下载文档原格式
Chapter 6 Machining AccuracyAbstractThe machining accuracy of machine parts is the quality foundation of the whole machine. It is usually measured with two indices: machining accuracy and surface quality and their value size will directly affect the performance and life of the whole machine. This chapter mainly involves the machining accuracy.The basic concepts,factors that are related with the machining accuracy, the comprehensive analysis of machining errors, and the methods of raising the machining accuracy are discussed in this chapter.Index7.1 Basic concepts of machining accuracy 7.2 factors affecting the machining accuracy 7.3 statistic analysis of machining errors7.4 methods of raising the machining accuracy6.1 Basic concept of machining accuracy6.1.1Machining accuracy and machining errorM achining accuracy：The degree of the practical dimensions after machining conforms to the desired geometrical parameters.The higher the degree conforms, the higher the machining accuracy.Machining error：The degree of the practical dimensions after machining deviates from the desired geometrical parameters. The size of the machining error denotes the size of the machining accuracy.“M achining accuracy”and“Machining error”are two different concepts to evaluate the accurate degree of the part geometries. In production practice, the accuracy is ensured by controlling the machining error or active adapting method 。
6.1.2 The method to study the machining accuracyThere are two methods to study the machining accuracy.One is the single factor analysis method,in which the machining accuracy is analyzed by calculation,experiment or inspection so as to decide the single factor’s influencing law to the single error.Another one is the statistic method,in which the statistical theory is used to process the inspected data of a batch of workpiece from the practice so as to control the process running in normal condition.These two methods are often used together in practice.Generally,the statistic method is first used to find out the law of the error,and judge the causes,then factor analysis method is used to find out the key factors which influence the machining accuracy.6.2 Factors affecting the machining accuracyPart machining happens in the processing system composed of machine tools, fixtures, cutting tool and workpiece.All the factors in the system that may cause machining errors are called the original errors.The existence of the original errors makes the positional or motional relationship deviated from its desired state, and brings about the machining error.If the original errors existed before machining,or in another word, it is causedwithout the cutting force, it is called the static processing error, otherwise, it is called the dynamic process error.Classification of original errorOrigianl error(原始误差）BeforemachiningIn machiningAfter machiningPrinciple errorAdjustment errorSetup errorMachine tool errorFixture errorCutting tool manu errorDeformation under forceThermal deformationCutting tool wearDeformation by inner stressInspection errorDynamic errorStatic errorDeformation under cutting force Geometric accuracy of the machine toolTool wear WorkpieceaccuracyControl system accuracyLocation and clampingaccuracyWeight deformationThermal deformation7.2.1 Error of machining principleMachining principle refers to the forming principle of the surface on the workpiece.The error of machining principle is caused by similar cutting motion of forming or similar cutting edge geometry. The purpose is to simplify the design and manufacturing of the machine tool and cutting tool, reduce the manufacturing cost, raise the production efficiency and facilitate the use.7.2.2 Error of the machine toolError of the machine tool refers to the manufacture error,setup error and wear come from the machine tool without cutting force.7.2.2.1spindle rotational error（1）concept of spindle rotational errorTheoretically,the position of the rotational axis of the spindle in the space is fixed, or in another word, the instant speed is zero.But in practice,there are a lot of factors influence the position of the spindle and make the spindle position change instantly. We call the maximum floating value in the sensitive direction of the error the rotational error of the spindle.There are three types of spindle rotational error, shown below：movieFig.1 the three types of the spindle error of the machine tool(a) Pure radial jump (b) pure axial jump (c) pure angular movePure radial jump ：The practical axis is parallel to the desired axis, but jump in constant amplitudePure axial jump：The practical axis moves along the desired axis with constant amplitude.Pure angular move：The practical axis moves inclined to the desired axis with a constant amplitude,and the joint point keeps constant.（2）influence of the spindle error on machining accuracy Different rotational error of the spindle has different influence on the machining accuracy,even the same rotational error has different influence when the machining method is different,shown as Fig.6.5、6.6、6.7、6.8and table 6.1.(a) Turning influenced by radial jump(b) Boring influenced by radial jumpmovie moviemovieEnd surface influenced by Angular moveBoring influenced by Angular movement error Table 1 influence of the spindle error on the surfaces under different machining method6.2.2.2 Error of the guidewayThe guideway of the machine tool is the main reference datum for the relative position and motion of the main components with the other components.It will influence the machining accuracy directly.（1）linear error in the vertical plane movieThe linear error in the vertical plane of the guideway on a h orizontal lathe or external cylindrical grinding machine can be omitted because the error is in the non-sensitive direction which is △R ≈△／2R .But for plane grinding machine or gantry planer,the error will directly reflect on the workpiece.2ZFig.6.9 The linear error of the guideway（2）linear error in horizontal planeFor a horizontal lathe,the linear error in horizontal plane is in the sensitive direction,shown as Fig.6.9(b),which will cause a cylindrical shape error△R＝△Y on the workpiece.But for plane grinding machine,vertical lathe and planer,the error can be ignored,as it is in the non-sensitive direction.（3）parallel error between the guidewaysThe parallel error(twisted)between the guideways will cause the shape error on the workpiece if a horizontal lathe is used.If the parallel error in the section of the cutting tool pass is△Z，the radial error on the workpiece is：△R≈△Y＝△Z·H/BNormally for lathe H／B≈2／3，for external grinding machine, H／B≈1. Therefore, it can not be ignored. movieFig.6.10 the parallel error between the guideways（4）parallel error between the guideways and the spindle If the error is in the horizontal plane,the cylindrical surface will have taper error；If it is in the vertical plane,the cylindrical surface will become a hyperbola shape(shown as figure).Because it is in the non-sensitive direction,it can be ignored.7.2.2.3 transmission errorWhen machining some surfaces such as helical surface, gear, worm wheel, the constant motional relationship between the cutting tool and the workpiece is ensured by the transmission system of the machine tool. Transmission error refers to the relative motional error of the elements at the two ends of the transmission chain.6.2.3 deformation of the machining systemthe machining system will deform under the cutting force,clamping force,transmission force,weight and inertial force，which will damage the correct position between the cutting tool and the workpiece and cause machining error.This phenomena can be explained by Fig.6.12(a),(b09c)Fig 6.12 error caused by deformation of the machining system （a）slim shaft deformation movie（b）short and big diameter workpiecedeformation moviesystemYsystem Y F K where ——the stiffness of the process system N ／mm ；——normal cutting force N ；——normal deformation mm 。
