Tolerance Analysis Supports Tolerance Assignment: The Industrial Case Study of Skillet

Tolerance Analysis Supports Tolerance Assignment: The Industrial Case Study of Skillet

Wilma Polini (University of Cassino and Southern Lazio, Cassino, Italy), Andrea Corrado (University of Cassino and Southern Lazio, Cassino, Italy) and Costanzo Cavaliere (CIEM SpA, Cassino, Italy)
DOI: 10.4018/IJMMME.2018010101
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This work presents a method to support product design, since it shows how to use together tolerance assignment and analysis for choosing among different set of tolerances assigned to the same product. It starts from tolerance assignment that produces different sets of tolerances for the product components which are all acceptable from a functional point of view. It translates each assigned set of tolerances into one or more groups of tolerances that are recognized by the software used for tolerance analysis. Therefore, the software for tolerance analysis is applied to each group of tolerances by means of a Monte Carlo simulation approach. Finally, the obtained results are intersected or compounded to obtain the trend of product functional requirements that allows to identify the best set of tolerances assigned to the product components. The developed method was applied to a skillet, a platform of an industrial plant that is made of five parts connected by screws. The obtained results show how the developed new method is a valid tool to support design for industrial application.
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The electromechanical systems for handling the car chassis and, generally, the mechanical components, are assemblies composed by a large number of parts with irregular geometry; this makes them complex to be designed, to be built and to be tested in a very short time, as industry requires.

A very sensitive and important issue in product and process development is tolerance design because of increasing demand for quality products and the growing requirements for automation in manufacturing. Tolerance design involves an interaction between design and manufacturing decision making (Singh et al., 2009). It is constituted by tolerance assignment and tolerance analysis. Tolerance assignment means to identify the kind of tolerances to apply to the different components of an assembly in order to ensure the effective assembly, the functionality of the assembled product and, at the same time, a low cost. Tolerance analysis verifies if the tolerances that are assigned to the assembly components, lead to assemble the product. Tolerance analysis predicts the cumulative effects of the tolerances that were assigned to the components, on the entire assembled product rather than on product's functional requirements.

This paper deals with tolerance assignment and tolerance analysis. In the literature, very few works dealing with tolerance assignment exist, since it shows as many different problems as there are in design projects and the solution of these problems is entrusted to the designers’ experience. The dimensional and form tolerances are assigned to internal and external features in (Lanzotti et al., 2000). A model that expresses several points of view: functional, standards, inspection and manufacturing, is proposed in (Ballu and Mathieu, 1995). Tolerances are generated from contact relations expressed as associations between surfaces (Technologically and Topologically Related Surfaces, TTRS) in (Clément et al., 1994, Salomons et al., 1996 and Weill, 1997). Rules involving the types of features and a user defined priority order among them generate geometric specification in (Anselmetti, 2001). Constraints to the relative motion among parts are used to define suitable geometric control on features in (Hu and Xiong, 2005). Geometric specifications defined on key components of a mechanism are shown in (Mejbri et al., 2005). (Armillotta and Semeraro, 2011) compares the methods available for the specification of geometric tolerances, from common engineering practice to the development of computer-aided support tools. A nonlinear combinatorial optimization problem based on assembly function requirement (AFR) in order to optimize the tolerance values is addressed in (Saravanan et al., 2014). A new general methodology is presented to assign dimensional and geometric tolerances to the all components of an assembly with a concurrent design approach (Polini, 2016). Recently, optimal Pareto set of tolerances is determined for pin and hole for minimum total cost and minimum clearance variation of a hole and pin assembly, given the design clearance, the process capability of the machines defined with their standard deviations and the mean diameter of either pin or hole (Jawahar et al., 2015).

In recent decades, many researchers have dedicated their studies to the development of the mathematical model for tolerance analysis of a mechanical assembly constituted by rigid bodies. In literature, there are some contributions tackling the issue of the comparison among different mathematical models. The most significant works on the comparison of the mathematical models are in (Hong and Chang, 2002, Shen et al., 2005, Polini, 2011, 2012 and Chen et al., 2014).

However, these methods are not easy to apply, especially for complex automotive assemblies, since they are created to deal with elementary features, such as planes, holes, pins, and so on.

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