Modelling and passive correction investigation of vibration induced machining errors on CNC machine tools
Machine tool vibration is a complex subject requiring a multi-disciplinary approach involving the identification and analysis of the vibration sources and characteristics, as well as its direct and indirect effects. Machine tool vibration is influenced and can be characterised by the machine's structural dynamics, the drive system performance and the cutting force generation. Its effect materialises in the form of poor surface finish of the workpiece, accelerated cuttingtool wear, and chatter during the machining process. This research project investigates vibration-induced errors on a Cincinnati Arrow 500 CNC vertical machining centre under dynamic conditions. Analyses and identifications of suitable experimental configurations for modal analysis and cutting process investigations are carried out to determine the most appropriate techniques for the aforementioned processes. The results are compiled into recommended metrology practices for determining the vibration modes. State-of-the-art practices are employed in the study to formulate and validate a machine tool axis drive model to examine the drive's individual element effects on the overall dynamic performance. The feasibility of an active control vibration technique employing the drive is also investigated. The hybrid modelling technique incorporating a new digital current control loop developed using the power system blockset and field-oriented control strategy was employed to construct the model. Analytical and experimental techniques for the validation of the digital drive system's position, velocity and current control loops utilising deterministic and non-deterministic signals from the internal machine drive system functions are also devised. The majority of machine tool vibration is generated while the machining process is taking place. Thus, the analysis and in-depth study of the machining process plays an important role in the investigation of machine tool vibration. In this study, vibration models of the cutting tool and workpiece are formulated and incorporated with an advanced cutting force generation model to create a machining process model constructed as part of an EPSRC project collaboration. The model is validated using various machining process conditions and correlated with the workpiece surface finish analysed using state-of-the-art 3D surface topography technique to identify salient vibration effects. In this study, a model of the machine structural dynamics is constructed using the Finite Element Method (FEM) for the comprehensive analytical investigation of the machine vibration behaviour accurately. The analytical model is validated against the measured results from the experimental modal analysis investigation obtained using the appropriate technique. Correlation analysis of the simulated and experimental modal analysis results is performed in order to improve the accuracy of the model and minimise modelling practice errors. The resulting optimised model then undergoes sensitivity analysis through parametric structural analysis and characterisation technique in order to identify potential vibration reduction technique by the passive methods.