Research On Mechanism Of Super-Thin Precision Cutting And Machined Surface Quality

With the development of the aerospace industry, high accuracy instrument, and precision mechanery, the demands for the service performance of components with complex small features are increasing more and more. The tool edge radius plays an important role in chip formation because undeformed chip thickness is very small in super-thin precision cutting. However, the performance of components will be failure due to damaged layer. Many researchers have put their attentions on reducing damaged layer, residual stress and improving machined surface quality with precision cutting technolagy. In this study, the effects of cutting paraments, tool geometries and cutting forces on surface roughness and damaged layer are studied in super-thin precision cutting 3J33.Firstly, the cutting force model of orthogonal super-thin precision cutting is studied with the cutting force divided into the chip formation force due to the primacy deformation zone and the ploughing force due to tool edge radius. By applying the minimum energy principle to cutting energy composed of the cutting and friction energy in the primary and secondary deformation zones on the basis of modified Oxley's machining model, the shear angle in the primary deformation zone is estimated and the chip formation force is calculated. With the aid of a slip line field model, ploughing forces due to the tool edge radius are studied. The orthogonal super-thin precision cutting forces model is extended to 3D cutting forces model of sharp-nosed tool and nose radius tool considering chip flow angle to lay a theoretical foundation for the subsequent research works.Secondly, the friction between the tool and the chip is assumed to follow a modified Coulomb friction law and the adaptive remeshing technique is used for the formation of chip. A two-dimensional and a three-dimensional finite elements model for super-thin precision cutting using the commercial software Msc.Marc on the base of thermo-elastic-plastic deformation theory and FEM. The effect of edge radius and rake angle on cutting forces, temperature distribution, and stress distribution are investigated in the two dimensional finite element analysis. The effect of edge radius and nose radius on cutting forces, temperature distribution, and stress distribution are investigated in the three dimensional finite element analysis. The simulation results demonstrate the behaviors of the non-uniform intense stress fields in deformation zones of precision cutting to give useful data for experimental analysis.Thirdly, a mathematical model for cutting force in super-thin precision cutting with sharp-nosed tool is developed in terms of cutting parameters by orthogonal experimental test. A second order mathematical model for cutting forces in super-thin precision cutting with nose radius tool is developed in terms of cutting parameters (feed rate and depth of cut) and nose radius by the quadratic rotary combination design. The experimental cutting forces are used to testify the theoretical model for turning with nose radius tool and simulation results obtained from finite element analysis.Moreover, the theoretical model for surface roughness, which is taken as a function of variables feed and tool corner radius, is modified by cutting forces using a Genetic Algorithmic (GA) approach. A second order predictive model for surface roughness in super-thin precision cutting with nose radius tool is developed in terms of cutting parameters (feed rate and depth of cut) and nose radius by the quadratic rotary combination design using GA approach. Lastly, the mechanical and thermal effects on microstructure and hardness of super-thin precision turned sub-surfaces are studied both with a scanning electron microscope (SEM) analysis and microhardness test using microindentation. A thin disturbed or plastically deformed layer is distinguished by the presence of grains that are elongated and rotated in the direction of cutting.No significant heat affected layer and phase transformation is found below the machined surface in all the tests. It also implies that mechanical deformation plays a larger role during super-thin precision turning. It was also found that the hardness of the plastically deformed layer differed from the bulk hardness of the workpiece. Larger cutting parameters (depth of cut and feed rate) usually give rise to deeper mechanical affected layers.