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.