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MACHINE TOOL Mechanical module interfaces for reconfi gurable machine tools Eberhard Abele Arno Wo rn Ju rgen Fleischer Jan Wieser Patrick Martin Robert Klo pper Received: 27 April 2007/Accepted: 19 September 2007/Published online: 20 October 2007 ? German Academic Society for Production Engineering (WGP) 2007 Abstract Reconfi gurable manufacturing systems (RMS) enable industrial companies to adapt to frequent and unpredictable changes of production requirements in a cost-effi cient way. RMS are constituted by modular machine tools that provide variable overall functions with the ability to add, remove, rearrange and replace functional sub-units. The performance of these machine tools as regards the quick and fl exible arrangement of modules and high work piece quality strongly depends on the properties of the mechanical module interfaces. In this paper, per- formance parameters for mechanical module interfaces were defi ned and their infl uence on the machine tools performance discussed. Then fl exibly arrange-able quick- coupling interfaces as a promising solution for module assembly were analyzed. Finally, tools for the determina- tion forthose interfaceperformance parametersare presented, which require technical testing. Keywords Machine tool ? Reconfi guration ? Mechanical interface 1 Introduction In the recent years, market changes have occurred with constantly increasing pace and in a hardly predictable manner. In order to stay competitive under these circum- stances, industrial companies must gain the ability to bring new products on the market quickly and to react effi ciently to quantitative fl uctuations of the demand. Therefore, there is a necessity of such manufacturing systems that combine a scalable output and an adjustable functionality and availability with a minimum lead time and high produc- tivity 1. Reconfi gurable manufacturing systems (RMS) are a promising approach to meeting this challenge. Modularity is one key characteristic of the RMS 2. Modular systems fulfi ll various overall functions with the combination of distinct building blocks. These building blocks, or modules, are mapped one-to-one to the systems sub-functions so that the physical and functional architec- tures remain similar 3, 4. The interaction between modules is minimized in order to avoid the infl uence of changes onto other modules which serves for the overall system to work correctly 4. With the ability to add, remove, rearrange, and replace functional units quickly, the modular approach allows RMS to provide adjustable functionality and capacity. The degree to which a manufacturing system is recon- fi gurable can be measured in terms of the possibility to integrate quickly modules (integrability), to modify the systems functionality (convertibility), to adapt the sys- tems capacity (scalability) and to restrict the fl exibility to the one that is needed for a given part family (customiza- tion). The customization helps to avoid unnecessary capacity and functionality, which makes RMS very cost- effi cient 2, 5. Since new RMS modules can be purchased or leased whenever the production program requires E. Abele ? A. Wo rn Institute of Production Management, Technology and Machine Tools, Darmstadt University of Technology, Darmstadt, Germany J. Fleischer ? J. Wieser ( laser operations like welding and hardening; tool and work-piece handling operations; and quality control tasks 6. Modules can be sub-divided into further sub-modules such as spindle systems or tools. The installation of machining modules onto the platform is performed with minimal functional congru- ence and interference, whereby machining functions can be executed simultaneously. The simultaneous task exe- cution leads to a signifi cant reduction of the primary machining processing time. The degree of modularity, rapid integrability, convert- ibility, and scalability of a RMT depends strongly on the properties of its module interfaces. These interfaces can be divided into mechanical interfaces and interfaces for the transmission of data, energy and auxiliary materials 2. Mechanical interfaces are of particular importance. Unlike the transmission interfaces, they not only determine which modules are to be connected, how easy and how quickly, but they also infl uence strongly the overall systems per- formance in the operating mode. The reason is their ability to transmit forces and moments and to align elements precisely. It is the purpose of this paper to discuss the infl uence of mechanical interfaces on the different aspects of the RMT performance. Section 2 deals with the defi nition of per- formance parameters for mechanical RMT interfaces and their infl uence on the overall-systems performance. Sec- tion 3 analyzes the cylindrical quick-coupling interfaces as a fl exible means of module assembly. Finally, in Sect. 4, testing tools for the performance evaluation of these interfaces are presented. RMS M X RMT 1 RMT 3 M 1M 2 PM SM 2SM 3 SM 4 SM 1 SM 5 RMT 2 SMRMT PM System Levels SM X RMT X Reconfigurable Manufacturing System Sub-Module Reconfigurable Machine Tool Platform Module Reconfiguration System Relation (undirected/directed) RMSRMS M 2M 2 RMT 1RMT 1 SM 2SM 2 SL 0 SL - 1 SL - 2 SL + 1 MModule + - M n SL System Level System Boundary Fig. 1 Example for the hierarchical structure of a RMS 5 Interface Receiver Device Plug Device M 1Platform Module (PM) M 2 M 3 PTW M 4 Sharing ModularitySwapping Modularity M 2 RMT SM1 SM3 SM4 SM2 Fig. 2 Example for the structure of a RMT 422Prod. Eng. Res. Devel. (2007) 1:421428 123 2 Performance criteria for mechanical RMT module interfaces 2.1 Parameters affecting the work piece quality The capacity of mechanical interfaces to position accu- rately RMT modules relatively to each other affects the positioning error of the tool relative to the work-piece. This tool positioning error has a static component x and a dynamic component x(t). The static component results in dimensional deviations of the work-piece, or fi rst order errors, whereas the dynamic component affects the work- pieces surface quality, i.e. the magnitude of second order errors. 2.2 Parameters for fi rst order errors Geometric positioning accuracy is the fi rst interface parameter that infl uences fi rst order work-piece errors, characterized by a mean error and dispersion 7. The geometric positioning accuracy stands for the component of the positioning error that does not depend on external factors such as forces or heat. However, the parameter comprises errors due to deformations caused by clamping forces, and therefore it cannot be accurately deduced from the part tolerances. In a working RMT, static loads resulting from cutting forces, forces of gravity and forces of inertia deform the mechanical interfaces. Yet, the knowledge of the elastic interface behavior is insuffi cient for predicting their effect on the overall systems stiffness, because interfaces bring about stress concentrations in the modules around the contact surface. This stress concentration then causes additional module deformation. Even if an interface com- ponent itself would be infi nitively stiff, the module assembly would still be weaker than an integral structure. The performance parameter, representing the interface stiffness, is therefore the effective interface elasticity deffective , which is defi ned as the difference in elasticity between two rigidly assembled modules di(integral struc- ture in Fig. 3) and the elasticity of the module assembly dm (modular structure in Fig. 3). Thus the elasticity d is defi ned as the ratio of defor- mation and load. deffective di? dm;1 dm xm F anddi xi F :2 Thus the parameter d comprises both the interface deformation and the interfaces effect on the module deformation. The effect on the module deformation, however, depends on the modules material and shape, and therefore d cannot be generalized. In the next chapter, we present a method that can resolve the issue for certain types of interfaces. The static positioning accuracy of the tool relative to the work-piece is further infl uenced by the interfaces thermal properties. To begin with, the interfaces expansion under a given temperature change directly causes a fi rst order error. The coeffi cient of thermal dilatation is the corresponding parameter. Unless for special cases, the thermal expansion affects only the degree of freedom (DOF) normal to the contact surfaces and can be easily calculated from the materials properties. Nevertheless, since interface components in general are relatively fl at, the effect is considered as negligible compared to the thermal dilatation that occurs in the modules. The thermal module dilatation however can depend on the interface conductivity, which specifi es how well heat is transferred from one module to another. 2.3 Parameters for second order errors Second order work piece errors are, among other things, a result of vibrations in the machine tool. The level of vibrations depends on the machine tools properties regardingstiffness,massdistribution,anddamping. While the interface mass in general is negligible com- pared to the module mass, the interface stiffness and its damping capacity can infl uence the systems dynamic behavior signifi cantly.Agooddampingcapacityis always desirable, as it increases the dynamic stiffness of the machine tool around the resonance frequencies and thus reduces the level of vibrations. The global effect of each of the parameters discussed above depends on the machine tool modules for which the interface is used, so that the importance of the different parameters cannot be assessed in a universally valid way (Fig. 4). Integral structureModular structure Interface integral x modular x FF Module deformation caused by interface x Fig. 3 Weakening effect of an interface on a structure: Interface elasticity and module elasticity caused by the interface Prod. Eng. Res. Devel. (2007) 1:421428423 123 2.4 Parameters characterizing an interfaces suitability for module reconfi guration Regarding the process of reconfi guration, the most impor- tant performance criteria for module interfaces are the time of assembly (expressed by the mean value and dispersion), the ease of assembly, and the compatibility 7. While the minimization of assembly times is a primary concern for RMT in general, the narrowness of the requirement depends on the frequency of reconfi guration. This fre- quency is highly variable for different module types and applications. Figure 5 gives an idea for replacement time require- ments depending on the average operating times of different modules types. The requirement of ease of assembly concerns the tools and skills necessary for RMT reconfi guration. Compatibility must be assured by the interfaces standardization, either on an open or on a pro- prietary basis 1. The issue of standards is both of strategic and of technical nature. Technological stability of the interfaces must be guar- anteed in order to last the standard for a long time, and therefore interfaces, unlike other technical products, should not be a subject to permanent innovation 8, 9. Compati- bility also requires fl exibility in a sense that an interface should be a suitable solution for the assembly of a maxi- mum variety of module types. 2.5 Other parameters The fatigue limit and the maximum load that an interface withstands are generally a small concern because high stiffness requirements bring about reserves as regards strength. Security requirements necessities fail save inter- face design, i.e., connections that do not depend on external sources of energy to be maintained. Since failure of the interface in locked mode should be impossible, the requirement for reliability concerns only the assembly process, e.g., the sensitivity of the locking mechanism to failure. An example for an interfaces need of maintenance is its sensibility to dirt. The need for accessibility with tools in case of reconfi guration, which for example is the case for bolted connections, and high space consumption are disadvantageous in that they restrict design choices. 2.6 Discussion Table 1 summarizes the performance parameters that is considered to be of primary importance. Three groups can be distinguished regarding the way the parameters can be obtained. The fi rst group of parameters requires physical testing with specifi c tools; the second group requires testing, with no specifi c tools; the third group requires no physical testing at all. The determination of the performance parameters is a precondition for the choice of the right interface for a modular machine tool among a set of candidate solutions. For reasons of com- patibility, this interface must be an optimum for the whole set of possible modules, including future ones, concerning work-piece quality, ramp-up time, reconfi guration cost and theinterfacesinitialcost.Ahigh fl exibilityis indispensable. T T g x x Q F x T x total x Damping capacity External factors Interface parameters Local effect F F(t )(t ) Modules Global effect x x(t ) Conductivity Dilatation Elasticity Geometrical positioning s F(t ) s Global effect Fig. 4 Effects of interface parameters on static and dynamic tool positioning errors Fig. 5 Assembly time requirements depending on the average operating time of a RMT module Table 1 Determinationoftheprincipalinterfaceperformance parameters Specifi c toolsTesting in situNo physical testing Stiffness Reconfi gurationCompatibility AccuracyTimeTools required DampingEase of assemblySpace consumption ConductivityReliabilityFlexibility MaintenanceFail save design Skills necessaryAccessibility 424Prod. Eng. Res. Devel. (2007) 1:421428 123 2.7 Compact quick-coupling adapter interfaces for RMT module assembly Types of mechanical interfaces range from simple bolted assemblies to sophisticated quick-coupling solutions. Bol- ted connections have been discussed extensively in the literature(e.g.,aspectsofpositioningaccuracyand assembly time in 7, stiffness considerations in 8, and the prediction of damping properties in 10). Bolted inter- faces, however, require generally long assembly times and must often be adjusted on site in order to achieve suffi cient positioning accuracy. In the following, cylindrical two-part adapter components with integrated quick-coupling mech- anisms will be discussed as a promising alternative to bolted joints. Similar interface types can be found for instance in quick-change pallet systems. Figure 6 illus- trates the basic principle and two examples for the use of these interfaces for the assembly of different types of modules. Due to their compactness, the cylindrical interface components have a low fl exural stiffness and for this rea- son at least three units are needed in order to guarantee suffi cient stiffness of a module assembly. In return, each of the three or more interfaces can be considered as not being solicited in bending and torsion, and can thus be modeled as a component that fi xes three translational but no rota- tional DOF. Yet for 3 + i interfaces an over-determination of 3(3 + i) 6 DOF must be dealt with. Leaving the otherwise over-determined DOF free is not an option because it would reduce signifi cantly the assemblys stiffness. Therefore, adjustment mechanisms are needed that position the interfaces relative to each other in every DOF that would be over-determined, before the interface parts are fi xed to the modules. Figure 7 shows the DOF that need to be adjusted if three interfaces are used for an assembly. As opposed to bolted connections, the adjustment must be performed only once during the life-time of a module, before it is deployed for the fi rst time. When the adjustment accuracy is high, the over-determination is compensated by the elasticity in modules and interfaces without resulting in high levels of residual stress. Figure 8 shows the SST-60 interface that was specifi - cally designed for RMT module assembly. Alignment of the two halves is facilitated by conic surfaces derived from HSK A-63 tool holder interface. Locking and release is performed by a hollow shank mechanism that can be actuated either by hydraulic pressure or a mechanical screw mechanism. Each of the two interface parts is bolted to a module on a precise position that must be adjusted only once, before the fi rst use of the module. The space con- sumption is considerable in the axial direction and the initial cost is high compared to conventional bolt connec- tions. For the SST-60 interface the maximum clamping force Fcconstitutes 18 kN at a clamping torque of 12 Nm. The position accuracy amounts 5 lm. Fig. 6 Application of multiple cylindrical two-part interfaces for RMT module assembly Interface 2 Interface 1 Interface 3 -2 DOF -3 DOF -1 DOF Module 1 Module 2 AdjustmentAdjustment -2 DOF -3 DOF -1 DOF ol Module 2 Fig. 7 Adjustment of the interface position in order to deal with over-determination Module Segment Platform Segment Fig. 8 Cylindrical two-part quick-coupling interface SST-60 Prod. Eng. Res. Devel. (2007) 1:421428425 123 2.8 Measurement tools for the performance evaluation of cylindrical RMT interfaces In order to make a judgment on whether cylindrical adapter type interfaces are a suitable solution for the connection of RMT modules and platforms, tools for the measurement of the interface stiffness are needed. 2.9 Stiffness measurement As discussed in Sect. 3, the bending stiffness and the tor- sion stiffness can be neglected if at least three compact interfaces are deployed. Thus, only the stiffness in three translational DOF needs to be measured. Due to the rota- tion symmetry of the interface, the stiffness is the same in all radial directions and the relevant stiffness parameters are reduced to one shear direction and the normal direction. The elastic behavior of the interface and