The suppression of the regenerative chatter phenomenon in turning process via the variation of the frequency and amplitude of the spindle motor excitation is carried out in this research. A programmable inverter is employed for variable periodic excitation of the spindle motor. A set of turning experiments is designed and performed on a number of conical work pieces both in constant spindle speeds and variable spindle speeds. The work-piece material was 1.7224 steel. These experiments are carried out in different spindle speeds so that the stability lobes diagram could be achieved. The results showed that the passive variations of the spindle speed increase the critical depth of cut in higher spindle speeds, while in lower spindle speeds it is case sensitive. It also showed that the increasing of the excitation frequency reduces the critical depth of cut, while increasing the amplitude increases the critical chip thickness in lower spindle speeds and reduces that in higher spindle speeds. ated that by decreasing the feeding rate and spindle speed, higher thickeners can be observed during shear spinning process. تفاصيل المقالة

A new dynamic friction model for modeling of elliptical vibration assisted turning (EVAT) was developed in this research. The periodic change of the friction force direction is known to be one of the most important causes of this phenomenon. In modeling of machining processes (including the EVAT process), static Coulomb friction model was employed by most of the researchers. Because of the periodic change in the friction force direction during the elliptical vibration path, the simple Coulomb friction model could not reflect the dynamic friction phenomena between the chip and the tool such as stick-slip and dynamic breakaway. In this research the LuGre dynamic friction model, which had been successfully employed for modeling of EVAT process, was modified in compliance to the machining process. To modify the LuGre model, the LuGre model coefficients were modified so that the effect of the temperature change of the contacting surfaces (which plays a key role in the friction behavior between the chip and the tool) could be reflected by the model. To determine the coefficients, orthogonal machining tests were performed and the Genetic Algorithm optimization method was carried out to derive the coefficients. Afterward, the results achieved from both dynamic models were compared with experimental results and Coulomb model results. The results from the new modified LuGre model were in better agreements with the experimental results. تفاصيل المقالة