机械制造专业英语.ppt

MACHINING TECHNOLOGY1.3 BASIC MOTIONS IN MACHINE TOOLS1.4 ASPECTS OF MACHINING TECHNOLOGY1.3 BASIC MOTIONS IN MACHINE TOOLSIn conventional machine tools,a large number of product features are generated or formed via thevariety of motions given to the tool or the WP.The shape of the tool plays a considerable role in thefinal surface obtained.Basically,there are two types of motions in a machine tool.The primary motion,generally given to the tool or WP,constitutes the cutting speed.While the secondary motion feeds the tool relative to the WP.In some instances,combined primary motion is given either to the tool or to the WP.A classification of machine tool movements used for traditional machining is given in Table 1.2.Table 1.3 gives a classification for nontraditional machining technology.It may be concluded that movements of nontraditional machine tools are simple and mainly in the Z direction,while traditional machine tools have a minimum of two axes,that is,X and Y directions in addition to rotational movements.1.4 ASPECTS OF MACHINING TECHNOLOGYMachining technology covers a wide range of aspects that should be understood for proper understanding and selection of a given machining technology.Tooling,accessories,and the machine tool itself determine the nature of machining operation used for a particular material.As shown on the right-hand side of Figure 1.7,the main objective of the technology adopted is to utilize the selected machining resources to produce the component economically and at high rates of production.Parts should be machined at levels of accuracy,surface texture,and surface integrity that satisfy the product designer and avoid the need for postmachining treatment,which,in turn,maintains acceptable machining costs.The general aspects of machining technology include:1.4.11.4.21.4.31.4.41.4.51.4.6MACHINE TOOLWORKPIECE MATERIALMACHINING PRODUCTIVITYACCURACY AND SURFACE INTEGRITYPRODUCT DESIGN FOR ECONOMICAL MACHININGENVIRONMENTAL IMPACTS OF MACHINING1.4.1 MACHINE TOOLEach machine tool is capable of performing several machining operations to produce the part required at the specified accuracy and surface integrity.Machining is performed on a variety of general-purpose machine tools that in turn perform many operations,including chip removal and abrasion techniques,by which cylindrical and flat surfaces are produced.Additionally,specialpurpose machine tools are used to machine gears,threads,and other irregular shapes.Finishing technology for different geometries includes grinding,honing,lapping,and superfinishing techniques.Figure 1.8 shows generalpurpose machine tools used for traditional machining in chip removal and abrasion techniques.Typical examples of general-purpose machine tools include turning,drilling,shaping,milling,grinding,broaching,jig boring,and lapping machines intended for specific tasks.Gear cutting and thread cutting are examples of special-purpose machine tools.During the use of general-or special-purpose manual machine tools,product accuracy and productivity depend on the operators participation during operation.Capstan and turret lathes are typical machines that somewhat reduce the operators role during machining of bar-type or chucking-type WPs at higher rates and better accuracy.Semiautomatic machine tools perform automatically controlled movements,while the WP has to be hand loaded and unloaded.Fully automatic machine tools are those machines in which WP handling and cutting and other auxiliary activities are performed automatically.Semiautomatic and automatic machine tools are best suited for large production lots where the operators interference is minimized or completely eliminated,and parts are machined more accurately and economically.NC machine tools utilize a form of programmable automation by numbers,letters,and symbols using a control unit and tape reader,while CNC machine tools utilize a stored computer program to perform all the basic NC functions.NC and CNC have added many benefits to machining technology,since small and large numbers of parts can now be produced.Part geometry can be changed through the flexible control of the part programs.The integration of CAD/CAM systems to machining technology has created new industrial areas in die,mold,aerospace,and automobile industries.Hexapods have added a new area to the machining technology in which complicated parts can be machined using a single tool that is capable of reaching the WP from many sides.The hexapod has six degrees of movement and is very dexterous like a robot,but also offers the machine tool rigidity and accuracy generally beyond a robots capability.The hexapods are used to help develop machining processes for WPs that need the dexterity offered by the hexapod design.For generalpurpose machining,the hexapod is an ideal machine tool formold-and-die machining applications.Its ability to keep a cutting tool normal to the surface being machined prootes use of larger radii ball nose end mills,which can cut more material with very small stepovers.In some applications,a flat nose end mill can be used very effectively for smooth surface finishes with little or no cusp.Nontraditional machining uses a wide range of machine tools such as ECM,USM,EDM,and LBM.Each machine tool is capable of performing a variety of operations,as shown in Figure 1.9.Nontraditional machining technology tackles materials that range from glass,ceramics,hard alloys,heat-resistant alloys,and other materials that are diffi cult to machine by traditional machining technology.machine toolgeneral-purposespecial-purposemachine toolsemiautomaticfully automaticNCCNCCAD/CAMhexapodsnontraditional machining ECMUSMEDMLBM1.4.2 WORKPIECE MATERIALThe WP material specified for the part influences the selection of the adopted machining method.Most materials can be machined by a range of processes,some by a very limited range.In any particular case,however,the choice of the material depends on the desired shape and size,the dimensional tolerances,the surface finish,and the required quantity.It must not depend only on technical suitability,but also on economy and environmental considerations.1.4.3 MACHINING PRODUCTIVITYThe choice of any machining method should take into consideration a rate of production that is inversely proportional to machining time.Methods of raising productivity include the use of the following:High machining speedsHigh feed ratesMultiple cutting toolsStacking multiple partsMinimization of the secondary(noncutting)timeAutomatic feeding and tool changing mechanismsHigh power densities1.4.4 ACCURACY AND SURFACE INTEGRITYThe selection of a machining technology depends on inherent accuracy and surface quality.Below the machined surface,some alterations occur as a result of the material removal mechanisms employed.Careful examination of such a layer is essential.It affects the technological performance of the machined parts in terms of fatigue strength,corrosion,and wear resistance.In some cases,a postfinishing technology may be adopted to solve such problems,which in turn raises production cost.1.4.5 PRODUCT DESIGN FOR ECONOMICAL MACHININGThis concept is very important to produce parts accurately and economically.Product design recommendations for each operation should be strictly followed by the part designer.Design complications should be avoided so that the machining time is reduced,and consequently the production rate is increased.Machine tool and operation capability in terms of possible accuracy and surface integrity should also be considered,so that the best technology,machine tool,and operation are selected.1.4.6 ENVIRONMENTAL IMPACTS OF MACHININGThe possible hazards of the selected machining technology may affect the operators health,themachine tool,and the surrounding environment.Reduction of such hazards requires careful monitoring,analysis,understanding,and control toward environmentally clean machining technology.The hazards generated by the cutting fluids have led to the introduction of the minimum quantity lubrication(MQL),cryogenic machining,and dry machining techniques.Strict precautionsare followed during laser beam machining(LBM)and abrasive jet machining(AJM),and these processes are covered in Chapter 11.Explain the importance of adopting an environmentally friendly machining technology.QUESTIONTHANKS