# Influence of Coolant Properties and Chip Former Geometry on Tool Life in Deep Drilling
## 1 Introduction  
The current time in the engineering industry places great demands on the reduction of the time needed for production preparation, preferably by rationalizing the process by applying new methods and techniques in the development process. One such technology is the process of deep drilling, where it is necessary to create holes up to ten times deeper than their diameters. Deep drilling is most often used in the automotive industry, utility vehicles, aerospace, medicine, and, of course, mechanical engineering manufacturing. Standard drilling tools demonstrate low service life in deep holes and do not meet the necessary cutting conditions. Their design does not satisfy the conditions for a sufficient supply of cooling agent, which is also associated with heat dissipation from the site of the cut, as well as insufficient chip removal, which is one of the most important principles in drilling deep holes. Conventional machines can be applied to deep drilling technology. Making holes of greater lengths is, however, limited by their design, and, therefore, for maximum depth and productivity, special machines are developed, the design and properties of which are tailored to deep drilling. Drilling is defined as a technological process resulting in holes of circular cross-section. Their depth depends on the design requirements, and it is often necessary to drill holes five times the depth of their diameter. This operation is called deep drilling. The deep drillingtechnology is much more demanding on the cutting process than the drilling of standard holes of less than 5 xD. In the past, holes with a length-to-diameter ratio greater than 3-5 xD were considered deep holes, but nowadays, thanks to modern procedures and technologies, we consider holes with a diameter of up to 10 xD to be deep holes. The constant effort to streamline production forces engineering companies involved in the development of drill bits to come up with new solutions for conventional helical drill bits capable of drilling holes that have a length-to-diameter ratio of up to 30 mm. Monolithic long screw drill bits, STS 22 1130 and STS 22 1150, or drill bits with internal cooling, STS 22 1154, are used for drilling deep holes. Because the drill bits' flutes are quickly filled with chips, the drill bits are pulled out of the cutting holes to cool and remove the chips. Such drilling is called intermittent work cycle drilling. Solid carbide (TK) drill bits work similarly to HSS drill bits. These are produced up to a maximum length of 30 xD and always come with internal cooling. Thanks to the drill bit's internal cooling, an intermittent work cycle need not be applied. In order to be able to drill in one cycle, it is necessary to pre-drill a pilot hole, the depth of which must be at least 3 xD. Since monolithic solid carbide drill bits are quite expensive and the solid carbide body is fragile and does not allow for application under any conditions, e.g., in older machines with increased vibration or with the product itself prone to vibration, it is possible to use a drill bit with a replaceable tip. The solid body of such a drill bit can withstand higher vibrations, but at the cost of reduced machining conditions. Another advantage is the simple replacement of the worn tip without further tool alignment. The gun drill is a tool with one cutting edge that contains internal flushing and external chip removal. It contains an external, direct V-shaped pipe and, at the same time, an inner hole for the cooling medium. The fluid flows through the axis of the tool, circling the cutting blade and flushing the emerging chips through an external path out of the hole. The gun drill has three basic parts: a drill head, a drill pipe, and a shank (Figure 1). The drill head is typically carved from carbide, but in the case of larger diameters, it can be composed of steel with carbide elements bonded thereto. The guide surfaces of the head are fluted along its circumference, and they are in contact with the drilled material in the course of drilling.