Investigation of Machining Stability Considering Thermal and Rotation Effect: Effectiveness of Impact Excitation for a Rotating Spindle

Abstract

This study investigates machining stability considering the rotational and thermal effects of a spindle-bearing system. Conventional method for analysis of machining stability only has regarded the static response of spindle system, which does not represent actual machining conditions as some investigations mentioned. Bearing stiffness change caused by a contact angle change is applied to the analytical approach for a dynamics of spindle system. At the same time, in a frequency response function (FRF) of rotating spindle, disturbance-induced signals are included such as a slip between hammer and tool, a spindle run-out and a noise due to insufficient impulse force. In this paper, a conventional impact exciting method which has been popular for characterization is refined in order to obtain dynamic characteristics of rotating spindle-bearing system on the consideration of coherence function. Those disturbances in the output signal are remarkably reduced through finding both the optimal conditions in terms of impulse force and the filtering of false responses. The proposed refined impact exciting tests, which can manage both rotational and thermal effects under rotating conditions, lead to provide FRFs matching more accurately. Stability lobe diagrams (SLDs) derived from FRFs obtained by experiments were derived to evaluate the effectiveness of the dynamic characteristic change in bearing-spindle systems. As a result, the axial depth of cut limitations at chatter occurrence points were well matched to actual depth limits with a deviation of 3% or less. Chatter vibration generation conditions can be well estimated, which proved that SLDs considering the dynamic characteristic of spindle-bearing system reveal the actual cutting condition since it includes thermal and rotation effects of spindle-bearing system during milling process.