| Traditionally, the dynamic characteristics of machine tool structures and the cutting process are generally identified by such methods as frequency response method, the impulse response method, and random excitation methods. All these methods use external exciting forces outside the machine tool in simulating the cutting process. This simulation is usually incomplete due to design and/or measurement difficulties and also too many simplifying assumptions, as a result inaccurate dynamic characteristics are obtained.;To overcome these problems, a new and more realistic method has been proposed in the present work and successfully applied for the identification of the dynamic characteristics of the closed loop system of a lathe.;Data Dependent System (DDS) methodology in the form of Auto Regressive Moving Average Vector (ARMAV) models representing two series of discrete data were applied to model the absolute displacement signals of the Headstock, Toolpost, and Tailstock units respectively in the radial (horizontal) direction and the radial component of the cutting force. All the signals were obtained under various idle running and actual cutting conditions of the lathe.;Detailed results and discussions about the identified receptances, dynamic cutting stiffnesses, and stability for the Headstock, Toolpost, and Tailstock, and about the computed limiting depth of cut based on Toolpost data are given. The effect of cutting conditions are also discussed. The results show that workpiece rotation frequencies and their lower harmonics are the dominant modes. Three types of realistic modes can be identified under working conditions, modes due to the machine tool structure, the cutting process, and workpiece rotation frequencies or imbalance. All these modes can not be realized under external excitation test. There are considerable differences between the results identified under idle running conditions and those identified under actual cutting conditions. Combination of cutting conditions were found to have significant effects on the identified dynamic characteristics, and optimum combinations have been recommended for finishing and roughing operations for the steel and cast iron workpiece materials. The computed limiting depth of cut were physically meaningful and comparable with published data. (Abstract shortened with permission of author.). |
