【発明の詳細な説明】[Detailed description of the invention]
本発明は型彫り放電加工機に使用する黒鉛電極
に係るものであり、その目的は15ミクロン以下の
放電加工仕上げ面粗度を高能率で得ることができ
る黒鉛電極を提供することにある。
従来、黒鉛電極（以下単に電極という）を使用
し、15ミクロン以下の放電加工仕上げ面粗度（以
下単に仕上面という）を得るためには電極極性を
陽極とし、ピーク電流値を5A以下にすると同時
に、被加工物の放電加工仕上り寸法精度を確保す
るためにパルスON時間巾を150μsec以上の電極
が消耗しないパルスON時間巾を設定し、１つ１
つの放電から被加工物へ与える電気エネルギーを
小さくし、その結果得られる放電痕跡を小さくす
る必要がある。
しかしながら、上記放電加工条件における電極
の放電加工仕状態は極めて不安定であることか
ら、15ミクロン以下の仕上面を得るためには長時
間を要し、その結果得られた仕上面には、同一個
所で放電が繰返えされたと思われる凹部が見散さ
れる結果、15ミクロン以下の仕上面を得ることが
できなかつた。このため電極は使用上の制約を受
けプラスチツク金型等の細かい仕上面の要求され
る分野では使用されていないなどの不都合があつ
た。
本発明者等は、5A以下の低ピーク電流による
放電加工状態の不安定になる理由を次の通り推察
し新たに知見した。放電加工中の電極と被加工物
との放電間隙は極間の抵抗によつて支配される。
極間抵抗は通常単位面積当りの加工速度すなわち
ピーク電流値とパルスON時間巾に支配されるこ
とになる、パルスON時間巾は電極消耗に大きく
影響することから放電間隙はピーク電流値の減少
と共に急激に狭くなり、5A以下の低ピーク電流
値における放電加工中においては発生する加工チ
ツプおよび加工液の分解により生じた気泡の逸散
が悪くなることが予測できる。その結果、5A以
上の放電加工中にみられる放電点が次から次へと
移動する安定放電ではなく、放電点は比較的に同
一個所に集中して発生するために不安定な放電状
況になるものと考えられる。一般に、加工チツプ
の排出は電極の強制上下動（ジヤンピングと言
う）により比較的に容易に排出される。万一排出
できないときは、アーク現象に至り放電加工は進
行しないことから、チツプ排出の問題は15ミクロ
ン以下の仕上面を得るための阻害要因ではないと
考えられる。15ミクロン以下の仕上面を得るため
の最大の阻害要因は電極表面への気泡の吸着と考
えられる。この気泡を除去するために、加工液量
を増加してみたが、5A以上の荒加工および中仕
上加工領域においてのみ効果があり、5A以下の、
15ミクロン以下の仕上加工領域では放電間隙も狭
く、充分な液量の供給が難かしい。又加工液穴の
孔けられないリブ等の薄物電極の放電加工におい
ては全く加工液が供給できないことから、要求す
る面粗度の粗細に関係なく、常に不安定な放電状
況により放電加工が困難となることが判つた。
本発明者等は放電加工状態の不安定さを解決す
るために、影響の最も大きいと考えられる、気泡
の逸散を良好にすることに着目し、幾多の研究を
重ねた結果、電極の持つている空隙の平均細孔径
の大きさによつて、気泡の逸散程度が異なり、そ
の結果、安定した放電加工状態が持続され、15ミ
クロン以下の仕上面が得られることを見い出し
た。電極の細孔径と5A以下の低ピーク電流によ
る放電状況の安定性との密接な関係については以
下の通り説明することができる。
与えられた放電エネルギーによつて熱分解され
た加工液は瞬時に気泡となるが、電極の持つてい
る平均細孔径が大きくなるに従がつて電極表面に
気泡が吸着され易く、次々と繰返えされている放
電は極間抵抗の低い所、すなわち炭化水素および
水素ガスの充満した気泡の吸着している所に起こ
り易く、必らずしも、安定放電の通念であるとこ
ろの極間距離の最も近い所から起こるとは限らな
い。この繰返し放電が同一場所に連続して起こる
と被加工物表面に凹部が発生し、これが仕上面を
粗くすることから、電極を使用した放電加工にお
いて15ミクロン以下の仕上面が得られないことが
判つた。
本発明者等は、放電加工液を放電エネルギーに
より分解されて生成した気泡が電極表面に吸着す
ることを減少させるために、電極又は電極成形前
の黒鉛基材中へ樹脂、金属あるいはピツチ等のベ
ンゼン環を有する炭化収率の高い化合物を真空含
浸あるいは加圧含浸し、更に必要に応じて、硬化
又は炭化することにより、黒鉛材料が元来持つて
いる気孔を減少あるいは埋め込むことにより、黒
鉛材の平均気孔径を4000Å以下にした。
電極に直接含浸できるのは樹脂の含浸であり、
金属あるいはベンゼン環を有する化合物の場合に
おいては、含浸后の電極表面は平滑でなく、高精
度を保つた含浸電極が得られないことから、機械
加工した電極への含浸はできない。このため、機
械加工前の黒鉛基材に含浸したのちに機械加工す
る必要がある。同時に、底付金型に要求される電
極の厚さは50〜200mmが標準である。
金属の含浸は融点の低い金属が適している。例
えば、銅等の高融点金属を、100Kg／cm2、1300℃
の苛酷な加圧含浸条件で含浸した場合においても
厚さ50mm以上含浸することはできなかつた。この
ことから、本発明で言う含浸金属は工業的に亜
鉛、アルミニウム、錫、鉛などの金属あるいはそ
れらの金属を含む合金に限られる。
次に本発明に従がつて得られた、4000Å以下の
平均細孔径を持つ電極あるいは細孔を有しない電
極により、15ミクロン以下の面粗度が得られ、且
つ電極の消耗しない放電加工条件、すなわちピー
ク電流3A、パルスON時間150μsec、被加工材SK
−５にて放電を行なつた結果を実施例に従つて説
明する。
実施例 １
平均細孔径7000Åを持つた黒鉛基材を機械加工
した電極へフルフリルアルコール樹脂を真空含浸
の常法に従がつて含浸した後、蒸気釜の中で150
℃に加熱硬化して得られた含浸電極の平均細孔径
は1100Åであつた。
実施例 ２
実施例１によつて得られた電極を600℃で熱処
理した含浸電極の平均細孔径は2800Åであつた。
実施例 ３
実施例２によつて得られた電極を更に3000℃で
熱処理された含浸電極の平均細孔径は3500Åであ
つた。
実施例 ４
平均細孔径7000Åを持つた黒鉛基材へ軟化点85
℃のコールピツチを220℃で真空含浸の常法に従
がつて含浸后550℃で熱処理をした。得られた含
浸黒鉛基材の平均細孔径は3200Åであつた。機械
加工して電極を得た。
実施例 ５
実施例４によつて得られた含浸黒鉛基材へ実施
例４と同じ操作にてコールピツチを再含浸、熱処
理して得られた再含浸黒鉛基材の平均細孔径は
2100Åであつた。機械加工して電極を得た。
実施例 ６
平均細孔径7000Åを持つた黒鉛基材に亜鉛を温
度650℃、圧力15Kg／cm2の条件下で加圧含浸の常
法に従がつて含浸した。得られた含浸黒鉛基材の
気孔はなかつた。機械加工して電極を得た。
実施例 ７
実施例６と同様にアルミニウムを温度850℃、
圧力20Kg／cm2の条件下で加圧含浸した。得られた
含浸黒鉛基材の気孔はなかつた。機械加工して電
極を得た。
実施例 ８
実施例６と同様に錫を温度500℃、圧力５Kg／
cm2で加圧加浸した。得られた含浸黒鉛基材には気
孔はなかつた。機械加工して電極を得た。
実施例 ９
実施例６と同様に黄銅を温度1050℃、圧力80
Kg／cm2で加圧含浸した。得られた含浸黒鉛には気
孔はなかつた。機械加工して電極を得た。
比較例 １
平均細孔径7000Åを持つた黒鉛基材から機械加
工して得られた電極。
比較例 ２
平均細孔径4300Åを持つた黒鉛基材から機械加
工して得られた電極。
使用電極の寸法は長さ100mm、幅100mm、高さ30
mmであり、上記放電条件により深さ１mm加工し
た。放電加工液は電極の中心から３mmの穴を通し
て0.05Kg／cm2の圧力で供給した結果を第１表に示
す。
The present invention relates to a graphite electrode used in a die-sinking electrical discharge machine, and its purpose is to provide a graphite electrode that can efficiently obtain an electrical discharge machining surface roughness of 15 microns or less. Conventionally, graphite electrodes (hereinafter simply referred to as electrodes) were used, and in order to obtain an electrical discharge machining surface roughness of 15 microns or less (hereinafter simply referred to as finished surfaces), the electrode polarity should be anode and the peak current value should be 5A or less. At the same time, in order to ensure the dimensional accuracy of the electrical discharge machining finish of the workpiece, the pulse ON time width is set to 150μsec or more so that the electrode does not wear out.
It is necessary to reduce the electrical energy applied to the workpiece from each discharge, and to reduce the resulting discharge traces. However, since the electrical discharge machining condition of the electrode under the above-mentioned electrical discharge machining conditions is extremely unstable, it takes a long time to obtain a finished surface of 15 microns or less, and the resulting finished surface is As a result, it was not possible to obtain a finished surface of 15 microns or less, as there were some concavities that appeared to have been caused by repeated discharges. For this reason, the electrodes were subject to limitations in use and were not used in fields requiring finely finished surfaces such as plastic molds. The present inventors speculated and newly discovered the reason why the electric discharge machining state becomes unstable due to a low peak current of 5A or less as follows. The discharge gap between the electrode and the workpiece during electrical discharge machining is controlled by the resistance between the poles.
The resistance between the electrodes is usually controlled by the machining speed per unit area, that is, the peak current value and the pulse ON time width. Since the pulse ON time width has a large effect on electrode wear, the discharge gap decreases as the peak current value decreases. It can be predicted that the gap becomes narrow rapidly, and that during electrical discharge machining at a low peak current value of 5A or less, the dissipation of air bubbles caused by the decomposition of machining chips and machining fluid will become worse. As a result, the discharge points are relatively concentrated in the same location, resulting in an unstable discharge situation, instead of the stable discharge that occurs during electrical discharge machining of 5A or more, where the discharge points move from one to the next. considered to be a thing. Generally, processed chips are relatively easily ejected by forced vertical movement of the electrode (referred to as jumping). If chips cannot be ejected, arcing will occur and electrical discharge machining will not proceed, so the problem of ejecting chips is not considered to be an impediment to obtaining a finished surface of 15 microns or less. The biggest impediment to obtaining a finished surface of 15 microns or less is thought to be the adsorption of air bubbles to the electrode surface. In order to remove these bubbles, we tried increasing the amount of machining fluid, but it was only effective in rough machining and semi-finishing machining of 5A or more, and
In the finishing area of 15 microns or less, the discharge gap is narrow and it is difficult to supply a sufficient amount of liquid. In addition, when machining fluid is not available for electrical discharge machining of thin electrodes such as ribs where holes cannot be drilled, no machining fluid can be supplied, so electrical discharge machining is always difficult due to unstable electrical discharge conditions, regardless of the required surface roughness. It turns out that. In order to solve the instability of the electric discharge machining state, the present inventors focused on improving the dispersion of bubbles, which is considered to have the greatest influence, and after conducting numerous studies, they found that the It was discovered that the degree of bubble dissipation varies depending on the average pore diameter of the voids, and as a result, stable electrical discharge machining conditions can be maintained and a finished surface of 15 microns or less can be obtained. The close relationship between the pore diameter of the electrode and the stability of the discharge state due to a low peak current of 5 A or less can be explained as follows. The machining fluid that is thermally decomposed by the applied discharge energy instantly turns into bubbles, but as the average pore diameter of the electrode increases, the bubbles are more likely to be adsorbed on the electrode surface, and the process is repeated one after another. This discharge tends to occur in areas where the resistance between the electrodes is low, that is, where bubbles filled with hydrocarbon and hydrogen gas are adsorbed, and the distance between the electrodes is not necessarily the same as the conventional idea for stable discharge. It does not necessarily occur from the closest location. If these repeated discharges occur continuously at the same location, depressions will occur on the surface of the workpiece and this will make the finished surface rough, which means that it is not possible to obtain a finished surface of 15 microns or less in electrical discharge machining using electrodes. I understand. In order to reduce the adsorption of air bubbles generated by decomposing the electrical discharge machining fluid by electrical discharge energy to the electrode surface, the present inventors added resin, metal, pitch, etc. to the electrode or the graphite base material before forming the electrode. By vacuum or pressure impregnation with a compound having a benzene ring and a high carbonization yield, and further hardening or carbonization as necessary, the graphite material is made by reducing or filling the pores that the graphite material originally has. The average pore diameter was set to 4000 Å or less. The electrode can be directly impregnated by resin impregnation.
In the case of a metal or a compound having a benzene ring, the electrode surface after impregnation is not smooth and an impregnated electrode with high precision cannot be obtained, so impregnation into a machined electrode is not possible. Therefore, it is necessary to impregnate the graphite base material before machining and then machine it. At the same time, the standard electrode thickness required for bottom molds is 50-200mm. For metal impregnation, metals with low melting points are suitable. For example, high melting point metal such as copper at 100Kg/cm 2 and 1300℃
Even when impregnated under severe pressure impregnation conditions, it was not possible to impregnate to a thickness of 50 mm or more. For this reason, the impregnated metal referred to in the present invention is industrially limited to metals such as zinc, aluminum, tin, and lead, or alloys containing these metals. Next, electric discharge machining conditions are provided in which a surface roughness of 15 microns or less is obtained using an electrode having an average pore diameter of 4000 Å or less or an electrode having no pores obtained according to the present invention, and the electrode is not consumed. In other words, peak current 3A, pulse ON time 150μsec, workpiece material SK
The results of discharging in -5 will be explained according to Examples. Example 1 An electrode machined from a graphite base material with an average pore diameter of 7000 Å was impregnated with furfuryl alcohol resin according to the conventional vacuum impregnation method, and then heated in a steam pot for 150 Å.
The average pore diameter of the impregnated electrode obtained by heating and curing at 1100 Å was 1100 Å. Example 2 The impregnated electrode obtained by heat-treating the electrode obtained in Example 1 at 600°C had an average pore diameter of 2800 Å. Example 3 The electrode obtained in Example 2 was further heat-treated at 3000° C. The average pore diameter of the impregnated electrode was 3500 Å. Example 4 Softening point of 85 to graphite base material with average pore diameter of 7000 Å
Coal pitch at 220°C was impregnated in a vacuum according to the conventional method, and then heat treated at 550°C. The average pore diameter of the obtained impregnated graphite base material was 3200 Å. Electrodes were obtained by machining. Example 5 The average pore diameter of the re-impregnated graphite base material obtained by re-impregnating and heat-treating the impregnated graphite base material obtained in Example 4 with coal pitch in the same manner as in Example 4 is
It was 2100Å. Electrodes were obtained by machining. Example 6 A graphite base material having an average pore diameter of 7000 Å was impregnated with zinc at a temperature of 650° C. and a pressure of 15 kg/cm 2 according to a conventional pressure impregnation method. The obtained impregnated graphite base material had no pores. Electrodes were obtained by machining. Example 7 Similar to Example 6, aluminum was heated to a temperature of 850°C.
Impregnation was carried out under pressure of 20 kg/cm 2 . The obtained impregnated graphite base material had no pores. Electrodes were obtained by machining. Example 8 Similar to Example 6, tin was heated at a temperature of 500°C and a pressure of 5 kg/
Pressure soaked at cm2 . The obtained impregnated graphite substrate had no pores. Electrodes were obtained by machining. Example 9 Brass was heated to a temperature of 1050°C and a pressure of 80°C in the same manner as in Example 6.
Pressure impregnation was carried out at Kg/ cm2 . The obtained impregnated graphite had no pores. Electrodes were obtained by machining. Comparative Example 1 Electrode obtained by machining a graphite base material with an average pore diameter of 7000 Å. Comparative Example 2 Electrode obtained by machining a graphite base material with an average pore diameter of 4300 Å. The dimensions of the electrode used are length 100mm, width 100mm, and height 30mm.
mm, and was machined to a depth of 1 mm under the above discharge conditions. The electrical discharge machining fluid was supplied through a hole 3 mm from the center of the electrode at a pressure of 0.05 Kg/cm 2 and the results are shown in Table 1.
【表】
以上の結果から本発明により平均細孔径を小さ
くした電極を使用して15ミクロン以下の仕上げ面
粗度を容易に得ることができると同時に安定な放
電状況が持続されることにより、放電加工速度も
大巾に向上できることが明らかとなつた。これら
のことから従来15ミクロン以下の仕上げ面を得る
ためには銅、銅タングステン等の高価な電極材料
が使用されていたが、電極製作の容易でしかも安
価な黒鉛電極を使用することができるので経済的
効果は極めて大きい。[Table] From the above results, it is possible to easily obtain a finished surface roughness of 15 microns or less by using an electrode with a small average pore diameter according to the present invention, and at the same time maintain a stable discharge condition. It has become clear that machining speed can also be greatly improved. For these reasons, conventionally expensive electrode materials such as copper and copper-tungsten were used to obtain a finished surface of 15 microns or less, but now graphite electrodes, which are easy to manufacture and are inexpensive, can be used. The economic effects are extremely large.