An experimental and theoretical analysis of surface generation in the ultra-precision grinding of hard and brittle materials

Abstract

This paper presents an experimental and theoretical study of surface generation in the ultra-precision grinding of hard and brittle materials. The study takes into account the material properties, the relative vibration between the grinding wheel and the workpiece, the machining parameters and the phase shift of the grinding process. The Taguchi approach is employed to study the influence of machining parameters on the surface quality and shows that the feed speed and rotational speed of the workpiece are key factors. Experiments have been conducted to study individual variables, and the results further show that the feed rate and the cross-feed distance have a significant effect on surface generation. It is found that the spirals around the central area of the workpiece are the primary mechanism for surface generation, which originates from the synchronous relative tool-work vibration. The integral part of the ratio of the rotational speed of the grinding wheel to rotational speed of the workpiece determines the number of spirals and its fractional part controls the spiral geometry. A theoretical model for predicting the single spiral generation has been developed to explain the accumulation of the phase shift and the geometry. The changeable feed speed near the end of grinding is also modelled, revealing the approximate straight lines around one circle in the central region. The simulated results indicate that the theoretical models and the ground surfaces are in close agreement. The scallop-height model is developed to calculate the influence of phase shift on surface quality, and it is found that the phase shift near the medium value can effectively improve surface quality. Finally, a comparison of different surface generation mechanisms in grinding mould steel, tungsten carbide (WC) and reaction bonded silicon carbide (RB-SiC) is made. It is interesting to note that the Spanzipfel effect contributes to the surface generation not only on ductile materials such as mould steel but also on brittle materials such as WC and RB-SiC. The Spanzipfel effect is the most significant in grinding mould steel. For WC and RB-SiC, the ground surface contains both a ductile region and a brittle region in the form of micro-fractures.