3D tolerance analysis system and methods

A computer-aided design system provides tolerance analysis of model assemblies. Constraints are applied to a model assembly based on user-selected surfaces. Overlay surfaces are defined at the assembly, where each of the overlay surfaces corresponds to a user-selected surface. One or more stacks are defined based on a user input, where the stacks are a measurement dimension encompassing two or more parts of the assembly. The system simulates 1) variation of the overlay surfaces across a range of variation values within a respective tolerance zone based on the tolerance specifications, and 2) alteration of the stack as a result of the variation. Based on the variation of the overlay surfaces, the system identifies a subset of the user-selected surfaces that, when varied within the respective tolerance zone, alter the stack. A database is configured to indicate 1) the user-selected surfaces and 2) respective contributor values indicating a relative contribution of each of the user-selected surfaces to the stack.