数控的发展数控中英文翻译.doc

英文资料翻译题目：NC Technology 系 别 中德机电学院 专 业 机电一体化技术 班 级 机电0902 姓 名 学 号 指导教师 2012年4月The developments of NC technologyOne of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control (NC). Prior to the advent of NC, all machine tools were manually operated and controlled. Among the many limitations associated with manual control machine tools, perhaps none is more prominent than the 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 step away from human control of machine tools.Numerical control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operating a machine tool, an NC technician writes a program that issues operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device for 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 than 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. Electron beam welding.Numerical control has also made machine tools more 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 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 from those used today. T he machines had hardwired logic circuit. 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 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 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 many 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. The 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 tied, via a data transmission link, to a host computer, Program for operating the machine tool 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 limitation as all technologies that depend on a host computer. When the host computer goes down, the machine tools also experience down time. 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 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 each individual machine tool. CNC solved the problems associated downtime of the host computer, but it introduced another problem known as data management. The same program might be loaded on 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.In the past, machine tools were kept as simple as possible in order to keep their costs down. Because of the ever-rising cost of labor, better machine tools, complete with electronic controls, were developed so that industry could produce more and better products at prices that were competitive with those offshore industries. NC is being used on all types of machine tools from the simplest to the most complex. The most common machine tools are the single-spindle drilling machine, lathe, milling machine, turning center, and machining center. 1. Single-Spindle Drilling Machine One of the simplest numerically controlled machine tools is the single-spindle drilling machine. Most drilling machines are programmed on three axes: a. The X-axis controls the table movement to the right or left. b. The Y-axis controls the table movement toward or away from the column. c. The Z-axis controls the up or down movement of the spindle to drill holes to depth. 2. Lathe The engine lathe, one of the most productive machine tools, has been a very efficient means of producing round parts. Most lathes are programmed on two axes: a. The X-axis controls the cross motion (in or out) of the cutting tool. b. The Z-axis controls the carriage travel toward or away from the headstock. 3. Milling Machine The milling machine has always been one of the most versatile machine tools used in industry. Operations such as milling, contouring, gear cutting, drilling, boring and reaming are only a few of the many operations that can be performed on a milling machine.The milling machine can be programmed on three axes: a. The X-axis controls the table movement to the right or left. b. The Y-axis controls the table movement toward or away from the column. c. The Z-axis controls the vertical (up and down) movement of the knee or spindle. 4. Turning Center Turning Centers were developed in the mid-1960s after studies showed that about 40 percent of all metal cutting operations were performed on lathes. These numerically controlled machines are capable of greater accuracy and higher production rates than the engine lathe. The basic turning centre operates on only two axes: a. The X-axis controls the cross motion of the turret head. b. The Z-axis controls the lengthwise travel (toward or away from the headstock) of the turret head. 5. Machining Center Machining centers were developed in the 1960s so that a part did not have to be moved from machine to machine in order to perform various operations. These machines greatly increased production rates because more operations could be performed on a work-piece in one setupThere are two main types of machining centers, the horizontal and the vertical spindle types. a. The horizontal spindle-machining center operates on three axes: (a) The X-axis controls the table movement to the right or left. (b) The Y-axis controls the vertical movement (up and down) of the spindle. (c) The Z-axis controls the horizontal movement (in or out) of the spindle. b. The vertical spindle-machining center operates on three axes:(a) The X-axis controls the table movement to the right or left. (b) The Y-axis controls the table movement toward or away from the column. (c) The Z-axis controls the vertical movement (up and down) of the spindle.A program for numerical control consists of a sequence of directions that caused a NC machine to carry out a certain operation, machining being the most commonly used process. Programming for NC may be done by an internal programming department, on the shop floor, or purchased from an outside source. Also, programming may be done manually or with computer assistance. The program contains instructions and commands. Geometric instructions pertain to relative movements between the tool and the work-piece. Processing instructions pertain to spindle speeds, feeds, tools, and so on. Travel instructions pertain to the type of interpolation and slow or rapid movements of the tools or worktables. Switching commands pertain to on/off position for coolant supplies, spindle rotation, direction of spindle rotation, tool changes, work-piece feeding, clamping and so on. (1). Manual programming. Manual part programming consists of first calculating dimensional relationships of the tool, work-piece and worktable based on the engineering drawings of the part, and manufacturing operations to be performed and their sequence. A program sheet is then prepared, which consists of the necessary information to carry out the operation, such as cutting tools, spindle tools, feeds, depth of cut, cutting fluids, power, and tool or work-piece relative positions and movements. Based on this information, the part program is prepared. Usually a paper tape is first prepared for typing out and debugging the program. Depending on how often it is to be used, the tape may be made of more durable mylar. Someone knowledgeable about the particular process and able to understand, read, and change part programs can do manual programming. Because they are familiar with machine tools and process capabilities, skilled machinists can do manual programming with some training in programming, however, the work is tedious, time consuming, and uneconomical and is used mostly in simple point-to-point applications. (2). Computer-aided Programming. Computer-aided part programming involves special symbolic programming languages that determine the coordinate points of corners, edges, and surfaces of the part.Because numerical control involves the insertion of data concerning work-piece materials and processing parameters, programming must be done by operators or programmers who are knowledgeable about the relevant aspects of the manufacturing processes being used. Before production begins, programs should be verified, either by viewing a simulation of the procession on a CRT screen or by making the part from an inexpensive material, such as aluminum, wood, or plastic, rather than the material specified for the finished parts. Principles of NC Machines An NC machine can be controlled through two types of circuits: open-loop and closed-loop. In the open-loop system (Figure 1 (a), the singles are sent to the servomotor by the controller, but the movements and final positions of the worktable are not checked for accuracy.The closed-loop system (Figure 1(b) is equipped with various transducers, sensors, and counters that measure accurately the position of the worktable. Through feedback control, the position of the worktable is compared against the signal. Table movements terminate when the proper coordinates are reached. The closed-loop system is more complicated and more expensive than the open-loop system.Figure 1 Schematic illustration of (a) an open-loop and (b) a closed-loop control system for an NC machineTypes of Control Systems There are two basic types of control systems in numerical control, point-to-point and contouring. (1) In a point-to-point system, also called positioning, each axis of the machine is driven separately by lead screws and, depending on the type of operation, at different velocities. The machine moves initially at maximum velocity in order to reduce nonproductive time, but decelerates as the tool approaches its numerically defined position. Thus, in an operation such as drilling (or punching a hole), the positioning and cutting take place sequentially. After the hole is drilled or punched, the tool retracts upward and moves rapidly to another position, and the operation is repeated. The path followed from one position to another is important in only one respect. It must be chosen to minimize the time of travel, for better efficiency. Point-to-point systems are used mainly in drilling, punching, and straight milling operations. (2) In a contouring system (also known as a continuous path system), the positioning and the operations are both performed along controlled paths but at diffe