现代模具制造-数控技术的大基础毕业课程设计外文文献翻译、中英文翻译

外文资料翻译资料来源：模具设计与制造专业英语文章名：Chapter 6Modern Mold Manufacturing书刊名：English for Die & Mould Design and Manufacturing作 者：刘建雄 王家惠廖丕博主编出版社：北京大学出版社，2002章 节：Chapter 6Modern Mold Manufacturing页 码：P108P118文章译名： 现代模具制造 Chapter 6Modern Mold Manufacturing6.1Fundamental of NC Technology6.1.1 Concept of NC and CNCNumerical control (NC) is a form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data. The alphanumerical data represent relative positions between a workhead and a workpart as well as other instructions needed to operate the machine. The workhead is a cutting tool or other processing apparatus, and the workpart is the object being processed. When the current job is completed, the program of instructions can be changed to process a new job. The capability to change the program makes NC suitable for low and medium production. It is much easier to write new programs than to make major alterations of the processing equipment.Numerically controlled (NC) machine tools were developed to fulfill the contour machining requirements of complex aircraft parts and forming dies. The first-generation numerically controlled units used digital electronic circuits and did not contain any actual central processing unit, thereby they were called NC or hardwired NC machine tools. In 1970s, computer numerically controlled (CNC) machine tools were developed with minicomputers used as control units. With the advances in electronics and computer technology, current CNC systems employed several high-performance microprocessors and programmable logical controllers that work in a parallel and coordinated fashion. Current CNC systems allow simultaneous servo position and velocity control of the axis, monitoring of controller and machine tool performance, online part programming with graphical assistance, in-process cutting process monitoring, and in-process part gauging for completely unmanned machining operations. Manufacturers offer most of these features as options. Today, virtually all the new machine control units are based on computer technology hence, when we refer to NC in chapter and elsewhere, we mean CNC.6.1.2 Basic Component of NC Machine ToolsThe control system of a numerically controlled machine tool can handle many tasks com-monly done by the operator of a conventional machine. For this, the numerical control system must “know” when and in what sequence it should issue commands to change tools, at what speeds and feeds the machine tool should operate, and how to work a part to the required size. The system gains the ability to perform the control functions through the numerical input information that is the control program, also called the part program.The work process of NC is shown in Fig. 6-1. The part programmer should study the part drawing and the process chart and then prepare the control program on a standard form in the specified format. It contains all the necessary control information. A computer-assisted NC part program for NC machining method is also available, in which the computer considerably facilitates the work of the programmer and generate a set of NC instructions. Next the part program is transferred into the control computer the wide accepted method is that the worker types the part program into the computer from the keyboard of the computer numerical control front panel. The computer converts each command into the signal that the servo-drive unit needs. The servo-drive unit drives the machine tool to manufacture the finished part.18Part drawing914 R10Part programPart3 10Servo-drive unitControl ComputerFig. 6-1 The work process of NCInput mediaA typical NC machine tool has five fundamental units. (1) the input media, (2) the machine control unit, (3) the servo-drive unit, (4) the feedback transducer, and (5) the mechanical machine tool unit. The general relationship among the five components is illustrated in Fig. 6-2.Fig. 6-2 Basic components of a CNC machine toolThe input media contains the program of instructions, it is the detailed step-by-step com-mands that direct the actions of the machine tool; the program of instructions is called a part program. The individual commands refer to positions of a cutting tool relative to the worktable on which the workpart is fixtured. Additional instructions are usually included, such as spindle speed, feed rate, cutting tool selection, and other functions. The program is coded on a suitable medium for submission to the machine control unit. For many years, the common medium was 1-inch wide punched tape, using a standard format that could be interpreted by the machine control unit. Today, punched tape has largely been replaced by newer storage technologies in modern machine shops. These technologies include magnetic tape, diskette, and electronic transfer of part programs from a computer.In modern CNC technology, the machine control unit (MCU) consists of a microcomputer and related control hardware that stores the program of instructions and executes it by converting each command into mechanical actions of machine tool, one command at a time. The MCU includes system software, calculation algorithm, and transition software to covert the NC parts program into a usable format for the MCU.The third basic component of an NC system is the servo-drive unit; the drives in machine tools are classified as spindle and feed drive mechanisms. Spindle and feed drive motors and their servo-amplifiers are the components of the servo-drive unit. The MCU processes the data and generates discrete numerical position commands for each feed drive and velocity command for the spindle drive. The numerical commands are converted into signal voltage by the MUC unit and sent to servo-amplifiers, which process and amplify them to the high voltage levels required by the drive motors.The forth basic component of an NC system is the feedback transducer. As the drives move, sensors measure their actual position. The difference between the required position and the actual position is detected by comparison circuit and the action is taken, within the servo, to minimize this difference.The fifth basic component of an NC system is the machine tool that performs useful work. It accomplishes the processing steps to transform the starting workpiece into a completed part. Its operations are directed by the MCU, which in turn is driven by instructions contained in the part program. In the most common example of NC, machine tool consists of the worktable and spindle.6.2Classifications of NC MachinesNumerical control machines are classified in different way: (1) the type of NC motion control system, (2) the servo-drive system, and (3) application of NC.6.2.1 Types of NC Motion Control SystemSome NC processes are performed at discrete location on the workpart (e.g., drilling, punching and spot welding). Others are carried out while the workhead is moving (e.g., turning, milling and continuous arc welding). If the workhead is moving, it may be required to follow a strai- ght-line path or a circular or other curvilinear path. These different types of movement are accomplished by the motion control system.Motion control systems for NC can be divided into two types: (1) point-to-point and (2) continuous path, whose features are explained below.1Point-to-Point Control SystemsPoint-to-point system, also called positing control systems, moves the worktable to a programmed location without regard for the path taken to get to the location. Once the move has been completed, some processing action is accomplished by the workhead at the location, such as drilling or punching a hole. Thus, the program consists of a series of point locations at which operations are performed, as depicted in Fig. 6-3.WorkpartFig. 6-3 Point-to-point (Positioning) control in NCThe machine control unit in a point-to-point system contains registers that hold the individual axis motion commands. In some systems, the X-axis command is satisfied initially, followed by Y-and Z-axis commands. This operation may produce a zigzag path that will ultimately terminate at the proper point location.Many NC point-to-point systems contain a more complex MCU. In these servos, positioning commands are evaluated simultaneously so that vector motion in two axes is possible. However,this vector motion is limited to a one-to-one pulse output. Therefore, only 45vectors may betraced. Such systems are some times called straight-cut.2Contouring Control SystemsThe contouring facility enables an NC machine to follow any path at any prescribed feedrate. The contouring control system, also called continuous path control systems, manages the simultaneous motion of the cutting tool in two, three, four, or five axes (the fourth and fifth axes are angular orientations) by interpolating the proper path between prescribed points. In this case, the tool performs the process while the worktable is moving, thus enabling the system to generate angular surfaces, two-dimensional curves, or three-dimensional contours in the workpart. This control mode is required in many milling and turning operations. A simple two-dimensional profile milling operation is shown in Fig. 6-4 to illustrate continuous path control.Tool profileFig. 6-4 Continuous path (contouring) control in NCWhen continuous path control is utilized to move the tool parallel to only one of the major axes of the machine tool worktable, this is called straight-cut NC. When continuous path control is used for simultaneous control of two or more axes in machine operations, the term contouring is used. All NC contouring systems have the ability to perform linear interpolation and circular interpolation.3. InterpolationOne of the important aspects of contouring is interpolation. The paths that a contouring type NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes. Some of these shapes can be defined mathematically by relatively simple geometric formulas, whereas others cannot be mathematically defined except by approximation. In any case, a fundamental problem in generating these shapes using NC equipment is that they are continuous, whereas NC is digital. To cut along a circular path, the circle must be divided into a serious of straight-line segments that approximate the circular path. The tool is commanded to machine each line segment in succession so that the machined surface closely matches thedesired shape. The maximum error between the nominal (desired) surface and the actual (ma- chined) surface can be controlled by the lengths of the individual line segments as explained in Fig. 6-5.Straight-line segment approximationOutside tolerance limitActual curveTolerance bandInside tolerance limitFig. 6-5 Approximation of a curved path in NC by a straight line segmentsIf the programmer were required to specify the end points for each of the line segments, the programming task would be extremely arduous and fraught with errors. Also, the part program would be extremely long because of the large number of points. To ease the burden, interpolation routines have been developed that calculate the intermediate points to be followed by the cutter to generate a particular mathematically defined or approximated path.A number of interpolation methods are available to deal with the various problems encountered in generating a smooth continuous path. They include: (1) Linear interpolation, (2) circular interpolation, (3) helical interpolation, (4) parabolic interpolation, and (5) cubic inter- polation. Each of these procedures permits the programmer to generate machine instructions for linear or curvilinear paths using relatively few input parameters. The interpolation module in the MCU performs the calculation and directs the tool along the path. In CNC systems, the interpolation is generally accomplished by software. Linear and circular interpolations are almost always included in modern NC systems, whereas helical interpolation is a common option. Parabolic and cubic interpolations are less common; they are only needed by machine shops that must produce complex surface contours.6.2.2 Types of NC Servo-Drive SystemsIn a NC machine, the MCU accepts information in the form of punched, magnetic tape codes or stored program. These input data must be transformed by the MCU into specific output codes in terms of voltages, or pulses per second. The transformed data, called output, is used todrive the motors to position the machine slides to the programmed position. These slides, or table drives, are commonly known as servo-drives. The principal function of NC is the positioning of the tool or the machine table in accordance with the programmed data. Industry has developed three different types of drives based on how the NC system accomplishes positioning. These are the open-loop, the closed-loop and half closed-loop drive system.1. Open-Loop Servo-DriveAn open-loop control system is the simplest and least cost form of servo-drive. It is characterized as a system that lacks feedback as in Fig. 6-6; that is, once an input control signal is sent, there is no sensing device to confirm the action of the control signal.WorktableDriving circuitPulse trainLeadscrewFig. 6-6 An open-loop control systemIn the open-loop control NC machine, the servomotor is usually stepping motor. The stepping motor output shaft rotates in direct proportion to pulses received. It has the advantages of high accuracy, easy implementation and compatible with digital signals, but it has the disadvantages of low torque, limited speed and risk of missed pulse under load. So the open loop control system is used in the economic NC machine. With an open loop system, there is always the risk that the actuator will not have the intended effect on the process, and that is the disadvantage of an open loop system. Open-loop systems are usually appropriate when the following conditions apply: (1) The actions performed by the control system are simple, (2) the actuating function is very reliable, and (3) any reaction forces opposing the actuation are small enough to have no effect on the actuation. If these characteristics are not applicable, then the feedback control system may be appropriate. There are two kinds of feedback control system, one is a closed-loop control and another is half-closed-loop system.The open-loop application in general, is restricted to smaller machines because of the limited power output availability with the stepping motors (a typical maximum is 45 kW and a torque of 200 Nm). Again the pulses per second restrict the speed of the drive. A typicalmaximum for stepping motors is 16,000 pulses per second. When this is applied to a system requiring 0.001 mm accuracy, the resultant maximum speed would be 0.96 m/min. Again for high-precision application like jig boring where an accuracy of 0.001 mm is to be maintained, an open-loop system does not serve the purpose. There is, of course, no doubt that on light duty machinery where the problems of instability are absent and also the requirements are not of high precision, open-close servo control does offer some cost saving solution. Usually the open-loop NC system is called economical NC system.2. Half-Closed-Loop Servo-DriveA half-closed-loop control NC system is one of the feedback control system as illustrated in Fig. 6-7, uses feedback measurements to ensure that the worktable is moved to the desired position. It is characterized as a system that the indirect feedback monitors the output of servomotor. Although this method is popular with NC systems, it is not as accurate as direct feedback. The haft-closed-loop system compares the command position signal with the drive signal of the servomotor.WorktableInstructionFig. 6-7 A half-closed-loop control systemIn operation, the half-closed-loop system is directed to move the worktable to a specified location as defined by a coordinate value in a Cartesian system. Most positioning system have at least two axes with a control system for each axis, but our diagram only illustrates one of these axes. A servomotor connected to a leadscrew is a common actuator for each axis. A signal indicating the coordinate value is sent from the MCU to the motor that drive the leadscrew, whose rotation is converted into linear motion of positioning table. As the table moves closer to the desired coordinate value, the difference between the actual position and the input value isreduced. The actual position is measured by a feedback sensor, which is attached to servomotor axis or leadscrew. This system is unable to sense backlash or leadscrew windup due to varying loads, but it is convenient to adjust and has a good stability.3. Closed-Loop Servo-DriveA closed-loop control system is another feedback control system as illustrated in Fig. 6-8. It is characterized as a system that the direct feedback monitors the output of servomotor. A feedback sensor directly measures the position of worktable. The closed-loop control system, with its drive signal originated by the worktable, is the preferred system because it monitors the actual position of the worktable on which the part is mounted. It is more accurate; however, its implementation costs are higher.WorktableInstructionFig. 6-8 A closed-loop control systemA half closed-loop or closed-loop system uses conventional variable-speed AC or DC motors, called servos, to drive the axes.6.3 Machining CentersMachining center has evolved from individual machin