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NUMERICAL CONTROL（数控技术）Numerical control(NC)is a form of programmable automation in which the processing equipment is controlled by means of numbers，letters，and other symbolsThe numbers，letters，and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or jobWhen the job changes，the program of instructions is changedThe capability to change the program is what makes NC suitable for low-and medium-volume productionIt is much easier to write programs than to make major alterations of the processing equipmentThere are two basic types of numerically controlled machine tools：pointtopoint and continuouspath(also called contouring)Pointtopoint machines use unsynchronized motors，with the result that the position of the machining head Can be assured only upon completion of a movement，or while only one motor is runningMachines of this type are principally used for straightline cuts or for drilling or boring The NC system consists of the following components：data input，the tape reader with the control unit，feedback devices，and the metalcutting machine tool or other type of NC equipmentData input，also called“mantocontrol link”，may be provided to the machine tool manually，or entirely by automatic meansManual methods when used as the sole source of input data are restricted to a relatively small number of inputsExamples of manually operated devices are keyboard dials，pushbuttons，switches，or thumbwheel selectorsThese are located on a console near the machineDials ale analog devices usually connected to a syn-chro-type resolver or potentiometerIn most cases，pushbuttons，switches，and other similar types of selectors aye digital input devicesManual input requires that the operator set the controls for each operationIt is a slow and tedious process and is seldom justified except in elementary machining applications or in special casesIn practically all cases，information is automatically supplied to the control unit and the machine tool by cards，punched tapes，or by magnetic tapeEightchannel punched paper tape is the most commonly used form of data input for conventional NC systemsThe coded instructions on the tape consist of sections of punched holes called blocksEach block represents a machine function，a machining operation，or a combination of the twoThe entire NC program on a tape is made up of an accumulation of these successive data blocksPrograms resulting in long tapes all wound on reels like motion-picture filmPrograms on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loopOnce installed，the tape is used again and again without further handlingIn this case，the operator simply loads and unloads the partsPunched tapes ale prepared on type writers with special tapepunching attachments or in tape punching units connected directly to acomputer systemTape production is rarely error-freeErrors may be initially caused by the part programmer，in card punching or compilation，or as a result of physical damage to the tape during handling，etcSeveral 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 ale done manuallyBefore the coded tape may be prepared，the programmer，often working with a planner or a process engineer, must select the appropriate NC 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 programA program 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 specificationsThe control unit receives and stores all coded data until a complete block of information has been accumulatedIt then interprets the coded instruction and directs the machine tool through the required motionsThe 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 storedWhen the entire number has been dialed，the equipment becomes activated and the call is completedSilicon photo diodes，located in the tape reader head on the control unit，detect light as it passes through the holes in the moving tapeThe light beams are converted to electrical energy，which is amplified to further strengthen the signalThe signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drivesSome photoelectric devices are capable of reading at rates up to 1000 characters per secondHigh reading rates are necessary to maintain continuous machinetool motion；otherwise dwell marks may be generated by the cutter on the part during contouring operationsThe reading device must be capable of reading data blocks at a rate faster than the control system can process the dataA feedback device is a safeguard used on some NC installations to constantly compensate for errors between the commanded position and the actual location of the moving slides of the machine toolAn NC machine equipped with this kind of a direct feedback checking device has what is known as a closed-loop systemPositioning 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 unitSignals thus received ale compared to input signals on the tape，and any discrepancy between them is automatically rectifiedIn an alternative system，called an openloop system，the machine is positioned solely by stepping motor drives in response to commands by a controllersThere are three basic types of NC motions, as follows: Point-to-point or Positional Control In point-to-point control the machine tool elements (tools, table, etc.) are moved to programmed locations and the machining operations performed after the motions are completed. The path or speed of movement between locations is unimportant; only the coordinates of the end points of the motions are accurately controlled. This type of control is suitable for drill presses and some boring machines, where drilling, tapping, or boring operations must be performed at various locations on the work piece. Straight-Line or Linear Control Straight-Line control systems are able to move the cutting tool parallel to one of the major axes of the machine tool at a controlled rate suitable for machining. It is normally only possible to move in one direction at a time, so angular cuts on the work piece are not possible, consequently, for milling machines, only rectangular configurations can be machined or for lathes only surfaces parallel or perpendicular to the spindle axis can be machined. This type of controlled motion is often referred to as linear control or a half-axis of control. Machines with this form of control are also capable of point-to-point control.Continuous Path or Contouring Control In continuous path control the motions of two or more of the machine axes are controlled simultaneously, so that the position and velocity of the can be tool are changed continuously. In this way curves and surfaces can be machined at a controlled feed rate. It is the function of the interpolator in the controller to determine the increments of the individual controlled axes of the machines necessary to produce the desired motion. This type of control is referred to as continuous control or a full axis of control.Some terminology concerning controlled motions for NC machines has been introduced. For example, some machines are referred to as four-or five-or even six-axis machines. For a vertical milling machine three axes of control are fairly obvious, these being the usual X, Y, Z coordinate directions. A fourth or fifth axis of control would imply some form of rotary table to index the work piece or possibly to provide angular motion of the work head. Thus, in NC terminology an axis of control is any controlled motion of the machine elements (spindles, tables, etc). A further complication is use of the term half-axis of control; for example, many milling machines are referred to as 2.5-axis machine. This means that continuous control is possible for two motions (axes) and only linear control is possible for the third axis. Applied to vertical milling machines, 2.5axis control means contouring in the X, Y plane and linear motion only in the Z direction. With these machines three-dimensional objects have to be machined with water lines around the surface at different heights. With an alternative terminology the same machine could be called a 2CL machine (C for continuous, L for linear control). Thus, a milling machine with continuous control in the X, Y, Z directions could be termed be a three-axis machine or a 3c machine, Similarly, lathes are usually two axis or 2C machines. The degree of work precision depends almost entirely upon the accuracy of the lead screw and the rigidity of the machine structureWith this systemthere is no self-correcting action or feedback of information to the control unitIn the event of an unexpected malfunction，the control unit continues to put out pulses of electrical currentIf，for example，the table on a NC milling machine were suddenly to become overloaded，no response would be sent back to the controllerBecause stepping motors are not sensitive to load variations，many NC systems are designed to permit the motors to stall when the resisting torque exceeds the motor torqueOther systems are in use，however，which in spite of the possibility of damage to the machine structure or to the mechanical system，ale designed with special hightorque stepping motorsIn this case，the motors have sufficient capacity to“overpowerthe system in the event of almost any contingency The original NC used the closedloop systemOf the two systems，closed and open loop，closed loop is more accurate and，as a consequence，is generally more expensiveInitially，openloop systems were used almost entirely for light-duty applications because of inherent power limitations previously associated with conventional electric stepping motorsRecent advances in the development of electro hydraulic stepping motors have led to increasingly heavier machine load applications数控技术数控是可编程自动化技术的一种形式，通过数字、字母和其他符号来控制加工设备。数字、字母和符号用适当的格式编码为一个特定工件定义指令程序。当工件改变时，指令程序就改变。这种改变程序的能力使数控适合于中、小批量生产，写一段新程序远比对加工设备做大的改动容易得多。数控机床有两种基本形式：点位控制和连续控制(也称为轮廓控制)。点位控制机床采用异步电动机，因此，主轴的定位只能通过完成一个运动或一个电动机的转动来实现。这种机床主要用于直线切削或钻孔、镗孔等场合。数控系统由下列组件组成：数据输入装置，带控制单元的磁带阅读机，反馈装置和切削机床或其他形式的数控设备。数据输人装置，也称“人机联系装置”，可用人工或全自动方法向机床提供数据。人工方法作为输人数据唯一方法时，只限于少量输入。人工输入装置有键盘，拨号盘，按钮，开关或拨轮选择开关，这些都位于机床附近的一个控制台上。拨号盘通常连到一个同步解析器或电位计的模拟装置上。在大多数情况下，按钮、开关和其他类似的旋钮是数据输入元件。人工输入需要操作者控制每个操作，这是一个既慢又单调的过程，除了简单加工场合或特殊情况，已很少使用。几乎所有情况下，信息都是通过卡片、穿孔纸带或磁带自动提供给控制单元。在传统的数控系统中，八信道穿孔纸带是最常用的数据输入形式，纸带上的编码指令由一系列称为程序块的穿孔组成。每一个程序块代表一种加工功能、一种操作或两者的组合。纸带上的整个数控程序由这些连续数据单元连接而成。带有程序的长带子像电影胶片一样绕在盘子上，相对较短的带子上的程序可通过将纸带两端连接形成一个循环而连续不断地重复使用。带子一旦安装好，就可反复使用而无需进一步处理。此时，操作者只是简单地上、下工件。穿孔纸带是在带有特制穿孔附件的打字机或直接连到计算机上的纸带穿孔装置上做成的。纸带制造很少不出错，错误可能由编程、卡片穿孔或编码、纸带穿孔时的物理损害等形成。通常，必须要试走几次来排除错误，才能得到一个可用的工作纸带。 虽然纸带上的数据是自动进给的，但实际编程却是手工完成的，在编码纸带做好前，编程者经常要和一个计划人员或工艺工程师一起工作，