1、The development trend of the numerical control technology1. NUMERICAL CONTROLNumerical control(NC)is a form of programmable automation in which the processing equipment is controlled by means of numbers，letters，and other symbolsThe numbers，letters，and symbols are coded in an appropriate format to de
2、fine a program of instructions for a particular workpart or jobWhen the job changes，the program of instructions is changedThe capability to change the program is what makes NC suitable for low-and medium-volume productionIt is much easier to write programs than to make major alterations of the proce
3、ssing equipmentThere are two basic types of 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 o
4、nly one motor is runningMachines of this type are principally used for straightline cuts or for drilling or boring The NC 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 NC equipmentData
5、input，also called“mantocontrol link”，may be provided to the machine tool manually，or entirely by automatic meansManual methods when used as the sole source of input data are restricted to a relatively small number of inputsExamples of manually operated devices are keyboard dials，pushbuttons，switches
6、，or thumbwheel selectorsThese are located on a console near the machineDials ale analog devices usually connected to a syn-chro-type resolver or potentiometerIn most cases，pushbuttons，switches，and other similar types of selectors aye digital input devicesManual input requires that the operator set t
7、he controls for each operationIt is a slow and tedious process and is seldom justified except in elementary machining applications or in special casesIn practically all cases，information is automatically supplied to the control unit and the machine tool by cards，punched tapes，or by magnetic tapeEigh
8、tchannel punched paper tape is the most commonly used form of data input for conventional NC systemsThe coded instructions on the tape consist of sections of punched holes called blocksEach block represents a machine function，a machining operation，or a combination of the twoThe entire NC program on
9、a tape is made up of an accumulation of these successive data blocksPrograms resulting in long tapes all wound on reels like motion-picture filmPrograms on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loopOnce installed，the tape is used again and
10、again without further handlingIn this case，the operator simply loads and unloads the partsPunched tapes ale prepared on type writers with special tapepunching attachments or in tape punching units connected directly to acomputer systemTape production is rarely error-freeErrors may be initially cause
11、d by the part programmer，in card punching or compilation，or as a result of physical damage to the tape during handling，etcSeveral 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 programming steps al
12、e done manuallyBefore the coded tape may be prepared，the programmer，often working with a planner or a process engineer, must select the appropriate NC machine tool，determine the kind of material to be machined，calculate the speeds and feeds，and decide upon the type of tooling needed. The dimensions
13、on the part print are closely examined to determine a suitable zero reference point from which to start the programA program manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the dr
14、awing specificationsThe control unit receives and stores all coded data until a complete block of information has been accumulatedIt then interprets the coded instruction and directs the machine tool through the required motionsThe function of the control unit may be better understood by comparing i
15、t to the action of a dial telephone，where，as each digit is dialed，it is storedWhen the entire number has been dialed，the equipment becomes activated and the call is completedSilicon photo diodes，located in the tape reader head on the control unit，detect light as it passes through the holes in the mo
16、ving tapeThe light beams are converted to electrical energy，which is amplified to further strengthen the signalThe signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drivesSome photoelectric devices are capable of reading at rates up to 1000 characters per secondHigh reading rates are necessary to maintain continuous machinetool motion；otherwise dwell m
