Self-tuning fuzzy control with a grey prediction for wire rupture prevention in WEDM

Wire rupture in the wire electrical discharge machining (WEDM) process is one of the most troublesome problems in practical applications. In this paper, the abnormal ratio R_ab, defined as the proportion of abnormal sparks in a sampling period, is taken to represent the gap state in machining. The grey predictor is adopted to compensate the time-delayed R_ab caused by the low pass filter data processing. A gain self-tuning fuzzy control system has been developed to cope with the conditions that often occur with wire rupture in the WEDM process, such as an improper setting of machining parameters, machining the workpiece with varying thickness, etc. Experimental results of several cases show that the proposed controller results in a satisfactory performance. Not only can it immediately suppress transient situation once there is a sudden change of workpiece thickness, but a stable performance can also be achieved during machining a workpiece of constant thickness. As a result, wire rupture problems in most WEDM processes can be successively solved by the proposed control strategy.     

Determining cutting parameters in wire EDM based on workpiece surface temperature distribution

The material removal process in wire electrical discharge machining (WEDM) may result in work-piece surface damage due to the material thermal properties and the cutting parameters such as varying on-time pulses, open circuit voltage, machine cutting speed, and dielectric fluid pressure. A finite element method (FEM) program was developed to model temperature distribution in the workpiece under the conditions of different cutting parameters. The thermal parameters of low carbon steel (AISI4340) were selected to conduct this simulation. The thickness of the temperature affected layers for different cutting parameters was computed based on a critical temperature value. Through minimizing the thickness of the temperature affected layers and satisfying a certain cutting speed, a set of the cutting process parameters were determined for workpiece manufacture. On the other hand, the experimental investigation of the effects of cutting parameters on the thickness of the AISI4340 workpiece surface layers in WEDM was used to validate the simulation results. This study is helpful for developing advanced control strategies to enhance the complex contouring capabilities and machining rate while avoiding harmful surface damage.     

航空薄壁件圆角的铣削加工试验研究

航空薄壁件圆角加工的质量控制一直是不易解决的难题。在圆角的走刀过程中,常常发生欠切、过切、振动等现象,一般要通过手工打磨来消除过切、欠切痕迹或振纹。其不仅降低了刀具的寿命,严重影响了工件的加工精度和加工效率。本文提出了细化圆角走刀路径的铣削方法,以解决航空薄壁件的圆角铣削加工问题。试验结果表明,该方法是有效、可行的。     

On-line Estimation of Workpiece Height by Using Neural Networks and Hierarchical Adaptive Control of WEDM

Wire breakage and unstable machining drastically reduce the machining efficiency and accuracy in wire electrical discharge machining (WEDM). When a stair-shaped workpiece is machined, poor electrolyte flow around the steps leads to wire rupture or unstable machining. This paper presents a WEDM adaptive control system that maintains optimal machining and improves the stability of machining at the stair section where workpiece thickness changes. A three-layer back propagation neural network is used to estimate the thickness of a workpiece. The developed adaptive control system is executed in the hierarchical structure of three control loops, using fuzzy control strategy. In the first control loop, the total sparking frequency is controlled within a safe level for wire rupture suppression. In the second control loop, the proportion of abnormal sparks is maintained at a pre-determined level for process control purposes. Based on the estimated thickness of a workpiece, adaptive parameter optimisation is carried out to determine the optimal machining settings and to provide the reference targets for the other two control loops. Experimental results demonstrate that the workpiece height can be estimated by using a feed-forward neural network. The developed adaptive control system results in faster machining and better machining stability than does the commonly used gap voltage control system.     

<Emphasis Type="Bold">Six-Axis Controlled Ultrasonic Vibration Cutting in Fabrication of a Sharp Corner</Emphasis>

The completion of machining in one setting leads to high accuracy and quality. Since ordinary machining is not capable of fabricating sharp corners, electrical discharge machining (EDM) is used. However, this requires a highly skilled machinist to set up the workpiece to maintain the accuracy of the product since two or more set-ups are involved in this process. In this study, a new manufacturing method for the fabrication of sharp corners is examined experimentally. Six-axis controlled cutting using a bore byte tool with the application of ultrasonic vibration has been applied in this method. Experimental resultshave shown the applicability of this method in the fabrication of sharp corners.     

Study of machining parameters optimization for different materials in WEDM

In process planning of wire electrical discharge machining (WEDM), determination of appropriate machining conditions is likely to face problems in many ways. In addition to the construction of the relationship between machining parameters and machining characteristics, optimization search technique, a large number of experiments must be conducted repeatedly to renew parameters for different workpiece materials. The concept of specific discharge energy (SDE) was employed in this paper to represent the WEDM property of workpiece materials as one of the machining parameters. Two kinds of materials with distinctive SDE values, i.e., higher and lower, respectively, were selected for our experiments. The experimental data obtained were used, and a neural network that can accurately predict the relationship between machining parameters and machining characteristics was constructed. It was found that the predicted error was less than 7 %. The optimization technique of genetic algorithms was employed, and the optimal combination of machining parameters that meet the required machining characteristics for different workpiece materials was obtained. The system proposed in this study is both user-friendly and practical. It can save considerable time and cost during the construction of the database for the expert system of process planning.     

Modeling and multiresponse optimization on WEDM for HSLA by RSM

Wire electric discharge machining (WEDM) is a nonconventional machining method to cut hard and conductive material with the help of a moving electrode. High-strength low-alloy steel (HSLA) is a hard alloy with high hardness and wear-resisting property. The purpose of this study is to investigate the effect of parameters on cutting speed and dimensional deviation for WEDM using HSLA as workpiece. It is seen that the most prominent factor for cutting speed and dimensional deviation is pulse-on time, while two-factor interactions play an important role in this analysis. Response surface methodology was used to optimize the process parameter for cutting speed and dimensional deviation. The central composite rotatable design was used to conduct the experiments. The analysis of variance was used for the investigation of significant factors.     

Highly efficient manufacture of groove with sharp corner on adjoining surfaces by 6-axis control ultrasonic vibration cutting

Rapid machining means the extreme reduction of the machining lead-time required in completing a product from the blank shape. One of the ways in which machining lead-time could be reduced drastically is by avoiding too many setup changes. This paper presents a new machining method to create a groove with sharp corner on adjoining surfaces in one setup. Grooves with sharp corner (GSC) on adjoined curved or overhanging surfaces are at present impossible to manufacture by conventional machining in one setup or even by the existing numerical control (NC) machining method especially if it is adjacent to the obstruction. The obstruction tends to hamper the flow of machining operation, thereby requiring two or more setups as well as additional expensive fixtures to machine such a product.

In the study, the GSC on adjoined curved or overhanging surfaces will be manufactured by 6-axis control milling using a non-rotational tool with the application of ultrasonic vibrations (USV) in one setup. The study also describes the machining method as well as the development of software for 6-axis control milling and the effect of ultrasonic vibrations in multi-axis machining. Based on the experimental results, the effectiveness of the new manufacturing method as well as the developed CAM software has been experimentally confirmed in the study.     

慢走丝电火花线切割粗加工仿真系统的开发

为对电火花线切割加工机理有更深刻的认识,了解加工参数对加工精度的影响规律,介绍了一个电火花线切割加工粗加工的仿真系统。仿真系统通过对放电点的探索、工件的去除以及对线电极丝振动的分析,将实际的加工现象形象地再现于计算机上。为证实仿真系统的正确性,将仿真结果与试验结果进行了比较。在试验结果的获取中,为能够得到很难测量的线电极周围的放电间隙和工件形状数据,提出一种新的三维形状测量方法。通过对仿真结果和试验结果的比较,定性地证明了仿真方法的正确性。研究结果表明:改变线电极张力及伺服电压值时,从仿真过程得到的加工形状结果与试验得到的加工形状结果其倾向基本一致;加工缝端部的形状呈凸形还是呈凹形,取决于加工过程中放电爆发力和静电引力的相对大小。     

Real-time P-H curve CNC interpolators for high speed cornering

In this paper, real-time Pythagorean hodograph (P-H) curve CNC interpolators are used for high speed corner machining. There are large contouring errors around sharp corners when low-bandwidth servo controllers (such as P-PI control) are used. In order to decrease the amount of the cornering errors, an improved interpolation method is proposed. The first deceleration phase of motion and the over-corner P-H curve constructed for regions with sharp corners are devised by quadratic and constant velocity interpolation algorithms, respectively. The geometric parameters of the over-corner P-H curve and the feed rate along the modified tool path are computed by pattern search algorithm in order to reduce the maximum cornering error. The proposed interpolation algorithm is implemented for symmetrical and unsymmetrical corners. The results of simulation, such as the cornering error and the total cornering time, are compared with previously published methods. It has been observed that the developed over-corner P-H approach can substantially reduce the amount of cornering error.     

Experimental study on elastic deformation machining process for aspheric surface glass

Elastic deformation machining is a fabrication method that exploits the elastic deformation properties of materials under stress. Coupled with plane lapping machining process, this new fabrication method is suitable for machining complex aspheric surfaces. Upon completion of the machining process, the workpiece under process will be shaped into a desired surface form. The elastic deformation machining has several advantages over traditional fabrication methods, i.e., high machining compatibility and high fidelity of material property during machining process. The subject of this study is to determine the surface shape of the finished glass workpiece after the lapping process of the elastic deformation machining. The experimental results were compared with theoretical calculations. In the case when the vacuum pressure is 50 kPa, the maximum deviation value between the deformation curves from the theoretical calculation and the experiment results is within 62 μm. In order to improve the precision of form surface, the vacuum pressure is modified from 50 to 42 kPa. This reduction corresponds to a change of workpiece thickness when it is lapped. The results of the change of vacuum pressure show that the form accuracy produced is improved significantly and agrees very well with theoretical calculations. The maximum deviation in this case is 1.6 μm. The study indicates that the experimental plane lapping setup that exploits the material elasticity property can be utilized to fabricate aspheric lenses with axisymmetric surface and low complexity.     

Improving the surface quality in wire electrical discharge machined specimens by removing the recast layer using magnetic abrasive finishing method

This study explores the feasibility of removing the recast layer formed on aluminum alloy cylindrical specimens machined by wire electrical discharge machining (WEDM) by using magnetic abrasive finishing (MAF). The WEDM is a thermal machining process capable of accurately machining parts with high hardness or complex shapes. The sparks produced during the WEDM process melt the metal’s surface. The molten material undergoes ultra-rapid quenching and forms a layer on the surface defined as recast layer. The recast layer may be full of craters and microcracks which reduce service life of materials tremendously, especially under fatigue loads in corrosive environments. This investigation demonstrates that MAF process, can improve the quality of WEDM machined surfaces effectively by removing the recast layer. The present work studies the effect of some parameters, i.e., linear speed, working gap, abrasive particle size, and finishing time on surface roughness and recast layer thickness using full factorial analysis. Three-level full factorial technique is used as design of experiments for studying the selected factors. In order to indicate the significant factors, the analysis of variance has been used. In addition, an equation based on regression analysis is presented to indicate the relationship between surface roughness and recast layer thickness of cylindrical specimens and finishing parameters. Experimental results show the influence of MAF process on recast layer removal and surface roughness improvement.     

Contouring of polished single-crystal silicon plates by wire electrical discharge machining

We investigate the effect of cutting by wire electrical discharge machining (WEDM) on the shape accuracy of polished single-crystal silicon. Single-crystal silicon plates are polished, and then contoured in deionized water or in oil by WEDM. The shape accuracy of the polished surfaces is measured with an interferometer. As a result, the polished surfaces are deformed into convex shapes by WEDM cutting. The polished surfaces tend to become flat as the roughness of the cut sections decreases, and the flatness is independent of the type of cutting liquid. Cutting in oil is advantageous for maintaining the smoothness of polished surfaces. These findings confirm that, in the contouring process of polished single-crystal silicon blocks, smooth and high-accuracy surfaces are achieved by conducting rough- and finish-cutting WEDM processes in oil.     

基于LabVIEW的电火花线切割放电加工状态的研究

电火花线切割加工时放电加工状态决定着加工的质量和速度,电极丝和工件之间的放电电压是对放电加工过程进行实时检测的重要参数.以LabVIEW为开发平台,以放电电压为检测参数,构建了电火花线切割放电加工状态识别仿真系统.该系统主要包括线切割放电状态识别模块、BP神经网络放电预测模块以及加工稳定性分析模块,对提高电火花线切割加工质量、加工效率及智能化加工有良好的效果.     

Prioritization analysis and compensation of geometric errors for ultra-precision lathe based on the random forest methodology

Geometric errors remarkably affect the dimensional accuracy of parts manufactured by ultra-precision machining. It is vital to consider the workpiece shape for the identification of crucial error types. This research investigates the prioritization analysis of geometric errors for arbitrary curved surfaces by using random forest. By utilizing multi-body system (MBS) theory, a volumetric error model is initially established to calculate tool position errors. An error dataset, which contains information of 21 geometric errors, workpiece shape, and dimensional errors, is then constructed by discretizing the workpiece surface along the tool path. The problem of identifying crucial geometric errors is translated into another problem of feature selection by applying random forest on the error dataset. Moreover, the influence extent of each geometric error on the dimensional accuracy of four typical curved surfaces is analyzed through numerical simulation, and crucial geometric errors are identified based on the proposed method. Then, an iterative method of error compensation is proposed to verify the reasonability of the determined crucial geometric errors by specifically compensating them. Finally, under compensated and uncompensated conditions, two sinusoidal grid surfaces are machined on an ultra-precision lathe to validate the prioritization analysis method. Findings show that the machining accuracy of the sinusoidal grid surface with crucial geometric error compensation is better than that without compensation.     

数控快走丝电火花线切割加工工艺方法研究

通过对数控快走丝电火花线切割加工中如何加工穿丝孔、装夹工件、编制程序、选择电参数、使用工作液、应用多次切割工艺方法以及诊断加工故障等工艺方法进行研究。形成了一套完整的工艺方法，效果良好。     

Corner Transition Toolpath Generation Based on Velocity-Blending Algorithm for Glass Edge Grinding

Sharp corners usually are used on glass contours to meet the highly increasing demand for personalized products,but they result in a broken wheel center toolpath in edge grinding.To ensure that the whole wheel center toolpath is of G1 continuity and that the grinding depth is controllable at the corners,a transition toolpath generation method based on a velocity-blending algorithm is proposed.Taking the grinding depth into consideration,the sharp-corner grinding process is planned,and a velocity-blending algorithm is introduced.With the constraints,such as traverse displacement and grinding depth,the sharp-corner transition toolpath is generated with a three-phase motion arrangement and with confirmations of the acceleration/deceleration positions.A piece of glass with three sharp corners is ground on a three-axis numerical-control glass grinding equipment.The experimental results demonstrate that the proposed algorithm can protect the sharp corners from breakage efficiently and achieve satisfactory shape accuracy.This research proposed a toolpath generation method based on a velocity-blending algorithm for the manufacturing of personalized glass products,which generates the transition toolpath as needed around a sharp corner in real time.     

Development of a virtual reality wire electrical discharge machining system for operation training

Wire electrical discharge machining (WEDM) uses a metallic thin wire to cut a programmed profile with high strength having sharp edges such as extrusion dies and blanking punches. However, the cost of purchasing and maintaining of WEDM equipment is very high for both the industry and general education institutions. Therefore, there are potential demands to reduce the expensive machine operation training cost, provide off-line collision-free simulation verification of the tool path, and examine the correctness of the programmed wire cutting NC codes. This paper presents the development of a virtual reality-based WEDM full machine simulation system which can emulate major functions of a real controller related to operation training and education. These functions include the tool path simulation, NC program interpretation and processing, kinematics of the machine mechanism, workpiece origin setting, etc. To demonstrate the developed system and illustrate the adopted method, the system capability is explained and shown in this paper. The research result can be used as a cost-effective interactive 3D digital tutoring system that has the benefits of improving on the inefficient, dangerous, and costly drawbacks in traditional learning and training for operating the real WEDM machine.     

运动学约束下直线转角非对称平滑加工规划

为实现数控加工中直线转角处的连续平滑进给,提出了一类直线转角的非对称平滑加工规划方法。针对转角初末速度、加速度大小不同的情况,考虑直线转角处的运动学约束和几何误差约束,并将高次转角模型求解化简为线性规划问题,可直接解算转角参数,实现进给速率平滑与刀具轨迹平滑的同步规划,提高规划效率与加工效率。与对称转角平滑的仿真对比实验结果表明,所提方法在保证加工过程平稳的情况下效率与精度有所提高,运动平台样机实验验证了该方法的可行性。     

Electrochemical machining analysis on grid cathode composed of square cells

During the electrochemical machining (ECM), the cathodes designed by the existing methods are mainly unitary cathodes, which can be only used to produce the workpieces with the same shapes. However, there are few researches on designing cathodes for machining the different workpieces with the different surfaces. This paper presents the grid cathode composed of the square cells to produce the workpieces with different shapes. Three types of the square cells, 2.5 mm′2.5 mm, 3 mm′3 mm, and 4 mm′4 mm, are utilized to construct the plane, the slant, and the blade cathode. The material of the cathode and the anode is CrNi 18 Ti 9 , and the ingredient of electrolyte is 15% NaCl and 15% NaNO 3 . The machining equilibrium machining current and time are acquired and analyzed, the machining process and the workpiece quality are compared between using the grid cathode and the unitary cathode. Moreover, the machining errors on the workpiece surface are measured and analyzed, and the error reasons are traced and discussed to obtain the better surface quality of the workpiece. The experiment and analysis results show that the grid cathode can be used to manufacture the workpieces with complex shapes in certain range of the error. The workpiece quality improves with the size of the square cell being reduced, and if the square element is small enough, the workpiece quality is almost equal to the one machined by the unitary cathode. The proposed research realizes a single cathode machining the different workpieces with the different surfaces.