Alternate methods for sampling in coordinate metrology

A variety of inspection equipment exists in industry to capture the points on a surface that will envisage manufacturing form errors. Manufacturing inspection faces a problem of finding optimal methods to capture an evenly spread distribution of points on the surface. Sampling does not yield complete information about a surface. Each material removal process leaves a unique pattern on the surface of the workpiece, which has to be taken into consideration while developing the sampling strategy. For instance, round patterns left by face milling at low feeds on flat surfaces, and spiral patterns left during surface milling and turning operations on surfaces of revolution. Two new methods have been developed to improve sampling using the coordinate measuring machine (CMM) for inspection of flat and revolved surfaces. These are the Spiral, and Hamspi sampling methods. The Spiral method focuses on the centre of the area and uses the Archimedean spiral. Hamspi is a method that combines both the Spiral and randomized Hammersley to measure points in the middle as well as the outer zone of the workpiece. Mathematical comparisons of these methods have been made to establish feasibility. An experiment was performed to determine the accuracy of these models using two dependent variables: inspection time and minimum zone. The minimum zone was (statistically) significantly affected by only two factors: sample size and workpiece shape. The sampling time was however affected by sample size, workpiece shape, and the interaction between them. This study observed that the beginning and ending cutting zones of spherical surfaces were the most significant regions to verify. It was found that the Spiral and Hamspi methods had similar point distributions as the Hammersley method while placing more emphasis on the origin of the workpiece.