The impact of fixturing error on feature tolerance allocation

Fixtures are used to locate, hold and support workpieces during the operation. An accuracy of the workpiece is decided by a relative location and orientation of the workpiece coordinate system with respect to that of the fixture. The impact of surface variability at contact points on the variability of workpiece location and orientation will be analyzed. Methods of estimating moments will be implemented to evaluate the distribution of the workpiece variability. When surface errors exist and the workpiece is fixtured for machining, inconsistency in feature location and orientation is expected. Hence, in order to derive efficient tolerance allocation for the feature, the fixturing error must be taken into account. A circular tolerance region based on a bivariate normal distribution will be used to obtain tolerance zones of a desired probability of rejection. By establishing tolerance efficiently, we can reduce a number of rejected parts, leading to some reduction in production cost and time.

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