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1、TEXT READING MATERIAL,Unit 22,TEXT Fundamentals of Numerical Control Controlling a machine tool by means of a prepared program is known as numerical control, or NC. NC equipment has been defined the Electronic Industries Association (EIA) as“A system in which actions are controlled by the direct ins
2、ertion of numerical data at some point. The system must automatically interpret at least some portion of this data”.,In a typical NC system the numerical data which is required for producing a part is maintained on a punched tape and is called the part program. The part program is arranged in the fo
3、rm of blocks of information, where each block contains the numerical data required to produce one segment of the work-piece. The punched tape is moved forward by one block each time the cutting of a segment is completed. The block contains, in coded form, all the information needed for processing a
4、segment of the work-piece: the segment length, its cutting speed, feed, etc.,Dimensional information (length, width, and radii of circles) and the contour form (linear, circular, or other) are taken from an engineering drawing. Dimensions are given separately for each axis of motion (x, y, etc.). Cu
5、tting speed, feed-rate, and auxiliary function (coolant on and off, spindle direction, clamp, gear changes, etc.) are programmed according to surface finish and tolerance requirements.,Preparing the part program for a NC machine tool requires a part programmer. The part programmer must possess knowl
6、edge and experiences in mechanical engineering. Part Programmers must be familiar with the function of NC machine tools and machining process and have to decide on the optimal sequence of operations.,In NC machine tools, each axis of motion is equipped with a separate driving device which replaces t
7、he hand-wheel of the conventional machine. The driving device may be a DC motor, a hydraulic actuator, or a stepping motor. The type selected is determined mainly by the power requirements of the machine.,The NC machine tool system contains the machine control unit (MCU) and the machine tool itself.
8、 The MCU has to read and decode the part program, to provide the decoded instructions to the control loops of the machine axes of motion, and to control the machine tool operation.,The MCU consists of two main units: the data processing unit (DPU) and the control loops unit (CLU). The function of th
9、e DPU is to decode the information received from the tape, process it and provide data to the CLU. Such data contains the new required position of each axis, its direction of motion and velocity, and auxiliary control signals to relays. On the other hand, the CLU provides a signal announcing that th
10、e previous segment is completed and that the DPU can read a new block of the part program. The CLU operates the drives attached of the machine lead-screws and receives feedback signals on the actual position and velocity of each one of the axes. Each lead-screw is equipped with a separate driving de
11、vice and a separate feedback device, but the latter exists only in a closed-loop system.,In CNC systems, the DPU functions are always performed by the control program contained in the CNC computer. The major part of the CLU, however, is always implemented in the most sophisticated CNC systems.,LEARN
12、 TO SPEAK Mr. Zhou:Good morning, Miss Yang. Miss Yang: Hi, Mr. Zhou. Mr. Zhou: What are you doing now? Miss Yang: I am turning a work-piece. Mr. Zhou: Is this lathe different from that lathe? Miss Yang:Yes. It is. Mr. Zhou: Where are the distinctions about them? Miss Yang:This is an NC lathe, and th
13、e other is a conventional lathe. Mr. Zhou: What is NC? Miss Yang:NC refers to numerical control, and controlling a machine tool by means of a prepared program.,Mr. Zhou: What does the NC machine tool system contain? Miss Yang:It contains the machine control unit and the machine tool itself. Mr. Zhou
14、: Do you like to machine a work-piece on NC machine tools or conventional machine tools. Miss Yang:I would prefer to use NC machine tools very much. Mr. Zhou: Why do you think that? Miss Yang:Because it has some advantages such as increased productivity, reduced the noise, improved operators safety
15、and product quality, and so on. Mr. Zhou: It sounds quite well. Miss Yang:You will like it in the future, too. Mr. Zhou: I think so. Thanks a lot. Miss Yang:You are welcome.,NEW WORDS AND PHRASES interpret In5t:prIt v. 解释说明 coolant 5ku:lEnt n. 冷却液 optimal 5BptIm(E)l adj. 最佳的，最理想的 decode di:5kEJd v.
16、解码，译码 implemented 5ImplImEnt v. 实行，执行 sophisticated sE5fIstIkeItId adj. 复杂的,numerical control (NC) 数字控制 the Electronic Industries Association (EIA) (美国)电子工业协会 auxiliary function 辅助功能 surface finish 表面粗糙度 D.C. motor 直流电机 stepping motor 步进电机 machine control unit (MCU) 机床控制单元 data processing unit(DPU)
17、数据处理单元 computer numerical control (CNC) 计算机数字控制,NOTES 1NC equipment has been defined as “A system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some portion of this data”. 数控设备被定义为：“采用在某些点直接插入数据来控制操作的系统，此系统至少
18、要能自动解读部分数据。” 这个句子中，A system 是as的宾语。A system后是由which引导的定语从句，修饰system。,2Part programmers must be familiar with the function of NC machine tools and machining process and have to decide on the optimal sequence of operations. 零件编程人员要熟悉数控机床的功能及加工工艺，并决定最佳的加工顺序。 句中，the optimal sequence of operations意思是最佳的加
19、工顺序。,3In NC machine tools, each axis of motion is equipped with a separate driving device which replaces the hand-wheel of the conventional machine. 在数控机床中，每个轴向的运动都要有单独的驱动装置代替普通机床中的手轮来驱动。 句中，which引导的定语从句修饰device。,4Each lead-screw is equipped with a separated driving device and a separate feedback de
20、vice, but the latter exists only in a close-loop system. 每个丝杠都有一个单独的驱动装置和反馈装置，不过反馈装置只出现在闭环系统中。,EXERCISES 1put the following into Chinese and recite them: (1) 1) NC: Numerical Control 2) CNC: Compute Numerical Control 3) MCU: Machine Control Unit 4) CPU: Center Process Unit 5) DPU: Data Processing Un
21、it 6) CLU: Control Loops Unit,(2) 1) dimensional information 2) auxiliary function 3) NC machine tools 4) The optimal sequence of operations 5) a DC motor 6) a stepping motor,2Fill in the blanks with proper words: machines controlled by means of prepared programs are called machines. In NC system, t
22、he part program is maintained on a or punched card. Each block of program contains some information in form. After one block is carried out, the punched tape is moved , so another block of information is read to the machine. The part program is often prepared by a part , who have to decide on the se
23、quence of . Programming may be done or with the aid of computer, which is usually called (CAP).,3Translate the following passage into Chinese: Numerical Control (NC) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols.
24、The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpart or job. When the job changes, the program of instruction is changed. The capability to change the program is what makes NC suitable for low- and medium-volume production
25、.,READING MATERIAL Numerical Control of Machining Process Numerical control of machine tools by computers has made great advances in recent years. Generally,automatic tools are efficient and accurate when large quantities of parts of the same shape are required,but changing from one automatic cycle
26、to another is a long and expensive process and would only be carried out for a long run.But there are many examples when only a few parts are required and it is in such work that the computer-controlled machine is valuable.,There are two types of computers-digital and analogue. A digital computer wo
27、rks with numbers given in digital form,a digit being one of a certain set of symbols used to show numbers.It is the digital computer which is used in electronic data processing.The analogue computer is based on an analogy of quantity.It deals with physical quantities and not numbers.,In a digital co
28、ntrol system information from a drawing is put on a disc which is put into a computer which computers from the information to the continuous motion of the cutting tool. The output is put on a disc which is put into the machine-tool control unit when required. One computer services a number of machin
29、es. Disc can be stored and used any number of times in future cases,simply by replaying them if the same parts are wanted again.For best results,the electronic equipment and the machine tool must be designed together.,This method has also been applied to the control of oxygen cutting of steel plates
30、.When irregular shapes are required,as in ship building,it has been the pracice to make a model and to follow its outline by hand control,a method which does not always give accuate results.Again,information is processed by a special computer which then prepares a disc for use in the control console
31、 of the cutting machine.With this disc the machine can automatically cut plates of complicated shapes very accurately.This development has particular importance for making steel plates and is a good example of automation applied to batch production.,PROFESSIONAL WORDS AND PHRASES analogue 5AnElB n. 模拟量 analogy E5nAlEdVI n. 类似，相似，模拟 symbol 5sImbEl n. 记号，符号 console kEn5sEul vt. 安慰，慰问 plate pleIt n. 金属板 automation C:tE5meiFEn n. 自动化 batch bAtF adj. 整批的 batch production 批量生产,