The paper investigates the role of planar datum features in tolerance analysis problems. Mating relations between datum planes are shown to involve translational and rotational errors, which are related to form tolerances and are usually neglected in tolerance analysis. To evaluate these errors, the contact between datum planes was simulated by a stochastic model, where two surface profiles are randomly generated and then registered to reproduce a mating condition. Concepts of fractal geometry were exploited to make the generation consistent with the autocorrelation properties of actual surfaces resulting from manufacturing processes. A simulation plan allowed to predict the amount of contact errors as a function of size, tolerance and process-related assumptions on the two features. An example of 3D tolerance chain is presented to demonstrate the relevance of form errors in the variation of assembly requirements.
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