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1、,English on Mechanical Engineering Lesson 3 Advantages of CNC,Text,Recent studies show that of the amount of time an average part spent in a shop, only a fraction of that time was actually spent in the machining process. Let us assume that a part spent 50 hours from the time it arrived at a plant as
2、 a rough casting or bar stock to the time it was a finished product. During this time, it would be on the machine for only 2.5 hours and be cut for only 0.75 hour. The rest of the time would be spent on waiting, moving, setting up, loading, unloading, inspecting the part, setting speeds and feed, an
3、d changing cutting tools.,CNC( computer numerical control) machines do not necessarily produce components more accurately than NC machines, and are only faster in operation owing to the speed with which the program blocks are transmitted to the control unit. CNC reduces the amount of non-chip-produc
4、ing time by selecting speeds and feeds, making rapid moves between surfaces to be cut, using automatic fixtures, automatic tool changing, controlling the coolant, in-process gaging, and loading and unloading the part. These factors, plus the fact that it is no longer necessary to train machine opera
5、tors, have resulted in considerable savings throughout the entire manufacturing process and caused tremendous growth in the use of CNC.,Text,Some of the major advantages of CNC are as follows: 1.The amount of input information is reduced. 2.There is automatic or semi-automatic operation of machine t
6、ools. The degree of automation can be selected as required. 3.Flexible manufacturing of parts is much easier. Only the part programming needs changing to produce another part. 4.Storage space is reduced. Simple work holding fixtures are generally used, reducing the number of jigs or fixtures which m
7、ust be stored.,Text,5.Small part lots can be run economically. Often a single part can be produced more quickly and better by CNC 6.Nonproductive time is reduced. More time is spent on machining the part, and less time is spent on moving and waiting. 7.Tooling costs are reduced. In most cases comple
8、x jigs and fixtures are not required. 8.Inspection and assembly costs are lower. The quality of the product is improved , reducing the need for inspection and ensuring that parts fit as required.,Text,Text,9. Machine utilization time is increased. There is less time that a machine tool is idle becau
9、se workpiece and tool changes are rapid and automatic. 10.Complex forms can easily be machined. The new control unit features and programming capabilities make the machining of contours and complex forms very easy. 11.Parts inventory is reduced. Parts can be made as required from the information wit
10、hin the control unit.,Text,Since the first industrial revolution, about 200 years ago, NC has had a significant effect on the industrial world. The developments in the computer and CNC have extended a per sons mind and muscle. The CNC unit takes symbolic input and changes it to useful output, expand
11、ing a persons concepts into creative and productive results. CNC technology has made such rapid advances that it is being used in almost every area of manufacturing. Such as machining, welding, pressworking and assembly.,Text,If industrys planning and logic are good, the second industrial revolution
12、 will have as much or more effect on society as the first industrial revolution had. As we progress through the various stages of CNC, it is the entire manufacturing process which must be kept in mind. Computer-aided manufacturing(CAM) and computer-integrated machining(CIM) are certainly where the f
13、uture of manufacturing lies, and considering the developments of the past, it will not be too far in the future before the automated factory is a reality.,Text,Developed originally for use in aerospace industries, NC is enjoying widespread acceptance in manufacturing. The use of CNC machines continu
14、es to increase, becoming visible in most metalworking and manufacturing industries. Aerospace, defense contract, automotive, electronic, appliance, and tooling industries all employ numerical control machinery. Advances in microelectronics have lowered the cost of acquiring CNC equipment. It is not
15、unusual to find CNC machinery in contract tool, die, and moldmaking shops. With the advent of low cost OEM(original equipment manufacturer) and retrofit CNC vertical milling machines, even shops specializing in one-of-a-kind prototype work are using CNCs.,Although numerical control machines traditio
16、nally have been machine tools, bending, forming, stamping and inspection machines have also been produced as numerical control systems.,Words and Expressions:,1. CNCCompute numerical control 2. fraction n.小部分, 片断, 分数 3. machining process 加工过程, 加工程序 4. rough casting/bar stock n. 原料,库存 5. finished product 成品,产品 6. program block block n. 块, 石块, 街区, 7. non-chip-producing time chip v. 削成碎片n. 芯片,碎片 8. automatic fixture/semiautomatic/jig automatic adj.自动的n.自动机械 fixture n.
