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A recent survey of the German metalworking industry showed that drilling was the most time-consuming process in machining workshops. In fact, 36% of all machining hours are spent on hole machining operations. This corresponds to 25% turning time and 26% milling time. Therefore, adopting high performance solid carbide bits instead of high speed steel and common carbide bits can greatly reduce the time required for drilling and thus reduce the cost of hole processing. In the past few years, the cutting parameters (especially the cutting speed) have been increasing, especially the cutting speed of the high performance solid carbide bit has been increasing obviously. Twenty years ago, the typical cutting speed of solid carbide bits was 60 ~ 80m/min. Nowadays, it is not surprising to drill steel parts at a cutting speed of 200m/min, provided that machine tools can provide sufficient power, stability and coolant transport capacity. However, compared with the general cutting speed of turning or milling, drilling has a great potential to improve the machining efficiency.
The solid carbide bit has a high requirement for the toughness of the matrix, and the wear of the bit is acceptable under the condition of controllable and uniform stability. Therefore, a typical drilling tool grade contains more cobalt than a turning or milling tool grade. The bit material is usually made of fine grain cemented carbide to improve the cutting edge strength and ensure uniform wear without breakage. The cutting edge temperature is not too high, but the drill bit is required to have thermal impact resistance. The best bit grades are typically pure tungsten carbide without the need for large amounts of tantalum carbide or titanium carbide. For solid carbide bits, the coating must play a greater role than simply improving surface hardness and wear resistance. The coating must provide a thermal insulation between the tool and the workpiece material and remain chemically inert; The bonding between the workpiece material and the coating must be minimized to reduce friction. The coating surface must be as smooth as possible; In addition, the coating of twist drill must be resistant to crack diffusion. The dynamic characteristics of drilling may cause microcracks, which must be prevented from spreading in order to maintain tool life. By selecting the right coating process and forming the appropriate coating microstructure, the coating material can be placed under compressive stress, thus greatly extending the tool life. Good effect can be obtained by using multi - layer coating. The multilayer coating can prevent the diffusion of microcracks between the layers. Even if some coatings are damaged and peeled, other coatings can still protect the cemented carbide substrate. For drilling tools, the use of nano coatings and precise custom coatings also has great potential for development. For example, a new TiAlN nano-coating with TiN on top could solve many of the problems encountered in drilling stainless steel. The smooth TiN top coating reduces the bonding and friction between the tool and the workpiece material, while the lower TiAlN nano coating provides hardness and wear resistance of the tool. This coating has excellent crack-proof diffusivity and thermal shock resistance, and the cutting speed of stainless steel drilling can reach 70 ~ 80m/min, almost twice that of conventional drills.
In order to give full play to the excellent performance of modern carbide substrate and surface coating, it is necessary to optimize the design of bit geometry parameters and drill type, and to adjust the drill point, drill Angle, blade shape, cutting edge preparation, chip groove type, chip groove and the number of blade belt reasonably according to the processing purpose. High-efficiency cutting bits generally adopt one of the four drill tip geometry. Among them, the tetrahedral drill tip with transverse edge is easy to be ground and grinding tolerance is easy to be controlled. However, its center clearance is small, and when the feed quantity is large, the rear knife surface will contact the hole bottom, thus affecting the improvement of feed rate. The other is the tapered bit tip, which has a larger central clearance than the tetrahedral bit tip, so the axial thrust generated during drilling is smaller. However, the geometric shape of this kind of drill tip is more complex, so it is not easy to ensure the consistency of tool manufacturing and management. In addition to the above two types of drill tips, there are also optional auger tips, which can be divided into two different types: traditional auger tips with a chip drain, chip can be discharged from the center; The new auger tip grinds out the chip drain groove and the back cutter surface at the same time, thus eliminating the cutting step and further improving the chip flow. Because the center clearance of these two kinds of drill tips is larger than that of other drill tips, they have high feeding capacity. In addition, the new auger tip has high speed cutting capability and can be drilled with less axial thrust. The only drawback to this drill tip geometry is the complexity of the grinding process required to make the drill bit.
In addition to tool life and processing speed, another major factor to consider when choosing a bit is the quality of the hole. In recent years, how to reduce burr has become the focus of attention. Deburring is a typical manual process that costs a lot of money and can cause serious problems if not done properly. The solid carbide drill will exert great pressure on the workpiece material when it rotates and feeds at high speed. Therefore, large burr will be produced at the outlet of the through hole when using conventional drilling design or drilling tip Angle machining. To solve this problem, the simplest method is to increase the drill tip Angle to 135°~145°. The drill tip Angle within this range can generate a disk at the outlet of the hole and keep the workpiece material under tensile stress at all times, making it easy to cut rather than just pushing it out of the workpiece. Cutting edge preparation, top chamfering and other geometric parameter optimization measures will also play a significant role in reducing burrs. Completely different problems arise when drilling grey cast iron and ductile cast iron. At the outlet of the through-hole, the material is more likely to collapse than to form burrs. Material breakage not only affects workpiece quality, but also may cause drill bit damage. Chamfering at the top of the drill, designed specifically for iron casting, helps prevent material breakage by allowing the bit to come out of the work piece in a very smooth manner and keeping cutting until the last turn. The drilling tip design needs to be adjusted continuously according to the geometric parameters of the chip removal groove. The solid carbide bit has a high requirement for the toughness of the matrix, and the wear of the bit is acceptable under the condition of controllable and uniform stability. Therefore, a typical drilling tool grade contains more cobalt than a turning or milling tool grade. The bit material is usually made of fine grain cemented carbide to improve the cutting edge strength and ensure uniform wear without breakage. The cutting edge temperature is not too high, but the drill bit is required to have thermal impact resistance. The best bit grades are typically pure tungsten carbide without the need for large amounts of tantalum carbide or titanium carbide. For solid carbide bits, the coating must play a greater role than simply improving surface hardness and wear resistance. The coating must provide a thermal insulation between the tool and the workpiece material and remain chemically inert; The bonding between the workpiece material and the coating must be minimized to reduce friction. The coating surface must be as smooth as possible; In addition, the coating of twist drill must be resistant to crack diffusion. The dynamic characteristics of drilling may cause microcracks, which must be prevented from spreading in order to maintain tool life. By selecting the right coating process and forming the appropriate coating microstructure, the coating material can be placed under compressive stress, thus greatly extending the tool life. Good effect can be obtained by using multi - layer coating. The multilayer coating can prevent the diffusion of microcracks between the layers. Even if some coatings are damaged and peeled, other coatings can still protect the cemented carbide substrate. For drilling tools, the use of nano coatings and precise custom coatings also has great potential for development. For example, a new TiAlN nano-coating with TiN on top could solve many of the problems encountered in drilling stainless steel. The smooth TiN top coating reduces the bonding and friction between the tool and the workpiece material, while the lower TiAlN nano coating provides hardness and wear resistance of the tool. This coating has excellent crack-proof diffusivity and thermal shock resistance, and the cutting speed of stainless steel drilling can reach 70 ~ 80m/min, almost twice that of conventional drills. In order to give full play to the excellent performance of modern carbide substrate and surface coating, it is necessary to optimize the design of bit geometry parameters and drill type, and to adjust the drill point, drill Angle, blade shape, cutting edge preparation, chip groove type, chip groove and the number of blade belt reasonably according to the processing purpose.
High-efficiency cutting bits generally adopt one of the four drill tip geometry. Among them, the tetrahedral drill tip with transverse edge is easy to be ground and grinding tolerance is easy to be controlled. However, its center clearance is small, and when the feed quantity is large, the rear knife surface will contact the hole bottom, thus affecting the improvement of feed rate. The other is the tapered bit tip, which has a larger central clearance than the tetrahedral bit tip, so the axial thrust generated during drilling is smaller. However, the geometric shape of this kind of drill tip is more complex, so it is not easy to ensure the consistency of tool manufacturing and management. In addition to the above two types of drill tips, there are also optional auger tips, which can be divided into two different types: traditional auger tips with a chip drain, chip can be discharged from the center; The new auger tip grinds out the chip drain groove and the back cutter surface at the same time, thus eliminating the cutting step and further improving the chip flow. Because the center clearance of these two kinds of drill tips is larger than that of other drill tips, they have high feeding capacity. In addition, the new auger tip has high speed cutting capability and can be drilled with less axial thrust. The only drawback to this drill tip geometry is the complexity of the grinding process required to make the drill bit. In addition to tool life and processing speed, another major factor to consider when choosing a bit is the quality of the hole. In recent years, how to reduce burr has become the focus of attention. Deburring is a typical manual process that costs a lot of money and can cause serious problems if not done properly. The solid carbide drill will exert great pressure on the workpiece material when it rotates and feeds at high speed. Therefore, large burr will be produced at the outlet of the through hole when using conventional drilling design or drilling tip Angle machining. To solve this problem, the simplest method is to increase the drill tip Angle to 135° ~ 145°. The drill tip Angle within this range can generate a disk at the outlet of the hole and keep the workpiece material under tensile stress at all times, making it easy to cut rather than just pushing it out of the workpiece. Cutting edge preparation, top chamfering and other geometric parameter optimization measures will also play a significant role in reducing burrs. Completely different problems arise when drilling grey cast iron and ductile cast iron. At the outlet of the through-hole, the material is more likely to collapse than to form burrs. Material breakage not only affects workpiece quality, but also may cause drill bit damage. Chamfering at the top of the drill, designed specifically for iron casting, helps prevent material breakage by allowing the bit to come out of the work piece in a very smooth manner and keeping cutting until the last turn. The drilling tip design needs to be adjusted continuously according to the geometric parameters of the chip removal groove.
