On-line calibration of positioning accuracy of CNC lathe using a double-frequency laser interferometer

Positioning accuracy is an important factor affecting the performance of CNC machine tools. Targeting a high positioning accuracy, a fast and effective calibration technique is in an increasing demand from both machine tool users and manufacturers. This paper introduces an on-line calibration approach to dynamically evaluate the positioning accuracy of a CNC lathe through statistical estimation. A double-frequency laser interferometer is configured to measure the respective motions in the lathe's two perpendicular axes during the machining process, triggered by a cutting force dynamometer. Positioning accuracy calibration is realised by comparing the nominal positions from the CNC motion controller to their actual values measured by the laser interferometer system. Compared to that achieved at the machine idle state or the free running state using conventional techniques, this on-line calibration approach provides a more efficient way to get a real picture of the actual performance of a CNC machine tool under its working condition. It also gives a truthful reference to effectively compensate positioning errors of the CNC machine tool.