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Saturday, June 5, 2010

What is Numerical Control ?

Numerical control (NC) is the operation of a machine tool by a series of coded
instructions consisting of numbers, letters of the alphabet, and symbols that the machine control
unit (MCU) can understand. These instructions are changed into electrical pulses of current that
the machine's motors and controls follow to carry out manufacturing operations on a workpiece.
The numbers, letters, and symbols are coded instructions that refer to specific distances,
positions, functions, or motions, that the machine tool can understand as it machines the
workpiece.

History

A form of NC was used in the early days of the industrial revolution, as early as 1725,
when knitting machines in England used punched cards to form various patterns in cloth. Even
earlier than this, rotating drums with prepositioned pins were used to control the chimes in
European cathedrals and some American churches. In 1863, the first player piano was patented;
it used punched paper rolls, through which air passed to automatically control the order in which
the keys were played.

The principle of mass production (interchangeable manufacture), developed by Eli
Whitney, transferred many operations and functions originally performed by skilled artisans to
the machine tool. As better and more precise machine tools were developed, the system of
interchangeable manufacture was quickly adopted by industry in order to produce large
quantities of identical parts. In the second half of the nineteenth century, a wide range of
machine tools were developed for the basic metal-cutting operations, such as turning, drilling,
milling, and grinding. As better hydraulic, pneumatic, and electronic controls were developed,
better control over the movement of machine slides became possible.

NC Evolves

In 1947, the U.S. Air Force found that the complex designs and shapes of aircraft parts
such as helicopter rotor blades and missile components were causing problems for
manufacturers, who could not keep up to projected production schedules. At this time, John
Parsons, of the Parsons Corporation, of Traverse City, Michigan, began experimenting with the
idea of making a machine tool generate a “thru-axis curve" by using numerical data to control
the machine tool motions. In 1949, the U.S. Air Material Command awarded Parsons a contract
to develop NC and in turn speed up production methods. Parsons subcontracted this study to the Servomechanism Laboratory of the Massachusetts Institute of Technology (MIT), which in 1952 successfully demonstrated a vertical spindle Cincinnati Hydrotel, which made parts through simultaneous three-axis cutting tool movements. In a very short period of time, almost all machine tool manufacturers were producing machines with NC.
At the 1960 Machine Tool Show in Chicago, over a hundred NC machines were
displayed. Most of these machines had relatively simple point-to-point positioning, but the
principle of NC was now firmly established. From this point, NC improved rapidly as the
electronics industry developed new products. At first, miniature electronic tubes were
developed, but the controls were big, bulky, and not very reliable. Then solid-state circuitry and,
eventually, modular, or integrated circuits were developed. The control unit became smaller,
more reliable, and less expensive. The development of even better machine tools and control
units helped spread the use of NC from the machine tool industry to all facets of manufacturing.

Data Processing

NC data processing (with numbers, letters, and symbols) is done in a computer or
machine control unit (MCU) by adding, subtracting, multiplying, dividing, and comparing. The
computer can be programmed to recognize an A command before a B command, an item 1
before an item 2, or any other elements in their sequential order. It is capable of handling
numbers very quickly; the addition of two simple numbers may take only one billionth of a
second (a nanosecond).

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