Cams are usually irregular revolving inclined planes. Figure 21, shows a cylinder or drum cam. A groove is cut in the cylindrical wall of the cam and an arm or lever is provided with a roller which rolls in the groove. When the cam is revolved the lever is constrained to follow all the twists and turns of the groove. A different form of cam is shown in Figure 22. It is formed with an irregular periphery against which the roller is pressed by a spring. As the cam wheel revolves, the roller and the arm to which it is attached must move in and out over all the hills and valleys of the periphery. The cam is one of the most useful elements in modern machinery, for it provides a very simple means of producing the most complicated and irregular motions.
FIG. 23.—ECCENTRIC BY WHICH ROTARY MOTION IS CONVERTED INTO RECTILINEAR MOTION
We cannot attempt to describe all the different types of cams, but reference should be made to the eccentric, which is a form of cam commonly used to operate the valves of a steam engine. The cam in this case is a perfectly circular disk, but the shaft that turns it does not lie at the center of the disk, consequently an object bearing against the periphery must move toward and away from the center as the disk revolves. Instead of using a spring-pressed roller to bear against one side of the disk, the whole disk is encircled with a ring of steel known as an eccentric strap. This strap is bolted to a valve rod and as the eccentric revolves the strap makes the valve rod move back and forth. (See Figure 23.)
A description of all the various combinations of gearing, link motions, ratchets, escapements, clutches, and miscellaneous movements would easily fill the rest of this book, and we must therefore content ourselves with this very brief survey of a few of the more important elements employed in the construction of modern machinery.
CHAPTER III
MACHINES FOR MAKING MACHINES
WHILE we may glory in the wonderful mechanical progress of to-day, we must not overlook the marvelous skill of the ancient artisan nor forget that it is to his inventive genius that we are indebted for practically every hand tool we possess. Only a few special tools owe their origin to the modern inventor. All the rest date back beyond the twilight of history. We have merely improved upon these tools by slight changes of design or the employment of better materials in their construction.