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Numerical control has been widely used in the industry or factory nowadays because it helps solve the problem of making manufacturing systems more flexible. In other words, numerical control can be defined as an operation of machine tools by means of specifically coded instructions to the machine control systems. In manufacturing field, especially in the area of metal working, numerical control technology has caused something of revolution. Advances in microelectronics and microcomputers have allowed the computer to be used as the control units on modern numerical control machinery, where the program is loaded into an executed from the machine's computer. These machines, also called as computer numerical control (CNC) machines.
The objectives of this unit are to :
1. Explain the historical background of Numerical Control (NC)
2. Describe the difference between a numerical control tape machine and a computer
numerical control machine.
3. Describe four ways that programs can be entered into a computer numerical controller.
4. Give the examples of industrial applications.
Numerical Control technology emerged in the mid 20th century. In 1947, John Parsons, who is closely associated with the invention of numerical control, began experimenting with the idea of using three-axia curvature data to control machines tool motion for the production of aircraft components. Parsons was awarded a U.S.Air force contract to built what was to become the first numerical control machine in 1949. In 1951, the project was assumed by the Massachusetts Institute of Technology in Cambridge, MA, USA. After further refinements, numerical control became available to industry by 1955.
In the past, NC machines ran off punched cards and tape, with tape becoming the more common medium. But later computer were introduced as aids in programming due to the time and effort required to change or edit tape. There are two forms of computer involvement, which are computer aided programming languages abd direct numerical control (DNC). Computer aided programming languages allowed a part programmer to develop an NC program using a set of universal commands, then the computer translated into machine codes and punched into the tape. Direct Numerical Control involved using a computer as a partial or complete controller of one or more numerical control machines.
NC VS CNC
The NC stands for the older and original Numerical Control technology, whereby the abbreviation CNC stands for the newer Computerized Numerical Control technology. Both systems perform the same tasks, which is manipulation of data for the sole purpose of machining a part. In both cases, the control system internal design contains all logical instructions that process the input data.
Numerical control (NC) can be defined as an operation of machine tools by means of specifically coded instructions to the machine control system. The NC system uses a fixed logical functions, which are built in and permanently wired within the control unit. The part programmer or machine operator cannot change these functions. The system does not allow any changes to the program at the control but it can intepret a part program. All required program changes must be made away from the control, typically in an office environment. NC system also requires the compulsory use of punched tapes for input of the program information.
The modern CNC system uses an internal micro processor ( i.e. a computer). This computer contains memory registers storing a variety of routines that are capable of manipulating logical functions. That means the program at the control unit (at the machine) can be changed by the part programmer or machine operator. This is the greatest advantage of CNC systems which contribute to a wide use of the technology in modern manufacturing. The CNC programs and the logical functions are stored on special computer chips, as software instructions, rather than used by the hardware connections, that control the logical functions. In other words, the CNC system is synonymous with the term ' softwired' while the NC is synonymous with the term ' hardwired'.
A CNC machine maybe used as stand alone units, in a network of machines such as flexible machining centers or machining cells. They have more programmable features than older NC tape machinery and easier to program, and most of them maybe programmed by more than one method.
In the CNC machine, the hardware is necessary to transfer the numerical control codes to the controller once the NC program is loaded into the computer's memory. Hence, it is also called as soft-wired controller. All machines can be programmed via an on board computer keyboard. Tape reader or electronic connector also used to allow the transfer of a program written elsewhere to the CNC machine.
The controller uses a permanent resident program called an executive program to process the codes into the electrical pulses that control the machine. In any CNC machine, the executive program resides in ROM memory, and the NC codes resides in RAM memory.
The information in ROM ( Read Only Memory) is written into the electronic chip and cannot be erased without special equipment. This is the reason why the executive program cannot be erased and is always active when the machine is on.
RAM ( Random access memory) can be accesses and altered by the computer. The contents of RAM are lost when the controlleris turned off. Thats why many CNC controllers use a battery backup system that powers the computer long enough for the programto be saved to other storage media when the power loss. Some of them use CMOS memory ( special type of RAM) which retains its contents even when the power is turned off.
Input media are used to electronically or machanically store the NC programs until they are needed. A program is simply read from teh input medium when it is loaded into the machine.
There are four ways to input programs into CNC machinery: MDI ( manual daa input),punched tape, magnetic tape and DNC ( direct numerical control/distributive numerical control).
The oldest medium for program storage is punched tape made of paper or mylar plastic. The NC program code is entered into the tape by use of a tape puncher which punches a series of holes that represent the NC codes. A tape reader employing electrical, optical, or mechanical means senses the holes in the tape and transfers the coded information into the machine computer.
Magnetic tape also used as a storage medium. The most commonly used style of magnetic tape is ¼ inch computer grade cassette tape, because the cassette case affords good protection and the small size is convenient for storage.
Now, floppy diskette widely used as a storage media for NC programs. A program can be saved on a floppy diskette, and transferred into the CNC control by means of a portable diskette drive attached to the communications port on the CNC machine.
Computers and computer controlled machinery do not deal in Arabic symbols or numbers. All of the internal processing is done by calculating or comparing binary numbers. Binary numbers contain only two digits, zero and one, as shown in Figure 1.1.
Figure 1.1 : Binary numbers compared to Arabic numbers
With the CNC controller, each binary digit " one" may represent a positive charge and a "zero" a negative charge, or a 'one' may be the presence of charge and a 'zero' the absence of charge. In either case, the CNC program code in binary form must be loaded int the computer. Programming formats and languages allow the NC code to be written using alphabetic characters and base-ten decimal numbers. When the NC program is punched or recorded on tape or other storage media, the information is translated into binary form.
NC was originally developed for use in aerospace industries, but now it is widely used in manufacturing. The use of CNC machines also becoming important especially metalworking and manufacturing industries. Besides that, aerospace, defense contract, aotomotive, electronic, appliance and tooling industries all employ numerical control machinery. With the advent of low cost OEM ( original equipment manufacturer) and retrofit CNC vertical milling machines, even shops specializing in one of a kind prototype work are using CNCs.
In this chapter we have studied:
1. A numerical control machine is a machine that is positioned automatically along a preprogrammed path by way of coded instructions.
2. CNC machines use an on-board computer as a controller.
3. There are four ways to input programs into CNC machinery: MDI ( manual daa input),punched tape, magnetic tape and DNC ( direct numerical control/distributive numerical control ).
4. Computers work with binary numbers. The CNC program must be loaded into the controller in binary form.