A Dual Difference Method for Identification of the Inherent Spindle Axis Parallelism Errors of Machine Tools

It is essential to identify spindle axis parallelism errors because such errors trigger volumetric errors when tools of different lengths are used. However, only a few works have addressed this issue. Thus, we identified the inherent spindle axis parallelism errors of machine tools relative to the end-point reference straight line of the Z-axis (according to ISO 230-1) using a dual difference method. Here, “inherent” refers to parallelism errors of the spindle axis that are not affected by the geometric errors of other axes controlled during the measurements, and “dual difference” refers to the difference in the differences of measuring data. The dual difference method uses two pairs of circular tests performed with the aid of a double ball-bar (DBB); the tool lengths differ during each test and the DBB set-up is shared by the pairs. Parallelism errors are then identified based on the dual differences within and between the two pairs. Experimentally, the maximum peak-to-valley (PV) values were 54.5 and 48.7 μm for differences in radial deviations within the two pairs when the parallelism errors were not compensated. After tool-center-point compensation by the identified errors, the PV values improved to 8.0 and 9.2 μm, respectively, showing that compensation was successful. In addition, the concentricity of two holes machined using tools of different lengths improved from 31.2 μm without compensation to 15.9 μm with compensation, further demonstrating the effectiveness of the method.