Papers by Author: Shinichi Ninomiya

Abstract: Use of a tool driven by a spin turning method on a 5 axis turning center has an advantage in suppressing a tool wear in the external turning of hard-to-machine materials. However, it is important not easy to control a radial runout of the tool periphery in the spin turning. In this research, a simple rod shaped tungsten carbide was applied as a rotating tool to the turning of a hardened steel workpiece (HRC63). In addition, on-machine grinding method was proposed as a measure to minimize a radial runout of the rotating tool. With this technique, a well suited rake angle, relief angle or chip breaker profile could be easily given to the tool. In spin turning of the hardened steel with a WC rod tool, roughness value of the worked surface was reduced and tool wear was suppressed in comparison with the turning without tool rotation. In addition, it was shown that correction of the tool runout and forming of the tool rake face could be done on the machine.

Abstract: In our previous research, precise groove machining could be achieved on a cemented carbide workpiece by utilizing a combined method of EDM and grinding using only a single tool, i.e. a rotary PCD disc tool acting as an electrode for EDM and as a wheel for grinding. In this study, the authors applied this method to forming a 3D shape on a cemented carbide workpiece. Firstly, an EC-PCD disc-shaped tool of φ33.4mm was mounted on a spindle of the EDM machine vertically to the workpiece. And the tool was rotated to make a spherical concave on the workpiece by EDM. The surface roughness of the workpiece was Rz=6.4μm and no electrode wear resulted at all. Secondly, the EDMed surface was ground with the same PCD tool. As a result, the improved surface roughness of Rz=1.7μm was obtained on the workpiece maintaining the precise tool profile and dimension.

Abstract: In order to cope with various problems associated with machining of PCD, development of a new PCD possessing excellent tool properties and good machinability at the same time has been demanded. From this point of view, the authors have developed a new PCD, “EC-PCD (Electrically Conductive PCD)”, composed of boron doped diamond particles in place of the standard non-conductive diamond. In this research, investigation into material properties of the newly developed EC-PCD is made. Through the tests, it was found that the boron doped diamond particles (the source material of the new PCD) had an electrical resistivity of 1.6×10^-4Ω·m and the thermal conductivity after sintering was lower than that of the standard PCD. In addition, it was confirmed that the electrical conductivity of the source diamond particles of the EC-PCD had not been lost even under the condition of high temperature and high pressure during the manufacture. As a result of the heat test, EC-PCD’s high resistance to oxidation at high temperatures was confirmed showing no changes in the surface condition even at 675oC while the surface of the S-PCD was largely changed at the same temperature. One of the reasons for this is guessed to be that EC-PCD is hard to react with the cobalt contained as a catalyst metal. Further, it was found in the friction tests using a steel ball that the friction coefficient of the EC-PCD was 50% higher than that of S-PCD at the room temperature though it dropped by 10-30% at the temperature of 80oC.

Abstract: Electrically conductive PCD (EC-PCD), which was made of electrically conductive diamond particles, was applied to an electrode for EDM. In this study, properties of EC-PCD electrode in the EDM of cemented carbide were investigated and the results were compared with those of copper-tungsten (Cu-W) electrode and standard PCD (S-PCD) electrode. In terms of EDM speed, 35μm/min with Cu-W electrode, 15μm/min with S-PCD electrode and 22μm/min with EC-PCD were recorded. As for the electrode wear rate, though Cu-W electrode wore by 20-30%, S-PCD and EC-PCD did not wear at all and even the electrode length became longer by several micro meters. Roughness value of the EDMed surface was Rz=8μm with both Cu-W and S-PCD, while it was Rz=3μm in the case of EC-PCD. From the above, it was found that EC-PCD showed excellent performance in the electrode wear rate and the roughness of the EDMed surface, though EDM speed was 0.7 times of Cu-W electrode.

Abstract: It has been made clear that the EC-PCD composed of boron doped diamond particles improves the performance in the die sinking EDM and wire EDM in comparison with the existing standard PCD (S-PCD). However, the effect of the property improvement could not been evaluated quantitatively in the research reports in the past. Therefore, in this study, wire EDM cutting tests were conducted on the specimens of S-PCD and EC-PCD, in addition to the evaluation of cutting efficiency and cutting PCD surface, a detailed investigation of the cut surface properties of the PCD has been performed under the color 3D laser scanning microscope. In order to investigate effects of a grain size of the source diamond, EDM cutting experiments were conducted on the EC-PCD specimens of 4 different grain sizes. As a result, it was found that the cutting speed was higher in the case of EC-PCD than the case of S-PCD, e.g., by 20% and 40% respectively in the sample of 10μm and 25μm particle size. Also, in the case of the cut S-PCD surface, a groove due to the discharge is formed in the boundary of the tungsten carbide layer and the PCD layer. However, in the case of the cut EC-PCD surface, the groove did not appear in the boundary.

Abstract: For a purpose to improve electric discharge machinability of PCD composed of coarse (25μm) diamond particles, which has been thought to be extremely difficult to EDM, in this study, a method (US-EDM) to give an ultrasonic vibration to an electrode in axial direction, flexural direction and complex direction that couples axial and flexural directions was attempted. As a result, it was found that EDM efficiency could be improved to 6 times higher (0.065mm³/min) than a standard efficiency (0.011mm³/min) obtained in machining conventional PCD (C-PCD) and the electrode wear could be reduced to 1/2 by giving vibrations to the electrode in axial direction (frequency f=28kHz, amplitude δ=18μm). Further, this method (US-EDM) was applied also to a new PCD (EC-PCD) composed of electrically conductive diamond particles. As a result, it was made clear that EDM efficiency could be improved to 0.22mm³/min. This value is equivalent to approximately 5 times higher efficiency obtained in the machining of ordinary die steel and cemented carbide materials.

Abstract: Grinding experiments were carried out in wet, with MQL, in dry, with a spray of dry ice particles and various gases to examine influence of grinding atmospheres on the grinding performance in the combination surface grinding of steel and WC with a diamond wheel. From the experimental results, it was found that the grinding with inert gas injection which reduces an oxygen concentration at the grinding point would proceed a processing at more stable grinding force, resulting in less wear of the diamond abrasives.

Abstract: Electrically conductive polycrystalline composite diamond (EC-PCD) material, which consists of electrically conductive diamond particles, has recently been developed for the purpose of providing the material with both excellent tool properties and machinability. This paper deals with effects of giving ultrasonic vibrations to an electrode on the EDM characteristics of EC-PCD with a view to achieve a great improvement in electro-discharge performance. In this series of EDM experiments, three types of ultrasonic vibration modes were selected (axial vibration, flexural vibration and complex vibration). The result showed that the removal efficiency of EC-PCD increased in any vibration modes. Especially when axial vibration with large amplitude and complex vibration composed of axial mode and flexural mode were given to the electrode, removal efficiency became four times higher than that obtained when no vibration was given. And, electrode wear rate was reduced to one third or 68% against the value of 187% obtained when no vibration was given. Furthermore it was shown that the effects resulted from not only the cavitation action of the working fluid but also the vibrational action of the electrode itself.

Abstract: With its high grit retention and easiness to true and dress, the vitrified bond is widely used as a bond material for cBN and diamond grinding wheels. By giving electrical conductivity to the vitrified bond, application of electrical discharge trueing/dressing and detection of a workpiece position by electrical contact sensing will become possible. And moreover, application of the vitrified bonded wheels to various types of electro-assisted grinding processes (electrochemical or electro discharge assisted methods) is expected. In this study, vitrified bonded diamond segments with electrical conductivity were manufactured experimentally by mixing the fine copper powders in the vitrified bond matrix. As a result of investigation into the electro discharge trueing performance in the die sinking and wire electro discharge machining, it was found that a vitrified bonded wheel could be formed by electro discharge machining only because the bond was electrically conductive. In addition, the electro discharge complex grinding utilizing electric discharge machining was applied to the PCD cutting tool materials using the electrically conductive vitrified bonded wheel, and confirmed that the grinding could be continued for a long time maintaining a stable grinding force.

Abstract: A vitrified bonded diamond wheel having electrical conductivity was manufactured on a trial basis. The electrically conductive (EC) vitrified bonded diamond wheel has enabled an electrodischarge truing and an electrical contact sensing. In this study, the EC vitrified bonded diamond wheel is applied to the EDM (Electro Discharge Machining) assisted grinding of PCD materials. The result shows that a lower and more stable grinding force can be realized and a better surface finish is attained compared with those achievable with a metal bonded diamond wheel.