Influence of the design of the hottest special hol

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The influence of the design of special hollow drill on cutting performance

hollow drill is a kind of hole machining tool more suitable for portable tools. However, due to the complex manufacturing process of hollow bits and the inability to process blind holes in the win-win western market, they are not widely used in metal cutting, and are usually only used when the power of through-hole or drilling equipment for processing some large-diameter or precious metal workpieces is limited. Since there are no standard products for hollow drills, most hollow drills used for processing special materials need to be developed by ourselves

we have developed a special hollow drill for users to process difficult materials. The processed material code is u-mn, and its main chemical components include: carbon (0.56% - 0.68%), manganese (1.35% - 1.65% 2. the pointer rotating shaft is dirty), silicon (0.2% - 0.35%), etc; When the tensile strength of the material is ≥ 882n/mm, the temperature rise of the former is much lower than the latter 2, and the hardness and wear resistance are higher. The drill is used to process 30+0.5mm through holes on 17mm thick materials. The portable drill has a power of 1000W and requires a drill life of 30min. The drill material is w6mo5gr4v2. In the process of developing the hollow bit, through repeatedly adjusting the design parameters of the bit and carrying out drilling tests, the geometric parameters of the bit are finally determined as: front angle g= 12 °, rear angle a= 9 °, auxiliary rear angle A1 = 3 °

the following is a brief analysis of the influence of hollow bit design on cutting performance

1 effect of rake angle change on cutting performance of drill

1) effect of rake angle on cutting force

the change of rake angle will affect the deformation degree of chip material, so as to change the cutting force. The larger the chip deformation, the greater the cutting force; The smaller the chip deformation, the smaller the cutting force. When the current angle changes in the range of 0 ° ~ 15 °, the change range of the cutting force correction coefficient is 1.18 ~ 1

2) influence of rake angle on bit durability

when the rake angle of the bit is increased, the strength and heat dissipation volume of the tip will be reduced, and the stress condition of the tip will be affected at the same time. When the current angle is positive, the tool tip is under tensile stress; When the front angle is negative, the tool tip is under pressure. If the selected rake angle is too large, it can increase the sharpness of the drill bit and reduce the cutting force, but the tensile stress on the tool tip is large, the strength of the tool tip is reduced, and it is easy to break. In the cutting test, many drill bits are damaged due to excessive rake angle. However, due to the high hardness and strength of the processed materials and the low rigidity of the main shaft and the whole machine of the portable drill, if the front angle is too small, the increase of cutting force during drilling will cause the main shaft to vibrate, obvious vibration lines will appear on the processing surface, and the durability of the drill bit will also be reduced

2 influence of the change of back angle on the cutting performance of drill bit

increasing the back angle can reduce the friction between the back face and the cutting material, and reduce the extrusion deformation of the machined surface. However, if the rear angle is too large, the strength and heat dissipation capacity of the blade will be reduced

the size of the back angle directly affects the durability of the drill bit. In the process of drilling, the main wear forms of drill bits are mechanical abrasion and phase change wear. Considering the mechanical abrasion, when the cutting life is certain, the larger the back angle is, the longer the available cutting time is; Considering the phase change wear, the heat dissipation capacity of the drill bit will be reduced with the increase of the back angle. After the drill bit is worn, as the wear zone of the back face is gradually widened, the cutting power is gradually increased, and the heat generated by friction will gradually increase, raising the temperature of the drill bit. When the temperature rises to the phase change temperature of the drill bit, the drill bit will wear rapidly

3 influence of bit design on grinding processing

hollow bits are used less and the processing batch is small. Therefore, the processing technology should be considered when designing bits, and the processing and grinding should be realized with common machining equipment and tools as far as possible

chips flow out through the rake face, so the shape of the rake face directly affects the chip shape and chip removal performance. The chip is squeezed and rubbed by the rake face in the outflow process, further causing deformation. The metal in the chip bottom layer deforms the most, and slides along the rake face, making the chip bottom layer longer, thus forming various curl shapes. When drilling with a hollow drill bit, it is hoped that the chips will become chips or banded chips to facilitate chip removal. In order to facilitate machining and grinding, the rake face must be designed as a plane without disconnecting the chip breaking groove. The rake face does not need to be reground in use

the flank of the hollow bit is the most easily reground surface and the surface with the fastest wear rate. Therefore, the sharpening of the hollow bit is realized by sharpening the flank

the auxiliary flank is divided into inner auxiliary flank and outer auxiliary flank. From the perspective of regrinding, regrinding the inner and outer auxiliary flanks is not easy to achieve, so the auxiliary flanks should be designed in the form of no regrinding

according to the above analysis, the blade of the hollow drill is designed as shown in Figure 1. Machining practice shows that the design can fully meet the requirements of use and tool regrinding

Figure 1 Schematic diagram of the blade design of hollow drill

4 the use of cutting fluid and its impact on the cutting performance of the drill

the main feature of hollow drill is that the inner core of the hole is not cut during machining, so the cutting amount of hollow drill is significantly reduced than that of fried dough twist drill, and the drill power required and the heat generated in cutting are also small

when drilling with high-speed steel hollow drill, because the temperature of the processing area has a great impact on the hardness of the drill, coolant must be used to cool down during the drilling process (if coolant is not used, the wear of the drill will be mainly phase change wear and rapid wear at the beginning). At first, we used the external spray cooling method, but because the bit station is processed in the horizontal axis direction, the coolant is not easy to enter the bit blade part, the coolant consumption is large, and the cooling effect is not ideal. By redesigning and changing the main shaft structure of the drill, the external spray cooling is changed into internal spray cooling, and the coolant is added from the core of the hollow drill bit, so that the coolant can smoothly reach the cutting part of the drill bit, thus significantly reducing the coolant consumption and improving the cooling effect

5 use effect of hollow drill

a well-designed hollow drill should meet the following requirements at the same time: ① it is easy to manufacture and can be processed by ordinary machine tools and general tools; ② It is convenient for re sharpening, and ordinary grinder can be used for sharpening; ③ High production efficiency and long service life; ④ Low price

the hollow bit we developed basically meets the above requirements. In practical use, the durability of the drill bit can stably reach 50 high nickel 3 yuan for King minutes, and the aperture tolerance and surface roughness meet the design requirements. Because it only needs to regrind the flank, the back angle of the drill bit is easy to control, and the grinding can be easily realized on the ordinary grinder. (end)

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