Study of the Diffusion of Carbon, Its Sources, and Effect on Finishing Micro-EDM Performance of Cemented Carbide

Apart from the necessity of surface modification based on different applications, in most of the cases, diffusion of carbon or foreign particles on the workpiece surface during micro-electrodischarge machining (micro-EDM) is avoidable, especially in finishing micro-EDM. This study aims to investigate different sources of materials that migrate to the machined surface during fine-finishing of micro-EDM of cemented tungsten carbide (WC-Co). The machined surfaces have been examined under scanning electron microscope and energy dispersive x-ray to investigate the changes in chemical composition. It has been observed that during finishing of micro-EDM, the major source of materials' transfer to both the workpiece and electrode is the diffusion of carbon that comes from the decomposition of the hydrocarbon dielectric. In addition, materials from both workpiece and electrode transfer to each other based on machining conditions and discharge energy. The migration occurs more frequently at lower gap voltages during die-sinking with micro-EDM because of low spark gap and stationary tool electrode. Milling micro-EDM results in lower amount of carbon migration and fewer surface defects that improve the overall surface finish significantly.     

Analysis of the electrochemical behaviors of WC–Co alloy for micro ECM

Micro electrochemical machining (ECM) of tungsten carbide with cobalt binder (WC–Co) was studied using ultrashort pulses. In ECM, the machining characteristics were investigated according to machining conditions such as electrolyte, workpiece potential, and applied voltage pulse. Using a mixture of sulfuric acid and nitric acid, microstructures with a sharp edge and good surface quality were machined on tungsten carbide alloy. The potentials of workpiece electrode and tool electrode were determined by considering the machining rate, machining stability, and surface quality of products. With the negative potential of the workpiece electrode, oxide formation was successfully prevented and shape with good surface quality in the range from R_a 0.069 μm to 0.075 μm were obtained by electrochemical machining. Moreover, the performance of ECM, which includes machining gap, tapering, surface roughness, and machining time, without tool wear was compared with that of electrical discharge machining (EDM). Microstructures of WC–Co with a sharp edge and good surface quality were obtained by electrochemical milling and electrochemical drilling. Micro electrochemical turning was also introduced to fabricate micro shafts.     

A study on the quality micro-hole machining of tungsten carbide by micro-EDM process using transistor and RC-type pulse generator

Tungsten carbide (WC) is an extremely hard and difficult-to-cut material used extensively in manufacturing because of its superior wear and corrosion resistance. Besides diamond-charged grinding wheels, micro-EDM is an effective method of machining this extremely hard and brittle material. Since micro-EDM is more generally an electro-thermal process, the supplied energy from a pulse generator is an important factor determining the performance of the micro-EDM process. This study investigates the influence of major operating parameters on the performance of micro-EDM of WC with focus in obtaining quality micro-holes in both transistor and RC-type generators. Experimental investigations were conducted with view of obtaining high-quality micro-holes in WC with small spark gap, better dimensional accuracy, good surface finish and circularity. In micro-EDM, the fabrication of micro-parts requires minimization of the pulse energy supplied into the gap which can be fulfilled using the RC-generator. It was observed that the RC-generator can produce better quality micro-holes in WC, with rim free of burr-like recast layer, good dimensional accuracy and fine circularity. Moreover, the smaller debris formed due to low discharge energy in RC-type micro-EDM can be easily flushed away from the machined area resulting in surface free of burr and resolidified molten metal. Therefore, RC-type micro-EDM could be more suitable for fabricating micro-structures in WC, where accuracy and surface finish are of prime importance.     

Effect of electrode material on electrical discharge machining of alumina

Technologies for machining advanced insulating ceramics are demanded in many industrial fields. Recently, several insulating ceramics, such as Si₃N₄, SiC and ZrO₂, have been successfully machined by electrical discharge machining (EDM). As unstable discharges occur during the machining of Al₂O₃ ceramics, inferior machining properties have been obtained. The formation mechanism of the electrical conductive layer on the EDMed surface is much different as compared to other ceramics. In addition to this, the electrically conductive layers are not formed sufficiently to adhere to the EDMed workpiece surface and keep a stable and continuous discharge generation on the ceramics. Graphite is widely used as electrode material in EDM. It is expected that carbon from graphite electrode implant and generate a conductive layer. Copper, graphite (Poco EDM-3) and copper-infiltrated-graphite (Poco EDM-C3) electrodes were used to compare the effects of generation of a conductive layer on alumina corresponding to EDM properties. The electrical discharge machining of 95% pure alumina shows that the EDM-C3 performs very well, giving significantly higher material removal rate (MRR) and lower electrode wear ratio than the EDM-3 and copper electrodes. The value of MRR was found to increase by 60% for EDM-3 with positive electrode polarity. As for EDM-C3, MRR was increased by 80% under the same condition. When the results were investigated with energy dispersive spectroscopy (EDS), no element of copper was observed on the conductive layer with both EDM-3 and EDM-C3. However, surface resistivity of a conductive layer created with EDM-C3 is less than with EDM-3. Surface roughness was improved to 25 μm with positive polarity of EDM-C3.     

Experimental characterization of material removal in dry electrical discharge drilling

Dry electrical discharge machining is one of the novel EDM variants, which uses gas as dielectric fluid. Experimental characterization of material removal in dry electrical discharge drilling technique is presented in this paper. It is based on six-factor, three-level experiment using L₂₇ orthogonal array. All the experiments were performed in a ‘quasi-explosion’ mode by controlling pulse ‘off-time’ so as to maximize the material removal rate (MRR). Furthermore, an enclosure was provided around the electrodes with the aim to create a back pressure thereby restricting expansion of the plasma in the dry EDM process. The main response variables analyzed in this work were MRR, tool wear rate (TWR), oversize and compositional variation across the machined cross-sections. Statistical analysis of the results show that discharge current (I), gap voltage (V) and rotational speed (N) significantly influence MRR. TWR was found close to zero in most of the experiments. A predominant deposition of melted and eroded work material on the electrode surface instead of tool wear was evident. Compositional variation in the machined surface has been analyzed using EDAX; it showed migration of tool and shielding material into the work material. The study also analyzed erosion characteristics of a single-discharge in the dry EDM process vis-á-vis the conventional liquid dielectric EDM. It was observed that at low discharge energies, single-discharge in dry EDM could give larger MRR and crater radius as compared to that of the conventional liquid dielectric EDM.     

A study on micro-hole machining of polycrystalline diamond by micro-electrical discharge machining

Polycrystalline diamond (PCD), with its superior wear and corrosion resistance, is an ideal material for micro-hole parts in the field of microfabrication. This study investigated the micro-hole machining performance for PCDs by micro-electrical discharge machining (micro-EDM). A series of experiments were carried out to investigate the proper machining polarity and the impacts of micro-EDM parameters on machining performance. Experimental results indicate that negative polarity machining is suitable for micro-EDM of PCDs because of the protection brought over by the adhesion sticking to the electrode. An appropriate volume of adhesion on the tool electrode can help to increase the material removal rate (MRR) and reduce the relative tool wear ratio (TWR). By contrast, an excessive volume of adhesion can lead the machining into a vicious circle in which micro-holes are drilled with overlarge diameters. An optimal set of machining conditions was chosen among the investigated ranges of nominal capacitance and electrode rotation speed. An exemplary PCD through-hole, machined under the chosen optimal machining conditions, shows satisfactory machining results.     

Experimental investigation and empirical modeling of the dry electric discharge machining process

Dry electric discharge machining (EDM) is an environment-friendly modification of the oil EDM process in which liquid dielectric is replaced by a gaseous medium. In the current work, parametric analysis of the process has been performed with tubular copper tool electrode and mild steel workpiece. Experiments have been conducted using air as the dielectric medium to study the effect of gap voltage, discharge current, pulse-on time, duty factor, air pressure and spindle speed on material removal rate (MRR), surface roughness (R_a) and tool wear rate (TWR). First, a set of exploratory experiments has been performed to identify the optimum tool design and to select input parameters and their levels for later stage experiments. Empirical models for MRR, R_a and TWR have then been developed by performing a designed experiment based on the central composite design of experiments. Response surface analysis has been done using the developed models. Analysis of variance (ANOVA) tests were performed to identify the significant parameters. Current, duty factor, air pressure and spindle speed were found to have significant effects on MRR and R_a. However, TWR was found to be very small and independent of the input parameters.     

A study on microgrinding of brittle and difficult-to-cut glasses using on-machine fabricated poly crystalline diamond (PCD) tool

Present study aims to investigate the feasibility of microgrinding difficult-to-machine glass materials with Poly Crystalline Diamond (PCD) tool, which is fabricated on-machine using micro-electrodischarge machining (micro-EDM). A detailed experimental investigation on the mechanism of the process including the effect of micro-EDM machining conditions on tool surface and the effect of grinding parameters on microgrinding performance are presented. In this context, a comparative study on the microgrinding performance of three glass materials (BK7, Lithosil and N-SF14) using on-machine fabricated PCD tool was carried out. It was found that during tool fabrication using micro-EDM process, higher discharge energy generates rougher surface and larger craters on the tool, which can provide higher material removal rate (MRR) during grinding but results in poorer surface finish on glass surface. In addition to micro-EDM conditions of tool fabrication, the roughness of the ground glass surface depends greatly on grinding parameters such as depth of cut, feed rate and tool rotational speed. The surface roughness increases with increasing axial depth of cut and feed rate, whereas higher rotational speed was found to improve the surface finish. Among three types of glass materials, BK7 glass was found to provide better performance in terms of the achieved surface finish and cutting force analysis.     

Rotational abrasive flow finishing (R-AFF) process and its effects on finished surface topography

The present study focus on abrasive flow finishing (AFF), a process that finishes complex internal and external geometries with the help of viscoelastic abrasive medium, while keeping in mind its low finish and material removal rates (MRR). Researchers have often strived to improve finishing rate and MRR. As an attempt to overcome the said limitations, this paper discusses rotational abrasive flow finishing (R-AFF) process wherein complete tooling is externally rotated and the medium reciprocates with the help of hydraulic actuators. In this study, preliminary experiments are conducted on Al alloy and Al alloy/SiC metal matrix composites (MMCs) at different extrusion pressures, and medium compositions are employed for finding optimum conditions of the same for higher change in roughness (ΔR_a). The same optimum conditions are used to study the effect of workpiece rotational speed on (ΔR_a), material removal (MR), change in workpiece hardness and surface topology. It is noted that as the workpiece rotational speed increases from 2 to 10 RPM, the experimental helix angle decreases from 22° to 9° and the helical path length increases from 67 to 160 mm. Based on these findings the mechanism of material removal of matrix and reinforcement in MMC using R-AFF have been proposed. Here the matrix material is removed by micro-cutting and three methods of material removal mechanisms for reinforcement are also explained. The scientific logic behind finishing mechanism of matrix and reinforcement, cross hatch patterns, helical path directions, micro-scratch (μ-scratch) width and depth variation with size, orientation and support that active abrasive grain obtains from neighboring abrasives is derived from scanning electron microscopy micrographs. Finally this study establishes that R-AFF can produce 44% better ΔR_a and 81.8% more MR compared to the AFF process. Accordingly, R-AFF generates micro cross hatch pattern on the finished surface that can improve lubricant holding capabilities.     

EDM performance of Cr/Cu-based composite electrodes

Electrode materials for electrical discharge machining (EDM) are usually graphite, copper and copper alloys because these materials have high melting temperature, and excellent electrical and thermal conductivity. The electrodes made by using powder metallurgy technology from special powders have been used to modify EDM surfaces in recent years, to improve wear and corrosion resistance. However, electrodes are normally fabricated at high temperatures and pressures, such that fabrication is expensive. This paper proposes a new method of blending the copper powders contained resin with chromium powders to form tool electrodes. Such electrodes are made at low pressure (20 MPa) and temperature (200 °C) in a hot mounting machine. The results showed that using such electrodes facilitated the formation of a modified surface layer on the work piece after EDM, with remarkable corrosion resistant properties. The optimal mixing ratio, appropriate pressure, and proper machining parameters (such as polarity, peak current, and pulse duration) were used to investigate the effect of the material removal rate (MRR), electrode wear rate (EWR), surface roughness, and thickness of the recast layer on the usability of these electrodes. According to the experimental results, a mixing ratio of Cu–0wt%Cr and a sinter pressure of 20 MPa obtained an excellent MRR. Moreover, this work also reveals that the composite electrodes obtained a higher MRR than Cu metal electrodes; the recast layer was thinner and fewer cracks were present on the machined surface. Furthermore, the Cr elements in the composite electrode migrated to the work piece, resulting in good corrosion resistance of the machined surface after EDM.     

Shear-thickening polishing method

A shear-thickening polishing (STP) method utilizing the shear thickening mechanism of non-Newtonian power-law fluid based slurry is proposed for curved surface polishing. The STP principle and micro-material removal action are analyzed. The high-performance STP slurry with the shear-thickening rheological behaviors has been prepared. To achieve the material removal mechanism of STP process, based on the Preston formula, fluid dynamics and shear thickening mechanism, the material removal rate (MRR) model is established and the difference of MRR between theoretical and experimental results is 6.12%. The experimental and theoretical tests of STP process are conducted to investigate the influences of polishing velocity, abrasive concentration and grain size on MRR and surface roughness. Compared with Newtonian fluid slurry, STP slurry can achieve much higher MRR and better surface quality due to shear-thickening effect. MRR of Cr12Mo1V1 (die steel) is up to 13.69 μm/h, and surface roughness is reduced from R_a 105.95 nm to R_a 5.1 nm within 0.5 h of processing. This indicates that STP is a promising processing method for precision finishing or polishing.     

A study on the machining of high-aspect ratio micro-structures using micro-EDM

Micro-electro-discharge machining (micro-EDM or μ-EDM) has been gaining popularity as a new alternative method to fabricate micro-structures. The main advantages of the micro-EDM method are its low set-up cost, high accuracy and large design freedom. Compared to etching or deposition techniques, micro-EDM has the advantage of being able to fabricate complex three-dimensional shapes with high-aspect ratio. However, there are many operating parameters that affect the micro-EDM process. The fabrication of micro-electrodes on the machine is also an important process to remove the clamping error to maintain high accuracy in the machined micro-structures.

In this paper, the machining of micro-structures is divided into two basic processes. One is the on-machine fabrication of the micro-electrodes with high-aspect ratio, and the other is the EDM of the workpiece in micrometer range. An optical sensor has been developed to measure and control the dimension of the thin electrode during the tool fabrication process. Different methods have been investigated to fabricate a thin electrode into the desired dimension without deflection. The performance of the micro-EDM process is evaluated in terms of the material removal rate (MRR), tool wear ratio (TWR), and the stability of the machining. Influences of the various operating parameters of the micro-EDM process, such as the operating voltage, gap control algorithm, and resistance and capacitance values in the R–C spark control circuit, are discussed.     

A comparative experimental study of machining characteristics in vibratory, rotary and vibro-rotary electro-discharge machining

This paper compares the effects of high- and low-frequency forced axial vibration of the electrode, rotation of the electrode, and combinations of these methods in respect of material removal rate (MRR), tool wear rate and surface roughness in die sinking electro-discharge machining (EDM) with a flat electrode (planing mode).

The results of the combined states of rotation and vibration at high and low frequency (vibro-rotary EDM) are compared in order to establish which combinations are most appropriate to different machining regimes (finishing, semi-finishing and roughing). It is found that the combination of high-frequency vibration and rotation of the electrode is effective in attaining a high MRR at a specified surface roughness (R_a). This case is modelled by stepwise linear regression. The significant parameters are found by analysis of variance (ANOVA) and the optimum machining parameter settings are obtained using overlay contour plots. The advantages of vibration and rotation are seen to combine in this new process when employed for die sinking EDM with a flat electrode.     

Probability of precision micro-machining of insulating Si3N4 ceramics by EDM

Electrical discharge machining (EDM) is used as a precision machining method for the electrically conductive hard materials with a soft electrode material. But recently we succeeded to machine on insulating material by EDM. The technology is named as an assisting electrode method. The EDMed surface is covered with the electrical conductive layer during discharge. The layer holds the electrical conductivity during discharge. For micro-EDM, the wear of tool electrode becomes lager ratio than the normal machining. So the micro-machining is extremely difficult to get the precision sample.

In this paper to obtain a fine and precise ceramics sample, some trials were carried out considering the EDM conditions, tool electrodes material and assisting electrode materials. Insulating Si₃N₄ ceramics were used for workpiece. The machining properties were estimated by the removal rate and tool wear ratio. To confirm the change of micro-machining process, the discharge waveforms were observed. The micro-machining of the Ø0.05 mm hole could be machined with the commercial sinking electrical discharge machine.     

Optimization of machining parameters in magnetic force assisted EDM based on Taguchi method

A versatile process of electrical discharge machining (EDM) using magnetic force assisted standard EDM machine has been developed. The effects of magnetic force on EDM machining characteristics were explored. Moreover, this work adopted an L18 orthogonal array based on Taguchi method to conduct a series of experiments, and statistically evaluated the experimental data by analysis of variance (ANOVA). The main machining parameters such as machining polarity (P), peak current (I_p), pulse duration (τ_p), high-voltage auxiliary current (I_H), no-load voltage (V) and servo reference voltage (S_v) were chosen to determine the EDM machining characteristics such as material removal rate (MRR) and surface roughness (SR). The benefits of magnetic force assisted EDM were confirmed from the analysis of discharge waveforms and from the micrograph observation of surface integrity. The experimental results show that the magnetic force assisted EDM has a higher MRR, a lower relative electrode wear ratio (REWR), and a smaller SR as compared with standard EDM. In addition, the significant machining parameters, and the optimal combination levels of machining parameters associated with MRR as well as SR were also drawn. Moreover, the contribution for expelling machining debris using the magnetic force assisted EDM would be proven to attain a high efficiency and high quality of surface integrity to meet the demand of modern industrial applications.     

Analysis and estimation of state variables in CNC face milling of AL6061

In this study, the effect of cutting parameters and machining forces on surface roughness and material removal rate of AL6061 in CNC face milling operation is investigated. Based on the experimental data, two different modeling techniques, namely regression analysis and multilayer perceptron, MLP, neural network, have been used to estimate the state variables (i.e. surface roughness, R _a , and material removal rate, MRR). Simulation results presented using machining data demonstrate that the MLP neural network possesses more powerful capacity than the regression analysis and performs the estimation of the R _a and MRR, simultaneously.     

电火花线切割加工铜钨合金切缝宽度及加工精度影响因素研究

铜钨合金是一种广泛应用于精密及难加工材料电火花成形加工的低损耗电极材料。在窄缝、清棱清角、高纵横比等结构的电极加工中,低速走丝电火花线切割加工较传统加工方法具有一定的优势。通过单因素实验研究了电火花线切割加工参数对铜钨合金粗加工切缝宽度、加工速度及表面粗糙度的影响规律,为实际加工提供理论指导。     

A study on the effect of tool nose radius in ultrasonic elliptical vibration cutting of tungsten carbide

Nowadays, ultrasonic elliptical vibration cutting (UEVC) technique is being successfully applied for ultraprecision machining of difficult-to-cut materials. Previous study reported that the tool geometry especially tool nose radius notably influences the performance of 1D ultrasonic vibration cutting (UVC). However, the effect of tool nose radius in the UEVC technique is yet to be studied. This study aims to investigate the effects of tool nose radius on the UEVC performance in terms of cutting force, tool wear and surface finish when machining a hard-to-cut material, sintered tungsten carbide (WC), using PCD tools. The experimental results show that the UEVC technique performs remarkably better in all aspects at a 0.6 mm nose radius compared to a lower (e.g. 0.2 or 0.4 mm) and a higher nose radius (e.g. 0.8 mm). When machining about 412 mm² surface area, an average surface roughness, R_a of 0.010 μm is achieved with a 0.6 mm nose radius. Analyses are conducted to justify the findings in this study.     

超声振动辅助气中放电加工实验分析

超声振动辅助气中放电加工技术避免了常用的煤油等工作液作为介质带来的环境污染问题，具有工作环境清洁、适用范围广、加工效率高、工具电极简单等优点。实验研究了电压、脉冲宽度、峰值电流、超声振幅及气体介质压力等参数对加工效率、工件表面粗糙度及电极损耗的影响．并对试验结果进行了分析。     

微细电火花加工损耗补偿的研究

针对三维微细结构的微细电火花加工,应用等损耗理论,采用分层加工的方法,建立了单道单层加工时底面形状随电极损耗而变化的轮廓模型,在此基础上,提出电极损耗适时补偿的方法.实验测定了钨电极加工不锈钢的损耗率,应用适时补偿方法加工得到的单层单道窄槽的尺寸精度良好.