Abstract:
A method of machining a pattern at a plurality of locations in a workpiece includes positioning a shaped electrode including the pattern at the plurality of locations in a sequence. In some embodiments, the sequence is random. The method further includes forming the pattern at each of the plurality of locations by passing electrical charges repeatedly between the shaped electrode and the workpiece and advancing the shaped electrode into the workpiece for a fraction of a full depth of the pattern. The method further includes repeating the positioning and forming steps a plurality of times until the full depth of the pattern has been formed at each of the plurality of locations.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/876,757, filed Dec. 22, 2006, entitled “Step-Down Plunge Electrodischarge Machining,” the disclosure of which is hereby incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    The invention relates generally to electrodischarge machining (EDM) processes. More particularly, the invention relates to plunge EDM for making a honeycomb extrusion die. 
         [0003]    Honeycomb bodies used in particulate filtration applications, such as diesel exhaust filtration applications, consist of a monolith body having longitudinal, parallel channels defined by longitudinal interconnected webs. The honeycomb bodies are typically made by extrusion from a ceramic material such as cordierite or silicon carbide. Extrusion dies used in making the honeycomb bodies have a die body with a discharge end including an array of longitudinal pins defined by interconnected slots. The array of longitudinal pins may include pins having any geometry useful in particulate filtration applications, such as square, triangle, or hexagon. The inlet end of the die body includes feedholes, which extend from the base of the die body to the slots and are used to supply batch material to the slots. To make a honeycomb body using the extrusion die, batch material is supplied to the feedholes and extruded through the slots. The batch material extruded through the slots form the interconnected webs of the honeycomb body. 
         [0004]    Honeycomb extrusion dies are commonly made by plunge EDM. In a typical plunge EDM process, a shaped electrode having the desired pin/slot pattern is placed in contact with a workpiece. A voltage is applied across the shaped electrode and the workpiece to cause current to flow between them through a fluid electrolyte. In some processes, the shaped electrode acts as a cathode and the workpiece acts as an anode; in other processes, and depending under some circumstances on the composition of the electrode, the current flow may be reversed. The pin/slot pattern is formed in the workpiece by a series of repetitive electrical charges discharged in a thin gap between the shaped electrode and the workpiece. The electrical charges generate enough heat to melt the workpiece and transfer the pin/slot pattern to the workpiece. While machining the workpiece, the workpiece is immersed in dielectric fluid, which acts as a conductor for the electrical charges and at the same time insulates the shaped electrode from the workpiece. The dielectric fluid also serves as a coolant and is used to flush machined chips out of the thin gap between the shaped electrode and the workpiece. 
         [0005]    While plunge EDM lends itself well to making machining pins and slots of various shapes and sizes within the same die body, a significant amount of variation in slot width and pin size within a machined die body has also been observed. This may be due to inadequate flushing of machined chips while forming the pin/slot pattern. Unflushed chips in a machining zone of the workpiece can create secondary discharges that could affect the width of the slots and size of the pins formed in that zone. Furthermore, the amount of unflushed chips varies from one machining zone to the next so that variation in slot width and pin size is not uniform across the workpiece, making it difficult to compensate for the effect of secondary discharges on slot width and pin size. Variation in slot width and pin size across the extrusion die translates to variation in web thickness and cell size across the honeycomb body formed with the extrusion die. Significant variation in web thickness and cell size across the honeycomb body can create an undesired thermal distribution profile and/or flow distribution profile in the honeycomb body during subsequent processing or use of the honeycomb body. 
         [0006]    Thus there is a desire for a plunge EDM process that forms patterns in a workpiece with uniform variation in pattern size across the workpiece, wherein the variation can be controlled to meet manufacturing tolerances. 
       SUMMARY 
       [0007]    In one aspect, the invention relates to a method of machining a pattern at a plurality of locations in a workpiece which comprises positioning a shaped electrode including the pattern at the plurality of locations in a randomized sequence, forming the pattern at each of the plurality of locations by passing electrical charges repeatedly between the shaped electrode and the workpiece and advancing the shaped electrode into the workpiece for a fraction of a full depth of the pattern, and repeating the positioning and forming steps a plurality of times until the full depth of the pattern has been formed at each of the plurality of locations. 
         [0008]    In another aspect, the invention relates to a method of plunge electrodischarge machining a pattern at multiple plunge locations in a workpiece which comprises determining a full depth of the pattern to be machined at the multiple plunge locations, selecting the total number of plunge steps to machine the pattern to the full depth, and determining a plunge depth for each plunge step as a fraction of the full depth. For each plunge step, the method includes randomly generating a sequence of plunge locations and machining the pattern in the workpiece at the plunge locations to the plunge depth associated with the plunge step according to the sequence of plunge locations. 
         [0009]    In another aspect, the invention relates to a plunge electrodischarge machining system which comprises a first fixture in which at least one shaped electrode having a pattern is arranged, a second fixture adapted for supporting a workpiece in opposing relation to the shaped electrode, a positioning device coupled to the shaped electrode, and a power supply configured to apply a voltage across the shaped electrode and the workpiece. The system further includes a control device which controls the positioning device such that the shaped electrode is repeatedly positioned at a plurality of locations in the workpiece in randomized sequences to form a fraction of a full depth of the pattern in the workpiece at the plurality of locations. 
         [0010]    Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0011]    The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
           [0012]      FIG. 1  is a simplified diagram of a plunge EDM system. 
           [0013]      FIG. 2  is a flowchart of a step-down plunge EDM process. 
           [0014]      FIG. 3A  depicts plunge locations on a workpiece. 
           [0015]      FIG. 3B  is a transverse cross-section of a pair of shaped electrodes. 
           [0016]      FIG. 4A  is a graph illustrating slot width variability produced by a step-down plunge EDM process. 
           [0017]      FIG. 4B  is a graph illustrating slot width variability produced by a one-shot plunge EDM process. 
           [0018]      FIG. 5  is a vertical cross-section of an extrusion die. 
       
    
    
     DETAILED DESCRIPTION  
       [0019]    The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements. 
         [0020]      FIG. 1  is a simplified diagram of a plunge EDM system  100  for machining patterns, such as slot and pin patterns, across a workpiece  102 . Typically, these patterns have features with high aspect ratio, but the system is not limited to features having a high aspect ratio. The system  100  includes a lower fixture  104  that holds the workpiece  102 . The system  100  further includes an upper fixture  106  arranged in opposing relation to the lower fixture  104 . The upper fixture  106  holds and supports one or more shaped electrodes  108  in opposing relation to the workpiece  102 . The shaped electrodes  108  may be held in the upper fixture  106  using any suitable means. For example, the shaped electrodes  108  could be inserted in apertures in the upper fixture  106  and fixed thereto by any suitable means, such as welds or threads. 
         [0021]    The upper fixture  106  is movable relative to the lower fixture  104  so that the shaped electrodes  108  can be positioned to machine features across the workpiece  102  and so that the shaped electrodes  108  can be advanced or plunged into the workpiece  102  as the features are machined to full depth. This could be implemented, for example, by coupling a positioning device  110 , such as an actuator(s) or translation stage(s), to the upper fixture  106 . In one example, the positioning device  110  provides motion along two orthogonal axes, e.g., in the x and y directions. This would allow lateral and vertical motion of the shaped electrodes  108  relative to the workpiece  102 . Alternatively, the positioning device  110  may provide motion along three orthogonal axes and may allow angular adjustment of the shaped electrodes  108  as necessary. Preferably, the positioning device  110  is controllable. Preferably, the positioning device  110  can position the shaped electrodes  108  across the workpiece  102  with high precision. It is also possible to couple a positioning device  112  to the lower fixture  104  so that the workpiece  102  is movable relative to the shaped electrodes  108 . In this case, the positioning device  110  need provide motion only in the vertical direction. 
         [0022]    The plunge EDM system  100  includes a power supply  113  for applying a voltage across the workpiece  102  and the shaped electrodes  108 . In the configuration illustrated in  FIG. 1 , the shaped electrodes  108  act as cathodes and the workpiece  102  acts as an anode. Preferably, the output of the power supply  113  is adjustable. The system  100  may include a control device  115  that determines the amount of voltage applied across the shaped electrodes  108  and the workpiece  102 . The voltage applied across a shaped electrode  108  and the workpiece  102  determines the electrical discharge between the shaped electrode  108  and the workpiece  102  and the size of the gap formed between the shaped electrode  108  and the workpiece  102 . The control device  115  may also control the positioning device  110  to move the shaped electrodes  108  to machine features at various locations in the workpiece  102 . The control device  115  may receive input from a position sensor  117  coupled to the upper fixture  106 , or from the positioning device  110 , and use the input to determine when features have been machined to full depth in the workpiece  102 . The control device  115  may include memory  115   a  for storing data and instructions, processor  115   b  for executing instructions, and input/output devices  115   c  for communicating with the various components of the plunge EDM system  100 . 
         [0023]    In the system illustrated in  FIG. 1 , the lower fixture  104  is provided with a fluid chamber  120  for holding dielectric fluid  124 . The plunge EDM system  100  may include an inlet line  122  through which dielectric fluid  124  from a dielectric fluid source  126  can be supplied into the chamber  120 . A pump  128  may be provided in the inlet line  122  to pump dielectric fluid  124  from the dielectric fluid source  126  into the chamber  120 . The system  100  may include a discharge line  130  through which dielectric fluid  124  can be withdrawn from the chamber  120  and returned to the dielectric fluid source  126 . A particulate filter  132  may be provided in the discharge line  130  to remove chips from the dielectric fluid  124  prior to returning the dielectric fluid  124  to the dielectric fluid source  126 . The lower fixture  104  supports the workpiece  102  in the chamber  120  such that the workpiece  102  is immersed in the dielectric fluid  124 . The dielectric fluid  124  is circulated through the chamber  120  while features are machined in the workpiece  102 . The dielectric fluid  124  conducts electrical charges from the shaped electrode  108  to the workpiece  102  while also insulating the shaped electrode  108  from the workpiece  102 . The dielectric fluid  124  also flushes chips from the gap formed between the workpiece  102  and the shaped electrode  108 . Although not shown in  FIG. 1 , system modifications are known that additionally provide for circulation of the dielectric fluid through channels in electrodes  108  as well as through open channels provided in workpiece  102 , such circulation being particularly effective to assist in the flushing of chips from that gap and from slot depressions being machined into the workpiece. 
         [0024]    The shaped electrodes  108  are positioned for machining features in the workpiece  102  through repetitive electrical charges discharged into a gap between the shaped electrodes  108  and the workpiece  102 . The shaped electrodes  108  are made of a conductive material. The workpiece  102  is also made of a conductive material. The material for the workpiece  102  may be selected based on the intended application of the machined workpiece. For example, for a workpiece  102  that is a blank for a honeycomb extrusion die, the workpiece  102  may be made of hard conductive materials, such as steel. The material of the shaped electrodes  108  may or may not be the same as the material of the workpiece  102  and need not be as hard as the material of the workpiece  102 . Typically, the shaped electrodes  108  are made of copper-tungsten, although other electrode materials such as graphite could alternatively be used. 
         [0025]    The shaped electrodes  108  include a pattern of the features to be formed in the workpiece  102 . For example, for a honeycomb extrusion die, the shaped electrodes  108  would include a honeycomb pattern or a portion of a honeycomb pattern. Each shaped electrode  108  may be configured to form a single feature, e.g., a pin or a slot, at a time or multiple features, e.g., rows and columns of pins and slots, at a time. In general, the shaped electrode  108  may be configured to form patterns with features of any desired shape. 
         [0026]    The plunge EDM process is a step-down process whereby multiple plunge steps in randomized sequences are used to machine features to full depth across a workpiece.  FIG. 2  illustrates the step-down plunge EDM process. First, the full depth of the pattern to be formed in the workpiece is determined ( 200 ). Next, the number of total number of plunge steps desired to machine each pattern to full depth is selected ( 202 ). This total number should be at least two, preferably greater than two, more preferably greater than five. Next, the plunge depth for each plunge step is determined ( 204 ). The plunge depth is a fraction of the full depth of the pattern that the shaped electrode would machine in the workpiece during a plunge step. The plunge depth may or may not differ from one plunge step to the next. Next, the plunge step is set to 1 ( 206 ). Next, a random sequence of plunge locations is generated ( 208 ). The plunge locations are the locations on the workpiece where the shaped electrodes would form patterns in the workpiece. For the current plunge step, patterns are machined in the workpiece for the plunge depth associated with the current plunge step according to the generated plunge sequence ( 210 ). The next step is to check if the plunge step is equal to the total number of plunge steps ( 212 ). If the plunge step is equal to the total number of plunge steps, the process is terminated ( 214 ). If the plunge step is not equal to the total number of plunge steps, the plunge step is incremented by 1 and the process is repeated from step  208 . The step-down plunge EDM process may be provided as instructions that could be executed by the processor ( 115   b  in  FIG. 1 ). 
         [0027]    In one example, the pattern to be formed in a workpiece is an array of square pins and slots. The pattern is formed in the workpiece using the step-down plunge EDM process described above. The full depth of the pins and slots is 0.23 inches, and the selected number of plunge steps is 10.  FIG. 3A  shows the plunge locations on the workpiece  102 . There are eight paired plunge locations, numbered  1 - 8 .  FIG. 3B  shows the paired electrodes  108  for machining the pattern in the workpiece ( 102  in  FIG. 3A ). The electrodes  108  are separated by an electrode width w. The plunge locations  1 - 8  indicated in  FIG. 3A  each have a width equal to the electrode width w. The electrodes  108  are moved between the plunge locations  1 - 8  in  FIG. 3A  to machine the pattern, i.e., the pins and slots, in the workpiece. Table 1 below indicates plunge depth and plunge sequence for an exemplary step-down plunge machining process of this type. 
         [0028]      FIG. 4A  presents slot width variability data taken from a series of six machined workpieces produced using the step-down sequence of Table 1. The graph of  FIG. 4A  plots average slot width versus measurement location for a number of locations across the machined faces of the workpieces. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Plunge 
                 Depth, 
                   
               
               
                 Step 
                 Inches 
                 Plunge Location 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0.025 
                 1 
                 8 
                 3 
                 6 
                 2 
                 7 
                 4 
                 5 
               
               
                 2 
                 0.050 
                 5 
                 4 
                 7 
                 2 
                 6 
                 3 
                 8 
                 1 
               
               
                 3 
                 0.075 
                 6 
                 3 
                 8 
                 1 
                 5 
                 4 
                 7 
                 2 
               
               
                 4 
                 0.100 
                 2 
                 7 
                 4 
                 5 
                 1 
                 8 
                 3 
                 6 
               
               
                 5 
                 0.125 
                 8 
                 1 
                 6 
                 3 
                 7 
                 2 
                 5 
                 4 
               
               
                 6 
                 0.150 
                 4 
                 5 
                 2 
                 7 
                 3 
                 6 
                 1 
                 8 
               
               
                 7 
                 0.175 
                 3 
                 6 
                 1 
                 8 
                 4 
                 5 
                 2 
                 7 
               
               
                 8 
                 0.200 
                 7 
                 2 
                 5 
                 4 
                 8 
                 1 
                 6 
                 3 
               
               
                 9 
                 0.225 
                 1 
                 8 
                 3 
                 6 
                 2 
                 7 
                 4 
                 5 
               
               
                 10 
                 0.230 
                 5 
                 4 
                 7 
                 2 
                 6 
                 3 
                 8 
                 1 
               
               
                   
               
             
          
         
       
     
         [0029]    For comparison purposes, arrays of square pins and slots as described above were machined in a series of nine additional workpieces using a one-shot plunge EDM process according to the sequence shown in Table 2 below. In the one-shot plunge EDM process, the pins and slots are machined to full depth in one step instead of incrementally as in the step-down plunge EDM process.  FIG. 4B  is a graph plotting average slot width as a function of measurement location across the faces of the nine workpieces produced using the one-shot plunge EDM sequence reported in Table 2. The target slot width for both machining processes is 0.0162 inches. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 (COMPARATIVE EXAMPLES) 
               
             
          
           
               
                 Plunge 
                 Depth, 
                   
               
               
                 Step 
                 Inches 
                 Plunge Location 
               
               
                   
               
             
          
           
               
                 1 
                 0.230 
                 1 
                 8 
                 3 
                 6 
                 2 
                 7 
                 4 
                 5 
               
               
                   
               
             
          
         
       
     
         [0030]    As can be observed from the graphs in  FIGS. 4A and 4B , slot width across the workpiece machined by the step-down plunge EDM process is considerably more uniform than slot width across the workpiece machined by the one-shot plunge EDM process. This is because the step-down plunge EDM process is thought to insure more consistent machining conditions across the workpiece through randomization of the plunging sequence over partial slot depth increments instead of plunging to full depth with each pass. In a one-shot plunge EDM process such as outlined in Table 2, average slot widths in the first four plunge locations (Locations  1 ,  8 ,  3  and  6  positioned as in  FIG. 3A ) can differ considerably from average slot widths in the second four plunge locations (Locations  2 ,  7 ,  4  and  5  in  FIG. 3A ). This difference may be attributed at least in part to the different flushing conditions between the first four plunging locations and the second four plunging locations, since the first four locations are bounded by solid material during machining while the second four locations are not. Thus it may be more difficult to flush machined chips from the first four plunging locations, particularly with a narrow and deep design of the slots, than it would be to flush such machined chips from the second four plunging locations that are open to the previously plunged regions of the workpiece. 
         [0031]      FIG. 5  shows the workpiece  102  after forming pins  500  and slots  502  in the workpiece using a step-down plunge EDM process as described above. The pins  500  and slots  502  may be formed in the workpiece  102  in one or more phases. The first phase may be a roughing phase, where the pins  500  and slots  502  are machined to full depth using the step-down plunge EDM process. The second and subsequent phases may be finishing phases, where the pins  500  are finished to full depth. The step-down or one-shot plunge EDM process may be used for the finishing phases. To complete formation of an extrusion die, feedholes  504  can be formed in the workpiece  102 . The feedholes  504  would typically extend from the base  506  of the workpiece  102  to the slots  502  in order to allow extrudable batch material to be supplied to the slots  502 . The workpiece  102  with the pins  500 , slots  502 , and feedholes  504  may serve as a template for other honeycomb extrusion dies. For example, the pins  500  may be modified as necessary to achieve other geometries more suitable for a particular application. 
         [0032]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.