1、Motors and Numerical Control Motors change electrical energy into mechanical energy . This is simply the mechanical forces between magnets .when two magnets (or electromagnets)approach each other ,one will be either pulled toward or pushed away from the other . 1.CONSTRUCTION Each type of motor is b
2、uilt to do its own special job .All of them have two main parts .One part is called the stator .The stator is stationary ；it does not move .The other part ,the rotor ,is mounted on bearings so it can rotate .The rotor shaft sticks out beyond the housing .For a motor ,the rotor shaft is coupled to a
3、mechanical load . 2.MOTORS The power input to a motor is electrical .Voltage is applied to a motors terminals resulting in a current .The power output of a motor is mechanical .This power is transmitted by the rotor shaft as a torque .This torque tends to rotate a load , such as a fan or pump . As a
4、 motor drives a load , mechanical power is drawn from the motor .The motor ,in turn ,draws electrical power from the source at the same rate 3.NUMERICAL CONTROL The major disadvantage of machine tool automation thus far described lies in the economics of the process .It is expensive to tool a instal
5、lations for automatic production .Therefore ,unless the part is to be made in very large numbers ,the cost becomes prohibitive .Great need exists for a method that permits rapid automatic production ,economical in job-lot amounts .The answer has been found in the numerical control of machine tools .
6、 Computer Numerical Control .Numerical control (NC) 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 par
7、ticular workpiece or job .When the job changes ,the program of the particular workpiece changes .The capability to change the program is what makes NC suitable for low-volume and medium-volume production ,and it is much easier to write new programs than to make major alterations to the processing eq
8、uipment . There is a wide area of performance duplication between numerical control and automatics .Numerical control , however ,offers more flexibility ,lower tooling cost ,quicker changes ,and less machine down-time . In machining contours ,numerical control can mathematically translate the define
9、d curve into a finished product ,saving time and eliminating templates .This can in turn improve accuracy .Another advantage appears to be great saving of machine time ,the equivalent og increasing productive capacity with no increase in facilities . Numerical control (N/C) is a form of programmable
10、 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 qppropriate format to define a program of instructions for a particular workpart or job .When the job changes ,the program of instructions i
11、s changed .The capability to change the program is what makes N/Csuitable 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 of numerically controlled machine tools :point-to-point and conti
12、nuous-path (also called contouring).Point-to-point machines use unsynchronized motors ,whit 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 . The N/C system consists of the following components :data input ,the
13、tape reader with the control unit ,feedback devices ,and the metal-cutting machine toll or other type of N/C equipment . Data input ,also called “man-to-control link” ,may be provided to the machine tool manually ,or entirely by automatic means .Manual methods when used as the sole source of input d
14、ata are restricted to a relatibely small number of inputs .Examples of manually operated devices are kryboard dials ,pushbuttons ,switches ,or thumbwheel selectors .These are located on a console near the machine .Dials are analog devices usually connected to a synchro-type resolver or potentiometer
15、 .In most cases ,pushbuttons ,switches ,and other similar types of selectors are digital input devices .Manual input requires that the operator set the controls for each operation .It is a slow and tedious process and is seldom justified except in elementary machining applications or in special case
16、s . In practically all cases , information is automatically supplied to the control unit and the machine too by cards ,punched tapes ,or by magnetic tape .Eight-channel punched paper tape is the most commonly used form of data input for conventional N/C systems .The coded instructions on the tape co
17、nsist 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 tape is made up of an accumulation of these successive data blocks .Programs resulting in long tapes are wound on reels like mot
18、ion-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 and again without further handling .In this case ,the operator simply loads and unloads the parts .Punched tapes are prepared o
19、n typewriters with special tape-punching attachments or in tape punching units connected directly to a computer system .Tape production is rarely error-free .Errors may be initially caused by the part programmer ,in card punching or compilation ,or as a result of physical damage to the tape during h
20、andling ,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 programming steps are done manually .Before the coded tape may be prepared ,the programmer ,often working with a planner or a
21、 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 needed .The dimensions on the part print are closely examined to determine a suitable zero reference point from which to
22、 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 cut the part to the drawing specifications . The function of the control unit may be better understood by comparing it to the
