Touch-Trigger Probe Error Compensation for Nonvertical On-Machine Measurement of Freeform Surface Workpieces

The on-machine measurement (OMM) accuracy for freeform surface workpieces is greatly reduced by the error of the touch-trigger probe. The existing methods typically perform probe error compensation under the assumption of vertical measurement. However, due to possible workpiece deformation or unknown workpiece models, the approaching direction inevitably deviates from the normal vector of the inspection point, leading to nonvertical OMM. In this article, the principal differences between vertical and nonvertical OMM processes are clarified first. Compared with vertical OMM, nonvertical OMM results in more complex probe pretravel errors and introduces the problem of ruby ball radius compensation. Then, a novel touch-trigger probe error compensation strategy for nonvertical OMM is proposed. The pretravel error of nonvertical OMM is divided into the delay error and the stylus deformation error. The former can be calculated by the approaching speed and OMM systems delay time. The latter could be figured out based on the 3-D deformation error map and interpolation methods. The normal vector approximation of the inspection point is iteratively calculated by the B-spline surface fitting to compensate for the stylus deformation error and ruby ball radius simultaneously. For a designed freeform surface workpiece, the proposed strategy improves the nonvertical OMM accuracy by 49% compared with the comparative method.