Manufacturing Assembly

2.6.1 What is Assembly and why is It Important?

The Oxford dictionary defines assembly as “a unit consisting of components that have been fitted together”. Various other definitions of assembly have been reported in the literature. For example, Linn (1997) describes assembly as “a series of tasks putting together a set of components to produce an end product”. In Baudin (2002)’s point of view “assembly consists of putting or fitting together different parts into a product”. Holland (1997) describes assembly as “a group of components merged together is called assembly”. Whitney (2004) argues that “assembly is more than putting parts together”.

It can be inferred from the above definitions that assembly is the combination of different parts held together using different assembly processes and resources.

The term assembly can be taken in two contexts: assembly as a process and assembly as a product (Linn, 1997). However in this work, the term assembly process has been used for the first context and the term product has been used for the second context (more details can be found in chapter 4).

Assembly is the most complex process in the industry (Delchambre, 1992) which has not been given attention in the past as compared to other manufacturing processes and therefore it is least understood (Whitney, 2004).

Assembly is also a major time and cost contributor towards the development of product as Cho (2005) noted that almost 53% of manufacturing time is consumed in carrying out assembly tasks; 10% to 30% of manufacturing cost is

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associated with the assembly of the majority of the products and 20-60% of total labour is involved in assembly in making a product in US.

The importance of assembly is further highlighted by Walker (2001), who points out that Boeing and Ford consume 5% to 7% and 10% to 12% of their cost in assembly respectively. He also argues that the assembly cost increases as the size of the company decreases. It implies that even large companies like Boeing and Ford are consuming a handful percentage of cost in product assembly and therefore assembly cost will increase for small and medium industries.

Martin-Vega et al. (1995) investigated that whether the investment in research and development activities, help to significantly reduce the cost and increase the effectiveness of manufacturing assembly. The research was carried out by the Department of Defence (DoD) in USA on 24 product lines in various companies whose annual sales volume ranges from $10 million to $2 billion.

They noted that the assembly cost accounts nearly 20% of the manufacturing cost in comparison to previous research which suggests a figure of 4.8%.

However their research findings suggest that the investment in assembly research is only significantly useful when it focuses on assembly integration and/or assembly support activities rather than considering assembly processes only.

Whitney (2004) reported that currently assembly is facing technical, economical and managerial challenges. He noted that the technical challenges exist due to increasing complexity and customization of products, economic challenges are caused due to increasing customer demands for high quality and low price products, and managerial challenges are due to increasing dependence on suppliers, and other stakeholders.

From the above discussion it is obvious that there is a need to investigate key aspects of assembly particularly the support activities required for collaboration across the assembly domain. It is also noted that a significant amount of

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resources (in terms of money, time, labour etc.) are consumed in the assembly of products therefore there is a potential need to undertake research in this area.

2.6.2 Assembly Structure

LV et al. (2011) classified product assembly structure into four layers named as assembly layer, part layer, feature layer and presentation layer as shown in the figure 2.6. The assembly layer represents information related to various aspects of subassemblies e.g. subassembly identification, subassembly relations etc.

The parts layer information describes information related to parts identification code, respective subassembly code and parts relationship. The feature layer represents feature related information e.g. feature type, feature name, feature identifier, etc. Finally the presentation layer represents face level assembly information.

Figure 2.6: Different structure levels of product assembly (LV et al., 2011)

A similar classification of assembly structure has been proposed by Hui et al.

(2006). The different levels described in his research are assembly, subassembly, part and feature levels. This thesis also takes similar view of the

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assembly structure and defines the related concepts e.g. product, component, subassembly, part, and feature which will be described in detail in chapter 4.

2.6.3 Integration of Assembly Design and Assembly Process Planning

Integration of various assembly related tasks including the assembly design, and assembly process planning results in an efficient assembly design (Lit and Delchambre, 2003). The integration of production engineering and design departments should be promoted at the early stages of product development (Sackett and Holbrook, 1988). It suggests that assembly planning knowledge should be introduced during early stages of product design to support the product development process. The existing research has shown an evidence of work done on the integration of assembly design and assembly process planning related tasks.

For instance, Zha and Du (2002) found that integration of assembly design and assembly process planning can support the product development. They proposed a STEP based model for the integration of assembly design and assembly process planning and have found that data in computer interpretable form reduces the product development time by diminishing the intervention of humans for knowledge sharing, and encourages information sharing on product data level rather than on document data level.

Dorador and Young (1999) suggested that the product and manufacturing information models can be linked together to support knowledge sharing between Design For Assembly (DFA) and Assembly Process Planning (APP) domains. They believe that DFA techniques only focus on assemblability and design issues of the product and they do not address assembly planning issues consequently causing lack of assembly planning knowledge during the design phase. Figure 2.7 displays an interaction between DFA and APP which will allow the designer to access the APP related information while doing design for assembly analysis.

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Figure 2.7: DFA and APP integration environment (Redrawn from (Doradar and Young, 1999))

The research carried out by Demoly et al. (2011) focus on the integration of assembly design and assembly process planning phases. They promoted the idea of introducing assembly process planning knowledge during the early product design phase and have found that it is a potential research area in an assembly oriented design. They further argue that semantic and knowledge based assembly models offer a better integration and understanding of assembly planner’s intents in early stages of product development.

However to capture semantics to form a knowledge base requires the use heavyweight ontologies as identified in sections 2.2 and 2.3. Therefore it is evident that semantic integration issues in the assembly domain need to be addressed.

2.6.4 Feature based Assembly Knowledge Representation

Features play a key role in the representation of assembly knowledge as Haasis et al. (2003) describe features as career of descriptive and semantic information of product development processes. In another study by Case and Harun (2000), features have been recognised as a better source of assembly knowledge representation as compared to components or parts themselves. Further evidence of features for assembly knowledge representation is given by Holland

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and Bronsvoort (2000) who say that “assembly features can be profitably used in assembly planning modules”. While Molloy et al. (1991) also stress that both Design For Assembly (DFA) and Assembly Process Planning (APP) rely on feature based information.

The previous research has also shown that features have the potential to support integration of assembly design and assembly process planning domains. For example, Bley and Franke (2004) found that features contain information related to different aspects of assembly domain and can support integration of assembly design and assembly process planning. Mantyla et al.

(1996) claim that features provide design and manufacturing reusable data repository and can support integration of these domains.

Xu (2009) found that although features have the potential to support integration of design and process planning domains however they are complicated in the sense that they have multiple contexts and definitions. This becomes further problematic as feature based approaches only capture single context of the information (Young et al., 2007). This requires the support of PLM systems which can facilitate the capture of multiple viewpoints of information and their relationships (Young et al., 2007). It is therefore can be established that there is a potential available to address the multiple viewpoints of information attached to assembly features and finding out the relationship between these viewpoints.

2.6.5 Ontologies in Manufacturing Assembly

Increasing recognition of ontologies as a source of knowledge management has attracted many researchers towards the use of ontologies in the assembly domain. However still, the work reported in this area is not very high. The following paragraphs provide an overview of existing research in the assembly domain.

Chakrbarty et al. (2009) developed an ontology to semantically enrich the Variation Reduction Advisor (VRA) system used in General Motor (GM). Their

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ontology was aimed to control the vocabulary related to the assembly problems and their solutions. Lohse et al. (2006) proposed an ontology to facilitate decisions related to the selection of assembly equipment and to support the reconfigurable assembly system. Lanz et al. (2008) proposed ontology to capture product and process related assembly knowledge by using the feature concept.

Majority of the researchers who carried out ontology based research in the assembly domain, have used OWL as ontological formalism. For instance, Delamer and Lastra (2006) developed an ontology for the modelling of assembly processes. The ontology enables reasoning and inferences of assembly knowledge representation based on OWL and SWRL rules. Kim et al.

(2006) proposed an assembly design ontology which provides formal specification of product design related knowledge using OWL and SWRL ontological formalisms. Mostefai et al. (2005) proposed an OWL based ontological approach to capture the product design knowledge to support the product development process. Demoly et al. (2012) developed an ontology to capture the product design and assembly sequence planning knowledge where they have used OWL and SWRL.

However Fiorentini et al. (2007) argued that the main purpose of using ontologies in modelling the assembly knowledge is to exploit the potential advantages of these semantic approaches to formally define the meanings of assembly concepts to enable interoperability across the assembly systems. As the assembly domain is very complex because of the multiple components and features involved in it therefore an ontological approach with high expressive power and reasoning capabilities is required.

Most of the approaches discussed above are either based on lightweight ontological approaches or they use the OWL based approaches. As discussed in section 2.4.2, OWL based approach lacks the capabilities to model complex domains like assembly therefore it requires the new methods for the exploration of semantic representation and interoperability in the assembly domain.

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Also the existing ontologies in the assembly domain focussed on specific application areas and no ontology provides a comprehensive set of assembly reference concepts which can be used as foundation to build semantic base to support interoperability across the assembly domain. The thesis targets this research opportunity to explore the role of reference ontologies in the assembly domain to probe the interoperability issues.