Development and evaluation of an on-machine optical measurement device

Demand for fabricating micro-features such as fine holes, micro-cavity for injection moulding, and micro-pin using both conventional (turning, milling, etc.) and non-conventional edge detection method (EDM), wire cut EDM, etc.) processes is increasing significantly. To successfully achieve micro-machining, development of a miniature machine tool, process technology, and on-machine measurement is essential. However, in such tool-based micro-machining processes, proper tool shape monitoring, precision processing, and dimensional control require significant attention. Since these are tool-based machining processes, tool shape monitoring and control are also important technologies to be established.In this study, an on-machine measuring device was developed based on non-contact optical method to inspect dimensions of the fabricated tools (e.g. electrodes for EDM) as well as the wear of tools used for the respective processes. The developed inspection system uses a laser light source and a photo-electronic device. To minimize errors due to the change of tool measurement position and the Fresnel diffraction of laser light, an edge detection algorithm using a linear discrimination function is proposed in this study. Furthermore, an intensity measuring method was added for specimen with a smaller diameter. The experimental results show that the developed on-machine optical inspection system has the accuracy and stability to effectively monitor the fine dimensions of tools and their wear.     

A study on the machining of high-aspect ratio micro-structures using micro-EDM

Micro-electro-discharge machining (micro-EDM or μ-EDM) has been gaining popularity as a new alternative method to fabricate micro-structures. The main advantages of the micro-EDM method are its low set-up cost, high accuracy and large design freedom. Compared to etching or deposition techniques, micro-EDM has the advantage of being able to fabricate complex three-dimensional shapes with high-aspect ratio. However, there are many operating parameters that affect the micro-EDM process. The fabrication of micro-electrodes on the machine is also an important process to remove the clamping error to maintain high accuracy in the machined micro-structures.

In this paper, the machining of micro-structures is divided into two basic processes. One is the on-machine fabrication of the micro-electrodes with high-aspect ratio, and the other is the EDM of the workpiece in micrometer range. An optical sensor has been developed to measure and control the dimension of the thin electrode during the tool fabrication process. Different methods have been investigated to fabricate a thin electrode into the desired dimension without deflection. The performance of the micro-EDM process is evaluated in terms of the material removal rate (MRR), tool wear ratio (TWR), and the stability of the machining. Influences of the various operating parameters of the micro-EDM process, such as the operating voltage, gap control algorithm, and resistance and capacitance values in the R–C spark control circuit, are discussed.     

A study on microgrinding of brittle and difficult-to-cut glasses using on-machine fabricated poly crystalline diamond (PCD) tool

Present study aims to investigate the feasibility of microgrinding difficult-to-machine glass materials with Poly Crystalline Diamond (PCD) tool, which is fabricated on-machine using micro-electrodischarge machining (micro-EDM). A detailed experimental investigation on the mechanism of the process including the effect of micro-EDM machining conditions on tool surface and the effect of grinding parameters on microgrinding performance are presented. In this context, a comparative study on the microgrinding performance of three glass materials (BK7, Lithosil and N-SF14) using on-machine fabricated PCD tool was carried out. It was found that during tool fabrication using micro-EDM process, higher discharge energy generates rougher surface and larger craters on the tool, which can provide higher material removal rate (MRR) during grinding but results in poorer surface finish on glass surface. In addition to micro-EDM conditions of tool fabrication, the roughness of the ground glass surface depends greatly on grinding parameters such as depth of cut, feed rate and tool rotational speed. The surface roughness increases with increasing axial depth of cut and feed rate, whereas higher rotational speed was found to improve the surface finish. Among three types of glass materials, BK7 glass was found to provide better performance in terms of the achieved surface finish and cutting force analysis.     

A review on the conventional and micro-electrodischarge machining of tungsten carbide

The capability of machining intricate features with high dimensional accuracy in hard and difficult-to-cut material has made electrodischarge machining (EDM) process as an inevitable and one of the most popular non-conventional machining processes. In recent years, both EDM and micro-EDM processes are being used extensively in the field of mould making, production of dies, cavities and complex 3D structures using difficult-to-cut tungsten carbide and its composites. The objective of this paper is to provide a state of the art in the field of EDM and micro-EDM of tungsten carbide and its composites. The review begins with a brief introduction on the EDM and micro-EDM processes. The research and developments in electrodischarge machining of tungsten carbide are grouped broadly into conventional EDM of tungsten carbide, micro-EDM of tungsten carbide and current research trends in EDM and micro-EDM of tungsten carbide. The problems and challenges in the area of conventional and micro-EDM of tungsten carbide and the importance of compound and hybrid machining processes are discussed. A summary of the future research directions based on the review is presented at the final section.     

Development of a reversible machining method for fabrication of microstructures by using micro-EDM

A new micromachining method for the fabrication of micro-metal structures by using micro-reversible electrical discharge machining (EDM) was investigated. The reversible machining combines the micro-EDM deposition process with the selective removal process, which provides the ability of depositing or removing metal material using the same micro-EDM machining system. From the discharge mechanism of micro-EDM, the process conditions of micro-EDM deposition were analyzed firstly. Using the brass and steel materials as a tool electrode, the micro-cylinders with 200 μm in diameter and height-to-diameter ratio of more than 5 were deposited on a high-speed steel surface. Then the machining procedure was transformed easily from deposition to selective removal process by switching the process conditions. Different removal strategies including micro-EDM drilling and micro-EDM milling were used in the machining. Micro-holes with 80 μm in diameter are drilled successfully in the radial direction of the deposited micro-steel cylinder. Also, a brass square column with 70 μm in side length and 750 μm in height, and a micro-cylinder with 135 μm in diameter and 1445 μm in height are obtained by using micro-EDM milling. Finally, the characteristics of the deposited material were analyzed. The results show that the material components of a deposited micro-cylinder are almost the same as those of the tool electrode, and the metallurgical bonding has been formed on the interface. In addition, the Vickers-hardness of 454Hv of the steel deposited material is higher when compared to the hardness of 200Hv of the raw steel electrode.     

微细组合电加工样机研制及实验研究

基于新研发的一套微细组合电加工样机μEM-200CDS2,介绍了研发过程中探索出的最小脉宽可以达纳秒级的双功能微能脉冲电源以及样机中的若干关键技术,包括放电状态的双参数检测技术、工具电极在位多功能磨削技术、工作液稳定供给控制技术等。其中,双功能微能脉冲电源具备主动消电离环节,可以减少脉间的残余电荷放电,有利于提高加工表面质量;组合电加工样机床身设计有利于提高系统的加工精度和效率。最后,结合小孔的加工试验研究了典型的组合电加工工艺过程,结果表明:该过程中,可以并行完成工具电极在位修整与零件加工,有利于提高微小特征的加工效率。     

三轴机床微细异型电极加工与尺寸误差分析

采用直线插补与圆弧插补算法,在三轴卧式微细电火花机床上加工出圆锥台、四棱台以及带有圆弧曲面的微细异型电极,并对加工结果尺寸误差及导致误差产生的原因进行了全面分析。结果表明,微细异型电极的尺寸误差均在2%以内,而放电间隙的随机性、机床运动精度、尺寸测量精度等均是导致误差形成的原因。     

A study on the quality micro-hole machining of tungsten carbide by micro-EDM process using transistor and RC-type pulse generator

Tungsten carbide (WC) is an extremely hard and difficult-to-cut material used extensively in manufacturing because of its superior wear and corrosion resistance. Besides diamond-charged grinding wheels, micro-EDM is an effective method of machining this extremely hard and brittle material. Since micro-EDM is more generally an electro-thermal process, the supplied energy from a pulse generator is an important factor determining the performance of the micro-EDM process. This study investigates the influence of major operating parameters on the performance of micro-EDM of WC with focus in obtaining quality micro-holes in both transistor and RC-type generators. Experimental investigations were conducted with view of obtaining high-quality micro-holes in WC with small spark gap, better dimensional accuracy, good surface finish and circularity. In micro-EDM, the fabrication of micro-parts requires minimization of the pulse energy supplied into the gap which can be fulfilled using the RC-generator. It was observed that the RC-generator can produce better quality micro-holes in WC, with rim free of burr-like recast layer, good dimensional accuracy and fine circularity. Moreover, the smaller debris formed due to low discharge energy in RC-type micro-EDM can be easily flushed away from the machined area resulting in surface free of burr and resolidified molten metal. Therefore, RC-type micro-EDM could be more suitable for fabricating micro-structures in WC, where accuracy and surface finish are of prime importance.     

Development of a grinding-drilling technique for holing optical grade glass

This study presents the development of a grinding-drilling technique for an innovative bench-top drill that combines micro-EDM with grinding and drilling to fabricate micro-holes in optical grade glass. Firstly, a novel diamond-tool, made with copper-based sintered alloy, is designed and fabricated on-line into a harbor-shaped structure with a hollow shaft and negative back rake-angles. Constructed reverse co-centric micro-hole EDM-drilling and reverse w-EDM facilitate on-line machining of the diamond-tool, which can then be directly utilized to drill micro-holes in optical glass and quartz. Application of a load-cell that detects the drilling force in real-time, providing feedback for fine tuning the feed-rate of the tool is proposed. Experimental results show that excellent geometric and dimensional accuracy of micro-holes can be achieved. The estimated reasonable tool life is determined at a machining number of 30 times. The proposed grinding-drilling technique is simple, cost effective, and can significantly contribute to the precision micromachining industry.     

Development of ultra-precision machining system with unique wire EDM tool fabrication system for micro/nano-machining

Aiming at quality machining of very hard materials with nanometer level surface quality and 0.1 μm dimensional accuracy, an ultra-precision machining system has been developed. The machine has feed axes accompanied by the counter motion mechanism driven at the center of gravity to eliminate the vibration caused by high acceleration/deceleration. A custom tool fabrication system consists of a 6-axis wire EDM machine and dedicated custom tool CAM system. High quality machining of sophisticated three-dimensional (3D) products made of tungsten carbide has successfully been demonstrated on the developed machine tool with custom-made PCD cutters fabricated by the tool fabrication system.     

CNC microturning: an application to miniaturization

Micromachining is the basic technology of microengineering for the production of miniature components. One group of tool based micromachining technology is CNC microturning. It is a conventional material removal process that has been miniaturized. The objective of this study is to asses the machinability of brass, aluminium alloy and stainless steel during external cylindrical longitudinal microturning process for different workpiece–tool combinations. Experiments were carried out by varying the depth of cut, feed rate and spindle speed. One parameter was varied while the other two were kept constant in order to identify the best combination of cutting parameters. Machinability assessment was done by force analysis, chip analysis and tool wear criterion. Microshafts were fabricated with brass, aluminium alloy and stainless steel. Finally, microturning process was successfully applied to fabricate compound shaped micropins of diameter less than 0.5 mm.     

Ultra-precision machining of Fresnel microstructure on die steel using single crystal diamond tool

With the increasing demand for the replication of structured optical elements such as Fresnel lenses and prism arrays, more attention is being paid to the development of ultra-precision diamond machining technology for the fabrication of die steel molds. However, the machining process would be a catastrophic failure because of rapid and excessive tool wear if a diamond tool is used to machine die steel. In the present paper, a micromachining method for fabricating microstructures on die steel using single crystal diamond tool is presented. The presented technology is based on a thermochemical technique that uses plasma nitriding treatment to suppress the rapid and excessive tool wear in the diamond machining of steel. Experimental findings revealed that severe chemical tool wear, which is the main wear mechanism in the diamond machining of steel, was reduced significantly after plasma nitriding treatment, and a mirror-quality surface with an average surface roughness of 20 nm root-mean-square (RMS) was achieved over a cutting distance of approximately 5.4 km. Furthermore, a Fresnel microstructure with surface roughness RMS better than 40 nm was precisely fabricated on AISI 4140 die steel using single crystal diamond tool.     

形状记忆合金非常规加工综述(英文)

形状记忆合金(SMAs)由于具有多种特殊性能,如伪弹性、形状记忆效应、生物相容性、高的比强度、高耐蚀性、高耐磨性、良好的抗疲劳性能,成为不断发展的先进材料。因此,形状记忆合金被广泛应用于航空航天、医疗和汽车等方面。然而,由于严重的加工硬化和伪弹性,形状记忆合金的传统加工会造成严重的刀具磨损、费时以及低维畸变。这些材料可以使用非传统的方法,如激光加工、水射流加工(WJM)和电化学加工(ECM)进行机械加工,但这些方法受限于该材料的复杂性和力学性能。而电火花加工(EDM)和线切割(WEDM)能够很好的加工具有复杂形状和精密尺寸的形状记忆合金。介绍大量关于使用电火花和线切割加工形状记忆合金的研究,分析不同研究的差异,并展望未来的研究趋势。     

Ultrasonic and electric discharge machining to deep and small hole on titanium alloy

Being a difficult-to-cut material, titanium alloy suffers poor machinability for most cutting process, let alone the drilling of small and deep holes using traditional machining methods. Although electric discharge machining (EDM) is suitable to handle titanium alloys, it is not ideal for small and deep holes due to titanium alloys’ low heating conductivity and high tenacity. This paper introduces ultrasonic vibration into micro-EDM and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-EDM has been developed. A wire electric discharge grinding (WEDG) unit which can fabricate a micro-electrode on-line, as well as a measuring unit, is set up on this equipment. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch was made along the electrode. Ultrasonic vibration is then introduced into the micro-EDM. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology.     

基于UKF薄壁件加工变形预测技术研究

薄壁件加工中的零件变形和让刀是影响加工精度的主要原因,首先,根据零件加工路径构建UKF预测模型;之后,把数控机床误差和在机测量系统误差作为已知噪声输入到UKF算法中,在机检测系统对序中尺寸进行测量作为过程转移噪声,上次过程转移噪声输入到下次预测中;最后,使用MATLAB预测出零件变形量。上次状态转移噪声输入到UKF算法中以提高预测对真实加工环境的模拟,运用在机检测技术把零件加工数据传输到UKF算法中提高变形预测精度,为薄壁件数控加工序中补偿提供数据依据。     

Probability of precision micro-machining of insulating Si3N4 ceramics by EDM

Electrical discharge machining (EDM) is used as a precision machining method for the electrically conductive hard materials with a soft electrode material. But recently we succeeded to machine on insulating material by EDM. The technology is named as an assisting electrode method. The EDMed surface is covered with the electrical conductive layer during discharge. The layer holds the electrical conductivity during discharge. For micro-EDM, the wear of tool electrode becomes lager ratio than the normal machining. So the micro-machining is extremely difficult to get the precision sample.

In this paper to obtain a fine and precise ceramics sample, some trials were carried out considering the EDM conditions, tool electrodes material and assisting electrode materials. Insulating Si₃N₄ ceramics were used for workpiece. The machining properties were estimated by the removal rate and tool wear ratio. To confirm the change of micro-machining process, the discharge waveforms were observed. The micro-machining of the Ø0.05 mm hole could be machined with the commercial sinking electrical discharge machine.     

Validation of volumetric error compensation for a five-axis machine using surface mismatch producing tests and on-machine touch probing

In order to validate volumetric error compensation methods for five-axis machine tools, the machining of test parts has been proposed. For such tests, a coordinate measuring machine (CMM) or other external measurement, outside of the machine tool, is required to measure the accuracy of the machined part. In this paper, a series of machining tests are proposed to validate a compensation strategy and compare the machining accuracy before and after the compensation using only on-machine measurements. The basis of the tests is to machine slots, each completed using two different rotary axes indexations of the CNC machine tool. Using directional derivatives of the volumetric errors, it is possible to verify that a surface mismatch is produced between the two halves of the same slot in the presence of specific machine geometric errors. The mismatch at the both sides of the slot, which materializes the machine volumetric errors is measured using touch probing by the erroneous machine itself and with high accuracy since the measurement of both slot halves can be conducted using a single set of rotary axes indexation and in a volumetric region of a few millimetres. The effect of a compensation strategy is then validated by comparing the surface mismatch value for compensated and uncompensated slots.     

日本放电加工的新方法和新技术

综述了日本在放电加工领域的最新研究动态，包括成形放电加工、微细放电加工和线切割放电加工三部分内容。在成形放电加工方面，主要介绍一项新技术——气中放电高速三维空腔铣的原理、实验装置及加工效果；在微细放电加工方面，主要介绍一种新近开发的可同时加工微细轴和微孔的自动微孔加工装置和目前已加工出的微细轴的最小尺寸；在线切割放电加工方面，主要介绍了提高圆角加工精度的新方法、提高放电位置可控性的新方法和新的硅锭切片方法。     

Application of micro-EDM combined with high-frequency dither grinding to micro-hole machining

This research presents a novel process using micro electro-discharge machining (micro-EDM) combined with high-frequency dither grinding (HFDG) to improve the surface roughness of micro-holes. Micro-EDM is a well-established machining option for manufacturing geometrically complex small parts (diameter under 100 μm) of hard or super-tough materials. However, micro-EDM causes the recast layer formed on the machined surface to become covered with discharge craters and micro-cracks, resulting in poor surface quality. This affects the diameter of the micro-hole machined and undermines seriously the precision of the geometric shape. The proposed method that combines micro-EDM process with HFDG is applied to machining high-nickel alloy. As observed in SEM photographs and surface roughness measurement, HFDG method can reduce surface roughness from 2.12 to 0.85 μm Rmax with micro-cracks eliminated. Our results demonstrated that micro-holes fabricated by micro-EDM at peak current 500 mA followed by HFDG at 40 V can achieve precise shape and good surface quality after 6–8 min of lapping.     

一种用于微细电火花加工的甚高频微能脉冲电源

作为微细电火花加工的关键技术之一,微能脉冲电源性能的优劣直接影响放电加工的精度、效率、稳定性等。从减小放电脉冲能量、增大放电间隙、可持续加工的需求出发,探索了一种基于电路共振原理的甚高频(频率在30~300 MHz)微能脉冲电源,突破了现有电火花脉冲电源的工作模式,能产生脉宽极窄、频率极高的脉冲波形,具有纳米级放电蚀除特性,提高了微细电火花加工的极限蚀除能力。放电频率为65 MHz时,相对于传统的微能脉冲电源,加工的孔边缘几乎没有重铸层,极大地减轻了在加工过程中的热损伤、重铸层和热影响区等常规缺陷,改善了工件加工的表面质量,实验结果证明所设计的甚高频微能脉冲电源具有良好的放电蚀除特性。