Patent Publication Number: US-7596424-B2

Title: Machining condition determining system and method for sinker electric discharge machining apparatus

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a sinker electric discharge machining apparatus for forming in the workpiece a cavity which is complementary in shape to a tool electrode by applying a series of current pulses. In particular, the present invention relates to a machining condition determining system for determining machining conditions. 
   2. Description of the Related Art 
   In general, a tool electrode for sinker electric discharge machining is advanced along an axis of advancement to shape the workpiece. The axis of advancement is mostly a z-axis. In preparation for sinker electric discharge machining, a tool electrode is designed according to a three-dimensional geometry of the cavity being formed. Then, optimum machining conditions are determined in consideration of a three-dimensional geometry of the cavity, material of the tool electrode and workpiece and etc. The setting of the machining conditions are included in an NC program for electric discharge machining. The machining conditions are, for example, current peak, on-time and off-time of current pulse to be supplied to a tool electrode. Current peak is a critical machining condition which determines material removal rate and surface roughness. In general, a machining condition determining system considers machining area or active area of the tool electrode to determine current peak. The machining area is obtained by projecting the machining surface of the tool electrode downward on a plane perpendicular to the axis of advancement. If a column-shaped tool electrode is used, the machining area is an area of the base. As the machining progresses and the tool electrode advances further in the negative direction of z-axis, the machining area may be changed. 
   With the spread of CAD/CAM (Computer Aided Design/Computer Aided Manufacture), a CAD device now produces three-dimensional geometries of the cavity and tool electrode, and a CAM device automatically produces an NC program using such 3D geometry data. The 3D geometry data of the tool electrode or cavity is also used to accurately calculate the machining area. 
   Japanese Laid-open patent application 2003-291032 discloses a CAD device which produces a three-dimensional solid model of the tool electrode, and prepares a surface model of the tool electrode from the solid model. Machining area is calculated from the surface model for a plurality of depth positions of the tool electrode using an iterative method such as Newton-Raphson method. The optimum machining conditions are determined based on the machining area. 
   In order to obtain the optimum machining conditions, accurate calculation of the machining area is needed. Machining area calculation methods depend on a three-dimensional geometry of the tool electrode. Therefore, a conventional machining condition determining system must store a plurality of computation methods for circular cone, cylindrical column and etc. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a machining condition determining system and method which accurately calculates machining area with a common way regardless of a three-dimensional geometry of a tool electrode. 
   Another object of the present invention is to provide a machining condition determining system and method which accurately calculates machining area even if a tool electrode has a complicated three-dimensional geometry. 
   According to one aspect of the present invention, a machining condition determining system for a sinker electric discharge machining apparatus in which a tool electrode is advanced along an axis of advancement to machine the workpiece comprises: 
   a cavity model producing means ( 22 ) for producing a cavity model (D) as a solid model of part of the tool electrode; 
   a tessellating means ( 23 ) for tessellating the cavity model into a mesh of polygons each of the polygons having an ordered set of vertices and having an orientation defined by the ordering of the vertices; 
   a projected area calculating means ( 32 ) for projecting the polygons on a plane surface (R) perpendicular to the axis of advancement and summing up areas of the projected polygons having the same orientation to calculate a machining area; and 
   a machining condition determining means ( 33 ) for determining machining conditions based on the machining area. 
   Preferably, the mesh of polygons may be a mesh of triangles. 
   The machining conditions may include current peak of current pulse to be supplied to the tool electrode. 
   According to another aspect of the present invention, a method of determining machining condition for a sinker electric discharge machining apparatus in which a tool electrode is advanced along an axis of advancement to machine the workpiece, comprises the steps of: 
   (a) producing a cavity model as a solid model of part of the tool electrode; 
   (b) tessellating the cavity model into a mesh of polygons each of polygons having an ordered set of vertices and having an orientation defined by the ordering of the vertices; 
   (c) projecting the polygons on a plane surface perpendicular to the axis of advancement; 
   (d) summing up areas of the projected polygons having the same orientation to calculate a machining area; and 
   (e) determining machining conditions based on the machining area. 
   According to yet another aspect of the present invention, a method of determining machining condition for a sinker electric discharge machining apparatus in which a tool electrode is advanced along an axis of advancement to machine the workpiece, comprises the steps of: 
   (a) producing a cavity model as a solid model of part of the tool electrode; 
   (b) tessellating the cavity model into a mesh of triangles each of the triangle having first, second and third vertices in order and having an orientation defined by the ordering of the vertices; 
   (c) projecting the first, second and third vertices (Q 1 ,Q 2 ,Q 3 ) to first, second and third points (P 1 ,P 2 ,P 3 ) on a plane surface (R) perpendicular to the axis of advancement; 
   (d) defining a first vector extending from the first point to the second point and a second vector extending from the first point to the third point; 
   (e) calculating the signed cross product of the first vector and the second vector; 
   (f) summing up the magnitude of the cross products having the same sign to calculate a machining area; and 
   (g) determining machining conditions based on the machining area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram illustrating an NC data producing system of the present invention. 
       FIG. 2  illustrates solid models of the tool electrode and workpiece. 
       FIG. 3  illustrates a triangle mesh representation of the cavity model of  FIG. 2 . 
       FIG. 4  illustrates a planar triangle of the mesh. 
       FIG. 5  illustrates the projection of the triangle. 
       FIG. 6  illustrates the projection of the triangle. 
       FIG. 7  is a flowchart illustrating a machining condition determining process of the NC data producing system. 
   

   PREFERRED EMBODIMENT OF THE INVENTION 
   A machining condition determining system of the present invention will now be described with reference to the drawings. 
   As illustrated in  FIG. 1 , an NC data producing system  1  includes a CAD device  2 , a CAM device  3  and a network  4  connecting the CAD device  2  and CAM device  3 . The CAD device  2  is provided for designing a tool electrode as well as a cavity to be formed in the workpiece. The CAM device  3  receives information on a geometry of the tool electrode, a position of the workpiece and etc., and produces an NC program for machining the workpiece. The CAD device  2  and CAM device  3  are implemented by running CAD program and CAM program on a computer such as a personal computer. The CAD and CAM programs are saved to secondary storage of the computer from a data storage medium such as a CD-ROM. These programs may be obtained over a network such as the Internet. A variety of networks such as LAN, WAN, Internet and etc. may be used. 
   The CAD device  2  includes an electrode model producing means  21 , a cavity model producing means  22  and a tessellating means  23 . The electrode model producing means  21  receives an operator&#39;s data input and produces an electrode model EL as a solid model of a tool electrode, and a work model W as a solid model of a workpiece to be shaped, as illustrated in  FIG. 2 . 
   In general, the workpiece is fixed parallel to an XY plane on an appropriate table. During electric discharge machining, a tool electrode is positioned extremely close to the workpiece with a gap of a few μm to a few tens of μm. A series of current pulses are supplied to the tool electrode to generate electric discharges at the microscopic gap at a high frequency. As the electric discharges repeatedly remove microscopic amounts of the workpiece material, the tool electrode is further advanced toward the workpiece in the direction of the z-axis. 
   The cavity model producing means  22  causes the electrode model EL to advance to a predetermined depth position h in the vertical downward direction t, as shown in  FIG. 2 . The direction t is a negative Z-axis direction. The cavity model producing means  22  calculates the product of the electrode model EL and work model W to produce a cavity model D as a solid model of part of the tool electrode. This part of the tool electrode is almost equals to a cavity to be formed in the work. The cavity is complementary in shape to part of the tool electrode and may be a hole, recess and groove depending on the shape of the tool electrode. 
   The tessellating means  23  tessellates the entire surface of the cavity model D into a mesh of simple planar polygons such as triangles to convert the cavity model D to a mesh model, as illustrated in  FIG. 3 . Each of the triangles faces outwards and a normal vector n represents its orientation. As illustrated in  FIG. 4 , each of the triangles has an ordered set of vertices Q 1 , Q 2  and Q 3  and its orientation is defined by the ordering of the vertices. When the vertices Q 1 , Q 2  and Q 3  are ordered in a counter-clockwise fashion, the triangle has a positive orientation. 
   The CAM device  3  includes a mesh model receiving means  31 , a projected area calculating means  32  and a machining condition determining means  33 . The mesh model receiving means  31  receives the mesh model with the mesh of planar triangles for the cavity from the CAD device  2  through the network  4 . The projected area calculating means  32  projects each planar triangle to a plane surface R to calculate machining area. The plane surface R is perpendicular to the z-axis and may be an xy plane. 
   As illustrated in  FIGS. 5 and 6 , the CAM device  3  projects each planar triangle to a plane surface R which is perpendicular to the axis of advancement of the tool electrode. The vertices Q 1 , Q 2  and Q 3  of a planar triangle are projected downward to respective points P 1 , P 2  and P 3  on a plane surface R. Actually a z-coordinate is removed from xyz coordinates of each vertex. The projected area calculating means  32  defines two vectors {right arrow over (A)} and {right arrow over (B)}. The vector {right arrow over (A)} points from P 1  (0,0) to P 2  (x2,y2) and a vector {right arrow over (B)} points from P 1  (0,0) to P 3  (x3,y3). The magnitude of the signed cross product {right arrow over (A)}×{right arrow over (A)} of the two vectors is the area of a shaded parallelogram in  FIGS. 5 and 6 . Therefore, the area S of the projected triangle P 1 P 2 P 3  is one-half the magnitude of the cross product {right arrow over (A)}×{right arrow over (B)}, as represented by the following equation: 
   
     
       
         
           
             
               
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   The orientation u of the projected triangle P 1 P 2 P 3  depends on the sign of the area S. If the projected area S is positive (S&gt;0), the triangle P 1 P 2 P 3  faces upward, as illustrated in  FIG. 5 . If the projected area S is equal to zero (S=0), the triangle P 1 P 2 P 3  is perpendicular to the axis of advancement. If the area S is negative (S&lt;0), the triangle P 1 P 2 P 3  faces downward, as illustrated in  FIG. 6 . The projected area calculating means  32  calculates the absolute value of the sum of the areas of the downward projected triangles to determine the machining area. Alternatively, the projected area calculating means  32  may calculate the sum of the areas of the upward projected triangles. 
   Material removal rate, surface roughness, shape accuracy and electrode wear rate depends on such machining conditions. The machining condition determining means  33  stores a table in which machining conditions such as current peak, on-time and off-time of current pulse are related to the machining area, shape accuracy and surface roughness. The machining condition determining means  33  determines the machining conditions based on the machining area as well as required shape accuracy and surface roughness. 
   With reference to  FIG. 7 , the machining condition determining process of the NC data producing system  1  will be described in detail. 
   At step S 100 , the CAD device  2  produces the electrode model EL and the work model W as solid models. At step S 101 , the CAD device  2  calculates the product of the electrode model EL and work model W to produce the cavity model D as a solid model. At step S 102 , the CAD device  2  tessellates the entire surface of the cavity model D into a mesh of planar triangles to generate a mesh model. The mesh model is transferred to the CAM device  3  from the CAD device  2  through the network  4 . 
   The CAM device  3  projects the vertices Q 1 , Q 2  and Q 3  of the triangles to respective points P 1 , P 2  and P 3  on a plane surface R which is perpendicular to the axis of advancement, at step S 103 . At step S 104 , the CAM device  3  defines a vector {right arrow over (A)} extending from the first point P 1  to the second point P 2  and a vector {right arrow over (B)} extending from the first point P 1  to the third point P 3 . At step S 105 , the signed cross product {right arrow over (A)}×{right arrow over (B)} of the two vectors {right arrow over (A)} and {right arrow over (B)} is calculated. If the cross product {right arrow over (A)}×{right arrow over (B)} is negative, at step S 106 , the process proceeds to step S 107 . Otherwise, the process skips to step S 108 . 
   The area S of the projected triangle defined by the ordered points P 1 , P 2  and P 3  is calculated using the equation (1). At the step S 107 , the CAM device  3  sums up the area S of the projected triangle to calculate a machining area. If the CAM device  3  projects all of the mesh of triangles and sums up the areas S of the projected triangles, at step S 108 , a machining area is obtained and the process proceeds to step S 109 . At the step S 109 , machining conditions such as current peak of current pulse is determined. 
   The present invention is not intended to be limited to the disclosed form. It is clear that many improvements and variations are possible with reference to the above description. The illustrated embodiment was selected to explain the essence and practical application of the invention. The scope of the invention is defined by the attached claims.