Probe Radius Compensation for On-Machine Measurement of Sculptured Surfaces

Accurate dimensional measurement of freeform surfaces is a key step in CNC machining. Usually, a CMM equipped with a touch trigger probe is employed for measurement of such surfaces. In such a precision measurement, probe radius error is a serious concern. Traditionally, probe radius compensation methods are based on using the design (CAD) surface. In specific instances, the inaccuracy of these compensation techniques could be one order magnitude higher than the repeatability of the probe. Probe radius compensation based on ideal machined surface (generated when the milling cutter accurately moves through the cutter location (CL) points) is superior in principle to the one based on design surface, since it is one level of abstraction closer to the final surface realized. On Machine Measurement (OMM), where the measurement process is accomplished on the machining center by replacing the milling cutter with the Touch Trigger Probe, affords such a possibility without resorting to computationally intensive surface localization. This paper presents the implementation and performance analysis of a proposed new probe radius compensation technique based on both the design surface data (CAD model) and machined surface data (CL points). The proposed method efficiently compensates the probe radius error of on-machine measurement data for sculptured surfaces. The model was implemented on the simulated CMM data of a bi-cubic Bezier surface machined by a ball-end mill and compared with the conventional compensation techniques. The results show that this method compensates the probe radius error better than the existing techniques and yields superior precision.