Optimization and Analysis of Nanocrystalline Diamond Coated Micro End Mills

This study compares the wear and performance of nanocrystalline diamond (NCD) coated and uncoated tungsten carbide (WC) micro end mills when machining 6061-T6 aluminum. Images of the tool profiles before and after milling are used to track changes in geometry caused by tool wear. Tool performance was characterized by the magnitude of cutting and thrust forces acting on 300 mum diameter end mills and through imaging and analysis of the tool and workpiece surfaces. Thin NCD coatings (<300 nm) allowed for an average of approximately 30 mm of milling before cutting edge fracture. Before edge fracture, the NCD coated tools produced highly uniform, burr free channels. In contrast, significant burring and surface irregularities were evident when using the uncoated tools. Even after cutting edge fracture, the NCD coated tools continued to produce cleaner channels with a lower amount of burring than the uncoated tools but fractured much more severely. However, NCD remaining within the tools' flutes resulted in lower cutting forces due to a reduced effective friction coefficient (friction and adhesion) between the tool surface and the cut chip. Carbon ion implantation (CII), one method to prevent the severe edge failure experienced by the NCD coated end mills, was used as a surface preparation technique to enhance both the cutting edge wear resistance and to increase the nucleation and growth of the diamond coating. Initial milling tests from the carbon ion implanted tools have indicated a drastic improvement in resistance to cutting edge fracture. The implantation of ions into the tool's surface induced compressive stress on the cutting edge, thereby increasing tool resistance. However, the inconsistency of CII has thus far resulted in poorly adhered diamond coatings. Additional stress analysis on the cutting edge has revealed the formation of detrimental bending stresses present during micro milling with an NCD coating. Increasing the cutting edge radius and the coating thickness can reduce this stress and prevent fracture.