The DBB (double ball bar) method is widely accepted as a tool to measure the motion accuracy of NC (numerically controlled) machine tools and to diagnose their error sources. The DBB method can only perform circular tests due to its nature. Motion errors due to the mistuning of CNC servo control systems are, however, often easier to observe on non-circular paths. In this paper, we present an overview of a method to diagnose motion errors due to servo control systems by measuring the machine's motion accuracy in arbitrary shape contouring, and then to re-tune CNC servo control systems such that the optimal contouring performance can be obtained. The cross grid encoder method, or the KGM method, is used for two-dimensional contouring error measurement. By comparing with contouring error profiles of position feedback signal, motion errors due to mechanical structures and those due to CNC servo control systems can be both identified.
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