To ensure the gear precision, industries need a coherent model to express, to analyse and to check geometrical specifications. Most gear tolerance representations are directly driven by the convenience of dimensional metrology and not by the convenience of the set of activities of the tolerancing process. Therefore, to ensure the coherence of all tolerancing process activities, there is a necessity to develop a complete gear tolerance model which should: represent standard tolerance practices; be integrated in the Computer-Aided systems of design, manufacturing and metrology; be controlled by CMM; and support automated tolerance analysis. The proposed model extends capabilities of a vectorial dimensioning & tolerancing model in order to satisfy the four requirements. This model is based on GeoSpelling [1]. Its coherence is illustrated by two applications: gear tolerance analysis and gear tolerance verification by CMM.
Keywords:Tolerancing Model, Functional Metrology, Gear
INTRODUCTIONAs technology increases and performance requirements continually tighten, the cost and required precision of assemblies increase as well. There is a strong need for increased attention to tolerance design to enable highprecision assemblies to be manufactured at lower costs. Indeed, tolerance analysis is a key element in industry for improving product quality. Designers want tight tolerances to assure product performance; manufacturers prefer loose tolerances to reduce cost. There is a critical need for a quantitative design tool for specifying tolerances. Tolerance analysis brings the engineering design requirements and manufacturing capabilities together in a common model, where the effects of tolerance specifications on both design and manufacturing requirements can be evaluated quantitatively. The inherent imperfections of manufacturing processes (forging, cutting or grinding) involve geometrical variations and a degradation of product quality. The geometrical variations of each part must be limited by geometrical specifications (tolerances) to ensure a certain level of product quality, which is defined by the functional requirements. In the case of gears, their geometrical variations impact the transmission error, the tooth contact position, meshing interference, and gap. To ensure a quality level, designers limit these parameters by requirements. Tolerancing decisions can profoundly impact the quality and cost of gears. To assess the impact of tolerance on gear quality, designers need to simulate the influences of tolerance with respect to the functional requirements. To do so, they use AGMA [2] or ISO tables. These tables are a set of discrete associations between tolerances and meshing quality. They do not take into account all tolerances, and they focus on the pitch error and the misalignment. In the case of a forged gear or not classical gear like WILDHABER-NOVIKOV, designers can not use them to allocate the gear tolerances to