Selection of layered manufacturing techniques by an adaptive AHP decision model

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Abstract

The paper describes a computer-based tool for the selection of techniques used in the manufacture of prototypes and limited production runs of industrial products. The underlying decision model, based on the AHP methodology, ranks available techniques by a score resulting from the composition of priorities at different levels, each considering homogeneous and independent evaluation criteria. At each level, priorities are calculated from pairwise comparisons among either manufacturing techniques or properties of the techniques with respect to different types of application. This approach is enhanced with a procedure that adapts the parameters of the decision model to prototype specifications. Compared to scoring procedures, the method reduces the ambiguity on the values of weighting factors and allows an easier interpretation of the rationale behind the selection process.

Introduction

The continuous growth of rapid prototyping and rapid tooling (RP/RT) technology is providing more and more options for one-off and limited production of manufactured parts. As a consequence, selecting the proper technique for a given application can become a fairly complex task. The selection problem can arise when a regular or occasional use of RP/RT processes is planned at a company, and can have different purposes:

  • compare layered manufacturing techniques to traditional model-making practices;

  • select proposals of service bureaus, each specialized on a restricted set of techniques;

  • evaluate the option to purchase one or more RP/RT systems.

The capabilities of currently available RP/RT techniques have been thoroughly discussed in several review papers [1], [2], [3]. Along with the many case histories reported in literature, these studies are helpful for a first orientation of users among preferential applications of the different techniques. The suitability of a technique to an application can be evaluated through several performance measures, related to both efficiency (costs, build speed, setup time) and quality (prototype strength, accuracy, and surface finish). The relative importance of these often conflicting criteria is strongly dependent on the specifications of each different application, and especially on prototype quantity and geometry.

In the selection of the technique to be used for a given application, most users rely upon information collected among experts and system vendors. No formal evaluation procedures are usually followed, even in the presence of highly integrated information flow from design to production. The use of a computer-based tool could help to make more informed decisions about the adoption of commercially available processes and systems. It would also help potential users without specific knowledge of RP/RT techniques to better understand their suitability on perspective applications.

In the development of such a tool, the selection problem is usually described as a multi-objective decision process, where the overall value of each alternative results from the trade-off of conflicting criteria with different priorities. In most cases, each alternative (process or system) is assigned a number of scores indicating the degree of fulfillment of prototype specifications (material, size, volume, quantity, etc.). A weighted sum of the scores provides the desired ranking of alternatives.

An approach based on weighted scores, often referred to as benefit value analysis, poses two critical requirements in order to ensure practical acceptance of decision support systems:

  • weighting factors of decision criteria need to be set as consistently as possible, with a structured use of available knowledge sources;

  • possible changes of weighting factors as a function of specific application requirements should be allowed.

In accordance with these considerations, the present work describes a formal procedure for the selection of RP/RT techniques and its implementation as a demonstrative software tool. The approach followed in the development of the procedure is based on the analytic hierarchy process (AHP), a multi-objective decision methodology that provides a logical formulation of the selection problem and reduces the inherent ambiguity of scoring methods [4]. For this purpose, it structures the selection process into hierarchical levels, each involving independent criteria resulting from a decomposition of more aggregate criteria at the upper level. Weights associated to criteria at each level are calculated, rather than set, from pairwise comparisons among criteria with respect to criteria at the upper level. The algorithm for weight estimation detects inconsistencies in the comparisons and makes later updates of the decision model easier.

The paper is organized as follows. After a review of background literature in Section 2, the selection problem is defined in Section 3 and further developed in Section 4 with a discussion of relevant decision criteria. The description of the proposed method in Section 5 clarifies the use of the AHP approach and introduces changes to the basic methodology in order to cope with application-dependent criteria. Section 6 reports the implementation of the software tool and its validation on some test problems, whose results are discussed in the conclusions of Section 7.

Section snippets

Related work

Criteria and rules for the comparison of RP/RT techniques in specific classes of applications are implicitly stated in all the above-cited technology reviews. One of the most detailed and complete [5] includes a chart for the selection of techniques from coarse prototype specifications (size, accuracy, shape complexity, material strength). The basic selection criteria introduced by this straightforward decision procedure have been further developed in literature with the aim of implementing

Problem statement

The purpose of the selection process is a quantitative comparison among the RP/RT techniques available for a given application, i.e. for the construction of single or multiple prototypes from given specifications. The following data are used as input for the selection:

  • the type of prototype, chosen among five categories (conceptual models, technical prototypes, sand castings, investment castings, injection molded parts);

  • the quantity of prototype parts to be manufactured, varying from one or few

Selection criteria

Each RP/RT technique has its strengths and weaknesses on both the technological and the economic ground: what has to be decided is which compromise is more suitable to a given application. As reminded in the following, different kinds of prototypes are likely to involve different selection constraints.

Decision model

Available alternatives are ranked for a given application by the method described below. The proposed decision model is based on three main features:

  • the organization of selection criteria in a hierarchy of relations among the above defined elements (application, prototype categories, attributes, alternatives);

  • the assignment of priority measures to alternatives according to the relative importance of elements at all levels of the hierarchy;

  • the proper setup and adaptation of model parameters.

Implementation

The hierarchical selection procedure has been applied to some test cases, in order to validate and improve its potential as a support to decisions concerning RP/RT techniques. A graphical software tool has been developed to allow an easy data input and a straightforward interpretation of calculated priority vectors.

Fig. 7 shows a run of the selection tool for the sand casting example discussed in Section 5.3. On the left side of the results area, global priorities of alternatives with respect

Conclusions

Test results seem to confirm the validity of choices made in the development of the selection procedure. The advantages of an AHP-based approach have been essential to deal with a problem with many options and decision criteria, which poses serious challenges to traditional scoring methods. Pairwise comparisons increase the robustness of relative priorities established among alternative solutions. The hierarchical structure of the decision process allows a better decomposition of problem

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