ELECTRIC SPARK MACHINING ELECTRODE AND METHOD OF PRODUCTION THEREOF

Abstract: An electrode for engraving electric spark machining which is produced by using a mixture of gypsum hemihydrate powder serving as both a principal material and a binder with a conductive metal powder as a filler, transferring, shaping and hardening the mixture into a desired shape to form electrode material and plating the electrode with the conductive metal. Gypsum hemihydrate powder, gypsum dihydrate powder of less than 1 vol % on the basis of the former and the conductive metal powder are kneaded in a vacuum. Water is added to the kneaded matter, which is again kneaded in a vacuum, poured into a mold to which a predetermined shape is transferred, shaped and hardened by hydration to obtain an electrode material. Then the electrode material is dried and plated with the conductive metal so as to reinforce electrical connection with the metal powder. Since the electrode is made of gypsum as the principal material, even an electrode for electric spark machining to which a precise and complicated shape is transferred can be produced easily, economically, and precisely by the use of a simple mold of, e.g. a silicone rubber, and hence it is particularly suitable for producing dental crowns one by one. Furthermore, gypsum hemihydrate as the principal body of the electrode material and the conductive metal film increase the strength of the electrode by virtue of their synergistic effect. The present invention makes possible machining of a hard material such as titanium for which precise electric spark machining at a desired speed has been difficult by the use of a conventional electrode subjected to impregnation treatment of a conductive solution in which graphite or the like is dispersed in place of plating treatment.

Description: [Technical Field] This invention relates to an electrode for electrical discharge machining, particularly an electrode for electrical discharge used for diesinking and to a process for manufacturing the same. [Background Art] (Prior Art) As the electrode for electrical discharge machining, those comprising graphite/Be/copper alloys processed by a three-dimensional machine tool are used. However, those electrodes for electrical discharge machining which primarily comprises graphite etc. suffer problems that they not only require considerable time for the machining with poor accuracy, but also they require expensive machinery, leading to rise in cost. Under such circumstances, the present inventors have developed an electrode for electrical discharge machining, which comprises a major body obtained by subjecting a binder of gypsum hemihydrate powder and a filler of conductive metal powder to molding and setting into a form of electrode, impregnated with a conductive solution to enhance continuity in the former conductive metal powder and filed a patent application as Japanese Patent Application No. 46069/1989. The JP-A 58-66630 describes a process for obtaining an electric discharge electrode, wherein an aluminium alloy is poured into a mold made of gypsum. The solidified aluminium alloy constitutes the electric discharge electrode main unit, to a part of which copper is then diffusibly infiltrated. To a surface of the thus formed diffusibly infiltrated layer, copper is further applied by plating or electroforming thereby forming an adherent layer. The manufacturing method of electric discharge electrodes as described in the JP-A 57 27623 differs in the fact that after having obtained a gypsum female form, the surface of that female form is plated with a metal and a lead is connected to this plating. The female form is then filled with epoxy resin or metallic material resin forming the core of the electrode. (Problems to be Solved by the Invention) Nevertheless, in an attempt of machining various types of products by means of electrical discharge machining using the electrode as disclosed in the previously filed Patent Application, problem has arisen that if such electrode is used as such, its durability is questionable due to the brittleness of the material. Besides, if such electrode is used for electrical discharge machining of a dental crown and bridge comprising a titanium material or for such machining of other hard materials into a complicated shape, it failed to attain machining with sufficient accuracy. Thus, improvements in the above points have been desired. (Means for Solving the Problems) This invention has been accomplished with a view to providing an electrode for electrical discharge machining used for diesinking having overcome the above problems and a process for manufacturing the same; which electrode comprises an electrode member produced by subjecting a base and binder of gypsum hemihydrate powder and a filler of conductive metal powder to modeling and/or molding and setting into a desired form of electrode, followed by plating the electrode member with a conductive metal to enhance continuity in the conductivity of the conductive metal powder particles dispersed as a filler in the electrode member and also forming a thin metal layer on the surface thereof. In the electrode according to this invention, the base gypsum hemihydrate powder, which is a material for imparting moldability and strength, also serves as a binder, and an α-type gypsum hemihydrate which is commercially available as a dental stone is preferably used since α-type gypsum hemihydrate has a characteristic that the expansion by the crystal growth of the gypsum dihydrate during the process of hydration and setting is greater than the contraction concomitantly occurring therewith, so that a molded product accurately modeled from the die face can be obtained without suffering contraction or distortion. Further, addition of a very small amount of gypsum dihydrate powder to serve as crystalline nucleus to the gypsum hemihydrate powder is particularly preferred since formation of needle crystals is accelerated when the gypsum hemihydrate is hydrated and set and the particles of the conductive metal powder can be dispersed to fill the gaps between the crystals. On the other hand, as the conductive metal powder to be blended with the gypsum hemihydrate powder, while copper, titanium, tungsten, nickel, brass, phosphorus bronze and lead can be mentioned, copper is preferred in view of feasibility of molding the electrode member, conductivity and cost, particularly preferably as a mixture of two copper powders which are different in the shape and particle size. It should be noted that the mixing ratio of the gypsum hemihydrate powder to the conductive metal powder is preferably 55 to 45 : 45 to 55 by volume in case of copper powder. The reason is that sufficient strength cannot be obtained in the molded electrode and further the electrode comes to have rough surface to have poor molding accuracy if the amount of the gypsum powder is less than 40 % and that molding procedures will be difficult if said amount is less than 30 %; whereas conductivity is lowered if said amount is more than 60 %. Since the electrode of this invention is required to have an ability of accurately machining a subtle shape of object such as a dental crown and bridge, it is primarily necessary to be produced with excellent moldability and have strength as well as to have excellent conductivity. The electrode of this invention can be prepared by adding a suitable amount of water, an accelerator for plaster, etc. to a material comprising a mixture of a gypsum hemihydrate powder and a conductive metal powder; kneading the resulting mixture with deaeration, usually in vacuo; modeling and/or molding the kneaded mixture by a method like casting into a desired shape using an inexpensive and easily operable mold made of a silicone rubber and the like to provide an electrode member; drying the electrode member; and subjecting the dried electrode member to impregnation with a surface treating solution containing a conductive metal such as copper, brass, silver, etc. (chemical plating) or to electroplating. The electrode member can as such be used for electrical discharge machining because of the conductivity by the presence of conductive metal powder incorporated therein as a filler. However, the subsequent plating treatment not only allows the conductive metal contained in the surface treating solution to intrude into the minute gaps present in the electrode member to enhance continuity between the particles of the conductive metal powder preliminarily dispersed as a filler in the electrode member but also forms a thin metal plating layer to reinforce the surface of the electrode member, whereby conductivity and strength of the electrode can be improved. Accordingly, "chipping" as occurred when the electrode member is used as such or when the electrode as disclosed in the prior application is used can be eliminated to provide the electrode with improved durability. Incidentally, if a surface preparation with a silver or copper conductive solution is applied to the electrode member prior to the plating treatment, a uniform metal plating layer can advantageously be formed even when the electrode is used for machining subtle shape of workpiece such as a dental crown and bridge having a considerably irregular surface. Even when the electrode for electrical discharge machining has a complicated transfer surface, it not only can be obtained with excellent moldability which is attributable to the gypsum hemihydrate powder incorporated therein as a binder but also has excellent conductivity due to the presence of the conductive metal powder incorporated therein as a filler and by the application of surface treatment, and further can retain the desired shape of machining surface over an extended period to enable extended electrical discharge machining. As described above, while a mixture of at least two such metal powders having different shapes and particle sizes is preferably used as the conductive metal powder, those having very small particle sizes of less than 1 µm will lead to reduced conductivity; whereas those of more than 50 µm cannot be dispersed well as a filler in the electrode member and also cause reduction in conductivity. Accordingly, when high degrees of conductivity and accuracy are required, it is preferred to blend powders with different particle sizes in the range of 1 to 50 µm such that the amount of the larger size particles may be slightly greater than that of the smaller size particles, for example, at a ratio of the former to the latter of 3 : 2, wherein a preferred combination is spherical larger particles and branched smaller particles in view of dispersibility as a filler. The amount of the gypsum dihydrate powder to be added as crystalline nucleus to the gypsum hemihydrate powder is suitably in an amount of 1 % by weight or less relative to the gypsum hemihydrate powder. The reason is because less than 0.2 % by weight of gypsum dihydrate powder incorporated cannot be expected to serve sufficiently as the crystalline nucleus; whereas more than 1 % by weight of gypsum dihydrate powder incorporated causes to increase the crystal size after hydration and setting to affect conductivity. [Brief Description of Invention] Figs. 1 and 2 each show both in cross section (A) and in plan view (B) an electrode for electrical discharge machining (for manufacturing a dental crown and bridge) according to the characteristics of this invention, and Fig. 3 shows in cross section how a dental crown and bridge can be obtained by electrical discharge machining using the electrode according to the characteristics of this invention; wherein the reference numeral (1) shows an electrode (member) having a transferred outer profile of a dental crown and bridge; (la) a register; (2) an electrode (member) having a transferred inner profile of the dental crown and bridge; (2a) a register; (3) a material to be subjected to electrical discharge machining; (4) a dental crown and bridge to be obtained by electrical discharge machining; (4a) the outer surface of the dental crown and bridge; and (4b) the inner surface of the dental crown and bridge. [Best Mode for Carrying Out the Invention] This invention will be described in detail by way of preferred embodiments, wherein part(s) all means part(s) by volume unless otherwise specified. First Embodiment In a 10 Torr vacuum kneader were kneaded well 50 parts of a dental stone (No. 80810 A, manufactured by Noritake Co., Ltd.), 0.5 part by weight of gypsum dihydrate particles having an average particle size of 15 µm, and 50 parts of a conductive metal powder comprising 30 parts of a branched copper powder having an average particle size of 10 µm (according to Fischer sub-sieve sizer method) and 20 parts of a spherical copper powder having a particle size of 325 mesh pass. Then, 3 parts of water to which additives including accelerator for plaster are incorporated was added to the kneaded mixture, followed by kneading under reduced pressure for 60 seconds to prepare a slurry form of deaerated kneaded mixture. The mixture was cast into predetermined silicone rubber molds having transferred thereon casting patterns of a model dental crown and bridge to mold cylindrical electrode members for electrical discharge machining each having a diameter of 10 mm and a length of 30 mm (see Fig. 1 showing, in cross section (A) and plan view (B), an electrode (l) having transferred thereon the outer surface (4a) of a final product of dental crown and bridge, and Fig. 2 showing, in cross section (A) and in plan view (B), an electrode (2) having transferred thereon the inner surface (4b) of the final product of dental crown and bridge, wherein (la) and (lb) are registers when these electrodes are set on an electrical discharge machine). Subsequently, after the electrode members were left for drying at room temperature for 24 hours, they were immersed in a 20 % copper sulfate solution for 30 to 300 minutes to effect chemical plating treatment. When a dental titanium material was subjected to electrical discharge machining using the thus obtained pair of electrodes at the set current of 6 A, on-time 100 µs, off-time 700 µs (see Fig. 3 in which the two electrodes (1,2) are aligned on the axis "X" of the electrical discharge machine to oppose each other, and a workpiece (3) is machined from the two opposing sides simultaneously, wherein the reference numeral (4) shows a dental crown and bridge to be finally obtained by the electrical discharge machining), electric discharging started immediately after application of electric current compared with the electrode of the prior invention which requires about one minute before discharging started. However, since the plating film was insufficiently formed, the electrodes yet worn away considerably, although not so serious as in the electrode of the prior invention. Moreover, the electrical discharge machining speed was 5.56 x 10⁻³mm/min which is far below the intended value of 3.13 x 10⁻²mm/min. Second Embodiment A pair of electrodes were prepared in the same manner as in first embodiment, except that the conductive metal powder comprising 5 parts of a branched copper powder having a particle size of 3 µm pass (according to Fischer sub-sieve sizer method), 20 parts of a spherical copper powder having an average particle size of 10 µm and a spherical copper powder having a particle size of 325 mesh pass, and that the immersion was carried out by means of electroplating in a 20 % copper sulfate solution containing a plating assistant at a current of 0.05 to 0.08 mA/cm² for 30 to 300 minutes. When dental titanium materials were subjected to electrical discharge machining using the thus prepared pair of electrodes under the conditions as shown in the following table, wear and tear of the electrodes was greatly improved by the sufficient thickness (50 to 120 µm) of plating film formed on the surface of the electrodes, and substantially desired machining speed was obtained. However, partial wear and tear occurred in the electrodes because of the nonuniformity in the thickness of the plating film on such portions having small degree of irregularity. Set current On-time Off-time Machining speed A 16 A 100 µs 700 µs 2.86 x 10⁻²mm/min. B 16 A 46 µs 322 µs 3.08 x 10⁻²mm/min. C 21 A 110 µs 330 µs 4.00 x 10⁻²mm/min. Third Embodiment A pair of electrodes were prepared in the same manner as in second embodiment, except that a surface preparation was applied on the dried electrode members using a silver conductive spray, and they were used in a machining test. Each of the electrodes had a smooth surface of uniform plating film with a thickness of 50 to 120 µm, and the intended machining speed was achieved with no partial wear and tear in the electrodes. [Industrial Applicability] As apparent from the foregoing description, the electrode for electrical discharge machining of this invention is free from the problem of molding contraction and can easily be molded if only a simple mold made of a silicone rubber and the like is available, so that it requires simple equipment for the molding and can be produced at a low cost. Thus, various types of electrodes can efficiently be produced even in small lots. Moreover, the present electrode can be produced with high dimensional accuracy to provide excellent machining performance which is equal to or even over the conventional electrodes for electrical discharge machining primarily comprising graphite which suffers poor processability. It should particularly be noted that this invention has enabled machining of hard materials such as titanium into complicated shapes of final articles using inexpensive electrodes with high accuracy. Compared with the electrodes of the prior invention, in which a gypsum hemihydrate powder is used as a binder and a conductive metal powder is used as a filler to prepare electrode members by modeling and/or molding and setting, which is then impregnated with a conductive solution containing a conductive powder such as graphite dispersed therein, the present electrodes are superior in any of the value of resistance in conductivity, surface accuracy and durability. Accordingly, the electrode of this invention, having overcome the problems inherent in this type of electrodes for electrical discharge machining, contributes very much to the development of the art.

Claims: An electrode for electrical discharge machining, comprising an electrode member that is plated with a conductive metal, characterized in that the main components of the electrode member are gypsum and conductive metal powder dispersed in it. An electrode as claimed in claim 1, characterized in that the conductive metal powder comprises a mixture of a spherical copper powder having a larger diameter and a branched copper powder having a smaller diameter. An electrode as claimed in claim 1 or 2, characterized in that the conductive metal plating is a chemically or galvanically applied copper layer. A process for manufacturing an electrode for electrical discharge machining, wherein an electric member is obtained by molding and wherein this electric member is then plated with a conductive metal forming a thin metal layer on the surface of the electrode member, characterized in that the steps for obtaining the electric member comprise kneading in vacuo a gypsum hemihydrate powder, a gypsum dihydrate powder in an amount of not more than 1 % by weight of the gypsum hemihydrate powder and a conductive metal powder; adding water to the kneaded product, followed by further kneading in vacuo; casting the further kneaded product into a mold having transferred a predetermined shape therein, effecting hydration, molding and setting, thereby providing an electrode member and drying the thus obtained electrode member. A process as claimed in claim 4, wherein prior to the plating with the conductive metal the surface of the electrode member is subjected to a surface preparation with a conductive metal solution. A process as claimed in claim 5, characterized in that the conductive metal solution is a silver or copper spray. A process as claimed in anyone of claims 4 to 6, wherein the conductive metal powder comprises a mixture of a spherical copper powder having a larger diameter and a branched copper powder having a smaller diameter. A process as claimed in anyone of claims 4 to 7, characterized in that the electrode member is chemically or galvanically plated by using a copper sulfate solution.

Inventors: BAN KIYOKO ROOM UMEMORI KOPORA, HORI TAKEO, NISHIMURO SATOSHI, BAN, KIYOKO, ROOM 305 UMEMORI KOPORASU, HORI, TAKEO, NISHIMURO, SATOSHI
Applicants: CUSP DENTAL SUPPLY CO LTD, JAPAN NUS CO LTD, CUSP DENTAL SUPPLY CO., LTD, JAPAN NUS CO., LTD.
Application year: 1990
Main classification: B23H1
IPC codes: B23H1, B23H9