# Influence of Coolant Properties and Chip Former Geometry on Tool Life in Deep Drilling
## 3 The Importance of Coolant in Deep Drilling Technology Using a Gun Drill  
The basic function of the cutting fluid in deep drilling is to reduce wear and extend the life of the tool. When drilling deep holes, cooling fluids and their correct selection for a particular workpiece material are extremely important. For example, if we were to drill into steel, the main requirement for cutting fluid would be its cooling ability, while in drilling into aluminum, the main requirement is lubrication. On special drilling machines, cutting oil is used, which is the best cooling agent. As a rule, mineral oil is used with additives that enhance its parameters. The main requirements for the cutting fluid function are the following:  
* Cooling of the cutting part of the tool;
* Reducing friction between the tool and the workpiece, thereby reducing torque values;
* Reliable chip removal from the drilled hole;
* Avoidance of expansion on the cutting part of the tool, thereby favorably affecting the roughness of the drilled hole;
* Damping of the tool's vibration. On conventional machines, oil and synthetic emulsions are used for gun drilling. The emulsion should account for between 10 and 15% since water does not have a good lubricating effect, which is very important for the guide surfaces. Another option is to use compressed air with oil aerosol (MQL). This method is the least effective, but it also allows for application in machines that are not equipped for internal cooling and do not have the necessary cladding for the work area. The air under pressure carries tiny droplets of oil into the cutting process, which ensures lubrication and, at the same time, performs the chip flushing function. The effect of the cutting fluid is enhanced if it is fed to the site of cutting under higher pressure. When drilling deep holes, cutting oils are used in some drilling operations. For drilling stability and tool life, the pressure and flow of the coolant are also important. The individual pressure values depend on the diameter of the hole and the hole's length. The recommended values are given in Figures 2 and 3. For oils and emulsions, filtration is also very important, as it is also involved in increasing the durability of the tool. It should be able to capture particles of 5-10 \(\upmu\)m in size, usually by centrifuge or magnet. The correct geometry of the tool is also important for the efficient supply of coolant to the site of cutting. The cutting speed acting on the cutting blade decreases from the circumference towards the axis of rotation. In the middle, the speed is zero, which poses a disadvantage for the tool. A classic two-point drill bit is significantly stressed in the place of the transverse blade. These adverse effects are reduced in a gun drill through its asymmetrical geometry, as it is a single-pointed tool, and the tip of the drill bit does not lie on the axis of rotation but, as a rule, in 1/4 of the diameter. As a result, the distribution of cutting forces (Figure 4) is unbalanced, and the resultant forces have a significant radial component that acts in the direction of the first guide surface. If it is necessary to reduce the vibration of the tool or improve the surface machining, it is necessary to increase the pressure on the guide surfaces, which can be achieved by grinding the damping flute slightly beveled on the outside or by moving the tip of the drill bit closer to the axis of the drill. Another adjusting option is to create a deeper oil flute in the stainless steel part of the tool. This will increase the space for the coolant to pass into the flush groove and improve chip removal. On the other hand, this will reduce the flow of fluid through the inner part of the cutting blade. In cases of chip-forming problems, a longitudinal or transverse chip-forming device is used. The longitudinal chip-forming device is sanded directly by the manufacturer and does not need to be adjusted during re-grinding. The transverse chip-forming device must be sanded with each re-grinding.