*4.1. Tool Manufacturing*

The S25R-MCLNR12 internal turning tool was used as the prototype tool. According to the optimization results, both the internal flow channels of toolholder and the nozzle were manufactured. Figure 10 shows the manufactured internal spray cooling turning tool. The main cutting-edge angle of the tool is 95◦, the rake angle is <sup>−</sup>6◦, and the relief angle is 5◦. The TiAlN coated VP15TF cemented carbide insert produced by MITSUBISHI (Tokyo, Japan) was adopted. Small holes with a diameter of 0.6 mm were drilled by the electrical discharge machining near the tip of insert for installing thermocouples to measure the cutting temperature.

**Figure 10.** The manufactured internal spray cooling turning tool and thermocouple position. (**a**) the internal spray cooling turning tool, (**b**) diagram of distance between thermocouple and main and minor cutting edges, (**c**) diagram of the distance between the thermocouple and the rake face.

#### *4.2. Experimental Conditions and Settings*

Figure 11 shows the internal turning experimental platform, which includes the lathe, spray cooling system, internal spray cooling turning tool and data measuring system. The experiments were conducted on HuaZhong CNC machine tool CK6136B (HOTON, Shandong, China), the maximum rotation diameter of the machine tool is 360 mm, and the maximum spindle speed is 6000 rpm. The workpiece is a hollow cylinder with inner and outer diameters of 45 mm and 100 mm, and its length is 200 mm. The workpiece is made of QT500-7 with a hardness of 170–230 HBS, and the strength is approximately 500 MPa. The coolant used for internal spray cooling is a mixture of water and oil with a volume ratio of 30:1. A K-type thermocouple was embedded into the small hole of the insert to measure the cutting temperature, and the thermocouple was connected with the JK808 temperature tester (Jinko Electronic Technology Co. Ltd., Changzhou, China), which was linked to the computer for collecting the real-time temperature data. The TR200 handheld surface roughness measuring instrument (JiTai Tech Detection Device Co., Ltd., Beijing, China) was used to capture the roughness of internal surface of workpiece. After each cutting test, the surface roughness was measured at six different locations along the circumferential direction of internal surface of workpiece, and the average value was taken as the output value surface roughness. Chips were also collected and the morphology were captured by a handheld microscope. The cutting experiments were carried out under both the dry and internal spray cooling conditions for comparison purposes, and the corresponding operation parameters of the experiment are listed in Table 4.

**Table 4.** The parameters of cutting experiments.


**Figure 11.** Internal turning experimental platform.

#### *4.3. Results and Discussion*

## 4.3.1. Cutting Temperature

The cutting temperature were measured by the thermocouple inserted in the thermocouple node in the insert as shown in Figure 10b,c. The experimental and CFD temperatures are shown in Figure 12a. By adjusting the heat flux (22 W/m<sup>2</sup> in this case) in the tool-tip contact surface, the errors between the CFD and experimental results could be minimized (5% in this case). In the CFD simulation, the effect of the chip on the cutting temperature was not considered, thus when the inlet pressure is smaller than 0.2 MPa (leading to lower air pressure), the experimental temperature is larger due to the poor chip removal ability. When the spray pressure is below 0.1 MPa, the measured temperature is 7.8 ◦C and 15.0 ◦C higher than that of the simulation in 0.1 MPa and dry cutting; when the pray pressure is larger than 0.2 MPa, the CFD temperature can be larger than the measure temperature due to the chip removal. Overall, the effects of the inlet pressure on the CFD and measured cutting temperatures are similar.

**Figure 12.** Effect of cooling conditions on cutting temperature. (**a**) cutting temperature under different spray pressure, (**b**) the influence of cutting speed on cutting temperature under dry cutting and internal spray cooling.

Figure 12b shows the cutting temperatures of the dry cutting and internal spray cooling at different cutting speeds. The experimental temperature gradually increases as the cutting speed increases. When the cutting speed are 60 m/min, 100 m/min and 140 m/min, the measured temperature of the internal spray cooling are 53.3 ◦C, 56.1 ◦C

and 58.7◦C, respectively, which are significantly lower than that of the dry cutting (89.9 ◦C, 98.7 ◦C and 100.9 ◦C). The cutting temperature under the internal spray cooling condition can be reduced by 41–44% compared to dry cutting with the cutting speed ranging from 60 m/min to 150 m/min.
