机械毕业设计英文外文翻译460数字控制 (2)

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科技译文：

Numerical Control

Numerical Control(NC) is a method of controlling the movements of machineComponents by directly inserting coded instructions in the form of numerical data(numbers and data ) into the system.The system automatically interprets these data and converts to output signals. These signals ,in turn control various machine components ,such as turning spindles on and off ,changing tools,moving the work piece or the tools along specific paths,and turning cutting fluits on and off.

In order to appreciate the importer of numerical control of machines ,let’s briefly review how a process such as machining has been carried out traditionally .After studying the working drawing of a part, the operator sets up the appropriate process parameters(such as cutting speed ,feed,depth of cut,cutting fluid ,and so on),determines the sequence of operations to be performed,clamps the work piece in a workholding device such as chuck or collet ,and proceeds to make the part .Depending on part shape and the dimensional accuracy specified ,this approach usually requires skilled

operators.Furthermore,the machining procedure followed may depend on the particular operators,and because of the possibilities of human error, the parts produced by the same operator may not all be identical .Part quality may thus depend on the particular operator or even the same operator on different days or different hours of the day. Because of our increased concern with product quality and reducing manufacturing costs,such variability and its effects on product quality are no longer acceptable this situation can be dliminated by numerical control of the mathining operation.

We can illustrate the importance of numerical concrol by the following example. Asume that holes have to be drilled on a part in the positions shown in the picture.In the traditional manual method of machining thid part, the operator positions the drill with respect to the workpiece, using as erfererence points any of the three of the method shown. The operator then proceeds to drill these holes. Let,s assume that 100 parts, having exactly the same shape and dimensional accuracy, have to be dielled. Obviously, this operation is going to be tediious because the operator has to go throuth the same motions again and again. Moreover, the probabiluty is high that, faor warious reasons, some of the paths machined will be different from

orthers. Let`s further assume that during this production run, the order for these paths is changed, so that 10 of the paths now require holes in difficult positions. The machinisst now has to reset the mathine , wehich will be time consuming and subject to error. Such operations can be perfomedeasily by numerical conctrol machines that are capable of producing parts reeatedly and accurately and of handling differeent parts by simply londing differeent part programs.

In numerical control ,data concerning all aspects of the machining operation,such as locations ,speeds,and cutting fluid,are stored on magnetic tape.,cassettes ,floppy or hard disks,or paper or plastic(Mylar,which is a thermoplastic polyester)tape,.Date are stored on punched 25 mm wide paper or plastic tape ,as originally developed and still used .The concept of NC control is that holes in the tape represent specific information in the form of alphanumeric codes ,The presence (on)or absence (off),of these holes is read by sensing devices in the contuol panel ,which then actuate relays and other devices (called hard-wired contuols),These devices control various mechanical and electrical system in the machine .This method eliminnated manual setting of machine positions and tool paths or the use of templates and other