## 4 Discussion  
From the determination of the temperatures at the outer edge of the cutting insert, a maximum temperature using drilling oil of between 450 K and 570 K is obtained, depending on the process parameters. The temperatures of the tool were investigated with a thermographic camera in a previous work, where no cooling lubricant was used. The highest spot of the temperature in the pictures of the thermograic camera are also at the outer edge of the cutting insert [10]. Compared to measurements of the temperature using a thermographic camera, the temperatures at that spot differ by an amount of about 400 K to 500 K. The difference in the achieved temperatures is mainly due to the use of cooling lubricant. The thermographic measuring method cannot be used in SLD processes with the usage of cooling lubricant, due to the influence of external disturbing factors, such as the exiting cooling lubricant covering the desired measuring area. Therefore, higher temperatures can be measured with the thermographic measuring system. The use of a cooling lubricant has a significant influence on the temperature of the tool. A steady flow of cooling lubricant changes the frictional conditions and does not produce as much heat at the cutting edge. In addition, hot chips are transported away from the cutting area and there is less heat transfer into the tool. The exact consideration of these factors is very difficult to implement in simulations. The heat transfer from the cutting edge can be taken into account via the factor of natural convection. Different values are given here for different ambient media. The use of the drilling oil \"r.rhenus R-OiL DD 10\" resembles the properties of mineral oil with an ester content of 5%. From this, the thermal  
Figure 17: Comparison of the temperature curve V001 of experiment and optimization. properties closest to the material properties viscosity and density are derived. Thus, the heat transfer coefficient for free thermal convection can be set to a value of \(\alpha=108.7\) W/m\({}^{2}\)K for a temperature of 40 \({}^{\circ}\)C [17]. The convection that results from the process can also be calculated indirectly from a drilling test that was carried out. During the process, cooling lubricant is fed through the tool at a pressure of \(p=20\) bar, which corresponds to a flow rate of \(\dot{q}=6.1\) L/min. Using the model of the temperature distribution at the cutting edge from Section 3.4, the cutting edge is heated to the process temperature. After drilling through the workpiece, the temperature drops because the cutting edge is no longer engaged and is additionally cooled by the fed cooling lubricant through the cooling channel. From this cooling curve, an analysis of the convection after the drilling process can be carried out. Varying the convection heat transfer coefficient results in a value of \(\alpha=139.3\) W/m\({}^{2}\)K for the free convection. In order to be able to determine the convection heat transfer coefficient during the process, the drill must be stopped on the inside of the workpiece. In this way, the coefficient can be determined flow-specifically in further work. Table 7 presents different values for thermal convection taken from the literature and from the optimization simulations. As the heat transfer coefficient is not a property of the fluid, but is dependent on the fluid properties, the geometry of the probe and the material properties of the probe, a direct comparison is difficult. [PERSON] et al. use a thermographic analysis of hollow aluminum cylinders in naming experiments. With a specific heat capacity of 896 J/kgK, aluminum has a better ability to store heat compared to tungsten carbide. The heat may be kept longer in the material resulting in lower heat transfer coefficients. Because of a higher specific heat and higher thermal conductivity of water in contrast to oil, water based coolants lead to enhanced heat transfer coefficients [16]. No literature values are available for thermal convection for processes using drilling oil under a certain pressure.