Step-height measurements on sand surfaces: A comparison between optical scanner and coordinate measuring machine

As with other manufacturing processes, modern casting is aiming at higher precision. To this, traceable measurements on sand moulds have become necessary. Optical 3D scanning offers a high degree of flexibility but suffers from lack of traceability, in particular when the object is sand. In this work step-heights ranging from 0.1 mm to 5.0 mm were made on 6 customised sand samples with an average grain size of 230 µm, produced using a hard binder that can be scanned both by tactile and optical probes.

The step-heights were measured using a CMM with ø8 mm tactile probe. An optical 3D scanner based on triangulation principle using fringe projection was also used to measure the step heights resulting in a point cloud for each scanning. A similar measurement procedure with scanner was made using the scanners software to simulate a ball probe with the same size of CMM probe and following the same routine to touch the different positions on the polygonised mesh.

Each measurement was repeated 5 times. The results of step height measurements on sand surfaces showed a maximum error of ± 12 µm for CMM, while scanner shows only ± 4 µm. Generally speaking, optical step height values were measured up to 27 µm larger than CMM measurements. A conclusion from this investigation is that CMM cannot be used to ensure traceability of optical measurements on this type of surface.