Straightness error compensation for large CNC gantry type milling centers based on B-spline curves modeling

Fast and accurate modeling for the straightness errors of machine tools is significant important for the implementation of error compensation. To decrease the straightness errors which generally exist in gantry type milling centers, a novel approach for real-time compensation of straightness errors is presented, including an accurate spatial straightness error model which based on B-Spline curves method and a real-time errors compensation system which can compensate multiple errors at the same time in real-time. An experiment is carried out to measure the straightness errors of a gantry type milling center by using a laser interferometer. There are two straightness errors for one direction movement of an axis. As a result, there are six straightness errors for a three-axis milling center. The straightness error model is established by the B-Spline curves method. According to these six straightness error models, a spatial straightness error model is obtained by superposition of these straightness error models. In addition, an external real-time errors compensation system is developed based on the function of external mechanical origin offset in Fanuc CNC systems. The compensation experiments are conducted to verify the accuracy of the spatial straightness error model and the effectiveness of the error compensation system. The experimental results show that the straightness errors of the machined work-pieces are proved to have been decreased over 90% compared with that of non-compensations. The B-Spline curves modeling combining the errors compensation system can be utilized as an effective approach to improve the accuracy of the CNC gantry type milling centers.