This paper presents an experimental investigation on the normal dynamic characteristics of several machined joint surfaces, i.e., the varying principle of the normal dynamic stiffness and damping with exciting frequency, relative displacement and static pre-load under different joint conditions, including joint materials, mediums, machining methods and surface roughness, etc.. The joint parameters are extracted from experimental data by establishing the theoretical model of the joint surfaces, and the mechanism is analyzed qualitatively. The studied results show that, in the test range of this paper, the stiffness and damping coefficient of the joint surfaces increase with the static pre-load; the stiffness for a dry joint is independent of the exciting frequency, while the damping coefficients for both a dry and an oiled joint decrease with the exciting frequency; little relative displacement has no marked effect on the dynamic characteristics. The amount of influence of exciting frequency and static pressure is related to the joint conditions. [S0739-3717(00)00804-7]

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2 This research took the large-scale thread whirling system as the object, to study the effect of supporting and holding way on the dynamic response of the workpiece system. Considering the specialties of large-scale thread whirling system and high-speed hard cutting technology, the theoretical dynamic model of the workpiece system was set up based on the Generalized Polynomial Assumed Mode Method after simulating cutting force by Deform 3D. Then, the natural frequency and the dynamic response characteristic were analyzed through solving the dynamic model with the Runge-Kutta method; the effect of hold devices as well as cutting position on the natural frequency of workpiece system were studied; and the vibration suppression capability of system damping and holding stiffness were also discussed. In the end, the large-scale thread whirling experiment was carried out to study the relationships between holding parameter, vibration amplitude and roughness. The results show that floating rests, holding stiffness and system damping play important roles in the vibration suppression during thread whirling process, and it is appropriate to set the holding ratio between 40% and 50% or between 65% and 70% in this case to improve machining quality.

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