Optimal tolerance allocation based on Difficulty matrix using FMECA tool

Abstract

The cost and quality of the product depend directly on the tolerance values. Especially, the tolerance allocation is an important engineering function involving a direct impact on compliance with functional and manufacturing requirements. The mastery of this function requires the use of a tool to analysis the reliability of the process as well as a method to optimize the tolerances. Thus, this work proposes an innovative tolerance allocation method based on the Failure Mode, Effects and Criticality Analysis (FMECA) tool as well as a Lagrange Multiplier (LM) approach.The difficulty of obtaining manufacturing dimensions is evaluated through a mathematical coefficient calculated using FMECA tool. The failure causes are the different sources of the manufacturing difficulty. The above sources are identified according to the diagram of Ishikawa. A suitable FMECA worksheet for the analysis is established for each type of manufacturing operation. A CAD model is developed to determine dimension chains as well as to identify manufacturing operations and influenced dimensions. The proposed LM method solves the tolerance allocation problems by quantifying manufacturing dimension difficulty and minimizing the tolerance cost. The results of a case study are compared with those of the standard methods using a cost model. This comparative study confirms that the proposed method reduces the total assembly cost considering manufacturing cost and quality loss.