Geometric accuracy evaluation during coordinated motion of rotary axes of a five-axis machine tool

Abstract

This paper presents a novel method to identify Position Independent Geometric Errors (PIGEs) of a five-axis machine tool with a tilting head and a rotary table using a Double Ballbar (DBB). A newly defined measuring trajectory only involving both rotary axes coordinated motion is proposed. The DBB is used to capture conic deviations that are used to generate error trace patterns. The asynchronisation between the resultant speed of the spindle tool cup during the B- and C-axes coordinated motion and the DBB capture speed has been resolved, enabling the true reflection of machine tool error conditions. The testing results are compared with simulation results established using the Homogeneous Transformation Matrices (HTMs) in conjunction with the rigid body kinematics. Different PIGEs were both estimated and measured using the simulation model and the proposed testing method respectively. Disagreements in the simulation and experimental results are analysed. The residual error after compensation of the PIGEs accounts for 8.76% of the total error budget, which might be due to thermal errors and kinematic errors. Experimental results show that the compensation of the PIGEs diagnosed in this paper will significantly enhance the coordinated motion of the B- and C-axes. Since the measurement involves both axes moving simultaneously, it can not only reflect the PIGEs of each rotary axes, but also assesses the geometric and kinematic coordination between the two rotary axes, which could comprehensively reflect the geometric accuracy of the five-axis machine tool.