Performance and Limitations of the Conventional Electrode Materials for Erosion of High Aspect Ratio Microcavities

The low electrode wear strategy based on a carbonaceous layer formation on electrodes considerably increases resource efficiency in the conventional die-sinking EDM. However, the smallest electrode projection area Ap for using the strategy is limited to 0.1 mm2 and maximum pulse current 3A, possibly due to the pulse re-opening phenomenon. In this work, using a novel generator circuit, pulse re-openings have been restricted up to pulse durations 100 μs and a current of 1A. Hence, the performance of conventional electrode materials is evaluated in order to push the limits of the low wear strategy. However, when using graphite and copper infiltrated graphite microelectrodes, bending of the electrodes is observed, especially in the tip region. The simulation of temperature in the microelectrodes suggests abnormal carbonaceous build up process. This explanation is also concurrent to the observation of a process instability resulting in irreproducible electrode wear behaviour. For copper microelectrodes, another phenomenon is observed where overcuts are produced in the eroded cavities. These overcuts are produced by sparks with high discharge voltage. Sliding of the plasma channel from the electrode corners to the side surfaces is proposed to cause such discharges and overcuts. Thus, underlying mechanisms limiting the low wear strategy in micro-EDM are identified.ISSN:2212-827

EDM Drilling of Non-Conducting Materials in Deionised Water

Electrical discharge machining (EDM) of non-conducting materials currently is only possible with the help of an auxiliary conductive layer, known as assisting electrode, which must be regenerated after each discharge. Therefore, an assisting conductive layer is applied to the surface of the workpiece to initiate the discharge process. In order to continue erosion, the eroded surface regions must have a new conductive layer in contact with the original assisting conductive layer, which is typically achieved by depositing carbonaceous material through dielectric oil decomposition. In this paper, the renewal of such conductive layer is proposed to be done by material deposition from the tool electrode. This alternative solution enables machining of non-conductive materials in deionized water instead of dielectric oil. The deposition of material from tool electrode is achieved through erosion in very narrow interelectrode gaps and controlled short circuiting. It is shown that machining of zirconium dioxide ceramic (ZrO2) is possible in deionised water using the alternative method. This method shows the feasibility to machine holes up to 1.5 mm depth in ceramic applying 1 mm diameter electrode in deionised water. Further improvements in the process may lead to industrial applications, where the cooling holes in ceramic coated turbine blades can be drilled by the same machine and process.ISSN:2212-827

Advanced die sinking EDM process monitoring based on anomaly detection for online identification of improper process conditions

Abstract Die sinking EDM processes are widely employed in advanced aerospace applications where part quality and machining time are main concerns. The aim of this research work is to develop an advanced EDM process monitoring procedure in the perspective of Zero Defect Manufacturing based on the identification of correlations between die sinking EDM process parameters and improper process conditions that could increase machining time and cause unacceptable part quality. To this purpose, the Real Time Acquisition (RTAQ) module installed on a AgieCharmilles FORM P 600 sinker spark erosion machine tool is utilized to monitor and acquire online data related to 8 selected process parameters with 32 ms sampling interval. An anomaly detection methodology is then applied to timely identify improper process conditions based on relevant features extracted from the EDM process parameters

Development of Process Chain for Micro-Injection Molding

In today’s continuously growing demand for components with increasingly smaller dimensions and features, micro-manufacturing is gaining more significance. For the mass production of plastic components with micro-features, injection molding is particularly suitable and still customary in order to keep target costs. Due to high requirements regarding lifetime and resistance to wear, the molds are made of hardened steel. The shaping of these molds involves electrical discharge machining (EDM). This process allows the generation of micro-structures in micrometer range having small inner radii, high dimensional accuracy and extreme aspect ratio independent of the workpiece hardness. As a work tool for EDM, electrodes made of pure copper (Cu) and tungsten reinforced copper (WCu) are commonly used. The shaping of the electrodes is conducted by micro-milling. For an overall understanding of this process chain, the interaction between micro-milling, micro-EDM, and micro-injection molding must be evaluated. This paper provides knowledge on the limits of each process with regard to burr formation, form accuracy, structure size and aspect ratio. Many factors throughout the process chain affect the size of an attainable micro-feature of a final product. The proper selection of the electrode material is a key factor in the process chain. As the feature size of the electrode defines the dimension of the final product shape, the smallest possible structures have to be found during micro-milling. The dimension of the eroded cavity is defined by the feature size of the electrode in combination with the lateral working gap. Eroded cavities with small inner corner radii and steep flanks can be generated when applying flawless and burr-free electrodes. However, using electrodes in inadequate conditions can lead to worse outcomes. The quality and reliability of the final product are determined to a great extent by the design of the injection molding process. Due to the arising vacuum when evacuating the remaining air in the mold, the inflowing melt is being distributed equally. It must be guaranteed that during injection molding, the mold is thermally-controlled. This prevents premature solidification. Through a combination of these two strategies, a form filling rate in micro-cavities up to 100 % is carried out. By deriving an impeccably adapted process chain, a precise micro-molding can be performed. This leads to the capability of manufacturing bars on a final plastic part with a width and height as low as 55 x 100 µm, and a length of 2 mm.ISSN:2212-827

Spark location adaptive process control in meso-micro EDM

Time-synchronised high-speed imaging and electrical signal measurements are performed in near-real erosion conditions to analyse the feasibility of determining spark location based on its electrical signals in die-sinking electrical discharge machining (EDM). Using this novel research platform, a correlation between the discharge voltage and the geometric location of a discharge on an electrode has been established. Through the derived understanding, a microsecond level spark location adaptive process control has been conceptualised and demonstrated. The parameter control of each spark according to its probabilistic location on the electrode results in low wear of micro- to macroscale electrode features, higher material removal rate and higher form precision despite of electrode complexity. Reduction in the required number of electrodes achieved through the novel spark location adaptive process control increases the economic and energy efficiency of die-sinking EDM

Die-sink EDM in meso-micro machining

Micro EDM has been identified since more than a decade as suitable process for machining complex shaped structures with high aspect ratio, though only through variations of EDM such as micro EDM milling, micro EDM drilling, coated electrodes etc. In the current paper, we present the research focused on analysing the capability for implementation of die-sink EDM in meso - micro scale machining (structures with surface area smaller than 10mm2 down to 0.05mm2) by concentrating on primary process parameters to obtain high material removal rate, low tool electrode wear with high form accuracy and precision. Graphite electrodes were mill machined in meso-micro scale with high precision and accuracy. Low tool wear technology was developed for using graphite electrodes in meso-micro EDM offering economical and energy efficient solution in meso-micro scale machining.ISSN:2212-827

Spark location adaptive process control in meso-micro EDM

Time-synchronised high-speed imaging and electrical signal measurements are performed in near-real erosion conditions to analyse the feasibility of determining spark location based on its electrical signals in die-sinking electrical discharge machining (EDM). Using this novel research platform, a correlation between the discharge voltage and the geometric location of a discharge on an electrode has been established. Through the derived understanding, a microsecond level spark location adaptive process control has been conceptualised and demonstrated. The parameter control of each spark according to its probabilistic location on the electrode results in low wear of micro- to macroscale electrode features, higher material removal rate and higher form precision despite of electrode complexity. Reduction in the required number of electrodes achieved through the novel spark location adaptive process control increases the economic and energy efficiency of die-sinking EDM.ISSN:0268-3768ISSN:1433-301