Research On Dominant Characteristics Of CNC Machine Tool Structure Dynamics Under Operational Conditions

The performance of machine tools is affected greatly by the dynamics of the structure. The whole machine structure influence on the dynamic characteristics of tip point received more and more attention in the study of machine dynamics. For heavy machine tool, the weak components are greatly changed in the processing of changing position or large stroke processing. The dynamic characteristic of machine tool structure will be greatly influenced by this change.Not all modes of machine tool structure effect the vibration under working conditions. Only the modes that are excited during cutting have to be taken into account. Analysis on the variation trend of the main modes, which is changing with machining position, is beneficial to the improvement of machine tool modification and processing stability analysis and also has important significance to improve the processing quality and efficiency. Combined with the above issues and related projects, the main research contents are as follows:A method to identify the weak components of machine tool structure by EMA and active excitation modal analysis is introduced. A CNC gantry milling machine, which has multi components and complex combinations, is tested to identify the various modes under static and dynamic conditions. The weak components are identified by comparing the modal distributions in the EMA and active excitation modal analysis experiments.After confirming the ram of machine tool as the weak component, the position-dependent dynamic characteristics of weak parts are discussed. The changing trend of the ram modal parameters is proved to be more sensitive in the Z direction by knock experiments at different locations. And the trend of first three modes at four different coordinate positions in the Z direction is obtained. A derivative mode which is based on the slightly change of mode shape is introduced to explain the above changing trend and is verified. A compliance map is introduced to express the position-dependent characteristics of machine tool parts. The correlation between the position-dependent of the tool and the ram is verified in the low frequency range.Then, the influence of position-dependent characteristic on the surface quality and stability of the machined surface is studied. The main modes of machine tool under processing conditions are obtained by combining the identification method of the main mode and random steps cutting method. The position-dependent changing trend of the main modes is also discussed. The modes of workpiece, which is corresponding to the maid modes of weak components, prove that the main mode changing with the machining positions can affect the stability of processing directly. And surface roughness values of workpiece at different positions show that the position-dependent main modes can affect the processing surface quality.