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Development of a High-speed Machining and Low-consumption Electrode for Electric Discharge

Machining

高速加工・低消耗を実現した放電加工電極の開発Hirofumi YAMAGUCHI Tomonari SUEYOSHI Momoyo OIE

山口　博文 末吉　知力也 尾家　百代

Key words; Electric discharge machining, high-speed machining, low-consumption electrode, copper-tungsten alloy, work-function, uniform and fine

キーワード；放電加工 ,　高速加工 ,　低消耗電極 ,　銅タングステン ,　仕事関数 ,　均一微細

Abstract

　In general, copper and graphite are frequently used as electrode materials for Electric Discharge Machining (EDM). Copper-tungsten alloy is used for special applications such as machining of cemented carbides and high-precision machining. Our company has been developed materials in order to improve the electric discharge machining property of copper-tungsten alloy. This paper introduces a new electrode, C-EDM, which has a better electric discharge machining property than that of conventional copper-tungsten alloys.

　放電加工用電極の材質として、銅やグラファイトなどの材質が多く用いられているが、高硬度の超硬合金の加工や高精度な加工などの特殊な用途では銅タングステンが使用されている。当社では、その銅タングステンのもつ放電加工特性を、より向上させるために材料の開発を進めてきた。本稿では、従来の銅タングステンよりも放電加工特性を向上させた新電極 C-EDM について紹介する。

1.Introduction　Copper and graphite are frequently used as electrode materials for Electric Discharge Machining (EDM). Copper-tungsten alloy is used for special applications such as machining of cemented carbides and high-precision machining. Our company manufactures copper-tungsten alloys having an improved electric discharge property by adding a small amount of a material with low work-function as well as other various copper-tungsten alloys with different compounding ratios. With a newly developed material, C-EDM, we achieved further high-speed machining and low consumption by uniformly and finely dispersing this additive.

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2.Electrodes to be used for EDM 　Electrodes for EDM are chosen depending on the workpiece material and the application. Table 1 shows generally-used materials, workability, characteristics and applications of electrodes for EDM. Table 2 shows characteristics and applications of three electrodes for EDM which are mainly handled by our company.

Table 1 Generally-used materials of electrodes for EDM

Material Copper Graphite Copper-tungsten alloy

Electrode workability

・Good for machining・Hard for grinding・Small-sized fine shape may be deformed due to low hardness.

・Available for both machining and grinding・A dedicated machining machine or a dust collector is required for collecting dust during the machining.

・Available for machining while the cutting tool is quickly worn.・Good for grinding・Fine shape can be maintained due to its high hardness.

Cost Low Relatively inexpensive Relatively expensive

Characteristics Mostly usedUsable as a large

electrode due to its small specific gravity.

・Low electrode consumption・Desired dimensions and shapes can be quickly machined.

ApplicationGenerally used for

steel, etc.

・ Generally used for steel, etc.・Machining of large shapes

・Especially suitable for cemented carbides・Machining of Fine shapes

Table.2 characteristics and applications of electrodes for EDM manufactured by our company

C30A2 C30H2 C30F2

CharacteristicsGeneral copper-tungsten alloy

・Easy to emit electrons by adding a small

amount of a material with low work-function・Used as a cathode

electrode

Less electrode consumption by adding

a small amount of a high-melting-point

material・Used as an anode

electrode

Application

Suitable for general applications such as steel and cemented

carbides

Suitable for cemented carbides

Suitable for steel

3．Characteristics of C-EDM　As same as C30H2 shown in Table 2, a material with low work-function is added to C-EDM. As shown in Fig.1, the additive is more uniformly and finely dispersed to C-EDM than C30H2. With this uniformly

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and finely dispersed additive, electrons are emitted from whole surface of the electrode and the workpiece is uniformly removed (machined). Further, the additive is a non-polluting material which does not contain any radioactive substance. In addition, the bonding strength of tungsten particles increases and the removal of the electrode by discharge energy decreases. Table 3 shows physical properties of three typical EDM electrodes and C-EDM handled by our company.

materials with low work - function

C-EDM C30H2Fig.1 Microstructures of C-EDM and C30H2

Table 3　 Physical properties

C30A2 C30H2 C30F2 C-EDM

Density 14.2 13.5 13.8 15.6

Electrical　Conductivity[IACS%]

48 48 43 20

Hardness[HRB]

93 93 93 103

4．Discharge machining performance 4-1．Rough　machining

　Fig. 2 and Table 4 show machining conditions. A workpiece was machined 7 mm long, 5 mm wide and 3 mm deep using an electrode. The electrode consumption rate, the machining speed and the appearance after the electric discharge machining of C-EDM were compared with those of C30A2 and C30H2. The electrode consumption rate means a ratio of the electrode consumption to the machined workpiece. The machining speed means the amount of machined workpiece per unit time. The score of the machining speed is assigned with a score of 1 representing C30A2.

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Table.4 Machining conditions

5mm

7mm-3mm

Electrode

Workpiece

　 Machining conditions

Workpiece Cemented carbide

Machine Transistor-type

Machining Rough　machining

Electrode polarity

Cathode

Fig. 2 Machining conditions

　Fig.3 and 4 show the electrode consumption rates and the machining speeds, respectively. Fig. 3 shows the appearance of the electrode after machining. The electrode consumption rate is 50 % less than that of C30A2 and 30% less than that of C30H2. As shown in Fig. 5, the electrode consumption of C-EDM is the least. The machining speed of C-EDM is 1.3 times that of C30A2 and 1.2 times that of C30H2, i.e. the machining speed of C-EDM is the fastest.

Fig.3 Electrode consumption rates Fig.4 Machining speeds

Fig. 5 Electrode consumption

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4-2．Finishing machining

　Fig. 6 and Table 5 show the machining conditions. A workpiece was machined using an electrode having a bottom 15 mm square. The surface roughness after machining was measured and compared with C30A2 and C30H2.

Table 5 Machining conditions

Workpiece

Electrode

15m

15m

　 Machining conditions

Workpiece Cemented carbide

Machining Finishing machining

Electrode polarity Cathode

Fig. 6 Machining conditions

　Fig.7 shows the surface roughness. The surface roughness of C-EDM is the best.

Fig.7 Surface roughness

5．Conclusions　With a newly developed electrode, C-EDM, we achieved a higher machining speed and a lower consumption than those of conventional copper-tungsten alloys by uniformly and finely dispersing an additive with low work-function. With this electrode, it is expected that the number of electrodes to be used for desired shapes and dimensions will decrease and the machining time will be reduced, i.e. the productivity will be improved.