先进制造技术第五单元

1、5 Numerical Control Numerical control (NC) is a method of controlling the movements ofmachine components by directly inserting coded instructions, in the form ofnumbers and letters, into the system. The system automatically interprets thesedata and converts them to output signals. These signals, in

2、turn, control variousmachine components-for example, by turning spindles on and off, changingtools, moving the workpiece or the tools along specific paths, or turningcutting fluids on and off. In order to appreciate the importance of numerical control of machines,lets briefly review how a process su

3、ch as machining has traditionally beencarried out. After studying the working drawings of a part, the operator sets upthe appropriate process parameters (such as cutting speed, feed, depth of cut,cutting fluid, and so on), determines the sequence of the machining operationsto be performed, clamps th

4、e workpiece in a workholding device (such as achuck or collet), and proceeds with the making of the part. Depending on part shape and on the dimensional accuracy specified, thisapproach usually requires skilled operators. The machining procedure followedmay depend on the particular operator; because

5、 of the possibilities of humanerror, even parts produced by the same operator may not all be identical. Part quality may, therefore, depend on the particular operator or (even withthe same operator) on the day of the week or the hour of the day. Because ofincreased concern with improving product qua

6、lity and reducing manufacturingcosts, such variability (and its effects on product quality) are no longeracceptable. This situation can be eliminated by numerical control of themachining operation. The importance of numerical control can be further illustrated by thefollowing example. Assume that se

7、veral holes are to be drilled on a part in thepositions shown in Fig.5. 1. In the traditional manual method of machining this part, the operatorpositions the drill bit with respect to the workpiece, using reference pointsgiven by any of the three methods shown in the figure. The operator thenproceed

8、s to drill the holes. Lets first assume that 100 parts, all having exactlythe same shape and dimensional accuracy, are to be drilled. Obviously, thisoperation is going to be tedious, because the operator has to go through thesame motions repeatedly. Moreover, the probability is high that, for variou

9、sreasons, some of the parts machined will be different from others. Lets now assume that during this production run, the order for these partsis changed, and ten of the parts now require holes in different positions. Themachinist now has to reposition the work table; this operation will be timeconsu

10、ming and is subject to error. Such operations can be performed easily by numerical control machinesthat are capable of producing parts repeatedly and accurately and of handlingdifferent parts (by simply loading different part programs, as will be describedlater). In operations under numerical contro

11、l, data concerning all aspects of themachining operation, such as locations, speeds, feeds, and cutting fluids, canbe stored on magnetic media, changing over time from tapes to hard disks. Theconcept of NC control is that specific information can be relayed from thesestorage devices to the machine t

12、ools control panel. On the basis of input information, relays and other devices (hardwiredcontrols) can be actuated to obtain a desired machine setup. Complexoperations (such as turning a part having various contours, or die sinking in amilling machine) can be carried out easily. Numerical control h

13、as had a major impact on all aspects of manufacturingoperations. NC machines are now used extensively in small andmedium-quantity production (typically 500 parts or less) of a wide variety ofparts, both in small shops and in large manufacturing facilities. Older machinescan often be retrofitted with

14、 numerical control.5.1 Historical Back The basic concept behind numerical control apparently was implementedin the early 1800s, when punched holes in sheet-metal cards were used toautomatically control the movements of weaving machines. Needles wereactivated by the sensing of the presence or absence

15、 of a hole in the card. Thisinvention was followed by automatic piano players (Pianola), in which thekeys were activated by air flowing through holes punched in a perforated rollof paper. The principle of numerically controlling the movements of machine toolswas first conceived in the 1940s by J. Pa

16、rsons in his attempt to machinecomplex helicopter blades. The first prototype NC machine was built in 1952at the Massachusetts Institute of Technology. It was a vertical-spindle, two-axiscopy milling machine retrofitted with servomotors, and the machiningoperations performed consisted of end milling

17、 and face milling on a thickaluminum plate The numerical data to be punched into the paper tapes were generated by adigital computer, another invention which was being developed at the sametime at MIT. In the experiments, parts were machined successfully, accurately,and repeatedly without operator i

18、ntervention. On the basis of this success, themachine-tool industry began designing, building, and marketing NC machinetools. Later, these machines were equipped with computer numerical controls(CNC) yielding greater flexibility, accuracy, versatility, and ease of operation.5.2 Computer Numerical Co

19、ntrol In the next step in the development of numerical control, the controlhardware (mounted on the NC machine) was converted to local computercontrol by software. Two types of computerized systems were developed: directnumerical control, and computer numerical control. In direct numerical control (

20、DNC), as originally conceived anddeveloped in the 1960s, several machines are directly controlled, step by step,by a central mainframe computer. In this system, the operator has access to thecentral computer through a remote terminal. In this way, the handling of tapesand the need for a separate com

21、puter on each machine are eliminated. WithDNC, the status of all machines in a manufacturing facility can be monitoredand assessed from the central computer. However, DNC has a crucialdisadvantage: If the computer shuts down, all the machines become inoperative. A more recent definition of DNC (now

22、meaning distributed numericalcontrol) covers the use of a central computer serving as the control systemover a number of individual computer numerical control machines havingonboard microcomputers. This system provides large memory and computationalcapabilities and offers flexibility while overcomin

23、g the disadvantage of directnumerical control. Computer numerical control (CNC) is a system in which a controlmicrocomputer is an integral part of a machine or a piece of equipment(onboard computer). The part program may be prepared at a remote site by theprogrammer, and it may incorporate informati

24、on obtained from draftingsoftware packages and from machining simulations, in order to ensure that thepart program is bug free. The machine operator can, however, easily andmanually program onboard computers. The operator can modify the programsdirectly, prepare programs for different parts, and sto

25、re the programs. Because of the availability of small computers having a large memory,microprocessor(s), and program-editing capabilities, CNC systems are widelyused today. The availability of low-cost programmable controllers also playeda major role in the successful implementation of CNC in manufa

26、cturing plants. Some advantages of CNC over conventional NC systems are the following: (1) Increased刀exibility -the machine can produce a specific part, followedby other parts with different shapes, and at reduced cost. (2) Greater accuracy -computers have a higher sampling rate and fasteroperation.

27、 (3) More versatility -editing and debugging programs, reprogramming,and plotting and printing part shape are simpler.5.3 Principles of NC Machines The basic elements and operation of a typical NC machine are shown inFig.5.2. The functional elements in numerical control and the componentsinvolved fo

28、llow. (1) Data input: The numerical information is read and stored in the tapereader or in computer memory. (2) Data processing: The programs are read into the machine control unitfor processing. (3) Data ou印ut: This information is translated into commands (typicallypulsed commands) to the servomoto

29、r (Fig.5.3). The servomotor then moves thetable (on which the workpiece is mounted) to specific positions, through linearor rotary movements, by means of stepping motors, leadscrews, and othersimilar devices. Types of Control Circuits. An NC machine can be controlled through twotypes of circuits: op

30、en-loop and closed-loop. In the open-loop system(Fig.5.3(a), the signals are sent to the servomotor by the controller, but themovements and final positions of the work table are not checked for accuracy. The closed-loop system (Fig.5.3(b) is equipped with various transducers,sensors, and counters th

31、at measure accurately the position of the work table.Through feedback control, the position of the work table is compared againstthe signal. Table movements terminate when the proper coordinates arereached. The closed-loop system is more complicated and more expensive thanthe open-loop system. Posit

32、ion measurement in NC machines can be accomplished throughdirect or indirect methods. In direct measuring systems, a sensing device readsa graduated scale on the machine table or slide for linear movement(Fig.5.4(a). This system is the more accurate because the scale is built into themachine, and ba

33、cklash (the play between two adjacent mating gear teeth) in themechanisms is not significant In indirect measuring systems, rotary encoders or resolvers (Figs.5.4(b)and S.4(c) convert rotary movement to translational movement. In this system,backlash can significantly affect measurement accuracy. Po

34、sition feedbackmechanisms utilize various sensors that are based mainly on magnetic andphotoelectric principles.5.4 Types 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

35、axis of themachine is driven separately by leadscrews an氏depending on the type ofoperation, at different velocities. The machine moves initially at maximumvelocity in order to reduce nonproductive time, but decelerates as the toolapproaches its numerically defined position. Thus, in an operation suc

36、h as drilling(or punching a hole), the positioning and cutting take place sequentially(Fig.5.5(a). After the hole is drilled or punched, the tool retracts upward andmoves rapidly to another position, and the operation is repeated. The pathfollowed from one position to another is important in only on

37、e respect: It must bechosen to minimize the time of travel, for better efficiency. Point-to-point systemsare used mainly in drilling, punching, and straight milling operations. (2) In a contouring system (also known as a continuous path system), thepositioning and the operations are both performed a

38、long controlled paths but atdifferent velocities. Because the tool acts as it travels along a prescribed path(Fig.5.5(b), accurate control and synchronization of velocities and movementsare important. The contouring system is typically used on lathes, millingmachines, grinders, welding machinery, an

39、d machining centers. Interpolation. Movement along the path (interpolation) occursincrementally by one of several basic methods (Fig.5.6). Examples of actualpaths in drilling, boring, and milling operations are shown in Fig.5.7. In allinterpolations, the path controlled is that of the center of rota

40、tion of the tool.Compensation for different types of tools, for different diameters of tools, orfor tool wear during machining can be made in the NC program. (a) In linear interpolation, the tool moves in a straight line from start toend (Fig.5.6(a), on two or three axes. Theoretically, all types of

41、 profiles can beproduced by this method, by making the increments between the points small(Fig.5.6(b). However, a large amount of data has to be processed in order todo so. (b) In circular interpolation (Fig.5.6(c), the inputs required for the pathare the coordinates of the end points, the coordinat

42、es of the center of the circleand its radius, and the direction of the toot along the arc. (c) In parabolic interpolation and cubic interpolation, the path isapproximated by curves, using higher-order mathematical equations. Thismethod is effective in 5-axis machines and is particularly useful in di

43、e sinkingoperations for sheet-forming of automotive bodies. These interpolations arealso used for the movements of industrial robots.5数控数字控制（NC）是控制运动的方法机组件由直接插入的编码指令，在形式数字和字母，进入系统。该系统自动地解释这些数据，并将它们转换为输出信号。这些信号，反过来，控制各种机部件，例如，通过接通主轴和关闭，改变工具，移动工件或沿着特定路径的工具，或转向切割和关闭流体。为了欣赏机数控的重要性，让我们简单回顾一下如何处理，如机加工历来执行

44、。学习的部分的施工图后，操作员设置适当的工艺参数（如切割速度，进给，切削深度，切削液，等），确定了加工操作的序列要执行，夹具在一个工件夹紧装置，工件（诸如一卡盘或夹头），并进行与零件的制造。取决于部位的形状和指定的尺寸精度，这方法通常需要熟练的操作人员。加工遵循的程序可以取决于特定运营;由于人类的可能性错误，即使零件用相同的操作者产生的可能并不全部是相同的。部质量可能，因此，依赖于特定的操作员或（甚至与在一周的某一天或一天中的小时同一运营商）。由于以提高产品质量，降低制造日益关注成本，例如可变性（及其对产品质量的影响）不再可以接受的。这种情况下，可以由数控被消除加工操作。数值控制的重要性可以通

45、过进一步说明下面的例子。假设几个孔被钻出的一部份在图5所示的位置。 1。在加工这一部分，操作员的传统手工方法相对于所述工件定位在钻头，使用参考点通过任何图中示出的三种方法进行说明。然后，操作者前进到钻孔。让我们先假设100份，均具有完全相同相同的形状和尺寸精度，将被钻出。显然，这操作将是繁琐的，因为操作员必须经过同样的动作重复。此外，该概率是高的，关于各种原因，一些加工的零件将是与众不同。现在，让我们假设该生产运行期间，这些零件顺序被改变，并且10的部件的现在需要在不同的位置上的孔。该机械师现在有重新定位的工作表;这个操作将是时间耗时并受错误。这样的操作可以很容易地通过数控机器进行其能够反复和

46、准确的操作和生产的零件不同的部分（通过简单地加载不同的部分节目，如将要描述更高版本）。在根据数字控制操作，有关的各方面的数据加工操作，如位置，速度，饲料，和切削液，可以被存储在磁性媒体，从磁带到硬盘随时间而改变。该NC控制的概念是具体的信息可以从这些中继存储设备到机床的控制面板。上的输入信息，继电器和其它设备的基础上（硬连线控制）可被致动以获得所需的机器设置。复操作（例如车削一部分具有各种轮廓，或开模在一个铣床）可以很容易地进行。数控已经对制造业的各个方面产生重大影响操作。数控机床，现在广泛使用的小型和中等数量生产（通常为500份或更少）的各种各样的部分，无论是在小商店和大型制造工厂。旧机器通

47、常可以加装数控。5.1历史回后面的数字控制的基本概念显然被实施在19世纪初，在钣金冲卡时，孔被用来自动控制织布机的动作。针头通过在卡的孔的存在与否的感测动作。此发明随后自动钢琴播放器（自动钢琴），其中，所述按键被空气流经眼儿的多孔辊筒激活纸。数值控制的机床的动作的原理在20世纪40年代首次设想由J.帕森斯在他企图机复杂直升机的螺旋桨。第一架原型机数控机床始建于1952年在麻省理工学院。它是一个垂直主轴，两轴仿形铣床加装伺服电机和机械加工执行的操作包括端铣和铣端面上厚厚铝板的数值数据被冲压成由一个被生成的纸带数字计算机，其被开发以相同的另一发明时间在麻省理工学院。在实验中，部分被成功地加工，准确

48、并多次无需操作员干预。在此成功的基础上，机床行业开始设计，建造和销售数控机床工具。后来，这些机器都配备了计算机数字控制（CNC）产生了更大的灵活性，准确性，多功能性和易于操作。5.2计算机数字控制在数控发展的下一步骤中，控制硬件（安装在数控机床上）转化为本地计算机由软件控制。两种类型的计算机系统被开发：直接数控和计算机数控。直接数字控制（DNC），按照当初的设想，并发达的20世纪60年代，几台机器直接控制，分步实施，由中央计算机主机。在这个系统中，操作员有权访问通过远程终端的中央计算机。以这种方式，带的处理并需要在每台计算机上的单独的计算机被消除。同DNC，所有机器的制造工厂的地位，可以监测并评估从中央计算机。然而，DNC具有至关重要缺点：如果计算机关机，所有的机器变得不可操作。DNC的更近的定义（现在的意分布数值控制）覆盖用的中央计算机的作为控制系统在具有多个单独的计算机数字控制机器的板载微型计算机。该系统提供了大容量内存和计算能力和报价，而直接克服的缺点灵活性数控。计算机数字控制（CNC）是一个系统，其中的控制微机是机器的或不可缺少的一部分一台设备（车载电脑）。零件程序可以在远程站点通过制备程序员，并且它可以包含从牵伸获得的信息软件包和从加工仿真中，为了确保零件加工程序是免费的错误。机器操作员可以，但是，容易且手动编程机载计算机。操