Accuracy evaluation of machine tools by modeling spherical deviation based on double ball-bar measurements

• The accuracy of a machine tool is evaluated by modeling spherical deviation.
• Accuracies of three linear axes are affects to the spherical deviation together.
• The spherical deviation is more reasonable for evaluation compared to the circular deviation in ISO 230-4 limited to two linear axes.
• Measurement uncertainty of measured spherical deviation is established for confidence interval.

In this study, the accuracy of a machine tool was evaluated by modeling the spherical deviation based on double ball-bar measurements under unloaded conditions. Circular measurement paths on the XY-, YZ-, and ZX-planes were planned, and three linear axis drives were commanded to follow the paths describing a nominal sphere. The spherical deviation, defined as the maximum radial range of deviations around a least-squares sphere, is affected by the accuracies of the three linear axes together. Therefore, the spherical deviation represents the accuracy of machine tools by quantifying the effect of the accuracies of three linear axes, whereas the circular deviation only quantifies the accuracies of two linear axes among the three linear axes. In this experimental study, spherical deviations of vertical/horizontal machine tools were measured and analyzed under various nominal lengths of a double ball-bar for various feed rates. The measurement uncertainty of the measured spherical deviation was investigated to determine the confidence interval.