A lathe, which is a machine used to make parts for other machines, is considered the oldest machine tool. This ability to reproduce machine parts reliably was one of the enabling technologies of the Industrial Revolution. One of the keys to this reproducibility is the control of the speed of the lathe. Essentially all lathes must have a means to vary the spindle speed. A variable speed lathe refers to those lathes that have a modern and precise means to control the spindle speed.
A spindle is the device that holds the piece to be worked. It is the axis around which the piece rotates. Spindle speed is critical because it affects the torque or power applied to the cutting action. Low speed generally applies more torque. The revolutions per minute (rpm) is one measure of spindle speed.
The cutting speed is related to the spindle rpm but is also a factor of the diameter of the piece of work. A larger-diameter piece will have a greater cutting speed, as a longer cutting pathway has been traveled in one revolution of the piece. The hardness of the material is also a consideration. Harder materials require more cutting force and thus lower speeds. Because of the need for greatly differing speeds, a variable speed lathe will be specified for a specific type of material, such as wood or metal.
Lathes powered by foot, such as a potter’s wheel, control speed by the frequency and force upon the foot pump. Small hobby lathes may use a simple three-step motor and may limit speed settings to a few finite levels. An earlier industrial variable speed lathe used a series of belts and pulleys, including conical pulleys, to vary spindle speed.
Variable speeds in direct-drive motors are achieved by uses of gears or pulleys. This type of a variable speed lathe will typically have spindle speeds that are multiples of each other, such as eight, 12, 16, and 24 times the base rpm. By changing pulleys, more speed options are available. Gears and pulley may be set by hand, or more often, by dials and levers similar to the mechanism of a 10-speed or 21-speed bicycle gear changer.
An infinite variety of speeds are available by the use of alternating current (AC) motors with infinitely variable speed transmissions or newer electronic controls. The electronic variable speed control is often combined with a variable speed motor to increase the operating range. A variable speed lathe incorporating the electronic controls will often be integrated with the ability to maintain constant cutting speed as the diameter of the piece changes, either as it is carved or along its length.