Modelling of surface finish,electrode wear and material removal rate in electrical discharge machining of hard-to-machine alloys

Hard-to-machine alloys are commonly used for industrial applications in the aeronautical, nuclear and automotive sectors, where the materials must have excellent resistance to corrosion and oxidation, high temperature resistance and high mechanical strength. In this present study the influence of different parameters of the electrical discharge machining process on surface roughness, electrode wear and material removal rate have been studied. Regression techniques are employed to model arithmetic mean deviation Ra (μm), peak count Pc (1/cm), material removal rate MRR (mm³/min) and electrode wear EW (%). All these parameters have been studied in terms of current intensity supplied by the generator of the electrical discharge machine I (A), pulse time t_i (μs), duty cycle η and open-circuit voltage U (V). This modelling allows us to obtain mathematical data and models to predict that the most influential factor in MRR and Ra is the current intensity and in the case of EW and Pc is the pulse time.     

An analytical model of the material removal rate between elastic and elastic-plastic deformation for a polishing process

This paper develops an analytical model for the material removal rate during specimen polishing. The model is based on the micro-contact elastic mechanics, micro-contact elastic-plastic mechanics and abrasive wear theory. The micro-contact elastic mechanics between the pad-specimen surfaces used the Greenwood and Williamson elastic model. The micro-contact elastic-plastic mechanics between specimen and particle, as well as the micro-contact elastic mechanics between particle and pad, are also analyzed. The cross-sectional area of the worn groove in the specimen is considered as trapezoidal area. A close-form solution of material removal rate from the specimen surface is the function of average diameter of slurry particles, pressure, the specimen/pad sliding velocity, Equivalent Young’s modulus, RMS roughness of the pad, and volume concentration of the slurry particle.     

Modeling and simulation of material removal in planarization process

In planarization processes, material removal analysis is essential to the estimation of the wear rate and non-uniformity. A model that describes the material removal of a pad with rough surface grinding by using abrasive grains is developed. A collection of micro-contact spots is identified and the deformation approach is subsequently calculated. Elastic-plastic theory and the wear model are used to construct the expression for the magnitude of material removal as a function of the indentation depth. First, the indentation depth of micro-contact spots in the asperity of the pad and the deformation of the flat part of pad are obtained by using elastic-plastic theory. Then, the material removal caused by individual micro-contacts is calculated with the help of wear theory. Finally, the macroscopic wear volume is found by summing the volumetric wear of each individual micro-contact. Moreover, the pad dressing process is introduced to demonstrate the developed model for material removal. A parametric study is conducted to explore the influence on the material removal results and the planarization interfacial phenomena of operational parameters. These parameters compose of the applied down force, rotational speed of dresser, and the density of abrasive grains. The results provide a detailed picture of the interface phenomena and yield an insight into the physical effects of the operating parameters in the planarization processes.     

Investigating material removal rate and surface roughness using multi-objective optimization for focused ion beam (FIB) micro-milling of cemented carbide

Cemented carbide has been investigated as a useful material for the fabrication of micro devices. Focused ion beam (FIB) micro-milling has been found to be one of the most appropriate methods for the fabrication of micro devices. The experimental FIB micro-milling on cemented carbide have been conducted according to the L₁₆ orthogonal array of Taguchi technique. Beam current, extraction voltage, angle of beam incidence, dwell time and percentage overlap between beam diameters have been considered as process variables of FIB micro-milling in experimental design. Material removal rate (MRR) and surface roughness have been determined experimentally for FIB micro-milling of cemented carbide and beam current has been identified as the most significant parameter. The minimum surface roughness of 5.6 nm has been reported on cemented carbide, which is not a usual practice to achieve on such polycrystalline material, and hence it may be considered as a significant research contribution. Maximum MRR of 0.4836 μm³/s has been reported. Moreover, genetic algorithm toolbox of MATLAB has been utilized for multi-objective optimization between MRR and surface roughness. The corresponding optimum values of MRR and surface roughness for multi-objective optimization have been represented by pareto optimum solution generated by genetic algorithm. The research work presented in this paper determines the setting of process parameters of FIB micro-milling for achieving a specific combination of MRR and surface roughness on cemented carbide.     

Surface roughness and material removal rate of lapping process on ceramics

Lapping is a widely used surface finishing process for ceramics. An experimental investigation is conducted into the lapping of alumina, Ni−Zn ferrite and sodium silicate glass using SiC abrasive to study the effect of process parameters, such as abrasive particle size, lapping pressure, and abrasive concentration, on the surface roughness and material removal rate during lapping. A simple model is developed based on the indentation fracture and abrasive particle distribution in the slurry to explain various aspects of the lapping process. The model provides predictions for the surface roughness,R _a andR _t , on the machined surface and rough estimation for the material removal rate during lapping. Comparison of the predictions with the experimental measurements reveals same order of magnitude accuracy.     

Roughness variability in the honing process of steel cylinders with CBN metal bonded tools

Herein are presented roughness results of honing of steel cylinders with cubic boron nitride (CBN) abrasive tools. Different honing parameters such as honing stone pressure against the workpiece, tangential speed of the honing head as well as grain size of the abrasive stone were varied in order to determine their influence on the surface roughness of cylinders. Four different strategies were implemented to measure and characterise roughness of inner surfaces of honed steel cylinders. The variability in results obtained with each strategy is compared. The first three strategies consist in measuring roughness at one, three or nine points, respectively, which are uniformly distributed over a circumference located at one of the cylinder ends. The fourth strategy is based on the third one: it entails measuring roughness at nine points, and then subsequently filtering results to eliminate any extreme roughness value lying outside the boundaries set by Chauvenet's criterion. At each point of measurement, roughness is measured in the direction of the cylinder generatrix. It was verified that the larger the number of measurement points, the lower the variability in roughness values obtained. Therefore, it is recommended to make at least nine measurements per cylinder and to filter values to avoid any non-representative, outlier value. The variability in these values was quantified firstly according to relative differences. Secondly, variability was quantified by means of the coefficient of variation which provides smaller values than the relative difference, although less intuitive and less direct.     

Analysis of tool wear and surface finish in hard turning

Hard turning is a profitable alternative to finish grinding. The ultimate aim of hard turning is to remove work piece material in a single cut rather than a lengthy grinding operation in order to reduce processing time, production cost, surface roughness, and setup time, and to remain competitive. In recent years, interrupted hard turning, which is the process of turning hardened parts with areas of interrupted surfaces, has also been encouraged. The process of hard turning offers many potential benefits compared to the conventional grinding operation. Additionally, tool wear, tool life, quality of surface turned, and amount of material removed are also predicted. In this analysis, 18 different machining conditions, with three different grades of polycrystalline cubic boron nitride (PCBN), cutting tool are considered. This paper describes the various characteristics in terms of component quality, tool life, tool wear, effects of individual parameters on tool life and material removal, and economics of operation. The newer solution, a hard turning operation, is performed on a lathe. In this study, the PCBN tool inserts are used with a WIDAX PT GNR 2525 M16 tool holder. The hardened material selected for hard turning is commercially available engine crank pin material.     

A review on material removal mechanism in electrochemical discharge machining (ECDM) and possibilities to enhance the material removal rate

The concept of electrochemical discharge machining (ECDM), also known as electrochemical spark machining (ECSM), was presented for the first time in 1968. Since then, this technology remains as research topic and was never explained seriously for industrial applications. The ECDM is a non-traditional machining technology used for machining of electrically non-conducting materials like glass, ceramics, quartz, etc. The literature reveals that the concept of mechanism of material removal in this machining process is not yet understood well. However, phenomena involved in the material removal needs to be investigated well in order to improve the process. In this paper, the concept of mechanism of material removal in ECDM is reviewed till date; scopes for further research have been identified. Possible future efforts to enhance the material removal rate in ECDM are also discussed.     

Effect of speed on material removal behavior in scribing of monocrystalline silicon

This paper investigates the effect of scribing speed on the surface morphology and material removal behavior in diamond wire sawing of monocrystalline silicon through specially designed high-speed diamond scribing experiments. High-speed scribing tests are performed on a (100) monocrystalline silicon wafer over a wide range of speeds. The results show that a higher scribing speed is prone to inducing more surface defects such as burrs and tearing in the ductile scribing region, and more radial cracks in the brittle scribing region. The critical scribing depth of ductile-to-brittle transition is found to decrease with increasing scribing speed. A strain rate hardening effect is evident in the experimental data, which explains the underlying mechanism for promotion of brittle fracture at higher scribing speeds.     

Indirect model for roughness in rough honing processes based on artificial neural networks

In the present paper an indirect model based on neural networks is presented for modelling the rough honing process. It allows obtaining values to be set for different process variables (linear speed, tangential speed, pressure of abrasive stones, grain size of abrasive and density of abrasive) as a function of required average roughness Ra. A multilayer perceptron (feedforward) with a backpropagation (BP) training system was used for defining neural networks. Several configurations were tested with different number of layers, number of neurons and type of transfer function. Best configuration for the network was searched by means of two different methods, trial and error and Taguchi design of experiments (DOE). Once best configuration was found, a network was defined by means of trial and error method for roughness parameters related to Abbott–Firestone curve, Rk, Rpk and Rvk.     

基于材料去除的微制造技术发展

这里将微机械制造技术分为材料去除技术、材料沉积(增材)制造技术和变形制造技术,对基于材料去除的微制造技术的原理、方法、特点、应用和发展现状进行了比较和综述,在此基础上对微制造技术的发展方向作出了预测。     

光学玻璃磁流变抛光液的研究与材料去除实验

根据磁流变抛光液的评价标准,合理选取了磁流变液的各成分,配制了三种型号磁流变抛光液。用配制的抛光液对K9玻璃进行了定点抛光实验,证明了该磁流变液性能良好,达到了预期的材料去除效果。得到了材料去除率曲线规律,以及单位时间材料去除深度微观空间形貌,为建立磁流变抛光材料去除函数模型提供了依据。     

Experimental investigation and analytical modelling of the effects of process parameters on material removal rate for bonnet polishing of cobalt chrome alloy

Cobalt chrome alloys are the most extensively used material in the field of total hip and total knee implants, both of which need highly accurate form and low surface roughness for longevity in vivo. In order to achieve the desired form, it is extremely important to understand how process parameters of the final finishing process affect the material removal rate. This paper reports a modified Preston equation model combining process parameters to allow prediction of the material removal rate during bonnet polishing of a medical grade cobalt chrome alloy. The model created is based on experiments which were carried out on a bonnet polishing machine to investigate the effects of process parameters, including precess angle, head speed, tool offset and tool pressure, on material removal rate. The characteristic of material removal is termed influence function and assessed in terms of width, maximal depth and material removal rate. Experimental results show that the width of the influence function increases significantly with the increase of the precess angle and the tool offset; the depth of the influence function increases with the increase of the head speed, increases first and then decrease with the increase of the tool offset; the material removal rate increases with the increase of the precess angle non-linearly, with the increase of the head speed linearly, and increases first then decreases with the increase of the tool offset because of the bonnet distortion; the tool pressure has a slight effect on the influence function. The proposed model has been verified experimentally by using different Preston coefficients from literature. The close values of the experimental data and predicted data indicate that the model is viable when applied to the prediction of the material removal rate in bonnet polishing.     

Effect of interfacial friction force on material removal in full aperture continuous polishing process

Full aperture continuous polishing using pitch lap is a key process of finishing large flat optical workpiece. The friction force of the workpiece and pitch lap interface significantly affects material removal. In this work, the friction force was determined by a measurement system that uses force transducers to support the workpiece. Experimental and theoretical analyses have been carried out to investigate the evolution of friction force with polishing time and its effect on material removal. Our results show that the friction coefficient of the workpiece/lap interface decreases during polishing, which is due to surface smoothing of the viscoelastic pitch lap by loading conditioner. In addition, the spatial average and uniformity of material removal rate (removal coefficient) increases with the increase of friction coefficient, which is due to rough lap surface, provides more sharp asperities to charge the polishing particles.     

Investigation of pad wear in CMP with swing-arm conditioning and uniformity of material removal

Chemical mechanical polishing (CMP) process plays the role of planarizing and smoothing the uneven layers after the material deposition process in the semiconductor industry. In this process, pad conditioning using a diamond disk is inevitable to attain a high material removal rate (MRR) and to ensure the stability of the process. Pad conditioning is performed for providing uniform surface roughness and opening up the glazed surfaces of the polishing pad. However, the uneven pad wear resulting from pad conditioning leads to changes in the uniformity of MRR and productivity of the device. In this study, we investigate the pad wear profile after swing-arm conditioning of the pad, based on measurements performed using a pad measurement system (PMS). Conditioning experiments are conducted with seven cases of profiles of the conditioner's duration time (PCDT). In all the cases, “W”-shaped pad profiles are generated through swing-arm conditioning. It is observed that a concave-shaped PCDT results in the lowest value of maximum pad wear rate. The average depth of pad wear (h_avg) is mainly related to the MRR, and the maximum depth of pad wear (h_max) and the horizontal distance from the wafer center to the position (e) where the maximum pad wear occurs affect the within-wafer non-uniformity (WIWNU). A concave-shaped PCDT results in longer life of the polishing pad by minimizing the variation in pad wear. This paper can provide a technical assistance in selecting the conditioning recipe and improving the lifetime of the polishing pad in the CMP process.     

Molecular dynamics simulation of the material removal mechanism in micro-EDM

Electrical discharge phenomena in EDM occur in a very short time period and in a very narrow space, thus making both observation and theoretical analysis extremely difficult. For this reason, the material removal mechanism in EDM has yet to be understood clearly. In this paper, the forming process of discharge craters in three dimensions was simulated, and material removal mechanism in EDM was analyzed using Molecular Dynamics (MD). It was found that material removal mechanism in EDM can be explained in two ways; one by vaporization and the other by the bubble explosion of superheated metals. It was also found that the metal removal efficiency is 0.02-0.05, leaving most of the melted pool resolidified. In addition, the influence of power density on the removal process was investigated, and the results showed that as the power density increases, the diameter and depth of the melted area increase, as does the metal removal efficiency. In this study, the forming mechanism of the bulge around discharge craters was also analyzed, and it was found that bulge is formed due to two mechanisms. The first is the shearing flow of the molten material caused by the extremely high pressure in the superheated material, and the second is the accumulation of the ejected material on the bulge formed by the first forming mechanism. It was also found that existence of micro pores in the workpiece material increases the depth of the discharge crater and melted area, thereby increasing the machining surface roughness. Simulation of the distribution of removed materials in the gap showed that some part of the removed material becomes debris ejected from the gap, while another part settles on the surface of the opposite electrode, and the last part returns to the surface of the electrode from which it was ejected.     

光学玻璃磁性复合流体抛光应力与材料去除率的实验研究

根据光学玻璃元件超精密加工技术的需求,研究自旋转式和行星旋转式磁性复合流体(MCF)抛光的应力分布和材料去除率。首先,设计可实现自旋转和行星旋转抛光装置,搭建抛光实验平台;然后,进行自旋转式和行星旋转式MCF抛光实验,通过自行设计抛光应力分布测试实验分析了两种抛光方式的应力分布规律;最后,通过定点抛光实验,对抛光前后的工件表面轮廓进行检测,计算并分析两种抛光方式的材料去除率。实验结果表明,立式的两种抛光方式,正应力均明显大于剪切应力,工件外侧受到的剪切应力大于中心受到的剪切应力,行星式抛光的材料去除率明显大于自旋转式抛光的材料去除率。     

Effects of working parameters on surface finish in ball-burnishing of hardened steels

Burnishing is a chipless finishing method, which employs a rolling tool, pressed against the workpiece, in order to achieve plastic deformation of the surface layer. Recent developments made possible burnishing of heat-treated steel components up to 65 HRC. Features of burnishing include a good roughness (comparable to grinding), as well as improvement of mechanical characteristics of the surface (fatigue strength, corrosion resistance, and bearing ratio), due to implementation of compressive stresses into the surface layer. This paper will present influences of certain burnishing parameters upon roughness, for a hardened steel component (64 HRC).     

固着磨料加工碳化硅反射镜的材料去除率及表面粗糙度的理论研究

本文基于固着磨料加工碳化硅反射镜的微观作用原理，理论上定量分析探讨了金刚石磨料压入碳化硅工件的深度对材料去除率、光学元件表面粗糙度的影响，分别获得了材料去除率数学模型及粗糙度的仿真计算结果。通过去除率及粗糙度的实验与理论模型的对比结果来看，实验值虽与理论值有偏差，但基本稳定在同一数量级内，由此验证了理论分析的正确性。固着磨料工艺的这一易于理论预测的特性对实际的光学加工有着极为重要的意义，因此这种较为新颖的固着磨料工艺在碳化硅反射镜加工领域内有着广阔的应用前景。