Experimental research on machining characteristics of SiC ceramic with end electric discharge milling

Silicon carbide (SiC) ceramic has been widely used in modern industry. However, the beneficial properties of SiC ceramic make machining difficult and costly by conventional machining methods. This paper proposes a new process of machining SiC ceramic using end electric discharge (ED) milling. The process is able to effectively machine a large surface area on SiC ceramic at low cost and no environmental pollution. The effects of emulsion concentration, emulsion flux, milling depth, copper electrode number, and copper electrode diameter on the process performance such as the material removal rate, electrode wear ratio, and surface roughness have been investigated. In addition, the microstructure of the machined surface is examined with a scanning electron microscope, and the material removal mechanism of SiC ceramic during end ED milling is obtained.     

Experimental research on effects of process parameters on servo scanning 3D micro electrical discharge machining

Servo scanning 3D micro electrical discharge machining (3D SSMEDM) is a novel and effective method in fabricating complex 3D micro structures with high aspect ratio on conducting materials.In 3D SSMEDM process,the axial wear of tool electrode can be compensated automatically by servo-keeping discharge gap,instead of the traditional methods that depend on experiential models or intermittent compensation.However,the effects of process parameters on 3D SSMEDM have not been reported up until now.In this study,the emphasis is laid on the effects of pulse duration,peak current,machining polarity,track style,track overlap,and scanning velocity on the 3D SSMEDM performances of machining efficiency,processing status,and surface accuracy.A series of experiments were carried out by machining a micro-rectangle cavity (900 μm×600 μm) on doped silicon.The experimental results were obtained as follows.Peak current plays a main role in machining efficiency and surface accuracy.Pulse duration affects obviously the stability of discharge state.The material removal rate of cathode processing is about 3/5 of that of anode processing.Compared with direction-parallel path,contour-parallel path is better in counteracting the lateral wear of tool electrode end.Scanning velocity should be selected moderately to avoid electric arc and short.Track overlap should be slightly less than the radius of tool electrode.In addition,a typical 3D micro structure of eye shape was machined based on the optimized process parameters.These results are beneficial to improve machining stability,accuracy,and efficiency in 3D SSMEDM.     

Experimental investigations into electrical discharge machining with a rotating disk electrode

In electrical discharge machining, when the provision of holes in the electrode is impracticable, flushing of the working gap poses a major problem. Use of a rotating disk electrode is proposed as a more productive and accurate technique than use of a conventional electrode. Material removal rate, tool wear rate, relative electrode wear, corner reproduction accuracy, and surface finish aspects of a rotary electrode are compared with those of a stationary one. The effective flushing of the working gap brought about by the rotation of the electrode remarkably improves material removal rate and machines surfaces with a better finish. Despite the prevalent high tool wear rate, the reproduction accuracy is least affected as the wear gets uniformly distributed over the entire circumference of the disk. Machining of a sharp corner is possible even with an aluminum electrode, whose relative electrode wear is greater than unity.     

Experimental study on micro electrical discharge machining of porous stainless steel

Various cutter strategies have been developed during milling freeform surface. Proper selection of the cutter path orientation is extremely important in ensuring high productivity rate, meeting the better quality level, and longer tool life. In this work, finish milling of TC17 alloy has been done using carbide ball nose end mill on an incline workpiece angle of 30°. The influence of cutter path orientation was examined, and the cutting forces, tool life, tool wear, and surface integrity were evaluated. The results indicate that horizontal downward orientation produced the highest cutting forces. Vertical downward orientation provided the best tool life with cut lengths 90–380 % longer than for all other orientations. Flank wear and adhesion wear were the primary wear form and wear mechanisms, respectively. The best surface finish was achieved using an upward orientation, in particular, the vertical upward orientation. Compressive residual stresses were detected on all the machined surfaces, and vertical upward orientation provided the minimum surface compressive residual stress. In the aspect of tool wear reduction and improvement of surface integrity, horizontal upward cutter path orientation was a suitable choice, which provided a tool life of 270 m, surface roughness (R _a ) of 1.46 μm, and surface compressive residual stress of ?300 MPa.     

An experimental study on the ultrasonic machining characteristics of engineering ceramics

Engineering ceramics have many unique characteristics both in mechanical and physical properties such as high temperature hardness, high thermal, chemical and electrical resistance. However, its machinability is very poor in conventional machining due to its high hardness and severe tool wear. In the current experimental study, alumina (Al₂O₃) was ultrasonically machined using SiC abrasives under various machining conditions to investigate the material removal rate and surface quality of the machined samples. Under the applied amplitude of 0.02 mm, 27 kHz frequency, three slurry ratios of 1:1, 1:3 and 1:5 with different tool shapes and applied static pressure levels, the machining was conducted. Using the mesh number of 240 abrasive, slurry ratio of 1:1 and static pressure of 2.5 kg/cm², maximum material removal rate of 18.97 mm³/min was achieved. With mesh number of 600 SiC abrasives and static pressure of 3.0 kg/cm², best surface roughness of 0.76 pm Ra was obtained.     

激光辅助加热切削氧化铝工程陶瓷的刀具磨损试验研究

氧化铝工程陶瓷具有较高的强度、硬度和较好的耐磨性,是典型的难加工材料,激光辅助加热切削是加工这种高硬度材料的有效方法之一。通过对氧化铝工程陶瓷在激光辅助加热切削时立方氮化硼(CBN)刀具的磨损试验,研究了激光功率、主轴转速和进给速率等不同工艺参数对刀具磨损所产生的影响。结果发现,CBN刀具的磨损形态主要表现为前刀面和后刀面磨损,造成刀具磨损的主要原因是磨料磨损和粘结磨损;同时还发现,与普通切削相比,采用辅助加热切削可以明显减小刀具的磨损。     

电火花机械复合磨削反应烧结SiC陶瓷的表面特征

研究了基于电火花机械复合磨削技术加工的反应烧结碳化硅(RB-SiC)陶瓷的表面特征。用电火花机械复合磨削(EDDG)、电火花磨削(EDG)以及普通磨削(CG)三种方法加工RB-SiC陶瓷,并采用激光共聚焦显微镜和扫描电子显微镜对加工后的SiC陶瓷的表面粗糙度、表面形貌及微观裂纹进行测量和对比试验,获得了RB-SiC陶瓷的EDDG加工特性。实验显示:EDDG加工的RB-SiC陶瓷的表面粗糙度优于EDG加工的表面粗糙度,为0.214 9μm,但比CG加工的表面粗糙度0.195 6μm略差。对加工后的SiC陶瓷表面形貌观察显示,传统磨削加工后的表面存在明显划痕,EDG加工表面主要由放电凹坑组成,而EDDG加工表面同时存在放电凹坑和磨削划痕;另外,传统磨削表面也存在磨削裂纹和晶界裂纹,但EDG加工后的表面只存在热裂纹,而EDDG加工后的表面存在磨削裂纹和热裂纹,不过热裂纹可以用金刚石磨粒磨削去除。对比实验显示RB-SiC陶瓷的EDDG加工与EDG和CG加工获得了不同的表面特征。     

Super-high-thickness high-speed wire electrical discharge machining

High-speed wire electrical discharge machining (WEDM-HS) of materials of super-high thickness (more than 1000 mm) is a challenging problem. First, sufficient energy is required to maintain the inter-electrode normal discharge. Next, there must be adequate inter-electrode dielectric fluid. Third, in order to generate a smooth cut surface, it is necessary to suppress the vibration of the wire electrode to reduce vibration lines on the cutting surface. To better understand these challenges, the energy and the flow of the medium between two electrodes were analyzed, allowing the establishment of a relevant model. The results indicated that for super-high-thickness machining, the pulse energy must be adequate to compensate for the energy consumed in the molybdenum wire and inter-electrode working liquid. In addition, the running speed of the wire electrode should be improved to ensure that there is a sufficiently high flow rate of the dielectric fluid. The servo control mode of the existing machine tools and dielectric fluid were improved and then a process experiment was performed. The experimental results show that the process can be carried out efficiently and stably and the workpiece surface can be cut smoothly using the improved working liquid and servo control mode.     

Development of parallel spark electrical discharge machining

This paper describes the development of parallel spark EDM method. In the discharge circuit, the electrode is divided into multiple electrodes, each of which is electrically insulated and connected to the pulse generator through a diode. A capacitor is inserted parallel to each discharge gap between each electrode and workpiece (here workpiece is common for each electrode). Compared with conventional EDM in which only a singular discharge can be generated for each pulse, multiple discharges can dispersively be generated for each pulse in parallel spark EDM. Results of experiments on parallel spark EDM and conventional EDM show that not only is the machining process more stable, but the machining speed and surface roughness can also be improved with parallel spark EDM.     

Study on surface characteristics using phosphorous dielectric on wire electrical discharge machining of polycrystalline silicon

In this study, wire electrical discharge machining (WEDM) was implemented to process polycrystalline silicon ingot (electric resistance, 2–3 Ω cm), and the influences on surface characteristics were examined. At first, two different dielectrics, pure water and pure water with sodium pyrophosphate powder, were experimented to compare their effects on cutting speed and surface roughness. In the experiment, pure water with sodium pyrophosphate powder has shown that it enhanced process efficiency and improved surface smoothness. Then, the effects on cutting efficiency with different concentrations were examined; 0.05 M was chosen for its best performance. After setting the concentration, several experiments were conducted to find out how different currents and pulse-on times affect the cutting efficiency and surface roughness. The findings in this study prove that using phosphorous dielectric on WEDM could be applied onto polycrystalline silicon cutting. In addition, pure water with sodium pyrophosphate powder increases both working efficiency and improves infiltration of the phosphorous element on the surface. These findings could be future references on researches of solar cell in both industrial and academic fields.     

Parametric analysis on electrochemical discharge machining of silicon nitride ceramics

The electrochemical discharge machining (ECDM) process has a potential in the machining of silicon nitride ceramics. This paper describes the development of a second order, non-linear mathematical model for establishing the relationship among machining parameters, such as applied voltage, electrolyte concentration and inter-electrode gap, with the dominant machining process criteria, namely material removal rate (MRR), radial overcut (ROC) and thickness of heat affected zone (HAZ), during an ECDM operation on silicon nitride. The model is developed based on response surface methodology (RSM) using the relevant experimental data, which are obtained during an ECDM micro-drilling operation on silicon nitride ceramics. We also offer an analysis of variance (ANOVA) and a confirmation test to verify the fit and adequacy of the developed mathematical models. From the parametric analyses based on mathematical modelling, it can be recommended that applied voltage has more significant effects on MRR, ROC and HAZ thickness during ECDM micro-drilling operation as compared to other machining parameters such as electrolyte concentration and inter-electrode gap.     

Parametric analysis on electrochemical discharge machining of silicon nitride ceramics

The electrochemical discharge machining (ECDM) process has a potential in the machining of silicon nitride ceramics. This paper describes the development of a second order, non-linear mathematical model for establishing the relationship among machining parameters, such as applied voltage, electrolyte concentration and inter-electrode gap, with the dominant machining process criteria, namely material removal rate (MRR), radial overcut (ROC) and thickness of heat affected zone (HAZ), during an ECDM operation on silicon nitride. The model is developed based on response surface methodology (RSM) using the relevant experimental data, which are obtained during an ECDM micro-drilling operation on silicon nitride ceramics. We also offer an analysis of variance (ANOVA) and a confirmation test to verify the fit and adequacy of the developed mathematical models. From the parametric analyses based on mathematical modelling, it can be recommended that applied voltage has more significant effects on MRR, ROC and HAZ thickness during ECDM micro-drilling operation as compared to other machining parameters such as electrolyte concentration and inter-electrode gap.     

Investigation of wire electrical discharge machining characteristics of Al6063/SiCp composites

In this investigation, the effect of wire electrical discharge machining (WEDM) parameters such as pulse-on time (T _ON), pulse-off time (T _OFF), gap voltage (V) and wire feed (F) on material removal rate (MRR) and surface roughness (R _a) in metal matrix composites (MMCs) consisting of aluminium alloy (Al6063) and silicon carbide (SiC_p) is discussed. The Al6063 is reinforced with SiC_p in the form of particles with 5%, 10% and 15% volume fractions. The experiments are carried out as per design of experiments approach using L₉ orthogonal array. The results were analysed using analysis of variance and response graphs. The results are also compared with the results obtained for unreinforced Al6063. From this study, it is found that different combinations of WEDM process parameters are required to achieve higher MRR and lower R _a for Al6063 and composites. Generally, it is found that the increase in volume percentage of SiC resulted in decreased MRR and increased R _a. Regression equations are developed based on the experimental data for the prediction of output parameters for Al6063 and composites. The results from this study will be useful for manufacturing engineers to select appropriate WEDM process parameters to machine MMCs of Al6063 reinforced with SiC_p at various proportions.     

Critical wall thickness in electrical discharge machining

Electrical discharge machining (EDM) is capable of fabricating thin wall structures due to the absence of physical contact between the tool and workpiece. However, the presence of impulsive force does exist during the sparking process, which could give rise to limitations in fabricating small features. The L9 Taguchi method together with the analysis of variance was used to determine the significance of various machining parameters on the specific impulsive force. Through the statistical analysis, it was observed that discharge energy was the most significant factor that influenced the specific impulsive force. Hence, it is important to understand the effects of specific impulsive force in EDM and to investigate its influence that limits the fabrication of thin wall structures. The limit in which the machined part is able to maintain its geometrical integrity is described as the critical wall thickness. The effects of specific impulsive force on thin wall structure were studied through determining the critical wall thickness for various discharge energies. The experimental work showed that both electrothermal erosion and brink erosion, which is attributed by the specific impulsive force was responsible for the loss of geometrical integrity of the wall structure. As the wall thickness falls below a critical value in a given discharge energy setting, the brink erosion was observed. The limit that established the critical wall thickness was elucidated by conflating the thermal erosion and brink erosion.     

电化学放电线切割加工方法实验研究

针对非导电硬脆材料的微细线切割加工,设计并搭建了电化学放电线切割加工装置,对电化学放电线切割加工过程中的电压-电流曲线与电极丝放电长度之间的关系进行了实验研究。实验结果表明,随着直流电压的增大,电压-电流曲线呈现出线性区、饱和区、跃变区和放电区等明显的区域性特征变化,稳定放电仅在电压高于一定值的放电区内发生,且电极丝放电长度越大,发生跃变放电时的临界电压越高。对普通玻璃和石英玻璃的加工实验表明,通过增大放电能量,能够获得更高的材料去除率,但也会导致工件加工精度和表面质量的下降。     

Three-dimensional shaping by dot-matrix electrical discharge machining

This paper deals with a new prototyping method called dot-matrix electrical discharge machining (EDM) with scanning motion. The machining process by the dot-matrix method is similar to printing motion with a dot-impact printer. This method can be applied not only to EDM but also electrochemical machining and forming. A prototype of the machining unit for the dot-matrix method has six feeding devices for thin wire electrodes. The electrodes of 300 μm in diameter are arranged with the pitches of 760 μm. To obtain a smooth surface, a planetary motion in the x-y-plane is added to the feeding of the machining unit in the z-direction, the same area is machined repeatedly, or the machining unit is moved with fine feed. By compensating for the wear of the electrode during the scanning EDM, various shapes with the accuracy of micrometers order can be obtained without a formed tool electrode.     

Non-linear mechanism in electrical discharge machining process

Electrical discharge machining (EDM) is an advanced non-traditional manufacturing technology that has many advantages over other machining methods. Many papers have discussed the machining mechanism and modeling of the EDM process. However, previous mechanism models have mainly been linear, which contradicts their precondition that EDM is a stochastic process. In this paper, a non-linear mechanism model is proposed for the EDM process. A threshold condition that leads to chaos is calculated using the Melnikov theory. The theoretical results indicate that the EDM system can generate varied chaos in the evolution of electrical discharge. To verify this conclusion, validation experiments are implemented. Several sets of complete EDM processes’ real-time series are analyzed by multiple chaotic numerical criteria, including power spectrum analysis, principle component analysis (PCA), correlation dimension analysis, and Lyapunov exponent analysis. The experimental results provide further qualitative and quantitative evidence that a complete EDM process has dynamical chaotic characteristics.     

基于Linux的微细电火花加工数控系统的研究

为了满足微细电火花加工高精度、高灵敏度和强实时性的运动控制要求,提出了以可编程多轴控制器为下位机、PC和Linux为上位机的体系结构,进行了六轴联动微细电火花加工数控系统的软硬件设计.为了解决微细电极在线制造和补偿等技术难题,该数控系统集成了机器视觉功能,基于V4L2应用程序接口开发了图像采集程序,基于OpenCV开发了图像处理程序,通过Canny边缘检测算法,提取了微细电极的边缘轮廓,可以在线观察和测量微细电极.此外,基于Lex和YACC设计并实现了数控代码解释器.     

微细电火花加工装置及加工工艺的研究

为适应现代加工小型化的需求,在开发微型电火花加工装置的基础上,通过加工微孔、微轴及微三维结构,对加工工艺进行系统研究;分析RC脉冲电源中参数R、C对加工效率、电极损耗比和表面质量的影响;验证电火花伺服扫描加工微三维结构的可行性.