Ball shooting tests for identification of modal parameter variation in rotating main spindles

Abstract

The dynamical properties of the main spindle of a milling machine can have essential effect on the stability of milling processes. These dynamical properties often change with the rotational speed of the spindle, e.g. the stiffness and damping properties of the angular contact bearings of the spindle vary significantly with the spindle speed. Our study focuses on the development of measurement devices which enable us to measure the dynamical properties of rotating spindles with high accuracy. The concept of the Fotonic Beam Reduction (FBR) laser sensor is presented, which is applied to measure the displacement of rotating milling tools in a contactless way. In order to determine the frequency response function (FRF) of the spindle, a ball shooter is constructed. The impulse generated by the bullet on the target is analysed experimentally. A force sensor is utilized to estimate the time interval of the impacting between bullet and surface. It is shown that the repeatability of the impact is very good, and the generated impulse can excite the natural frequencies up to 30 kHz. The FBR sensor with the ball shooter is applied to identify the FRF of a milling spindle versus its rotational speed. The variation of the vibration frequencies is demonstrated by means of the experiments.