Numeric Control : Nedians (NED University)

Free Web Hosting Provider - Web Hosting - E-commerce - High Speed Internet - Free Web Page

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 sponsored a research project at Massachusetts Institute 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.

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 program 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 flexibility.

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

The program of instructions serves as input to the controller unit, which in turn commands the machine tool or other process to be controlled.

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 picture film.

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 auxiliary 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.

The second basic component of NC system is the controller unit. This consists of electronics and hardware that read and interpret the program of instructions and 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 sequence controls to coordinate 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 servomotors and other controls in machine tools.

Most N.C. tools today are provided with positive feedback controls for this purpose and 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 readability of the system is great enough that the feedback controls are not needed.

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 motors and controls necessary to drive them. It also includes the cutting tools, work fixtures and other auxiliary equipment needed in machining operation.

When classified according to the machine tool control system, there are three basic types of NC systems :

The classification is concerned with the amount of control over the relative motion between the workpiece and cutting tool . The least control is exerted over the tool motion with the point to point systems. Contouring represents the highest level of control.

Point to point is also sometimes called a positioning system. In PTP the objective of the machine tool control system is to move the cutting tool to predefined location. The speed or path by which this movement is accomplished is not important in point to point NC. Once the tool reaches the desired location, the machining operation is performed at that position. NC drill presses are a good example of PTP systems. The spindle must first be positioned at a particular location n the workpiece. This is done under PTP control . Then the drilling of the holes is performed at that location, the tool is moved to the next hole location, and so forth. Since no cutting is performed between holes there is no need for controlling the relative motion of the tool and workpiece between hole locations. On positioning systems the speeds and feeds used by the machine tool are often used by the machine operator rather than by the NC tape. Positioning systems are the simplest machine tool control systems and therefore the least expensive of the three types. However for certain processes such as drilling operations and spot welding. PTP is perfectly suited to task and any higher level of control is unnecessary.

Straight cut control systems are capable of moving the cutting tool parallel to one of the major axes at a controlled rate suitable for machining . It is therefore appropriate for performing milling operations to fabricate workpieces of rectangular configurations. With this type of NC systems it is therefor appropriate for performing milling operations to fabricate workpieces of rectangular configurations . With this type of NC system it is not possible to combine movements in more than single axis direction. Therefore angular cuts on the workpiece would not be possible. An NC machine tool capable of performing straight cut movements is also capable of point to point movements.

Contouring is the most complex flexible and the most expensive type of machine tool control . It is capable of performing both PTP and straight cut operations . In addition the distinguishing feature of the of contouring NC system is their capacity for simultaneously control of more than one axis movement of machine tool Figures below illustrate the versatility of continuous path NC. Milling and Turning are the common examples of the use of contouring control.