4.3.2. Surface Roughness

Figure 13a shows the effect of the inlet pressure on the surface roughness of the workpiece internal surface at a cutting speed of 100 m/min. With the increase in the inlet pressure, the surface roughness of the workpiece decreases from 2.92 μm in dry cutting to 2.63 μm with a tool inlet pressure of 0.6 MPa. When the tool inlet pressure is lower than 0.2 MPa, the surface roughness decreases insignificantly with the increase in inlet pressure. This is mainly due to the low airflow velocity when the inlet pressure is small, which leads to the insignificant cooling and lubricating effect, and deteriorates the surface quality. However, the velocity of the air and coolant droplets increase when the tool inlet pressure is higher than 0.2 MPa, which improves the permeability of the coolant droplets and the chip removal, thereby enhancing the lubrication effect and convective heat transfer, and the surface quality as well.

**Figure 13.** Effect of cooling conditions on machined surface roughness of workpiece. (**a**) Surface roughness of workpiece under different spray pressure at a cutting speed of 100 m/min, (**b**) The spray cooling and dry cutting roughness at different cutting speeds.

Figure 13b shows the effect of the cutting speed on the surface roughness of the workpiece internal surface. The surface roughness decreases with the increase in cutting speed ranging from 60 m/min to 140 m/min. Higher cutting speed leads to higher the cutting temperature, which subsequently softens the material and reduces the cutting force and vibration. Compared to the dry cutting, the internal spray cooling method could reduce the surface roughness of the workpiece by 0.1–0.25 μm.
