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( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C Numerical ControlNumerical Control One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical controlled (NC).Prior to the advent of NC; all machine tools were manually operated and controlled. Among the many limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major stop away from human control of machine tools. Numerical control means the control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device error accepting and decoding the programmed instructions, known as a reader. Numerical control was developed to overcome the limitation of human operators, and it has done so Numerical control machines are more accurate the manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology: 1. Electrical discharge machining 2. Laser cutting. 3. Electrin beam welding. Numerical control has also made machine tools more ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes. Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U.S. Air Force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively. However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter is the straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated. This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry , describe the cutting tool configuration, and specify the necessary motions . The development of the APT language was a major step forward in the further development of NC technology. The original NC systems were vastly different ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development. A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader , If it was necessary to produce 100 copies of a given part , it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use. This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and breakage. However, it still left two other problems. The most important of these was that it was difficult ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new tape, It was also still necessary to run the tape through the reader as may times as there were parts to be produced, Fortunately, computer technology became a reality and soon solved the problems of NC associated with punched paper and plastic tape. Te development of a concept known as direct numerical control (DNC) solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions. In direct numerical control, machine tools are tide, via a data transmission link, to a host computer. Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend on a host computer. When the host computer goes down, the machine tools also experience downtime. This problem led to the development of computer numerical control. The development of the microprocessor allowed for the development of programmable logic controllers (PLCs) and microcomputers. These two technologies allowed for the development of computer numerical control (CNC). With CNC, each machine tool has a PLC or a microcomputer that serves ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C the same purpose. This allows programs to be input and stored at each individual machine tool. It also allows programs to be developed off-line and downloaded at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced another problem known as data management .The same program might be loaded in ten different microcomputers with no communication among them. This problem is in the process of being solved by local area networks that connect microcomputers for better data management. Numerical control (n/c) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpants or job .When the job changes, the program of instructions is changed .The capability to change the program is what makes N/C suitable for low-and medium-volume production. It is much easier to write programs than to make major alterations of the processing equipment. There are two basic types of numerically controlled machine tools: point-to-point and continuous-path (also called contouring). Point-to-point machines use unsynchronized motors, with the result that the position of the machining head can be assured only upon completion of a moment, or while only one motor is running. Machines of this type are principally used for straight-line cuts or for ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C drilling or boring.fig.20-1 illustrates a typical sequence of a point-to-point movement from xyz coordinates 2,0,0(point A) to 0,1,3(point D).In this example, all three servomotors would begin operating and then each would shut off as it reached the proper station for this axis. the action would start at coordinate 2,0,o(point A).The three motors operating together would carry the machining head to the vicinity of 1, 1,1(point B),where the y motor would stop. The head would then continue to 0, 1, and 2(point C), where the x motor would stop. Finally, the head would complete its movement to 0,1,3(point D) under the action of motor Z. Machine tools with point-to-point system controls are the simplest and least expensive. A comparison of continuous-path motion to point-to-point motion is illustrated in Fig.20-2.In this example, the motors would run continuously at proportional speeds. A straight line would be generated from A to D .Machine tools equipped with continuous-path capabilities are normally operated by computers. The N/C system consists of the following components: data input, the tape reader with the control unit, feedback devices, and the metal-cutting machine tool or other type of N/C equipment. Data input, also called “man-to-control link“, may be provided to the machine tool manually or entirely by automatic means. Manual methods when used as the sole source of input data are restricted to a relatively small number of ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C inputs. Examples of manually overstated devices are keyboard dials, pushbuttons, switches, or thumbwheel selectors. These are located on a comps; e mere tie , caromed. Dials are analog devices usually connected to a synchronic-type resolved or potentiometer. In most cases, pushbuttons, switches, and other similar controls for each operation. It is a slow and tedious process and is seldom justified except in elementary machining applications or in special cases. In practically all cases, information is automatically supplied to the control unit and the machine tool by cards, punched tapes, or by magnetic tape. Eight-channel punched paper tape is the most commonly used form of data input for conventional N/C systems .The coded instructions on the tape consist of sections of punched holes called blocks .Each block represents a machine function, a machining operation, or a combination of the two .The entire N/C program on a tape is made up of an accumulation of these successive data blocks. Programs resulting in long tapes are wound on reels like motion-picture film. Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop .Once installed, the tape is used again and again without further handling. In this case, the operator simply loads and unloads the parts. Punched tapes are prepared on typewriters with special tape-punching attachments or in tape punching units connected directly to a computer system. Tape production is rarely error-free, Errors may be initially caused by the part programmer, in card ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C punching or compilation, or as a result of physical damage to the tape during handling, etc. Several trial runs are often necessary to remove all errors and produce an acceptable working tape. While the data on the tape is fed automatically, the actual programming steps are done manually. Before the coded tape may be prepared, the programmer, often working with a planner or a process engineer, must select the appropriate N/C machine tool, determine the kind of material to be machined, calculate the speeds and feeds, and decide upon the type of tooling needed. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program. Aerogram manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the drawing specifications. The control unit receives and stores all coded data until a complete block of information has been accumulated. It then interprets the coded instruction and directs the machine tool through the required motions. The function of the control unit may be better understood by comparing it to the action of a dial telephone, where, as each digit is dialed, it is stored. When the entire number has been dialed, the equipment becomes activated and the call is completed. Silicon photo diodes, located in the tape reader head on the control unit, detect light as it passes through the holes ( Word to PDF Converter - Unregistered ) http:/www. Word-to-PDF-C in the moving tape. The light beams are converted to electrical energy, which is amplified to further strengthen the signal .The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives. Some photoelectric devices are capable of reading at rates up to 1000 characters per second. High reading rates are necessary to maintain continuous machine-tool motion; other-wise dwell marks may be generated by the cutter on the part during contouring operations .the reading device must be capable of reading data blocks at a rate faster than the control system can process the data. A feedback device is a safeguard used on some N/C installations to constantly compensate for errors between the command position and the actual location of the moving slides of the marching tool. An N/C machine equipped with this kind of a direct feedback checking device has what is known as a closed-loop system. Positioning control is accomplished by a sensor which, during the actual operation, records the position of the slides and relays this information back to the control unit. Signals thus received are compared to input signals on the tape, and any discrepancy between them is automatically rectified. In an alternative system, called an open-loop system, the machine is positioned solely by stepping motor drives in response