# A Study on the Effect of Gear Hobbing Process Parameters on the Residual Stress of the Tooth Root
## 4 Results and Discussion
### Single-Factor Experimental Analysis  
Through the measurement of the above methods, the measurement results for the root residual stress of the single factor test are shown in Table 10.  
In order to further research the influence of each process parameter on the residual stress of the tooth root, a single-factor experiment was carried out. The influence trends of the hob speed, axial feed speed, and radial cutting depth on the residual stress at the root are shown in Figure 9. According to the findings presented in Figure 9a, with the increase in the hob speed, the residual stress value of the tooth root gradually increases, indicating that the residual tensile stress increases. With the increase in the hob speed, there will be more heat generated in the cutting zone, resulting in more drastic chip deformation and an increased chip compression ratio. Additionally, the chip temperature is significantly higher than the temperature of the corresponding separation position on the gear, which is due \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Serial Number** & **Data (MPa)** & **Serial Number** & **Data (MPa)** \\ \hline
1 & 129.60 & 7 & 183.23 \\
2 & 215.73 & 8 & 176.33 \\
3 & 184.70 & 9 & 169.83 \\
4 & 238.57 & 10 & 154.10 \\
5 & 154.77 & 11 & 121.03 \\
6 & 159.47 & 12 & 68.27 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Single-factor experiment results for residual stress at tooth root. to the large deformation of the workpiece material and the great friction between the hob and the gear in the second deformation area, which is called the greatest heat concentration area [20]. The temperature variation in the cutting area affects the bond between the chip and the gear, with a higher temperature facilitating stronger bonding. Therefore, with the hob speed increases, the chip with a high heat load separates from the gear, leading to a rise in residual tensile stress on the gear surface. Figure 9: (**a**) Single-factor test results of gear hobbing (effect of hob speed on residual stress at tooth root). (**b**) Single-factor test results of gear hobbing (effect of axial feed velocity on residual stress at tooth root). (**c**) Single-factor test results of gear hobbing (effect of radial cutting depth on root residual stress). According to the findings presented in Figure 9b, with the increase in the hob axial feed speed, the residual stress value of the tooth root gradually increases, indicating the residual tensile stress increases, but the overall range is between 150 and 190 MPa. The main reason for this phenomenon is that when the axial feed speed increases, the volume of material that is cut by the cutter teeth per unit of time will increase, resulting in a gradual rise in the cutting force and cutting temperature, which jointly increase the force and cutting temperature. Moreover, the dominant factor in this increase is the cutting temperature. It is noteworthy that the range of variation in residual stress is relatively small, which is due to the axial feed speed having a lesser impact on the cutting stress. It has been proven by finite element simulation in reference that the effect of the axial feed speed on the cutting stress is not obvious [37], which explains the small variation range of the residual stress in this experiment. According to the findings presented in Figure 9c, with the gradual increase in the radial cutting depth, the residual stress value of the tooth root gradually decreases, indicating that the residual tensile stress decreases. The increase in the radial cutting depth corresponds to a decrease in horizontal feed times, which in turn reduces the cumulative cutting length and load, resulting in a reduction in residual stress [21]. [PERSON] et al.