Ph.D. Dissertation Defense by Jong-Suh Park
Thursday, April 1, 2004

( Dr. Ye-Hwa Chen, Chair)

"The Prediction of Chatter Stability in Hard Turning"

Abstract

The development of more wear-resistant tool materials such as Polycrystalline Cubic Boron Nitride (PCBN) and ceramics have made hard turning a potential alternative to grinding operations in the finishing of hard materials. However, hard turning is more sensitive to chatter than conventional mild turning. The reasons include both a high precision requirement in finishing as well as the relatively brittle property of PCBN cutting inserts. Therefore, the ability to predict chatter free cutting conditions is very important for hard turning in order for it to be an economically viable manufacturing process. Despite a large demand from industry, a realistic chatter model for hard turning has not been developed due to the complexity of the modeling, which is mainly caused by the flank wear and non-linearity in hard turning.
This thesis attempts to develop a chatter model for predicting chatter stability conditions in hard turning. The salient features of this chatter model include the consideration of the effects of flank wear and non-linearity. We first develop a linear model by introducing non-uniform load distribution on a tool tip to account for the flank wear effect. We then develop a nonlinear model by further incorporating the non-linearity in the structure and cutting force. A stability analysis based on the root locus method and the harmonic balance method is conducted to determine a critical stability parameter. To validate the models, a series of experiment has been carried out to determine the stability limits as well as certain characteristic parameters for facing and straight turning. Chatter in hard turning has the feature that the critical stability limits increase very rapidly when the cutting speed is higher than 12 rev/sec for all feed directions. The nonlinear model provides more accurate predictions than the linear model, especially in the high-speed range. However, there is a minor disagreement with experimental data at the low speed range. Furthermore, the stabilizing effect of the flank wear has been confirmed through a series of experiments. To fully account for the validity of linear and nonlinear models, finally, an empirical model is proposed to fit in with the experimental stability limits in the full range of cutting speed.

The main contributions of the thesis are threefold. First, chatter-free cutting conditions are predicted and can be used as a guideline for designing tools and machines. Second, the characteristics of chatter in hard turning, which is observed for the first time, helps to broaden our physical understanding of the interactions between the tool and the workpiece in hard turning. Third, experimental stability limits for different flank wear can contribute to lead more reasonable ways to consider the flank wear effect in chatter models of hard turning. Based on these contributions, the proposed chatter model will help to improve the productivity in many manufacturing processes. In addition, the chatter experimental data will help to develop other valid theoretical chatter models in hard turning.