Integrated Design Method to Improve Producibility based on Product Key Characteristics and Assembly Sequences

L. Mathieu1(2), B. Marguet’ L.U.R.P.A, E.N.S Cachan, Cachan, France C.C.R, E.A.D.S - France, Suresnes, France

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Abstract Producibility improvement is a major challenge for design and manufacturing firms. For aircraft or car body, this improvement has to go through a better product assemblability. In order to reach this goal, effects of manufacturing variations have to be managed. Two questions are asked about variation reduction: how to control effect of variations on functional requirements and what product characteristics are most sensitive to variations. No global answers exist to these questions. Therefore, we propose to describe in this paper an industrial method. The main idea of this method is that geometrical variation flows belong to the assembly process. During the preliminary design, the first step of our method is to identify where are the Key Characteristics for the product. The second step of the method performs a qualitative product analysis in order to eliminate the worst assembly sequences. At last a quantitative analysis allows to select the most promising assembly sequences. This integrated method has been applied with success to aircraft assembly.

Keywords: Assembly sequencing, Integrated design, Tolerancing analysis.

1 INTRODUCTION Car or aircraft industries are characterized by large and complex organizational structures and by an extremely aggressive business environment where customers are asking for a better product at a lower price. Product producibility as “a measure of the relative ease of manufacturing a product” is a philosophy of designing a product so that it can be produced in an extremely efficient manner with the highest levels of quality. This influence on design concept to define a product easier to make is a vital relationship between manufacturing shop floor and design office in order to produce a complex product. This product‘s producibility must now be considered early in the design phase to avoid problems that impact production schedule and cost. As complex product is made up from a high number of parts (a typical civil aircraft contains over 100,000 individual engineered parts) organized by various product levels, assemblability as a measure of the relative ease of product assembly, plays a prominent role for producibility. The study of the ease of product assembly during the design step is generally referred to “Design For Assembly” (DFA) [I]. The following items can describe DFA methods: Reduce part count within the assembly, improve the modularity of the assembly, utilize standard part wherever possible and minimize the probability of assembly failures. This last item is directly related to part variations. Because there is no way to produce parts that fully conform to their nomina I definition, geometrical variations wil I always appear on parts. Impact of these variations can be more or less important for the product’s assemblability. A level of geometrical variation out of tolerance on one part can lead to turn down the product quality or the product may fail to be assembled. In this case, part has to be reworked or repaired, adding extra cost for the product and increasing time required for the production process. Therefore reducing and managing part variations are a key step for improving product assemblability and then prod uci bi li ty . However two questions are generally asked when variations reduction is looked for during the design cycle:

what product characteristics are the most sensitive to variations and how to control effect of variations on functional requirements ? The goal of this paper is to give answers to these two questions by presenting a new integrated design method taking into account geometrical variation in the product development cycle as soon as possible. By managing variation reduction on product key characteristics and by analysing assembly sequence, this method will improve the product prod uci bi li ty . This paper will begin by describing the basic concepts used in our method. These concepts will allow us to propose an integrated design method for producibility improvement. Following the method proposal, different analysis tools will be described in order to facilitate its deployment. At last we will illustrate the results of this method on a real complex industrial application.

2 BASIC CONCEPTS

2.1 Assembly cost For complex product, assembly is the major manufacturing process far away from detail part manufacturing operations. It has been argued that more than 30% of manufacturing cost or financial value of a civil aircraft is introduced through assembly. Moreover the industrial companies responsible for delivering the complex product have generally sub-contracted largely all detail part manufacturing operations keeping only very large machined parts manufacturing and structural assemblies. By focusing on easy assembly, these industries will concentrate their producibility benefits. Efforts to improve product producibility have to be first put on assembly operation improvement.

2.2 Functional requirements To design products easy to assemble is feasible only with a concurrent product definition through Integrated Product Teams (IPTs). These teams gather together production, design, stress, facility equipment, manufacturing and tool engineering representatives.

The first task dedicated to the IPTs is to fully understand how the product is supposed to work and to be used. Functional analysis allows achieving this goal by defining customer requirements, technical requirements and product con strain ts . These functional require men ts and their cascades through all the component details allow focusing on what is really necessary about operation and performance for the product. Moreover the functional analysis allows for understanding how the design is related to the customer requirements. The product design process can also evolve with a mapping of the functional product specifications into the realization of these specifications using appropriate geometric concepts.

2.3 Assembly Design Amongst all geometric requirements related to functional specifications, assembly requirements will be related directly to the following assembly functions: “parts have to fit perfectly together”. From a design point of view, assembly requirements are generally viewed as alignment or gap. They can be modeled through assembly links between two assembly surfaces. These assembly links will link two different parts and will limit one or more degrees of freedom between the related parts. D. Whitney goes further and defines two types of assembly link: the mate and the contact [2]. A mate is an assembly link that establishes constraints and dimensional relationships between parts. While a contact is an assembly link that provides partial constraint or just supports and fastens the part once it is located. The type of assembly link, mate or contact will be related to the sequence of assembly link realization as mates are realized first until part is fully located on the product [3].

2.4 Tolerance propagation and assembly sequence Assemblability between two parts is defined during assembly process by the realization of all assembly links (mate and contact) existing in the product. Two causes may appear which will lead to fail the assembly link: design errors or geometrical variations on assembly features. In the first case, mates or contacts are not theoretically feasible (i.e : the 2 axes between an hinge and a peg are not aligned in the nominal model). This case is not unusual in complex product. However a virtual mock-up of the product could be used to verify the integrity of the nominal assembly. The second cause of failing the assembly links results from impact of geometrical variations on assembly features. This impact will be directly related to assembly sequence and assembly link types [4]. Indeed, as mate establishes positional constraints between two components, any geometrical variation on this link has a direct effect on the position of the two components. But, if we consider only rigid parts, as contacts just supporting and fastening the part, geometrical variation has no impact to the relative position of the two parts. So, geometrical variations on assembly flow through parts by mates only. Selection of which assembly links will be mate and which will be contact allows the IPTs to lead variation flows through the product and hence to minimize geometrical variation impact on assembly links. Based on the previous concepts we can propose now an integrated design method to improve product prod uci b ili ty .

3 INTEGRATED DESIGN METHOD Our design method related to product assemblability was defined following three main axes: Admissible Assembly Sequences, Product Key Characteristics and Tolerance Analysis. A diagram of our method based on these axes is on the following picture (Figure 1).

3.1 Assembly sequence As we have seen it before, analysis the assembly sequence and the related assembly link types allows understanding how variations flow through the product. Selection of the optimal assembly sequence gives us the product configuration that will minimize impact of variations on product requirement. This is worth noting that the use of this optimal assembly sequence to assemble the produce can be realized without extra manufacturing cost.

Figure 1 : Integrated design method. The first step of our integrated method is then to identify all admissible assembly sequences for the product. This identification will be based on product decomposition, assembly feasibility, industrial constraints etc.. . All admissible assembly sequences will correspond to our solution group and would be identified by IPT experts or by assembly planning software [5]. Selection of the product assembly sequence is decided early in the design process of a complex product, as this input is very important for manufacturing investments. The analysis of all admissible assembly sequence has then to be performed as soon as possible of the product design.

3.2 Key Characteristics Geometrical requirements are related to the product specifications by one or more geometrical characteristics. In case of a complex product, these characteristics are too numerous for the IPTs to be able to analyse all of them. In order to be efficient, IPTs need to manage by hierarchy which characteristics are the most important. We call these characteristics “Key Characteristics” (KCs). Key Characteristics are geometrical characteristics on the product for which a minute deviation from nominal specifications has a significant impact on the product’s

functions. Key Characteristics identification allows then to focus variation reduction only where it is really necessary for the product. Producibility improvement will be realized based on these KCs. The identification of KCs is performed first by a functional analysis of the product from top to down levels. This functional analysis gives to the IPTs, the geometrical requirements on each part. Based on these requirements coming from to the product design and the assembly sequence, Key Characteristics are identified.

3.3 Tolerance Analysis In order to select optimal assembly sequence and to satisfy KCs, tolerance analysis has to be realized on each admissible assembly sequence and for each assembly requirement. In order to achieve tolerance analysis, level of geometrical variation has to be identified on each part related to the manufacturing capabilities. Furthermore types of assembly links (mate or contact) have to be specified related to the studied assembly sequence. Once all assembly sequences have been evaluated, the optimal one can be selected. Two solutions can be imagined : the optimal assembly sequence allows to respect the geometrical requirements or not. If not, geometrical variation level have to be revalued based on new manufacturing process capabilities or functional analysis have to be modified based on new product design. Both of these two solutions introduce will have a significant impact on product development cost.

4 ASSOCIATED TOOLS In order to apply this method, different graphical and numerical tools have been defined. We propose to describe in the following these tools.

4.1 Assembly Oriented Graph Assembly Oriented Graph (AOG) is an extension of the Datum Flow Chain [2] and of the liaison graph [6]. The AOG will be used in order to give picture of the location dependencies of parts and surfaces. It allows representing geometrical variation flows in the product. For the purpose of our paper, we can restrict a product to a set of elementary parts, a set of assembly features and a set of geometrical requirements that the product has to deliver. Each assembly surface is related to another assembly surface in order to perform an assembly link. Depending on the assembly sequence, these links will be mate or contact. All components are assumed rigid and we will consider only static and sequential assemblies. As we consider only sequential assemblies, a component can be used to locate another component only if it has been completely located before. An AOG is a directed acyclic graphical representation of an assembly. This graph is composed of nodes and oriented arcs. Each node represents assembly surface brought together by component. Oriented arcs represent mate between two assembly surfaces. The direction of the arcs specifies which component locates the others. The arrow points on the positioned component. Non oriented arcs represent contact between two assembly surfaces and dash line represents geometrical requirement. Figure 2 depicts an AOG for a product composed from three parts A, B and C. Part A is the base part, this part gives the position of parts B and D and the position of part C is given by B. The AOG is characterized by the following properties: 0 The graph has one and only one root node that has

no arcs oriented towards it. This node represents the base component.

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There is always one oriented way on the graph going from the base node to another node. The graph cannot contain oriented cycles or loops, as loops will imply that parts locate themselves.

Figure 2 : Assembly oriented graph. Each AOG will represent a different assembly sequence and will allow the IPT to easily understand how parts are positioned, where are the geometrical requirements and how variations flow through the product.

4.2 Propagation Chain & Risk Analysis Variations on geometrical requirements depends on variations on mates. The analysis of each AOG allows studying the path of variations on geometrical requirement. We call this path of variations, a propagation chain (Figure 3). All links participating to the propagation chain will have an impact to the realization of the geometrical requirement. Risk analysis on these links allows then identifying which link will be a Key Characteristics for the studied geometrical requirement. The magnitude of variations of the end part is increased by the number of links belonging to the chain between this component and the base part. A qualitative criterion to select the best admissible AOG is the shortest propagation chain for a geometrical requirement.

Figure 3 : Propagation chain

4.3 Tolerance analysis software Based on KCs and Assembly Oriented Graphs, optimal assembly sequence can now be identified. This identification will be realized by 3D-tolerance analysis software. Tolerance analysis is used to assess the impact of assembly sequence and detail part tolerances on the overall assembly form, fit andlor function. In case of

complex 3-D assemblies, computer simulation tools for variation analysis are used [7]. This software begins with the development of functional feature models with datum and geometric dimensioning / tolerancing capability defined and attached to them. After completing the simulation based on assembly sequence, the tool shows the ability to meet specification at the identified measurement location and provide statistical measures of such capability. By analysis, results from all the best family simulations allows identifying the optimal assembly sequence . The link between these three tools are described on figure 4.

Figure 4 : Associated tools.

5 The integrated design method presented above has been used to improve assemblability of a section on the future Airbus A380. This section is composed by four major components. See figure 5. These major components are in turn decomposed in many sub-components. The first task dedicated to the IPT in charge of this section was to identify the geometrical requirements. Alignment of the upper and lower sub-sections and minimal gaps between the two door panels and windows panels were two examples of these geometrical requirements.

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Figure 5 : Application study.

Based on these geometrical requirements, the IPT was able to select the KCs related to the design of this section. The selection has been made by a risk analysis and based on the knowledge of similar products. Quality engineers have used these KCs in order to insure that these characteristics will be under control during the manufacturing process. After working on KCs, two assembly processes have been identified to realize this section. Assembly Oriented Graphs and tolerance analysis software have allowed the IPT to understand how variations will flow in this section and what variation impact was expected on geometrical requirements. Based on these two evaluations, the optimal assembly sequence has been chosen in order to get the best producibility for this section.

6 CONCLUSION This paper presented an integrated design method taking into account geometrical variation in order to improve product assembly. This method is based on four main concepts: 0 Importance of assembly in complex product process. 0 Need to identify product functions and geometrical

require men ts . 0 Difference between mate and contact link for an

assembly link. 0 Importance of assembly sequence in the variation

propagation through the product. This method will start by identifying Key Characteristics for the product that is the product characteristics, which will have the most important impact on product requirements. Following, the method will select the optimal assembly sequence, which will minimize tolerance impact on geometrical requirement. Based on KC identification and optimal assembly sequence selection, it will be possible to greatly improve product assembly and producibility without spending a lot of money on new manufacturing means or new product design.

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Marguet B., Mathieu L., 1999, Aircraft Assembly Analysis Method Taking into Account Part Geometric Variations, Proceedings of the 6th ClRP Seminar on CAT, University of Twente, 365-374. Santochi M., Dini G., Failli F., 1995, STC'A Cooperative Work on Assembly-Planning Software Systems, Annals of the CIRP, vol. 44, n"2, 651- 658. Ballu A,, Mathieu L., 1999, Choice of functional specifications using graphs within the framework of education, Proceedings of the 6th ClRP Seminar on CAT, University of Twente, 197-206. Salomons 0. W., Van Houten F., Kals H., 1997, Current status of CAT systems, Proceedings of the 5th ClRP Seminar on CAT, Toronto, 438-452.

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