Introduction to Numerical Control Machining

Historical perspective

The development of numerical control owes much to the United States air force,which  recognized the need to develop more efficient manufacturing methods for modern aircraft. Following
World War II, the components used to fabricate jet aircraft became more complex and required more machining. Most of the machining involved milling operations,so the Air Force sponsered
a research project at Massachusetts Institue of Technology to develop a prototype NC milling machine. This prototype was produced by retrofitting a conventional tracer mill with numerical
control servomechanisms for the three axes of the machine. In March 1952, the MIT  Labs held the first demonstration of the NC machine.  The machine tool builders gradually began developing their
own projects to introduce commercial NC units. Also,certain industry users, especially airframe builders,worked to devise numerical control machines to satisfy their own particular production needs.
The Air force continued its encouragement of NC development by sponsoring additional research at MIT to design a part programming language that could be used in controlling N.C. machines.

What is Numerical Control ?

Numerical control can be defined as a form of programmable automation in which process is controlled by numbers,letters and symbols. In NC,the numbers form a programme of instructions designed for a particular workpart or job. When job changes the program of instruction changes. This capability to change a program for each new job gives NC its flexibilty.

Numerical control should be considered as a possible mode of controlling the operation for any production situation possesing the following characteristics :

Similar workparts in terms of raw material (e.g. metal shock for machining)
The work parts are produced in various sizes and geometries.
The workparts are produced in batches of small to medium size quantities.
A sequence of similar processing steps is required to complete the operation on each workpiece.
Many machining jobs meet these conditions. The machined workparts are metal, they are specified in many different sizes and shapes,nad most machined parts produced in the industry today are made in small to medium size lots sizes. To produce each parts a sequence of drilling operations may be required ar aseries of turning or milling operations. The suitability of NC for these kinds of jobs is the reason of tremendous growth of numerical control in metal working industry over the last 25 years.

Basic components of NC system

An operational numerical control system consists of the following three basic components:

1. Program of instructions.
2. Controller unit, also called machine tool unit.
3. Machine tool or other controlled process.
The program of instructions serves as input to the controller unit, which in turn commands the machine tool or other process to be controlled.

Program of Instructions.
The program of instructions is the detailed step by step set of instructions which tell the machine what to do. It is coded in numerical or symbolic form on some type of input medium that can be interpreted by the controller unit. The most common one is the 1-inch-wide punched tape. Over the years,other forms of input media has been used,including punched cards, magnetic tape, and even 35mm motion picturefilm.
There are two other methods of input to the NC system which should be mentioned. The first is by manual entry of instructional data to the controller unit. This is time consuming and is rarely used
except as an auxillary means of control or when one or a very limited no. of parts to be made. The second method of input is by means of a direct ling with the computer. This is called direct numerical
control, or DNC.

Controller Unit
The second basic component of NC system is the controller unit. This consists of electronics and hardware that read and interpret the program of instructionsand convert it to mechanical actions of the machine tool. The typical elements of the controller unit include the tape reader, a data buffer, signal output channels to the machine tool,and the sequencecontrols to coodinate the overall operation
of the foregoing elements.
The tape reader is an electrical-mechanical device for the winding and reading the punched tape containing the program of instructions. The signal output channels are connected to the servomotores
and other controls in machine tools. Most N.C. tools today are provided with positive feedback controls for this purposeand are referred as closed loop systems. However there has been growth in the open loop systems which do not make use of feedback signals to the controller unit. The advocates of the open loop concept claim that the realiblity of the system is great enough that the feedback controls are not needed.

Machine Tool
The third basic component of an NC system is the machine tool or other controlled process. It is part of the NC system which performs useful work. In the most common example of an NC system,one designed to perform machining operations, The machine tool consists of the worktable and spindle as well as the motorsand controls necessary to drive them. It also includes  the cutting tools,work fixtures and other auxillary equipment needed in machining operation.

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