Coordinate Measuring Machine Measurement Planning

Once a measuring instrument has been chosen for the control of the quality of a part of known design and specifications, the measurement process must be planned. For coordinate measuring instruments implementing point-based measurement, planning implies appropriately choosing the number and placement (pattern) of the points to be measured. In fact, coordinate measuring instruments sample points on features to be measured, but how should points be located on the feature itself? This problem is particularly relevant with measuring instruments which require a long time to sample dense clouds of points, e.g., most coordinate measuring machines (CMMs). In this chapter the problem of planning the inspection strategy, i.e., defining the number and pattern of sampling points, provided the measuring systems allow the operator to define the inspection strategy, will be addressed, with particular reference to CMMs. Sample size planning will be approached as an economic problem, because as the sample size increases, uncertainty is reduced and measurement cost rises, and a trade-off has to be searched for. Then, a few different criteria for defining the sampling pattern are proposed; these differ in terms of the accuracy and the information required for their application. These criteria can be categorized as blind, adaptive, and process-based sampling strategies. A few examples are proposed, outlining the effectiveness of different approaches to sampling strategy planning. In order to better understand the problem of strategy planning, a brief description of the main CMM features is provided.