Machines may be divided into the two classes: (1) rotating machines, such as lathes, boring mills, drills, presses, milling and grinding machines; and (2) reciprocating machines, such as slotters, planers, and shapers.

The speed of machines of the first class is constant throughout each operation, and most of their energy is used to overcome the friction of boring, grinding, or cutting metal, the amount of power consumed by the friction of the machine itself being comparatively small. In machines of the second class, the power is subject to great fluctuations because, in addition to the friction of the work, friction and loss of energy are caused by the retardation, return, and acceleration of the machine. The work of shapers, slotters, and other machines, is done by the stroke of a reciprocating tool in one direction During the working stroke the tool must move at a suitable cutting speed, while on the return stroke, when no work is performed, it is desirable that it should travel as rapidly as possible.