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OPEN DESIGN OF MANUFACTURING EQUIPMENT R. Vallance*, S. Kiani†, and S. Nayfeh‡ *
‡
Mechanical Engineering Department, University of Kentucky † Open Design Foundation, Inc. Department of Mechanical Engineering, Massachusetts Institute of Technology
ABSTRACT In this paper, we discuss open design of manufacturing equipment as an approach to the design, test, and continuous improvement of modular and reconfigurable manufacturing equipment. Open design fosters collaborative efforts by providing a framework for freely sharing information such as design documentation and performance data. This paper presents the principles of open design by introducing the Open Design Definition, outlining the requirements for license agreements, and illustrating the application of open design with two current projects. The first project is the open design for a linear positioning stage intended as a foundational element in modular machines or machine tools. The second project is an aggregate machine being redesigned and reconfigured for micro EDM. Open design may become a beneficial alternative to proprietary development of machinery and manufacturing processes, especially in core or foundational elements of agile systems. KEYWORDS:
open design, agile manufacturing, modular machines, reconfigurable machines
INTRODUCTION Recent advances in Information technology (IT) and networking now enable corporations to exchange and synchronize information regarding design and manufacture, enabling designers and manufacturers to reduce the time between product conception and full-scale production. Designers routinely practice concurrent engineering, use modern computeraided design (CAD) and engineering (CAE) software, and employ rapid-prototyping processes. Manufacturers routinely operate with computer-numerically-controlled (CNC) processes, flexible production systems, cellular manufacturing, and just-in-time (JIT) deliveries. Despite these crucial improvements, the specification, design, fabrication, start-up, and validation of automated manufacturing systems remains a major barrier to dramatically reducing time-to-market. To address this issue, we often emphasize a need for agility, the ability to rapidly adapt or reconfigure production systems. Achieving agile production systems requires a new style of manufacturing equipment and processes. Equipment must be modular, easily reconfigured, and computer controlled through standard interfaces. Designing, characterizing, and using a new generation of manufacturing equipment remains a formidable challenge to implementing agile systems in industry.
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In this paper, we present open design of manufacturing equipment as a new approach to the design, test, and continuous improvement of modular and reconfigurable manufacturing equipment. Open design integrates conventional design techniques with a new perspective on sharing of information and collaborating. Open design enables collaborative efforts by providing incentives and methods for freely sharing information. Design documentation (e.g. CAD data, FEA analyses, spreadsheets, simulations) and performance data (e.g. reliability, precision, accuracy, yields) are freely shared. Furthermore, the documentation can be freely modified, and artifacts produced using original or modified documentation can be distributed. If the modified design is distributed or artifacts are sold for profit, then design documentation must be publicly disclosed. This approach may become a beneficial alternative to closed machinery and manufacturing processes, especially in core or foundational elements of an agile system. BACKGROUND AND PRIOR WORK The principles of open design are derived from a development model known in the software industry known as "Open Source", "Free Software", or "Copyleft." This development model has been in widespread use among computer programmers around the world since the early 1980s and gained commercial recognition during the mid 1990s [Raymond]. Examples of commercially supported Open Source applications include the LINUX operating system, the Apache web-server, and the Mozilla web-browser (formerly Netscape). The concept of Open Source software is often attributed to Richard Stallman, who began advocating Free Software in the early 1980s. Free Software is free in the sense of liberty and not gratis, like "freedom of speech, not free beer" [Stallman]. Stallman proposed that Free Software should provide the following liberties to software developers and users: • • • •
the freedom to run the software, for any purpose, the freedom to modify the software to suit your needs, the freedom to redistribute copies of the original software (either gratis or for a fee), and the freedom to distribute modified versions of the software (either gratis or for a fee).
Like the scientific method, Free or Open Source software facilitates continuous improvement by sharing. The scientific method advances models for natural phenomena through hypotheses, experiments, validation of hypotheses, and theories that might progress to laws. The scientific method requires an ethic for sharing hypotheses, experimental setups, experimental results, anomalies, and even expenses. Sharing information enables other scientists to reproduce experimental results, vary the experiment, or continue the scientific effort. Free Software treats source code like other forms of scientific knowledge by guaranteeing that source code is publicly available to anyone who wishes to use or modify the source code for their own purpose. On these principles, Stallman established the GNU project at MIT’s Artificial Intelligence (AI) Lab in 1984 and later established the Free Software Foundation. GNU and the Free Software Foundation develop and distribute several successful applications (for fee and free). By the mid 1990s, many programmers recognized the benefits of Free Software development, and a handful of Free Software businesses were successfully established. Page 2 of 12
However, corporations seeking profit through limited-use licenses were slow to accept it as an alternative business model. Free Software struggled for acceptance because many mistakenly confused liberty for gratis and thought that Free Software could not be sold for a fee. To eliminate this confusion, a group of industry leaders including Todd Anderson, Chris Peterson, John Hall, Larry Augustin, Sam Ockman, and Eric Raymond adopted the term "Open Source" on February 3, 1998. OPEN DESIGN FOUNDATION Recognizing that mechanical design might benefit from a similar open exchange of information, the authors established the Open Design Foundation (ODF) as a non-profit corporation in 1999. The ODF seeks to support open design, evaluate open design licenses, distribute documentation for open designs via the Internet, and sponsor projects. The ODF maintains the Open Design Definition and evaluates license agreements suitable for use with design documentation and manufactured artifacts derived from open design projects. The ODF conducts activities primarily through Internet services, which include a web site (http://www.opendesign.org) for general information and links to project web pages. Additional services include basic tools for online collaboration such as an email-based list server with archiving system and a product data management system (PDM) for exchanging data. The ODF also provides instructional documents for techniques used in open design. In addition to these services, the ODF recognizes the need for foundational infrastructure projects. These projects can help reduce barriers to participation in open design projects. For instance, access to specific software should not prevent any designer or manufacturer from leading or contributing to new projects or building an openly designed artifact. OPEN DESIGN DEFINITION AND LICENSES Open design is easily confused with public domain, open architecture, and even the development of standards. Outwardly, open design seems to oppose the maintenance of intellectual property or trade secrets, but open design licenses have been designed to allow “proprietary” and “free” subsystems to be integrated harmoniously. The principal purpose of intellectual property is to promote the development of technical innovation for the benefit of society. In exchange for disclosing an invention, patents grant exclusive rights to the inventor for a set period, with the intent that the inventor will commercialize the technology for profit. After the exclusive right expires, the invention becomes available within the public domain, and anyone or organization can use the invention for their purpose or profit. Most machine elements and even entire machines are available in the public domain. It is important to recognize that open design is more than a forum for promoting design in the public domain. The first difference between open design and the public domain is the requirement to share documentation for a modified design if any embodiment of the modified design is redistributed for profit or gratis. This ensures that early contributors to open designs are always guaranteed access to revised versions. The second difference is that documentation is generally not available for reusing public domain designs, and so they
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must be reverse engineered. Open design eliminates this by providing documentation in its original form (e.g. native solid model files), be made publicly available. Many machine elements within the public domain are specified by national or international standards (e.g. gears, fasteners, and bearings). The governing standards are written by experts in the technical area, and they ensure the quality of the standard. This approach has produced many successful and necessary standards for essential machine elements such as ABEC bearings and NEMA [NEMA] motors. While this type of standard plays an essential role in machine elements, it is less appropriate for standardizing complete machines. We believe that open design may accomplish de facto standards in aggregate machinery, where the publicly available documentation serves as the standard. Open architecture refers to systems composed of independent modules that mutually operate according to standard interfaces between the modules [Koren]. Open architecture requires that the interfaces’ standards be available for implementation by suppliers or system integrators. Open design differs from open architecture in two fundamental ways. First, open design requires that the complete documentation for the design be publicly disclosed, not just the interface between modules. Second, license agreements provide for adaptations, enhancements, or modifications to any portion of the design, including the modules and the interface. Therefore, manufacturing equipment developed under open design methods may be freely adapted as needed by end-users, system integrators, or module suppliers. To reduce confusion regarding the principles of open design, we follow the method used successfully by the Open Source software community. The principles of Open Source software are established in a document titled the Open Source Definition [OSI(a)]. This document serves as the standard by which Open Source license agreements are measured. Following their approach, we have written a living document (titled the Open Design Definition) intended to serve as the standard for suitable license agreements [ODF]. License agreements ensure that the freedoms described in the Open Design Definition are protected as the design is modified and redistributed. Anyone is free to use on open design as a functional element of a proprietary or non-proprietary system as long as the open design element is clearly identified. However, any individual or organization using or modifying an open design must agree to the terms specified within an Open Design license:
documentation of a design is available for free, anyone is free to use or modify the design by changing the design documentation, anyone is free to distribute the original or modified designs (for fee or for free), and modifications to the design must be returned to the community (if redistributed).
Because design encompasses more than just hardware, the recommended licenses for open design depend upon the particular aspect being licensed. For software, we recommend the GNU General Public License (GPL) [FSF(b)] or the Lesser GPL (LGPL) [FSF(c)]. These licenses are well established and maintained by the Free Software Foundation. It is the license model used for projects like Sun’s sponsorship of OpenOffice (http://www.openoffice.org). For documentation such as written reports, papers, or drawings, we recommend the Free Documentation License [FSF(a)], which also originated
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in the software community. What remains is the need for a license applicable to design, revision, and distribution of hardware and manufactured artifacts. Any open design license should satisfy the requirements in the Open Design Definition [ODF]. The Open Design Definition allows any individual or organization to use an open design as an element in a proprietary machine, as long as they inform purchasers that a portion of the proprietary machine is an open design. The total machine remains proprietary, but redistribution of modified open design elements requires that revised design documentation be publicly available. An organization or individual can produce or sell an open design if the purchaser is informed that they are purchasing an open design and the seller publicly distributes or provides a simple path to all the design documentation. CURRENT OPEN DESIGN PROJECTS This section illustrates the principles of open design for manufacturing equipment through two initial projects. The first project is a modular, high-precision, linear motion stage, and the second project is an aggregate micro EDM machine. The documentation for these designs, including solid models, drawings, software, documentation, and bills of materials, are available for free to anyone that wishes to use, modify or redistribute these designs. Linear Motion Stages and Modular Machine Tools Linear and rotary stages are the functional building blocks of the vast majority of manufacturing systems. An equipment maker usually has the choice of purchasing modular stages or designing them “in house.” Modular stages require less design effort (and are often less expensive) than custom stages, but are more difficult to package and integrate into a complex machine. Concerns about performance also play a role in this make-or-buy decision: thermal errors and vibration in a built-up system of modular components are difficult to predict. Hence, modular stages often load parts onto a machine while integrated stages usually do the precision “value added” work at the heart of a process. At MIT, we are developing a family of open design linear-motion stages suitable for construction of three-axis machine tools. An open-design stage can be used in proprietary machine development in the same way as a modular stage. It can also be modified for better integration and fabricated in-house or by a third party. More importantly, open-design stages are designed to deterministically attain dynamic performance in a built-up system through discrete mounting points, substructure analysis, damping, and module scaling. Discrete Mounts and Substructuring: Consider the integration of a ballscrew-driven stage (such as that shown in Figure 1) into a machine tool by bolted joints at eight “hard points,” four on the base and four on the carriage. Using the method of substructuring [Mead], we can obtain a vibration model of the machine tool if we know the blocked and free receptance (dynamic compliance) at each hard point of each component of the system. We therefore design each element of the system with discrete hard points and take care to maximize their stiffness. Once a prototype has been built, we can directly measure the receptances to refine our model. The receptances
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used in a system-level “impedance budget” can be obtained from analysis, experiment, or their fusion.
Figure 1: Linear-motion module with eight hard points labeled MB1 through MB4 and SC1 through SC4. Damping: The principal difficulty in designing a machine for dynamic performance is the low and difficult-to-predict level of damping attained from metals, joints, and rolling-element bearings. We therefore design each component with embedded viscoelastic dampers. These dampers are designed into stiffeners (e.g., constrained-layer damping) and secondary bearing supports to provide a deterministic level of damping without introducing significant creep. Without this damping, it would be difficult for modular components bolted together at discrete mount points to attain dynamic performance comparable to an integrated machine structure. Module Scaling The first generation of linear-motion modules (currently under development) feature a conventional construction involving a rotary motor, ballscrew, and modular linear guides mounted to a welded base. This design is encapsulated in a parametric solid model driven by a set of numerical scripts that produce a plausible design (employing standard bearings and motors) from inputs such as the length of travel, payload, attainable speed, desired impedances, and so on. But the payload that a given stage must carry will depend on the
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sizes of the other components of the machine. Moreover, the closed-loop performance of a stage will also depend on the dynamics of the machine as a whole, so that we cannot neatly perform the system design in a top-down sequence. Rather, we will have to iterate back and forth between system and component design. Microfabrication and Nanofabrication Equipment To support research in micromachining [Masuzawa, 2000] and nanotechnology [Corbett et al.], a new initiative, Open Microfabrication and Nanofabrication Equipment (OMNE), was initiated by the Precision Systems Laboratory at the University of Kentucky. OMNE’s objective is to provide a variety of designs for micro/nano fabrication equipment through open design projects. The first OMNE project, still in its infancy, is the development and construction of a new machine for 3D-micromachining using electro-discharge micro milling [Hii et al.]. We describe this project to illustrate essential aspects in applying open design to manufacturing equipment. In electro-discharge micro milling, material is removed with electrical discharges between a rotating cylindrical electrode and a workpiece [Yu et al., 1998a; and Yu et al. 1998b]. 3D-micromachining is achieved by positioning the electrode in the x, y, and z directions using a CNC controller. The electrode is often manufactured using Wire Electro Discharge Grinding (WEDG) [Masuzawa, 1985]. Both of these processes are currently embodied in a machine from Panasonic Factory Automation. Unfortunately, proprietary machines limit scientific investigation of the process and make enhancements or modifications to the equipment or process difficult. Reconfiguration of Existing XY Stage We wish to avoid developing the OMNE µEDM machine entirely from scratch, and therefore adapt and reuse hardware from an existing machine. A surplus xy-stage from a mask-aligner (commercially available during the mid-1980s) was selected as the starting point. The project milestones are to rebuild portions of the stage and eventually replace much of the machine structure. To facilitate reconfiguration of the machine and redesign activities, 3D solid models of the stage and machine elements were constructed. Figure 2 illustrates the original elements in the positioning stage. The foundation of the stage is a granite slab supported on pneumatic self-leveling pads. The y-stage is a T-shaped structure that is supported on five vacuum-preloaded air-bearing pads (two on the edge of the granite and three on the top surface of the granite). The x-stage is a rectangular structure that is coupled to the granite surface and the y-stage using vacuum-preloaded air-bearings. The location of the x-stage in the x and y directions is measured using a plane-mirror laser interferometer. The y-stage is actuated by a DC servomotor (mounted on the y-stage) and a friction-drive transmission that couples the y-stage to a column rigidly mounted to the granite. The x-stage is actuated by another DC servomotor (mounted on the y-stage) and a friction-drive transmission that connects the x-stage to the y-stage. The home positions of the y-stage and x-stage are established with LVDT sensors.
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z-stage and spindle support x-stage
servo motors laser
y-stage granite air bearings
Figure 2: XY Positioning Stage for the OMNE Micro EDM Machine We are adapting the positioning stage for micro EDM, replacing outdated technology, and adding a z-axis and spindle [Hii et al.]. For flexibility in machine and process control, we are developing an open-architecture CNC machine controller [Koren]. The controller uses a three-channel, 16-bit motion I/O card (Precision Micro Dynamics, Inc. MFIO-3A) operating on a personal computer running a real-time operating system (QNX). Similar to NIST’s Enhanced Machine Controller [Proctor], the position and velocity loops are closed through the host computer’s Intel Celeron CPU rather than through a DSP on the controller card. This prevents developing control software for a proprietary DSP that is not portable to an alternative controller card. The original laser interferometer is being replaced with a new interferometer system (Zygo 510). The pneumatic and vacuum systems for the air bearings are also being renovated. Discussion This project illustrates several aspects of open design for manufacturing equipment that warrant discussion. The first topic is whether open design is suitable for creatively originating new designs or whether the method is better suited to incremental improvements. To avoid this issue, we decided to adapt and modify a pre-existing machine. Ideally, the new machine would have been based on a positioning stage already available under an open design license. By nature of being one of the first open design projects, this was not possible. We therefore selected a positioning stage that was originally manufactured by a corporation that is no longer in business; we intend to eventually replace the original elements in the positioning stage with open designs.
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This project illustrates the potential for open design to increase the longevity of useful designs. Although the positioning stage was originally designed during the early/mid 1980s, its technology is still useful for many applications. Too frequently, useful designs dissolve into history because a project is terminated before completion, a product life cycle is complete, or a company goes out of business. For instance, the laser interferometer used in the positioning stage is no longer produced or supported by its original manufacturer. Due to its proprietary nature, information for operating and maintaining the interferometer is no longer available; it should therefore be replaced. Since open design provides a public repository of design information, it can function like an archived journal. Open design can preserve design knowledge, prevent useful designs from becoming dead, and allow designers to avoid continuously reinventing the wheel. Although open manufacturing equipment would ideally consist entirely of non-proprietary technology, this is not practical. In fact, proprietary technology often enables significant technical advantage. Therefore, the Open Design Definition and licenses accommodate the inclusion of proprietary technology within open designs. It is preferred, however, that any proprietary elements should either conform to accepted standards (e.g. NEMA motors [NEMA]) or be open architecture. Specifications for mechanical and electrical interfaces of open architecture elements should be publicly available and incorporated within the design documentation. This ensures that open designers can replace any proprietary element with another alternative, and it prevents open designs from becoming obsolete should a proprietary element become unavailable. SOFTWARE TOOLS FOR OPEN DESIGN To facilitate collaboration, open designers should agree to abide by certain standards in sharing and exchanging design information. These include standard design processes, design software (CAD/CAM/PDM), units of measure, drawing formats, etc. Some standards, such as an agreement to use SI units, are easily established. Other standards, such as the software tools, are controversial due to personal preferences and software availability. Unlike software development, much of the software required for concurrent design is not economically affordable for all designers. For collaboration purposes, we suggest that software tools for design be available and accessible to any that wish to participate. Thus, Open Source software is often preferable, since it is free to all users. Since geometric descriptions form the foundation of any machine design, we begin by discussing the exchange of 2D and 3D CAD data. Proprietary CAD applications, which dominate industry, traditionally use custom or proprietary data formats that reflect the nature of the application’s geometry kernel. Therefore, native CAD data is generally not portable among CAD applications, and this makes concurrent design with more than one CAD application difficult and inefficient. Standard interchange formats such as DXF, IGES, and STEP enable data to be exchanged, but generally require some loss of modeling information (such as parametric representation or the ability to modify the data). Therefore, these translations are generally performed in unidirectional exchanges and not sufficient for concurrent design. For these reasons, 3D CAD data remains far from portable and is the weakest software tool available for open design.
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Open Source CAD applications may eventually resolve these issues. Two open source 2D CAD packages already under development, QCAD (http://www.qcad.org) and FreeDraft (http://freeengineer.org/Freedraft/index.html). Unfortunately, Open Source solid modeling software does not exist at this time, but there may be light at the end of the tunnel. OpenCascade (http://www.opencascade.org) is a project to develop a solid modeling kernel. An open source solid modeling kernel might establish a standard file format that can be exchanged in collaborative open design projects. Until a standard file format or Open Source solid modeling exists, open design must depend on translation tools using prescribed processes [Kiani] and proprietary CAD systems. Other tools for office documents and analyses are readily available and mature. These tools include complete suites of office applications (e.g. word processing, spreadsheets, presentations) such as OpenOffice, and numerical programming tools such as SciLab (http://www-rocq.inria.fr/scilab/) and Octave (http://www.octave.org/). Although these applications were not developed specifically for design purposes, they suit designers needs. These applications are Open Source, so the opportunity exists to adapt them to the specific needs of the Open Design community. Server Side Tools - Product Data Management (CVS) - Design "How To’s" - Project Web Sites - Email List Hosting
ODF Internet Server
it m
it Su bm
ch
Fe t
ch
t Fe b Su
Client Design Tools - OpenCascade - FreeDraft - QCAD - Octave - Scilab - Gnumeric - JCVS - gEDA
Designer A
Documentation Tools
Designer B
- OpenOffice - LaTek - KOffice
Figure 3: Open Source Software Tools for Open Design Once designers create CAD data, documents, and analyses, they must be stored centrally in a repository or product data management (PDM) system. ODF uses the Concurrent Versions System (CVS) (http://www.cvshome.org) in a client/server architecture as shown in Figure 3. CVS is a back-end tool that provides versioning, data vaulting, checkout, locking, and branching capabilities similar to many commercial PDM packages. It is used extensively for concurrent software development. While the basic tool has a command line front-end, several graphical or web-based front-ends are available (http://www.cvsgui.org). Though
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CVS is a powerful tool it is designed primarily for software development and specific procedures must be followed to use it for PDM purposes. Since CVS is open source, it can be adapted to accommodate the needs of open design. CONCLUSIONS Both the designers and users of manufacturing equipment can benefit from open design. For example, the designer of a PCB inspection machine may incorporate an open-design loading robot rather than designing it “in house” or purchasing a proprietary design. Integration of the loading robot may require that its controller interface as well as some of its mechanical details be modified. This modification may be carried out “in house” or by a third party, speeding development and reducing the cost of the inspection machine. Some time after the inspection machine is purchased, the user may wish to change the way that PCBs are loaded and unloaded from the machine. Because the loading robot is an open design, the user has all of the information required to perform the modification or have it done by another party. Open design gives manufacturers the freedom to modify equipment in response to product or production-system changes, to continuously improve equipment as well as manufacturing processes. REFERENCES Corbett, J., P.A. McKeown, G.N. Peggs, R. Whatmore. 2000 "Nanotechnology: International Developments and Emerging Products". Annals of the CIRP. Vol. 49, No. 2. p. 523-545. FSF(a) Free Software Foundation. "Free Documentation License". http://www.gnu.org/copyleft/fdl.html. FSF(b) Free Software Foundation. "GNU General Public License (GPL)". http://www.gnu.org/copyleft/gpl.html FSF(c). Free Software Foundation. "Less General Public License (LGPL)". http://www.gnu.org/copyleft/lesser.html. Hii, K.F, X. Zhao, and R.R. Vallance. 2000 "Design of a Precision Electro Discharge Micro Milling Machine". Proceedings of the American Society of Precision Engineers Annual Conference. Scottsdale, Arizona. October 22-27. Kiani, S. 2000 "Collaborating in a Mixed Environment". Open Design Foundation. http://www.opendesign.org/MixedCAD.html. Koren, Y. 1998 “Open Architecture Controllers for Manufacturing Systems”. Open Architecture Control Systems---Summary of Global Activity. edited by Koren, Y. et al, Institute for Industrial Technologies and Automation. Masuzawa 1985. “Wire Electro-Discharge Grinding for Micro-Machining”. Annals of the CIRP. Vol. 34, No. 1. p. 431-434. Masuzawa 2000. "State of the Art of Micromachining". Annals of the CIRP. Vol. 49, No. 2. p. 473-488. Mead, D. J., Passive Vibration Control, Wiley, New York, 1998.
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NEMA. 1996, Motion/Position Control Motors, Controls, and Feedback Devices, NEMA Standard MG-7 Rev. 1. National Electrical Manufacturers Association (NEMA). Rosslyn, VA. September 1996. ODF. 2000 Open Design Foundation. "Open Design Definition". http://www.opendesign.org/odd.html OSI(a). Open Source Initiative. "Open Source Definition". http://www.opensource.org/osd.html. OSI(b). Open Source Initiative. "History of the Open Source Initiative". http://www.opensource.org/history.html. Proctor, F.M., and J. Michaloski. 1993 "Enhanced Machine Controller Architecture Overview". NIST Internal Report 5331. National Institute of Standards and Technology. Raymond, E.S. 1999 "A Brief History of Hackerdom". in Open Sources: Voices from the Open Source Revolution. O’Reilly & Associates. Also available at http://www.gnu.org/gnu/thegnuproject.html. Stallman, R. 1999 "The GNU Operating System and the Free Software Movement". in Open Sources: Voices from the Open Source Revolution. O’Reilly & Associates. . Also available at http://www.gnu.org/gnu/thegnuproject.html. Yu et al. (a). Yu, Z., T. Masuzawa, and M. Fujino. 1998a "3D Micro-EDM with Simple Shape Electrode, Part 1: Machining of Cavities with Sharp Corners and Electrode Wear Compensation". International Journal of Electrical Machining. No. 3. p. 7-12. Yu et al. (b). Yu, Z., T. Masuzawa, and M. Fujino. 1998b "3D Micro-EDM with Simple Shape Electrode, Part 2: Machining and Error Analysis of conical and Spherical Cavities". International Journal of Electrical Machining. No. 3. p. 71-78.
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