# Influence of Coolant Properties and Chip Former Geometry on Tool Life in Deep Drilling
## 4 Testing the Impact of Changing the Coolant Parameters on the Service Life of the Gun Drill
### Dependence of the Drill Bit Life on the Coolant Properties  
The main function of the coolant is effective heat dissipation from the site of cutting and--through lubrication--to reduce internal and external friction. The main requirements for a coolant to be effective are pressure, emulsion percentage, temperature, and the cutting fluid flow rate. These parameters have been tested to determine their impact on tool life. The fluid flow rate has not been tested as the cooling equipment used does not allow it to be controlled and has been set to the constant flow rate of 22.3 L/mm, as recommended by the manufacturer. The testing was completed at the moment of damage to the drill bit, which can be seen in Figure 6.  
In general, the mechanism of wear of the drill bit mainly involves diffuse wear, adhesive wear, oxidative wear, and breakout. The cutting heat comes from the shear deformation of the materials, the friction between the contact surfaces, between the drill bit and the workpiece, and between the continuously removed chips. Heat formation increases above the melting point temperature of the alloy elements (for example, Co and W) and the hardness. The chemical resistance of the tip of the drill will decrease dramatically due to the peeling off of the alloy elements, too. Cyclic stress will cause mechanical and thermal shock during high-speed drilling. Mechanical shock subsequently gives rise to parallel cracks, and thermal shock gives rise to perpendicular or oblique cracks in the blade. These cracks expand, merge, and break the blade. The first test was aimed at monitoring the life of the gun drill at different concentrations of cooling emulsion, namely 6%, 9%, 12%, and 15%, respectively, under a pressure of 35 bar. The resulting values of the gun drill life at different emulsion percentages are shown in Figure 7.  
Figure 6: Example of damage to the drill bit. Figure 7: Plotted number of drilled holes up to the point of tool damage at different concentrations of the cooling emulsion. Life values show a decreasing lifespan at an emulsion percentage below 9%. From the resulting life values, it is recommended to set the emulsion percentage value to 9%. The higher percentage of emulsion did not have a significant impact on the tool life. The second test was aimed at monitoring the life of the gun drill under a change in pressure of the cooling emulsion ranging from 20 to 45 bar at its 9% concentration. The resulting values of the gun drill life under different emulsion pressures are shown in Figure 8.  
Tool life values show an increasing trend under emulsion pressure above 30 bar. From the resulting lifespan values, it is recommended to set the fluid pressure value to 35 bar. Higher emulsion pressure has only a small impact on the tool's life. The last test was aimed at monitoring the life of the gun drill under a change in the temperature of the cooling emulsion ranging from 25-32 \({}^{\circ}\)C at 9% emulsion concentration and a constant pressure of 35 bar. The resulting values of the gun drill life under different temperatures are shown in Figure 9. The life values do not show an increased lifespan with the change in temperature that we were able to achieve on the refrigeration equipment. Figure 8: Plotted number of drilled holes up to the point of damage to the tool under different coolant pressures. Figure 9: Plotted number of drilled holes up to the point of damage to the tool at different coolant temperatures. During the testing, the influence of the pollution of the cooling medium was also manifested by adhesive damage on the guide surface of the gun drill. When it comes to adhesive damage (Figure 10) to the guide surface of the gun drill, it is necessary to immediately check the coolant filter and the percentage value of the cooling emulsion. This damage reduces the number of re-grindings of the drill bit and its reuse. Subsequently, the impact of changing the geometry of the chipformer on its service life was also examined.