## 4 Discussion  
In the fine blanking of metallic sheets under narrow clearance, the work material is sheared with nearly a full burnished surface area ratio due to hydrostatic pressure [20]. This suggests that the work material under shearing is compressed to the punch side surface during the piercing process under narrow clearance. Since the clearance is 4% of the sheet's thickness under the present piercing conditions, the nanotextures on the punch side surface are coined onto the burnished surface by this hydrostatic pressure. Macroscopically, the pierced hole surface is smooth, metallic, and shiny since the fine surface profile of the laser-trimmed punch was transferred onto the hole surface. Together  
Figure 11: SEM image on the nanotextured side surface of the punch from the punch edge along the punch length after piercing 100 holes into the AISI316L sheets. Figure 10: Comparison of hole surfaces pierced by the WC (Co) punch and by the trimmed diamond-coated WC (Co) punch. (**a**) Piercing by the bare WC (Co) punch and (**b**) piercing by the trimmed and diamond-coated WC (Co) punch. with this transcription, the nanotextures on the punch side surface were also coined microscopically. This simultaneous transcription of the punch surface structure through the piercing process forms a multi-dimensional hole surface profile. This finishing process is useful in the production of automotive parts. A macroscopically smooth hole surface is essential to function as an orifice for fuel injection [21]. Microscopic textures are expected to offer new functionality to the orifice hole surface to control the flow pattern of liquid fuel before fogging it into fine droplets. One of the most effective features for nanotextures lies in the enhancement of the optical responses of the nanotextured surfaces. Many studies on the enhancement of optical properties by surface nanotexturing revealed that the geometric regularity of nanotextures significantly reflects this nanotexturing effect [22; 23]. Optical reflectivity is greatly reduced even by irregular nanotextures on the surface [22]. The regular alignment of nano-dots and nano-grooves is necessary to induce a surface plasmon effect onto the nanotextured surface [23]. As shown in Figures 7 and 9, semi-regular nano-grooves are densely coined onto the hole surface by fine piercing with a laser trimmed punch. The surface plasmon in a blue color is locally seen on the nanotextured area with regular alignment on the hole surface. This surface plasmonic state is greatly improved by narrowing the clearance between the piercing punch and the core-die so that more regularly aligned nanotextures can be coined onto the hole surface. In standard metal forming, the geometry and dimensions are transcribed onto the metallic members by fine stamping with mechanically finished dies and punches. In standard piercing, the clearance between the die and punch is optimized to increase the burnished surface area ratio and to prolong tool life. As shown in Figure 10a, the maximum roughness on the hole surface reaches 3 \(\upmu\)m due to the scratched traces using a normal punch. By using the laser trimming process, the diamond-coated punch has a smooth head and side surfaces with a lower maximum roughness than 0.5 \(\upmu\)m, as well as nanotextures on the trimmed side surface. This suggests that the diamond coated punch has geometric functionality, as well as a fine surface profile. Through the fine piercing process, the low roughness surface of the punch is transcribed onto the metallic-shining hole surface; e.g., the maximum roughness on the pierced hole surface in Figure 10b is less than 0.1 \(\upmu\)m. Concurrent with this piercing, the semi-regular nanotexture alignment on the punch is coined onto this smooth hole surface. This proves that multi-dimensional, geometric features are formed on a product surface by micro-manufacturing with the use of laser-decorated tools.