A recent survey conducted in the German metalworking industry indicates that drilling is one of the more time-consuming operations in the machine shop. In fact, of all the machining operations, 36% of the machining time is consumed in hole-making operations. Corresponding to this is that 25% of the machining time is consumed in turning operations and 26% in milling operations.

　　Therefore, the use of high-performance solid carbide drill bits instead of high-speed steel and ordinary carbide drill bits can significantly reduce the labor hours required for drilling operations, thereby reducing the cost of hole machining.

　　In recent years, cutting speeds have been continuously increased, especially for high-performance solid carbide end mills.20 years ago, the typical cutting speed for solid carbide end mills was60～80m/min. Nowadays, it is no longer surprising to use 200m/min cutting speeds to drill steel parts under the condition that the machine tool can provide sufficient power, stability and coolant delivery capability. However, compared with the general cutting speeds of turning or milling operations, there is still a lot of room for improvement in the cutting efficiency of drilling operations.

　　The overall hard alloy drill has high toughness requirements for the substrate, and the wear of the drill can be accepted if it is controllable and uniformly stable. Therefore, the typical drilling tool grade contains more cobalt than turning or milling cutter.

　　The material of the drill bit is usually micro-grained hard alloy to improve the strength of the cutting edge and to ensure uniform wear without chipping. When using a hard alloy drill bit, it is usually necessary to use a water-based cutting fluid, so the temperature at the cutting edge is not too high, but it is required to have resistance to thermal shock. The better performing drill bit grades are typical pure tungsten carbide materials, without the need for a large amount of tungsten carbide or titanium carbide added.

　　For the overall hard alloy drill, the coating must perform a function greater than simply increasing the surface hardness and wear resistance. The coating must provide an insulating layer between the tool and the work material and must be chemically inert; it must reduce friction by reducing the adhesion between the coating and the work material; the surface of the coating must be as smooth as possible; in addition, the coating of the twist drill must also have the ability to resist the propagation of cracks. The dynamic characteristics of drilling may cause micro cracks, and in order to maintain the tool life, the crack propagation must be prevented. By selecting the correct coating process and generating the appropriate coating microstructure, the coating material can be placed in a state of compressive stress, thereby greatly extending the tool life.

　　Good service can be obtained by using multilayer coating. The multilayer coating can prevent micro cracks from spreading between each layer of coating, and even if individual layers of coating are damaged and peeled off, the other layers of coating can still play a protective role on the hard alloy substrate. For drilling tools, the use of nano coating and precisely customized coating also has great development potential.

　　For example, a new kind ofTiAlNnanocoating withTiNas the top layer can solve many problems encountered in drilling stainless steel. The smoothTiNtop coating can reduce the adhesion and friction between the tool and the workpiece material, while theTiAlNnanocoating on the bottom layer can provide hardness and wear resistance for the tool. This coating has good crack propagation resistance and heat shock resistance, and the cutting speed can reach70～80m/minin drilling stainless steel, which is nearly2times of the conventional drill.

　　In order to give full play to the excellent performance of modern hard alloy base and surface coating, it is necessary to optimize the geometric parameters and drilling type of the drill, and it is necessary to make reasonable adjustments to the drill tip, drill tip angle, blade shape, cutting edge preparation, chip removal groove type, the number of chip removal grooves and blades, etc., according to the processing purpose.

The efficient cutting insert is generally of one of the four geometric forms. Among them, the tetrahedral insert with a cross edge is easy to grind and it is also easy to control the grinding tolerance, but its center clearance is small, and the back face of the cutter will contact the bottom of the hole when the feed is large, which affects the improvement of the feed rate.

　　The other type is a conical drill tip, which has a larger central clearance compared to a tetrahedral drill tip, resulting in less axial force during drilling. However, the geometry of this drill tip is more complex, making it difficult to ensure consistency in tool manufacturing and management. In addition to the above two types of drill tips, there are also spiral drill tips available, which are divided into two different types: traditional spiral drill tips with a chip flute for chip removal from the center; and new spiral drill tips that are ground with a chip flute and a rear face simultaneously, eliminating drilling steps and further improving chip flow. Due to the larger central clearance designed for these two types of drill tips, they have a high feed capacity. Moreover, the new spiral drill tips also have the ability for high-speed cutting and can drill with less axial force. The disadvantage of this drill tip geometry is that the grinding process required to manufacture the drill bit is more complex.

　　　When selecting a drill bit, another major factor to consider, in addition to tool life and cutting speed, is the quality of hole finish. In recent years, how to reduce burrs has become a key concern. De-burring is a typical manual process, which is very expensive in terms of machining cost, and can also cause serious problems if operated improperly.

　　　The overall hard alloy drill bit will exert a large pressure on the workpiece material when it is rotating at high speed and feeding. Therefore, when using conventional drill design or drilling point angle processing, a larger burr will be produced at the exit of the through hole. To solve this problem, a simpler method is to increase the drilling point angle to135°～145°, and the drill bit with a drilling point angle in this range can produce a disk at the exit of the hole, and the workpiece material is always under the action of tensile stress, which makes the material easy to machine rather than just pushing it out of the workpiece. The preparation of the cutting edge, the chamfering of the drill top, and other geometric parameter optimization measures will also play a great role in reducing the burr.

　　When drilling grey iron and ductile iron, a completely different problem is encountered. These materials are more brittle and are more likely to break away from the hole exit than to form a burr. Material breakage not only affects the quality of the workpiece but can also lead to the breakage of the drill. A chamfered and specially designed drill point can help the drill to bore the workpiece in a very stable manner and maintain the cut until the last revolution, thereby helping to avoid material breakage.

　　The design of the drill bit needs to be continuously adjusted according to the geometric parameters of the chip flute. The number of cutting edges, the thickness of the transverse edge, the width of the chip flute, the width of the land, etc., are all factors that need to be considered in the design of the drill bit. In addition, the influence of the workpiece material should not be overlooked.

　　When drilling holes in steel, the two-groove twist drill is usually a better tool choice. This type of drill is convenient to use, easy to grind again, and has excellent fault tolerance, which can reduce the radial jump error to a smaller extent, and can tolerate the instability of the machine tool and the workpiece.

　　With2or more chip slots, the drill has performance advantages when drilling long-length holes or in materials with internal stresses (such as cast steel). The three-groove drill has3flutes and3 cutting edges, providing better guidance and self-centering during drilling. However, since this type of drill cannot withstand too much torque, it is only recommended for cutting gray cast iron and non-ferrous materials. The2-sided and4-fluted drill is also an alternative cutting tool option (especially when internal cooling of the cutter is required).

A twist drill with4flutes performs excellently when drilling steel and cast iron materials because of its excellent tolerance and can achieve a high feed rate of more than twice that of a single-flute drill. This drill is also a good choice for deep hole drilling with a depth-to-diameter ratio of up to30, and its drilling speed is about5times that of conventional gun drills.

　　For the processing of aluminum alloy materials, the use of a straight groove drill bit can achieve better drilling accuracy, and can also process complex stepped hole types in a relatively simple way. The disadvantage of the straight groove drill bit is that it requires extremely high precision for tool clamping, and such drill bits lack fault tolerance for radial jump, excessive cutting speeds and feed rates or low coolant pressure.

　　In drilling operations (especially deep hole drilling), a very serious problem is that if the drill deviates from the center line of the hole at the beginning (deviation), it is almost impossible to correct the deviation during the subsequent processing, and the flute will guide the drill to drill down along the eccentric position until the bottom of the hole. However, since the drill has a spiral angle, the hole drilled will also be spiral. To avoid this problem, it is important to have a properly pointed drill with good self-centering capability. In addition, improving the guidance of the drill also helps to prevent deviation.2Flutes on a drill with two flutes can only achieve25% support at the beginning of drilling, so it is prone to deviation and can move in most directions under the influence of even a small force. However,4flutes on a drill can achieve support in all directions, so it can process holes with better roundness and cylindrical accuracy.4Flute drills also provide better support performance in non-uniform drilling or through-hole drilling, which are very common in the processing of parts such as hydraulic parts.

　　In today's drilling operations, chip removal must be fully controlled, rather than the way it was in the past, where the operator could simply withdraw the drill and peck at it whenever they felt the drilling force increasing. A crucial issue is that chip formation and breakage must be achieved in such a way that the chips can easily match the chip flute and be smoothly discharged from the hole with minimal friction, starting from the moment the chips begin to form at the drill tip.

　　The kinematic principle of drilling actually helps in controlling the chips, since the chip speed is zero at the center of the drill point, the chips flow more or less around the cross edge and are completely formed in the chip groove, provided the chip groove has the correct geometry. In addition, the chip groove is ground smooth up to the negative cross edge cone at both ends, which also helps in forming a free chip flow and in achieving drilling under controlled conditions.

　　The use of the correct drill bit and reasonable drilling process parameters can improve production efficiency and reduce machining costs. But how about the tool cost? First of all, the geometry of these advanced drill bits is more difficult to manufacture compared to traditional drill bits, so in general, the price of new drill bits is also more expensive than traditional drill bits. However, this new type of drill bit can be regrinded4～5 times, although each time the tool life will decrease by about10％, it is still possible to achieve a saving of more than50％ in tool costs.

　　　However, the reduction in the life of the cutter after each grinding may also cause some problems. In order to ensure the machining safety, only cutters with high safety factor can be used for machining, so users must use a monitoring and tracking system to replace the ground cutters in time. To solve this problem, the method is to use "use and throw" products, but it is usually very uneconomical to use disposable solid carbide cutters.

A new modular drill design effectively avoids the above problems. This drill features a replaceable tungsten carbide tip, which offers comparable cutting performance and tool life as efficient solid carbide drills. The tip is secured to the steel drill shank with a positive fit, without the use of a threaded connection or other methods that are difficult to operate on small diameter drills.

　　Since the grinding requirement is not considered in the design of the drill tip, the geometry of the drill tip can be optimized. A positive relief angle is adopted in the cross edge area of the drill tip to reduce the cutting force and improve the self-centering ability. Since the rigidity of the alloy steel drill body is reduced compared to the solid carbide drill, it is important to compensate for the rigidity of the drill bit by adopting a positive relief angle.

　　Unlike conventional hard alloy drill bits, the chip space of the modular drill bit does not have the same spiral angle from front to back, but uses a right-handed spiral in the front part of the chip space to accelerate chip flow, and a small negative spiral angle in the rear part of the chip space, which is particularly useful for increasing the stability of the drill bit and reducing vibration, and it is also very important for compensating the decrease in rigidity of the steel drill body.

　　Another difference between the modular and solid carbide bits is the location of the chip space at the bit bottom. On the modular bit, the chip space is located within the chip pocket and directly faces the front face of the cutting edge (on the solid carbide bit, the chip space is located on the side of the bit bottom). The importance of this design is that the front face of the cutting edge is usually the area with the higher temperature during machining and is in great need of effective cooling between the chip and the tool material. This chip space design not only optimizes the cooling effect but also applies a thermal shock to the chip, which helps improve chip control. Since the transverse edge is not directly exposed to the impact of the chip space, it is also conducive to the formation of chips in the transverse edge area at very low cutting speeds.

　　In addition to the consistency of tool life, another important advantage of using a modular drill is the significant reduction in tool inventory. When using conventional hard alloy drill bits, a large inventory of drill bits is required as many bits are constantly in the process of being regrind and retoothed. With disposable bits, the process of regrind and retooth is eliminated and the tool inventory is equal to the number of bits in machine (perhaps with a small number of spare bits on the tool rack).