Research on ECM process of micro holes with internal features

Micro holes with internal features are widely used as spray holes and cooling holes nowadays, which are usually required to be with high aspect ratio and shape accuracy, as well as good surface quality. An electrochemical machining (ECM) process is presented to machine these micro holes with diameter <200 μm. A quantitative relation between micro-hole diameter and machining parameters including voltage, duty ratio and feedrate is obtained through orthogonal experiments. According to the designed shape of internal features, change rules of machining parameters for varied diameters in different depth are obtained, and then micro holes with internal features are shaped precisely. Taking reverse tapered hole as an example, ECM experiments by varying parameters of voltage, duty ratio and feedrate (called varying voltage machining, varying duty ratio machining and varying feedrate machining, respectively) are carried out. Micro holes with inlet diameter of 178 μm and taper angle of 1.05° are shaped on a 1.0 mm-thick workpiece of 18CrNi8. The deviation of inlet is <3 μm and the taper-angle error is <0.1° in varying voltage machining. The corresponding dimensional accuracy of taper angle is improved by 51% than that of varying duty ratio machining under the same efficiency. The machining efficiency of varying voltage machining is increased by 36% compared to the efficiency in varying feedrate machining. In addition, the micro holes with complex features of funnel shape and bamboo shape are machined.     

Micro electro discharge machine with an inchworm type of micro feed mechanism

A micro electro discharge machine with an inchworm type of micro feed mechanism has been developed. The prototype of micro electro discharge machine is comprised of a wire electro discharge grinding unit, a rotating unit of electrode, RC circuitry for micro electro discharge generation and a subsystem detecting and controlling machining process, in addition to the inchworm mechanism. In the design of the inchworm mechanism, a novel clamp mechanism with force magnifying structure is devised to increase its thrust capability and a pair of guide sleeves together with the clamps are used to decrease yawing error. The inchworm mechanism prototype has 60 mm stroke only limited by the length of the shaft, less than 2 μm yawing error and reaches to 30N output thrust force. The machining experiments carried out on the micro EDM prototype are also described. The techniques to machine micro electrode, micro holes with high aspect ratio, micro structures on stainless steel and silicon materials are discussed. Micro electrode diameter as small as 25 μm and micro holes with minimum size of less than 50 μm are obtained. And the maximum aspect ratios of micro electrodes and micro holes exceed 20 and 10 respectively.     

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.     

Micro electrical discharge machining using high electric resistance electrodes

In micro electrical discharge machining (EDM), because the material removal per single pulse discharge mainly determines the minimum machinable size of a micro EDM, decreasing the material removal per single pulse discharge is important. In this study, in order to decrease the material removal per single pulse discharge, high electric resistance materials such as single-crystal silicon are used for electrodes. Analytical results show that when the electrode resistance increases, the peak value of the discharge current decreases, whereas the pulse duration increases. In addition, the discharge energy decreases when increasing the resistance. Silicon is used as a tool electrode, and the effect of resistivity of the silicon tool electrode on the diameter of discharge craters generated on the stainless steel workpiece is examined. Experimental results reveal that with increasing silicon electrode resistivity, the diameter of discharge craters decreases. Because the diameter of discharge craters can be decreased to 0.5 μm, improved finished surfaces of R_z 0.03 μm are obtained.     

Micro-scale hole profile measurement using rotating wire probe and acoustic emission contact detection

The development of a new probing method to inspect the inner diameter of micro-scale holes is presented in this paper. This was accomplished by contact detection using acoustic emission with a Ø170 μm rotating wire probe tip. Contact is detected when the rotating probe approaches and impacts the hole’s inner surface. The effective diameter of the rotating probe is calibrated by using a high precision grade 0 Mitutoyo gauge block. The wire rotating probe used was fabricated with micro stainless steel wire and micro tubes. The probe’s effective diameter was compensated for in the measurement of the hole. The probe was used to measure the diameter and the roundness of micro-scale holes. Probes used in previous publications have different geometry than the probe in this paper and are used almost exclusively for external dimensions. Micro-scale holes of less than 1.0 mm in diameter and 10 mm in depth are successfully measured and the 3D profile is created accordingly. Also, the out-of-roundness values of each level spacing, 50 μm apart in height, are calculated.     

High precision self-alignment using liquid surface tension for additively manufactured micro components

Self-assembly of components using liquid surface tension is an attractive alternative to traditional robotic pick-and-place as it offers high assembly accuracy for coarse initial part placement. One of the key requirements of this method is the containment of the liquid within a designated binding site. This paper looks to expand the applications of self-assembly and investigates the use of topographical structures applied to 3D printed micro components for self-assembly using liquid surface tension. An analysis of the effect of edge geometry on liquid contact angle was conducted. A range of binding sites were produced with varying edge geometries, 45–135°, and for a variety of site shapes and sizes, 0.4–1 mm in diameter, and 0.5 mm × 0.5 mm–1 mm × 1 mm square. Liquid water droplets were applied to the structures and contact angles measured. Significant increases in contact angle were observed, up to 158°, compared to 70° for droplets on planar surfaces, demonstrating the ability of these binding sites to successfully pin the triple contact line at the boundary. Three challenging self-assembly cases were examined: (1) linear initial component misplacement >0.5 mm, (2) angular misplacement of components, and (3) misplacement of droplet. Across all three assembly cases the lowest misalignments in final component position, as well as highest repeatability, were observed for structures with actual edge geometries <90° (excluding 45° nominal), where the mean magnitude of misalignment was found to be 31 μm with 14 μm standard deviation.     

微细电火花加工设备技术研究

微细电火花加工的关键设备技术涉及电极的微进给伺服机构、电极和工件的附加树对运动机构、微小能量放电电源以及加工状态检测与控制系统等。文章围绕微细电火花加工系统的设计系统，介绍基于压电致动原理以及摩擦传动的微进给机构、工具电极的线放电磨削机构和旋转主轴、以及微小能量放电电源的设计等，并指出需要进一步研究的课题。     

Fabrication of disk microelectrode arrays and their application to micro-hole drilling using electrochemical micromachining

Minimal-taper microholes are widely used in modern industries. Electrochemical micromachining (EMM) has been demonstrated to be a feasible method to fabricate these microholes. In this study, based on its unique processing properties and productivity, a disk microelectrode array was fabricated via electrolysis for producing micro-holes. The dimensions of the cathode for hydrolysis were optimized by applying the finite element method to the constructed physical model. A 3 × 3 disk microelectrode array and a 5 × 5 cylindrical microelectrode array with uniform dimensions were then fabricated using the optimized cathode. Micro-holes were drilled on stainless-steel plates using both disk and cylindrical microelectrode arrays. The taper of the resulting micro holes obtained using the new disk microelectrode array was lower than that of the holes formed using the cylindrical microelectrode array. The effects of EMM parameters, including the applied voltage, feeding speed, and pulse-on time, on the hole diameter and taper were also investigated. The results suggest that appropriate machining parameters should be selected in consideration of the effects of these parameters on hole diameter, taper, localization, and material removal rate.     

A long-stroke 3D contact scanning probe for micro/nano coordinate measuring machine

This paper presents a long-stroke contact scanning probe with high precision and low stiffness for micro/nano coordinate measuring machines (micro/nano CMMs). The displacements of the probe tip in 3D are detected by two plane mirrors supported by an elastic mechanism, which is comprised of a tungsten stylus, a floating plate and two orthogonal Z-shaped leaf springs fixed to the outer case. A Michelson interferometer is used to detect the vertical displacement of the mirror mounted on the center of the floating plate. An autocollimator based two dimensional angle sensor is used to detect the tilt of the other plane mirror located at the end of the arm of the floating plate. The stiffness and the dynamic properties are investigated by simulation. The optimal structural parameters of the probe are obtained based on the force-motion model and the constrained conditions of stiffness, measurement range and horizontal size. The results of the performance tests show that the probe has a contact force gradient within 0.5 mN/μm, a measuring range of (±20 μm), (±20 μm), and 20 μm, respectively, in X, Y and Z directions, and a measurement standard deviation of 30 nm. The feasibility of the probe has preliminarily been verified by testing the curved surface of a convex lens.     

Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup

Lubrication conditions and blank holder force (BHF) are two key processing parameters in deep drawing. This is more obvious in micro forming because of the miniaturization of the specimen size. Micro conical–cylindrical cups with internal conical bottom diameter of only 0.4 mm were well formed. The influences of lubrication conditions and BHF on micro deep drawing of micro conical–cylindrical cups were investigated using a micro blanking–deep drawing compound mold. Pure copper C1100 with a thickness of 50 μm, which was annealed at 450 °C for 2 h in vacuum condition, was chosen as the specimen material. The experiments were conducted on a universal testing machine with a forming velocity of 0.05 mm/s under 4 kinds of lubrication conditions and BHF. The experimental results showed that a micro conical–cylindrical cup with internal conical bottom diameter of only 0.4 mm was well formed, and the limiting drawing ratio (LDR) reached 2.1. The polyethylene (PE) film, which decreased the drawing force and increased the drawing ratio (DR), was superior to castor oil, petroleum jelly and dry friction, and can be chosen as a proper lubricant for micro deep drawing. The rim of the micro cup seriously wrinkled when BHF was less than 4.2 N. The bottom of the micro cup cracked when the BHF was larger than 5.6 N.     

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.     

Experimental study on micro EDM-drilling of Ti6Al4V using helical electrode

There is a growing interest in the machining of micro-holes with high aspect-ratio in difficult-to-machine alloys for the aerospace industry. Processes based on electro discharge machining (EDM) and developed for the manufacture of both micro-electrode and micro-hole are actually used, but most of them involve micro-EDM machines. In this work, the influence of EDM parameters on material removal rate, electrode wear, machining time and micro-hole quality when machining Ti6Al4V is studied. Due to an inefficient removal of debris when increasing hole depth, a new strategy based on the use of helical-shaped electrodes has been proposed. The influence of helix angle and flute depth with respect to process performance has been addressed. Main results include 37% reduction in machining times (hole diameter 800 μm) when using electrode helix angle of 45° and flute-depth of 50 μm, and an additional 19% with flute-depth of 150 μm. Holes of 661 μm diameter and as much as 6.81 mm depth, which yields in aspect ratio of 10:1, have successfully been machined in Ti6Al4V.     

Research on improvement of machining accuracy of micro-rods with electrostatic induction feeding ECM

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns can easily be obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. To improve the machining accuracy, the machining characteristics when the workpiece is fed in the axial and radial directions were investigated using NaCl aqueous solution as the electrolyte. When fed in the axial direction, the machinable length of the micro-rods was found to peak at the optimum feed speed because of the influence of pitting corrosion and collision between electrodes. When the workpiece was fed in the radial direction, the influence of pitting corrosion decreased, however, the micro-rod was shortened with increasing feed distance in the radial direction because of the stray current flowing through the end of the micro-rod. The simulation results of the material removal process agreed qualitatively with experimental results. Next, machining characteristics were compared between the electrolytes, NaCl and NaNO₃ aqueous solutions, by feeding the workpiece in the axial direction. It was found that the influence of pitting corrosion was eliminated with the NaNO₃ aqueous solution, and there was no machinable length limitation with suitable feed speeds. In addition, the taper angle and gap width were smaller with the NaNO₃ aqueous solution, compared with those of the NaCl aqueous solution. Stainless steel micro-rods of 100 μm in diameter with a high aspect ratio of 20 were fabricated with the NaNO₃ aqueous solution. According to the preliminary research results, the machinable minimum diameter of the micro-rods was investigated and micro-rods with an average diameter of 9 μm and length of 78 μm were machined successfully.     

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.     

Approaches for improvement of EDM cutting performance of SiC with foil electrode

Electrical discharge machining by foil electrode serves as an alternative method for SiC slicing. This technology uses a highly tensioned thin foil as the tool electrode. The main advantages over wire EDM are that the foil thickness can be made smaller than the wire diameter, vibrations can be avoided by applying high tension, and higher current can be supplied since there is less risk of tool breakage. However, due to the large side surface area of the foil electrode, there is a high occurrence probability of side surface discharges and high concentration of debris, which affects kerf width accuracy and machining stability. In the aim to overcome both problems, this study proposes two foil electrode designs: a foil electrode in which holes are machined and the insulation of the side surface areas by a resin coating layer of 5 μm thickness. The influences of both foil electrodes were tested with three different slicing strategies: no strategy, applying jump motion of the tool electrode, and applying reciprocating motion. From machining experiments and comparative studies of the discharge delay time, it was found that with both foil tools, the occurrence probability of side surface discharges can be reduced. In addition, the chip pocketing effect of the holes enhance the flushing conditions, resulting in a higher cutting speed.     

Cost effective refractive index sensor based on optical fiber micro cavities produced by the catastrophic fuse effect

We propose a refractive index optical fiber sensor based on the micro cavities generated through the fiber catastrophic fuse effect. This sensor was tested in the measurement of solutions with refractive indices ranging from 1.3320 to 1.4280. The linear dependence of the reflection spectra modulation period as function of the surrounding environment refractive index leads to a resolution of 3 × 10⁻⁴ RIU. The proposed sensor is an innovative solution based on optical fiber damaged by the fuse effect, resulting in a cost effective solution.     

An investigation of accuracy,repeatability and reproducibility of laser micromachining systems

Component technologies of laser micro machining systems are key factors affecting their overall performance. The effects of these technologies on accuracy, repeatability and reproducibility (ARR) in different implementations of such systems have to be investigated to quantify their contributions to the overall processing uncertainty, especially those with the highest impact on beam delivery sub-systems. The aim of this research was to evaluate the capabilities of state-of-the-art machining platforms that were specially designed and implemented for laser micro structuring and texturing. An empirical comparative study was conducted to quantify the effects of key component technologies on ARR of four state-of-the-art systems. In particular, the capabilities of the optical and mechanical axes were investigated when they were utilised separately or in combination for precision laser machining. Conclusions are made about the positional accuracy of the mechanical and optical axes and the importance of their proper calibration on the systems’ overall performance is discussed. It is shown that the laser machining platforms can achieve repeatability and reproducibility better than 2 μm and 6 μm, respectively.     

Miniaturized liquid film sensor (MLFS) for two phase flow measurements in square microchannels with high spatial resolution

Two miniaturized liquid film sensors (MLFS) based on electrical conductance measurement have been developed and tested. The sensors are non-intrusive and produced with materials and technologies fully compatible and integrable with standard microfluidics. They consist of a line of 20 electrodes with a purpose-designed shape, flush against the wall, covering a total length of 5.00 and 6.68 mm. The governing electronics achieve 10 kHz of time resolution. The electrode spacing of the two sensors is 230 μm and 330 μm, which allows measurements of liquid films up to 150 μm and 400 μm for sensors MLFS_A and MLFS_B, respectively. The sensor characteristics were obtained by imposing static liquid films of known thickness on top of the actual sensor. Further dynamic measurements of concurrent air-water flow in a horizontal microchannel were performed. The line of electrodes is placed across the flow direction with an angle of 3.53° from the direction of flow, allowing for a spatial resolution perpendicular to the flow of 14.2 μm for sensor MLFS_A and 20.5 μm for sensor MLFS_B. The high time and spatial resolution allows for fast and accurate detection of the presence of bubbles, and even measurement of film thickness and bubble velocity. Further information, such as the bubble shape, can be gathered based on the shape of the liquid layer underneath the bubble, which is particularly important for heat transfer studies in microchannels.     

An investigation of tube and rod electrode wear in micro EDM drilling

This paper studies the influence of various factors contributing to micro electrode wear during electrical discharge machining (EDM) drilling with micro rod and micro tube electrodes. In this paper, a simple method for calculating volumetric wear ratios is proposed based only on geometrical information obtained from the process. The objective of the research is to investigate the wear behaviour of electrodes and the suitability of electrode wear compensation methods. Electrode shape deformation and random variation of the volumetric wear are studied as the main factors affecting the applicability of wear compensation methods and as an indicator of the accuracy achievable with the micro EDM process.     

Algorithms and machining experiments to reduce depth errors in servo scanning 3D micro EDM

In servo-scanning 3D micro electro discharge machining (SS-3D MEDM), the depth errors of 3D micro cavities are accumulated layer by layer due to the contour scanning process with keeping discharge gap for compensating axial electrode wear in real time. In this research, the errors’ causes were analyzed, and then a layer depth constrained algorithm (LDCA) and an S-curve accelerating algorithm (SCAA) were proposed to reduce the depth errors. By LDCA, over-cutting errors can be avoided by controlling a tool-electrode feed maximum at every scanning spot. As a supplementary algorithm for LDCA, SCAA can compensate insufficient-machining errors at start and end of scanning paths. Implementation process and control strategy of the algorithms were also described. The purpose of this research is to efficiently machine complex 3D micro-cavities with high accuracies of shape and surface. By use of computer-aided manufacturing software of Pro/Engineer to plan complex 3D scanning paths, machining experiments were carried out to verify the proposed algorithms. The experimental results show: Typical 3D micro cavities <800 μm can be automatically machined, and the machining accuracies of micro surfaces and edges are obviously improved, and the depth errors can be controlled within 2 μm, and the material removal rate reaches 2.0 × 10⁴ μm³/s with tool electrode of ∅80 μm and its rotational speed of 1000 r/min. In addition, the 3D micro cavities designed on unknown edge or hollow workpieces can be successfully formed.