Machine tool, any stationary power-driven machine that is used to shape or form parts made of metal or other materials. The shaping is accomplished in four general ways: (1) by cutting excess material in the form of chips from the part; (2) by shearing the material; (3) by squeezing metallic parts to the desired shape; and (4) by applying electricity, ultrasound, or corrosive chemicals to the material. The fourth category covers modern machine tools and processes for machining ultrahard metals not machinable by older methods.
Machine tools that form parts by removing metal chips from a workpiece include lathes, shapers and planers, drilling machines, milling machines, grinders, and power saws. The cold forming of metal parts, such as cooking utensils, automobile bodies, and similar items, is done on punch presses, while the hot forming of white-hot blanks into appropriately shaped dies is done on forging presses.
Modern machine tools cut or form parts to tolerances of plus or minus one ten-thousandth of an inch (0.0025 millimetre). In special applications, precision lapping machines can produce parts that are within plus or minus two millionths of an inch (0.00005 millimetre). Because of the precise dimensional requirements of the parts and the heavy cutting forces exerted on the cutting tool, machine tools combine weight and rigidity with delicate accuracy.
Before the Industrial Revolution of the 18th century, hand tools were used to cut and shape materials for the production of goods such as cooking utensils, wagons, ships, furniture, and other products. After the advent of the steam engine, material goods were produced by power-driven machines that could only be manufactured by machine tools. Machine tools (capable of producing dimensionally accurate parts in large quantities) and jigs and fixtures (for holding the work and guiding the tool) were the indispensable innovations that made mass production and interchangeable parts realities in the 19th century.
By the end of the 19th century a complete revolution had taken place in the working and shaping of metals that created the basis for mass production and an industrialized society. The 20th century has witnessed the introduction of numerous refinements of machine tools, such as multiple-point cutters for milling machines, the development of automated operations governed by electronic and fluid-control systems, and nonconventional techniques, such as electrochemical and ultrasonic machining. Yet even today the basic machine tools remain largely the legacy of the 19th century.
All machine tools must provide work-holding and tool-holding devices and means for accurately controlling the depth of the cut. The relative motion between the cutting edge of the tool and the work is called the cutting speed; the speed in which uncut material is brought into contact with the tool is called the feed motion. Means must be provided for varying both.
Because an overheated tool may lose its cutting ability, temperatures must be controlled. The amount of heat that is generated depends on the shearing force and the cutting speed. Because the shearing force varies with the material being cut and the tool material varies in its tolerance for high temperatures, the optimum cutting speed depends both on the material being cut and the cutting-tool material. It is also influenced by the rigidity of the machine, the shape of the workpiece, and the depth of the cut.