| Summary: | 碩士 === 國立高雄應用科技大學 === 模具工程系碩士班 === 102 === This research attempts to achieve a synchronized hybrid process of micro-EDM grinding (micro-EDMG, or μEDMG) on a single machine, it is aimed at saving the secondary precision grinding process after general micro-EDM process in micro-machining fields. First, the micro-tool of tungsten carbide was dressed to 0.3 mm diameter by a WEDG (wire electrical discharge grinding) system, and co-deposited in the nickel electro-plating bath with 6~12 μm of diamond abrasives. A low speed spindle is used for electro-plating, and then, another high speed spindle up to 20 KRPM is adopted to perform synchronous process of micro-EDM grinding. In our previous studies, it had been confirmed the feasibility of the synchronized micro-EDM and micro grinding process with 10 μm of lateral depth into work piece by precision positioning. However, there were severe problems of residual craters and debris accumulation, causing by the secondary discharge between both electrodes.
This study proposes two methods to overcome the defects and increase machining quality compared to the previous researches. One of them is to dress standard cylindrical micro-tool into the helical groove with a short taper, making the spiral grooves serve as the chips pocket to increase debris expelling efficiency. By appending cobalt ion into the plating bath to achieve Ni-Co alloy co-deposition, it is found that both more uniform dispersion of abrasives on the tool’s surface and richer diamond grains and stronger mechanical property in wearing resistance are achieved. By doing so, significant improvement of the precision quality and machining efficiency for such a hybrid process, and the anti-worn out capability of the micro-tool are verified through the conducted surface of micro-holes. Moreover, only one step of processing is required on the same machine and both higher efficiency and better quality by applying micro diamond tools to fabrication of under 0.33 mm micro-holes co-axially and precision circle with 0.3 mm thick of SUS-304 is achieved.
Fabrication conditions of micro diamond tools by Ni-Co co-deposition is conducted under Ni-Co/Diamond bath with 5 g/l of abrasives concentration, 2 RPM of rotation, 7 ASD of current density, and electro-plating interval of 5 min. It was verified from EDS analysis that the diamond amount occupied 31.77 wt(%) and cobalt element occupied 11.31 wt(%). Such a micro-tool with processes parameters of 20 KRPM, 30 μm/min and feeding depth of 2 mm depth achieved the best surface roughness of Ra 0.107 μm. Its diameter difference between entrance and exit also appeared to be the value of 9 μm. From oscilloscope observed EDM waveform and the 10 μm scaled SEM picture of tool’s profile, it is confirmed that synchronous micro-EDMG occurred during the operation processes.
Furthermore, it is found from the machined surface that there are three phases of EDMG processes contributed to this novel strategy. Namely, the first phase during machining depth of 0~1.0 mm along z-direction, it is dominated by micro-EDM drilling with roughing current of 0.5A. There is almost no grinding, or the grinding chips are totally re-discharged again by the second discharge process. The second phase during 1.0~1.5 mm is contributed to rough EDM grinding of the micro holes, some recast layers are pulling-pushed or scratched, and some craters are refilled to some sort of level. There seems some kind of hybrid machining effects around EDM spark’s spot. Finally, the third phase during 1.5~2.0 mm, there are some sort of complete grinding to remove the recast layers and craters to a fine surface in this finishing stage.
Besides, from SEM photo of 1 μm scale, it is found that not only simply EDM or simply mechanical grinding, but also plastic machining regions observed. They are contributed from micro-scale removing mechanism during micro-EDMG processes, in which recast layers are pulling-pushed and scratched to longer chips, and some craters are refilled again. On the other way, due to such a short period of EDM pulse on-time, the shoveled chip by grinding abrasive is the most possible position to be re-discharged again to shape a non-typical EDM crater.
Keywords: micro-EDM (electrical discharge machining), micro-EDMG (micro-EDM grinding), synchronous processes, hybrid Ni-Co co-deposition, Diamond Tool
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