## 4 Discussion
That is a huge problem in simulations. At the moment, cutting simulations are mostly carried out for dry cutting processes or with an estimated value for the convection heat transfer coefficient of the temperature. In future work, the influence of the drilling oil on the temperature can be considered in more detail. Neglecting the cooling lubricant in the CEL simulation leads to inaccuracies of the absolute temperature but also of the temperature distribution. A reasonable coupling of the 3D cutting simulation with a fluid simulation representing the cooling lubricant supply is currently not yet reasonable, or merely possible with very high computing power. By neglecting the cooling lubricant, the absolute temperatures of the cutting insert are shifted towards higher temperatures. With the linear adjustment in the back calculation covered in Section 3.4, it may occur that the minimum temperatures decrease below the ambient temperature when looking at the nodes of the center axis of the drill. Here, the assumption must be made that the temperature cannot fall beneath the ambient temperature, but if it does, the calculated temperature has to be set to the ambient temperature. The CEL simulation also features the issue that an engagement of the complete cutting edge leads to convergence problems. The machined material is shifted unfavorably along the bore wall, causing unrealistic temperatures. Accordingly, the simulation time that influences the engagement height of the tool is adjusted to avoid convergence problems and \"smearing\" of the workpiece material. This leads to deviations in the temperature distribution between the simulation and drilling process. However, the final comparison of the measured temperatures at the sensor positions T1-3 with the corresponding temperatures T1-3 opt shows no significant delta, as can be seen in Figure 17. Because the simulation mainly \begin{table}
\begin{tabular}{c c c} Heat Transfer Coefficient \(a\)/W/m\({}^{2}\)K & Literature & Optimization \\ \hline free in air \(\alpha_{air-free}\) & 22 [16] & 139.3–313.8 \\ air pressure of \(p=6\) bar \(\alpha_{air-other}\) & 294 [16] & - \\ free in MQL \(\alpha_{MQL}\) & 200–300 [17] & - \\ drilling oil \(\alpha_{oil}\) & - & further work \\ emulsion \(\alpha_{max}\) & 5230 [16] & - \\ \end{tabular}
\end{table} 
Table 7: Comparison of the thermal convection with literature data. focuses on investigating the temperature distribution with realistic process parameters, there is no particularly nice chip formation to be seen, referred to in Figure 10. The material is removed from the simulation after exiting the Euler room, which represents the chip removal by the drilling oil. The heat conduction from the chip to the tool and corresponding friction conditions are changed. Since the temperature distribution was determined at the nodes of the cutting edge, the effect is negligible considering only the temperatures at the cutting edge for further investigation. Accordingly, not only the methodical procedure is justified, but also the determined analytical equations for the presented process. A further investigation looking at the correlation between subsurface properties and the thermo-mechanical condition shows a variation of the process parameters in a CEL simulation according to the design of experiments used above. It was shown that the general behavior of the simulation can be correctly approximated in terms of feed force and drilling torque. Figure 18 shows a comparison between the temperature of the calculated CEL simulations and the temperatures determined by the calculation of the temperature at the outer edge of the cutting insert form the sensoriy SLD tool. The general height of the temperature has a delta of \(\delta T\) about 100-180 K. Here too, the temperatures of the CEL simulations have a higher gradient [11]. Further investigations can use the equations obtained to calculate a controlling equation that can be used to keep the temperature constant during the process by varying the feed and cutting speed.