Research On Analytical Prediction Of Cutting Forces In Stainless Steel Machining

Metal cutting is the most traditional material processing technology in mechanicalmanufacturing industry. As the material property improvment and some special serviceenvironment demand (high termperature， high pressure， irradiation， corrosion), themanchining quality need be more strictly controled. This dissertation focuses on cutting forceprediction for the difficult-cut-materials (304stainless steel) of nuclear reactor coolant pump,some principle problems about orthogonal cutting and oblique cutting are researched. Thematerial property, nolinear large deformation and thermomechanical coupling of cuttingprocesss are also considered together. These work and achievements are shown below:First of all, the modeling process of the primary shear zone is discussed. A new modelbased on unequally divided shear zone is presented. The material constitutive relationship inthe primary shear zone is represented using Johnson-Cook equation. The chip formation issupposed to occur mainly by shearing within a primary shear zone, and the tool edge radiuseffects can be neglected under the assumption of a sharp cutting edge. A piecewise power lawdistribution assumption of the shear strain rate is adopted according to Oxley’s experimentalresualts. Based on plastic mechanics and thermodynamics, the governing equations of thevelocity, strain, stress and temperature of chip flow through the primary shear zone areestablished. At the tool/chip interface, a chip speed-dependent friction law is introduced.While calculating the flow stress numerically, the thermomechanical equations under thestrong coupling condition are solved simultaneously, so the coupling effect of workinghardening and thermal softening on flow stress are considered. Finally, the cutting forces fordifferent machining conditions were predicted, the effect of cutting parameters and toolgeometry were investigated. Besides, the distributions of strain rate, velocity, strain, stressand temperature for the proposed model are also simulated.Using the equivalent plane approach, orthogonal cutting theory based on unequaldivision shear zone is extended and applied to oblique cutting. The equivalent plane angle is defined to determine the orientation of the equivalent plane. The geometrical parametersassociated to oblique cutting are analyzed using the coordinate transformation approach.Similarly, the governing equations of the shear velocity, shear strain, shear stress andtemperature distribution in the primary shear zone are established. The forces applied on chipare analysed, the calculated expression of cutting forces and chip flow angle are deduced onthe basis of chip force equilibrium. The cutting force prediction process of oblique machiningis implemented using a program, the influence of cutting parameters and inclination anglewere investigated.The helical flutes are decomposed into a set of differential oblique cutting edges. Thecutting parameters of discrete edges are characterized by local cutting angle and theundeformed chip thickness. To every engaged tooth element, the flow stress is obtained fromthe material constitutive equation, the thermomechanical governing equations within theprimary shear zone, the shear angle formula, the mean friction expression at the tool-chipinterface and so on, then differential cutting forces are computed by the flow stress. Cuttingforces were predicted using oblique cutting transformation into end milling. The value of themilling force is calculated through the numerical integration. Using the proposed method, theeffect of tool and cutting parameters on milling force are analysed. The results show that it’spossible to get an ideal milling force distribution (To meet the continuity, stability, andminimum conditions) by choosing the appropriate machining parameter, which can reducetool deflection error and improve the maching precision. This work is the foundation ofmachining parameter optimization and toolpath planning based on cutting force.This dissertation discusses the limitations of the traditional oblique cutting model andproposes a more general oblique cutting model. The improved model considers the impact ofmain cutting edge angle and non-free cutting. Based on the coordinate transformations, weanalyse the geometry relationships of discrete cutting edges and deduce the mathematicalexpressions of the local cutting angles and cutting parameters. Every discrete cutting elementcan be considered as an improved oblique cutting process, which takes into account theinteraction between the chip elements. The relationship of the global chip flow angle and the turning force can be derived based on force equilibrium of each local chip element and theglobal chip. The proposed model has been applied to predict and analyse the turning force inthe three distinct cases, especially in the case of non-free factors of the corner radius and maincutting edge angle. The chip flow in turning may scratch the machined surface, under thecircumstances, the surface will have a poor quality. We investigated the influence of the tooland cutting parameters on the local and global chip flow angle. The results helpmanufacturing engineers to choose the cutting angle, nose radius and cutting depth to controlchip flow direction towards the outside of the workpiece axis. Finally, the tool and cuttingparameter effect on the local geometric and physical parameters along the cutting edge areconsidered as coupled. These local information is important to the optimization of cuttingparameters and the cutting edge geometry.The proposed method in this dissertation has been implemented in Matlab7.0. Based on aset of orthogonal cutting tests, the material and friction coefficients are indentified. Predictedresults were compared with experiment results of cutting forces for304stainless steel andfound in reasonable agreement. So this model can be used to analyse the cutting process of thenuclear reactor coolant pump. This model is also validated by the experimental results ofother materials, such as literature results of42CrMo4steel and316L stainless steel, millingexperimental results of45steel. It should be noted that although this work is limited tomilling and turning, the present method also be applied to other machining operations such asdrilling and boring.