Modelling and simulation of nano-surface generation in ultra-precision machining

Abstract

In ultra-precision diamond turning, the quality of a machined surface that can be generated is important in assessing the performance of the machining system. This process of surface generation has attracted a lot of research interest. However, most of the work to date is based on empirical studies. Relatively little quantitative work has been reported. Although some attempts have been made in the development of machining models to simulate surface topography of a workplace, most of them focused on the synthesis of surface topography from the data derived from interferometry or Scanning Electron Microscopy (SEM). Few deterministic models have been found to simulate the generation of surface topography based on machine kinematics, material science and cutting theories. The influences of crystallographic orientation and other properties of the work materials, and their interaction with the surface generation have been overlooked in most of the current models. In this research, an investigation has been conducted into the factors affecting the surface generation in ultra-precision diamond turning. Experimental results indicate that the quality of a diamond turned surface is affected by both the process factors and material factors. The former involves cutting conditions like spindle speed, feed rate, tool geometry as well as the relative tool-work vibration due to machine vibration and spindle error motions. These process factors are related to the cutting geometry and the dynamic characteristics of the cutting system. The material factors considered are material anisotropy, swelling, and crystallographic orientation of the work materials. This study shows that the influence due to external factors like tool wear or machine chatter can be suppressed or even eliminated through a proper selection of operational settings and control of the dynamic characteristics of the machine. However, the influence of material factors would still persist even if cutting is performed under an optimal cutting condition. In order to measure quantitatively the effect of material swelling and anisotropy on the surface generation, a Multi-spectrum Analysis Method has been adopted in the study. In this method, various features of a diamond turned surface are extracted and analyzed by the spectrum analysis of its surface roughness profiles measured at a finite number of radial sections of the diamond turned surface. It is found that the tool feed rate, the spindle rotational speed, the tool geometry, the material properties as well as the relative tool-work vibration are not the only dominant components contributing to the generation of surface roughness. The vibration induced by the variation of crystallographic orientation of the workpiece material is another major factor. Such a vibration can cause a significant variation in the frequency of surface modulation formed on the machined surface. However, the multi-spectrum analysis method is incapable of determining the exact contribution of each factor upon the overall surface roughness. To overcome this shortcoming, a Multiple Data Dependent Systems (MDDS) analysis method is proposed. The metal cutting dynamics are characterized by the natural frequency, the damping ratio, and the relative contribution of the central wavelength components which make up the roughness profiles at a finite number of radial sections of a workpiece. Experimental results indicate that the cutting dynamics are dominated by the relative vibration between the tool and the workpiece, the spindle axial error motion and the swelling of the work materials. The contribution and the natural frequency of the tool-work vibration components are found to vary with the crystallographic orientation of the workpiece. Based on the results of the experimental findings and the quantitative analysis, a 3-D surface topography simulation model for ultra-precision diamond turning is proposed. The model takes into account the effect of tool geometry, machining conditions and tool-workpiece vibration. It makes use of the surface roughness profiles predicted at a finite number of radial sections of the workpiece to construct the surface topography of a diamond turned surface. The model has been evaluated through a series of cutting experiments. Satisfactory results have been achieved in the prediction of surface roughness parameters and the 3-D surface topography in diamond turning of polycrystalline aggregates. A captioned model has been further developed to account for the influence of materials induced vibration in diamond turning of highly anisotropic single crystal materials. This leads to the establishment of a model-based simulation system. It is composed of several model-elements which include a microplasticity model, a dynamic model and an enhanced surface topography model. The microplasticity model is used for predicting the variation of micro-cutting forces with the changing crystallographic orientations of the workpiece during cutting. A dynamic model is built for determining the vibration induced by the variation of the cutting forces. The influence of this vibration on the surface roughness is estimated by an enhanced surface topography model. The system has been successfully implemented and evaluated through a series of cutting experiments. The simulation results are found to agree well with the experimental ones. Ultra-precision diamond turning is an expensive process. Nowadays, the achievement of a super mirror finish in many current industrial applications still depends much on the experience and skills of the machine operator through an expensive trial and error approach when new materials or new machine tools are used. The successful development of the surface topography model and the model-based simulation system can help to identify the optimal cutting conditions for different work materials without the need for costly trial and error cutting tests. It also helps to find the best surface quality that can be achieved under particular dynamic conditions for a specific machine. Moreover, this is the first of its kind in which a deterministic model-based system has been successfully built which accounts for the effect of materials induced vibration in diamond turning anisotopic materials. This contributes significantly to the knowledge of ultra-precision machining and the further improvement of the performance of ultra-precision machines.