《数控技术》双语 电子教案 Chapter 1_数控技术双语
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河南科技大学教案首页
课程名称 数控技术（双语）任课教师 韩建海
Chapter 1 Introduction to Numerical Control 计划学时 6 教学目的和要求：
Through the study of this chapter ,students should grasp the basic concepts of NC, CNC ,NC machine tools and so on;understand the characteristics and main technical indicators of CNC machine tools;master the composition and claification of CNC machine tools;know the prospects of Numerical Control.重点：
1.Basic Components and claification of NC machine tools.2.Application of NC machine tools.3.Prospects of Numerical Control.难点：
1.Concept of interpolation．
2.Control principle and composition of NC machine tools.思考题：
1.What are the components of NC machine tool，and the function of each component? 2.What is the Point-to-point control，contouring control NC machine tool? 3.What is the open-loop control，half-closed-loop control and closed-loop control NC machine ? 4.What are the Advantages and Disadvantages of NC? 5.What work parts are suited for machining by NC machine?
内容提要
This chapter introduces the basic concepts of the numerical control technology and NC machine tools;components of CNC machine tools;characteristics of CNC machining;Control principle and claification of CNC machine tools;scope of application of CNC machine tools;history and prospects of NC technology and NC machine tools.Chapter 1 Introduction to Numerical Control 1.1 FUNDAMENTALS OF NC TECHNOLOGY 1.1.1 Development History of NC The concept for NC dates from the late 1940s．The numerical data was stored in the punched cards．
The first NC machine was developed in 1952．MIT met the challenge succefully，and in 1952 demonstrated a Cincinnati Hydrotel milling machine equipped with the new technology，which was named Numerical control(NC)and used a pre-punched tape as the input media．
Since 1952，practically every machine tool manufacturer in the Western world has converted part or its entire product to NC．
The first NC machines used vacuum tubes，electrical relays，and complicated machine control interfaces(1952)．The second generation of machines utilized improved miniature electronic tubes(1959)，and later small scale integrated circuits(1965)．
As computer technology improved，NC underwent one of the most rapid changes known in history．The fourth generation used much improved integrated circuit(1970s)．
The fifth generation is microproceor CNC(1980s)．
Among the strengths of the fifth generation microproceor CNC(MCNC)are added part program memory storage，reduction of printed circuit boards，programmable interface，faster memory acce，parametric subroutines，and macro capabilities．
1.1.2 Concept of NC and CNC Numerical control(NC)is a form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data．
The capability to change the program makes NC suitable for low and medium production．It is much easier to write new programs than to make major alterations of the proceing equipment．
Numerically controlled(NC)machine tools were developed to fulfill the contour machining requirements of complex aircraft parts and forming dies．The first-generation numerically controlled units used digital electronic circuits and did not contain any actual central proceing unit；thereby they were called NC or hardwired NC machine tools．In 1970s，computer numerically controlled(CNC)machine tools were developed with minicomputers used as control units．With the advances in electronics and computer technology，current CNC systems employed several high-performance microproceors and programmable logical controllers that work in a parallel and coordinated fashion．Current CNC systems allow simultaneous servo position and velocity control of all the axis，monitoring of controller and machine tool performance，online part programming with graphical aistance，in-proce cutting proce monitoring，and in-proce part gauging for completely unmanned machining operations． Manufacturers offer most of these features as options．Today，virtually all the new machine control units are based on computer technology；hence，when we refer to NC in chapter and elsewhere, we mean CNC．
1.1.3 Basic Components of NC Machine Tools 1.The work proce of NC.2.A typical NC machine tool has five fundamental units：
(1)the input media，(2)the machine control unit，(3)the servo-drive unit，(4)the feedback transducer，and(5)the mechanical machine tool unit．The general relationship among the five components is illustrated in Figure 1.2．
(1)The input media contains the program.of instructions，it is the detailed step-by-step commands that direct the actions of the machine tool；the program of instructions is called a part program.The individual commands refer to positions of a cutting tool relative to the worktable on which the work part is fixtured.Additional instructions are usually included, such as spindle speed , feed rate, cutting tool selection, and other functions．The program is coded on a suitable medium for submiion to the machine control unit．For many years，the common medium was 1-inch wide punched tape，using a standard format that could be interpreted by the machine control unit．Today，punched tape has largely been replaced by newer storage technologies in modern machine shops．These technologies include magnetic tape，diskette，and electronic transfer of part programs from a computer．(2)In modern CNC technology，the machine control unit(MCU)consists of a microcomputer and related control hardware that stores the program of instructions and executes it by controlling each command into mechanical actions of machine tool，one command at a time．The MCU includes system software，calculation algorithm，and transition software to covert the NC parts program into a usable format for the MCU．
(3)The third basic component of an NC system is the servo-drive unit；the drives in machine tools are claified as spindle and feed drive mechanisms．Spindle and feed drive motors and the。servo-amplifiers are the components of the servo-drive unit．The MCU procees the data and generates discrete numerical position commands for each feed drive and velocity command for the spindle drive．The numerical commands are converted into signal v01tage by the，MUC unit and sent to servo-amplifiers, which proce and amplify them to the high voltage levels required by the drive motors．
(4)The forth basic component of an NC system is the feedback transducer．As the drives Move, sensors measure their actual position．The difference between the required position and the actual position is detected by comparison circuit and the action is taken，within the servo． to minimize this difference．
(5)The fifth basic component of an NC system is the machine tool that performs useful Work.It accomplishes the proceing steps to transform the starting workpiece into a completed part.Its operations are directed by the MCU，which in turn is driven by instructions contained in the part program．In the most common example of NC，machine tool consists of the worktable and spindle．
1.2 CLASSIFICATIONS OF NC MACHNINES Numerical control machines are claified in different way：(1)the types of NC motion control system，(2)the type of servo-drive system，(3)application of NC．
1.2.1 Types of NC Motion Control System Some NC procees are performed at discrete locations on the workpart(e．g．，drilling，punching and spot welding)．Others are carried out while the workhead is moving(e．g.turning，milling and continuous arc welding)．If the workhead is moving，it may be required to follow a straight-line path or a circular or other curvilinear path．These different types of movement are accomplished by the motion control system．
Motion control systems for NC can be divided into two types：
(1)point-to-point,(2)continuous path，those features are explained below．
1.2.1.1 Point-to-Point Control Systems Point-to-point system，also called Positing control systems.Moves the worktable to a programmed location without regard for the path． Such as drilling or punching a hole． As depicted in Figure 1．3．
Phenomenon : zigzag path ,straight-cut(45 vectors)．
01.2.1.2 Contouring Control Systems The contouring facility enables an NC machine to follow any path at any prescribed feed-rate．The contouring control system，also called continuous path control systems，manages the simultaneous motion of the cutting tool in two，three，four，or five axes(the fourth and fifth axes are angular orientations)by interpolating the proper path between prescribed points． In this case，the tool performs the proce while the worktable is moving，thus enabling the system to generate angular surfaces，two-dimensional curves，or three-dimensional contours in the workpart．This control mode is required in many milling and turning operations．A simple two-dimensional profile milling operation is shown in Figure 1.4 to illustrate continuous path contro1．
When continuous path control is utilized to move the tool parallel to only One of the major axes of the machine tool worktable，this is called straight-cut NC．When continuous path control is used for simultaneous control of two or more axes in machine operations，the term contouring is used．All NC contouring systems have the ability to perform linear interpolation and circular interpolation．
1.2.1.3 Interpolation One of the important aspects of contouring is interpolation．The paths that a contouring type NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes．Some of these shapes can be defined mathematically by relatively simple geometric formulas，whereas others cannot be mathematically defined except by approximation．In any case，a fundamental problem in generating these shapes using NC equipment is that they are continuous，whereas NC is digital．To cut along a circular path，the circle must be divided into a serious of straight-line segments that approximate the circular path．The tool is commanded to machine each line segment in succeion So that the machined surface closely matches the desired shape．The maxim error between the nominal(desired)surface and the actual(machined)surface can be controlled by the lengths of the individual line segments as explained in Figure 1.5．
If the programmer were required to specify the endpoints for each of the line segments，the programming task would be extremely arduous and fraught with errors．Also，the part program would be extremely long because of the large number of points．To ease the burden，interpolation routines have been developed that calculate the intermediate points to be followed by the cutter to generate a particular mathematically defined or approximated path.A number of interpolation methods are available to deal with the various problems encountered in generating a smooth continuous path．They include：(1)Linear interpolation，(2)circular interpolation，(3)helical interpolation，(4)parabolic interpolation，and(5)cubic interpolation．Each of these procedures permits the programmer to generate machine instructions for linear or curvilinear paths using relatively few input parameters．The interpolation module in the MCU performs the calculation and directs the tool along the path．In CNC systems，the interpolation is generally accomplished by software．Linear and circular interpolations are almost always included in modern NC systems，whereas helical interpolation is a common option．Parabolic and cubic interpolations are le common；they are only needed by machine shops that must produce complex surface contours． 1.2.2 Types of NC Servo-Drive Systems
In a NC machine，the MCU accepts information in the form of punched，magnetic tape codes or stored program．These input data must be transformed by the MCU into specific output codes in terms of voltages，or pulses per second．The transformed data，called output is used to drive the motors to position the machine slides to the programmed position．These slides，or table drives，are commonly known as servo-drives．The principal function of NC is the positioning of the tool 0r the machine table in accordance with the programmed data．Industry has developed three different types of drives based on how the NC system accomplishes positioning．These are the open-loop，the closed-loop and half closed-loop drive system．
1.2.2.1 Open-Loop Servo-Drive An 0pen-100p control system is the simplest and least cost form of servo-drive．It is characterized as a system that lacks feedback as in Figure 1．6；that is，once an input control signal is sent.there is no sensing device to confirm the action of the control signal．
In the open-loop control NC machine，the servomotor is usually stepping motor.The stepping motor output shaft rotates in direct proportion to pulses received．It has the advantages of high accuracy，easy implementation and compatible with digital signals，but it has the disadvantages of low torque，limited speed and risk of mied pulse under load．So the open 100p contr01 system is used in the economic NC machine．With an open loop system，there is always the risk that the actuator will not have the intended effect on the proce，and that is
the disadvantage of an open loop system．Open-loop systems are usually appropriate when the following conditions apply：(1)The actions performed by the control system are simple，(2)the actuating function is very reliable，and(3)any reaction forces opposing the actuation are small enough to have no effect on the actuation．If these characteristics are not applicable，then the feedback control system may be appropriate．There are two kinds of feedback control system，one is a closed-loop control and another is half-closed-loop system．
The open-loop application in general，is restricted to smaller machines because of the limited power output availability with the stepping motors(a typical maximum is 4～5kW and a torque of 200Nm)．Again the pulses per second restrict the speed of the drive．A typical maximum for stepping motors is 1 6000 pulses per second．When this is applied to a system requiring 0.001 mm accuracy，the resultant maximum speed would be 0.96m／min．Again for high-precision application like jig boring where an accuracy of 0.001mm is to be maintained，an open-loop system do-se-s not serve the purpose．There is，of course，no doubt that 0n light duty machinery where the 15roblems of instability are absent and also the requirements are not of high precision ,open-close servo control does offer some cost saving solution．Usually the open-loop NC system is called economical NC system．
1.2.2.2 Half-Closed-Loop Servo-Drive A half-closed-loop control NC system is one of the feedback control system as i11ustrated in Figure 1．7，uses feedback measurements to ensure that the worktable is moved to the desired position．It is characterized as a system that the indirect feedback monitors the output of servomotor．Although this method is popular with NC systems，it is not as accurate as direct feedback．The half-closed-loop system compares the command position signal with the drive signal of the servomotor． In operation，the half-closed-loop system is directed to move the Worktable to a specified location as defined by a coordinate value in a Cartesian system．Most positioning system have at least two axes with a contro1 system for each axis，but our diagram only illustrates one of these axes．A servomotor connected to a leadscrew is a common actuator for each axis．A signal indicating the coordinate value is sent from the MCU t0 the motor that drive the leadscrew, whose rotation is converted into linear motion of positioning table．As the table moves closer to the desired coordinate value，the difference between the actual position and the input
value is reduced．The actual position is measured by a feedback sensor，which is attached to servomotor axis or lead screw．This system is unable to sense backlash or lead screw windup due to varying loads，but it is convenient to adjust and has a good stability．
1.2.2.3 Closed-Loop Servo-Drive A closed-loop control system is another feedback control system as illustrated in Figure 1．8．It is characterized as a system that the direct feedback monitors the output of servomotor．A feedback sensor directly measures the position of worktable．The closed-loop control system，with its drive signal originated by the worktable，is the preferred system because it monitors the actual position of the worktable on which the part is mounted．It is more accurate；however，its implementation costs are higher． A half closed-loop or closed-loop system uses conventional variable-speed AC or DC motors，called servos，to drive the axes。1.2.3 Application of CNC The operating principle of CNC has many applications．There are many industrial operations in which the position workhead must be controlled relative to part or product being proceed．The applications are divided into two categories：(1)machine tool applications and(2)non-machine tool applications．Machine tool applications are those usually aociated with the metalworking industries．Non-machine tool applications comprise a diverse group of operations in other industries．It should be noted that the applications are not always identified by the name“numerical control”；this term is used principally in the machine tool industry 1.2.3.1 Machine Tool Applications The most common applications of NC are in machine tool control，machining was the first application of NC．Machining is a manufacturing proce in which the geometry of work IS produced by removing exce material．By controlling the relative motion between a cutting tool and the workpiece，the desired geometry is created．
There are four common types of machining operations：(1)turning，(2)drilling，(3)milling，and(4)grinding．Each of the four machining procees is traditionally carried out on a machine tool designed to perform that procees．Turning is performed on a lathe, drilling is done drilling pre ,milling on a milling machine ,and so on ．The common NC done machine tools are listed in the following along with their typical features．
NC lathe，either horizontal or vertical axis．Turing requires two-axis continuous path control，either to produce a straight cylindrical geometry or to create a profile．
NC boring。mill，horizontal or vertical spindle．Boring is similar to turning，except that an internal cylinder is created instead of an external cylinder．The operation requires continuous path，two-axis contr01．
NC drill pre．These machines are use point-to-point control of workhead(spindle containing the drill bit)and two axis(*y)control of the worktable．Some NC
drill prees have turrets containing six or eight drill bits．The turret position is programmed under NC control，thus allowing different drill bits to be applied to the same workpart during the ma-chine cycles without requiring the machine operator to manually change the tool．
NC milling machine．Milling machines require continuous path control to perform straight cut or contouring operations．
NC grinding machine．NC grinding machine is intended for finishing treatment of work-parts．The grinding machine includes cylindrical-，surface-，internal-，spindle-，thread-，gear and tool grinding machines．
Machining center．Machining centers have been defined as multifunction NC machines with automatic tool changer and tool storage．Since their introduction in the late l 950s，they have become one of the most common of all cutting machines．Increased productivity and versatility are the major advantages of machining center．The ability to perform drilling，turning，reaming，boring，milling，contouring，and threading operations on a single machine eliminates the need for a number of individual machine tools，thus reducing capital equipment and labor requirements．Additional savings result from reduced materials handling，fixture costs，and floor space requirements．Substantial time conventionally spent moving work from machine t0 machine is saved，and throughput is much faster．Also，in-proce inventory，represented by kinds of work-pieces normally seen at several machines，is replaced by work at only one ma-chine．
Machining centers，are claified as either vertical or horizontal．Vertical machining centers continue to be widely accepted and used，primarily for flat parts and where three-axis machining is required on a single part face such as in mold and die work．Horizontal machining centers are also widely accepted and used，particularly with large，boxy，and heavy parts． 1.2.3.2 Non-Machine Tool Applications In addition to the machining proce，NC machine tool have also been developed for other metal working procees，these machines inc Punch prees for sheet metal hole punching.The two-axis NC operation is similar to that of a drill pre except that holes are produced by punching rather than by drilling.Prees for sheet metal bending．Instead of cutting sheet metal，these systems bend sheet metal according to programmed commands．
Welding machine．Both spot welding and continuous arc welding machines are available With automat IC controls based on NC．
Thermal cutting machine．Such as oxyfuel cutting，laser cutting，and plasma arc cutting． The stock is usually flat：thus two-axis control is adequate．Some laser cutting machines can cut holes 1n preformed sheet metal stock，requiring four or five axis control．
Tube bending machine．Automatic tube bending machines are programmed to control the location(along length of the tube stock)and the angle of the bend．Important applications include frames of bicycles and motorcycles． 1.3 FEATURES OF NUMERICAL CONTROL AND ITS
APPLICATION AREAS 1.3.1 Advantages and Disadvantages of NC When the production application satisfies the characteristics：(1)batch production in small or medium lot sizes；(2)repeat orders at random or periodic intervals；(3)complex part geometry；(4)Much metal needs to be removed from part；(5)many separate machining operations on the part and(6)the part is expensive．NC yields many benefits and advantages over manual production methods．These benefits and advantages translate into economic savings for the user company．However，NC is more sophisticated technology than conventional production methods，and there are drawbacks and cost that must be considered apply the technology effectively．In this section，we examine the advantages and disadvantages of NC．
The advantages generally attributed to NC，with emphasis on machine tool applications，are the following：
(1)Greater accuracy and repeatability．Compared with manual production methods，NC reduces or eliminates variations that are due to operator skill differences，fatigue，and other factors attributed to inherent human variability．Parts are made closer to nominal dimensions，and there is le dimensional variation among parts in the batch．
(2)More complex part geometries are poible．NC technology has extended the range of poible part geometries beyond what is practical with manual machining methods．1 his is an advantage in product design in several ways：①More functional features can be designed into a single part，thus reducing the total number of parts in the product and the aociated cost of aembly；②mathematically defined surfaces can be fabricated with high precision；and③the space is expanded within which the designer’s imagination can wander to create new part and product geometries．
(3)Nonproductive time is reduced．NC cannot optimize the metal cutting proce itself，but it does increase the proportion of time the machine is cutting metal，reduce the workpiece handling time，and carry out automatic tool changes on some NC machines．This advantage translates into labor cost savings and lowers elapsed times to produce parts．
(4)Lower scrap rates．Because greater accuracy and repeatability are achieved，and human errors are reduced during production，more parts are produced within tolerance．As a consequence，a lower scrap allowance can be planed into the production schedule，so fewer Darts are made In each batch with the result that production time is saved．
(5)Inspection requirements are reduced．Le inspections are needed when NC is used because parts produced from the same NC part program are virtually identical．Once the pro-gram has been verified，there is no need for the high level of sampling inspection that is required when parts are produced by conventional manual methods．Except for tool wear and equipment malfunctions，NC produces exact replicates of the part each cycle．
(6)Engineering changes can be accommodated more gracefully．Instead of making alterations in a complex fixture so that the part can be machined to the engineering change，revisions are made in the NC part program to accomplish the change.(7)Simpler fixtures are needed．NC requires simpler fixtures because accurate positioning of the tool is accomplished by the NC machine t001．Tool positioning does not have to be designed into the jig．
(8)Shorter manufacturing lead times．Jobs can be set up more quickly and fewer setups are required per part when NC is used．This results in shorter elapsed time between order re-lease and completion．
(9)Reduced parts inventory．Because fewer setups are required and jig changeovers are easier and faster，NC permits production of parts in smaller lot sizes．The economic lot size is lower in NC than in conventional batch production．Average parts inventory is therefore reduced．
(10)Le floor space required．This results from the fact that fewer NC machines are required to perform the same amount of work compared to the number of conventional machine tools needed．Reduced parts inventory also contributes to lower floor space requirements．
(11)Operator skill-level requirements are reduced．The skill requirements for operating an NC machine are generally le than those required to operate a conventional machine t001． Tending an NC machine usually consists only of loading and unloading parts and periodically Changing tools.The machining cycle is carried out under program control.Changing tools for some NC machine tools can even be carried out by program contr01．Performing a comparable machining cycle in a conventional machine requires much more participation by the operator，and a higher level of training and skill are needed．
On the opposing side，the disadvantages of NC include the following：
(1)Higher investment cost．An NC machine tool has a higher first cost than a comparable conventional machine tool.There are several reasons why:①NC machines include CNC controls and electronics hardware；②software development costs of the CNC controls and manufacturer must be included in the cost of the machine；③more reliable mechanical components are generally used in NC machine；and④NC machine tools often poe additional features not included on conventional machines，such as automatic tool changers and part changers．
(2)Higher maintenance effort．In general，NC equipment requires a higher level of maintenance than conventional equipment requires，which translates to higher maintenance and repair costs．This is due largely to the computer and other electronics that are included in modern NC system．The maintenance staff must include the persons who are trained in maintaining and repairing this type of equipment．
(3)Part program．NC equipment must be programmed．To be fair，it should be mentioned that proce planning must be accomplished for any part，whether or not it is produced 0n NC equipment．However，NC part programming is a special preparation step in batch production that is absent in conventional machine shop operations．
(4)Higher utilization of NC equipment．To maximize the economic benefits of an NC machine tool，it usually must be operated multiple shifts．This might mean adding one or two extra shifts to the plant’s normal operations，with the requirement for supervision and other staff support． 1.3.2 Application Areas for NC Machine Tools As with other expanding technologies，there is tendency to consider numerical control as a final solution to a broad range of manufacturing problems；however，NC application in certain manufacturing situations would be highly undesirable.Figure 1.9(a)illustrates the appropriate application area for NC，based on the criteria of number of parts to be produced and their complexity；(b)illustrates the relations between the cost of machining and the number of parts．
An NC machine is most efficiently used in an environment that takes advantage of inherent flexibility of NC．The precise level of control attributed to a numerically controlled device enables it to perform complex operations often beyond the capability of a human operator． For these reasons，NC is best suited to relatively low volume runs of complex and varied component s．However，NC can also be used to produce large numbers of complex components and／or small numbers simple ones．
The application areas of general-purpose conventional machines and special purpose automated machines are also illustrated in Figure 1．9．The lines separating the application areas represent general boundaries only．Depending on manufacturing requirements and available equipment，significant variations can occur． 本章小结
Generally, this chapter introduces the basic concepts of the numerical control technology and NC machine tools;components of NC machine tools;characteristics of NC machining;Control principle and claification of NC machine tools;scope of application of NC machine tools;and the history and prospects of NC technology and NC machine tools.This chapter is the one of the focus of this course.
