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摩擦 球阀 设计

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NUMERICAL CONTROL 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 equipment There 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 cases In 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 a computer 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 specifications The 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 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 storedWhen 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 in the moving tapeThe light beams are converted to electrical energy，which is amplified to further strengthen the signalThe signals are then sent to registersin 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 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 data A 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 rectified In 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 themechanical 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 electrohydraulic stepping motors have led to increasingly heavier machine load applicationsMILLING Milling is a basic machining process in which the surface is generated by the progressive formation and removal of chips of material from the workpiece as it is fed to a rotatingcutter in a direction perpendicular to the axis of the cutterIn some cases the workpiece isstationary and the cutter is fed to the workIn most instances a multipletooth cutter is used so that the metal removal rate is high，and frequently the desired surface is obtained in a single pass ofthe work The tool used in milling is known as a milling cutterIt usually consists of a cylindrical body which rotates on its axis and contains equally spaced peripheral teeth that intermittently engage and cut the workpieceIn some cases the teeth extend part way across one or both ends of the cylinder Because the milling principle provides rapid metal removal and can produce good surface finish，it is particularly wellsuited for mass-production work，and excellent milling machines have been developed for this purposeHowever，very accurate and versatile milling of a general-purpose nature also have been developed that are widely used in job-shop and tool and die workA shop that is equipped with a milling machine and an engine lathe can machine almost any type of product of suitable size Types of Milling OperationsMilling operations can be classified into two broad categories，each of which has several variations： 1In peripheral milling a surface is generated by teeth located in the periphery of the cutter body；the surface is parallel with the axis of rotation of the cutterBoth flat and formed surfaces san be produced by this methodThe cross section of the resulting surface corresponds to the axial contour of the cutterThis procedure often is called slab milling 2In face milling the generated flat surface is at right angles to the cutter axis and isthe combined result of the actions of the portions of the teeth located on both the periphery and the face of the cutterThe major portion of the cutting is done by the peripheral portions of the teeth with the face portions providing a finishing actionThe basic concepts of peripheral and face milling are illustrated in Fig161Peripheral milling operations usually are performed on machines having horizontal spindles，whereas face milling is done on both horizontaland vertical-spindle machines. Surface Generation in MillingSurfaces can be generated in milling by two distinct1y different methods depicted in Fig16-2Note that in up milling the cutter rotates against the direction of feed the workpiece，whereas in down milling the rotation is in the same direction as the feedAs shown in Fig162，the method of chip formation is quite different in the two casesIn up milling the chip is very thin at the beginning，where the tooth first contacts the work，and increases in thickness,becoming a maximum where the tooth leaves the workThe cutter tends to push the work along and lift it upward from the tableThis action tends to eliminate any effect of looseness in the feed screw and nut of the milling machine table and results in a smooth cutHowever，the action also tends to loosen the work from the clamping device so that greater clamping forcers must be employed. In addition the smoothness of the generated surface depends greatly on the sharpness of the cutting edges In down milling，maximum chip thickness occurs close to the point at which the tooth contacts the workBecause the relative motion tends to pull the workpiece into the cutter，all possibility of looseness in the table feed screw must be eliminated if down milling is to be usedIt should never be attempted on machines that are not designed for this type of millingInasmush as the material yields in approximately a tangential direction at the end of the tooth engagement，there is much less tendency for the machined surface to show tooth marks than when up milling is usedAnother considerable advantage of down milling is that the cutting force tends to hold the work against the machine table，permitting lower clamping force to be employedThis is particularly advantageous when milling thin workpiece or when taking heavy cuts Sometimes a disadvantage of down milling is that the cutter teeth strike against the surface of the work at the beginning of each chipWhen the workpiece has a hard surface，such as castings do，this may cause the teeth to dull rapidly Milling CuttersMilling cutters Can be classified several waysOne method is togroup them into two broad classes，based on tooth relief，as follows： 1Profile-cutters have relief provided on each tooth by grinding a small land back of the cutting edgeThe cutting edge may be straight or curved 2In form or cam-relieved cutters the cross section of each tooth is an eccentric curve behind the cutting edge，thus providing reliefAll sections of the eccentric relief，parallel with the cutting edge，must have the same contour as the cutting edgeCutters of this type are sharpened by grinding only the face of the teeth，with the contour of the cutting edge thus remaining unchanged Another useful method of classification is according to the method of mounting the cutterArbor cutters are those that have a center hole so they can be mounted on an arborShank cutters have either tapered or straight integral shankThose with tapered shanks can be mounted directly in the milling machine spindle，whereas straightshank cutters are held in a chuckFacing cutters usually are bolted to the end of a stub arborTypes of Milling CuttersPlain milling cutters are cylindrical or diskshaped，having straight or helical teeth on the peripheryThey are used for milling flat surfacesThis type of operation is called plain or slab millingEach tooth in a helical cutter engages the work gradually，and usually more than one tooth cuts at a given timeThis reduces shock and chattering tendencies and promotes a smoother surfaceConsequently, this type of cutter usually is preferred over one with straight teeth Side milling cutters are similar to plain milling cutters except that the teeth extend radially part way across one or both ends of the cylinder toward the centerThe teeth may be either straight or helicalFrequently these cutters are relatively narrow，being disklike in shape. Two or more side milling cutters often are spaced on an arbor to make simultaneousparallel cuts，in an operation called straddle milling Interlocking slotting cutters consist of two cutters similar to side mills，but made to operate as a unit for milling slotsThe two cutters are adjusted to the desired width by inserting shims between them Staggered-tooth milling cutters are narrow cylindrical cutters having staggered teeth，and with alternate teeth having opposite helix anglesThey are ground to cut only on the periphery，but each tooth also has chip clearance ground on the protruding sideThese cutters have a free cutting action that makes them particularly effective in milling deep slots Metal-slitting saws are thin，plain milling cutters，usually from 132 to 316 inchthick，which have their sides slightly dished” to provide clearance and prevent binding.They usually have more teeth per inch of diameter than ordinary plain milling cutters and are used for milling deep，narrow slots and for cutting-off operations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，选择合适的数控机床，决定加工材料，计算切削速度和进给速度，决定所需刀具类型，仔细阅读零件图上尺寸，定下合适的程序开始的零参考点，然后写出程序清单，其上记载有描述加工顺序的编码数控指令，机床按顺序加工工件到图样要求。 控制单元接受和储存编码数据，直至形成一个完整的信息程序块，然后解释数控指令，并引导机床得到所需运动。 为更好理解控制单元的作用，可将它与拨号电话进行比较，即每拨一个数字，就储存一个，当整个数字拨好后，电话就被激活，也就完成了呼叫。 装在控制单元里的纸带阅读机，通过其内的硅光二极管，检测到穿过移动纸带上的孔漏过的光线，将光束转变成电能，并通过放大来进一步加强信号，然后将信号送到控制单元里的寄存器，由它将动作信号传到机床驱动装置。 有些光电装置能以高达每秒1000个字节的速度阅读，这对保持机床连续动作是必须的，否则，在轮廓加工时，刀具可能在工件上产生划痕。阅读装置必须要能以比控制系统处理数据更快的速度来阅读数据程序块。 反馈装置是用在一些数控设备上的安全装置，它可连续补偿控制位置与机床运动滑台的实际位置之间的误差。装有这种直接反馈检查装置的数控机床有一个闭环系统装置。位置控制通过传感器实现，在实际工作时，记录下滑台的位置，并将这些信息送回控制单元。接受到的信号与纸带输入的信号相比较，它们之间的任何偏差都可得到纠正。 在另一个称为开环的系统中，机床仅由响应控制器命令的步进电动机驱动定位，工件的精度几乎完全取决于丝杠的精度和机床结构的刚度。有几个理由可以说明步进电机是一个自动化申请的非常有用的驱动装置。对于一件事物，它被不连续直流电压脉冲驱使,是来自数传计算机和其他的自动化的非常方便的输出控制系统。 当多数是索引或其他的自动化申请所必备者的时候，步进电机对运行一个精确的有角进步也是理想的。因为控制系统不需要监听就提供特定的输出指令而且期待系统适当地反应的公开- 环操作造成一个回应环，步进电机是理想的。 一些工业的机械手使用高抬腿运步的马乘汽车驾驶员，而且步进电机是有用的在数字受约束的工作母机中。 这些申请的大部分是公开- 环 ,但是雇用回应环检测受到驱策的成份位置是可能的。 环的一个分析者把真实的位置与需要的位置作比较，而且不同是考虑过的错误。 那然后驾驶员能发行对步进电机的电脉冲，直到错误被减少对准零位。在这个系统中，没有信息反馈到控制单元的自矫正过程。出现误动作时，控制单元继续发出电脉冲。比如，一台数控铣床的工作台突然过载，阻力矩超过电机转矩时，将没有响应信号送回到控制器。因为，步进电机对载荷变化不敏感，所以许多数控系统设计允许电机停转。然而，尽管有可能损坏机床结构或机械传动系统，也有使用带有特高转矩步进电机的其他系统，此时，电动机有足够能力来应付系统中任何偶然事故。最初