3. Automated Production Machines
3.4 Reconfigurable Machine Tools: Design Methods
The philosophy of Reconfigurable Machine design for intermediate flexibility between CNCs and DMTs was first proposed by researchers at the University of Michigan. This philosophy was applied to the creation of the worlds first Reconfigurable Machine Tool (RMT), the Arch Type RMT (see Figure 3.2.a).The objective of creating the Arch Type RMT was to illustrate the principle of constructing machine tools around part families. The machine was not restricted to a single part as in the case of a DMT, nor did the machine possess general-purpose flexibility as in CNC machines.
The flexibility of the machine was restricted by designing it around a family of V6 and V8 engine blocks [37]. The machine could perform milling operations on discreet inclined planes.
Figure 3.2.b illustrates the reconfiguration of the machine by reorienting the machine spindle in 15o steps through a range from 15o degrees below the line of the work table to 60o above the line of the work table. The discreet reorientation of the machine spindle reduced the number of active DOF during machine operation, simplifying the machine control system. Benefits demonstrated by this method of machine design are:
lean design by a part family approach to flexibility,
the realisation of design simplicity by use of non-orthogonal machine axes,
a lean mechanical and control design by discreet reorientation of machine axes,
the potential for high production volumes by mechanically optimised machine configurations.
The literature survey has indicated that the Arch Type RMT has been the only significant attempt at the development of a serial reconfigurable machine tool to date.
3.4 Reconfigurable Machine Tools: Design Methods 3.4.1 Virtual Modularity in RMT Design
Shinno and Ito [38, 39, 40 ] developed a methodology whereby the structural configuration of machine tools may be generated from simple geometric objects. This introduced the notion of structurally generating machinery from elementary modular subsystems. The structural synthesis of machines from simple virtual modules has been conducted by Chen and Yan [41]. Researchers at the University of Michigan further extrapolated this methodology for the fabrication of a virtual library of precompiled mechanical modules from which multiple types of machine tools could be assembled [42,43]. The structural synthesis of machines from a precompiled library of modules has been identified as an important advancement in facilitating the rapid design of machines.
The concept of modular design of machine tools has been discussed in academia since the 1980‟s.
It has thus far been successfully used for the virtual synthesis of machine tools. The methodology has however, not been practically adopted for the creation of machines that are physically modular.
3.4.2 Kinematic Optimization of RMTs
Landers et al [30] and Moon and Kota [42, 44] have developed a mathematical framework for the synthesis of a kinematically viable machine tool from a library of mechanical building blocks.
The methodology is intended to yield optimized and kinematically exact solutions in reference to the part being produced.
18 | P a g e
3.4 Reconfigurable Machine Tools: Design Methods
Figure 3.3: RMT Kinematic Design Methodology [30]
The sequence of steps in this methodology is illustrated in Figure 3.3. The first step in the methodology is the use of a screw theory based mathematical representation to obtain a task matrix from process plans and CAD drawings. The motion of any rigid body may be represented by a screw, which is comprised of a rotation about an axis and a translation about that axis. In this method discreet tool trajectories are represented by a screw. The general format of a screw is given by equation 3.1:
$ = 𝑀𝑀 𝑀𝑚 𝑀𝑐 𝑃𝐴+ 𝜀𝑃𝑇 𝑆 + 𝜀𝑆 𝑜 (3.1) MM, Mm and Mc represent the maximum, minimum and current positions of the tool in the trajectory. The term (PA + ePT) represents the pitch of the motion and lastly the term 𝑆 + 𝜀𝑆 𝑜 represents the direction of motion. The complete set of screws, used to describe the desired operation is then condensed into a Homogenous Transformation Matrix (HTM). Elements of the task transformation matrix are represented by templates in a Function Structure Graph as illustrated in Figure 3.4. The graph gives the overall topology of the required machine tool, and suitable library modules are assigned to appropriate place holders or templates on the graph.
Figure 3.4: An Example of a Function Structure Graph [42]
19 | P a g e
3.4 Reconfigurable Machine Tools: Design Methods
It is recommended for RMT design that mechanical modules are combined into a library and parameterised according to individual HTMs [44]. The individual HTMs of the modules assigned to the Function Structure Graph are then concatenated according to equation 3.2 to yield a feasible machine transformation matrix:
𝑤𝑜𝑟𝑘𝑡𝑜𝑜𝑙𝑇= 𝑀1𝑀2𝑀3… 𝑀𝑛 (3.2) If the resultant machine transformation matrix is in the image of the task transformation matrix then the current selection of modules is kinematically viable. This method is capable of yielding multiple, kinematically viable module sets for the construction of a machine tool. A library of modules may include a range of kinematically equivalent units; in this instance the optimal machine configuration is obtained by examining different module combinations while considering factors such as: mechanical stiffness, actuator speeds and torque.
3.4.3 Software Aids in Virtual RMT Synthesis
The rapid development of machine tools is necessary for shortening the production lead time on newly developed products. Software systems intended to shorten the machine design process have been developed at the University of Michigan (USA) and Fraunhofer Institute for Machine Tools and Forming Technology (Germany). The software package developed by the Fraunhofer Institute is called VRAx, which was created to give German machine tool companies a competitive advantage over their Japanese, South Korean and Chinese counterparts [45]. The VRAx system was not created with the intention of developing RMTs; however it does feature related design techniques. The software possessed the ability to create and store common machine components in a database. These components can be accessed at a later stage and used to create multiple types of machinery rapidly in a virtual environment. The concept employed in this software is a direct parallel to that of virtual modular machine design.
Figure 3.5: Screen shots of PREMADE (PRogram for REconfigurable MAchine DEsign) [37]
The University of Michigan developed a software package called PREMADE (PRogram for REconfigurable MAchine DEsign). The PREMADE package was designed specifically for the development of RMTs and contained a library of virtual mechanical modules for the rapid design of machines [37]. The software package incorporated the mathematical methodology developed by Moon and Kota [42, 44] and was capable of generating feasible machine designs based on the input of relevant part data.
20 | P a g e