WO2007133258A2

WO2007133258A2 - Edm electrodes by solid free - form fabrication

Info

Publication number: WO2007133258A2
Application number: PCT/US2006/045485
Authority: WO
Inventors: Michael L. Rynerson; James Randall Gilmore; Jianxin Liu
Original assignee: The Ex One Company
Priority date: 2005-11-28
Filing date: 2006-11-27
Publication date: 2007-11-22
Also published as: EP1958292A2; WO2007133258A3

Abstract

Solid free-form fabrication is used to make EDM electrodes (40, 60, 70) having improved structural and/or performance features. Included among the features are flushing conduits (78) that terminate upon work surfaces (46, 66) upon which such termination was previously unavailable or upon porosity that itself terminates on a work surface (46, 66). Also included among such features are conduit wall protrusions (96) which reduce or eliminate sprues on the workpiece surface and the inclusion of chambers (64) that reduce the amount of material needed to make the EDM electrode (40, 60, 70).

Description

Title: EDM Electrodes by Solid Free-Form Fabrication

Inventors: Michael L. Rynerson, James Randall Gilmore, and Jianxin Liu

Priority Claim: This application claims priority to U.S. Pat. Appl. No. 60/740,026, which was filed on

November 28, 2005, and is incorporated herein by reference in its entirety.

Technical Field:

The present invention relates to methods for producing EDM electrodes by solid free- form fabrication techniques.

Background Art:

Electrical discharge machining (hereinafter referred to as "EDM") is a method of machining wherein material is removed from an electrically conductive workpiece by a series of rapidly reoccurring electrical spark discharges between the workpiece and a forming electrode across a small gap filled with an electrically insulative liquid, i.e., a dielectric fluid. The forming electrode is referred to as the "EDM electrode." EDM is widely used in the aerospace, mold making, extrusion, and die casting industries for manufacturing molds for forming plastic materials, forging dies, extrusion dies, die casting dies, and for making engine components, such as compressor blades for jet turbine engines. EDM is capable of creating complex shapes with very high accuracy detail and controlled surface roughness finishes. EDM thermally affects only a very thin layer near the workpiece surface without affecting the metallurgical condition of the underlying material. EDM is used for roughing and finishing operations. In EDM, the material removal results from the erosive effect produced when electrical discharges, i.e., sparks, occur in the dielectric fluid filled, small gap between the electrically conductive workpiece and the EDM electrode. The dielectric fluid-filled gap between the workpiece and the EDM electrode is referred to hereinafter and in the appended claims as the "spark gap." The sparks occur in briefly existing plasma channels that form in the spark gap as the dielectric fluid is locally ionized by the voltage potential between the points of closest proximity of the surface of the workpiece and the EDM electrode. The spark gap between the workpiece and the EDM electrode is typically on the order of about 10 to about 100 microns. The voltage difference between the workpiece and the EDM electrode is usually in the range of about 30 to about 300 V and the electrical discharge usually lasts in the range of about 0.1 microseconds to about 8 milliseconds. Hundreds of thousands of sparks may occur each second and seem to form a cloud of sparks across the workpiece surface throughout the spark gap even though each spark occurs at a different instant in time. The temperature in the plasma channel of a spark reaches between about 8,000 and about 12,000 degrees Celsius. Proximity to this high temperature plasma channel along with resistive heating in the small area where the spark contacts the workpiece surface causes the workpiece surface to melt in that area to form a small molten pool. The electrical current is switched off briefly after the spark initiates. This causes the plasma channel across which the spark flowed to collapse which in turn causes a portion of the molten material to be ejected from the molten pool into the dielectric fluid. The ejection leaves a small crater on the surface of the workpiece. It is by the accumulation of uncountable numbers of such craters that the surface of the workpiece gradually erodes away to form the desired machined configuration that is a mirror image of the EDM electrode surface. The molten material that is ejected into dielectric fluid solidifies into small particles which are collectively referred to as swarf. The swarf must be flushed from the spark gap or it may cause an electrical short between the workpiece and the EDM electrode. Such shorts may ruin the surface of the workpiece and/or the EDM electrode.

Erosion also occurs on the work surface of the EDM electrode, i.e., the surface of the EDM electrode which is across the spark gap from the workpiece. However, the EDM electrode material and the electrical conditions are usually selected to minimize the erosion of the EDM electrode. Nonetheless, EDM electrode work surface wear is not insignificant and determines the effective lifetime of the EDM electrode.

There are two discrete types of EDM - wire EDM and sinker EDM. In wire EDM, the EDM electrode is a wire, usually a brass or copper wire. A wire EDM machine superficially resembles a bandsaw because the wire electrode is continuously fed between two guides as it is advanced into the workpiece. Unlike a bandsaw, however, the wire electrode never actually contacts the workpiece. The wire electrode is fed from a supply reel to a takeup reel so that the spark erosion of the workpiece is always being done by fresh, uneroded wire. The present invention is not concerned with this type of EDM.

The present invention is concerned with sinker EDM. Sinker EDM is also variously known in the art as "vertical EDM" and "ram EDM." All three of these names reflect the most commonly used arrangement of this type of EDM in which the EDM electrode is placed above the workpiece and is "sunk," "rammed," or "vertically" fed into the workpiece. Although neither this type of EDM nor the present invention is restricted to such a top - bottom juxtapositioning of the EDM electrode and workpiece nor to a strictly vertical direction of relative travel between the workpiece and the EDM electrode, for convenience of description, the conventional arrangement for this type of EDM is used herein as the model for describing the prior art and the present invention. Also for convenience of description, the term "EDM" is used hereinafter in this description and in the appended claims to refer only to this type of EDM to the exclusion of wire EDM.

FIG. 1 schematically illustrates a commonly used arrangement of EDM equipment. The EDM machine 2 shown comprises a pedestal station 4 and a command station 6. The pedestal station 4 includes a vertically positionable tool holder 8 which is mounted at the end of a servo-controlled feed mechanism 10. An EDM electrode 12 is in turn mounted on the bottom of the tool holder 8. Directly below the EDM electrode 12, a workpiece 14 is rigidly fixed in place upon a positionable work table 16 within a tub 18. The tub 18 contains a sufficient amount of a dielectric fluid 20 to completely submerge the workpiece 14. A reservoir 22 of dielectric electric fluid 20 is located in the bottom portion of the pedestal station 4. The dielectric fluid 20 is supplied from the reservoir 22 by a pump 24 through supply lines 26 and returns from the tub 18 to the reservoir 22 through a return line 28. A filter 30 is interposed between the reservoir 22 and the tub 18 to remove swarf and other contaminants from the dielectric fluid 20 before it enters or reenters the tub 18. The command station 6 includes a power supply 32 for providing the electrical power for the cutting process and a control unit 34 for controlling the cutting process.

FIG.l shows the EDM electrode 12 as being negatively charged with respect to the workpiece 14. Although this is a common arrangement, the opposite polarity arrangement is also sometimes used.

During operation of the EDM machine 2, the control unit 34 and the power supply 32 are activated. Activating the power supply 32 creates a voltage difference between the EDM electrode 12 and the workpiece 14. The EDM electrode 12 is advanced into the dielectric fluid 20 toward the workpiece 14 until electrical discharge sparking begins to occur between the two. The servo-controlled feed unit 10 actively adjusts the distance between the EDM electrode 12 and the workpiece 14 so that the electrical discharge sparking continues. As the workpiece 14 gradually erodes, the EDM electrode is advanced or "sunk" into the workpiece 14. When the machining operation is completed, the EDM electrode 12 is withdrawn leaving an exact negative duplicate of the EDM electrode 12 work surface imparted into the workpiece 14.

EDM electrodes may have complex geometries. Fabrication of EDM electrodes by conventional machining methods can be expensive and time consuming. Solid free-form fabrication methods have been used with some success to produce EDM electrodes. Solid free-form fabrication methods are also referred to generically as rapid prototyping methods.

Solid free-form fabrication methods involve creating either a three-dimensional article or a preform, pattern, or model of the article one layer at a time from an electronic representation of the article. Typically, the electronic representation is virtually sliced into a stack of thin layers. The bottom-most layer is made upon a supporting surface, then each successive layer is created upon a lower layer until the three-dimensional article or a preform, pattern, or model of the three-dimensional article has been formed. Where what is being formed is a preform of the article, post-forming operations such as sintering and/or infiltration are usually conducted to complete the article and to enhance its physical properties.

Prior art references teach or suggest various approaches to the application of solid free-form fabrication methods to the making of EDM electrodes. For example, European published patent application EP 0 649 695 Al of the National Research Council of Canada discloses using three-dimensional printing and other solid free-form fabrication methods to form a three-dimensional solid replica an EDM electrode, which may have a flushing hole on a work surface that is perpendicular to the direction of intended electrode travel, or feed direction, from a nonconductor or a conductor and then applying an electrically conductive layer to at least a part of the replica to create the work surface of the EDM electrode.

B. Yang et al., in an article entitled "Rapid Electroforming Tooling," Material Research Society Symposium Proceeding, Volume 625 (2000) 57-66, reports the use of stereolithography to create a master having the negative geometry of an EDM electrode, then metallizing a surface of the master, electroforming a copper shell onto the metallized surface, removing the copper shell, backfilling the obverse side with a molten metal, and solidifying the molten metal to complete the EDM electrode. F. Gillot et al. in an article entitled "Dimensional Accuracy Studies of Copper Shells

Used for Electo-Discharge Machining Electrodes Made with Rapid Prototyping and the Electroforming Process," Journal of Materials Processing Technology, 159 (2005) 33-39, first presents a survey of various automated layer-wise methods for making an EDM electrode directly and indirectly, and then reports results of using an indirect method that involved the electroplating of copper onto a positive form to create the work surface of the EDM electrode.

B. Stucker et al., in an article entitled "Zirconium Diboride/Copper EDM Electrodes from Selective Laser Sintering," Proceedings of the Solid Freeform Fabrication, 1997, pages 257-266, report using zirconium diboride powder coated with a thermoplastic binder to make EDM electrodes via the Selective Laser Sintering process followed by infiltration with a copper alloy. Similar teachings are contained in an article by H. Zaw et al. entitled, "Formation of a New EDM Electrode Material Using Sintering Techniques," Journal of Materials Processing Technology, 89-90 (1999) 182-186. Both the Brent Stucker et al. and H. Zaw et al. articles report that the performance of the EDM electrodes made by selective laser sintering was inferior to that of EDM electrodes of the same material made by other means.

H. Dϋrr et al., in an article entitled "Rapid Tooling of EDM Electrodes by Means of Selective Laser Sintering," Computers in Industry, 39 (1999) 35-45, also describes the use of selective laser sintering to make EDM electrodes. This article reports making the EDM electrodes from a powder mixture of bronze, nickel, and copper phosphate, wherein the copper phosphate interacts with the bronze as a low melting binder. EDM cutting tests were conducted with as-sintered electrodes and infiltrated electrodes using two infiltrants of different melting points. The electrode wear rate for each of these EDM electrodes was significantly higher than that of the reference solid copper EDM electrode. Also, the higher melting point infiltrant significantly improved the wear rate over the uninfiltrated electrode, but the lower melting point infiltrant significantly worsened the wear rate.

Y. Tang et al., in an article entitled "Laser Cladding of Copper-Based Materials for Building Electrical Discharge Machining Electrode," Materials and Design, 22 (2001) 669- 678, report the use of a laser cladding method for making EDM electrodes from copper/tungsten/nickel mixtures and copper/boron carbide mixtures. This article notes that the nickel in the former mixture created porosity that was detrimental to the EDM electrode performance. L. Lillander et al., in an article entitled "EDM Electrodes Made by RPT," European

Action on Rapid Prototyping Newsletter, No. 8, May 1996, pp. 3-5 and 8-9, observes that complex EDM electrodes need more frequent flushing than simpler EDM electrodes. This article suggests that SFFF should be able to be used to provide very efficient flushing through more complex channeling. However, the article does not describe such channeling and notes that more knowledge of EDM materials, flushing and EDM parameters will have to be developed before SFFF is capable of making EDM electrodes for engineering parts.

U.S. Pat. No. 5,728,345 of Hlavaty et al. discloses using a stereolithography process to make a cured resin model of an EDM electrode having a network of interconnected supporting members defining the shape of the electrode and leaving channels between the supporting members to drain away uncured resin from the model. After the model is fully cured and the uncured resin is drained away, a slurry mixture of graphite and resin is infiltrated into the voids of the model. The resin is then heat cured so that the resin is burned away leaving an electrode of pure carbon, or alternatively, the graphite and resin mixture is cured sufficiently to harden it and then an electrically conductive metallized coating is applied to the outer surface. Hlavaty et al., however, does not teach or suggest how to the stereolithography process can be used to include flushing channels in the resulting EDM electrode. Although solid free-form fabrication has been used to make conventional EDM electrodes, it has not been used to make EDM electrodes with improved structural and/or performance features.

Disclosure of the Invention: It is an object of the present invention to provide EDM electrodes with improved structural and/or performance features and methods for making such EDM electrodes. In particular, it is an object of the present invention to provide EDM electrodes having structures which provide improved capabilities for flushing the swarf from the spark gap.

It is also an objective of the present invention to provide EDM electrodes having reduced amounts of costly electrode material.

It is another objective of the present invention to provide EDM electrodes having flushing structures which reduce or eliminate the sprues that form during the EDM process. The term "sprue" is used herein to refer to the unmachined, pencil-like structures that remain on the workpiece surface after EDM machining. These structures form where flushing holes on the work surface of the EDM electrode leave behind areas of the workpiece surface that were not subjected to spark erosion.

It is a further objective of the present invention to utilize solid free-form fabrication methods to achieve the foregoing objectives. According to one aspect of the present invention, EDM electrodes are provided which have improved flushing capabilities. EDM electrodes produced by conventional machining methods are sometimes provided with drilled conduits that- conduct fresh dielectric fluid from a reservoir into the spark gap. The dielectric fluid typically enters the conduit from a non- work surface of the EDM electrode and exits the conduit through an orifice on a work surface that is perpendicular to the feed direction of the EDM electrode. Thus, conventional EDM electrodes may have inlets on their top surfaces and outlets on their bottom surfaces and drilled conduits connecting the inlets and outlets. The present invention, however, employs solid free-form fabrication methods to manufacture EDM electrodes which have dielectric fluid conduits which terminate on work surface locations which are unavailable for conduit termination placement by conventional fabrication methods. The term "unavailable" as used herein means that it would be at least impractical in terms of cost, time, or effort and at most impossible. For example, although it may be technically possible to place an exit on a small, angled work surface by conventional fabrication methods, the surface would still be unavailable for conduit termination placement if making such a placement would significantly increase the cost of the EDM electrode or the time to make the EDM electrode. In contrast, in some embodiments of the present invention, dielectric fluid conduits terminate at small angled work surfaces, which may be non-perpendicular to the cutting direction, without significantly affecting the cost, time, or effort needed to make the EDM electrode. Unavailable surfaces include portions of work surfaces which would not be accessible by line-of-sight hole forming techniques, e.g., mechanical or laser drilling, for creating dielectric fluid conducting conduits.

In some embodiments of the present invention, flushing is augmented by drawing swarf-laden dielectric fluid out of the spark gap through a conduit or conduits in the EDM electrode. Also, in some embodiments of the present invention dielectric fluid may be fed through one or more conduits in the EDM electrode into the spark gap through a region of the EDM electrode that contains fluid-conducting porosity which terminates on a work surface. In these embodiments, fresh dielectric fluid enters the spark gap via the porosity at the work surface of the EDM electrode. According to another aspect of the present invention, the EDM electrode is provided with one or more chambers whose combined volume is a significant percentage of the volume of the EDM electrode. Preferably, the combined volume of the chambers is in the range of about 50 to about 90 percent of the volume of the EDM electrode, and more preferably, in the range of about 75 to about 90 percent. The presence of these chambers significantly reduces the amount of costly electrode material contained in the EDM electrode. One or more of the chambers may be part of or connected to the conduits which conduct dielectric fluid through the EDM electrode. However, the chambers need not be so connected. Such chambers can be independent of other chambers and conduits. They also may be interconnected without being connected to or part of the dielectric-fluid conduits. Such chambers also may be provided as part of an internal structure that gives the EDM electrode a significantly improved strength to weight ratio. Such an internal structure may including ribbing, strutting, or a honeycomb arrangement of internal structural features. Some other embodiments of the present invention minimize the amount of electrode material contained in the EDM electrode. In these embodiments, the EDM electrode is made to net shape or to near-net shape wherein the shape is essentially that of a hollow article having the desired working surface and a region that is adapted to be held, directly or indirectly, by a tool holder. Such EDM electrodes may contain ribbing, strutting, or a honeycomb arrangement of structural features in the hollow region to provide structural integrity.

According to another aspect of the present invention the EDM electrode is provided with structures which reduce or eliminate the sprues that may be formed during EDM machining on the workpiece surface. In some preferred embodiments of the present invention, a dielectric fluid conducting conduit that terminates on a work surface has a wall protrusion that partially obstructs its termination orifice. The wall-protrusion may extend partway into the orifice or it may extend across the orifice. Multiple wall-protrusions may intersect each other. One or more wall protrusions may be used to make a cross-hair or a grid across the orifice. The wall protrusions may terminate within the conduit a preselected distance from the orifice. Such a termination may be abrupt or gradual. Wall-protrusions may be attached to a conduit wall along the full length of the conduit or only a part of its length. Wall protrusions may taper either away from or toward the conduit walls. A wall protrusion, being a part of the EDM electrode, spark erodes the workpiece in the vicinity of the orifice while allowing the dielectric fluid to flow through the orifice. In some preferred embodiments of the present invention, the structures which reduce or eliminate the sprues comprise a narrowed end of the conduit in the vicinity of its termination on a work surface. The present invention also comprises methods for producing EDM electrodes that achieve one or more of the foregoing objectives and produce the above described EDM electrodes. Although any SFFF method that involves layer-wise fabrication using a particulate material may be employed in the method embodiments of the present invention, selective laser sintering and three dimensional printing are the preferred SFFF methods.

In some of these method embodiments, the present invention includes the steps of: (1) creating an electronic representation of a preform of an EDM electrode, wherein the EDM electrode has a work surface, a non-work surface, and a conduit providing fluid communication between the non-work surface and a portion of the work surface that is unavailable for conduit termination orifice placement by conventional fabrication methods, the conduit terminating at an orifice on the work surface portion; (2) fabricating the preform in a layer-wise fashion from a particulate material using the electronic representation to guide the fabricating; and (3) infiltrating the preform with a molten metal. It is preferred that the particulate material used be a metal or other electrically conductive material, e.g., tungsten, and the infϊltrant be a metal, e.g., copper.

Some particularly preferred method embodiments of the present invention further include the steps of: (1) providing a tungsten powder; (2) providing a second powder, wherein the second powder is selected from the group consisting of a nickel powder, a cobalt powder, an iron powder, and combinations thereof; (3) mixing the tungsten powder with the second powder to make a mixture, wherein the second powder comprises between about 1 and about 10 percent of the mixture by weight; and (4) providing the mixture as at least a portion of the particulate material of the EDM electrode fabricating step. It is also preferred that the tungsten powder comprises a major weight fraction of a relatively coarser tungsten powder and a minor weight fraction of a relatively finer tungsten powder. Preferably, the step of infiltrating comprises infiltrating the preform with molten copper.

Some preferred embodiments of the present invention include the steps of: (1) creating an electronic representation of a preform of an EDM electrode, wherein the EDM electrode has a work surface, a non-work surface, a region of fluid-conducting porosity terminating at a work surface of the EDM electrode, and a conduit providing fluid communication between the non-work surface and the region of fluid-conducting porosity; (2) fabricating the preform in a layer-wise fashion from a particulate material using the electronic representation to guide the fabricating; and (3) infiltrating the preform with a molten metal. Some preferred embodiments of the present invention include the steps of: (1) creating an electronic representation of a preform of an EDM electrode, wherein the EDM electrode has a work surface, a non-work surface, and a conduit providing fluid communication between the non-work surface and the work surface, the conduit terminating at an orifice on the work surface and having a sprue-reducing wall protrusion partially obstructing the orifice; (2) fabricating the preform in a layer-wise fashion from a particulate material using the electronic representation to guide the fabricating; and (3) infiltrating the preform with a molten metal.

The present invention also includes among its embodiments methods which include a step of using the inventive EDM electrodes disclosed herein to EDM machine workpieces.

Brief Description of the Drawings:

The criticality of the features and merits of the present invention will be better understood by reference to the attached drawings. It is to be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the present invention.

FIG. 1 is a schematic elevational view of a prior art EDM machine.

FIG. 2 is a schematic cross sectional view of a hollow EDM electrode embodiment of the present invention.

FIG. 3 is a schematic cross sectional view of a reinforced hollow EDM electrode embodiment of the present invention.

FIG. 4 is a schematic cross sectional view of an EDM electrode embodiment of the present invention having flushing conduits terminating on portions of the EDM electrode's work surface which are unavailable for conduit termination placement by conventional means.

FIGS. 5 A and 5 B are, respectively, schematic plan and cross sectional views of an EDM electrode embodiment of the present invention illustrating conduit wall protrusions which act to reduce or eliminate the formation of sprues on the workpiece surface.

Modes for Carrying Out the Invention:

Some embodiments of the present invention employ SFFF methods to produce EDM electrodes that reduce or minimize the amount of electrode material necessary for the fabrication of the EDM electrodes. These SFFF methods accomplish this by making the EDM electrodes as a net or near-net shape article or preform. This is an important economic factor when the EDM electrode material is costly, e.g., tungsten.

For example, referring to FIG. 2, there is shown a cross sectional view of a hollow EDM electrode 40 according to an embodiment of the present invention. If viewed from the top, the EDM electrode 40 would be seen to have a circular shape. The EDM electrode 40 has an outer surface 42 and an inner surface 44. The EDM electrode 40 is designed to be advanced into a workpiece up to the points A. Thus, the portion of the outer surface 42 below points A is the work surface 46 of the EDM electrode 40. All or some of the portion of the outer surface 42 above points A may serve as a tool holder region 48. The wall thickness T at the work surface 46 region of the EDM electrode 40 is chosen to be thick enough to give the EDM electrode 40 a preselected useful life taking into consideration the type of workpiece that the EDM electrode 40 is to be used to machine, the operational parameters that will be used during the machining, and the anticipated wear rate of the work surface 46. The thickness T is also chosen to provide the EDM electrode 40 with sufficient structural integrity to maintain its shape to the end of its useful life. Although no flushing conduits which terminate on the work surface 46 are shown in FIG. 2, such conduits are preferably included.

Referring to FIG. 3, there is shown a cross section view of a hollow EDM electrode 60 according to another embodiment of the present invention. The EDM electrode 60 is similar to the EDM electrode 40 described above except that the EDM electrode 60 includes a honeycomb arrangement of struts 62 that provide additional structural integrity to the EDM electrode 60 and form chambers 64 within the EDM electrode 60. Although no flushing conduits which terminate on the work surface 66 are shown in FIG. 3, such conduits are preferably included.

Some embodiments of the present invention have improved capabilities for flushing the swarf from the spark gap. In some cases the improvement is made by providing the EDM electrode with dielectric fluid conduits which terminate on work surface locations of the EDM electrode which are unavailable for conduit termination placement by conventional fabrication techniques. An example of such an embodiment is shown in FIG. 4 which shows a cross section of an EDM electrode 70 according to an embodiment of the present invention. If viewed from the top, the EDM electrode 70 would be seen to have a circular shape. The EDM electrode 70 comprises a work surface 72, a tool holder region 74, a plenum 76, and dielectric fluid conduits 78. The conduits 78 begin at plenum 76 and terminate on work surface 72. Some of the conduit terminations, e.g., conduit termination 80, are at work surface locations which are unavailable for conduit termination placement by conventional means. During operation, the EDM electrode 70 is gripped by a tool holder at tool holder region 74. Dielectric fluid enters into plenum 76 through the tool holder and flows into the conduits 78 and out through the conduit terminations, e.g., termination 80, to flush swarf from out of the spark gap around the work surface 72.

The present invention also includes embodiments which have structures which reduce or eliminate the sprues that may form on the workpiece surface during machining. One such structure is a narrowed end of a dielectric fluid conduit in the vicinity of its termination on a work surface. Such a structure is shown in FIG. 4, wherein conduits 78 have narrowed ends, e.g., narrowed end 82, in the vicinity of their termination on work surface 72. Also referring to FIG. 4, it should be understood that the placement of the conduits 78 on portions of the work surface 72 that are not perpendicular to the feed direction 84 of the EDM electrode 70 will tend to reduce or eliminate sprue formation. For example, the uninterrupted portion 86 of work surface 72 as it follows conduit 88 into the work piece will tend to cut away at any sprue that may have been left by conduit 88.

FIGS. 5 A and 5 B illustrate another embodiment of the present invention which has structures for reducing or eliminating sprues. FIG. 5A shows a plan view of a portion of a work surface 92 in the vicinity of a conduit 94. Wall protrusions 96 extend from the conduit wall 98 to form a cross-hair grid across the end of the conduit 94. During EDM machining, the wall protrusions 96 cut away material that would otherwise be left as sprue material on the surface of the workpiece. FIG. 5B shows a cross-sectional view of conduit 94 and illustrates that in this particular embodiment of the present invention the wall protrusions 96 extend along only a portion of the conduit wall 98, thus minimizing its effect on the flow rate of dielectric fluid through conduit 94. The dashed lines 100 in FIG. 5B are meant to indicate the location the conduit wall 98 would have had if the wall protrusions 96 were not present.

In some embodiments of the present invention dielectric fluid may be fed through one or more conduits in the EDM electrode into the spark gap through a region of the EDM electrode that contains fluid-conducting porosity which terminates on a work surface. In these embodiments, fresh dielectric fluid enters the spark gap via the porosity at the work surface of the EDM electrode. The fluid conducting porosity may be retained in preselected areas of the EDM electrode despite the molten metal infiltration of other areas of the EDM electrode through the use of infiltration stilting techniques which are well known in the art. In embodiments that utilize three-dimensional printing to create the EDM electrode preform, the techniques described in U.S. Pat. No. 5,775,402 of Sachs et al. of utilizing infiltration stop materials in appropriate areas to prevent infiltration may also be employed to maintain the fluid-conducting porosity in preselected areas of the EDM electrode.

While only a few embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that many changes and modifications may be made thereunto without departing from the spirit and scope of the present invention as described in the following claims. All United States patents and patent applications cited herein are incorporated herein by reference in their entireties.

Claims

What is claimed is:

L A method comprising the steps of: a) creating an electronic representation of a preform of an EDM electrode, wherein said EDM electrode has: i) a work surface; ii) a non-work surface; and iii) a conduit providing fluid communication between said non-work surface and said work surface of said EDM electrode, said conduit terminating at an orifice on said work surface and having a sprue-reducing wall protrusion; b) fabricating said preform in a layer-wise fashion from a particulate material using said electronic representation to guide said fabricating; and c) infiltrating said preform with a molten metal.

2. The method of claim 1, wherein said conduit has a plurality of sprue-reducing wall protrusions and said plurality form a grid across said orifice.

3. The method of claim 1, further comprising the steps of: d) providing a tungsten powder; e) providing a second powder, wherein said second powder is selected from a group consisting of a nickel powder, a cobalt powder, an iron powder, and mixtures thereof; f) mixing said tungsten powder with said second powder to make a mixture, wherein said second powder comprises between about 1 and about 10 percent of said mixture by weight; and g) providing said mixture as at least a portion of said particulate material of said fabricating step.

4. The method of claim 3, wherein said tungsten powder comprises a major weight fraction of a relatively coarser tungsten powder and a minor weight fraction of a relatively finer tungsten powder.

5. The method of claim 1, wherein said step of infiltrating comprises infiltrating said preform with molten copper.

6. The method of claim 1, wherein said EDM electrode has one or more chambers and the combined volume of said one or more chambers is in the range of about 50 to about 90 percent of the volume of said EDM electrode.

7. The method of claim 6, wherein said combined volume of said one or more chambers is in the range of about 75 to about 90 percent of said volume of said EDM electrode.

8. The method of claim 1, wherein said EDM electrode has a second conduit, said second conduit providing fluid communication between said non-work surface and said work surface of said EDM electrode, the method further comprising the steps of: h) machining a workpiece with said EDM electrode in a dielectric fluid, wherein there is a spark gap between said workpiece and said EDM electrode during said machining; and i) injecting said dielectric fluid into said spark gap during said step of machining through at least one of said conduit and said second conduit.

9. The method of claim 8, further comprising the step of extracting said dielectric fluid from said spark gap during said step of machining through at least one of said conduit and said second conduit.

10. The method of claim 1, wherein said step of fabricating includes fabricating said preform using one selected from a group consisting of three-dimensional printing and selective laser sintering.

11. The method of claim 1, wherein at least a portion of said work surface is not perpendicular to an intended feed direction of said EDM electrode and said conduit terminates on said portion of said work surface.

12. A method comprising the steps of: a) creating an electronic representation of a preform of an EDM electrode, wherein said EDM electrode has i) a work surface; ii) a non-work surface; iii) a region of dielectric fluid-conducting porosity, said region terminating at said work surface of said EDM electrode; and iv) a conduit providing fluid communication between said non-work surface and said region of dielectric fluid-conducting porosity; b) fabricating said preform in a layer-wise fashion from a particulate material using said electronic representation to guide said fabricating; and c) infiltrating said preform with a molten metal.

13. The method of claim 12, further comprising the steps of: d) providing a tungsten powder; e) providing a second powder, wherein said second powder is selected from a group consisting of a nickel powder, a cobalt powder, an iron powder, and mixtures thereof; f) mixing said tungsten powder with said second powder to make a mixture, wherein said second powder comprises between about 1 and about 10 percent of said mixture by weight; and g) providing said mixture as at least a portion of said particulate material of said fabricating step.

14. The method of claim 13, wherein said tungsten powder comprises a major weight fraction of a relatively coarser tungsten powder and a minor weight fraction of a relatively finer tungsten powder.

15. The method of claim 12, wherein said step of infiltrating comprises infiltrating said preform with molten copper.

16. The method of claim 12, wherein said EDM electrode has one or more chambers and the combined volume of said one or more chambers is in the range of about 50 to about 90 percent of the volume of said EDM electrode.

17. The method of claim 16, wherein said combined volume of said one or more chambers is in the range of about 75 to about 90 percent of said volume of said EDM electrode.

18. The method of claim 12, wherein said EDM electrode has a second conduit, said second conduit providing fluid communication between said non-work surface and said work surface of said EDM electrode, the method further comprising the steps of: h) machining a workpiece with said EDM electrode in a dielectric fluid, wherein there is a spark gap between said workpiece and said EDM electrode during said machining; and i) injecting said dielectric fluid into said spark gap during said step of machining through at least one of said conduit and said second conduit.

19. The method of claim 18, further comprising the step of extracting said dielectric fluid from said spark gap during said step of machining through at least one of said conduit and said second conduit.

20. The method of claim 12, wherein said step of fabricating includes fabricating said preform using one selected from a group consisting of three-dimensional printing and selective laser sintering.

21. The method of claim 12, wherein at least a portion of said work surface is not perpendicular to an intended feed direction of said EDM electrode and said region of dielectric fluid-conducting porosity terminates on said portion of said work surface.

22. A method comprising the steps of: a) creating an electronic representation of a preform of an EDM electrode, wherein said EDM electrode has: i) a work surface; ii) a non-work surface; and iii) a conduit providing fluid communication between said non-work surface and a portion of said work surface that is unavailable for conduit termination orifice placement by conventional fabrication methods, said conduit terminating at an orifice on said work surface portion; b) fabricating said preform in a layer-wise fashion from a particulate material using said electronic representation to guide said fabricating; and c) infiltrating said preform with a molten metal.

23. The method of claim 22, wherein said portion is not accessible for making a conduit by line-of-sight hole forming techniques.

24. The method of claim 22, further comprising the steps of: d) providing a tungsten powder; e) providing a second powder, wherein said second powder is selected from a group consisting of a nickel powder, a cobalt powder, an iron powder, and mixtures thereof; f) mixing said tungsten powder with said second powder to make a mixture, wherein said second powder comprises between about 1 and about 10 percent of said mixture by weight; and g) providing said mixture as at least a portion of said particulate material of said fabricating step.

25. The method of claim 24, wherein said tungsten powder comprises a major weight fraction of a relatively coarser tungsten powder and a minor weight fraction of a relatively finer tungsten powder.

26. The method of claim 22, wherein said step of infiltrating comprises infiltrating said preform with molten copper.

27. The method of claim 22, wherein said EDM electrode has one or more chambers and the combined volume of said one or more chambers is in the range of about 50 to about 90 percent of the volume of said EDM electrode.

28. The method of claim 27, wherein said combined volume of said one or more chambers is in the range of about 75 to about 90 percent of said volume of said EDM electrode.

29. The method of claim 22, wherein said EDM electrode has a second conduit, said second conduit providing fluid communication between said non-work surface and said work surface of said EDM electrode, the method further comprising the steps of: h) machining a workpiece with said EDM electrode in a dielectric fluid, wherein there is a spark gap between said workpiece and said EDM electrode during said machining; and i) injecting said dielectric fluid into said spark gap during said step of machining through at least one of said conduit and said second conduit.

30. The method of claim 29, further comprising the step of extracting said dielectric fluid from said spark gap during said step of machining through at least one of said conduit and said second conduit.

31. The method of claim 22, wherein said step of fabricating includes fabricating said preform using one selected from a group consisting of three-dimensional printing and selective laser sintering.

32. The method of claim 22, wherein at least a portion of said work surface is not perpendicular to an intended feed direction of said EDM electrode and said conduit terminates on said portion of said work surface.

33. An EDM electrode fabricated by the method of claim 1.

34. An EDM electrode fabricated by the method of claim 12.

35. An EDM electrode fabricated by the method of claim 22.

36. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 33.

37. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 34.

38. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 35.

39. An EDM electrode comprising: a) a work surface b) a non-work surface; and c) a conduit providing fluid communication between said non-work surface and said work surface of said EDM electrode, said conduit terminating at an orifice on said work surface and having a sprue-reducing wall protrusion.

40. The EDM electrode of claim 39, wherein said conduit has a plurality of sprue-reducing wall protrusions and at least some of the wall protrusions of said plurality intersect to form a grid across said orifice.

41. An EDM electrode comprising: a) a work surface b) a non- work surface; c) a region of dielectric fluid-conducting porosity, said region terminating at said work surface of said EDM electrode; and d) a conduit providing fluid communication between said non-work surface and said region of dielectric fluid-conducting porosity.

42. An EDM electrode comprising: a) a work surface b) a non-work surface; c) a non-work surface; and d) a conduit providing fluid communication between said non-work surface and a portion of said work surface that is unavailable for conduit termination orifice placement by conventional fabrication methods, said conduit terminating at an orifice on said portion;

43. The EDM electrode of claim 42, wherein said portion is not accessible for making a conduit by line-of-sight hole forming techniques.

44. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 39.

45. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 40.

46. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 41.

47. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 42.

48. A method comprising the step of EDM machining a workpiece using the EDM electrode described in claim 43.