数控专业外文翻译--数控机床的组成部分.doc

附录英文原文N/C Machine Tool Elements FromEnglish for Machinery N/C machine tool elements consist of dimensioning systems, control systems, servomechanisms and open-or closed-loop systems. It is important to understand each element prior to actual programming of a numerically controlled port.The term measuring system in N/C refers to the method a machine tool uses to move a part from a reference point to a target point. A target point may be a certain locating for drilling a hole, milling a slot, or other machine operation. The two measuring systems used on N/C machines are the absolute and incremental. The absolute measuring system uses a fixed reference point. It is on this point that all positional information is based. In other words, all the locations to which a part will be moved must be given dimensions relating to that original fixed reference point. Figure shows an absolute measuring system with X and Y dimensions, each based on the origin. The incremental measuring system has a floating coordinating system. With the incremental system, the time the part is moved. Figure 16.2 show X and Y values using an incremental measuring system. Notice that with this system, each new location bases its values in X and Y from the preceding location. One disadvantage to this system is that any errors made will be repeated throughout the entire program, if not detected and corrected.There are two types of control systems commonly used on N/C equipment: point-to-point and continuous path. A point-to-point controlled N/C machine tool, sometimes referred to as a positioning control type, has the capability of moving only along a straight line. However, when two axes are programmed simultaneously with equal values a 45 angle will be generated. Point-to-point systems are generally found on drilling and simple milling machine where hole location and straight milling jobs are performed. Point-to-point systems can be utilized to genetate arcs and angles by programming the machine to move in a series of small steps. Using this technique, however, the actual path machined is slightly different from the cutting path specified.Machine tools that have the capability of moving simultaneously in two or more axes are classified as continuous-path or contouring. These machines are used for machining arcs, radii, circles, and angles of any size in two or there dimensions. Continuous-path machines are more expensive than point-to-point systems and generally require a computer to aid programming when machining complex contours.N/C servomechanisms are devices used for producing accurate movement of a table or slid along an axis. Two types of servos are commonly used on N/C equipment: electric stepping motors and hydraulic motors. Stepping motor servos are frequently used on less expensive N/C equipment. These motors are generally high-torque power servos and mounted directly to a lead screw of a table or tool slide. Most stepping motors are actuated by magnetic pulses from the stator and rotor assemblies. The net result of this action is that one rotation of the motor shaft produces 200 steps. Connection the motor shaft to a 10-pitch lead screw allows 0.0005-in. movements to be made. Hydraulic servos produce a fluid pressure that flows through gears or pistons to effect shaft rotation. Mechanical motion of lead screws and slides is accomplished through various values and controls from these hydraulic motors. However, they are more expensive and noisy. Most larger N/C machines use hydraulic servos.N/C machines that use an open-loop system contain no-feedback signal to ensure that a machine axis has traveled the required distance. That is, if the input received was to move a particular table axis 1.000 in, the servo unit generally moves the table 1.000 in. There is no means for comparing the actual table movement with the input signal, however, The only assurance that the table has actually moved 1.000 in. is the reliability of the servo system used. Open-loop systems are, of course, less expensive than closed-loop systems. A closed-loop system compares the actual output with the input signal and compensates for any errors. A feedback unit actually compares the amount the table has been moved with the input signal. Some feedback units used on closed-loop systems are transducers, electrical or magnetic scales, and synch. Closed-loop systems greatly increase the reliability of N/C machines.Machining CentersMany of todays more sophisticated lathes are called machining centers since they are capable of performing, in addition to the normal turning operations, certain milling and drilling operations. Basically, a machining center can be thought of as being a combination turret lathe and milling machine. Additional features are sometimes included by manufacturers to increase the versatility of their machines.Numerical ControlOne 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:Electrical discharge machining.Laser cutting.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 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 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 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 taps, 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. 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 lost 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 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 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. Cutting Tool GeometryShape of cutting tools, particularly the angles, and tool material are very important factors. Angles determine greatly not only tool life but finish quality as well. General principles upon which cutting tool angles are based do not depend on the particular tool, Basically, the same considerations hold true whether a lathe tool, a milling cutter, a drill, or even a grinding wheel are being designed. Since, however the lathe tool, depicted in Fig. 18.1, might be easiest to visualize, its geometry is discussed. Tool features have been identified by many names. The technical literature is full of confusing terminology. Thus in the attempt to cleat up existing disorganized conceptions and nomenclature, this American Society of Mechanical Engineers published ASA Standard B5-22-1950. What follows is based on it.A single-point tool is a cutting tool having one face and one continuous cutting edge, Tool angles identified in Fig. 18.2 are as follows:Tool angle 1, on front view, is the back-rank angle. It is the angle between the tool face and a line parallel to the tool base of the shank in a longitudinal plane perpendicular to the tool base. When this angle is downward from front to rear of the cutting edge, the rake is positive; when upward from front to black, the rake is negative. This angle is most significant in the machining process, because it directly affects the cutting force, finish, and tool life.The side-rake angle, numbered 2, measures the slope of the face on a cross plane perpendicular to the tool base. It, also, is an important angle, because it directs chip flow to the side of the tool post and permits the tool to feed more easily into the work.The end-relief angle is measured between a line perpendicular to the base and the end flank immediately below the end cutting edge; it is numbered 3 in the figure. It provides clearance between work and tool so that its cut surface can flow by with minimum rubbing against the tool. To save time, a portion of the end flank of the tool may sometimes be lest ungrounded, having been previously forged to size. In such case, this end-clearance angle, numbered 4, measured to the end flank surface below the ground portion, would be larger than the relief angle.Often the end cutting edge is oblique to the flank. The relief angle is then best measured in a plane normal to the end cutting edge angle. Relief is also expressed as viewed from side and end of the tool.The side-relief angle, indicated as 5, is measured between the side flank, just below the cutting edge, and a line through the cutting edge perpendicular to the base of the tool. This clearance permits the tool to advance more smoothly into the work.Angle 6 is the end-cutting-edge angle measured between the end cutting edge and a line perpendicular to the side of the tool shank. This angle prevents rubbing of the cut surface and permits longer tool file.The side-cutting-edge angle, numbered 7, is the angle between the side cutting edge and the side of the tool shank. The true length of cut is along this edge. Thus the angel determines the distribution of the cutting forces. The greater the angle, the longer the tool life; but the possibility of charter increases. A compromise must, as usual, be reached.The nose angle, number 8, is the angle between the two component cutting edges. If the corner is rounded off, the arc size is defined by the nose radius 9. The radius size influences finish and chatter. Sand CastingThe first stage in the production of sand castings must be the design and manufacture of a suitable pattern. Casting patterns are generally made from hard word and the pattern has to be made larger than the finished casting size to allow for the shrinkage that takes place during solidification and cooling. The extent of this shrinkage varies with the type of metal or alloy to be cast. For all but the simplest shapes the pattern will be made in two or more pieces to facilitate molding. If a hollow casting is to be made the pattern design will include extension pieces so that spaces to accept the sand core are molded into sand. These additional spaces in the mould are termed core prints.Sand moulds for the production of small and medium-sized castings are made in a molding box. The mould is made in two or more parts in order that the pattern may be removed.The drag half of the mould box is placed on a flat firm board and the drag half of the pattern placed in position. Facing sand is sprinkled over the pattern and then the mould box is filled with molding sand. The sand is rammed firmly around the pattern. This process of filling and ramming may be done by hand but mould production is automated in a large foundry with the mould boxes moving along a conveyor, firstly to be filled with sand from hoppers and then to pass under mechanical hammers for ramming. When ramming of the sand is complete, excess sand is removed to leave a smooth surface flush with the edges of the molding box.The completed drag is now turned over and the upper, or cope, portion of the moulding box positioned over it. The cope half of the pattern is placed in position, correct alignment being ensured by means of small dowel pins. Patterns for the necessary feeder, runner and risers are also placed so as to give an even distribution of metal into the mould cavity. The risers should coincide with the highest readily escape from the mould. The sizes of risers should be such that the metal in them does not freeze too rapidly. An important function of a riser is to act as reservoir of liquid metal to feed solidification within the mould. A thin coating of dry parting sand is sprinkled into mould at this stage. This is to prevent the cope and drag sticking together when the cope half is moulded. The cope is now filled with moulding sand and this is rammed firmly into shape in the same manner as in the making of the drag.After the ramming of sand in the cope is completed the two halves of the moulding box are carefully separated. At this stage venting of the moulding box are carefully separated. At this stage venting of the mould can be done, if necessary, to increase the permeability of the mould. After venting the patterns are carefully removed from both cope and drag, and a gate or gates are carefully cut to connect the runner channel with the main cavity. Gates shou