Evaluation of 3-D feature relating positional error

Position tolerance is a three dimensional geometric specification used to control the location of features of size with respect to a set of datum features or with respect to each other. Among features of size, hole is perhaps most frequently encountered in the production of industrial parts. To control the relationship between a set of holes, feature relating position tolerance (referred to as FRTZ) is used. FRTZ is applied to ensure proper assembly between mating parts. Assessment of feature relating positional error is a difficult task due to the absence of a datum reference frame whose function is to impose translational and rotational constraints on the part. Existing techniques for the assessment of FRTZ are either too expensive, or time consuming and inaccurate. This paper presents a mathematical model of three-dimensional feature-relating positional error and proposes a sound methodology for its evaluation. The results of performance analysis indicate that the proposed methodology is robust, accurate and efficient. The procedure has been implemented in a computer-aided inspection system.