氏名　CHAIYA PRANEETPONGRUNG
学位の種類　博士（工学）
学位記番号　博甲第563号
学位授与の日付　平成22年9月30日
学位論文題目　Effects of Added Ultrasonic Vibration on the Electrical Discharge Machining Properties for Cemented Carbide and Si3N4 Insulating Ceramics　（超硬材料および絶縁性Si3N4の放電加工特性に及ぼす超音波振動付加の影響）
論文審査委員
　主査　教授　淺井　達雄
　副査　准教授　中村　和男
　副査　准教授　三上　喜貴
　副査　准教授　五島　洋行
　副査　筑波技術大学産業情報学科准教授　谷　貴幸

• 論文目次
• 論文要旨
• 論文審査の結果の要旨

［平成22（2010）年度博士論文題名一覧］ ［博士論文題名一覧］に戻る．

Acknowledgements
Abstract
List of Figures
List of Tables

CHAPTER 1　INTRODUCTION AND BACKGROUND OF RESEARCH
　1.1　INTRODUCTION　p.1
　1.2　HISTORY OF EDM　p.1
　1.2.1　Type of EDM　p.2
　1.2.2　Principle mechanism of EDM　p.3
　1.2.3　EDM process parameters　p.5
　1.2.3.1　Discharge voltage　p.6
　1.2.3.2　Peak current （IP）　p.6
　1.2.3.3　On time and Off time　p.7
　1.2.3.4　Polarity　p.7
　1.2.4　Servo feed control （Gap control）　p.7
　1.2.5　Discharge waveform　p.9
　1.2.6　Evaluation of EDM characteristics　p.12
　1.2.7　Surface topography （Surface finish）　p.13
　1.2.8　Conductive layer generation in insulatiing ceramics machining　p.13
　1.2.8.1　Machining phenomena and their mechanism　p.14
　1.3　ULTRASONIC MACHINING （USM）　p.16
　1.3.1　The machining system　p.17
　1.3.1.1　Acoustic transducers　p.18
　1.3.1.2　Acoustic horn （Mechanical amplifiers or concentrator）　p.18
　1.3.1.3　Abrasive slurry　p.18
　1.3.1.4　Material removal process　p.19
　1.3.2　Ultrasonic polishing　p.19
　1.4　LITERATURES SURVEY　p.20
　1.5　THE ULTRASONIC SYSTEM OF THIS RESEARCH　p.23
　1.5.1　Measuring frequency and amplitude　p.25
　1.6　PURPOSE OF RESEARCH　p.25
　1.7　STRUCTURE OF THIS DISSERTATION　p.26

CHAPTER 2　MACHINING OF CEMENTED CARBIDE MATERIAL
　2.1　BACKGROUND OF MACHINING OF CEMENTED CARBIDE MATERIAL　p.27
　2.2　OBJECTIVE　p.28
　2.3　USEDM ON MACHINING PROCESS　p.28
　2.3.1　Experimental procedure　p.28
　2.3.1.1　Effect of ultrasonic vibration frequency　p.31
　2.3.1.2　Experimental results and discussions　p.31
　2.3.1.3　Investigation of EDMed surface　p.32
　2.3.1.4　Observation of debris　p.33
　2.3.1.5　Observation of debris element　p.35
　2.3.2　Effect of the ultrasonic vibration directions　p.35
　2.3.2.1　Effects of electrode polarity　p.36
　2.3.2.2　Ratio of longtitudinal and transverse amplitude　p.37
　2.3.2.3　Vibration type （ normal, rotation, longtitudial, transverse, and combined）　p.38
　2.3.2.4　EDMed surface profiles　p.41
　2.3.2.5　Observation of debris moving behaviour on each vibration　p.43
　2.3.2.6　Observation of debris　p.46
　2.3.2.7　Effects of ultrasonic vibration on EDMed hole size　p.49
　2.4　GAP DISTANCES AND VIBRATION AMPLITUDE　p.51
　2.4.1　Measuring method of gap distance　p.51
　2.4.2　Experiment result of purpose method　p.52
　2.4.3　Effect of gap distance　p.54
　2.4.4　Surface roughnesses of the bottom hole　p.58
　2.5　CONCLUSIONS　p.61

CHAPTER 3　MACHINING OF Si3N4 INSULATING CERAMIC MATERIAL
　3.1　INTRODUCTION　p.62
　3.2　BACKGROUND OF MACHINING Si3N4 WITH THE ASSISTING ELECTRODE　p.62
　3.2.1　Concept of EDM process of insulating ceramic material　p.63
　3.3　USEDM ON MACHINING PERFORMANCE OF Si3N4　p.64
　3.3.1　Experimental procedure　p.64
　3.3.1.1　Effect of frequency and amplitude on machining efficiency　p.64
　3.4　RESULT AND DISCUSSION　p.65
　3.4.1　Result of high-and low frequency on machining efficiency　p.66
　3.4.1.1　Material removal rate and discharge waveform　p.66
　3.4.1.2　Adhesion of conductive layer　p.68
　3.4.1.3　Observation conductive layer　p.69
　3.4.1.4　EDMed surface　p.70
　3.4.2　Effects of ultrasonic vibration on transition characteristics　p.71
　3.4.2.1　Effect of electrode vibration on transition time　p.73
　3.4.3　Effects of electrode polarity　p.75
　3.4.4　Effects of ultrasonic vibration on material removal rate　p.78
　3.4.5　Observation of conductive layer　p.79
　3.4.6　EDMed Surface profiles　p.80
　3.4.7　Effects of ultrasonic amplitude on machining efficiencies　p.81
　3.4.8　Adhesion condition of conductive layer　p.82
　3.4.9　EDMed surface conductive layer　p.84
　3.4.10　Composition of conductive layer　p.85
　3.5　POLISHING EDMed BY ULTRASONIC AND ABRASIVE　p.85
　3.6　CONSLUSIONS　p.92

CHAPTER 4　CONCLUSIONS
　4.1　SUMMARY　p.93
　4.2　FURTURE STUDY　p.96

　References　p.98
　List of publications and conferences　p.102

　Cemented carbide and Si3N4 insulating ceramics are widely used in engineering industries as cutting tools and die manufacturing materials. They are considered to be too hard to machine with traditional mechanical machining systems. To overcome this problem, many machining methods have been investigated, such as laser, water jet, ultrasonic vibration, electron beam and electrical discharge machining （EDM） methods. However, many disadvantages still exist for each method that concern the accuracy of the size and shape, removal rate, resistivity to both corrosion and erosion as well as high cost problems. It is well-known that the EDM method is a precise machining method for electrically conductive materials that uses a soft, cheap tool electrode but has a slow rate of material removal.

　To obtain the productivity, product accuracy and surface quality, a new hybrid machining process was devised with EDM and USM （ultrasonic method）, which is called USEDM. In this research, to improve the machining properties of the EDM for cemented carbide and Si3N4 insulating ceramics, new methods were designed and proposed that use the ultrasonic vibration technique.

　Two types of USEDM machining systems were produced. One had a low frequency of 29 kHz with a large vibration amplitude, while the other had a high frequency of 59 kHz with a small amplitude. The Cu-W tool electrode was synchronized with the devised vibration system, and several discharge generation conditions were carried out on the cemented carbide material and the Si3N4 insulating ceramics. When machining the Si3N4 ceramics, the assisting electrode method, which was proposed and named by the author's research group about 20 years ago, was applied.

　The following experimental factors were investigated: the polarity of the tool electrode, servo gap, vibration amplitude, discharge ampere, on and off time, vibration frequency, vibration direction, rotational machining and so on. The machining properties were observed and included the material removal rate （MRR）, the wear ratio of the tool electrode and the surface roughness of the machined surface. The discharge waveforms were observed and analyzed by our designed observation system. SEM, laser optical microscope and EDS were used to investigate the EDM-treated surface.

　At first, the machining efficiencies of the USEDM were evaluated when machining the cemented carbide of the WC-Co （ED30） material. Better machining properties were obtained with a negative polarity in the tool electrode. The effects of the frequency were investigated in the direction of the longitudinal vibration. It was revealed that the machining efficiencies of the 29-kHz ultrasonic frequency were better than the 59-kHz ultrasonic frequency. Because of the large amplitude, the 29-kHz frequency had a higher pumping force than the small amplitude of the 59-kHz frequency. The debris was easier to eject out of the discharge gap with the large pumping force.

　The machining efficiencies of the vibration directions were also investigated with the 29-kHz machine. The machining was carried out against the normal EDM with no spindle rotation （EDM-NR） and with a normal EDM-spindle rotation （EDM-R）. The directions of the electrode’s vibration were tested for the longitudinal vibration, the transverse vibration and the complex vibration. The results showed that the highest machining efficiencies were obtained for the complex vibration. The MRR of the complex vibration was two times higher than the EDM-R approximation and four times higher than the EDM-NR in the finishing condition. A higher pumping force could be obtained for the complex vibration. The correlation between the gap distance and the servo value was estimated by the steel ball method, which was proposed by our research group. The results revealed that the superior machining efficiency could be obtained for a difference value between the gap size and the amplitude were larger than the debris size approximation two times.

　When machining the sintered Si3N4 insulating ceramics, the results revealed that the 59-kHz ultrasonic frequency with a small amplitude of 3.5 μm affected the machining efficiencies. The large amplitude of 18 μm for the 29-kHz ultrasonic frequency interfered with deposition of a conductive layer on the workpiece’s surface, which led to an unstable machining condition. The machining efficiencies of the USEDM on the Si3N4 insulating ceramics were investigated and compared to the results with the normal EDM. The MRR of the USEDM was two times higher than the normal EDM approximation. The ultrasonic vibration affected the deposition of a conductive layer on the workpiece’s surface during the transition process in which the discharge generation changed from the electrical conductive area to the insulating material. Thus, the ultrasonic vibration must be added after the transition process. The EDM-treated surface of the Si3N4 was polished to remove the conductive layer by using the ultrasonic method with an abrasive slurry. The conductive layers and the craters were removed by the proposed method. The surface roughness was greatly improved. It was clarified that the ultrasonic vibration could assist the material removal behavior of the discharge.

　本論文は「Effects of Added Ultrasonic Vibration on the Electrical Discharge Machining Properties for Cemented Carbide and Si3N4 Insulating Ceramics（超硬材料および絶縁性Si3N4の放電加工特性に及ぼす超音波振動付加の影響）と題し、4章より構成されている。第1章「序論」では、既存の機械的加工法の多くの手法において、難加工性材料としてされている、高硬度・高強度の脆性材料であるWC系超硬材料および絶縁性窒化珪素（Si3N4）セラミックスに対する形状加工法の概要を示すとともに、高精度の複雑形状加工の加工が可能な放電加工法の適用に関する問題点を述べて、本研究で開発した超音波法を付加した新たな放電加工法開発の経緯を説明している。さらに、開発した装置の機能について説明している。

　第2章「超硬材料の加工」では、超硬材料（WC-Co系、G5相当）を被加工物として、本研究で開発した2種類の放電加工システムの適用の可能性を調べ、超硬材料には、29kHz・大振幅の装置が適していること示している。さらに、電極極性は負とすることで良好な安定な加工が行えることを明らかにしている。電極の回転、振動付与方向が加工特性に及ぼす影響を調べ、電極を縦および横方向に同時に振動させることにより、加工速度は通常加工の4倍程度向上することを明らかにした。この値は、これまでに報告されている他の報告より10倍程度向上している。

　さらに、電極と被加工物間の極間距離と振動振幅値の関係を適当に選択することにより加工状態は向上することを示し、条件の選定指針を明らかにしている。　これらの、加工現象を解明するために、高速ビデオを用いた加工穴内部での加工屑の移動状態記録装置を考案し、各振動モードでの加工屑の運動・排出現象を解析している。加工屑の形状・寸法と運動状態と放電の波形解析結果を相互比較することにより、超音波を付加することにより、放電加工特性が向上する現象解明している。

　第3章「絶縁性窒化珪素（Si3N4）の加工」では、絶縁性材料の放電加工法である“補助電極法”を用いて、超音波付加の影響を検討して、59kHz・小振幅の装置の適用が良好であることを明らかにしている。この手法では、導電性の被膜を形成しながら加工が行われることから、超音波付加は、補助電極材料の加工時　および導電性被膜形成時の遷移領域が過ぎてからの加工領域で行うことにより、加工特性は、通常加工の2倍程度まで向上することを示している。ただし、表面粗さは、超音波法を付加することにより、通常加工より若干劣化するので、放電加工後、電極と被加工物間に硬質の粉末を挿入して超音波法を付加することにより、表面粗さは、Raで1μm以下のすることが可能であることを明らかにした。

　第4章「総括」では、本研究で得られた結果を要約し、総括している。

　以上のように、本論文で得られた知見は、難加工性材料の高精度複雑形状加工においては、超音波を付加した放電加工法が有効であることを示している。よって、本論文は工学上及び工業上貢献することが大きく、博士（工学）の学位論文として十分な価値を有するものと認める。