Design Verification through Tolerance Stack up Analysis of Mechanical Assembly and Least Cost Tolerance Allocation

Abstract

Geometric dimensioning and Tolerancing (GDT) constitutes the dominant approach for design and manufacture of mechanical parts that control inevitable dimensional and geometrical deviations within appropriate limits. The stack up of tolerances and their redistribution without hampering the functionality is very important for cost optimization. This paper presents a methodology that aims towards the systematic solution of tolerance stack up problem involving geometric characteristics.Conventional tolerance stack up analysis is usually difficult as it involves numerous rules and conditions. The methodology presented i.e. generic capsule method is straightforward and easy to use for stack up of geometrical tolerances of components and their assembly using graphical approach. In the work presented in this paper, angularity tolerance has been considered for illustration of the methodology. Two approaches viz. Worst Case (WC) and Root Sum Square (RSS) have been used. An example of dovetail mounting mechanism has been taken for purpose of stack up of angularity. Based on the stacked tolerance, it can be verified with the design tolerance of the assembly. Based on the comparison, designer has to reassign the appropriate tolerances to fulfil the functionality if required. If the stacked tolerance is as per designer requirement, then reallocation of tolerances on individual components should be done. Costs versus tolerance data are available for each component. With optimization technique, the optimized cost has been calculated for the assembly.

Allocation of geometric tolerances in one-dimensional stackup problems

“…For hole patterns, the definition of position tolerance with maximum material condition (MMC) is used to develop equations for such tasks as calculating process capabilities [23] or reconstructing datums and basic dimensions from inspection data [24]. In [25], the allocation of angularity tolerances on a dovetail joint is carried out by a solution search method using a graphical procedure for stackup analysis.…”

Statistical geometric computation on tolerances for dimensioning

“…al. [29] that describes a systematic solution for the tolerance stack up problem involving geometric characteristics on both the worst case model and the statistical model. Gayton et.…”

An extended form of the reciprocal-power function for tolerance allocation

“…More complex expressions, such as the Michael-Siddall function [14] and the combined and polynomial functions [15] are potentially more accurate but require a larger amount of cost data for a reliable estimation of their parameters. Comparative studies on cost-tolerance functions [16][17][18][19][20] focus on the choice of the correct function as a compromise between accuracy and suitability to specific allocation methods. As highlighted in [5], an often neglected task is the selection of function parameters, which is seldom related to the properties of toleranced features.…”