Development of helical electrode insulation layer for electrochemical microdrilling

The microhole is an important basic element in microparts and devices. When ECM is used in electrochemical microdrilling (ECMD), the tool electrode is very small and the flow channel cannot be provided for electrolyte in electrode, so the electrolyte cannot be renewed rapidly. Therefore, the electrolyte cannot rapidly carry away the reaction products, such as gas bubbles, sludge and Joule heat, from the machining zone. The ions that are depleted during pulse-on period cannot easily be replenished by convection and diffusion of the electrolyte. These phenomena cause the resistance to be changed in the inter-electrode gaps. Variation in the resistance affects the machining accuracy and surface quality of microholes. It may sometimes cause short-circuiting, damaging the tool and workpiece during ECMD process because the inter-electrode gaps are small. However, the machining accuracy will be affected by stray current in ECM. The most effective way to improve is to coat the insulation layer on the electrode sidewall, so that current can only be accurately released from the bottom end. The life of the electrode would be reduced by thin-film cracks if the bonding force between the film and the electrode is too weak to machine a precious microhole. Therefore, this study proposed to use the helical electrode in order to promote the processing of product exclusions. The electrode is made by using the sol-gel method and dip coating method. The innovative ceramic and epoxy double layer films is used to avoid side machining. The study results found that the electrode surface corrosion and adding the surfactants into sol-gel can be used to improve film adhesion on the helical edge. The electrode has voltage resistance up to 19.4 V by indirectly etching of 10 seconds. Finally, the coated electrode is tested to know the ECMD performance. The process parameter EB10C01E could obtain effective improvement on average hole diameter from 426.5 μm to 232.0 μm. The objective of this paper is to develop a low price and high quality of the helical electrodes to achieve high shape accuracy for ECMD process.