Study On Constant Pressure Grinding Process Using Finite Element Analysis And Molecular Dynamics Method

Belt grinding process is a new machining technology with high efficiency, adaptability and low cost. It is widely used in various fields of modern manufacturing. Recently, with the development of new abrasive material and grinding equipments, much progress has been made in both high precision and high efficiency belt grinding technology. For example, some advanced countries have developed nano belt grinding techniques for ultra-precision processing. Ultra processes using belt grinding method will receive more and more attention, since it can be use to produce components which require high precision and high quality efficiency. One example is that some metal substrates, such as non-corrosive steel substrates and monocrystal metal substrates, used in optical instruments are required at nano-scale surface roughness to get better performance. Therefore, it is very useful to study the working mechanism of ultra belt grinding processes for the purposes of designing grinding process and optimizing its quality and efficiency.There are some differences between wheel grinding process and belt grinding process. The current research of grinding process mainly focuses on wheel grinding process. In contrast, the related research on belt grinding processes under constant pressure needs further exploration. Based on our observation, we first focus on thermal analysis for constant pressure grinding using finite element method. Next, in order to study the mechanism of constant pressure grinding process in ultra-precision machining, we build up a model for better analyzing single grit nano-grinding process using molecular dynamics method.The main contributions of this paper are as follows:(1) We propose a molecular dynamics (MD) model for single grit constant pressure grinding process. Based on the model, we also investigate some major factors that affect the grinding process, such as grinding forces, stress and temperature distribution. In the MD modeling, we first model the interaction among atoms of the grit and the workpiece using embedded atom method (EAM), Tersoff and Morse potential functions. Then, the grit is set to be driven by a driving source with constant speed and damping connection. The simulation results indicate that in the grinding process, the axial grinding forces show some amplitudes of vibration, but within a stable range; the resistance force from the workpiece becomes stable as the grinding speed reaches at a certain high level. The equivalent stress experimental results indicate that the region of stress concentration occurs not only near the contact area but also below and in front of the contact area. The temperature results show that although different grinding forces act on the workpiece, the highest grinding temperatures are stable. In addition, the highest grinding temperature locates in the chip stack area. It is consistent with the macro phenomenon. The aforementioned results conclude that in nano grinding process, pressure forces should be controlled at a proper value to avoid deep stress concentration. In addition, a higher grinding speed can help to improve the grinding efficiency.(2) Using MD method, we study the plastic deformation of anisotropy single metal crystal under different grinding directions using the atom adjacent changing ratio and central symmetry parameter distributions. The Adjacency changing ratio proposed in this paper is used to describe the deformation degree of atoms within a certain interval. Central symmetry parameter which describes the symmetry degree of atom neighbors is very suitable to distinguish crystal defect types, especially for planer defect analysis. The adjacency changing ratio distribution experimental results show that when the forces that act on the {111} crystal plane reach a certain level, the slipped phenomenon will occur. Meanwhile, its deformation direction and strength is related to pressure forces and grinding direction. In addition, the degree of deformation will vary under different strength of the interaction among metal atoms. For Mg material which has week atom interaction, the deformation mostly occurs in front of the grit. For W material with strong atom interaction, the plastic deformation is only found on the contact area while other area is mainly affected by elastic deformation. The results show that the higher pressure on the grit, the stronger stress concentration will occur in the deep place of the workpiece. In addition, in order to avoid oscillation of the grain and less deformation appeared, the low dense crystal face shoud be selected as grinding surface and the grinding direction should push less pressure on {111} crystal plane.(3) We propose a finite element (FE) thermal model for constant pressure grinding process. This model is based on the analysis of the elastic contact pressure distribution and the grinding heat energy partition. In modeling the pressure distribution between the contact wheel and the workpiece, we improve the Signorini finite element model. The grinding energy partition is obtained based on energy analysis of the chip ground and single grit contact model of grinding process. The surface grinding experiments show that our FE model can produce satisfied results. Our model generates about 4% error rate in subsurface temperature and only 3~5% error rate in surface grinding temperature. It indicates that our model is reliable to some extend. Since we take into account several important factors, such as grinding speed, properties of abrasive belt and workpiece and elastic property of contact wheel, our FE thermal model and its simulation results can help to optimize the grinding process and predict the grinding thermal results so that the grinding burn can be avoided.