JPH074696B2 - Electric discharge machining electrode and its manufacturing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrode for electrical discharge machining, particularly an electrode for die sinking electrical discharge machining, and a method for manufacturing the same.

BACKGROUND ART Die-sinking electrical discharge machining electrodes are generally made of graphite-beryllium-copper alloys processed with a three-dimensional machining machine. However, electrical discharge machining electrodes primarily made of graphite or the like have problems such as time-consuming machining, low forming accuracy, and high costs due to the expensive machining equipment. Therefore, the inventors of the present invention previously developed an electrical discharge machining electrode in which an electrode material made of hemihydrate gypsum powder as a main material and binder and conductive metal powder as a filler is transferred, molded, and hardened into a desired shape, and then impregnated with a conductive solution such as graphite solution (ultrafine powder of natural graphite suspended in ammonia, such as "Hitazol" manufactured by Hitachi Chemical Co., Ltd.), thereby enhancing the mutual conductive interconnections of the conductive metal powder dispersed and filled in the electrode material. This electrode was filed as Japanese Patent Application No. 1-46069. However, when attempts were made to manufacture various products by electrical discharge machining using the previously filed patent, the material was found to be brittle when used as is (the metal powder had loose connections in some areas, resulting in low current resistance density, prone to internal discharges, and only a machining current of a few amperes could be passed), which created durability problems.Furthermore, when electrical discharge machining was performed on delicate dental crowns and other products made of hard materials such as titanium, there was a significant wear on the electrodes, and the machining precision was insufficient, making it impossible to obtain the desired products.

"Disclosure of the Invention" The present invention was completed with the objective of providing an electrode for die-sinking electrical discharge machining that solves the problems described above, and a method for manufacturing the same. The electrode material is made of hemihydrate gypsum powder as the main material and binder, and conductive metal powder as the filler. The electrode material is transferred, molded, and hardened into a desired shape, and is then plated with conductive metal, thereby improving the electrical connection between the conductive metal powder particles dispersed and filled in the electrode material, and the surface of the electrode material is reinforced by being coated with a thin layer of the plated metal.

In the electrode of the present invention, the hemihydrate gypsum powder, which is the main material, is used as a molding and strength-imparting factor for the electrode material and as a binder for the filler. Therefore, it is preferable to use α-type hemihydrate gypsum, which is commercially available as dental ultra-hard gypsum (α
Mold hemihydrate has the characteristic that the expansion due to the growth and development of crystals during hydration and setting is greater than the shrinkage that accompanies hydration and setting, so there is no molding shrinkage or molding distortion, and a precise product that fits the mold surface can be obtained.Furthermore, if a small amount of gypsum dihydrate powder is added to the gypsum hemihydrate powder as crystal nuclei during the manufacturing process, the formation of needle-like crystals is promoted as the hemihydrate hydrates and hardens, and the conductive metal powder is dispersed and filled into the gaps between the crystals, making its use particularly preferred.

On the other hand, as the conductive metal powder to be mixed with the hemihydrate gypsum powder, copper, titanium, tungsten, nickel, brass, phosphor bronze, lead, etc. are listed as candidates, but copper is preferred in terms of the formability, conductivity, and price of the electrode material. Furthermore, mixing two or more types of powders with different shapes and particle sizes is particularly preferred because the conductive properties are greatly improved when these are mixed.

Here, in the case of copper powder, the mixing ratio of hemihydrate gypsum powder to conductive metal powder is preferably in the range of 55-45:45-55 by volume. This is because if the gypsum powder is 40% or less, sufficient molding strength cannot be obtained and the molding surface becomes rough, resulting in poor molding precision of the electrode material; if it is 30% or less, molding itself becomes difficult; and if it is 60% or more, conductivity decreases. The main use of the present invention is to produce electrodes that are precisely molded into and molded into the shape of a complex, precise object, such as a dental crown, so in addition to conductivity, moldability and molding strength are primarily required.

The electrode of the present invention is manufactured by adding an appropriate amount of water and a setting accelerator, etc. to a mixture of hemihydrate gypsum powder and conductive metal powder, kneading the mixture while degassing (a method of kneading under vacuum conditions is generally used), and then obtaining an electrode material by a method similar to cast molding, for example, transferring and molding the kneaded mixture into a desired shape using an inexpensive and easily manufactured mold made of silicone rubber or the like, and then drying the electrode material and immersing it in a surface treatment liquid containing copper, brass, silver, or other conductive metals (chemical plating) or by electroplating.

However, the subsequent plating process allows the conductive metal contained in the surface treatment solution to penetrate into the fine gaps in the electrode material, improving the interconnection between the conductive metal powder particles previously dispersed and filled in the electrode material, and the surface of the electrode material is reinforced with a thin plated metal layer, thereby not only increasing the conductivity but also increasing the strength of the electrode, thereby eliminating the problem of electrode chipping that occurs when the electrode material itself is used as an electrode or when the electrode of the prior application is used, and improving durability. Furthermore, prior to plating, it is preferable to perform a base treatment on the electrode material with a silver- or copper-based conductive solution, as this allows for a uniform deposition of a thin plated metal layer even on an electrode for use in EDM with highly irregular shapes, such as a delicate dental crown.

The electric discharge machining electrode obtained in this manner has good formability provided by the hemihydrate gypsum powder as a binder, and also has excellent conductivity due to the presence of conductive metal powder as a filler and the application of a surface treatment to the electrode material, even when the electrode has a complex transfer surface.In addition, the machining surface can be maintained in the required shape for a long period of time, so electric discharge machining can be continued for a long period of time.

As mentioned above, it is preferable to use two or more kinds of conductive metal powders having different shapes and particle sizes in combination. However, fine powders of less than 1 μm have a low conductivity.
If the diameter exceeds 1 μm, the dispersibility and packing properties in the electrode material will be lost and the conductivity will decrease at the same time. Therefore, in cases where high conductivity and molding precision are required, it is preferable to mix in particles in the range of 1 to 50 μm, in an appropriate ratio such that larger diameter particles are slightly more prevalent than smaller diameter particles, for example, a ratio of about 2:3.
As for the shape, a combination of large diameter particles being spherical and small diameter particles being dendritic is preferable from the viewpoint of dispersion and packing properties. The amount of gypsum dihydrate powder added as crystal nuclei to the gypsum hemihydrate powder is 0.2 to 1 times the amount of the gypsum hemihydrate powder.
The reason is that if it is less than 0.2% by weight, it is difficult to expect it to function as a crystal nucleus, and if it is less than 1% by weight, it is difficult to expect it to function as a crystal nucleus.
If the temperature exceeds this range, the crystal grain size after hydration and hardening will become too large, adversely affecting the electrical conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are a cross-sectional view (A) and a plan view (B) showing the shape of an electric discharge machining electrode (for producing a dental crown) according to a feature of the present invention, and FIG. 3 is a cross-sectional view showing how a dental crown is electric discharge machined using an electrode according to a feature of the present invention.

1: Electrode (material) with the surface of the dental crown transferred onto it 1a: Alignment mark 2: Electrode (material) with the inner surface of the dental crown transferred onto it 2a: Alignment mark, 3: Workpiece for EDM 4: Dental crown resulting from EDM 4a: Outer surface of the dental crown, 4b: Inner surface of the dental crown.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to examples.

(Example 1) Dental super hard gypsum (No. 80810A, manufactured by Noritake Company)
50 parts (parts by volume; the same applies below except for gypsum dihydrate), 0.5 parts of gypsum dihydrate particles with an average particle size of 15 μm, and dendritic copper powder (particle size:
50 parts of conductive metal powder consisting of 30 parts of copper powder (particle size: 325 mesh or less) and 20 parts of spherical copper powder (particle size: 325 mesh or less) was mixed under a pressure of 10 To
After thoroughly mixing in a vacuum mixer, 3 parts of water containing additives such as a hardening accelerator was added and mixed under reduced pressure for 60 seconds to form a bubble-free slurry. This was then poured into a silicone rubber mold with a previously prepared dental crown model transferred onto it to form a cylindrical shape (outer diameter: D = 10 mm, length: L = 30 mm).
The electrode material for electrical discharge machining (see Fig. 1 showing electrode 1 on which the surface 4a of a dental crown, which is the final product of electrical discharge machining, has been transferred and molded, and Fig. 2 showing electrode 2 on which the inner surface 4b of the dental crown has been transferred and molded. Here, (A) is a cross-sectional view of the electrode, (B) is a plan view of the electrode, and 1a and 2a are "alignment marks" for setting the electrode in the electrical discharge machine) was formed. Next, the electrode material was naturally dried at room temperature for 24 hours, and then the concentration
The pieces were immersed in a 20% copper sulfate solution for 30 to 300 minutes for chemical plating.

Using the above electrode, electrical discharge machining of titanium material for dental crowns was carried out with a set current of 6A, a current application time of 100μs, and a rest time of 700μs (see Figure 3. The electrodes 1 and 2 were placed opposite each other on the axis "X" of the electrical discharge machine, and the workpiece 3 was machined simultaneously from two directions. Here, 4 is a dental crown, which is the final product of the electrical discharge machining). With the electrode of the prior invention, it took about one minute to start discharging after current application, but with the electrode of the present invention, immediate discharge was possible. However, because the formation of the plating film was insufficient, the electrode was still worn out, although not as much as in the prior invention, and the electrical discharge machining speed was 5.56× ^10-3
mm/min, which was far from the development target of 3.13×10 ^-2 mm/min.

(Example 2) In forming an electrode material for electrical discharge machining, the conductive metal powder was mixed by volume with 5 parts of dendritic copper powder (particle size: under 3 μm, measured by the Fisher subsieve sizer method), 20 parts of spherical copper powder (particle size: 10 μm average), and 10 parts of spherical copper powder (particle size: 32 μm).
An electrode was prepared in the same manner as in Example ¹ , except that the electrode material was prepared from 25 parts of copper foil (5 mesh under) and 25 parts of copper foil (5 mesh under). After drying, the electrode material was electroplated for 30 to 300 minutes in a plating solution prepared by adding a plating aid to a 20% copper sulfate solution, by passing a current of 0.05 to 0.08 mA/cm² through the electrode material.

Using the above electrode, electrical discharge machining of titanium material for dental crowns was carried out under the following conditions.
The formation of the plating film (m) significantly improved electrode wear and achieved the desired machining speed, but the plating film thickness was uneven in the finely uneven portions of the electrode, resulting in partial electrode wear.

(Example 3) The same procedure as in Example 2 was repeated except that the dried electrode material was subjected to a surface treatment using a silver-based conductive spray prior to plating.
Electrodes were fabricated in the same manner as in 1. and subjected to machining tests. Even in areas with severe irregularities, electrodes with smooth surfaces and uniform plating films of 50 to 120 μm in thickness were obtained, and the target machining speed was achieved without partial wear of the electrode.

[Industrial Applicability] As is clear from the above description, the present invention is free from the problem of molding shrinkage and can be molded using a silicone rubber mold or other simple mold. This simplifies molding equipment, enabling cost reductions and making it suitable for small-lot production of a wide variety of products. Furthermore, the present invention has good dimensional accuracy and machining performance equivalent to or superior to that of conventional electrical discharge machining electrodes primarily made of graphite, which have machining problems (it is particularly noteworthy that an inexpensive electrode can be used to accurately machine an electrode having a complex shape made of a hard material such as titanium). Furthermore, the present invention is superior in terms of conductive resistance, surface accuracy and durability compared to the prior art in which an electrode material using hemihydrate gypsum powder as a binder and conductive metal powder as a filler is transferred, molded and hardened into a desired shape, and then impregnated with a conductive solution in which conductive powder such as graphite is dispersed. This solves the problems of conventional electrical discharge machining electrodes of this type, and will make a significant contribution to the development of the industry.

──────────────────────────────────────────────────── Continued from the front page (72) Inventor: Takeo Hori 7-175 Ichinokura-cho, Tajimi City, Gifu Prefecture 1 (72) Inventor: Kiyoko Saka 701 Kitadamen, Umemori, Nisshin-cho, Aichi-gun, Aichi Prefecture 305 Umemori Corpus (72) Inventor: Satoshi Nishimuro 1481-7 Yoshida-cho, Totsuka-ku, Yokohama City, Kanagawa Prefecture (56) References: Japanese Patent Application Publication No. 152023 (JP, A) Japanese Patent Application Publication No. 152024 (JP, A)

Claims (10)

[Claims] [Claim 1] An electrode for electrical discharge machining, characterized in that the electrode material is made of hemihydrate gypsum powder as a main material and binder, and conductive metal powder as a filler, and is transferred, molded, and hardened into a desired shape, and then plated with conductive metal, thereby improving the electrical connection between the conductive metal powder particles dispersed and filled in the electrode material, and the surface of the electrode material is coated and strengthened with a thin layer of the plated metal. 2. An electrode for electrical discharge machining according to claim 1, wherein said conductive metal powder comprises a mixture of at least one type of copper powder having a large particle diameter and one type of copper powder having a small particle diameter and having a spherical shape. 3. The electrode according to claim 2, wherein said plating is by chemical plating using a copper sulfate solution. 4. The electrode according to claim 2, wherein said plating is performed by electroplating using a copper sulfate solution. [Claim 5] A method for manufacturing an electrode for electric discharge machining, comprising: vacuum-kneading gypsum hemihydrate powder, gypsum dihydrate powder containing 1% or less by weight of the powder, and conductive metal powder; adding water to the mixture and further vacuum-kneading the mixture; pouring the mixture into a mold onto which a predetermined shape has been transferred, and hydrating, shaping, and hardening the mixture to obtain an electrode material; drying the electrode material, and then plating the electrode material with a conductive metal, thereby improving the electrical connection of the conductive metal powder dispersed and filled in the electrode material and strengthening the surface of the electrode material by coating it with a thin layer of the plated metal. 6. The method according to claim 5, wherein the surface of the electrode material is subjected to a surface treatment with a conductive metal liquid prior to plating with the conductive metal. 7. The method according to claim 6, wherein the conductive metal liquid is a silver-based or copper-based spray. 8. The method according to claim 5, wherein the conductive metal powder is a mixture of at least one type of copper powder having a large particle size and one type of copper powder having a small particle size and a spherical shape. 9. The method according to claim 8, wherein said plating is carried out by a chemical plating method using a copper sulfate solution. 10. The method according to claim 8, wherein said plating is carried out by electroplating using a copper sulfate solution.