ORIGINAL ARTICLE
Development of a virtual reality wire electrical discharge
machining system for operation training
Yung-Chou Kao&Jo-Peng Tsai&Hsin-Yu Cheng&
Chia-Chung Chao
Received: 5 November 2009 / Accepted: 13 September 2010 / Published online: 21 September 2010 # Springer-Verlag London Limited 2010
Abstract Wire electrical discharge machining (WEDM) uses a metallic thin wire to cut a programmed profile with high strength having sharp edges such as extrusion dies and blanking punches. However, the cost of purchasing and maintaining of WEDM equipment is very high for both the industry and general education institutions. Therefore, there are potential demands to reduce the expensive machine operation training cost, provide off-line collision-free simulation verification of the tool path, and examine the correctness of the programmed wire cutting NC codes. This
paper presents the development of a virtual reality-based WEDM full machine simulation system which can emulate major functions of a real controller related to operation training and education. These functions include the tool path simulation, NC program interpretation and processing, kinematics of the machine mechanism, workpiece origin setting, etc. To demonstrate the developed system and illustrate the adopted method, the system capability is explained and shown in this paper. The research result can be used as a cost-effective interactive 3D digital tutoring system that has the benefits of improving on the inefficient, dangerous, and costly drawbacks in traditional learning and training for operating the real WEDM machine.
Keywords Wire electrical discharge machining . Virtual reality (VR) . Virtual machine tool . Digital education . Operation training
1 Introduction
The cutting theory of the wire electrical discharge machining (WEDM) process can be traced back to 1955. In the 1960s, the non-traditional manufacturing process WEDM was introduced to the manufacturing industry [1]. The operating theory of a wire cutting machine uses a continuous moving wire as an electrode, placing the workpiece on an X–Y worktable so the machine could cut the workpiece to a precise profile. There is no direct contact between the wire and the workpiece, so the wear resulted from direct contact between the cutter and workpiece in the general machining process could be eliminated. Therefore, WEDM can generate a more precise profile for high-strength heat-treated steel such as a cutter [2,3]. The wire electrical discharge machine
Y.-C. Kao (*)
Department of Mechanical Engineering,
National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Sanmin District,
Kaohsiung 80778 Taiwan, Republic of China e-mail: [email protected]
J.-P. Tsai
:
H.-Y. ChengDepartment of Computer Science and Information Engineering, Far East University,
No.49, Chung Hua Rd., Hsin-Shih.,
Tainan County 744, Taiwan, Republic of China J.-P. Tsai
e-mail: [email protected] J.-P. Tsai
Department of Information Management, National Sun Yat-Sen University,
No. 70, Lienhai Rd.,
Kaohsiung 80424 Taiwan, Republic of China C.-C. Chao
General Manager Office, Precision Machinery Research Development Center,
No.27, 37th Road, Taichung Industrial Park, Taichung, Taiwan, Republic of China e-mail: [email protected] DOI 10.1007/s00170-010-2939-1
has thus become an indispensable CNC machine tool in the metal mold-making industry. However, the real WEDM cutting speed is generally very slow, resulting in an expensive mold-making process, even though the tool path could be created and the NC program could also be easily post-processed through off-the-shelf WEDM CAD/CAM software. A correct WEDM NC program might generate an incorrect profile if the workpiece origin is not set correctly by the operator. Therefore, the need for a WEDM full machine simulation system emerged to bridge the gap between an NC program and operating a real WEDM machine tool. This system could also be used to reduce the cost of WEDM in education, training, and maintenance.
Recently, the application of 3D computer graphics such as virtual reality (VR) has been becoming more popular in the digital learning and training domains. The character-istics of the three I’s (immersion, interaction, and imagina-tion) [4] could allow people to experience a realistic scene through stimulating the senses through sound, touch, space, etc. There are already many studies focused on applying VR in education [5,6]. From the economic point of view, it can decrease the training cost of using real machine operation practices [7]. Furthermore, it can also enhance both the learning interest and teaching effectiveness for students [8].
Currently, the price of a real wire EDM machine tool is generally high and its operation speed is slow. Therefore, it is insufficient for the hands-on practice of such equipment in the education field and results in poorer educational quality. Some teachers might also worry that improper operation by inexperienced students could damage the machine and injure the novice. Therefore, the development of a safer WEDM machine tool learning and training environment is expected.
There have been several researches related to the development of machine tool simulation systems consisting of a virtual machine tool and a virtual controller function. For example, Wang et al. [9] adopted Java 3D to develop a virtual machine tool. A virtual CNC machine tool located at a remote site is connected to a real machine through the Internet, and both the virtual and real machines can be operated almost synchronously. The team led by Ong developed an Internet-based virtual milling machine [10,
11] based on VRML (Virtual Reality Modeling Language) and Java EAI (External Authoring Interface). This system emphasized the general functions of a machine tool and could perform a virtual cutting simulation and collision detection. Machining parameters were also incorporated to estimate cutting force and tool life. Suh et al. [12] also used VRML and Java EAI to construct a web-based virtual machine tool putting more emphasis on the machine tool model and configuration. This system has the simulation function of the NC code and tool path. Lee et al. [13]
developed a virtual cutting system for the turning process and NC tool path animation. Acal Pérez and Lobera [14] also created a virtual milling system through C++ for controlling a virtual reality milling machine built by 3D Max software. Tang et al. [15] used Microsoft Visual C++ and OpenGL to develop and simulate the collision detection of a five-axis machine tool. Zhou et al. [16] presented a virtual injection molding system in simulating the real process of injection molding and in evaluating various influences from product design to manufacturing. Moreover, Bruno et al. [17] proposed a framework to dynamically simulate virtual prototypes in an immersive environment, which is generally very expensive and difficult for popular applications in general educational institutions. However, it seems that a virtual reality-based WEDM machine emulating integrated operational functions such as edge finding, four-axis (XYUV) simultaneous movement, colli-sion detection, water level, and operation panel has not yet been developed for more realistic virtual machine tool systems. To provide a more realistic training environment which could be used by students or novices to learn the correct setup process prior to operating a real WEDM machine, a virtual control panel of the WEDM machine is expected to provide simultaneous real-time controller operation and cutting simulation.
In order to achieve more precise profiles in WEDM machining, many scholars already focused on a variety of studies. For example, Han et al. [18] studied a corner error simulation based on the wire vibration analysis. Yang and Lee [19] used a data structure and an R-map algorithm to verify WEDM NC program. Puri and Bhattacharyya [20] adopted the Taguchi method to find out the main parameters that affect different machining criteria such as average cutting speed, surface roughness, and the geomet-rical inaccuracy caused by wire lag. These precision issues are very important in actual manufacturing stage; however, in this paper, the focus was on developing an integrated virtual 3D interactive environment including operation of a control panel, kinematic motion of a WEDM machine, and a preliminary wire cut profile verification and collision-free tool path during the training stage for novices. Therefore, the precision issues of workpiece profile are neglected in this paper and could be considered for further development. The main objective of this research was to provide a virtual 3D interactive system platform so the user can experience the operating process of a virtual WEDM machine prior to operating a real machine. With the interactive 3D virtual learning and training system, this research could enhance the learning interest, reduce the danger from operating a real machine, and avoid accidents and the subsequent maintenance costs resulting from improper operation.
This research adopted the object-oriented analysis and design tool, Unified Modeling Language (UML)
[21], to analyze the virtual wire electrical discharge machining system. Commercial CAD software was used to build the solid models of the components for a WEDM machine tool. Virtual reality software, EON Studio™, was adopted to construct the virtual scene such as the light source, material, texture, and the relative motion and interactivity of the VR-based WEDM (VR-WEDM). The VR-WEDM controller and its human machine interface were developed by Microsoft Visual Basic 6.0. The interactivity of the virtual controller, virtual scene, and virtual machine was then integrated and tested. The development process proposed in this paper is shown in Fig.1.
This paper is organized as follows: Section 2 is dedicated to system analysis and constructing the VR-WEDM machine. Section 3 explains the approach for the development of the VR-WEDM controller. Section4 out-lines the adopted method for the wire cutting simulation. System realization and its functions along with related demonstrations are illustrated in Section 5. Section 6
provides a discussion and suggests possible extensions. The last section highlights the results and contribution of this paper.
2 System analysis and construction of the VR-WEDM machine
This section describes the software system and construction of the proposed VR-WEDM machine in three stages. First, UML was used for the system analysis so the system could be represented in a specifying, visualizing, and documenting form. Second, the geometric model of the virtual machine was designed and constructed with 3D CAD software. The related geometric model was exported from the CAD software and then imported by the adopted VR application for the texture mapping process reflecting the machine appearance, for the design of the virtual scene, and for setting the interactivity. Lastly, the kinematic relations of the machine components were set and mapped in the “Routes window” of the adopted VR software. Details of the processes are illustrated in the following sections.
2.1 System analysis
UML is a standardized formal language integrating the methods proposed by Booch, Rumbaugh (OMT), and Jacobson (OOSE). In 1997, it passed the audit of the Object Management Group and became a standard. UML inherits the concept of the Object Orientation so it is used as a standardized blueprint to communicate with all the stakeholders when developing a software system. Owing to its reusable and maintainable characteristics, it can increase the efficiency of the software development process.
This research used UML to analyze the system frame-work. The use case diagram of the virtual system in this research is shown in Fig.2. There are several use cases and actors in the system. An actor could be a person or external system possessing an interactive relation with the analyzed system. This paper treated the user, virtual controller screen, and the virtual machine display as actors. The actors and use cases have interactive relations, as shown in Fig.2. The use cases in this research are explained as follows:
1. Select Device: the functions include importing the machine tool, locating the workpiece, selecting view, etc. 2. Operate Machine: used for manual operation. The functions include “move the axis,” “auto-thread,” “return to the initial location,” “set the workpiece origin,” “automate the edge-finding,” etc.
3. Setting Machine Parameters: used to set the parameters before the machine starts.
4. Interpret NC program: used to interpret the NC blocks and send commands to the virtual operation panel. These commands are used to drive the virtual machine. 5. Manipulate String Command: the function imports an
external NC program to be read, edited, and saved. 6. Display Dynamic Cutting: displays the dynamic solid
cutting simulation process on the virtual machine screen.
2.2 Constructing the virtual reality machine
The construction of the virtual reality WEDM in this research referenced the configuration of the original real machine manufactured by an assisting company. The 3D CAD files of the machine tool components were exported from the CAD software “Solidworks” in the STL format. A VR software “EON Studio™” was adopted for the interface design of the interactive 3D functions of the virtual machine and also for constructing the virtual scene for vivid virtual reality machine operation. For the appearance of the machine model, the “texture mapping” and “material” functions were adopted to make the virtual machine appear more realistic. Figure 3ashows the real wire electrical discharge machining machine, while Fig. 3b displays the initial machine model imported from CAD.
2.3 Setting the kinematic relations of the machine components
There were two stages in this process. First, the hierarchical relation and the kinematic relations of the machine components were designed. Figure4ashows the hierarchi-cal diagram of the components in the VR-WEDM. The components were divided into movable parts and fixed parts. In this figure, the movable components are the X-axis, Y-X-axis, Z-X-axis, U-X-axis, and V-axis. Figure 4a also shows an important message, i.e., the relation between the parent node and the child node. There is a parent–child relation in the X-axis and Y-axis, the same as that in the V-axis, U-V-axis, and Z-axis. For example, if the V-axis (parent node) moves, the U-axis and Z-axis (child nodes) will follow the V-axis to the new V location. After setting the kinematic relations of the machine components, these relations were mapped into the Routes window of the
(a) picture of the referenced real WEDM (b) the created virtual WEDM
Fig. 3 Real machine and corre-sponsive virtual machine model Fig. 2 Use case diagram of the developed system
adopted VR software. Figure 4b shows the interactive relations of the machine axes in the Routes window.
3 Development of the VR-WEDM controller
The VR-WEDM controller was developed through several steps: (1) designing and developing the functions of the virtual control panel, (2) interpreting the NC codes, and (3) computing the interpolation of the line and arc movement. These steps are described as follows:
3.1 Designing and developing the functions of the virtual control panel
The human machine interface (HMI) of the virtual control panel in the VR-WEDM machine was designed according to the specification of a real machine. The aim of this research was to develop a virtual reality system to be used for teaching and training assistance, so the HMI was designed as similar as possible to the real machine, as shown in Fig.5. There are four groups of buttons, including “Function Button Area (A),” “Operational Panel Area (B),” “Character & Number Edit
Fig. 5 Control panel of the VR-WEDM system
(a) hierarchical diagram
(b) interactive relations defined in Route window
6 Discussion
Through the development of the VR-WEDM system in this research, some valuable insights were gained. However, there are several points worth discussing:
1. The virtual machine system was designed by referring to a real machine. An operable prototype system was developed and implemented; however, more advanced functions such as setting electrical pulse parameters or integrating a real controller should be considered as important research directions in the future.
2. The user can learn to understand the correct wire cutting process of a wire EDM machine prior to operating the real machine in person through learning the VR-WEDM. Moreover, the developed VR-WEDM facilitates the operation, simulation, and error checking of the NC program so the user can understand the WEDM machin-ing knowledge and become more confident in further operation. The integrated VR-WEDM system including a virtual machine and virtual controller, together with intelligent functions such as edge finding, is helpful for educating and training students and novices in factories. 3. The virtual machine and virtual controller were
integrated so their operation was similar to the real one. However, there are still many areas to be researched in the future, for example, considering the physical characteristics of the workpiece and dielectric liquid, the electric discharge parameters, etc. Further-more, if the system could add a knowledge base to store valuable practical experiences, the capability of the VR WEDM system and the effectiveness of learning and training could be enhanced.
4. Though some commercial software includes a WEDM module and full machine construction module such as Vericut™, the control panel simulator function is not popular yet. In the proposed VR-WEDM in this paper, an integrated simulation environment suitable for education and/or training purposes was provided. The adopted theory or algorithm of the commercial software is not disclosed to the users, limiting further application and development. The developed VR-WEDM could be used as a 3D platform and further wire cutting domain knowledge could be integrated in the future.
5. Even though there are many studies on the VR milling machine, there are many distinct differences in domain knowledge between a real milling machine and a real WEDM machine. Therefore, the development methods for different kinds of VR machines differ according to different cutting algorithms, simulation methods, NC program interpretations, and the kinematics of the machine. Table 3 shows the differences in some characteristics between a three-axis VR milling machine
and a VR-WEDM machine. Besides, there are still many differences such as cutting force, machine parameter setting, and precision issues.
7 Conclusions
In this paper, a VR-WEDM system was designed by integrating the construction of a virtual machine and the development of a virtual controller. Realization of the system was demonstrated in Section5. The research results have been preliminarily tested with regard to the feasibility to assist 3D digital training and/or learning prior to operating the WEDM machine. Further study can be conducted on the effectiveness and confidence with regard to education and training. The methodology and related theory presented in this paper might provide a research reference for academics and industrial practices.
Acknowledgments The authors appreciate the grant support from the Precision Machinery Research and Development Center and the assisting company CHMER EDM for the kindly providing the Wire EDM machine tool models and operation specifications. Thanks also extends to Professor Mu-Tian Yan, Dept. of Mechatronic Engineering, Huafan University, Taiwan, R. O. C. for providing pictures of a real maple-leaf shaped product and its wire cut NC codes.
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