# Cutting Performance and Wear/Damage Characteristics of PCBN Tool in Hard Milling
## 3 Experimental Analysis
### Chip Formation  
The friction and thermal conductivity between the tool and the chip are different due to the different tool materials in the process of high-speed milling of hardened steel, which leads to a difference of the contact surface between the chip and the tool. Because the majority of the heat generated during cutting is carried away by the chips, the high-temperature chips would react with oxygen in the air and produce temper colors of the chips. Therefore, chip color can be a characteristic indicator of cutting temperature [28; 29; 30]. The color of the chips can reflect the cutting temperature. The deeper the chip color is, the higher the cutting temperature was [30]. The chip temperature of the PCBN tool and carbide tool for the intermittent machining of hardened steel is shown in Figure 9. The overall temperature of chips was higher during the intermittent machining of hardened steel with the cemented carbide tool, and the contact surface between the tool and chips was rough.  
When the PCBN tool was used to machine hardened steel, the cutting temperature was higher at the smaller chip thickness. The reason was that the action mechanism between the cutter and the workpiece was mainly ploughing and extrusion in the place where the chip thickness was small. Because the size of chip section was small, the excessive heat generated by ploughing and extrusion could not be discharged in time and, thus, the chip color was deepened. When the cutting thickness was large, the cutting heat was mainly caused by the shear action and the friction between the cutter and the chip.  
The microscopic characteristics of the chips of two tools under the same cutting conditions are shown in Figure 10. It was found that the chip contact surface of the PCBN cutter was smoother than that of the carbide cutter. The reason was that the PCBN tool had a smaller friction coefficient, which decreased the friction between the tool and chip. There were pits in the middle of the chip contact surface machined by the PCBN tool. The chip contact surface processed by the carbide cutter involved mainly parallel scratches.  
Figure 9: Chip temperature distribution (Test 5): (**a**) PCBN tool; (**b**) carbide cutting tool.  
The free surface topologies of the chips processed by the two tools are shown in Figure 11. _Pc_ represents the sawtooth pitch of chip. The sawtooth degree and the sawtooth pitch of chip machined by the PCBN tool were higher than those of the carbide cutting tool. The reason was that the larger flow velocity of the chip in the process using the PCBN cutting tool led to a stronger separation trend in the process of sawtooth chip formation, which further led to the increase of sawtooth pitch.  
The free surface morphology of the chip processed by the PCBN cutter with different cutting speeds is shown in Figure 12. It was found that the sawtooth pitch of the chip (_Pc_) increased with the increase in cutting speed. The reason was that the increase in cutting speed made the shear slip more easily during chip formation, which led to the increase of sawtooth degree.  
Figure 11: Chip free surface topography (Test 6, v = 500 m/min): (**a**) PCBN tool; (**b**) carbide cutting tool.  
Figure 10: Chip contact surface topography (Test 5): (**a**) PCBN tool; (**b**) carbide cutting tool.