Analysis of Tool Wear and Roughness of Graphite Surfaces Machined Using MCD and NCD-Coated Ball Endmills

Abstract

The high-purity G5 graphite material is widely used for glass moulding and provides high hardness and brittleness because it is sintered to fine particles unlike other graphite materials. Hence, tool cutting of a G5 workpiece is performed by local fracture instead of plastic deformation of the machined surface. Although a diamond-coated tool with outstanding hardness is used to machine very hard graphite, the tool shows variability regarding the service life and machining performance depending on the grain size, even in the same machining environment. We investigated the wear and change trend of machined surface roughness considering microcrystalline diamond (MCD) and nanocrystalline diamond (NCD)-coated tools, which are generally used to machine graphite materials, and analysed their relation with coating. For rough machining, the MCD-coated tool, for which the delamination of coating occurred later, showed less wear and improved machined surface roughness. For precision machining, the NCD tool showed less tool wear rate relative to the cutting length, leading to a small difference in the machined surface roughness between the two tools. We conclude that if rough and precision machining processes are performed using the same cutting tool, the MCD-coated tool is advantageous in terms of service life, while the difference in roughness of the final machined surface between the tools is negligible.

Effect of Coating Thickness on Abrasion and Cutting Performance of NCD-Coated Ball Endmills on Graphite Machining

“…Thus, graphite is stable in temperatures of 1000 °C and higher; therefore, as of recently, it is being widely utilized as a mold material for hot compression molding of glass. Among various types of graphite, grade 5 (G5) graphite is fabricated into particles of a few μm or less, unlike general graphite, and it has high strength and hardness, which makes it favorable for surface abrasion and transformation according to the force applied to the mold during hot compression molding [ 3 ]. However, microparticles or hard fragments are crushed and ejected instead of cutting chips during mechanical processing using cutting tools.…”

“…In this study, tool lifespan and change in cutting performance were analyzed based on the following: our previous research [ 3 ] and according to the change in thickness of the layer of NCD coating, as well as the coating abrasion of the tool when applied for rough milling and finish milling. As mentioned in Section 1 , the coating thickness varied with respect to the radius of the cutting edge, the dimensional accuracy of the tool, and the roughness of the flank and rake surfaces to study its impact on the machining performance of the tool.…”

“…Therefore, a detailed analysis on the variable that most impacts the degradation of machining performance and the lifespan of the NCD-coated ball endmill was necessary. Tools with identical geometrical data as those used by Lee et al (our previous research) [ 3 ] were selected. These tools were coated with nano-diamond crystals at 8 and 12 μm deposition thicknesses, and the experiment was performed.…”

“…Additionally, because the use of lubricant can cause the graphite particles to fuse to the workpiece and the tool surface, machining is performed in a dry environment. The machining system in this research uses a 3-axis CNC high speed machine tool (Roeders, RXP500, Soltau, Germany), identical to that used in the our previous research, which was also referenced for the recommended cutting conditions provided by the tool manufacturer [ 3 ]. The major specifications of the machine tool and the cutting conditions are listed in Table 4 and Table 5 .…”

“…While the delamination of NCD coating is easier than that of MCD coating, and the lifespan of NCD coated tools are shorter than those of MCD coated tools, the geometric error of the tool is, relatively, smaller. Hence, NCD coated tools are, relatively, more advantageous in precision cutting [ 3 , 4 ]. Thus, many studies have attempted to improve the lifespan of NCD-coated tools [ 4 , 5 , 6 , 7 , 8 , 9 ].…”