Abstract
The stability of high-speed machining operations is crucial in machining process and presents a key issue for insuring better surface quality, increasing productivity and protecting both machines and safe workpiece. Stability prediction in milling is based on experimental modal analysis by the estimation of frequency response functions using a tap test. One limitation of accurately estimating the stability using such approach is the change in process and the dynamic characteristics of the machine tool under operation. This paper proposes a signal processing procedure applied to vibrations in machining process in order to obtain spindle’s modal variations in operation. The novelty of the proposed approach consists in removing “virtual modes”, caused by harmonic excitations, from the system response before performing operational modal analysis. Thus, the proposed procedure combines two existing techniques that are the Cepstral Editing Procedure and the Least Square Complex Exponential. The importance of the developed methodology is in adjusting the chatter stability criterion for material removal on a dynamic basis. The main work is given as follows: first of all, the Cepstral Editing Procedure (CEP) algorithm is applied on the acceleration signals for removing deterministic vibrations caused by harmonic excitations. The residue signal is the system response to white noise excitation. The frequency response functions (FRF) are then calculated from these signals at different cutting conditions. The outcome is compared to the result of impact test on the spindle under static condition. Similarities in the form of FRFs obtained in static and operational conditions validate the proposed approach while variations of modal properties under different cutting conditions are successfully captured. Secondly, the Least Square Complex Exponential (LSCE) method in operational modal analysis is invoked to find the natural frequencies and damping ratios of the system at different spindle speeds and cutting depths. Then, the dynamic chatter stability lobes diagrams (SLD) are established which account for spindle’s speed-dependent modal variations. A significant change in the stability border is observed which is interesting in machining fields. It will be shown that some depths of cut that are stable with static stability lobes become unstable with dynamic stability lobes and vice versa.
Highlights
In last years, manufacturers have shown great interest to increase productivity and surface quality of pieces by using high speed milling machine centers.Many problems that occur during high speed milling are related to the instability of the cutting process
The construction of stability lobes diagrams requires prior information on: (1) the response function of the system that presents the key variable in the dedication of the stability limit [6], and (2) the cutting coefficients that depend on the material used
For the diagnosis of dynamic cutting performance, we proposed a new tool in signal processing to characterize the modal parameters that vary with speed
Summary
Manufacturers have shown great interest to increase productivity and surface quality of pieces by using high speed milling machine centers. Many problems that occur during high speed milling are related to the instability of the cutting process. This instability causes poor surface finish, an increase rate of tool wear, a bad dimensional accuracy and a possible breakage of cutting tool and spindle tool unit. The construction of stability lobes diagrams requires prior information on: (1) the response function of the system (cutting tool/ tool holder/ machine tool) that presents the key variable in the dedication of the stability limit [6], and (2) the cutting coefficients that depend on the material used.
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