Fabrication accuracy analysis of micromilling tools with complicated geometries by wire EDM

The micromilling tool is one of the key factors affecting micromilling performance. The design and fabrication of micromilling tools are still behind the increasing requirements in miniature component fabrication. How to estimate the fabrication accuracy of a newly designed micromilling tool is one of the urgent issues for micro tooling. This paper introduces an accuracy analysis method in the fabrication of micromilling tools by wire electrical discharge machining (WEDM) process. Taking two typical micro ball end mills into consideration, the micro tool fabrication process is kinematically modeled and analyzed. Analytical results show that the final fabrication accuracy has a close relationship with the designed micro tool geometry. The fabrication procedures can be arranged based on the kinematical analysis, and the final fabrication accuracy also is affected by it. The radius errors of the fabricated micro ball end mill prototype are within ±2μm, which is higher than that of commercially available similar ones. It verifies the proposed accuracy analysis method.     

Comparative study of conventional and micro WEDM based on machining of meso/micro Sized Spur Gear

This paper discusses the comparison of micro machining process using conventional and micro wire electrical discharge machining (WEDM) for fabrication of miniaturized components. Seventeen toothed miniaturized spur gear of 3.5 and 1.2 mm outside diameter were fabricated by conventional and micro WEDM respectively. The process parameters for both conventional and micro WEDM were optimized by preliminary experiments and analysis. The gears were investigated for the quality of surface finish and dimensional accuracy which were used as the criteria for the process evaluation. An average surface roughness (R_a) of 50 nm and dimensional accuracy of 0.1–1 μm were achieved in micro WEDM. Whenever applied conventional WEDM for meso/micro fabrication, a R_a surface roughness of 1.8 μm and dimensional accuracy of 2–3 μm were achieved. However, this level of surface roughness and dimensional accuracy are acceptable in many applications of micro engineering. A window of conventional WEDM consisting of low energy discharge parameters is identified for micromachining.     

Fabrication of micro rods by twin-mirroring-wire tangential feed micro electrical discharge grinding

In micro electrical discharge machining (micro-EDM), the precision fabrication of cylindrical micro rods is difficult to achieve with a high processing efficiency. In order to overcome this challenge, this paper proposes a new processing method, which is denoted as twin-mirroring-wire tangential feed micro electrical discharge grinding (TMTF-WEDG). The machining principle, characteristics, and realization of the new method are firstly introduced. Then, the advantages of TMTF-WEDG in terms of machining efficiency and accuracy are demonstrated. The experimental results have shown that the machining efficiency can be increased to more than 70% in comparison with conventional tangential-feed wire electrical discharge grinding. It has also been proved that a minimum removal of material corresponding to a reduction of less than 1 μm in the diameter of a micro rod can be obtained by TMTF-WEDG. This considerably helps in improving the accuracy and repeatability of the machining process. A deviation of less than 1 μm on the diameter of a micro rod has been obtained in a length range of 800 μm. The process repeatability in machining five micro rods has been established to be below 2 μm. The proposed method is therefore of great significance for improving the machining efficiency and ensuring a high precision in the shaping process of cylindrical micro rods.     

Vibration assisted wire electrochemical micro machining of array micro tools

Micro structures and components are widely used in modern industries, and micro machining has therefore become a popular research topic. As micro tools are essential in micro machining, wire electrochemical micro machining is introduced in the fabrication of micro tools in this paper, and micro square column tool arrays are fabricated using wire cathodes by two steps. In order to improve the machining efficiency and quality, an electrode vibration technique is used, and the effects of bubble behaviour on slit width homogeneity and edge radius are studied through simulations of the electric field. The influences of various machining parameters such as vibration conditions, electrical properties, electrolyte concentration and feedrate on the standard deviation of the slit width and on the value of the edge radius are investigated. In addition, the micro dimple array is fabricated using electrochemical micro machining by employing the micro square column tool array as the cathode.     

复合运丝型电火花线切割加工参数分析与研究

提出了一种新型电火花线切割机床,即电极丝作往复直线运动的同时还绕自身轴线高速旋转的复合运丝型线切割机床。介绍了该类机床与其他线切割机床加工的基本工艺指标。通过与高速走丝电火花线切割机床比较实验,分析了脉冲宽度、脉冲间隔、脉冲峰值电流等电参数对加工工艺指标的影响,实验表明这种独特的复合运丝方式在降低表面粗糙度、提高加工精度等方面较传统运丝方式具有较大的优越性,且机床结构较为简单,对于各种工艺参数和电参数具有更加广泛的适用性,具有进一步研究和推广价值。     

Study on the generative design method and error budget of a novel desktop multi-axis laser machine for micro tool fabrications

Micro end mills made of hard or ultra-hard materials are mainly fabricated by grinding or by wire electrical discharge machining (WEDM). However, with the advances of new tool materials from ultra-hard to super-hard together with lower or no electrical conductivity such as the material of nano-polycrystalline diamond, the grinding or the WEDM method cannot be used for machining due to their ultra-low process efficiencies for such materials. Laser machining has been tested an effective method. Accordingly, multi-axis laser machines need to be designed for micro tool fabrications. In the paper, a typical micro ball end mill with relatively complex features has been analyzed by the generative design method to generate the number and properties of needed motion axes. Based on the analysis, a novel five-axis laser machine has been designed. Aiming at high-quality micro tool fabrications, the kinematics model has been derived for this five-axis laser machine and error budge has been studied for the subsequently optimum selection of key motion components.     

Design and development of PCD micro straight edge end mills for micro/nano machining of hard and brittle materials

One of the biggest challenges for mechanical micro/nano milling is the design and fabrication of high precision and high efficiency micro milling tools. Commercially available micro milling tools are either too expensive (around several hundred US dollars) or simply made from downsizing of macro milling tools, which is sometimes not appropriate for the specific micro/nano milling requirements. So the design and fabrication of custom micro milling tools are necessary. In this paper, a micro straight edge endmill (SEE) is designed. Static and dynamic FEM analyses have been done for the SEEs with different rake angles trying to identify their stiffness and natural frequencies. By wire electrical discharge machining (WEDM), the SEEs made of polycrystalline diamond (PCD) with three different rake angles have been fabricated. The evaluation milling on tungsten carbide (WC) and silicon wafer have processed on a nano milling center. Experimental results show the SEEs have a good ability to simultaneously micro/nano milling of both the side and bottom surfaces with submicron surface roughness, and the SEE has high accuracy for large aspect ratio thin wall machining. The milling experiments on silicon wafer have successfully demonstrated that ductile mode machining was achieved and the coolant played an important role in silicon wafer milling.     

Tool fabrication system for micro/nano milling—function analysis and design of a six-axis Wire EDM machine

The dies or molds used for the fabrication of micro products usually are made of ultra-hard materials such as tungsten carbide or silicon carbide and have sophisticated three-dimensional geometries. Such kind of dies or molds can only be fabricated by milling operations instead of grinding processes with ultra-hard milling tools made of PCD or CBN. Electrical discharge machining (EDM) is a good choice for the fabrication of such ultra-hard tools. In this paper, a function analysis and design of a six-axis Wire EDM (WEDM) machine is introduced. Based on the typical micro/nano cutting tool geometry features, a mathematical model between the cutting tool and the electrode wire is built. Then, the mathematical model is analyzed and it turns out that six axes are needed for cutting such complicated tool geometries. According to the WEDM features, first the axes are allocated to the workpiece side and the electrode wire side. The workpiece is assigned three linear motions and one rotary motion around its center line and the wire has two rotary motions. Second, the axis sequences are defined. At last, the best concept of the mechanical structure for the six-axis WEDM machine is selected.     

The design and optimization of micro polycrystalline diamond ball end mill for repairing micro-defects on the surface of KDP crystal

Micro-milling is a promising approach to repair the micro-defects on the surface of KH₂PO₄ (KDP) crystal. The geometrical parameters of micro ball end mill will greatly influence the repairing process as a result of the soft brittle properties of KDP crystal. Two types of double-edged micro ball end mills were designed and a three-dimensional finite element (FE) model was established to simulate the micro milling process of KDP crystal, which was validated by the milling experiments. The rake angle of −45°, the relief angle of 45° and the cutting edge radius of 1.5–2 μm were suggested to be the optimal geometrical parameters, whereas the rake angle of −25° and the relief angle of 9° were optimal just for micro ball end mill of Type I, the configuration with the rake angles ranging from 0° to 35°, by fully considering the cutting force, and the stress–strain distribution over the entire tool and the cutting zone in the simulation. Moreover, the micro polycrystalline diamond (PCD) ball end mills adopting the obtained optimal parameters were fabricated by wire electro-discharge machining (WEDM) and grinding techniques, with the average surface roughness Ra of tool rake face and tool flank face ∼0.10 μm, and the cutting edge radius of the tool ∼1.6 μm. The influence of tool's geometrical parameters on the finished surface quality was verified by the cutting experiments, and the tool with symmetric structure was found to have a better cutting performance. The repairing outlines with Ra of 31.3 nm were processed by the self-fabricated tool, which could successfully hold the growth of unstable damage sites on KDP crystal.     

Precision machining of micro tool electrodes in micro EDM for drilling array micro holes

Micro electro discharge machining (micro EDM) is suitable for machining micro holes on metal alloy materials, and the micro holes can be machined even to several microns by use of wire electro discharge grinding (WEDG) of micro electrodes. However, considering practicability of micro holes <Φ100 μm in batch processing, the controllable accuracy of holes’ diameter, the consistency accuracy of repeated machining and the processing efficiency are required to be systematically improved. On the basis of conventional WEDG method, a tangential feed WEDG (TF-WEDG) method combined with on-line measurement using a charge coupled device (CCD) was proposed for improving on-line machining accuracy of micro electrodes. In TF-WEDG, removal resolution of micro-electrode diameter (the minimum thickness to be removed from micro electrode) is greatly improved by feeding the electrode along the tangential direction of wire-guide arc, and the resolution is further improved by employing negative polarity machining. Taking advantage of the high removal resolution, the precise diameter of micro-electrode can be achieved by the tangential feed of electrode to a certain position after diameter feedback of on-line measurement. Furthermore, a hybrid process was presented by combining the TF-WEDG method and a self-drilled holes method to improve the machining efficiency of micro electrodes. A cyclic alternating process of micro-electrode repeated machining and micro holes’ drilling was implemented for array micro holes with high consistency accuracy. Micro-EDM experiments were carried out for verifying the proposed methods and processes, and the experimental results show that the repeated machining accuracy of micro electrodes was less than 2 μm and the consistency accuracy of array micro holes was ±1.1 μm.     

A study on the micro tooling for micro/nano milling

Many researches have studied the micro tools either by simulations or experiments showing that the micro tools play very important roles in micro/nano machining, and micro tool geometries have a direct impact on the final machining quality. Commercially available micro milling tools are usually simply made from downsizing of macro milling tools, which may not be able to be accurately fabricated. Custom micro milling tools have been created by some researchers, but no design criteria for micro milling tools have been introduced. The above factors constrain the development of effective micro milling tools and consequently constrain the researches on micro/nano milling. Based on former researcher??s work, this paper tries to introduce the design criteria for the custom micro tooling. Firstly, the extent typical micro milling tools are studied, and their drawback/limitations are summarized. Secondly, experimental evaluations of the key drawback/limitations are processed. Thirdly, the design criteria for custom micro milling tools are proposed, and corresponding design process is introduced. Then, a new micro hexagonal end mill has been designed based on these criteria. Finally, a polycrystalline diamond micro hexagonal end mill with a diameter of 0.5?mm was fabricated by wire electrical discharge machining, and the evaluation experiments for the hexagonal end mill have been processed on a nano milling center. Experimental results show the newly designed hexagonal end mill can achieve submicron level surface roughness and has simultaneously high-accuracy side and bottom machining capabilities.     

An investigation into wire electrochemical micro machining of pure tungsten

Tungsten-based microstructures have attracted great interest in many industrial advanced applications. Nevertheless, with a disadvantageous combination of high hardness, toughness and brittleness, the micro machining of pure tungsten poses significant difficulty. In this paper, an investigation into the wire electrochemical micro machining (WEMM) of pure tungsten at low alkaline electrolyte concentration and small pulse duration is presented. Under the optimal machining conditions, tungsten-based microstructures with a side gap of 4 μm, slit width of 18 μm and aspect ratio of 5.6, as well as with a side gap of 5 μm, slit width of 20 μm and aspect ratio of 15, were obtained. In order to improve productivity in the machining of multi-slit microstructures, multi-wire electrochemical micro machining of tungsten was introduced. Using a 3-wire electrode, a 9-slit microstructure with a slit width of approximately 24 μm was produced and the machining efficiency was improved by a factor of three. The results revealed that it was a promising method for the fabrication of tungsten-based periodic or quasi-periodic microstructures, such as the gratings used in the X-ray absorption contrast system of imaging.     

An experimental investigation on machining parameters of AISI D2 steel using WEDM

The performance of the wire electrodischarge machining (WEDM) machining process largely depends upon the selection of the appropriate machining variables. Optimization is one of the techniques used in manufacturing sectors to arrive for the best manufacturing conditions, which are essential for industries toward manufacturing of quality products at lowest cost. As there are many process variables involved in the WEDM machining process, it is difficult to choose a proper combination of these process variables in order to maximize material removal rate and to minimize tool wear and surface roughness. The objective of the this work is to investigate the effects of process variables like pulse on time, pulse off time, peak current, servo voltage, and wire feed on material removal rate (MRR), surface roughness (SR), gap voltage, gap current, and cutting rate in the WEDM machining process. The experiment has been done using Taguchi’s orthogonal array L27 (3⁵). Each experiment was conducted under different conditions of input parameters and statistically evaluated the experimental data by analysis of variance (ANOVA) using MINITAB and Design Expert tools. The present work also aims to develop mathematical models for correlating the inter-relationships of various WEDM machining parameters and performance parameters of machining on AISI D2 steel material using response surface methodology (RSM).The significant machining parameters and the optimal combination levels of machining parameters associated with performance parameters were also drawn. The observed optimal process parameter settings based on composite desirability (61.4 %) are pulse on time 112.66 μs, pulse off time 45 μs, spark gap voltage 46.95 V, wire feed 2 mm/min, peak current of 99.99 A for achieving maximum MRR, gap current, gap voltage, cutting rate, and minimum SR; finally, the results were experimentally verified.     

Development of surface roughness optimisation and prediction for the process of wire electro-discharge grinding

This paper investigates the technological capabilities of a hybrid micro machining process for performing wire electro-discharge grinding (WEDG). In particular, micro wire electrical discharge machining (μWEDM) is employed in combination with a rotating submergible spindle to perform WEDG. In this research, first a machining strategy for workpiece preparation is presented. Then, the effects of different machining setup parameters on the achievable surface finish after WEDG are investigated. In particular, an experimental study was conducted to identify the most statistically significant setup parameters for performing the main cut that affect the resulting surface quality. A signal-to-noise (S/N) ratio analysis was conducted to optimise the technological parameters for performing WEDG. By modifying the discharge energy for main cuts when performing WEDG surface finish comparable to that of μWEDM can be achieved. In addition, a simple and cost-effective method for on-the-machine estimation of resulting surface roughness is proposed. Especially, by applying inductive learning a surface roughness prediction model for WEDG can be generate based on data acquired by monitoring on-line the process.     

基于线电极原位制作的微细电解线切割加工

微细电解线切割加工是一种微细加工新方法。从理论上分析了线电极直径大小对微细电解线切割加工精度的影响,提出了原位制作微米尺度线电极的方法,并制作出直径5μm的钨丝线电极。通过电解线切割加工试验,加工出缝宽为20μm左右的微型桨叶结构和曲率半径在1μm以下的微细尖角结构。     

Experimental study on fabrication and evaluation of micro pyramid-structured silicon surface using a V-tip of diamond grinding wheel

A mechanical fabrication of micro pyramid-structured silicon surface is proposed using crossed grooving with a 60° V-tip of diamond grinding wheel. It can obtain high form-accuracy, good surface quality and efficient productivity in contrast to laser machining and etching, and also assure a high aspect ratio in contrast to other mechanical processes. In order to describe its micro-structured topography, a white-light interferometer was employed, and its measured point cloud was matched using an Iterative Closest Point (ICP) algorithm. In micro grinding, a novel CNC mutual-wear truing was first developed to sharpen the wheel V-tip; then, the effects of microscopic wheel topography, silicon crystal-orientation and grinding parameter were investigated on ground micro-topography, truing ratio and material removal ratio; finally, its form-accuracy, pyramid top radius, groove tip radius, surface roughness and aspect ratio were evaluated. It is shown that better microscopic grain protrusion topography on wheel V-tip produces much larger material removal ratio and much better micro-structured topography in micro grinding, but it leads to much less truing ratio in finer GC truing. In micro grinding, silicon crystal-orientation has little effect on micro-structured topography due to diamond crystal-orientations that are randomly distributed on wheel V-tip. Although the micro pyramid-structured form error is only about 3.4 μm, its V-groove bottom and pyramidal top have very large form errors (23.1-47.9 μm) due to the sharpness of wheel V-tip and the frangibility of micro pyramid top. On increasing feed speed, its pyramid top radius decreases and its groove tip radius slightly increases, ultimately leading to an increase in aspect ratio, whereas its surface quality descends. It is concluded that the micro-pyramid arrays may be precisely patterned on silicon surface using a SD600 wheel with crossed tool paths, on-machine V-tip truing and the depth of cut in 1 μm.     

Study on the Variations of Form and Position of the Wire Electrode in WEDM-HS

The electrode, a tool which is used for wire electrical discharge machining (WEDM), is a tiny flexible metal wire, therefore, in WEDM machining process, its form and position will inevitably vary because of the action of many forces, and these dynamic variations have a direct influence on machining precision and stability, especially in the high-speed WEDM (WEDM-HS) machine process; the variations of form and position have been a critical obstacle to improving machining quality and implementing multi-cut processing. In this paper, the variation regularity of wire-electrode has been investigated by experimental analysis, and some corresponding measures have been suggested to stabilize the form and position of the wire electrode in the WEDM-HS machining process.     

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.     

Optimum design of cam-roller follower mechanism using a new evolutionary algorithm

Selection of tooling to perform specific operations like drilling and milling on ceramic materials using rotary ultrasonic machining process is an important aspect to meet stringent dimensions on workpiece as well as intended performance of tool. This phenomenon is more critical for micro rotary ultrasonic machining. In the present study, an effort was made to do micro drilling operation of Ø0.3 mm tool with varying geometry, having different wall thicknesses and abrasive grain sizes using design of experiments. The effect of tool-based parameters like grain size and wall thickness has been studied on axial cutting force, radial cutting force, tool wear, edge chipping area and taper. After examining axial and radial cutting forces, it has been concluded that lower wall thickness (80 μm) tool is good for drilling operation; and higher wall thickness (100 μm) tool is good for milling operation under same material removal rate conditions. It has been also investigated that lower wall thickness (80 μm) tool has less edge chipping area and less taper and can impart high drilling depth as compared to higher wall thickness (100 and 150 μm) tool. It is also concluded that lesser grain size (15 μm) tools are advantageous in terms of edge chipping area and cutting force for drilling and milling operations as compared to higher grain size (30, 35 and 45 μm) tool at constant material removal rate. Higher grain size tools have been broken at 1.13 mm³/h material removal rate conditions due to bad profile accuracy. But higher grain size tools have worked fairly well at less material removal rate condition. Higher grain size tools produced less wear. Tool wear was found minimum in higher wall thickness (100 μm) tool having higher abrasive grain size (30 μm). Using inferred results, Ø0.3 mm drilling experiments have been carried out on six aerospace ceramic materials. Also, groove of 0.5 mm size using Ø0.3 mm optimised tool has been successfully carried out in sintered SiC.     

微细工具电极在线制作技术及实验研究

本文提出了一种在线制作微型工具电极的新方法。该方法是基于电化学微加工,通过反复交换工具电极和工件的极性,进行相互加工获得与电化学微加工相适应的微型工具电极。由于该过程在线进行,因此可以避免因两次夹紧工具电极造成的位置误差和夹紧误差,并可以大大提高加工精度。该方法对于电化学展成微加工来说是非常重要的。通过实验,稳定的加工出了最小直径为20μm的工具电极。