Effects of a drawbar design and force on multipurpose aerostatic spindle dynamics

Abstract

The tool point frequency response function (FRF), or receptance, is an important dynamic condition for machining processes, especially milling and finishing, as it enables users to calculate the most appropriate operating conditions, namely spindle speed and depth of cut, for achieving the best spindle performance. In this paper, we evaluate the effects of the drawbar on the tool point FRF of a multipurpose aerostatic spindle using a new procedure. The drawbar was considered to be nested inside the shaft and the receptance of the assembly was predicted based on a multiple-point receptance coupling approach. We coupled the tool holder and the tool to the receptance, so two types of contact parameter were considered simultaneously. We determined the contact parameters between the shaft and the tool holder, and between the tool holder and the tool, and included the values obtained when coupling with different combinations of tool holder and tool. We also evaluated the shaft-tool holder contact parameter for three different axial pulling forces. Moreover, we investigated the aerostatic bearing dynamics, including the stiffness and damping factor, in three different cases of supplied air pressure. Finally, we estimated the FRF of the tool under several aerostatic bearing and drawbar conditions and calculated stability lobe diagrams based on the estimated FRF. These results show that considering the drawbar improves the estimate of the tool point FRF and, subsequently, the stability lobe diagram. The effect of varying the length and density of the drawbar on the tool point FRF was also investigated.