Polycrystalline Composite Diamond Research Articles

Efficient electromagnetic micropunching technology for producing well-ordered, high-density micropits

ABSTRACT Micropits are favorable for interfacial lubrication and cell culturing. This study developed an AC electromagnetic-driven high-speed micropunching mechanism capable of rapidly fabricating a large-numbered array of well-ordered micropits. The designed mechanism converts 60-Hz AC into 120-Hz electromagnetic force, which in turn drives a punching-shaft to complete a high-speed punching movement at 120-Hz. The micropunching mechanism is installed on a desktop gantry framework to create a stable, high-speed CNC micropunching system. Combining an eddy current sensor with an oscilloscope determines optimal spring stiffness for the system. A micropunching experiment is performed using a micropunching tool comprised of electrically conductive polycrystalline composite diamond (ECPCD) and an annealed aluminum alloy workpiece. The results revealed that a 120-Hz punching-frequency, a spring stiffness of 2.7 N/mm, and a workpiece moving speed of 2160 mm/min generate micropits with a high-integrity profile. It takes only 3.4 seconds to complete 400 high-integrity, well-organized micropits, with a depth of 23.0–23.3 μm that are burr-free and almost chipless, indicating the proposed high-speed electromagnetic micropunching technology is highly precise and efficient while also being energy-saving.

Microgroove grinding of monocrystalline diamond using medium-frequency vibration-assisted grinding with self-sensing grinding force technique

Abstract This study presents an approach to extra-hard material removal using medium-frequency vibration-assisted grinding with grinding force for producing a microgroove array on monocrystalline diamond (MCD). Vibration mode at nano-amplitude is employed to vibrate the MCD workpiece against the polycrystalline composite diamond (PCD) wheel-tool during grinding. This approach is used to break the covalent atomic bonds of diamond, creating a nanocracked subsurface layer thereby reducing grinding during material removal. The PZT stage’s grinding system is designed with a low center of gravity, bilateral symmetry, and an in-situ co-shaft grinding devise. This setup provides a well-defined nanoscale depth of grinding. Expentmental results show that a highly consistent, smooth crisscross microgroove array can be successfully produced on MCD with no apparent burrs and chipping. It is confirmed that medium-frequency vibration-assisted grinding with grinding force remarkably reduces grinding and improves the surface finish on a MCD workpiece. In addition, these experiements prove that diamond’s innate charateristic of thin-brittle resistance self-generates a nanocrack growth boundary stopping further crack expansion. Using a self-sensing grinding force design allows the system to determine whether the nanocracked subsurface layer has been completely removed. This ensures high quality grinding, which produces a flat, solid microgroove surface on MCD. Furthermore, detailed discussions are conducted for the following aspects: effects of dressing using spark erosion, influences of amplitude and grinding mode, geometry and topography of microgrooves, PCD wheel tool wear, and an assesment criterion for available nasal-tip radius.

Improvement of Wire Electrical Discharge Machining Characteristics in Machining Boron-doped Polycrystalline Diamond Using a Novel Iso-pulse Generator

This study aims to present a novel iso-pulse generator for improving machining characteristics of boron-doped polycrystalline composite diamond (BD-PCD) by wire electrical discharge machining (wire-EDM). The transistor-controlled pulse generator composed of a high-power source circuit, a low-power source circuit, a snubber circuit and a complex programmable logic device based pulse control circuit was devised to provide a high open voltage during the ignition period and a low peak current with a fixed discharge duration for sparking as well as identify gap states as normal sparks and abnormal sparks based on the duration of ignition delay time. Pulse on-time and pulse interval of each spark are regulated in real-time to achieve the stability of machining operation while preventing wire electrode from breaking according to the identified gap state. Compared to a conventional iso-frequency pulse generator, the developed iso-pulse generator can provide smaller variation of peak current and discharge duration and thus, the latter can enable the wire-EDM process to produce smaller surface damages and better surface quality than the former. Experimental results not only verify the iso-pulse generator can achieve a fine surface finish as low as Ra 0.247 μm on machined surface of BD-PCD through one rough and one finish machining operation, they also demonstrate that the developed system can enable a table-top wire-EDM machine to successfully sharpen the cutting edge of BD-PCD end milling tools.

Development of a novel micro w-EDM power source with a multiple Resistor-Capacitor (mRC) relaxation circuit for machining high-melting point, -hardness and -resistance materials

This study presents the development of a novel micro-wire electric discharge machining (w-EDM) power source for machining high-melting point, -hardness and -resistance materials. The relaxation circuit power devised with a multiple Resistor-Capacitor (mRC) is triggered by a Programmable Logic Device (PLD) to generate a high-peak, short-pulse-time current train. The mRC relaxation circuit is characterized by high density discharge-spots per unit of time for swiftly removing material and creating fine discharge craters. Three microstructures made of tungsten carbide, WC-based cermet, and Boron-Doped Polycrystalline Composite Diamond (BD-PCD), respectively, are machined to validate the novel relaxation circuit. Experimental results demonstrate the 3RC relaxation circuit reduces machining times by 25%–35%, resulting in a more time efficient micro w-EDM process. Compared with a transistorized power source, the heat-affected zone is smaller and surface finish is better when using the 3RC relaxation circuit. A set of metrics for helping measure important electro-thermal machining related properties during micro w-EDM is proposed in this study. These are the ‘Spark Erosion Rate (SER)’, ‘Electro-Thermal Machinability (ETM)’, and ‘Cutting Performance (CP)’. An extensive examination of the quantitative and qualitative properties of material erosion mechanisms, material removal rates (MRR), and cobalt deposition for machining of difficult-to-machine materials is undertaken and the results discussed in detail.

Development of a PCD Burnishing Tool and its Burnishing Performance

In this study, a polycrystalline composite diamond (PCD) was adopted instead of a single crystal diamond as a burnishing tool. At first, EDM machinability was compared between two kinds of PCDs (10μm grain size), i.e. standard PCD (S-PCD) and electrically conductive PCD (EC-PCD (which was made of boron doped diamond particles). EDM machining time to make a hemispherical shape was 17 minutes for EC-PCD and 37 minutes for S-PCD. Further experiments were conducted with EC-PCD of different grain sizes such as 2μm and 5μm. As a result, the smaller the grain size of EC-PCD the higher the machining efficiency was. Surface roughness values of the EC-PCD after finish EDM were Rz=1.7μm for EC-PCD010, Rz=1.4μm for EC-PCD005, and Rz=1.2μm for EC-PCD002. As a result of burnishing a carbon steel bar (S45C, φ30mm) with the EC-PCD tool, the surface roughness value after machining was improved from 10μm to 1μm.

Development of a Pulse Generator for Rough Cutting of Oil-based Micro Wire-EDM

This paper aims to develop a specific pulse generator for rough cutting of oil-based micro wire electrical discharge machining (wire-EDM). A two-stage current-limiting resistance power supply was presented to suppress thermal damage on the machined surface of polycrystalline diamond (PCD) while achieving stable machining by supplying high open voltage and low discharge current pulse waveforms. Tests revealed that the two-stage resistance power supply could achieve lower thermal damaged layer, better surface quality and surface finish than a conventional pulsed power supply since the former could provide smaller peak current and shorter discharge duration than the later. Experimental results demonstrate that boron-doped polycrystalline composite diamond (BD-PCD) is superior to polycrystalline diamond in the smoothness of cutting edge, surface quality and surface roughness since the former has lower specific resistance and thermal conductivity than the later. This study has indicated that the proposed power supply could enable a prototype micro wire-EDM machine to achieve a surface finish as low as 0.45μm Ra and a thermal damaged layer as small as 6μm on the machined surface of BD-PCD after one rough machining operation.

A force controlled grinding-milling technique for quartz-glass micromachining

This paper presents a novel approach to the micromachining of quartz glass using an intellectualized grinding-milling technique to overcome the difficulties in machining hard-brittle materials. A bench-type linear 3-axis CNC machine tool providing grinding-milling at depths of several nanometers is constructed to realize ductile-regime material removal during quartz-glass milling. Finite element analysis (FEA) is conducted on the deformation and resonant frequency of the developed machine tool. A micro-tipped grinding-tool made of boron-doped polycrystalline composite diamond (BD-PCD) and designed with a double-negative back rake angle (DN-BRA) to create compressive stress grinding-milling is proposed and employed. To sense the force at which grinding-milling is conducted and provide real-time feedback on the milling tool's feed-rate, load-cells are devised on 3 axes. Using an appropriate grinding-milling technique in combination with proper feedback to control the machining feed-rate, quartz glass is machined layer-by-layer under a ductile regime. A miniature 3-step-shaped pyramid made of quartz glass of 0.3mm in height and of Ra0.66μm surface roughness with very little brittle fracturing is achieved. The optimum grinding depth, milling speed and corresponding grinding-milling force are 1μm, 50–70m/min, and 0.4N, respectively. A comprehensive examination of the quantitative and qualitative properties of the BD-PCD tool was undertaken. Experimental confirmation of the proposed approach is presented. Additionally, the following aspects are discussed in detail: the spark erosion rate of the machined diamond tool, milling feed-rate, grinding depth, graphitization of diamond, and tool wear.

Successful fabrication of a convex platform PMMA cell-counting slide using a high-precision perpendicular dual-spindle CNC machine tool

This study presents a novel approach to the fabrication of a biomedical-mold for producing convex platform PMMA (poly-methyl-meth-acrylate) slides for counting cells. These slides allow for the microscopic examination of urine sediment cells. Manufacturing of such slides incorporates three important procedures: (1) the development of a tabletop high-precision dual-spindle CNC (computerized numerical control) machine tool; (2) the formation of a boron-doped polycrystalline composite diamond (BD-PCD) wheel-tool on the machine tool developed in procedure (1); and (3) the cutting of a multi-groove-biomedical-mold array using the formed diamond wheel-tool in situ on the developed machine. The machine incorporates a hybrid working platform providing wheel-tool thinning using spark erosion to cut, polish, and deburr microgrooves on NAK80 steel directly. With consideration given for the electrical conductive properties of BD-PCD, the diamond wheel-tool is thinned to a thickness of 5 µm by rotary wire electrical discharge machining. The thinned wheel-tool can grind microgrooves 10 µm wide. An embedded design, which inserts a close fitting precision core into the biomedical-mold to create step-difference (concave inward) of 50 µm in height between the core and the mold, is also proposed and realized. The perpendicular dual-spindles and precision rotary stage are features that allow for biomedical-mold machining without the necessity of uploading and repositioning materials until all tasks are completed. A PMMA biomedical-slide with a plurality of juxtaposed counting chambers is formed and its usefulness verified.

A002 Development of A High-Precision Hybrid CNC Machine Tool for In-Situ Fabricating A Biomedical-Slide Mold

The study presents a novel approach to the fabrication of a biomedical-slide mold for in-situ producing convex platform quantitative cell-counting slides made of PMMA. The process incorporates three important procedures: 1) the development of a high-precision hybrid dual-spindle CNC machine tool; 2) the formation of a boron-doped polycrystalline composite diamond (BD-PCD) wheel-tool on the machine tool developed; and 3) the cutting of a multi-groove -biomedical-slide mold array using the formed diamond wheel-tool in-situ on the developed machines. The factors influencing formation of the wheel-tool and generation of the microgroove array are discussed in detail.

EDM Properties of Newly Developed PCD Made Up of Electrically Conductive Diamond Particles

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 property and machinability. This paper deals with material properties and electrical discharge machinability of EC-PCD. The result shows that the EC-PCD is superior in heat resistance to conventional PCD by a factor of about 200°C. And the EC-PCD shows low frictional wear at high temperature in a sliding test against a stainless steel disk. Furthermore, processing speed of EC-PCD is more than 7 times higher than that of conventional PCD in EDM.

Development of an ultrathin BD-PCD wheel-tool for in situ microgroove generation on NAK80 mold steel

This study presents the development and application of an ultrathin diamond wheel-tool that combines in situ reverse RWEDM (rotary wire electrical discharge machining) with in situ HSFSG (high-speed and fast-shallow grinding) technique for directly generating precision microgrooves onto NAK80 mold steel. The mechanism for reverse RWEDM involves the brass wire being located beneath the disk workpiece to upwardly machine it by spark erosion. The method is employed for thinning a diamond wheel-blank made of boron-doped polycrystalline composite diamond (BD-PCD). The good electrical conductivity of BD-PCD allows for an ultra-thin grinding-edge of 5-μm thickness to be produced on the wheel blank. The HSFSG technique is used to successfully grind micro-grooves in NAK80 mold steel. The method overcomes the traditional obstacles to using diamond to machine steel by having excellent metal removal rates. Experimental results prove the precision of the microgrooves generated. Nanometric grinding depth results in much less friction allowing for the cold machining conditions and preservation of diamond's sp3 bond structure. The grinding-edge has an extremely low wear rate of just 0.1μm per 280mm of grinding. The factors influencing formability, thermal machinability, graphitizing of diamond and the wear process of the BD-PCD wheel-tool are discussed in detail.

Performance of Ultrasonic Electro-Discharge Machining on Electrically Conductive PCD

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.

Study on Thinning of a Boron-Doped Polycrystalline Diamond Wheel-Tool by Micro Rotary W-EDM Approach

This study presents a novel approach for using a micro rotary wire Electrical Discharge Machining (micro w-EDM) to thin the grinding-edge of a wheel-tool made from boron-doped polycrystalline composite diamond (PCD). For thinning the PCD, two discharge circuits (a Resistance-Capacitance (RC) circuit and a transistor) were used as power sources to obtain a grinding-edge of less than 10 µm in thickness and high surface quality. The wheel-blank is vertically mounted on a spindle and while rotating is thinned by micro w-EDM along a planned computer numerically controlled path. Experimental results verify that boron-doped PCD can be successfully thinned down to 5 µm in edge-thickness. The study shows it is possible to break (cut) diamonds of 10-µm grain size, leaving smooth surface-exposed diamonds at the cutting edge of the wheel tool. The dimensional and geometrical accuracy of the wheel-tool can be exactly controlled. Raman analysis reveals graphitizing of the PCD caused by local high temperature spark erosion at a peak of 1593 cm-1 in RC discharge circuit machining. The peak at 1332 cm-1 for the transistor circuit method indicates diamond sp3 structure. The surface degenerating layer produced by transistor circuit machining gives a suitably thin grinding edge with exposed diamond grains.

EDM Properties of EC-PCD Manufactured Using Electrically Conductive Diamond Particles

Electrically conductive polycrystalline composite diamond (EC-PCD), which consists of electrically conductive diamond particles manufactured by doping boron atoms, has recently been developed for the purpose of providing material with both excellent tool properties and good machinability. This paper deals with an investigation into the machinability of EC-PCD by EDM with a copper electrode. As a result, it was found that the material removal rate of EC-PCD made up of diamond particles of 10 and 25 µm in diameter respectively was far higher than that of the conventional PCD. Furthermore, it was confirmed in the single pulse EDM experiment with a needle electrode that the EC diamond region in the EC-PCD was removed together with the cobalt region whereas only the cobalt region of C-PCD was removed when the electrode was set at plus polarity.

Effect of Complex Electrodischarge Grinding for Electrically Conductive PCD

Polycrystalline Composite Diamond (PCD) is excellent in chipping resistance despite its very high hardness. However, it is not easy to EDM or grind PCD. To realize high efficiency and high quality processing of PCD simply and at low cost, the authors devised new PCD (EC-PCD) by using electrically conductive diamond particles and applied a complex electrodischarge grinding method. In this study, investigation is made on effective grinding condition to realize high efficiency, low and stable grinding force and low wheel wear in complex electrodischarge grinding. As a result, superior grinding property was obtained when the grinding wheel was set at minus polarity, and set peak current of iP = 4 and 6 A was applied. Furthermore it also became clear that additional conventional grinding process followed after complex electrodischarge grinding improved the surface condition.

Investigation on Material Property of Electrically Conductive Polycrystalline Composite Diamond (EC-PCD)

Electrically conductive polycrystalline composite diamond (EC-PCD) material, which consists of electrically conductive diamond grits, has recently been developed for the purpose of providing the material with both excellent tool property and machinability. This paper deals with the material property and the wear characteristics of EC-PCD. The result shows that the EC-PCD is superior in heat resistance compared with the conventional PCD. And the EC-PCD shows low frictional wear at high temperature in sliding test against stainless steel disk. Furthermore, in a cutting test of aluminum alloy, the EC-PCD cutting tool shows the same wear characteristics as the conventional PCD tool.

EDM Properties of EC-PCD Using a Copper Electrode

Electrically conductive polycrystalline composite diamond (EC-PCD), which consists of electrically conductive diamond grits, has recently been developed for the purpose of providing the material with both excellent tool property and good machinability. This paper deals with an investigation of machinability of EC-PCD by EDM with a copper (Cu) electrode. As a result, it was found that the EDM speed (Material removal rate) for EC-PCD was higher than that of the standard PCD. Although the surface roughness of the standard PCD was 13µm Rzjis at set current ip=3A, the surface roughness of the EC-PCD was 5µm Rzjis at the same current condition. Furthermore, it was observed that the EC diamond particle in EC-PCD was machined by single discharge EDM. Moreover, it was confirmed that EC-PCD was able to be applied a fine discharge profiling or a making fine holes using a rotating electrode.

Complex Grinding Assisted with Electrical Discharge Machining for Electrically Conductive PCD

A new PCD material named EC-PCD (Electrically conductive polycrystalline composite diamond), which consists of electrically conductive diamond grits, has recently been developed. This paper deals with an investigation of a complex grinding assisted with electrical discharge machining (EDM) to realize high efficiency, low and stable grinding force and low wheel wear for the new EC-PCD. The effect of complex grinding assisted with EDM is compared experimentally with the standard PCD (S-PCD). The result shows that, in the complex grinding, lower and more stable grinding force is realized thanks to the material removal action in EDM and that lower wheel wear and better surface finish are attained, just when the EC-PCD is selected as a workpiece.