1、外文翻译 NUMERICAL CONTROL Numerical control(N C)is a form of programmable automation in which the processing equipment is controlled by means of numbers，letters， and other symbols The numbers， letters， and symbols are coded in an appropriate format to define a program of instructions for a particular w
2、ork part or job When the job changes， the program of instructions is changed The capability to change the program is what makes N C suitable for low-and medium-volume production It is much easier to write programs than to make major alterations of the processing equipment There are two basic types o
3、f 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 running Machines of thi
4、s type are principally used for straightline cuts or for drilling or boring The N C 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 N C equipment Data input， also called “mantocontrol
5、 link”， may be provided to the machine tool manually， or entirely by automatic means Manual methods when used as the sole source of input data are restricted to a relatively small number of inputs Examples of manually operated devices are keyboard dials， pushbuttons， switches， or thumbwheel selector
6、s These are located on a console near t he machine Dials ale analog devices usually connected to a syn -chro-type resolver or potentiometer In most cases， pushbuttons， switches， and other similar types of selectors aye digital input devices Manual input requires that the operator set the controls fo
7、r each operation It 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 tape Eightchannel
8、 punched paper tape is the most commonly used form of data input for conventional N C systems The coded instructions on the tape consist of sections of punched holes called blocks Each block represents a machine function， a machining operation， or a combination of the two The entire N C program on a
9、 tape is made up of an accumulation of these successive data blocks Programs resulting in long tapes all wound on reels like motion-picture film Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop Once installed， the tape is used again a
10、nd again without further handling In this case， the operator simply loads and unloads the parts Punched tapes ale prepared on typ e writers with special tapepunching attachments or in tape punching units connected directly to a computer system Tape production is rarely error-free Errors may be initi
11、ally caused by the part programmer， in card punching or compilation， or as a result of physical damage to the tape during handling， etc Several 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 progr
12、amming steps ale done manually Before the coded tape may be prepared， the programmer， often working with a planner or a process engineer, must select the appropriate N C machine tool， determine the kind of material to be machined， calculate the speeds and feeds， and decide upon the type of tooling n
13、eeded. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program A program manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to
14、 cut the part to the drawing specifications The control unit receives and stores all coded data until a complete block of information has been accumulated It then interprets the coded instruction and directs the machine tool through the required motions The function of the control unit may be better
15、 understood by comparing it to the action of a dial telephone， where， as each digit is dialed， it is stored When 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
16、passes through the holes in the moving tape The light beams are converted to electrical energy， which is amplified to further strengthen the signal The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives Some photoelectric devices ar
17、e capable of reading at rates up to 1000 characters per second High reading rates are necessary to maintain continuous machinetool motion； otherwise dwell marks may be generated by the cutter on the part during contouring operations The reading device must be capable of reading data blocks at a rate
18、 faster than the control system can process the data A feedback device is a safeguard used on some N C installations to constantly compensate for errors between the commanded position and the actual location of the moving slides of the machine tool An N C machine equipped with this kind of a direct
19、feedback checking device has what is known as a closed -loop system Positioning 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 unit Signals thus received ale compared to input signals on the tape，and any discrepancy between them is automatically rectified
