There exist a great number of variational cutting force models for the case of plunge cutting but the analytical prediction of the parameters in these models is still elusive at best. The case of traverse cutting is even more intractable from an analytical point of view. A knowledge of the coefficients in the variational cutting force model is necessary to predict the borderline of chatter stability in machining operations. This paper describes the application of a sequential equation error minimization technique to determine empirically the optimum parameter values in a predetermined set of force component models from dynamic cutting data. The identification technique was verified on an analog computer simulation of the dynamic behavior of a machine tool system. The identified parameter values were compared with the actual simulated values. Even in the presence of noise inputs, the identification was accurate to within two to three percent. Experimental plunge cutting tests were performed on an aluminum workpiece. The results of the identification technique applied to these tests were analyzed against the backdrop of the simulation results. Conclusions drawn from the cutting tests were for the most part consistent with other researchers’ results and with intuition. The important contributions of this work are not the conclusions drawn from the cutting tests but the methodologies developed for obtaining results such as these. Specifically, no special inputs are required as in frequency response testing. Identification for both wave generating cuts and wave removing cuts is carried out in the same test. This situation is more indicative of a real cutting situation than either wave generating cuts or wave removing cuts considered separately.
A major problem in the active control of the boring process is developing a practical method of providing small-amplitude tool-tip positioning. The main thrust of the underlying research work is the design, development and evaluation of a new actuation concept for active control of the boring operation. The actuation concept was implemented using a special boring bar with two internal longitudinal hydraulic chambers. A pressure difference between these two chambers provides the driving force to create the desired tool-tip motion. Using a measure of the dynamic cutting force, the controlled boring bar system was successful in making improvements over the uncontrolled boring bar’s cutting performance in terms of regenerative chatter control. The cutting tests used in this thesis were plunge cuts in mild steel. The controlled level of improvement was smaller than was anticipated. The problem was considered not a fault of the actuation scheme, but a result of a non-optimal combination of the servovalve, measurement system and controller. Further work in these areas should yield considerably improved results using the new actuation concept.
A previously established theoretical basis for controlling both the static stiffness as well as dynamic vibrations and regenerative chatter in particular was implemented in the work described by this paper. It was found that the experimental boring bar setup did exhibit two principal modes but that the controller could be synthesized largely independent of these modes. Cutting tests were performed using Delrin acetal plastic on a lathe equipped with a pivoted boring bar which was controlled by an electro-hydraulic servo system. It was found that the theory established earlier did in fact predict qualitatively the new stability borderline. Width of cut was improved by a factor of twelve and equivalent static stiffness was increased without bound.
The properties of low stiffness and low structural damping in boring bars are widely known to be factors resulting in chatter and inaccurate machining. The application of active control offers a new alternative to improving the performance of a boring bar. This paper presents the theoretical basis for such an active control system. The analysis includes the practical consideration of principal modes in the boring bar model and discusses this influence on controller design. Simulation results using data from an experimental system illustrate some important factors of system design.
The analysis of machine tool chatter from frequency domain considerations is generally accepted as a valid representation of the regenerative chatter phenomenon. However, active control of regenerative chatter is still in its embryonic stage. It was established in reference [2] that a measurement of the cutting force could be effectively used in conjunction with a controller and a tool position servo system to increase the stability of an engine lathe and to improve its transient response. This paper presents the design basis for such a system, including both analytical and experimental considerations. The design procedure stems from a real part stability criterion based on the work by Merritt [1]. Because of the unknown variability in the dynamics of a machine tool system, the controller parameters were chosen to accomodate some mismatch between structure and tool servo dynamics. Experimental tests to determine the stability zone of the controlled machine tool system qualitatively confirmed the analytical design results. The experimental results were consistent in that the transient response tests confirmed the frequency domain stability tests. It was also demonstrated experimentally that the equivalent static stiffness of a flexible work-piece system could be substantially increased.
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