A technique for accuracy improvement of squareness estimation using a double ball-bar

Squareness measurement and its compensation are important for improving the performance of machine tools. A double ball-bar is widely used to measure squareness because of its simple, convenient setup and measurement procedures. This study addresses two important issues affecting the accuracy of squareness estimation when a simplified model of the geometric errors is used and estimation is carried out over constrained local ranges of the moving axes. A simplified model is frequently applied to geometric error estimation using a double ball-bar to avoid singularity problems while solving the double ball-bar equation. However, the use of a simplified error model significantly affects the accuracy of the squareness estimation. In this article, a squareness estimation technique using double ball-bar measurements and a laser interferometer is proposed. In addition, a convenient squareness estimation method involving only single double ball-bar measurements is suggested for the full travel range utilizing the straightness error data. The validity of the proposed method is verified by comparison with laser interferometer measurement results using an optical square on the XY-plane of a three-axis machine tool. (paper)