Electrical discharge machining of electron beam melting formed shrouded blisk

Shrouded blisk components are one of the core parts in aerospace engines. Five-axis or six-axis electrical discharge machining is once regarded as the best solution to manufacturing these components. The machining process, however, is time consuming and money cost. In this paper, an electron beam melting (EBM) and electrical discharge machining (EDM) combined method for shrouded blisks manufacturing was presented. In the new method, EBM was used to generate the near-net forming blank of the shroud blisk, and EDM was responsible for finish machining. The mechanical properties of the EBM formed Ti6Al4V blank was tested to be good enough for shrouded blisks. A visual identification method was proposed to establish the machining reference on the EBM formed blisk blank for the subsequent EDM process. The given example showed the required blade accuracy of ±0.03 mm can be achieved by the visual identification method. Through the combination of EBM and EDM method, the machining time of the shrouded blisk was greatly reduced by 54.8 %.     

隐曲线的线性和旋转插补

基于数控五轴刀具运动分析,提出了旋转插补的概念,给出了旋转插补的相关定义,在此基础上提出了隐曲线的线性和旋转插补原理和方法,并进一步提出了改进的插补方法。     

Performance Evaluation of a Real-Time Interpolation Algorithm for NURBS Curves

This paper presents a real-time interpolation algorithm for NURBS curves. In contrast to the existing linear and circular interpolators, the proposed interpolator can maintain small contour errors and feedrate fluctuations. Feedrate components, acceleration components and the servomotor driving force for each axis are precalculated from the given curve shape and themachine tool dynamic properties. As a sudden change in the geometrical properties of the tool path can increase contour errors and cause a sudden change of driving force of each servomotor, a new strategy of variable feedrate machining based on the geometrical properties of tool path is suggested. Real-time performance measurement of this interpolator is performed to demonstrate its practical feasibility.     

Adaptive feedrate interpolation with multiconstraints for five-axis parametric toolpath

A good adaptive feedrate will be helpful for improving machining accuracy and efficiency, as well as avoiding the excess of the machine’s physical capabilities and feed fluctuations during machining. Therefore, it is highly desirable to consider the constraints of geometric error, cutting performance, and drive constraints in the feedrate scheduling of the parametric curve interpolator for five-axis computer numerical control machining. In this paper, a novel multiconstraints feedrate scheduling method is proposed for the parametric curve interpolator in five-axis machining. In the method, the feed optimization model is first built with the constraints of geometric error, the maximum feedrate and acceleration of cutter tip, and the maximum feedrate and acceleration of five-drive axes. Then, the relations between each constraint and the cutter tip feedrate are derived by means of near arc length parameterization. After that, a linear programming algorithm is applied to obtain the optimal feed profile on the sampling positions of the given tool path. Finally, illustrated examples are given to validate the feasibility and applicability of the proposed feedrate scheduling method. The comparison results show that the proposed method has an ability of the simultaneous guarantees of geometric accuracy, cutting performance, and drive characters of machine tools.     

空间曲面零件电火花线切割加工新技术

开发了能实现空间曲面工件电火花线切割加工的多轴联动加工系统。阐明了系统的组成及运动规律。通过计算机仿真和加工实践。研究验证了数学模型的正确性。从而拓宽了电火花线切割加工的工艺范围。解决了空间曲面工件的加工难题。     

多轴加工系统闭链刚度场建模与刚度性能分析

在复杂曲面多轴加工中,刀具可以在一定范围内以不同姿态加工复杂曲面,刀具姿态的改变直接影响整体加工系统的综合刚度特性,从而影响到加工性能。建立与刀具位姿相关的综合刚度场模型,可用于分析整体加工系统的综合刚性分布规律。提出多轴加工系统刚度场半解析计算方法,其中针对机床运动轴及其传动部位、刀具、工件等关键部件的力学特性分别采用了雅可比矩阵法、点传递矩阵法、有限元法等方法建立相应的刚度矩阵,并根据多体小变形理论建立了多轴加工工艺系统闭链刚度场模型。根据该模型解耦得到三维空间的力椭球,从力椭球提取整体加工系统的刚度性能指标,绘制刚度性能等值线图,分析工作空间中多轴加工系统的刚度特性分布规律,可用于指导刀具运动规划。     

Investigating and removing the re-sticky debris on tungsten carbide in electrical discharge machining

Electrical discharge machining (EDM) is an excellent method to machine tungsten carbide with high hardness and high toughness. However, debris from this material produced by EDM re-sticking on the workpiece surface remarkably affects working efficiency and dimension precision. Therefore, this study investigated the re-sticky phenomenon of tungsten carbide and how to reduce the debris re-sticking on the workpiece surface. In general, the polarity in EDM depended on the different electrical parameters of the machine input and the different materials of the tool electrode. The first item of investigation observed the re-sticky position of the debris to study the effect of different polarities during the EDM process. Next, the tool electrode was set at different conditions without rotation and with a 200 rpm rotational speed to evaluate the rotating effect in EDM. Finally, different lift distances of the electrode and different shapes of electrode with rotation were utilized to investigate the improvement for reducing debris re-sticking on the machining surface. The results showed that only negative polarity in EDM could cause the re-sticky phenomenon on tungsten carbide. On the other hand, debris would notably re-stick on any machining position when the tool electrode was not rotated in EDM. Besides, debris significantly stuck on the center of the working area with rotation of the electrode. Additionally, a larger lift distance of the tool electrode could reduce debris re-sticking on the working surface, but this process would decrease material removal rate in EDM. In the end, a special shaped design of the tool electrode resulted in the re-sticky debris completely vanishing, when the electrode width was 0.6 times the diameter of this cylindrical electrode.     

微通道结构带状电极电火花加工方法研究

针对大厚度、高深宽比金属微通道结构加工的难题,提出一种带状电极电火花加工方法,利用厚度30～100 μm的带状电极,在金属基体上高效制造微通道结构。研究了带状电极电火花加工机理,建立了带状电极在加工间隙中的运动模型,分析了影响带状电极运动的主要因素,搭建了带状电极电火花加工装置,开展了微通道结构带状电极加工实验研究,获得了带状电极电火花加工基础工艺规律。利用带状电极电火花加工方法成功加工出的具有200条微通道的反应器结构和44×45微换热器阵列结构,表明带状电极电火花加工可以实现窄宽度(100 μm以下)、大厚度(35 mm以上)、高深宽比(10以上)和高精度(缝宽标准差3 μm以内)的大批量微通道结构的高效加工,相关方法和技术有望在微模具、微散热器、微反应器等领域获得推广和应用。     

Optimal curvature-smooth transition and efficient feedrate optimization method with axis kinematic limitations for linear toolpath

In industrial areas, the machining performance with linear G01 code is a crucial indicator to evaluate the computer numerical control (CNC) systems and many researchers have presented various methods to deal with the corner tracking issue. However, the axis jerk limitations are not satisfied well and the different axis kinematic limitations are not considered in most researches, which will reduce the machining efficiency and machining quality simultaneously. In this paper, a novel method including trajectory planning, feedrate scheduling, and interpolating is proposed to obtain better machining quality and higher machining efficiency. In trajectory planning, a B-spline curve is utilized to smooth the linear toolpath and obtain a curvature-smooth trajectory, which is third-order geometry continuous. Thereby, a time-optimal method for the geometric continuous trajectory is proposed based on linear programming algorithm in the feedrate scheduling and the bounded multi-constraints, including axis velocity, axis acceleration, axis jerk, and feedrate. Moreover, it can be seen that the proposed method is near Bang-bang control. To reduce the computation time of the optimal numerical method, an efficient method with a look-ahead window around the transition B-spline curve is applied. In the interpolation stage, a novel interpolation method about arc-length is proposed to improve computation efficiency. Finally, simulation and experiment are conducted to show superiorities of the proposed method to the already existing approaches. The results show that the cycling time of the proposed method is reduced by more than 7% than G² method and 20% than G³ method with better contour performance.     

An adaptive feedrate scheduling method of dual NURBS curve interpolator for precision five-axis CNC machining

Non-uniform rational b-spline (NURBS) tool path is becoming more and more important due to the increasing requirement for machining geometrically complex parts. However, NURBS interpolators, particularly related to five-axis machining, are quite limited and still keep challenging. In this paper, an adaptive feedrate scheduling method of dual NURBS curve interpolator with geometric and kinematic constraints is proposed for precision five-axis machining. A surface expressed by dual NURBS curves, which can continuously and accurately describe cutter tip position and cutter axis orientation, is first used to define five-axis tool path. For the given machine configuration, the calculation formulas of angular feedrate and geometric error aroused by interpolation are given, and then, the adaptive feedrate along the tool path is scheduled with confined nonlinear geometric error and angular feedrate. Combined with the analytical relations of feed acceleration with respect to the arc length parameter and feedrate, the feed profiles of linear and angular feed acceleration sensitive regions are readjusted with corresponding formulas and bi-directional scan algorithm, respectively. Simulations are performed to validate the feasibility of the proposed feed scheduling method of dual NURBS curve interpolator. It shows that the proposed method is able to ensure the geometric accuracy and good machining performances in five-axis machining especially in flank machining.     

A strain energy minimization method for generating continuous NURBS-based motion curves in free-form surface machining

In spline-based free-form surface machining, segmented toolpaths are inevitable, and the discontinuities between segments cause large feedrate errors in the generated motion commands. Methods of the geometric design to create smooth composite curves may introduce new error sources in the original curve, and thus they are not suitable for motion planning in high precision free-form surface machining. In this paper, a strain energy minimization model (SEMM) and a non-uniform rational B-spline (NURBS)-based motion planning algorithm is proposed for free-form surface machining. The proposed SEMM and motion planning algorithm are capable of creating composite NURBS-based motion curves with up to C2 integral continuity without oscillations. Results from simulation show that with the proposed energy minimization model and motion curve generation algorithm, the feedrate errors caused by discontinuities in toolpaths can be effectively removed.     

空气中微细电火花沉积与去除可逆加工技术研究

论述了一种新的电火花加工方法。它使用通用的电火花成形加工机床,利用常见的电极材料,在空气介质中,通过脉冲放电在工件表面上沉积生长电极材料,再通过反转极性和适当的轨迹控制对所生成的沉积材料进行有选择的去除加工,进而实现材料的生长与去除可逆加工。通过对电火花加工理论的研究,预测和论证了实现这一新加工方法的可能性和实现条件。通过试验成功地将钢、铜和钨三种电极材料沉积到工件上,形成直径为 100~240 mm、高度为1 000~2 500 mm的微小圆柱体。并对沉积物进行了选择去除,实现了在同一设备上的可逆电火花加工。对沉积材料的致密性、硬度及其与工件的结合强度等进行了系统的分析,表明沉积物组织致密、坚硬,可以满足功能材料的要求。     

Investigation of emulsion for die sinking EDM

Machining fluid is a primary factor that affects the material removal rate, surface quality, and electrode wear of electrical discharge machining (EDM). Kerosene is the most commonly used working fluid in die sinking EDM, but it shows low ignition temperature and high volatility; if the improper operations are undertaken, it can cause conflagration. Using distilled water or pure water as the machining fluid in EDM, no fire hazard occurs, and the working environment is well; however, using distilled water or pure water as the machining fluid in EDM, the material removal rate of machining large surface is low, and the machine tool is easily eroded. Emulsion-1 and emulsion-2 used as working fluid in die sinking EDM are developed. The compositions of emulsion-1 and emulsion-2 are introduced. In comparison with kerosene, emulsion-1 and emulsion-2 used in EDM show high material removal rate, low surface roughness, high discharge gap, and good working environment. The electrode wear ratio in emulsion-1 is lower than that in kerosene. The electrode wear ratio in emulsion-2 is higher than that in kerosene. The effects of composition and concentration of emulsifier on the emulsion property and EDM performance have been investigated. The comparative tests of EDM performance with kerosene, emulsion-1, and emulsion-2 have been done.     

Design of a real-time NURBS interpolator with constant segment length for milling EDM

The utilization of the non-uniform rational B-spline (NURBS) toolpaths becomes more important than ever before. However, in traditional milling electrical discharge machining (EDM) of parametric curves, there commonly exist problems such as speed loss and an overincreased sampling period, which directly cause a decrease in machining efficiency. Moreover, traditional approaches normally suffer from a complex toolpath planning. In this paper, a real-time interpolator with a constant segment length is proposed to improve the milling EDM of NURBS curves. The proposed interpolator can directly process NURBS curves with their definition information. The toolpath planning can thus become simpler. A new two-stage interpolation method is adopted such that both a high speed accuracy and a relatively short sampling period can be achieved, and the magnitude of chord errors can also be reduced. While the first-stage interpolation samples a NURBS curve with a constant-length segment, the second-stage interpolation, or the re-interpolation, executes multiple interpolations in a sampling period to generate the required feed rate. A loop buffer is designed for the implementation of the real-time interpolator. Experimental results show that the proposed interpolator demonstrates a superior machining performance to that by traditional interpolators, especially in terms of chord errors and erosion rate.     

基于数控刀位轨迹的电火花成形加工中电极平动补偿和修正

在电火花成形加工(EDM)中,电极的平动产生了加工误差。针对EDM中电极平动的不同类型,提出并实现了基于数控刀位轨迹的电极平动补偿修正方法。该方法根据EDM的电极平动类型和平动量对用于电极加工的数控程序进行补偿处理,达到对电极平动进行修正的目的,从而减少电极平动所产生的加工误差。试验表明,该方法可以有效地提高EDM加工精度、保持加工质量的稳定性以及降低劳动程度和缩短生产周期。     

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.     

Feasibility study of micro-slit EDM machining using pure water

This study addresses micro-slit EDM machining feasibility using pure water as the dielectric fluid. Experimental results revealed that pure water could be used as a dielectric fluid and adopting negative polarity EDM machining could obtain high material removal rate (MRR), low electrode wear, small slit expansion, and little machined burr, compared to positive polarity machining. In comparing kerosene versus pure water, pure water was observed to cause low carbon adherence to the electrode surface. Also discharge energy does not decrease and the discharge processes are not interrupted. Therefore, MRR was higher, and related electrode wear ratio compared to kerosene use was lower. In a continual EDM with multi-slit machining, kerosene will cause carbon element adherence, creating an initially high MRR and electrode wear, with rapid decline. However, pure water will not cause carbon element adherence on the electrode surface, so MRR and electrode wear is always stable in this process.     

Micro-hole Maching of Copper Using the Electro-discharge Machining Process with a Tungsten Carbide Electrode Compared with a Copper Electrode

This paper describes micro-hole machining of a copper plate using the electro-discharge machining (EDM) process. Tungsten carbide was selected as the material for the electrode and compared with a copper-electrode. A precision centreless grinding process was employed to grind the electrode down to the desired diameter. A series of experiments were performed on a traditional EDM machine to investigate the effects of electrode material polarity setting and of a rotating electrode. Results have shown that electrode wear and hole enlargement are both smaller when positive polarity machining is selected; whereas electrode wear is larger and machining speed is higher when negative polarity machining is selected. High-quality micro-hole machining in copper can be achieved by the proposed method.     

高精度硬质合金套类零件的加工方法

介绍硬质合金套类零件电解和电解磨削加工的基本原理,综合运用了以下多种新工艺:螺旋电极电解扩孔、螺旋 电极电解磨孔、中极法电解磨削外圆及端面。