Design principles of reconfigurable machines

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431required for mass production lines, flexible machinesdesigned for high volume production are relativelyexpensive. In contrast, the RM is designed for customizedflexibility, i.e., the flexibility needed to produce a particularpart family [7]. The RM can perform a pre-designed set ofrequired operations as specified for the specific part familywith high reliability, repeatability and high productivity.The limited, customized flexibility allows reduction ofinvestment costs on the one hand and fast response when aproduct changes on the other, both representing economicalbenefits.The innovative idea of RMS and its six core characteristicswas originally developed for “systems”, namely,mass production lines. Development of new designprinciples based on RMS philosophy was required inorder to design RMs. These machines are to be used onRMS lines, thereby extending the RMS concept from asystem level to the machine level.In the literature, several general principles of machinedesign have been proposed and discussed. For example,Doubbel describes principles of embodiment design suchas: principle of division of tasks, force and energytransmission, as well as safety and reliability principles[8]. Norton defines engineering design as “The process ofapplying the various techniques and scientific principlesfor the purpose of defining a device, a process or a systemin sufficient detail to permit its realization” and alsodescribes the phases of the design process [9]. Suh presentsa thorough discussion of the design process in engineeringusing mathematical tools and shows examples of machinedesign [10]. He presents the design equation that relates thefunctional requirements vector {FR} to the design parameters{DP} vector using the design matrix [A]. Using thisapproach, he studies different cases of coupled design,redundant design and ideal design. Other researchers focuson design principles of machines which are used primarilyfor manufacturing. Altintas presents fundamental principlesof designing CNC machine tools [11]. It includessizing and selecting drive motors, configuration of physicalstructure and modeling of servo control. Design principlesof CNC machine tools were also presented in detail byKoren [12]. Design principles of machining systems aswell as an upgradeable multi spindle RM are discussed in[13]. RM design concepts are studied in [14] with the goalof developing “modular reconfigurable machine”. Asstated there, the key characteristics of modular machinedesign were focused on decomposition, standardizationand exchangeability. According to [15], the design of RMSis based on a construction kit principle that enables it toadjust to new production requirements by substitution,addition or removal of machine systems. A synthesismethodology for designing reconfigurable machine tools(RMT) [16, 17] takes a set of functional requirements and aset of process plans as the input and generates a set ofkinematically viable RMTs to meet the given specifications.A thorough study of machine tools scalability in thecontext of RMS is presented and discussed in [18]. Currentstudies are focused on improving dynamic design capabilitiesof RMTs [19] and on a modular approach forRMT servo axis modeling [20]. The derived machine toolmodels can be used for design and control of RMT servoaxes. When high volumes of parts are produced, there is aneed for rapid and low cost inspection equipment formeasuring geometrical and dimensional tolerances as wellas surface quality. Typically, dedicated gages that aredesigned for high precision and high repeatability areexpensive and are not flexible to product changes [21].Therefore, manufacturers prefer using flexible coordinatemeasuring machines (CMM) that can measure manydifferent parts or features [22]. An interesting applicationof the general design principles proposed by Suh [10]demonstrates a methodology for selecting a measuringsystem for inspection of mechanical parts [23]. Utilizingthis methodology, the authors selected a flexible CMMbasedsystem for completing their task.The goal of this paper is to introduce and explain thedesign principles of RMs. These design principles followthe concepts and vision of the RMS philosophy presentedin [6, 7] and introduce a complete set of practical designprinciples. Based on these design principles, several RMswere designed. The paper presents three examples of suchmachines designed for machining, inspection and assemblyoperations. We explain how the design principles arereflected in the actual design of each machine. Metalcutting, metrology and assembly represent differentmanufacturing operations; however, similar design principleshave been used to design RMs for each of theseoperations. Two full-scale prototypes of RMs were built(Sections 3 and 4). These RMs were experimentally testedto evaluate their reconfigurability features as well as theirfunctional performance. The paper briefly describes someof these studies and refers the readers to more completedocumentation of our research efforts.Section 2 presents and explains the design principles ofRMs. Sections 3, 4 and 5 describe each RM that wasdesigned based on the design principles discussed inSection 2. First a brief description of the machine isprovided. Then, the application of the design principlesduring the design phase as well as the studies andvalidation performed for each machine is discussed.Section 6 summarizes the paper and presents concludingremarks.2 Design principlesAn RM is a machine that is specifically designed to handleproduct variants within a specific part family. A gooddesign of an RM is a design that makes it proficient inhandling changes and simplifies the changeover procedure.The design principles of RMs follow the philosophy ofreconfigurable manufacturing systems. RMs are designedmainly for mass production applications. RMs are designedto allow customized flexibility and a cost-effectiveproduction and inspection of a family of parts.A machine is classified as an RM if its design follows thenecessary principle and several of the primary principlesstated below.
432Necessary principle:1. A reconfigurable machine is designed around aspecific part family of products.Primary principles:2. A reconfigurable machine is designed for customizedflexibility only.3. A reconfigurable machine is designed for easy andrapid convertibility.4. A reconfigurable machine is designed for scalability:allows addition or removal of elements that increaseproductivity or efficiency of operation.5. A reconfigurable machine is designed to allowreconfiguration of the machine to operate at severallocations along the production line performing differenttasks at different locations, using the same basicstructure.6. A reconfigurable machine should be designed applyingmodularity concepts, namely, using common “buildingblocks” and common interfaces.Clarifying remarks:1. The first principle is necessary in defining a machine asa RM. The other five principles are the key principlesthat specify the essence of RMs.2. Part family, in principle 1, is a set of parts with similarcharacteristics. A characteristic is a distinguishableproperty of a part e.g. material, geometry, shape orcolor. Similarity can be difficult to measure dependingon the property. Two parts can be similar based on oneset of properties, but different when a second set isconsidered. When machined parts are considered, thechoice of geometric and shape properties is common.Cylinder heads for different engines may be regarded apart family. Similarly, several types of engine blockscan form a part family. However, a cylinder head andan engine block of the same engine may not belong tothe same part family. Rigorous definitions anddiscussion of part families can be found in [24]. Thedefinition of a part family in our context is broad toallow each manufacturing business to define its ownpart family and design a RM according to its specificneeds.3. Customized flexibility, mentioned in principle 2,means that a machine possesses only a limited amountof flexibility related to several specific features asrequired from the design specifications. General flexibilitymeans that a single flexible machine can dealwith a large variety of features such as in a computernumerically controlled (CNC) machine tool or in ageneral-purpose coordinate measurement machine(CMM).4. The third principle of “easy and rapid convertibility”suggests that the configuration should be designed toallow easy and fast change of machine elements, rapidaddition or removal of elements and quick set up time.The designer should design means for rapid reconfigurationof the machine in advance. He or she shoulddecide how to allow fast access to fasteners andconnectors, how to design several optional locationsfor different machine elements and how to automatethe process in order to speed it up and keep it precise.5. Principle 5 refers to the requirement that the basicstructural design of a RM will allow changeableconfigurations in order to place the machine at differentlocations along the production line. At each locationthe RM will be configured to perform specific tasksthat are required for that location. In other words thesame basic RM may include different structural(hardware) elements such as spindles, sensors orgrippers as well as different software configurations.6. Design for modularity is a broad principle related to thegood practice of machine design in general. In ourcontext, modularity should allow efficient reconfigurabilityof the machine. Standard electrical, mechanical,control and software interfaces should allow rapidintegration of common elements or “building blocks”which were designed or selected in advance.3 Reconfigurable machine tool (RMT)The researchers at the ERC/RMS suggested an innovativeconcept of a machine tool [25] and developed and studiedseveral RMT concepts. Two of them have been built,demonstrated and are currently used for research [26, 27].In this paper we will focus on one machine only, the “archtype RMT” [27].3.1 Brief description of the arch type RMTThe arch type RMT, shown in Fig. 1a, was built around apart family of products with inclined surfaces, which existin some automotive engine blocks or cylinder heads. It wasdesigned for a mass production line for both milling anddrilling on inclined surfaces.The machine tool has three controlled degrees offreedom along its column, along the spindle axis andalong the table axis. One additional passive motion is theangular reconfiguration motion of the spindle, whichallows reconfiguring the spindle’s angular position intofive pre-designed locations to allow machining on differentinclined surfaces. Therefore, the arch type RMT is a nonorthogonalmachine that may have different characteristicsat each configuration of the machine.The machine is designed to drill and mill on an inclinedsurface in such a way that the tool is perpendicular to thesurface.3.2 Application of RM design principles in arch typeRMTPrinciple 1 The arch type RMT was built for a part familywith inclined surfaces found in V6 and V8 automotivecylinder heads shown in Fig. 2. During manufacturing of
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