Coordinate stitching measurement of highly steep freeform surfaces

Abstract

Abstract A coordinate measuring machine (CMM) is widely used for surface measurement during milling and grinding of workpieces. However, the measurement uncertainty increases with the surface slope for highly steep freeform surfaces. In addition, local features sometimes exist on the surface out of the accessible space of the CMM. This study presents a coordinate measurement method based on stitching multiple subpatches. The complex freeform surface of a large slope is first divided into subpatches which are individually tilted and measured with smaller uncertainty due to reduced slopes. A global stitching algorithm is then proposed to compensate the motion error and stitch all the subpatches together simultaneously, based on configuration optimization to minimize the overlap inconsistency. The coordinate stitching measurement method is first validated through simulation, which shows that the motion error of the order of 0.05°/0.5 mm is tolerable for a highly steep surface. It is then experimentally verified using the stitching measurement of an optical spherical surface and a steep freeform surface. An ordinary multi-axis adjustment stage is suitable for subpatch repositioning because the stitching algorithm compensates the motion error to a level lower than the CMM uncertainty.