Prod. Eng. Res. Devel. DOI 10.1007/s11740-007-0057-1
MACHINE TOOL
Mechanical module interfaces for reconfigurable machine tools Eberhard Abele Æ Arno Wo¨rn Æ Ju¨rgen Fleischer Æ Jan Wieser Æ Patrick Martin Æ Robert Klo¨pper
Received: 27 April 2007 / Accepted: 19 September 2007 German Academic Society for Production Engineering (WGP) 2007
Abstract Reconfigurable manufacturing systems (RMS) enable industrial companies to adapt to frequent and unpredictable changes of production requirements in a cost-efficient 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 flexible arrangement of modules and high work piece quality strongly depends on the properties of the mechanical module interfaces. In this paper, performance parameters for mechanical module interfaces were defined and their influence on the machine tool’s performance discussed. Then flexibly arrange-able quickcoupling interfaces as a promising solution for module assembly were analyzed. Finally, tools for the determination for those interface performance parameters are presented, which require technical testing. Keywords Machine tool Reconfiguration Mechanical interface
E. Abele A. Wo¨rn Institute of Production Management, Technology and Machine Tools, Darmstadt University of Technology, Darmstadt, Germany J. Fleischer J. Wieser (&) Institute of Production Science, Universita¨t Karlsruhe (TH), Karlsruhe, Germany e-mail:
[email protected] P. Martin R. Klo¨pper Laboratoire de Ge´nie Industriel et Production Me´canique, E´cole Nationale Supe´rieure d’Arts et Me´tiers CER Metz, Paris, France
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 circumstances, industrial companies must gain the ability to bring new products on the market quickly and to react efficiently to quantitative fluctuations 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 productivity [1]. Reconfigurable manufacturing systems (RMS) are a promising approach to meeting this challenge. Modularity is one key characteristic of the RMS [2]. Modular systems fulfill various overall functions with the combination of distinct building blocks. These building blocks, or modules, are mapped one-to-one to the system’s sub-functions so that the physical and functional architectures remain similar [3, 4]. The interaction between modules is minimized in order to avoid the influence 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 reconfigurable can be measured in terms of the possibility to integrate quickly modules (integrability), to modify the system’s functionality (convertibility), to adapt the system’s capacity (scalability) and to restrict the flexibility to the one that is needed for a given part family (customization). The customization helps to avoid unnecessary capacity and functionality, which makes RMS very costefficient [2, 5]. Since new RMS modules can be purchased or leased whenever the production program requires
123
Prod. Eng. Res. Devel.
RMT M4 M3 Interface Plug Device
Receiver Device
Platform Module (PM)
M1
SM1 SM2
M2
SM3 SM4
Sharing Modularity
Swapping Modularity
Fig. 2 Example for the structure of a RMT
RMS RMT X
RMT 2
RMT 3
SL
RMT 1
RMT 1 MX
M1
PM
SM 1
…
SM 5 SM 2
SM 4
PM
0
SL
SM 3
SL
SM 2
RMS
1
+
SL
Mn
M2
M2 SM X
-2
Reconfigurable Machine Tool
SM
Sub-Module
Platform Module
Module
SL
System Level
M
System Relation (undirected/directed)
+
1
Reconfigurable RMT Manufacturing System
System Boundary
123
M2
System Levels
Fig. 1 Example for the hierarchical structure of a RMS [5]
© PTW
the transmission interfaces, they not only determine which modules are to be connected, how easy and how quickly, but they also influence strongly the overall system’s performance 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 influence of mechanical interfaces on the different aspects of the RMT performance. Section 2 deals with the definition of performance parameters for mechanical RMT interfaces and their influence on the overall-system’s performance. Section 3 analyzes the cylindrical quick-coupling interfaces as a flexible means of module assembly. Finally, in Sect. 4, testing tools for the performance evaluation of these interfaces are presented.
additional or different functions, flexibility is no longer part of the initial investment [6]. Likewise, RMS can be easily upgraded with the integration of new modules, which allows manufacturers to keep pace with technological advances without having to replace entire production systems. The architecture of RMS can be structured hierarchically as proposed in Fig. 1. On the highest system level, reconfigurable machine tools (RMT) are linked into sequential or parallel manufacturing systems. Each RMT consists of modules, which can be arranged by means of a platform, for instance (Fig. 2). Module functions include: machining operations such as milling, drilling, turning and grinding; 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 congruence and interference, whereby machining functions can be executed simultaneously. The simultaneous task execution leads to a significant reduction of the primary machining processing time. The degree of modularity, rapid integrability, convertibility, 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
Reconfiguration
-
Prod. Eng. Res. Devel.
2 Performance criteria for mechanical RMT module interfaces
Integral structure
Modular structure Interface
2.1 Parameters affecting the work piece quality The capacity of mechanical interfaces to position accurately 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 first order errors, whereas the dynamic component affects the workpiece’s surface quality, i.e. the magnitude of second order errors.
2.2 Parameters for first order errors Geometric positioning accuracy is the first interface parameter that influences first 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 insufficient for predicting their effect on the overall system’s 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 component itself would be infinitively 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 defined as the difference in elasticity between two rigidly assembled modules di (integral structure in Fig. 3) and the elasticity of the module assembly dm (modular structure in Fig. 3). Thus the elasticity d is defined as the ratio of deformation and load. deffective ¼ di dm ; xm xi and di ¼ : dm ¼ F F
ð1Þ ð2Þ
Thus the parameter d comprises both the interface deformation and the interface’s effect on the module deformation. The effect on the module deformation,
xintegral
xmodular
F
F Module deformation caused by interface
Fig. 3 Weakening effect of an interface on a structure: Interface elasticity and module elasticity caused by the interface
however, depends on the module’s 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 influenced by the interface’s thermal properties. To begin with, the interface’s expansion under a given temperature change directly causes a first order error. The coefficient 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 material’s properties. Nevertheless, since interface components in general are relatively flat, 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 specifies 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 tool’s properties regarding stiffness, mass distribution, and damping. While the interface mass in general is negligible compared to the module mass, the interface stiffness and its damping capacity can influence the system’s dynamic behavior significantly. A good damping capacity is 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).
123
Prod. Eng. Res. Devel.
2.4 Parameters characterizing an interface’s suitability for module reconfiguration Regarding the process of reconfiguration, the most important 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 reconfiguration. This frequency is highly variable for different module types and applications. Figure 5 gives an idea for replacement time requirements depending on the average operating times of different modules types. The requirement of ease of assembly concerns the tools and skills necessary for RMT reconfiguration. Compatibility must be assured by the interfaces standardization, either on an open or on a proprietary basis [1]. The issue of standards is both of strategic and of technical nature. Technological stability of the interfaces must be guaranteed 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]. Compatibility also requires flexibility in a sense that an interface External factors s
∆T T
F F( t )
Interface parameters Conductivity
Q
Dilatation
xT
Geometrical positioning
xg
Elasticity
xF
Damping capacity
Global effect
Local effect
x total
Modules s
x x (t )
x( t )
Fig. 4 Effects of interface parameters on static and dynamic tool positioning errors
should be a suitable solution for the assembly of a maximum 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 interface 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 interface’s need of maintenance is its sensibility to dirt. The need for accessibility with tools in case of reconfiguration, 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 first group of parameters requires physical testing with specific tools; the second group requires testing, with no specific 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 compatibility, this interface must be an optimum for the whole set of possible modules, including future ones, concerning work-piece quality, ramp-up time, reconfiguration cost and the interfaces’ initial cost. A high flexibility is indispensable.
Table 1 Determination of the principal interface performance parameters
Fig. 5 Assembly time requirements depending on the average operating time of a RMT module
123
Specific tools
Testing in situ
No physical testing
Stiffness
Reconfiguration
Compatibility
Accuracy
Time
Tools required
Damping
Ease of assembly
Space consumption
Conductivity
Reliability
Flexibility
Maintenance
Fail save design
Skills necessary
Accessibility
Prod. Eng. Res. Devel.
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. Bolted connections have been discussed extensively in the literature (e.g., aspects of positioning accuracy and assembly time in [7], stiffness considerations in [8], and the prediction of damping properties in [10]). Bolted interfaces, however, require generally long assembly times and must often be adjusted on site in order to achieve sufficient positioning accuracy. In the following, cylindrical two-part adapter components with integrated quick-coupling mechanisms 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 illustrates 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 flexural stiffness and for this reason at least three units are needed in order to guarantee sufficient 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 fixes three translational but no rotational 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 significantly the assembly’s 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 fixed 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 first 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 specifically 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 first use of the module. The space consumption is considerable in the axial direction and the initial cost is high compared to conventional bolt connections. For the SST-60 interface the maximum clamping force Fc constitutes 18 kN at a clamping torque of 12 Nm. The position accuracy amounts £5 lm.
Interface 3
Interface 2 -2 DOF
-1 DOF Mo du
Adjustment
Mo du le
1
le 2
-3 DOF
Adjustment
Interface 1
Fig. 7 Adjustment of the interface position in order to deal with over-determination
Module Segment
Platform Segment
Fig. 6 Application of multiple cylindrical two-part interfaces for RMT module assembly
Fig. 8 Cylindrical two-part quick-coupling interface SST-60
123
Prod. Eng. Res. Devel.
2.8 Measurement tools for the performance evaluation of cylindrical RMT interfaces
Measurement of shear stiffness
Measurement of normal stiffness
F
F/2
F
F
F/2
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. Interface 1 Interface 2
Ring component
2.9 Stiffness measurement
Fig. 10 Principle of the test setup for stiffness measurement
As discussed in Sect. 3, the bending stiffness and the torsion 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 rotation 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 of the assembly can then be modeled as illustrated in Fig. 9. The concept is based on a stiff frame and on two interfaces of the same type that are symmetrically arranged (Fig. 10). The setup is convertible in a way that allows the measurement in the shear direction and in the normal direction with a single actuator and a maximum of common components. The stiffness is obtained in the form of load– displacement-curves; the load itself is generated by a pneumatic mechanism that uses a diaphragm in order to avoid stick–slip effects. A special ring component is used to distribute the shear force evenly over the interfaces’ circumference. Sliding surfaces had to be avoided in order to prevent friction from perturbing the measurement. A test setup has been developed for the physical realization of the elasticity measurement (Fig. 11). Since the stiffness reduction of the overall system, caused by an interface, depends on the module geometry and the module material, a method is needed that takes the
1 Testing bridge load cell
5 Tie rod
2 Testing bridge distance sensor
6 Distance sensor µ EU05, Resolution
