Deviation model based method of planning accuracy inspection of free-form surfaces using CMMs

Measurements of free-form surfaces are performed with the use of numerically controlled CMMs on the basis of a CAD model, which results in obtaining coordinates of discrete measurement points. The local geometric deviation, i.e. the distance of a particular measurement point from the CAD model of the nominal surface, is established for each point. The measurement aims at evaluating the form deviation and thus the greatest deviation of the actual surface from the CAD model. An effective measurement is one in which the probability of locating the greatest deviation is highest with the smallest possible number of measurement points. The present article suggests a method of planning a measurement strategy for objects with free-form surfaces. Repeatability of deterministic deviations on surfaces processed under the same conditions was applied. A CAD model of the product, built on the measurement points, and including the deterministic component of these deviations, was constructed. Effective surface measurements were planned locating the measurement points in the critical areas. The devised model was used for performing the measurement.

► The method of planning CMM sampling for inspecting of freeform surfaces is proposed.
► CAD models of patterns left by the machining process on surfaces were used.
► A small number of points in critical areas were measured.
► The probability of locating the greatest deviation is much higher.
► The measurement standard error is smaller; the time of measuring is radically reduced.