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.
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
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