Virtual Arch Type Reconfigurable Machine Tool Design

Virtual Arch Type Reconfigurable Machine Tool Design:Principles and MethodologyReuven Katz, Yong-Mo MoonThe University of MichiganNSF ERC for RMSAnn Arbor, MI 48109September, 2000AbstractThe report presents a new approach for designing a Reconfigurable Machine Tool (RMT). The design methodology presented in the report was developed in ERC/RMS at the University of Michigan and it generates all kinematicaly feasible RMT designs. The design of a new RMT is not just configuring the existing modules for a specific task, but rather a design of a new type of machine, which can illustrate the ideas of reconfiguration science. This report illustrates the characteristics of RMTs by showing design procedures of virtual arch type RMT. Finally, the report justifies the need for building a RMT prototype by reviewing the academic and economic value of the machine.1 IntroductionThe goals of this report are: a. To present the concept of “Highly Reconfigurable Non-Orthogonal Machine Tool”. b. To present the design concepts of a “Virtual Arch type RMT”. c. To check to what extent its features represent and reflect the basic principles and characteristics of a typical Reconfigurable Machine Tool as defined by [Koren, 1999], and d. to present a general methodology for the design of RMTs.Although the machine is experimental, it reflects specific need that may arise in the practical machining production line. Virtually we consider engine block heads as the target product of the virtual arch type RMT. Therefore, the machine is not designed with the intention to fit directly into an industrial environment. It was rather designed to function in the ERC testbed as an experimental prototype to prove the concepts of reconfiguration. We believe that the industry may and will apply the general concepts demonstrated by the Virtual Arch RMT, but may use modified reconfigurable machine tool design to meet its production line needs and some specific machining operations.The Arch Type RMT is an experimental prototype machine that is designed to demonstrate basic design and functional concepts of reconfigurable machine tools. As explained later in the report, the machine should be designed around a family of functional features derived from the requirements of producing part family in a production line. The production line represents our machining system, it shows cost effective in many cases to build RMT around part family for such system, rather than building a general-purpose CNC and adapt the part to the machine. The reconfigurable production line as well as the reconfigurable machine tool is designed at the outsetwith clearly defined part family and the role of each machine tool that comprises the line. The suggested concept should decrease cost and enhance reliability of the machine, since it reduces complexity by reducing the number of active axes of motion during machining.The philosophy and the evolution of “Curve shaped” machine tool design is discussed in one of the chapters. The methodolo gy of RMT design and its application for Virtual Arch RMT design are explained in details. The benefits and risks associated with the building process of an original and innovative RMT are discussed. Conclusion and recommendation conclude the report.As a result of the presented study, we conclude that ERC/RmS may benefit from building the Virtual Arch RMT, therefore, we recommend to build it.For readers who want to understand more about the RmS, the references such as Bollinger [1998], Koren [1997,1998, 1999] and Ulsoy [1998] would be helpful. Also references Lee [1997] and Rogers [1997] may help to understand the reconfiguration science.2 RMT principles and characteristics as applied in the design of Virtual Arch Type RMTIn this chapter we shall analyze to what extent the RMT principles and characteristic[Koren, 1999 ] are reflected in the design of “Virtual Arch Type RMT”? We shall try to address this topic in a systematic way in order to gain better understanding.2.1 Principles2.1.1The Reconfigurable Machine Tools should be designed for reconfiguration at theoutset.The “Virtual Arch Type RMT” is designed at the outset for reconfiguration and the amount of reconfiguration is determined from the part family. We allow reconfiguring the machining angle in a discrete manner in the range of –15 up to 45 degrees. We have to remember that our specific machine is an experimental machine tool, which is not designed to meet any particular industrial requirements. However, the design took into consideration, a well defined “part family” with a well defined “family of functional features” such as some milling and drilling operations which take place on an inclined surface as shown in figure 5.1. Drilling and milling operation on inclined surfaces are typical to m any mechanical parts in the automotive industry and in many other industries. Therefore, it represents a wide span of realistic options to utilize the idea of angular reconfiguration and to apply this idea in a design of some other reconfigurable machine tools that will be built at the outset to enable the change of the machining angle.2.1.2 The Reconfigurable Machining Systems should have a modular structure, for bothmachines and controls, designed for easy integration.Although the principle is a “system level” or RmS principle, the RMT, being part of the system, should posses modularity features. RMT modularity concepts will be discussed in later chapters.2.1.3 The Reconfigurable Machine Tools should be designed for a feature family withbuilt-in customized flexibility.The Virtual Arch type RMT is designed for either drilling or milling on an inclined aluminumsurfaces of some part which belongs to a specific “part family” where the part family and feature family determine the reconfigruation of RmSs and RMTs respectively. The customized flexibilityof the Virtual Arch type RMT implies that it was designed at the outset to perform a “family offunctional features”. For example, a hole on an inclined surface or a groove or any othermachining operation on an inclined surface the designer knew about in advance.In an industrial application the Virtual Arch type RMT may be installed in a ReconfigurableMachining System, or the reconfigurable production line. Usually it will serve as one out ofmultiple of m achine tools. The line is designed at the outset to produce some specific “partfamily” where the change of an inclination angle of the surface is required. Reconfiguration requirements in practical industrial application should reflect the knowledge and t he vision of engineers and marketing executives to predict future market demands for new products. Based on those predictions the manufacturing line should be designed at the outset for flexible reconfiguration utilizing RmS and RMT principles.2.1.4 The RMS is designed for a shorter cycle time with production done in batches, with ashort conversion time between batches. The RMTs installed in the system should be designed to meet this requirement.The Virtual Arch type RMT is designed to meet short conversion time between batches. The reconfiguration process is planned to be short, less than an hour including calibration if needed.2.1.5 The RMT and its controller are designed for diagnostics to allow errors due toconversion & Reconfiguration to be diagnosed systematically.A built-in process of the Virtual Arch type RMT measurement and calibration is designed at the outset. In addition to the precise fixture of the spindle platen to the arch plate, we shall design an Electro-optical device to allow tool t ip positioning and error correction. Other means of diagnostics, such as tool wear sensors or maintenance data collecting sensors may be added.2.2 CharacteristicsThe main characteristic of any RMT is its customized flexibility. This property makes it unique and different from both the dedicated machine and the CNC machine that may share with RMT characteristics such as modularity, integrability or diagnostability.2.2.1 CustomizationThis characteristic defines two aspects: customized flexibility and customized control. Customized flexibility means that the Virtual Arch RMT is built around family of functional features. An example of such family of functional features is the drilling of several holes on an inclined surface done in one station of a production line. We design at the outset for the possibility that the inclination angle of the surface may change when the next generation of the same part family will be introduced. Customized flexibility represents the ability of the machine to change its spindle angular position or to provide only the flexibility needed for those specific parts.At another station along the same production line, a second Virtual Arch RMT may be used for face milling of an inclined surface. When a new part generation with a di fferent inclinationangle is introduced, once again the customized flexibility of the RMT may be applied for a lowcost reconfiguration process of the milling RMT.Customized control of Virtual Arch RMT is achieved by integrating control modules with theaid of open-architecture technology, providing the exact control functions needed for controling non-orthogonal machine. For example, the cosine functions will be fed in to the controller forinclined surface milling with predefined angles.2.2.2 ModularityThe Virtual Arch type RMT will be designed to meet modularity concepts such as: applyingmodular structural elements of the spindle assembly in order to allow spindles interchangability;designing unified fasteners and connectors; applying the X-axis design out of “Lamb” modulslibrary; applying modular and open control design; enabling interface to the integration of thePetri-Net-based discrete logic control; using modular software.2.2.3 ConvertibilityIn a reconfigurable system the optimal operating mode is configured in batches that should becompleted during one day, with short conversion times between batches. Conversion requireschanging tools, part-programs, and fixtures, and also may require manual adjustment of passivedegrees-of-freedom of the machine. As stated the conversion of Virtual Arch RMT should beshorter than one hour.2.2.4 IntegrabilityThe Virtual Arch type RMT mechanical elements and control modules are designed withinterfaces for easy component integration and replacement. The integrated machine performancemay be predicted according to the performance of its components and the interfaces of bothsoftware and machine hardware modules.2.2.5 DiagnosabilityThe Virtual Arch RMT will include built in and add-on metrology devices that measure theaccuracy of the machine following a reconfiguration process and built in methodology to calibrate the machine tool on-line. The Virtual Arch RMT may include in the future sensors to measure changes in machine parameters that may affect either machine performance such as: machine tool wear or thermal errors or machine maintainability parameters such as changes in motors moments or oil leakage. (It is a budgetary decision only, whether to include them or not).2.3 SummaryAs shown, the Virtual Arch Type RMT meets most of the principles and characteristics of RMT according to ERC basic definitions.The main idea of RmS principles suggests that there are many other RMT concepts and configurations, which may be designed at the outset for the machining of a different part family and a different family of functional features that meet similar set of principles and characteristics.There is no one preferred machine design that represents the reconfigurability principles in an optimal and exclusive way. Therefore, it is for the ERC to choose one concept that represent originality as well research and education benefits, at the same time the conceptualdesign may be applied in practice is a production line with relatively minor redesign and investment.3 RMT Design Methodology3.1 Introduction to RMT design methodology [Moon, 2000]The RMT design methodology was developed [Moon and Kota, 1998, 1999, and Moon, 2000]. The idea of Reconfigurable Machine Tool (RMT) goes beyond the concept of modularity in that a RMT allows mass customization, facilitates easy integration of new technologies, is cost-effective, and provides high-speed capability. RMT design methodology presents a scientific basis for design of RMTs starting process requirements. The references address the following issues:?The basic building blocks (or modules) of RMTs? A method of representation of modules that allows computational synthesis? A method of representation of desired machining tasks?Systematic enumeration of alternate designs?Development of new module.s3.2 RMT design methodology overviewFigure 3.1 Overview of RMT design methodology [Moon and Kota, 1999]The key feature of this methodology is the use of screw-theory based mathematicalrepresentation to transform a given description of machining tasks to be performed (processplanning data) into a machine tool concept that is capable of performing the prescribed tasks. Starting from machining operations data, a set of feasible structural configurations of the machineis determined using graph theory. Various kinematic functions, (motions and base position) arethen mapped to individual entities in each structural configuration. Using a precompiledparameterized library of commercially available machine modules, each function i s then mappedto a feasible set of modules. This provides a set of kinematically feasible machine tools thatprovide desired motions.4 Design of Virtual Arch Type RMT4.1 The Evolution of Arch Design IdeaThe virtual arch type RMT design is derived idea from the RMT patent [Koren and Kota,1998]. The concept of non-orthogonal machine should be convertible to the existing machine toolto increase the reusability of RMT.The goal of this chapter is to present the evolutionary design process of only one p ossiblefamily of reconfigurable machine tools suitable for machining on inclined surfaces whichrepresents only one “family of functional features” required in machining processes.There is no single preferred machine design that represents the reconfigurability principles. One may suggest for example a RMT concept that uses an “add-on strategy” of orthogonal axes modules in order to increase the number of degrees of freedom for cutting.The Virtual Arch type RMT originated from a different need related to angular positioning ofthe spindle and machining features such as drilling holes on an inclined surface or milling on aninclined surface. The evolution process is described below.Starting with the “Igloo” approach of a machine with spindles located on a hemispherepointing at different angles towards the workpiece (Figure 4.1). There are several advantages to the Igloo machine related to its geometry however, it does not fit a production line where there is a transfer line and a flow of parts.Figure 4.1 Igloo machine conceptual designTo overcome the problems of the igloo concept, the next idea was the “Tunnel shaped” machine tool (Figure 4.2). It kept the circular shape and enabled material flow. Such a machine is difficult to implement in practice and difficult to maintain.Figure 4.2 Tunnel shaper machine conceptual designTo overcome the maintenance problem, circular and the arch type machines were suggested (Figures 4.3).(a) Single arch configuration (b) Double arch configurationFigure 4.3Concept of circular arch type machineThe idea of moving the arch along a column in order to relocate its position is shown in (Figure 4.4) [Koren and Kota, 1999]Figure 4.4 Concept of arch type machine with moving columnIn order to decrease the moments applied on the structure and size of the machine a “Side Mounted Arch type RMT” was suggested. A circular bearing is used for continuous reconfiguration location possibility. (Figure 4.5) The machine is non-orthogonal and milling on an inclined surface can be only achieved by combined motion of two axes.Figure 4.5 Arch type machine with circular bearing conceptual designThe Virtual Arch type RMT is an evolution of the former concept, eliminating the unique circular bearing and suggesting discrete angular positioning of the spindle at pre-defined location along a “Virtual arch”. (Figure 4.6). This machine has a non-orthogonal geometry as well.Figure 4.6 Virtual arch type machine conceptual designMastering the non-orthogonal properties of machine tools may lead to the next step that may be an application that represents a machine with a general planar curvilinear path (Figure 4.7).Figure 4.7 Curve linear machine conceptual designAn idea to eliminate the need of a non-orthogonal moti on while cutting on an inclined surface (was suggested by Dr. Min from ERC/RMS) as shown (Figure 4.8). However, the tool is always orthogonal to workpiece and moves like in any other orthogonal machines.ArbitrarySpindle PathFigure 4.8 Arch type machine with orthogonal motion conceptual design Summary: The final Virtual Arch type RMT design hardly resembles the curved structures of some of the machines that were described herein, however, we have shown how the evolutionary process leading us to the final design concept.4.2 Task ClarificationAt this section, we will show the design procedures of RMT using PREMADE (Program for REconfigurable MAchine tool DEsign) which is RMT design software.The input for PREMADE is operation plan which includes the machining operation information and machining feature family data. We are assuming that the target part family is engine blocks head as in figure 4.9.(a) V-6 engine block head (b) V-8 engine block headFigure 4.9 Engine block part familyFrom the operation information and design variables of the engine block heads in figure 4.9, we made our sample part family as in figure 4.10. It has different inclined surfaces from –15? to 45?. The operation plan for the virtual arch RMT contains both milling and drilling operations performed on four different parts of one parts family (Figure 4.10).TX TYTZTXTYTZTXTYTZ TXTYTZ(a) 45?(b) 30?(c) 15?(d) -15?Figure 4.10 Machining feature familyBy the developed kinematic analysis method, the required motions for machining features were selected to be a translation along the surface (TX) and a translation normal to surface (TZ). The configuration difference between features needs a rotational motion (RX). The configuration changes between machining features imply reconfigurations of a machine tool which can be achieved ether by active motion(s) or passive motion(s).Figure 4.11 PREMADE task clarification moduleThe list of required motions uses screw dual number form which was developed to represent the motions of tool [Moon and Kota, 1999, Moon, 2000, Chen, 1993].4.3 Structure DesignThe structural design of RMT uses graph theory to represent the machine tools’ functional and structural topology [Shinno,1981,1984,1987, Moon, 2000].Virtual arch RMT needs three active motions and there already many machine tools are available for three DoF cases, known as 3-Axis CNCs. In this example, we are about to use oneof those CNCs, Lamb’s Jaguar Series. But we have additional rotational motion and it is assigned as in figure 4.12.Figure 4.12 PREMADE structure design module4.4 Module SelectionAs we mentioned before, PREMADE has functionality to access the network and review the available modules’ information to select all the candidate modules.Figure 4.13 PREMADE supplier selection moduleOnce the module library includes a virtual arch plate as a machine module then the design theory can include it as a part of configurations. As shown in figure 4.1, the design theory canT XT Y T Z T RBasegenerate a new set of RMT configurations around a given family of machining features or it can generate a set of configurations from an existing machine configuration for the new family of machining features. Including a virtual arch plate in parameterized module library enalbes the design of virtual arch type machine.Figure 4.14 PREMADE module selection moduleAfter selection of all the candidate modules to meet the functional and structural requirements, a solution graph can be generated as in figure 4.14. This graph contains all the feasible solutions for the required set of machining features.4.5 Configuration EvaluationFigure 4.15 PREMADE configuration evaluationSince the evaluati on function of the PREMADE is not fully functional yet, we used traditional evaluation method for dynamic/static stiffness analysis.After reviewing all the feasible configurations shown in figure 4.16, the design in figure 4.17 was selected as the final design. The process of selecting a preferred configuration is descried in Katz and Chung’s work [2000].(a) Configuration #1 (b) Configuration #2Figure 4.16 Generated configurations(c) Configuration #3 (d) Configuration #4(e) Configuration #5 (f) Configuration #6Figure 4.16 Generated configurationsFigure 4.17 Selected virtual arch RMT configuration4.6 FEA of Selected ConfigurationFigure 4.18 FEA Model of Virtual Arch RMTTo verify the dynamic and static characteristics of virtual arch RMT, we used FE method using comecial software (I-DEAS 6). When the spindle assembly is at 45? angle the natural frequencies and mode shapes are as in figure 4.19.All the interfaces are assumed to be rigid thus the natural frequencies are estimated higherthan the real values.(a) First Mode (82.0 Hz)(b) Second Mode (87.5 Hz)Figure 4.19 FEA result when spindle module is 45?(c) Third Mode (143 Hz)Figure 4.19 FEA result when spindle module is 45?From the modeshape of the first mode, it is clear that the tip of the arch plate is the most flexible (weak) part. Thus to improve the dynamic chracteristics of the virtual arch RMT, stiffning of the arch plate is the most efficient method.(a) First Mode (94.0 Hz)Figure 4.20 FEA result when spindle module is 0?(b) Second Mode (96.4 Hz)(c) Third Mode (150 Hz)Figure 4.20 FEA result when spindle module is 0?To verify the effect of spindle module’s position changes, we performed FE analysis after moving spindle modules to 0?. The modeshapes and the natural frequencies are shown in figure 4.20.The first modes of 45? and 0? are 82 Hz and 94Hz respectively. It implies that the position of the spindle module contributes about 13% to the first natural frequency of virtual arch RMT. It also suggests that the machining stability of virtual arch RMT will be affected by the spindle module’s position. When the spindle module is at 45? angle, it behaves like added mass at the end of cantilever beam and lowers the dynamic stiffness of the whole system.5 Benefits and Risks of Building the Arch Type RMT5.1 An Original Machine – An Outcome of Original ResearchThe Virtual Arch RMT is a unique machine, which reflects the RmS philosophy and vision. As shown, the design methodology of ERC Project 4.1 was used to design it. The academic community is looking forward to the ERC/RMS for fresh machine tool concepts, and this is an opportunity to demonstrate one. Virtual Arch RMT is the first testbed for the RMT concept. It would not demonstrate all the characteristics of RMT but it will show most of them.5.2 Virtual Arch RMT as a research tool5.2.1 Providing an Opportunity to Test Theory versus PracticeThe arch-type RMT is a prototype for testing the new RMT design methodology. The new phase of Project 4.1 includes not only generation of kinematics, but also the design and analysis of the machine dynamics.In order to evaluate the possibility of building industrial machines based on reconfiguration concepts it is necessary to build this prototype. The machine will serve also as a research platform for additional aspects of machine reconfiguration as explained below.5.2.2 Machine Tool MetrologyOn-line measurements and calibration process of the RMT during the assembly of the machine and following reconfiguration, is more challenging than that of a conventional machine due to angular positioning errors of a spindle axis. The ERC will develop a novel method for the calibration of the RMT. This method and the sensors may be applied to commercial machine tools to enhance their precision.5.2.3 ControlThe control of the machine will be based on open-architecture principles. This will enable the ERC to integrate real-time machine diagnostics that can be studied for future plant impl ementation. In addition the machine will be designed with an interface to allow the future integration of the Petri-Net-based discrete logic control that was developed in Project 3.1.The conventional CNC control for orthogonal axes does not fit this machi ne. A new control strategy that can enable non-orthogonal operations with a changing position of the spindle axiswas developed by ERC researchers and shows promising results in simulations. The building andtesting of the machine will enable us to validate this novel control methodology.5.3 Educational BenefitsThe Virtual Arch RMT will serve as an educational tool to present and teach the concept ofmachine tool reconfiguration, system design and integration. There is no other machine that maydescribe and demonstrate these concepts. The RMT will enable students to undertake practicalprojects for the course ME-583: Science Base for Reconfigurable Machining Systems. The RMTdemonstration can be integrated in several courses: IOE-425 Manufacturing Strategies; ME-381 Manufacturing Processes; ME-450 Design Projects; ME-584 Control of Manufacturing Systems;ME-585 Machining Dynamics; and IOE-601 In-Process Quality. During machining workshopclasses the RMT will be used as an educational tool for machining of different parts using thereconfiguration ability of the machine.We believe that the RMT will serve many “generations” of ERC graduate students as avaluable research platform.5.4 Risk assessment of building Virtual Arch RMTLike in any development of an experimental and novel approach, there is some risk involved.First we would like to define possible modes of failure related to technical issues:Critical Failure– The RMT is built and integrated, however, due to major errors themachine does not operate or shows severe safety hazards.Moderate Failure – The RMT is built and integrated, but does not meet some performance requirements.Minor Failure- The RMT is built and integrated but still needs some redesign of itselements in order to operate and meet main operational requirements.Second family of mode failure classification may be related to the “Marketing” process ofreconfiguration concepts by ERC, using the RMT as a demonstration tool. We shall not definemodes of failure here, however, we shall address this important topic later.7.5What is the risk of building the Virtual Arch RMT and how to reduce it?5.5.1 Any design process of an innovative system includes risky elements; therefore, riskmanagement is essential.Risk reduction process related to the technical aspects is directly related to the ability ofmanaging efficiently and professionally the project development process.The process described herein is typical for a new R&D prototype construction.First, all required machine specifications have to be well defined.Conceptual design of the machine as a system and all of its major elements, is the secondimportant step. About 70% of the important features and properties of the machine and itsprobability to operate successfully are defined at this stage. In order to reduce the risk, theconceptual and detailed design should be accompanied by analyses, simulations and documentedcalculations and reasoning.Professional design review process is an additional measure to eliminate major errors andreduce risk.。