The effect of grinding machine stiffness on surface integrity of silicon nitride

A theoretical model based on mechanics and machine dynamics is presented to describe the effect of machine stiffness on surface integrity of ground silicon nitride. The model accounts for both the static and dynamic structural loop stiffnesses of a precision-grinding machine. Experimental results are also presented to verify the model. A unique workholder with an adjustable compliance is used to achieve a structural loop stiffness in the range of 5-40 N/mum. Silicon nitride is ground with cup-type diamond wheels of vitrified and cast iron fiber bonds. To effectively stabilize the cutting performance of a cast iron fiber bond wheel, the ELID technique is adopted for in-process dressing. The damage depth of ground work-pieces is assessed against machine stiffness. The modeling and experimental results demonstrate that there exists a critical machine stiffness in grinding of ceramics. When machine stiffness is higher than the critical stiffness, no chatter should occur in the grinding process. In this case, damage depth increases with the increase of set depth of cut. In contrast, if machine stiffness is lower than the critical stiffness, chatter can occur in the grinding process that may induce grinding damage. The model can also be used to predict the critical machine stiffness for other types of structural ceramics.