their ability to perform work or withstand service. Such study is a study of the mechanism of a machine, and must always precede the study of design to provide the proper strength.
A mechanism, therefore, is a combination of parts so formed and connected as to produce a desired motion, but not necessarily to perform any specific work.
A machine is a working mechanism, or a combination of mechanisms, suitably designed for the performance of specific work.
Mechanism drawing is really the first step in machine design; and all the familiar parts of machines, such as springs, screws, cams, pulleys and belts, gears, etc., are dependent for their existing practical form upon their theoretical layouts as mechanisms, involving exact mathematical principles. The student should pursue carefully the study of motion as applied to the development of the common machine parts, as this study is fundamental to the advanced work which follows it.