1、1在線教務輔導網：在線教務輔導網：http:/教材其余課件及動畫素材請查閱在線教務輔導網教材其余課件及動畫素材請查閱在線教務輔導網QQ:349134187 或者直接輸入下面地址：或者直接輸入下面地址：http:/Chapter1 Introduction1 Background 2 Construction and working principles3 Types and characteristics 4 Future development1 Background1.1 The development of NC machine tool After World War II, With

2、 the increasing complexity of aircraft components, in 1949, US air force asked Parsons company to develop a special equipment to automatically machine those components. With the concept of numerically controlled machine tool by Parsons company, in 1952, a control device that based on vacuum tube and

3、 relay has been developed for milling machine control. Vacuum tube, known as applied on first generation of NC machine tool.Vacuum tubeBy the end of 1950s, the second generation NC machine tool whose control circuits are consisted exclusively by transistors were successfully developed. After that, t

4、hose expensive and fragile vacuum tube NC machine tools were gradually replaced.With the development of NC machine tools, practicability, flexibility, convenient maintenance and adaptability were required by users.transistorHowever, transistor circuits cannot meet all these requirements due to econo

5、my. In 1965, with the development of integrated circuits, the third generation of NC machines provide solutions for those requirements.Stage1 -1970s Numerical control (NC)Stage2 1970s Computer numerical control (CNC)Stage3 1970s-1980s Microprocessor control. It had significantly improved characteris

6、tics and reliability of machine tools. From then on, NC technology were rapidly developed and widely applied all over the world. Stage4 1990s- PC platform. 1.2 Definition of NC machine toolNC technology- controlling mechanical movement and working process by numerical signals；NC machine tool- machin

7、e tools controlled by program system that can logically execute special codes. Compared with conventional machine tools, a NC machine tool is a kind of automatic machining facility. When machining with a conventional machine tool, operators manipulate hand wheels to move the cutting tool along the d

8、esired contour of work piece. While using a NC machine tool, its NC system can complete the machining process guided by program. NumericalizedCutting pathNC equipmentinputdecodingcalculatingMachine toolmovement2. Construction and working principle of NC machine tools2.1 Construction of NC machine to

9、olsNC machine tools have different purposes in various fields, however, each type of NC machine tool has similar constructions.Control mediumNC deviceServomechanismMachine toolAssistant control1) Control mediumNC machine tool requires no direct manual operation during working, but must perform as op

10、erator required. Therefore, a bridge between human being and machine tool must be established. The bridge is known as control medium, carrying all required machining information for NC device.The exact type of control medium has punched tape, punched card, magnetic tape, magnetic disk and USB hubs,

11、depends on the NC device. Machining information must be transferred to NC device through readers, such as photoelectrical tape readers, tape machines, disk drivers and USB ports.Punched tapePunched cardMagnetic diskTape reader2) NC control equipmentNC device is the brain of a NC machine tool. Today,

12、 most NC machining tools are controlled by microprocessors.NC device requires software to read and decode information, and requires hardware to run the software. Construction of NC deviceControl mediumReaderInput portProcessor&ControllerMemoryOutput portServo mechanism3) Servo systemAs the execu

13、tor of a NC system, servo system consists of servo motor and servo drive system. Servo system drive cutting tools or worktable, and/or actuates accessories to machine workpieces that required. The operating information is in the form of pulses. Each pulse produces a displacement of a movable part, w

14、hich is known as pulse equivalent, usually equals to 0.001mm. 4) Assistant control systemAssistant control system is a sort of electric control equipment connecting NC device and mechanical components. It receives instruction signals (e.g. spindle speed, tool changing, coolant supply) from NC device

15、. After decoding, logic judgment and power amplification for the signals, the machine tool will be acting as desired driven by its electric, hydraulic, pneumatic or mechanical appliances. Moreover, assistant control systems feedback some switching signals to CNC.Oil pump5) Machine tool realityMachin

16、e tool reality is the main body of a NC machine tool, consisting of basis (e.g. base and bed) and movable components (e.g. worktable, carriage （床鞍）, spindle). It is the mechanical components to perform machining processes, similar to conventional machine tools.It features of:Driven by high performan

17、ce spindle and servo transmissions；Better rigidity and anti-vibration of structure；Efficient transmission components, e.g. lead screw pairs, rolling ways, etc. CJK6136 NC latheProgramming according to drawingsInput program into NC deviceDecodingServo mechanisms control machining process (spindle, fe

18、eding, tool changing, clamping, cooling, lubricating, etc.)DrawingsProgrammingNC deviceMachiningProductsServo mechanismProgram listMedium2.2 Working principle of NC machine tools3 Classifications and features of NC machine tools3.1 Types1. By functions1) General NC machine toolsSimilar to convention

19、al machine tools, NC lathe, NC milling machine, NC borer, NC driller, NC grinder, etc. were developed for different machining purposes. Each of them can be further classified, e.g. vertical milling machine, horizontal milling machine, gantry milling machine（龍門銑床）, etc.They have similar functions as

20、conventional machine tools, but can perform machining for parts with much higher accuracy and complex contour.2) Machining centerMachining center is a kind of NC machine tool with magazine and cutting tool changing mechanism. Typical machining center including milling center and turning center.Machi

21、ning center has better accuracy, efficiency, automatization and lower running cost due to concentrated machining processes, because of:Reducing the number of machine tools required;Reducing preparing time (e.g. tool alignment) before machining;Reducing errors between clampings.2By motion control typ

22、es1) Point-to-pointFor this type, only the end point of motion need to be precisely controlled, i.e. precise locating from a point to another. The path and speed of motion are not accurately controlled, and there is no machining process during moving and locating.Motion contrail of point-to-point co

23、ntrolTo reduce time for moving and locating, moving parts usually rapidly approaches end point, and then precisely reaches target point at low speed to ensure accuracy. Point-to-point control is applied on NC driller, NC punch, NC spot welding machine, etc.2) Straight line controlBesides precise loc

24、ation of start point and end point, straight line control must ensure moving path and speed (for different feedrate)between them. Machining is performed during moving, usually, the cutting paths are parallel to coordinate axes, and sometimes inclined straight lines.Straight line control is usually a

25、pplied on NC lathes, NC grinders and NC milling machines. 3) Contouring controlContouring control is also known as continuous control, which is applied on most NC machine tools. It controls at least two axes at the same time, and has interpolation. Besides start point, end point and moving path, mov

26、ing speed at each point along the path needs to be controlled.Contour ofwork pieceInterpolationUnder contouring control, parts with complex curves and curve surfaces can be machined. The control type is usually applied on NC lathe, NC milling machine and machining center.ThinkingWhat are their motio

27、n types?3By servo mechanismsAccording to their inspection and feedback components, servo system on machine tools can be classified intoOpen loop servoClosed loop servoSemi-closed loop servo1) Open loop controlUnder open loop control, moving speed and displacement of worktable is determined by freque

28、ncy and number of pulse, because their is no inspection and feedback component. Therefore, the accuracy of the system depends on driving components and step motor.StepmotorInstructionpulseWorktableLead screwBearingsSpindleCutting toolOpen loop machine tool has following advantages:Simple constructio

29、nLow costConvenient adjustmentStable performanceOpen loop control system is suitable for comparatively low accuracy applications, e.g. economical machine tools.BearingLead screwLocationcomparingTachometerSpeedcontrolServomotorSpeed feedbackWorktableInstructionLocation feedbackSpindleCutting toolLoca

30、tion detectingBearing2) Closed loop controlInspection components were built in worktable. NC device compares instruction to real location of worktable, and then corrects errors. Closed loop system can eliminate the effects of transmission errors. However, in fact, mechanical components have non-line

31、ar parameters, e.g. friction, rigidity, clearance, etc. it made parameter adjustment difficult. Any defect may result in system instability.Closed loop control system is usually applied on large machine tools where high precision is required, e.g. NC milling/boring machine, super-precision lathe, su

32、per-precision grinder, etc.3) Semi-closed loop controlIt is applied on most NC machine tools. For this type, inspection components are fixed at the end of motor shaft. With inspection components that have high resolution (高分辨率) , the system is able to provide reasonable accuracy, which is usually be

33、tween that of open loop and closed loop. Moreover, the system is stable because of short control circuit. Lead screwLocationcomparingSpeed & location detecting componentSpeedcontrolServomotorSpeed feedbackWorktableBearingInstructionLocation feedbackSpindleCutting tool3.2 Features of NC machine t

34、oolsFor mass production, automatic machining may obtain better cost efficiency. The basis of automatic mass production is stringent machining process.However, for small quantity production, varying products require different machining process, therefore, conventional automatic production may be repl

35、aced by flexible machining.Flexible machining technology is known as computer controlled automatic production, including single CNC machining equipment and accessories. Therefore, NC machine tools are the most important part for flexible automation. Compared to conventional machining facility, NC ma

36、chine tools have following advantages:(1) Good adaptability. Especially for machining parts with complex contour and in small quantity. Reprogramming is enough for machining a completely different part.(2) High accuracy and stable production quality. Pulse equivalent of 0.001mm, high rigidity, therm

37、al stability and clearance eliminating mechanism ensure high machining accuracy.Besides, NC machine tools automatically perform machining thus avoid the manual error.(3) High production efficiency. Great rigidity allows large feedrate and cutting depth. Automatic changing machining position and cutt

38、ing tools eliminates transporting, clamping and measuring between machining procedures.(4) Reduced manual labor and improved working environment. Automatic machining requires little manual operation.40(4) Reduced manual labor and improved working environment. Automatic machining requires little manu

39、al operation.(5) Cost-efficient.Though expensive capital cost increases depreciation (折舊) , for small quantity production, NC machine tools save time and cost for preparing, fixture, adjustment, and inspection;Stable accuracy reduces wasters;A NC machine tools can perform many tasks to save total number of machine tools. (6) Providing possibility for modern production management.NC machine tools transfer and process information by st