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A functional approach for the formalization of the fixture design
process
R.?Huntera, J.?Riosb,? J.M.?Pereza, A.?Vizana
A Department of Mechanical and Manufacturing Engineering, Escuela Tecnica Superior de Ingenieros Industriales, Universidad Politecnica de Madrid, Jose Gutierrez Abascal, 2, 28006 Madrid, Spain
B Currently in Enterprise Integration (Bldg 53), Cranfield University, Cranfield, MK43 0AL, UK Received 14 January 2005; Accepted 14 April 2005. Available online 26 August 2005.
Abstract
The design of machining fixtures is a highly complex process that relies on designer experience and his/her implicit knowledge to achieve a good design. In order to facilitate its automation by the development of a knowledge-based application, the explicit definition of the fixture design process and the knowledge involved is a prior and a fundamental task to undertake. Additionally, a fundamental and well-known engineering principle should be considered: the functional requirements and their associated constraints should be the first input to any design process. Considering these two main ideas, this paper presents the development undertaken to facilitate the automation of the fixture design process based on a functional approach.
In this context, the MOKA methodology has been used to elicit fixtures knowledge. IDEF0 and UML have been used to represent the fixture design process. A methodology based on the function concept and aiming to formalize such design process is proposed. Fixture functional requirements have been defined and formalized. Functional fixtures elements have been used to create a functional design solution, the link of these elements with the functional requirements and with typical commercial fixture components has been defined via tables and rules mapping. And finally, a prototype knowledge-based application has been developed in order to make an initial validation of the proposed methodology.
Keywords: Fixture design process; Fixture knowledge modeling; Fixture functional requirements
Article Outline
1. Introduction
2. Analysis of machining fixtures requirements
2.1. Functional requirements
3. Proposed methodology for the formalization of the fixtures design process
4. Fixture functional design process model
5. Information model, instantiation, and methodology validation
6. Conclusions
References
1. Introduction
The main objective of any design theory is to provide a suitable framework and methodology for the definition of a sequence of activities that conform the design process of a product or system [1]. In general, all of them identify requirements as the starting point in the design process. In fact, the engineering discipline dealing with product design can be defined as the one that considers scientific and engineering knowledge to create product definitions and design solutions based on ideas and concepts derived from requirements and constraints [2], [3] and [4].
For this research, a relevant issue when considering requirements, taking this as a general concept, is to make explicit the meaning of two main terms: Functional Requirement (FR) and Constraint (C). A ‘functional requirement’, as it stated by different authors, ‘represents what the product has to or must do independently of any possible solution’, [2] and [4]. A FR is a kind of requirement, and considering some basic principles widely recognized in the field of Requirements Engineering, we could add ‘it ISA unique and unambiguous statement in natural language of a single functionality, written in a way that it can be ranked, traced, measured, verified, and validated’. A ‘constraint’ can be defined as ‘a(chǎn) restriction that in general affects some kind of requirement, and it limits the range of possible solutions while satisfying the requirements’. So, a constraint should be always linked to a requirement, and its purpose is to narrow the design outcome to acceptable solutions.
Considering the Theory of Axiomatic Design [4], functional requirements should be defined in the functional domain, which brings on the scene the issue of how to define and represent the functionality of a product. The way used to represent it will affect the reasoning process of the designer, and in that sense, the mapping between the functional and the physical domains, being the later the one where the design solutions are developed. Several authors have investigated the concept of functionality and functional representation [2], [5] , [6], [7] and [8]. Their design approach provides a view based on the ‘Function-Behavior-Structure’ framework, where ‘function is defined using structure and behavior’ [6]. The objective is to fill the gap that allows a designer to progress from Frs. to physical design solutions. The approach is that product functions are achieved by means of its structure, which seems to lead to an iterative causal approach, where solutions are sought until the selected structure causes the intended functionality. The approach adopted in the research presented in this paper is based on the definition of Fixture Functional Components (FFC), which can satisfy the fixture functionality, and on the mapping between such FFC and fixture commercial elements.
An advanced approach to the definition of requirements and functions comes from the creation of ontologism. The ontological approach pursues the definition of the meaning of terms making use of some kind of logic, and the definition of axioms to enable automatic deduction and reasoning [9]. The ontological approach has got a higher relevance since the representation of knowledge is considered the key factor in whatever engineering process, and it has been recognized as a way to facilitate the integration of engineering applications [10], to describe functional design knowledge [7], and to define requirements [11]. Considering a purist approach, if an ontology does not include axioms to enable reasoning then it could be considered more like an information model, and in this sense, this is the approach developed in the work presented in this paper.
When considering the methodologies developed for the design of fixtures, it can be stated that in general they are rational and propose a series of steps to follow. For example, the methodologies proposed by Scallan and Henriksen [12] and [13] , make use of this approach to describe in general terms the information needed in each stage of the fixture design process. Basically, the importance of modeling in detail such information, which mainly is related to fixture requirements, fixture functionality, fixture components, manufacturing resources, manufacturing processes, and design rules; resides on the possibility to automate the design process through the development of a knowledge-based application or system. It is relevant to mention that several authors have already aimed to develop knowledge-based applications for fixture design, none of them based on a functional approach, some of the most recently published works can be found in the Refs. [14], [15], [16], [17], [18] and [19].
In the following sections, this paper presents a methodology to formalize the design process of machining fixtures based on the engineering concepts of functional requirements and fixture functions [20]. The formalization of the functional requirements is achieved through the application of a structured way of specification via natural language. Additionally, IDEF0, MOKA methodology, and UML diagrams are used to capture, represent and formalize knowledge, being the ultimate goal to facilitate the automation of the fixture design process.
The IDEF0 method is used to create an activity model of the fixture design process, allowing the identification of the information used in each one of the different tasks it comprises. UML has been used to complement the IDEF0 model by representing the interaction between the activities of the process. The MOKA methodology together with UML, are used to capture and represent knowledge involved in the fixture design process. Finally, to validate the proposed methodology, partial results obtained from the development of a prototype knowledge-based application are presented.
2. Analysis of machining fixtures requirements
In Section 1, two terms have been defined: functional requirement and constraint. Requirements have always existed, the way in which they are expressed, and how they are integrated in the product design process, are aspects that are addressed from different disciplines, for example: product design engineering and requirements engineering among others. In general, Requirements Engineering refers to the discipline dealing with the capture, formalization, representation, analysis, management and verification of requirements fulfillment. However, all these aspects need to be integrated in the product design process, and requirements should lead to the definition of the possible product design solutions, which in general demands an integrated view of the requirements issue. It is important to keep in mind that the development of such discipline is strongly related to Software Engineering and Systems Engineering, and in fact much of the research related to requirements come from authors from these engineering areas [21], [22] and [23].
When considering the analysis of requirements, probably, the first aspect to think about is how the requirements are represented or declared. As it has been previously mentioned, the way of expressing requirements definitively affects their interpretation and the creation of a design solution. In this sense, it is widely accepted, that the use of natural language is the most common way of expressing requirements and in consequence, their writing becomes an important issue. The anatomy proposed by Alexander et al. [24] to write requirements in a semi-structured way is used as basis to declare the functional requirements and constraints of fixtures [20].
In machining, work holding is a key aspect, and fixtures are the elements responsible to satisfy this general goal. In their design process, the starting point is the definition of the machining fixtures functional requirements and constraints. Usually, a fixture solution is made of one or several physical elements, as a whole the designed fixture solution must satisfy all the Frs. and the associated Cs. Centering, locating, orientating, clamping, and supporting, can be considered the functional requirements of fixtures, what a fixture must do independently of any particular solution’. In terms of constraints, ‘what limits the range of possible solutions‘, there are many factors to be considered, mainly dealing with: shape and dimensions of the part to be machined, tolerances, sequence of operations, machining strategies, cutting forces, number of set-ups, set-up times, volume of material to be removed, batch size, production rate, machine morphology, machine capacity, cost, etc. At the end, the solution can be characterized by its: simplicity, rigidity, accuracy, reliability, and economy.
2.1 Functional requirements
From the literature review [25], [26] and [27], and from the interview with designers of machining fixtures [28], it can be concluded that basic functional requirements that any fixture solution must satisfy are related to: centering, locating, orientating, clamping, and supporting.
However, the way that designers deal with these Frs. is far from being independent of the solution they are considering, and in general the Frs. are not explicit but implicit in the design process. Chakrabarti et al. [29] point out some of the problems that appear in relation to requirements during the design process, for example ‘requirements during conceptual and embodiment designs result mainly from analysis of proposed designs’, which in fact it is in contradiction with the basic principle, presented by different authors, of functional definition prior to any design solution identification. Adopting the ideas of ‘Toyota's Set-based Concurrent Engineering’ [30] and Axiomatic Design Theory [4], it seems logical to state that the Frs. should be clearly identified and defined prior to selecting any possible design solution and as the design progresses the different constraints linked to the Frs. should be refined to narrow the set of possible solutions.
Chakrabarti et al. [29] also conclude that ‘in order for requirements to be adequately fulfilled by the final design, they must be identified, understood, remembered and used’. This conclusion is not new, and in this sense, it demonstrates how actual and relevant this issue is. It also reinforces a couple of ideas widely recognized in engineering design, one is the need to capture, formalize and document knowledge, and the second is to make use of it in the development of Knowledge-Based Engineering (KBE) applications that could help the designers to carry out their job and make use in an automatic way of as much scientific knowledge as possible [31]. In this particular case applied to the design of machining fixtures.
When addressing the development of a KBE application, there are two different sorts of Frs. that need to be identified and documented. One kind is the functional requirements of the application itself; in this case a KBE application for the design of machining fixtures; and the second one is the functional requirements of the components subject of the application; in this case machining fixtures. An example of the former ones for an application developed in collaboration with an industrial partner is presented by Rios et al. [28]. For this kind of Frs. specification, UML seems to be a good methodology: activity, component, and use case diagrams help to specify and give a view of the system. However, when getting into the logical view where class and interaction diagrams have to be defined, it is needed to have a complete understanding of the object of the application: machining fixtures. With this objective, and considering that the design of machining fixtures based on functional requirements would be the aim of a KBE application, the capture and documentation of the machining fixtures FRs is part of the subject of the work developed [20], and it is commented next.
In this context, the approach adopted was to use part of the tools provided by the MOKA methodology [31], the named: Illustrations, Constraints, Activities, Rules and Entities forms, to elicit knowledge about machining fixtures as a first step in the formalization of the FRs and Cs. Based on these forms, it is possible to represent the main components linked with the fixture design process: non-functional requirements, functional requirements, constraints, design rules, fixture functional elements, fixture commercial components, etc. [20]. Fig. 1 and Fig. 2 present an example of application to the definition of fixture FRs and Cs.
MOKA Entity form for fixture FRs.
After this first phase, the requirements capture is completed with the formalization of the functional requirements syntax. At this point, it is important to remember that the declaration of a FR is a sentence written in a way that allows the FR to be measured, verified, and validated. The structure proposed is based on Alexander's anatomy [24], and it has similarities with the function representation presented by Takeda et al. [6], where it is stated that a function is a combination of a ‘function body’ (verb), an ‘objective entity’ (on which the function occurs on or to), and ‘functional modifiers’ (adverb). The structure proposed in this research is made up of four main components: action, object, resource, and qualifiers (Fig. 3). And unlike with the Takeda approach, all the modal adverbs (i.e.: firmly, precisely, in general all the adverbs ending with the suffix) are not considered as a modifier, since they do not have a quantitative value, and in consequence they cannot be measured neither validated.
The Action component is expressed by an active verb that refers to the function of the fixture. As named previously, these functions are: centre, position, orientate, clamp, and support. A noun expresses the Object component, and refers to the physical object on which the action is performed. In the first level of fixture FRs definition, Object will be the part to be machined. A noun expresses the Resource component, and it refers to where the action will be performed. In the first level of fixture FRs definition, Resource will be the machine tool on which the machining is performed. A quantitative adjective group or noun group expresses a Qualifier for the action. The Qualifiers components refer to limits of the FRs, and allow representing the constraints (Cs) associated with them. Each quantitative qualifier must have at least a nominal numerical value, a unit of measure, and a tolerance. Each FR must have at least one quantitative qualifier. Considering the previous concept of constraints refinement to narrow the set of possible solutions, the specification of the qualifiers may not have numerical values when they are initially defined, but in the final stage, when the constraints have to be considered to select a candidate solution the numerical values need to be declared. The proposed structure of the fixture FR was then modeled in UML (Fig. 4).
The class functional requirements have as attributes the four components previously defined: Action, Object, Resource and Qualifier. Considering an Object Oriented implementation, each instance of the class will have a unique identifier that allows tracing that particular FR. With this capability, it is possible to modify and update any of the attributes of such FR at any time during the design process. As an example, instances of fixture FRs for orientating and supporting are represented in the Table 1.
Table 1. Instances of fixture FRs
Action
Object
Qualifiers
Qualifier type
Orientate
Part
In the machine tool (M0)
(Resource)
Respect to the coordinated system of the part (M1)
(How)
On the orientation part activity (M2)
(When/Where)
Modifier (M0)
Respect to system axis of machine tool
Modifier (M0)
In a vertical milling machine
Modifier (M1)
Respect to the tool path
Constraints
Machine tool type (Vertical or horizontal mill)
Work area: lengths in X, Y, Z
Support
Part
In machine tool (M0)
(Resource)
On static equilibrium (M1)
(How)
On the support part activity (M2)
(When/Where)
Modifier (M0)
In a vertical milling machine
Modifier (M1)
When the sum of forces is equal to zero
Modifier (M2.1)
Vertical degree of freedom of the part
Modifier (M2.2)
When the orient activity propose a result
Constraints
Work area: lengths in X, Y, Z Shape and size of the base plate
3 Proposed methodology for the formalization of the fixtures design process
The methodology proposed in this research for the fixtures design process is based on five main design phases (numbered 1–5), named: Functional Requirements development (FR), definition of Fixture design Functions (FF), Functional Design fixture solution (FD), Detailed Design fixture solution (DD) and Fixture final design solution Validation (FV). These stages aims to define a process with continuous feedback, which allows developing the fixture design in a systematic, structured and concurrent way (Fig. 5).
Phase 1: The first phase, development of functional requirements (FR), comprises the capture of the knowledge needed to perform the design process for machining fixtures. It has two main tasks, first filling in the MOKA forms, and second formalization of the functional requirements according to the structure defined in Section 2.
Phase 2: The second phase, definition of fixture functions (FF), is aimed to complete a set of high level software function templates that implemented in a knowle
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