Internal Surface Finishing in AMed Bending Holes by EDM Using Suspended Ball Electrode

Metal Additive Manufacturing (AM) technology, i.e., 3D printing, now makes it possible to fabricate complex shapes of metals, including bending holes and lattice structures. However, when such hole shapes are formed using metal AM technology, the downskin surface, which is formed facing downward during layering, has a considerably larger surface roughness than other surfaces. When bending holes are applied as cooling channel, the large surface roughness may cause a reduction in strength of the hole internal surface, and therefore it is necessary to finish the surface. In order to reduce the surface roughness of internal surface, the feasibility of internal surface finishing by electrical discharge machining (EDM) using a suspended ball electrode was experimentally discussed in this study. The suspended ball electrode consists of a ball electrode and a flexibly deformable foil. First, the internal surface finishing of AMed straight hole was investigated by EDM using the suspended ball electrode, and the stability of electric discharges during the process was also discussed. Next, hole finishing of AMed bending holes was tried, and self-alignment effect of suspended ball electrode was confirmed, in which the ball electrode was autonomously guided to the center of AMed hole. Then, the self-alignment effect of ball electrode in EDM finishing using suspended ball electrode was experimentally investigated. Experimental results showed the self-alignment effect of ball electrode becomes higher as the AMed initial hole diameter is larger. Furthermore, the hole finishing on bending holes were discussed in detail for practical use of this finishing method. When surface finishing of hole was performed on bending holes, it was found that an unfinished surface remained inside the hole in the case of large bending angle, since the self-alignment effect of ball electrode would decrease. This is probably because the self-alignment effect is weakened by transient changes of ball weight component and tilting of foil part when the foil contacts with the hole internal surface during the machining of bending parts. Then, these factors on the self-alignment action was investigated. As a result, EDM finishing of internal surface of bending holes with large bending angles was succeeded by reducing the weight of ball electrode and the foil angle.