US 3585342 A
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United States Patent John C. Kosoo St. Marys, Pa.
June 3, 1970 June 15, 1971 Stackpole Carbon Company St. Marys, Pa.
Inventor App]. No. Filed Patented Assignee EDM ELECTRODE 3 Claims, No Drawings 11.8. CI 2l9/69E, 2l9/145,29/l82.2
Int.Cl 823k 9/16 Field ofSearc 219/69 E, 145; 204/293, 294; 29/1822, 182.5; 252/503;
Primary Examiner-J. V. Truhe Assistant Examiner-Gale R. Peterson Attorney-Brown, Murray, Flick and Peckham ABSTRACT: Electrodes for EDM (electric discharge machining) of improved wear performance are provided by consolidated bodies of, by volume, 50 to 90 percent of graphite with the remainder consisting essentially of chromium-iron alloy containing from about one to about 99 volume percent of chromium.
EDM ELECTRODE In the EDM process shapes are produced, as is well known, by striking an are between an electrode and a metallic workpiece, both of which are immersed in a dielectric fluid. When the system is operating properly, i.e., when properly biased, melting occurs primarily at the workpiece and small molten droplets are discharged from it into the fluid. In this way there is formed in the workpiece a female cavity which reproduces the male electrode form.
A variety of difl'erent materials have been used as EDM electrodes. Commonly copper, brass, copper-tungsten alloys, silver-tungsten alloys, graphite, and copper impregnated graphite have been used as such electrodes.
The specific electrode material used in a given case is dictated by the surface finish and tolerance required of the cavity produced in the workpiece and other factors involved in electrode selection are ease of its machinability and the economics of the operation.
In the EDM operation the electric discharge of an are between an anode and a cathode results in wear, not only of the workpiece but also of the electrode. To maintain permissible tolerances and to minimize electrode useage, it is desirable to achieve maximum wear ratio, i.e., ratio of workpiece wear to electrode wear. Other factors being equal, the wear ratio is favorably influenced by high melting point, the high conductivity, and high strength of the electrode.
A major object of the present invention is to provide EDM electrodes that accomplish maximum ratio of workpiece wear to electrode wear, which are readily made by well-known powder metallurgy procedures, which possess the desirable easy machinability of graphite while at the same time supplying graphite-metal composites without the necessity of resorting to undesirably expensive liquid metal impregnation presently used in making metal impregnated graphite EDM electrodes.
The EDM electrodes in accordance with this invention comprise from about 50 to 90 percent by volume of graphite with the remainder consisting essentially of, by volume, about I to 99 percent of chromium and the remainder iron except for incidental impurities that exert no unfavorable action upon the electrodes in use.
The electrodes provided by this invention are made by standard powder metallurgical procedures. That is, graphite, iron and chromium in powder form are blended together with a binder, such as a phenolic resin although a variety of other binders are known'and may be used. The blended mixture is compacted under a pressure that, as is well known in the art, will depend upon the particular composition of the blend but it ordinarily will range from about to about 40 psi. to form an electrode blank of the desired size and shape. The blank is then sintered by firing in a nonoxidizing atmosphere or in vacuum at temperatures of the order 1 100 to 1800 C. for a time that is primarily dependent upon the size of the blank.
Experience with the invention has shown that the fired products are characterized by strengths 1.5 to 4 times higher than most graphites, with the strength increasing with the metal content. Additionally, electrical resistivity is of the order of 5 to times lower than conventional EDM graphite materials.
Typical comparative properties of electrodes made in accordance with this invention are given in the following table:
TABLE I.ELECTRICAL RESISTIVITY AND STRENGTH FOR VARIOUS EDM MATERIALS I now believe that the inclusion of chromium in these new electrodes serves several important purposes. For one, firing at the elevated temperature range ,stated results in the production of a chromium carbide which wets the graphite but does not exude from the surface. The high melting, refractory nature of the carbide formed and its presence in the electrode is believed likewise to improve the EDM characteristics as compared with graphite compositions containing metallic chromium alone.
Electrodes made in accordance with this invention have been used in cutting tests carried out on hardened tool steel, nickel alloy, and WC-Co workpieces as well as with several commercial EDM electrode materials for purposes of comparison. The results of these tests are given in the following table:
TABLE IL-EDM WEAR RATIOS FOR VARIOUS WORK- PIECES--ELEOTRODE COMBINATION Percentages by volume.
As will be understood from the foregoing test data the graphite-chromium-iron alloy EDM compositions of this invention are shown to be very good EDM electrode materials with cutting results comparable or better than had with copper impregnated graphite electrodes, which presently is a premium EDM electrode material. The new graphite EDM electrodes of this invention are thus shown also to provide favorable wear ratios, high strengths, and low electrical resistivity when compared with standard graphite EDM grades. A further and important aspect of the invention is the avoidance of expensive metal impregnation procedures coupled with excellent machinability of my new electrodes.
1. An EDM electrode consisting essentially of a sintered body of, by volume, 50 to percent of graphite and the remainder iron and chromium, the chromium constituting from 1 to 99 percent of the remainder.
2. Am EDM electrode according to claim 1, the body of the electrode containing chromium carbide reaction product of said chromium and said graphite formed during firing of the compact.
3. An EDM electrode consisting essentially of a sintered body of, by volume, 80 percent of graphite and the remainder about percent of ion and about 10 percent of chromium.