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The lathe is the oldest machine tool and a key technology of the industrial revolution. Variable speed lathes are important for controlling spindle speed, which affects torque and cutting speed. Different materials require different speeds, and variable speed lathes can be operated by foot, belts and pulleys, or electronic controls.
A lathe, which is a machine used to make parts for other machines, is considered to be the oldest machine tool. This ability to reliably reproduce machine parts was one of the enabling technologies of the industrial revolution. One of the keys to this reproducibility is speed control of the lathe. Essentially all lathes must have some means of varying spindle speed. A variable speed lathe refers to those lathes that have a modern and accurate means of controlling the speed of the spindle.
A chuck is the device that holds the work piece. 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. Revolutions per minute (rpm) is a measure of spindle speed.
The cutting speed is related to the spindle speed but is also a factor of the diameter of the workpiece. A larger diameter workpiece will have a higher cutting speed, because a longer cut path has been traveled in one revolution of the workpiece. The hardness of the material is also to be taken into consideration. Harder materials require more cutting force and therefore lower speeds. Due to the need for widely varying speeds, a variable speed lathe will be specified for a specific type of material, such as wood or metal.
Foot-operated lathes, such as the potter’s wheel, control speed based on the frequency and force on the foot pump. Small hob lathes may use a simple three-phase motor and may limit speed settings to a finite few levels. An earlier variable speed industrial lathe used a series of belts and pulleys, including bevel pulleys, to vary the speed of the spindle.
Variable speeds in direct drive motors are achieved through the use of gears or pulleys. This type of 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 the pulleys, more speed options are available. The gears and pulley may be set by hand, or more often, by dials and levers similar to the mechanism of a 10- or 21-speed bicycle shifter.
An infinite variety of speeds are available through the use of alternating current (AC) motors with infinitely variable speed drives or newer electronic controls. Electronic variable speed control is often combined with a variable speed motor to increase range. A variable speed lathe incorporating electronic controls will often be integrated with the ability to maintain a constant cutting speed as the diameter of the workpiece varies, both during machining and along its length.
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