72页Machine Tools Objectived.Machine tools are the main engines of the manufacturing industry. This chapter covers a few of the details that are common to all classes of machine tools discussed in this book. After completing the chapter, the reader will be able to>understand the classification of the various machine tools used in manufacturing industries.>identify the differences between generating and forming of surfaces. > identify various methods used to generate different types of surfaces. >distinguish between the different accuracies and surface finishes that are achievable with different machine tools.>understand the different components of the machine tools and their functions.>learn about the different support structures used in the machine tools. >understand the various actuation systems that are useful to generate the required surfaces.>Learn the different types of guideways used in the machine tools.>understand the work holding requirements.3.1 INTRODUCTIONThe earliest known machine tools are the Egyptian foot-operated lathes.These machine tools were developed essentially to allow for the introduction of accuracy in manufacturing.A machine tool is defined as one which while holding the cutting tools, would be able to remove metal from a workpiece in order to generate the requisite job of given size, configuration, and finish. It is different from a machine, which is essentially a means of converting the source of power from one form to the other. The machine tools are the mother machines since without them, no components can be produced in their finished form. They are very old and the industrial revolution owes its success to them.A machine tool is required to provide support to the workpiece and cutting tools as well as provide motion to one or both of them in order to generate the required shape on the workpiece. The form generated depends upon the type of machine tool used.In the last two centuries, the machine tools have been developed substantially. The machine tool versatility has grown to cater to the varied needs Of the new inventors coming with major developments. For example,James Watt's steam engine could be proven only after a satisfactory method was found to bore the engine cylinder with a boring bar by Wilkinson around 1775.A machine tool is designed to perform certain primaryfunctions,but the extent to which it can be exploited to perform secondary functions is a measure of its flexi bility.Generally,the flexibility of the machine tool is inc reased by the use of secondary functional attachments,s uch as radius or spherical turning attachment for a cent re lathe.Alternatively,to improve productivity,special atta chments are added,which also reduce the flexibility.3.2CLASSIFICATION OF MACHINE TOOLSThere are many ways in which the machine tools can be classified.One such classification based on the produc tion capability and application is shown below:1.General purpose machine tools(GPM)are those designed to perform a variety of machining operations on a wide range of components.By the very nature of generalisation,the general purpose machine tools are thou gh capable of carrying out a variety of tasks,would no t be suitable for large production,since the setting time for any given operation is large.Thus,the idle time on the general purpose machine tools is more and the mac hine utilisation is poor.The machine utilisation may be termed as the percentage of actual machining or chip g enerating time to the actual time available.This is much lower for the general purpose machine tools.They m ay also be termed as the basic machine tools. Further,skilled operators would be required to run the general purpose machine tools.Hence,their utility is in job shops,such as catering to small batch and large v ariety job production,where the requirement is versatility rather than production capability.Examples are lathe,shaper,and milling machine.2Production machine tools are those where a number of functions of the machine tools are automated such t hat the operator skill required to produce the component is reduced.Also,this would help in reducing the idle t ime of the machine tool,thus improving the machine ut ilisation.It is also possible that a general purpose machi ne tool may be converted into a production machine to ol by the utilisation of jigs and fixtures for holding the workpiece.These have been developed from the basic m achine tools.Some examples are capstan lathes,turret la thes,automats,and multiple spindle drilling ma chines. The setting time for a given job is more.Also,tooling design for a given job is more time consuming and ex pensive.Hence the production machine tools can only beused for large volume production.3.Special purpose machine tools(SPM)are those mac hine tools in which the setting operation for the job a nd tools is practically eliminated and complete automationi s achieved.ms greatly reduces the actual manufacturing t ime of a component and helps in the reduction of cos ts.These tools are used for mass manufacturing.These machine tools are expensive compared to the general pur pose machines since they are specifically designed for the given application,and are restrictive in their application c apabilities.Examples are cam shaft grinding machine,conn ecting rod twin boring machine,and piston turning lathe.4.Single purpose machine tools are those,which are designed specifically for doing a single operation ona class of jobs or on a single job.These tools ha ve thehighest amount of automation and are used for really high rates of production.These are used specifically for one product only,and thus have the least flexibili ty.However,these do not require any manual interven tion andare most cost effective.Examples are transfer linescomposed of unit heads for completely machining any given product.The application of the above four types can be shown graphically in Fig. 3.1.Fig. 3.1Application of machine tools based on the capability. 3.3GENERATING AND FORMINGGenerally,the component shape is produced in machine tools by two different techniques,generating and forming. Generating is the technique in which the required pr ofile is obtained by manipulating the relative motionsof the workpiece and the cutting tool edge.Thus,the obtained contour would not be identical to the shape of the cutting tool edge.This is generally used for a majority of the general profiles required.The type of surface generated depends on the primary motion ofthe workpiece as well as the secondary or feed motio n of the cutting tool.For example,when the workpiece is rotated and a single point tool is moved along a straight line paralle l to the axis ofrotation of the workpiece,a helical s urface is generated,as shown in Fig. 3.2(a).If the pitch of the helix or feed rate is extremely small,or the surface generated may be approximated to a cylin der.This is carried out in ladles and is called turning or cylindrical turning.Fig. 3.2Generating and forming of surfaces by machine tools.An alternate method of obtaining the given profile is called forming in which,the shape of the cutting toolis impressed upon the workpiece,as shown in fig. 3.2 (b).Thus,the accuracy Of the obtained shape dependupon the accuracy of the form of the tool used.However,many of the machine tool operations areactually combinations of the above two.For example. when a dove tail is cut,the actual profile is obtained by sweeping the angular cutter along the straight line. Thus,it involves forming(angular cutter profile)and gene rating(sweeping along a line),as shown in Fig. 3.3.Fig3.3Generation of surface.3.4METHODS OF GENERATING SURFACESFig. 3.4Classification of machine tools using single point cuttingtools.A large number of surfaces can be generated or formed with the help of the motions given to the tooland the workpiece.The shape of the tool also makesa very important contribution to the final surface obtaine d Basically,there are two types of motions given in a machine tool.The primary motion given to the workpiece or cutting tool constitutes the cutting speed,which cause s a relative motion between the tool and workpiece suc h that the face of the cutting tool approaches the mat erial to be ually,the primary motion consum es most of the cutting power.The secondary motion is one which feeds the tool relatively past the workpiece. The combination of the primary and secondary motions is responsible for the generation of specific surfaces.Someti mes,there would be a tertiary movement in between thecuts for specific surfaces.A classification of machine tools based on the motions is shown in Fig. 3.4,for single point tools,an d Fig. 3.5for multi-point tools.In the case of job rot ation,cylindrical surfaces would be generated,as shown i n Fig. 3.6,when a tool is fed in a direction parallelto the axis of rotation.When the feeding direction is not parallel to the axis of rotation,complex surfaces, such as cones(Fig. 3.7),or contours(Fig. 3.8)can begenerated.The tools used in the above cases are of si ngle point.If the tool motion is perpendicular to the a xis of rotation,a plane surface would be generated,as shown in Fig. 3.9.However,if a cutting tool of a giv en form is fed in a direction perpendicular to the axis of rotation,also called plunge cutting,a contour surface of revolution would be obtained,as shown in Fig. 3.10.Fig. 3.5Classification of machine tools using multi-point cutting tools. Plane surface generation in shaping Plane surfaces can be generated when the job or tool reciprocates for the primary motion,as shown in Fig. 3.11,without any rota tion.With the multi-point tools generally plane surfaces aregene rated,as shown in Fig. 3.12.However,in this situation, a combination of forming and generating,is used to get a variety of complex surfaces,which are otherwise i mpossible to get through the single-point tool operations. Some typical examples are the spur gear hobbing and spiral milling of formed cavities.3.5ACCURACY AND FINISH ACHIEVABLEIt is necessary to select a given machine tool or m chining operation for a job such that it is the lowest cost option.There are various operations possible for a given type of surface and each one has its own charac teristics in terms of possible accuracy,surface finish,and cost.This selection is made at the time of process pla nning.The obtainable accuracy for various types of machi ne tools is shown in Table 3.1.The surface finish expe cted from the various processes is shown in Fig. 3.13.The values presented in Table 3.1and Fig. 3.13areonly a rough guide.The actual values greatly vary depe nding on the condition of the machine tool,the cutting tool used,and the various cutting process parameters.80Manufacturing TechnologyBASIC ELEMENTS OF MACHINE TOOLS3.6 BASIC ELEMENTS OF MACHINE TOOLSThe various components that are present in all the mac hine tools may be identified as follows:•Work holding device to hold the workpiece in the correct orientation to achieve the required in manufacturin g,for example chuck.•Tool holding device to hold the cutting tool in the correct position with respect to the workpiece,and provi de enough holding force to counteract the cutting forces acting on the tool,example tool•Work motion mechanism to provide the necessary sp eed to the workpiece for generating the surface,example head stock.•Tool motion mechanism to provide the various motio ns needed for the tool in conjunction with workpiece m otion in order to generate the required surface profiles, example carriage.•Support structure to support all the mechanisms sho wn above,and maintain their relative position with respe ct to each other,and allow for relative movement betw een the various parts to obtain the*requisite part pr ofile and accuracy,example bed.The type of device or mechanism used varies depending on the type of machine tool and the function it is expected to serve.In this chapter,someof the more common elements would be discussed.How ever,further details may be found in the chapters wher e the actual machine tools are discussed.The various motions that need to be provided in the machine tool are cutting speed and feed.The range of speed and feed rates to be provided in a given machi ne tool depends on the capability of the machine tool and the range of work materials that are expected to be processed.Basically,the actual speed and feed chosen depends upon the•work material,•required production rate,•required surface finish,and•expected accuracy.The drive units in a machine tool are expected to provide the required speed and convert the rotational sp eed into linear motion.Details of these may be foundin books dealing with machine tool design.3.7 SUPPORT STRUCTURESThe broad categories of support structures found in vario us machine tools are shown in Fig. 3.14.They may be classified as beds(horizontal structures)or columns(vertic al structures).The main requirements of the support structure are •Rigidity•Accuracy of guideways•Impact resistance•Wear resistanceBed provides a support for all the elements presentin a machine tool.It also provides the true relative po sitions Of all units in machine tools.Some of these un its may be sliding on the bed or fixed.For the purpo se Of sliding,accurate guideways are provided.Bed weig ht is approximately half the total weight of the machine tool.The basic construction of a bed is like a box,to provide the highest possible rigidity with low weight.To increase the rigidity,the basic box structure is added wi th various types of ribs,as shown in Fig. 3.15.The a ddition of ribs complicates the manufacturing process for the beds.Beds are generally constructed using cast iron or alloy c ast iron consisting of alloying elements,such as nickel,c hromium,and molybdenum.With cast iron,because of t he intricate designs of the beds,the casting defects may not be fully eliminated.Alloy steel structure is also used for making beds. The predominant manufacturing method used is welding.T he following advantages can be claimed for steel constru ction:(a)With steels,the wall thickness can be reduced .Thus,greater strength and stiffness for the same weight would be possible with alloy steel bed construction.(b)Walls of different thicknesses can be conveniently welded.Whereas in casting,this would create problems.(c)Repair of welded structures would be easier.(d)Large machining allowances would have to be provi ded for casting to remove the defects and hard Concrete is also tried as bed material.Its choice is ma inly because of the large damping capacity.For precision machine tools and measuring machines,granite is also us ed as the bed material.The major types of bed styles used in the machine tools are shown in Fig. 3.16.。