Electrical discharge machining using simple and powder-mixed dielectric: The effect of the electrode area in the surface roughness and topography

Electrical discharge machining (EDM) is one of the most widely disseminated manufacturing technologies, in particular as regards the generation of accurate and complex geometrical shapes on hard metallic components. Nevertheless current EDM technologies have major limitations when dealing with fine surface finish over large process area. Indeed this is one reason that explains the need of final manual polishing of mould cavities performed by EDM. Recently EDM with powder-mixed dielectric (PMD-EDM) has been a focus of an intense research work in order to overcome these technological performance barriers. This paper presents a research work within the objective to acquire deep knowledge on EDM technology with powder mixed dielectric and to compare its performance to the conventional EDM when dealing with the generation of high-quality surfaces. In particular the analysis of the effect of the electrode area in the surface quality measured by the surface roughness and craters morphology was carried out for both technologies. The results achieved evidenced a linear relationship between the electrode area and the surface quality measures as well as a significant performance improvement when the powder mixed dielectric is used.     

State of the art electrical discharge machining (EDM)

Electrical discharge machining (EDM) is a well-established machining option for manufacturing geometrically complex or hard material parts that are extremely difficult-to-machine by conventional machining processes. The non-contact machining technique has been continuously evolving from a mere tool and die making process to a micro-scale application machining alternative attracting a significant amount of research interests.In recent years, EDM researchers have explored a number of ways to improve the sparking efficiency including some unique experimental concepts that depart from the EDM traditional sparking phenomenon. Despite a range of different approaches, this new research shares the same objectives of achieving more efficient metal removal coupled with a reduction in tool wear and improved surface quality.This paper reviews the research work carried out from the inception to the development of die-sinking EDM within the past decade. It reports on the EDM research relating to improving performance measures, optimising the process variables, monitoring and control the sparking process, simplifying the electrode design and manufacture. A range of EDM applications are highlighted together with the development of hybrid machining processes. The final part of the paper discusses these developments and outlines the trends for future EDM research.     

Applications of acoustic mapping in electrical discharge machining

The spatial distribution of discharges in electrical discharge machining (EDM) comprises valuable process information, which is not accurately obtained from electrical signals that are utilized extensively for process monitoring and control. This research hence explored the application of acoustic emission (AE) to map the discharges, in consideration of the acoustic time lag. In particular, the work refers to realistic process conditions, wherein AE from successive discharges cause repeated signal interference, which is detrimental to reliable time lag estimation. The applications of this capability for the respective identification of electrode length and workpiece height in fast-hole EDM and wire EDM are presented.     

An investigation of ultrasonic-assisted electrical discharge machining in gas

This study focuses on using ultrasonic to improve the efficiency in electrical discharge machining (EDM) in gas medium. The new method is referred to as ultrasonic-assisted electrical discharge machining (UEDM). In the process of UEDM in gas, the tool electrode is a thin-walled pipe, the high-pressure gas medium is applied from inside, and the ultrasonic actuation is applied onto the workpiece. In our experiment, the workpiece material is AISI 1045 steel and the electrode material is copper. The experiment results indicate that (a) the Material Removal Rate (MRR) is increased with respect to the increase of the open voltage, the pulse duration, the amplitude of ultrasonic actuation, the discharge current, and the decrease of the wall thickness of electrode pipe; and (b) the surface roughness is increased with respect to the increase of the open voltage, the pulse duration, and the discharge current. Based on experimental results, a theoretical model to estimate the MRR and the surface roughness is developed.     

Thermal stresses due to electrical discharge machining

The high temperature gradients generated at the gap during electrical discharge machining (EDM) result in large localized thermal stresses in a small heat-affected zone. These thermal stresses can lead to micro-cracks, decrease in strength and fatigue life and possibly catastrophic failure. A finite element model has been developed to estimate the temperature field and thermal stresses due to Gaussian distributed heat flux of a spark during EDM. First, the developed code calculates the temperature in the workpiece and then the thermal stress field is estimated using this temperature field. The effects of various process variables (current and duty cycle) on temperature distribution and thermal stress distribution have been reported. The results of the analysis show high temperature gradient zones and the regions of large stresses where, sometimes, they exceed the material yield strength.     

Simulation model of debris and bubble movement in consecutive-pulse discharge of electrical discharge machining

Debris concentration and bubble volume fraction in the bottom gap between the electrode and workpiece affect the state of consecutive-pulse discharge and the efficiency of electrical discharge machining (EDM). Thus, the mechanisms of debris and bubble movement during consecutive-pulse discharge should be elucidated. However, these mechanisms have not been fully understood because of debris and bubble movement in the machining gap are difficult to simulate and observe. This study proposes a three-dimensional model of flow field with liquid, gas, and solid phases for machining gap in EDM. The mechanisms of debris and bubble movement in the machining gap during consecutive-pulse discharge were analyzed through the model. Debris and bubble movement in consecutive-pulse discharge was observed through experiments. The results showed that the proposed simulation model is feasible. The bubble expansion is the main way that the bubbles exclude from machining gap. Much debris moves outside the machining gap following the excluded bubbles, which is the main way that the debris excludes from machining gap. The bubble expansion becomes strong with the increase of the discharge current and pulse-on time.     

加工间隙对电火花加工质量影响的分析

在电火花加工中要降低各种干扰因素的影响,确保加工质量和稳定的加工过程,放电间隙的一致性是非常重要的一个技术指标。详细分析了影响放电间隙状态变化的因素,提出了维持放电间隙大小一致性的相应措施,对进一步改善电火花加工质量具有一定的参考价值。     

混粉电火花镜面加工技术及应用

利用自行研制的混粉电火花镜面加工装置，对混粉电火花镜面加工影响因素，加工表面性能等进行了深入研究，找出了粉末特性，工件材料，粉末浓度及放电参数等对混粉电火花镜面加工的影响规律，给出了合理的混粉电火花镜面加工工艺条件，以文中确定的加工工艺条件对铸造和压铸模具进行混粉电火花镜面加工，结果表明，该技术能提高模具制造质量，降低工人劳动强度，缩短模具制造周期。     

Near dry electrical discharge machining

This study investigates the near dry electrical discharge machining (EDM) process. Near dry EDM uses liquid–gas mixture as the two phase dielectric fluid and has the benefit to tailor the concentration of liquid and properties of dielectric medium to meet desired performance targets. A dispenser for minimum quantity lubrication (MQL) is utilized to supply a minute amount of liquid droplets at a controlled rate to the gap between the workpiece and electrode. Wire EDM cutting and EDM drilling are investigated under the wet, dry, and near dry conditions. The mixture of water and air is the dielectric fluid used for near dry EDM in this study. Near dry EDM shows advantages over the dry EDM in higher material removal rate (MRR), sharper cutting edge, and less debris deposition. Compared to wet EDM, near dry EDM has higher material removal rate at low discharge energy and generates a smaller gap distance. However, near dry EDM places a higher thermal load on the electrode, which leads to wire breakage in wire EDM and increases electrode wear in EDM drilling. A mathematical model, assuming that the gap distance consists of the discharge distance and material removal depth, was developed to quantitatively correlate the water–air mixture's dielectric strength and viscosity to the gap distance.     

Performance of electrical discharge textured cutting tools

Tool face friction adversely affects chip formation and consumes about 25% of the total cutting energy. Friction in cutting can be controlled by introducing a lubricant into the tool-chip interface, the effectiveness of which may be enhanced by surface texturing the tool. This paper details the innovative application of electrical discharge machining for generating an isotropic texture on the tool rake face, with a view to facilitating lubricant penetration and retention. A significant reduction in feed and cutting forces that ensues from said texturing is demonstrated, followed by a presentation of the features and application areas of the technology.     

Analysis and performance of slotted tools in electrical discharge drilling

Accumulation of machining debris due to inadequate gap flushing severely limits the material removal rate and impairs the quality of the machined surface in electrical discharge machining. This is particularly pronounced in electrical discharge drilling of holes with a high aspect-ratio, wherein conventional flushing techniques essentially cease to be effective. To this end, this paper proposes the application of novel tool electrodes comprising geometric features specifically designed to promote tool rotation-induced debris egress. The corresponding flow fields are modelled numerically to optimize said designs. Relative to conventional rotating cylindrical tools, removal rate enhancements on the order of 300% are demonstrated.     

Fabrication of hollow nickel micro-spheres with high degree of hollowness by silicon powder-mixed spark erosion

For the purpose of developing a simple, fast and low cost method to fabricate the hollow nickel micro-spheres, the hollow nickel micro-spheres with high degree of hollowness were fabricated by silicon powder-mixed spark erosion (SPMSE) in the present paper. Morphologies and loose packed densities of hollow nickel micro-spheres fabricated by the SPMSE and conventional spark erosion (CSE) respectively were studied. The results showed that the hollow nickel micro-spheres fabricated by the SPMSE are bigger and exhibit higher degree of hollowness as compared to the CSE. It is expected that the powder-mixed spark erosion can fabricate hollow nickel micro-spheres with satisfactory particle size and hollowness through adjusting the particle size of silicon powders and machining parameters, even selecting other kinds of semi-conductive or conductive powders. In addition, the powder-mixed spark erosion can also fabricate other kinds of hollow metallic micro-spheres.     

气中放电加工电极夹具的研制

介绍了一种气中放电加工电极夹具的结构及工作原理，阐述了设计要点，给出了设计过程和应用实例。该电极夹具克服了一般电极夹具不能旋转、不能密封的缺点，实际应用效果良好。     

High-speed dry electrical discharge machining

A novel high-speed dry electrical discharge machining (EDM) method was proposed in this study. Using this method, the material can be rapidly melted by extremely high discharge energy and flushed out of the discharge gap by high-pressure and high-speed air flow. The material removal rate (MRR) of dry EDM was significantly improved by the proposed method. The MRR of dry EDM is usually in tens mm³/min, whereas the MRR of the proposed method can be as high as 5162 mm³/min, which improves the MRR by 2nd to 3rd order of magnitude. Investigation was conducted systemically. The influences of work piece polarity, discharge current, pulse duration time, gas pressure, and electrode rotation speed on machining performance were studied. The machining mechanism of this method was thoroughly analyzed. Moreover, the re-solidified layer, surface morphology, elementary composition, and phase of AISI 304 stainless steel for high-speed dry EDM were also investigated. Theoretical and technical foundations were laid for the industry application of dry EDM.     

Geometric modeling of the linear motor driven electrical discharge machining (EDM) die-sinking process

Based on Z-map method, in this paper the development of a geometric model for the linear motor equipped EDM die-sinking process is described. Firstly discussed are the advantages and benefits of the introduction of the linear motor into EDM die-sinker. The current model has been employed to calculate the minimum gap distance for sparks to occur and to analyze the possibility of spark generation between the workpiece and electrode surfaces. Also calculated is the crater formed by a single spark. The final machined surface topography is predicted. The influence of peak current and discharge duration on the average surface roughness is simulated. The experiment has been conducted to study the effects of machining conditions as well as to verify the effectiveness of the developed geometric model.     

Micro hole machining by conventional penetration electrical discharge machine

This work aims to study the parameters that affect the micro hole machining process (diameter smaller than 0.1 mm and thickness/diameter relation bigger than 20), by electro-erosion penetration process in sheets. To make the execution of the micro holes in conventional machines possible, the construction of two devices is proposed, a mechanical and an optical one. From results experimentally obtained, the process proved to be technically and economically viable, mainly if compared with mechanic drilling, as it presents advantages when cost per tool, number of holes per tool, drilling length, and precision of the holed dimension are considered. The circularity deviation provided by the experiment was smaller than 0.01 for 0.1 mm diameter holes made in 2.4 mm thick SAE 1010 steel sheets. It was also concluded that the quality of the hole directly depends on the cleaning process.     

Investigation of the spark cycle on material removal rate in wire electrical discharge machining of advanced materials

The development of new, advanced engineering materials and the need for precise and flexible prototypes and low-volume production have made the wire electrical discharge machining (EDM) an important manufacturing process to meet such demands. This research investigates the effect of spark on-time duration and spark on-time ratio, two important EDM process parameters, on the material removal rate (MRR) and surface integrity of four types of advanced material: porous metal foams, metal bond diamond grinding wheels, sintered Nd-Fe-B magnets, and carbon–carbon bipolar plates. An experimental procedure was developed. During the wire EDM, five types of constraints on the MRR due to short circuit, wire breakage, machine slide speed limit, and spark on-time upper and lower limits are identified. An envelope of feasible EDM process parameters is generated for each work-material. Applications of such a process envelope to select process parameters for maximum MRR and for machining of micro features are discussed. Results of Scanning Electron Microscopy (SEM) analysis of surface integrity are presented.     

Reliability of electrode wear compensation based on material removal per discharge in micro EDM milling

This paper investigates the reliability of workpiece material removal per discharge (MRD) estimation for application in electrode wear compensation based on workpiece material removal. An experimental investigation involving discharge counting and automatic on the machine measurement of removed material volume was carried out in a range of process parameters settings from fine finishing to roughing. MRD showed a decreasing trend with the progress of the machining operation, reaching stabilization after a number of machined layers. Using the information on MRD and discharge counting, a material removal simulation tool was developed and validated.     

Wire electro discharge trueing and dressing of fine grinding wheels

In this paper fundamental investigations are presented regarding the capabilities of Wire-EDM for trueing and dressing of fine grained metal bonded diamond grinding wheels. These wheels are often used for precision grinding operations of hard and brittle mould materials like ceramics or cemented carbides. They are characterised by high profile constancies and wear resistances. Due to the electrical conductivity of the bond material, Wire-EDM offers an efficient and powerful alternative to conventional trueing and dressing. Achievable grit protrusion and possible thermal damage to smallest diamond grits are theoretically and experimentally examined. Additionally, machining strategies for high profile accuracy are presented.     

Micro-electrical discharge machining of polycrystalline diamond using rotary cupronickel electrode

Cupronickel was used as the electrode material to fabricate microstructures on polycrystalline diamond by electrical discharge machining (EDM). The electrodes were shaped into tiny rotary wheels driven by the flow of EDM fluid. Results showed that material removal rate was improved by a factor of five compared to conventional electrode materials. Raman spectroscopy and energy dispersive X-ray spectroscopy indicated that graphitization of diamond and diffusion-based chemical reactions between nickel and diamond dominated the EDM process. Effects of electrode rotation rate and discharge energy on the EDM characteristics were clarified. High form accuracy (∼0.5 μm/1 mm) and low surface roughness (∼0.1 μm Ra) were obtained.