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1、MechanicalMechanicalManufacturingManufacturingTechnologyTechnologyThe influence of residual stress on machining accuracy and its controlResidual stress refers to the stress that remains inside the workpiece after the external load is removed.Residual stress is caused by uneven volume changes in the
2、macro or micro structure inside the metal.The external factors come from hot processing and cold processing.Parts with residual stress are in an unstable state.Once the equilibrium condition of its internal stress is broken,the distribution of internal stress will change,causing new deformation and
3、affecting the processing accuracy.1.The source and characteristics of internal stressThe influence of residual stress on machining accuracy and its controlThe influence of residual stress on machining accuracy and its control(1)residual stress generated in blank manufacturingDuring hot processing su
4、ch as casting,forging,welding and heat treatment,due to the uneven thickness of each part of the workpiece,the inconsistent cooling rate and shrinkage,and the volume change during the transformation of the metallographic structure,etc.,the residual stress will be generated inside the blank,and the s
5、tructure The more complex and uneven the wall thickness,the greater the difference in heat dissipation conditions and the greater the residual stress generated inside the blank.The blank with residual stress is temporarily in balance.When a layer of metal is cut,this balance is broken,the residual s
6、tress is redistributed,and the workpiece will obviously deform until it reaches a new balance.As shown in the figure is an example of casting deformation due to internal stress.As shown in the figure,the casting has compressive stress at the 1 and 2 positions and tensile stress at the 3 position.Thi
7、s is because the cooling rate of parts 1 and 2 is faster than that of part 3 after casting.If a gap is made in part 2 and the stress is redistributed,the workpiece will be bent and deformed as shown.Casting deformation due to internal stress(2)Residual stress caused by cold straighteningFor some par
8、ts with low stiffness and easy deformation,such as slender shaft,crankshaft and lead screw,cold straightening method is often used for straightening.The straightening method is to apply a force in the opposite direction of the bending,as shown in the figure.Under the action of external force F,the d
9、istribution of residual stress inside the workpiece is shown in the figure,producing compressive stress above the axis(represented by a minus sign)and tensile stress below the axis(represented by a plus sign).Between the axis and the two double-point lines is the elastic deformation area,and beyond
10、the double-point line is the plastic deformation area.When the external force F is removed,the plastic deformation zone of the outer layer prevents the recovery of the internal elastic deformation and redistributes the residual stress,as shown in the figure.At this time,although the cold alignment r
11、educes the bending,but the workpiece is in an unstable state,such as reprocessing,will produce a new deformation.Therefore,the processing of high precision lead screw does not allow cold straightening,but many artificial aging to eliminate the residual stress.Internal stress caused by cold alignment
12、（3）Residual stress caused by machiningUnder the action of cutting force and cutting heat,the machining surface will have different degrees of plastic deformation and change of metallographic structure,and will also produce internal stress,and cause the workpiece deformation after machining.2、Measure
13、s to relieve or reduce internal stressIn order to improve and ensure the machining accuracy of the workpiece,attention should be paid to reduce and eliminate the internal stress.（1）Design parts structure properly.Minimize the size difference between parts,so as to reduce the blank in the manufacturi
14、ng process of the internal stress.（2）Arrange the process reasonably.Separate roughing and finishing in different processes to allow sufficient time for deformation after roughing and redistribution of residual stress to reduce the impact on finishing.For large workpieces that need to be completed in
15、 one process in roughing and finishing,the workpiece should also be loosened after roughing to allow the deformation of the workpiece to recover,and then clamp the workpiece with a small clamping force for finishing.（3）Arrange the process of eliminating internal stress.One is to the blank or in the
16、large workpiece rough processing,let the workpiece stay under natural conditions for a period of time to process,using the natural change of temperature to make it many times hot expansion and cold contraction,natural aging.The second is through artificial aging heat treatment process,such as castin
17、g,forging,welding parts for annealing or tempering;Parts are tempered after quenching;For high precision parts,such as bed,screw,or precision spindle box,making low temperature tempering after rough machining,even for screw cold processing,precision spindle after finishing,etc.Third,some casting,for
18、ging,welding parts in the form of vibration will be added to the mechanical energy on the workpiece,vibration aging treatment,causing the internal lattice creep of the workpiece,so that the metal internal structure stability,the elimination of internal stress.Ways to improve and guarantee machining
19、accuracy 1 Direct elimination and reduction of errors For example,three measures for turning slender shafts：Use the reverse cutter(feed)to cutUsing large feed and large main Angle(9093)turning tool 2Error compensation methodArtificially create a new kind of original error or use the original error t
20、o offset another kind of original error.For example:preload lotus finishing grinder lathe bed guide rail;By correcting mechanism improve the precision of screw lathe transmission chain.3 Error average methodFor example,group the previous process dimensions with large errors.The machining accuracy ca
21、n be guaranteed without improving the accuracy of the previous procedure and the capability of this procedure.4Error transfer methodTransfer the geometric error,stress deformation and thermal deformation of the process system to the direction that does not affect the processing accuracy.Thanks for watching