Mechanism of high-speed machining of graphite

Scholars both domestically and internationally have conducted preliminary analysis and research on the formation and expansion of cracks during chip formation in conventional and high-speed machining of graphite, as well as on chip morphology and size, and the interaction between tool and workpiece materials. Das believes that when using a tool with a positive rake angle for graphite turning, a large chip will first be generated, leaving an arc-shaped groove on the surface of the workpiece to be machined, and then it will be removed in the form of small chips; whereas when using a tool with a negative rake angle for machining, due to the simultaneous expansion of multiple slip planes, the generated

  The chips are primarily small particles. When studying the turning process and characteristics of carbon-graphite materials, it is noted that the cutting tool does not simply strip the graphite billet from its surface. Instead, depending on the properties of the cutting tool and billet, as well as the cutting factors of the tool and the sharpness of the cutting edge, the tool exerts effects such as "cutting and stripping" or "crushing and stripping" on the particles on the cutting surface. The cutting force during the turning process of carbon-graphite materials is irregular, intermittent, and high-frequency impact force. Furthermore, there is considerable friction between the carbon particles and the cutting edge during high-speed relative motion. It is believed that the cutting process of graphite materials is primarily due to the surface of the processed material being squeezed and fractured (crushed) by the squeezing force of the tool's cutting edge. This squeezing force is actually the frictional effect between the cutting edge 1:3 and the processed material. It is mentioned that the chip powder particles during the turning of sintered graphite are concave-convex and fragmented, and the chip shape of newly sharpened tools is irregular, with many edges on the surface, measuring tens to hundreds of micrometers in size. As the tool wear increases, the average diameter of the chip particles becomes approximately spherical, and the concavity and convexity of the powder surface decrease by 14%.

During the high-speed turning of graphite materials, Masuda observed through high-speed photography that the initial cracks generated inside the graphite material propagate along the cutting direction, causing the graphite material to fragment. Most of the chips slide along the rake face, thus resulting in crater wear of the tool. Sato classified the graphite chips generated during the high-speed turning of sintered graphite into four grades: 500μm, 250μm, 125μm, and 63μm. Among them, graphite chip particles with a size less than 250μm account for the majority of the total chip weight, and as the feed rate increases, the proportion of large particle chips also increases. After studying the high-speed milling process of graphite, K6nig believed that the formation process of graphite chips is quite similar to that of brittle materials such as ceramics; the graphite material is crushed and fragmented at the tool tip, forming small chips. The cracks generated by cutting first extend downward and forward of the tool tip, and then extend to the free surface, forming large fragmented chips and creating fracture pits on the machined surface of the graphite. The contact state between the chips and the tool's rake face is divided into a cutting contact impact zone and a sliding zone along the rake face, which lead to different tool wear patterns, respectively. The cutting force and its fluctuation amplitude during the turning of graphite materials increase with the increase of feed rate, and the ratio of maximum cutting force to average cutting force can reach 2.0, fully reflecting the fluctuation characteristics of cutting force during the machining of brittle materials.