## 2 Materials and Methods  
### Determination of Thermal Properties  
The main problem determining the in-process temperature in drilling tools is the inaccessibility of the impact zone of the tool. That is why only indirect temperature measurements are possible. The used tool is a SLD tool with a diameter of \(D=18\) mm and a length of \(l=700\) mm with a cutting insert and screwed guiding pads. The temperature propagation in the cutting insert is to be measured. Figure 1 shows the drill head with the PT1000 resistance sensors S1 to S3, placed beneath the cutting insert of the SLD tool. A load-free installation of the sensors is a condition for obtaining proper temperature values. Pressure applied to the sensors during cutting process leads to a deviation of the measured values. PT1000 sensors have the advantage of a fast response characteristic and small installation space specifications. The circuit board equipped with the sensors was placed in a slightly deeper pocket under the cutting edge. The sensors were then completely covered with 2C epoxy resin and cured. This setup avoids pressure-related measurement inaccuracies and isolates the electric measuring equipment from the cooling lubricant. A Seria rotarX slip ring from B-COMMAND GmbH (Hamburg, Germany) was used for data transmission. The cables passing through the cooling channel are connected to the slip ring and thus led the sensor data to the measuring computer. Embedding the temperature sensors and assembling the cutting insert changes the heat transfer characteristics from the cutting edge to the sensors. With a heat transfer simulation, the thermal quantities of conduction, specific heat and the heat transfer coefficient due to convection were adjusted in a preliminary test. In this way, the heat transfer properties of the tool were identified in order to enable a recalculation of the applied temperature at the cutting edge. The cutting insert was implemented in the heat transfer simulation and discretized with 4-node linear heat transfer tetrahedral (DC3D4) elements. For the reverse engineering experiment, the air mass flow of a hot air soldering gun was set to a volumetric flow rate of \(\dot{V}=60\) L/min. The temperature of the soldering gun \(T_{\text{heat}}=300\)was checked with the test of a 935 temperature measuring device at a working distance of 2 mm. The air flow was then directed at the same working distance to the outer surface of the cutting edge. For the heat transfer simulation representing the preliminary test, a defined convection was applied to the cutting insert. A temperature of \(T_{\text{airflow}}\) = 200 \({}^{\circ}\)C was measured with the thermocouple at the working distance. Thus, the heat transfer coefficient due to free convection \(\alpha\) can be determined approximately via the analytical equation. \[v=\frac{\dot{V}}{A}=\frac{\dot{V}}{\pi\cdot r^{2}}=141.5\ \frac{\text{m}}{ \text{s}}, \tag{1}\]  
\[\alpha=12\cdot\sqrt{v}+2=144.75\ \frac{\text{W}}{\text{m}^{2}\text{K}}, \tag{2}\]  
In addition, the surface area at which the airflow is impinging the cutting edge was determined via the shear layer, which is caused by the expansion of the airflow after exiting the nozzle. Via a defined heat flow onto the calculated surface, the cutting insert is heated according to Figure 2. With a diffusion angle of \(\Theta=10^{\circ}\), at which the air flow velocity attenuates to 1%, the working distance of 2 mm and a nozzle diameter of 2.6 mm the impact area results in a radius of 1.617 mm with \(T_{\text{airflow}}\). A heat radiation from the cutting insert was applied to the free surfaces with a factor of 0.9. Figure 1: PT1000 temperature sensors S1–3 attached to SLD tool. Figure 2: Heat transfer simulation in the cutting insert for determining the heat transfer properties. The heat transfer simulation uses the material parameters of tungsten carbide with a carbide content of 15%. The set material parameters can be taken from Table 1.  
With these parameters, first a heating simulation is performed.