Abstract:
A system, method, and computer program for designing an electrode for electric discharge machining, comprising identifying a cavity in a three-dimensional design; calculating a direct negative boolean of said cavity to define a general form for an electrode; determining an orbit path for said electrode, wherein said orbit path has a plurality of vertices corresponding to a plurality of instances with said three-dimensional design; subtracting a plurality of instances from said general form for said electrode whereby an orbit gap is removed from said general form electrode; and applying a constant face offset to said general form for said electrode having said orbit gap and appropriate means and computer-readable instructions.

Description:
TECHNICAL FIELD 
       [0001]    This presently preferred embodiment relates generally to EDM (electro-discharge machining). More specifically, the presently preferred embodiment relates to the method of undersizing electrodes for polygonal orbit EDM. 
       BACKGROUND 
       [0002]    Manufacturers use electrical discharge machining (EDM) to remove portions of metal too hard to machine with conventional milling techniques, or to form intricate cavities that could not be easily machined using conventional milling processes in a cost effective manner. During a series of consecutive sparks that produce a series of micro-craters on an electrically conductive work piece in the presence of an energetic dielectric fluid, the EDM process vaporizes material along a cutting path. Wire-cut EDM and die-sinking EDM are the two common types utilized by manufacturers today. In wire-cut EDM, a thin single strand metal wire is fed through a work piece that is constantly fed from a spool and held between an upper and lower guide. 
         [0003]    In die-sinking EDM, a graphite or copper electrode is machined into a desired shape and fed into a work piece to erode a cavity in a dielectric fluid. The eroded cavity is bigger than the electrode because of spark gap and EDM orbit. Spark gap is caused by the EDM erosion process itself and is proportional to the amount of current used. The EDM orbit moves the electrode in a programmed path which creates room for the dielectric fluid and eroded material to escape. 
         [0004]    The shape and size of the orbital path is based primarily on the size and shape of the electrode. The most commonly used orbits are spherical, circular, square, star and custom. The shape of the cavity being machined is dependent on the shape and size of the orbit. For example, if an electrode with a square cross section is moved in a circular orbit, the resulting cavity would have rounded corners along the vertical edges. Because the size and shape of the resulting cavity should meet particular dimensions, the electrode is often under-sized based on the chosen orbit. 
         [0005]    To virtually mill a part, a CAD application, for example NX(tm) by UGS Corp., is utilized to define the electrode path of orbit. To design the actual electrode, however, there are commercially available electrode design packages, for example PS-Electrode by Delcam and Quick Electrode by Cimatron, that completely ignore the steps required for orbit and gap compensation. When designing the electrodes, features in cavities are identified, a negative core is modeled, tangential extensions are formed, and then electrode base and holders are selected. The most important and challenging step in the electrode design process is compensating the geometry for spark gaps and EDM orbits, which said commercially available packages fail to provide. The user is therefore left to use manual offsets and manipulate machining tools to under-size the electrode and trick the milling machine to form an electrode that is smaller than the electrode originally designed to compensate for the inherent deficiencies present today. For example, in circular and spherical orbits, the electrode making process is tricked by under-sizing programmed tools and over-sizing actual milling tools. Circular and spherical orbits are limited since they cannot produce sharp corners in the cavity. 
         [0006]    Spherical orbit under-sizing is the easiest to manage, as the spark gap and orbit gap are uniformly applied to the entire electrode geometry. This uniform application is accomplished by using either face offsets or applying negative stock at the machine tool. (during NC programming, negative stock is applied by programming with a smaller tool.) There are severe limitations to both the methods: (1) face offsets are not always reliable, (2) negative offsets can only be applied with tools with corner radii bigger than the offset value, (3) the final cavity will have corner radii as big as 50% of orbit_distance+finish_spark_gap, and (4) smaller corner radii in the original cavity could result in sharp corners on the under-sized electrode that could increase electrode wear and (5) Spherical orbits take at least twice as long as other orbits to erode the desired cavity. 
         [0007]    A second category of orbit path is the circular orbit that is technically more complicated to achieve than under-sizing for spherical orbits. In the circular orbit, the orbit gap is applied uniformly in a X-Y plane. There is no orbit gap along a Z axis. Rather, the spark gap is applied uniformly in all directions. If the electrode geometry is simple, most users manage the electrode geometry by offsetting select faces from the electrode. The management of electrode geometry is also accomplished by manipulating the programmed tool while machining the electrode. The limitations are: (1) face offsets are less reliable than in spherical orbit under-sizing, (2) face offsets have to be individually calculated and administered for inclined faces, which makes it even less reliable, (3) the final cavity will have vertical corner radii as big as 50% of orbit_distance+finish_spark_gap, (4) small vertical corner radii in the original cavity could result in sharp corners on the under-sized electrode that could increase electrode wear, and (5) manipulating tool diameter while machining the electrode is only applicable to flat end mills. 
         [0008]    The third category of orbit path is the square orbit, which is the most commonly used polygonal planar orbit in Sink EDM. Square orbits produce superior results because they are capable of creating corner radii as small as the finish spark gap in the final cavity. Square orbits also do not cause the sharp corners in the electrode thereby increase electrode longevity. However, designing an electrode for a square orbit is very complicated except in very simple cases. And hence, it is very seldom employed. 
         [0009]      FIG. 1  is an orthogonal orientation depicting a prior art technique to undersize an electrode to account for orbital path and spark gap, under-sizing the programmed tools is done manually to account for spark gap and orbit size. For example, a user desires to mill an electrode  100  and uses a cylindrical tool  105 , to that end. The tool is 40 millimeters long and 10 millimeters in diameter, for example. Given a spark gap of 0.02 millimeters, and an orbit size of 0.1 millimeter, the user “cheats” the CAM system by programming the tool to be 10 mm−2(spark gap+orbit size), or 9.76 mm. Likewise the length is shortened by just the spark gap to result in 39.98 mm (40 mm−0.02 mm) programmed tool length. Therefore, the milling machine thinks it is using a programmed tool  110 , but it is really using the cylindrical tool  105 . 
         [0010]    There is a need for a solution that can easily undersize electrodes for polygonal orbits to produce sharp corners and without needed manual modification to trick the tools in manufacturing the electrodes. 
       SUMMARY  
       [0011]    To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading in accordance with the purpose of the presently preferred embodiment as broadly described herein, the presently preferred embodiment discloses a method for designing an electrode for electric discharge machining, the method comprising identifying a cavity in a three-dimensional design; calculating a direct negative boolean of said cavity to define a general form for an electrode; determining an orbit path for said electrode, wherein said orbit path has a plurality of vertices corresponding to a plurality of instances with said three-dimensional design; subtracting a plurality of instances from said general form for said electrode by boolean subtraction whereby an orbit gap is removed from said general form electrode; and applying a constant face offset to said general form for said electrode having said orbit gap. The method further comprising the step of adding tangential extensions to said electrode whereby said tangential extensions provide relief. The method, wherein said orbit path is polygonal. The method, wherein said orbit path is tessellated into a plurality of discrete vertices. The method, wherein said orbit path is polygonal and the method further comprises the step of adding tangential extensions whereby said tangential extensions provide relief. The method, wherein said orbit path is tessellated into a plurality of discrete vertices and the method further comprises the step of adding tangential extensions whereby said tangential extensions provide relief. 
         [0012]    Another advantage of the presently preferred embodiment is to provide a computer-program product tangibly embodied in a machine readable medium to perform a method for designing an electrode for electric discharge machining, comprising instructions for identifying a cavity in a three-dimensional design; instructions for calculating a direct negative boolean of said cavity to define a general form for an electrode; instructions for determining an orbit path for said electrode, wherein said orbit path has a plurality of vertices corresponding to a plurality of instances with said three-dimensional design; instructions for subtracting a plurality of instances from said general form for said electrode by boolean subtraction whereby an orbit gap is removed from said general form for said electrode; and instructions for applying a constant face offset to said general form for said electrode having said orbit gap. The computer-program product, further comprising the instruction for adding tangential extensions whereby said tangential extensions provide relief. The computer-program product, wherein said orbit path is polygonal. The computer-program product, wherein said orbit path is tessellated into a plurality of discrete vertices. The computer-program product, wherein said orbit path is polygonal and the method further comprises the instruction for adding tangential extensions whereby said tangential extensions provide relief. The computer-program product, wherein said orbit path is tessellated into a plurality of discrete vertices and the method further comprises the instruction for adding tangential extensions whereby said tangential extensions provide relief. 
         [0013]    Another advantage of the presently preferred embodiment is a computer data signal for computer aided modeling, said computer data signal comprising code configured to cause a designer to implement on a computer to employ a method comprising generating an electrode design from a general form for an electrode having an orbit path with a plurality of instances directly related to a plurality of vertices of said orbit path, where a boolean subtraction of said plurality of instances defines an orbit gap, wherein said electrode design is said general form for said electrode less said orbit gap and a constant face offset; formatting signals to transmit to a milling machine to form a physical electrode based on said under-sized electrode; utilizing said physical electrode to erode a cavity in an electrically conductive physical workpiece. 
         [0014]    Still another advantage of the presently preferred embodiment is an electrode for eroding an electronically conductive workpiece to form a cavity by die-sinking, wherein a software application computes said electrode such that said electrode is a negative of said cavity less an orbit gap and a constant face offset, wherein said orbit gap is calculated from an orbit path having a plurality of vertices. 
         [0015]    And yet another advantage of the presently preferred embodiment is a molded part formed by a core and a cavity wherein said core and said cavity are milled by at least electric discharge machining having an electrode designed by a software application such that said electrode is a negative of a design cavity less an orbit gap and a constant face offset. 
         [0016]    A further advantage of the presently preferred embodiment is a data processing system having at least a processor and accessible memory, comprising means for identifying a cavity in a three-dimensional design; means for calculating a direct negative boolean of said cavity to define a general form for an electrode; means for determining an orbit path for said electrode, wherein said orbit path has a plurality of vertices corresponding to a plurality of instances with said three-dimensional design; means for subtracting a plurality of instances from said general form for said electrode by boolean subtraction whereby an orbit gap is removed from said general form electrode; and means for applying a constant face offset to said general form for said electrode having said orbit gap. 
         [0017]    Other advantages of the presently preferred embodiment will be set forth in part in the description and in the drawings that follow, and, in part will be learned by practice of the presently preferred embodiment. 
         [0018]    The presently preferred embodiment will now be described with reference made to the following Figures that form a part hereof, and which is shown, by way of illustration, an embodiment of the presently preferred embodiment. It is understood that other embodiments may be utilized and changes may be made without departing from the scope of the presently preferred embodiment. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    A preferred exemplary embodiment of the presently preferred embodiment will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and: 
           [0020]      FIG. 1  is an orthogonal orientation depicting a prior art technique to under-size an electrode to account for orbital path and spark gap; 
           [0021]      FIG. 2  is a block diagram of a computer environment in which the presently preferred embodiment may be practiced; 
           [0022]      FIG. 3  is a logic flow diagram depicting a method disclosed in the preferred embodiment; 
           [0023]      FIG. 4  is a three-dimensional design of a workpiece; 
           [0024]      FIG. 5  is an illustration of an electrode body that has a direct boolean negative of a cavity in a three-dimensional design of a workpiece extending therefrom; 
           [0025]      FIGS. 6   a - 6   d  is a series of illustrations depicting the formation of an electrode formed following the method disclosed; 
           [0026]      FIG. 7  is an axonometric orientation for an electrode undersized for orbit; 
           [0027]      FIG. 8  is an orthogonal orientation for an electrode undersized for orbit in a work piece; 
           [0028]      FIG. 9  is a close-up in an orthogonal orientation for an electrode undersized for orbit and spark gap; and 
           [0029]      FIG. 10  is an orthogonal orientation for an electrode undersized for orbit and spark gap in a work piece. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    The numerous innovative teachings of the present application will be discussed with particular reference to the presently preferred embodiments. It should be understood, however, that this class of embodiments provides only a few examples of the many advantageous uses of the innovative teaching herein. The presently preferred embodiments provide, among other things, a system and method for undersizing electrodes for polygonal orbit electrical discharge machining.  FIG. 2  and the following discussion are intended to provide a brief, general description of a suitable computing environment in which the presently preferred embodiments may be implemented. Although not required, the presently preferred embodiments will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer. Generally program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Likewise, the presently preferred embodiment may be performed in any of a variety of known computing environments. 
         [0031]    With reference to  FIG. 2 , an exemplary system for implementing the presently preferred embodiments includes a general-purpose computing device in the form of a computer  200 , such as a desktop or laptop computer, including a plurality of related peripheral devices (not depicted). The computer  200  includes a microprocessor  205  and a bus  210  employed to connect and enable communication between the microprocessor  205  and a plurality of components of the computer  200  in accordance with known techniques. The bus  210  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The computer  200  typically includes a user interface adapter  215 , which connects the microprocessor  205  via the bus  210  to one or more interface devices, such as a keyboard  220 , mouse  225 , and/or other interface devices  230 , which can be any user interface device, such as a touch sensitive screen, digitized pen entry pad, etc. The bus  210  also connects a display device  235 , such as an LCD screen or monitor, to the microprocessor  205  via a display adapter  240 . The bus  210  also connects the microprocessor  205  to a memory  245 , which can include ROM, RAM, etc. 
         [0032]    The computer  200  further includes a drive interface  250  that couples at least one storage device  255  and/or at least one optical drive  260  to the bus. The storage device  255  can include a hard disk drive, not shown, for reading and writing to a disk, a magnetic disk drive, not shown, for reading from or writing to a removable magnetic disk drive. Likewise the optical drive  260  can include an optical disk drive, not shown, for reading from or writing to a removable optical disk such as a CD ROM or other optical media. The aforementioned drives and associated computer-readable media provide non-volatile storage of computer readable instructions, data structures, program modules, and other data for the computer  200 . 
         [0033]    The computer  200  can communicate via a communications channel  265  with other computers or networks of computers. The computer  200  may be associated with such other computers in a local area network (LAN) or a wide area network (WAN), or it can be a client in a client/server arrangement with another computer, etc. Furthermore, the presently preferred embodiment may also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices. All of these configurations, as well as the appropriate communications hardware and software, are known in the art. 
         [0034]    Software programming code that embodies the presently preferred embodiment is typically stored in the memory  245  of the computer  200 . In the client/server arrangement, such software programming code may be stored with memory associated with a server. The software programming code may also be embodied on any of a variety of non-volatile data storage device, such as a hard-drive, a diskette or a CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory of one computer system over a network of some type to other computer systems for use by users of such other systems. The techniques and methods for embodying software program code on physical media and/or distributing software code via networks are well known and will not be further discussed herein. 
         [0035]    The presently preferred embodiment discloses a method for designing an electrode for eroding an electrically conductive workpiece to form a cavity therein. Referring now to the steps illustrated in  FIG. 3  and cross-referencing those steps with illustrative examples in the subsequent figures  FIG. 4  through  FIG. 10 , a designer begins by creating a three-dimensional design  400  of a virtual workpiece utilizing a computer aided drafting software application, like NX(tm) by UGS Corp. (Step  300 ). The designer then identifies a cavity  405  to erode utilizing a process of electro-erosion known as electrical discharge machining (EDM) by die sinking (Step  305 ). 
         [0036]    Continuing, an electrode  500  is designed beginning with an electrode starting block and the direct negative boolean of the cavity  405 , which results in the electrode starting block with a direct negative boolean of the cavity extending therefrom (Step  310 ). Next determine the size and shape of the orbit in roughing and finishing EDM processes, where the shape of the orbit approximately resembles the cavity itself (Step  315 ). The presently preferred embodiment discloses a polygonal orbit, however it is understood that the polygonal orbit can have many sides and many vertices. For example, the presently preferred embodiment can be utilized with circular orbits by tessellating the circle into a polygon, or any orbital path that can be deconstructed into discrete points. 
         [0037]    Now to under-size the electrode, given a square orbit  600  as proposed by the designer based upon the square-like nature of the cavity to be milled, the three-dimensional design  400  is moved to a first vertex  605  in a manner that orbits the cavity  405  itself, instead of the electrode, in the desired path (Step  320 ). At the first vertex  605 , the three-dimensional design  400  performs a boolean-subtraction operation on the electrode  500  from an instance  607 , where an the instance denotes an associated copy of the same part at a different location (Step  325 ). The three-dimensional design  400  is moved to a next vertex  610  of the square orbit  600  (Step  330 ), after which another boolean-subtraction operation on the electrode  500  occurs (Step  335 ). Successive boolean-subtraction operations occur on the electrode  500  at each subsequent vertex  615 ,  620  until the first vertex  605  is reached (Step  340 ). 
         [0038]    An orbit gap  700 , also identified as an orbit-adjusted electrode or an electrode offset, is the result of the foregoing steps where the presently preferred embodiment then further adds a plurality of tangential extensions shown at  800  from the corresponding edges of the cavity  405  to provide relief (Step  345 ). As depicted in  FIG. 8 , an orthogonal orientation for an electrode undersized for orbit in a work piece, it is important to note that at this step the orbital gaps are different on the vertical and inclined faces, and there are no gaps between the horizontal faces. Finally, a spark gap is formed by a constant face offset  900  applied to all faces to compensate for a pre-determined spark-gap (Step  350 ) to create the end product of an undersized electrode  1000  for the proposed polygonal orbit. 
         [0039]    In another embodiment, the electrode geometry is derived from a subset of faces from the cavity geometry. In such cases, the solution is still valid by creating a negative of the derived electrode and intersecting all of the instances of the electrode itself. Or put another way, the electrode is moved around and the alternative embodiment intersects all of the instances of the electrode itself. 
         [0040]    Following the disclosed presently preferred embodiment, the electrode is milled using conventional techniques. And is put into production to erode the necessary electrically conductive workpiece. The electrically conductive workpiece is used in physical manufacturing to create a core and a cavity that will be used for a manufacturing technique, like injection molding for example, to make parts. 
         [0041]    The presently preferred embodiment may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof. An apparatus of the presently preferred embodiment may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a programmable processor; and method steps of the presently preferred embodiment may be performed by a programmable processor executing a program of instructions to perform functions of the presently preferred embodiment by operating on input data and generating output. 
         [0042]    The presently preferred embodiment may advantageously be implemented in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. The application program may be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language may be a compiled or interpreted language. 
         [0043]    Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of nonvolatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed ASICs (application-specific integrated circuits). 
         [0044]    A number of embodiments of the presently preferred embodiment have been described. It will be understood that various modifications may be made without departing from the spirit and scope of the presently preferred embodiment. Therefore, other implementations are within the scope of the following claims.