Research on Optimization of Universal Drill Bit
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Research on Optimization of Universal Drill Bit
1 Type of Drill
Physical drilling with a drill bit is called drilling. The most commonly used drill bit is a twist drill. The twist drill is characterized by two twisted spiral grooves for chip removal. The intersection of the spiral groove and the top cone of the drill bit constitutes the main cutting edge, and the side edge of the groove constitutes the secondary cutting edge. It plays the role of guiding support and finishing light.
The most commonly used materials are high-speed steel and cemented carbide. There are three types of cemented carbide drill bits: integral type, welded type and indexable blade type. Generally, the hardness and rigidity of high speed steel drills are low, and the cutting speed is about 30m/min or less, and the feed rate is about 0.25mm/r. Its low price and high toughness make it suitable for machine tools with low rigid speeds, such as general For drilling, turning, and milling machines, the hole diameter is wide, about Ø0.2 to Ø7.5mm. The drillable depth is often expressed as the ratio of the depth L and the diameter D, up to L/D=10, the processing size The accuracy is below IT1I level. The diameter range of solid carbide drills is usually Ø0.1 to Ø20mm, and the diameter of welded carbide drills is usually Ø8 to Ø40mm. The cutting speed is generally about 100m/mim, which is much higher than that of high speed steel drills. The volume is also slightly higher than that of HSS drills. The cutting efficiency of cemented carbide drills for external cutting fluid is about 3 to 5 times higher than that of HSS drills. The cutting fluid for internal cutting fluids can be 5 to 10 times higher. Carbide indexable inserts The drill is mainly used for the processing of medium-sized holes (about Ø20-Ø56mm). The benefit can be 3 times higher than that of high-speed steel drills. The machining accuracy of carbide drills can reach IT11 level or even IT9 level. Refer to Figure 1 for the diameter range and drillable depth L/D range of various drills of general structure.
Figure 1 Relationship Between Drilling Depth Ld and Bit DiameterIn order to give full play to its high efficiency, carbide drills are suitable for use on advanced and efficient machine tools such as CNC machine tools and machining centers. Although high-speed steel drills can be used on these machines, they have not played the role of advanced machines. Generally speaking, when emphasis is placed on machining efficiency, machining accuracy, drill bit life and machining batches, drill holes on CNC machine tools and machining centers and use carbide drills. When producing single pieces in small batches, special consideration should be given to the toughness in processing, and when the tool cost is expected to be as low as possible, high-speed steel drills can be used.
These types of drills, if they can drill most workpiece materials such as carbon steel, cast iron, stainless steel, non-ferrous metals and alloy steel, are called universal drills. Only suitable for a certain type of material efficiently and economically, a drill bit with specific processing requirements for a certain part is called a special drill bit. In order to meet the ever-increasing requirements for efficiency, high quality and environmental protection, universal drill bits and special drill bits are continuously improved on the basis of the original twist drill structure and shape.
2 Overview of Twist Drills
Although the structure of the twist drill can be used for both drilling and chip removal, it is widely used, but there are also some inherent problems. For example, the spiral groove surface at the front end constitutes the rake face of the drill bit, and the top angle cone surface constitutes the flank face. The intersection of the two faces is the main cutting edge. It is longer, has a wider chip width, and is more resistant to cutting forces. The cutting speed varies with the radius of each point on the main cutting edge. Since the rake face of the main blade is not a flat surface but a spiral space curved surface, the rake angle of each point of the main blade is unequal (see Figure 2).
Figure2 Different Rake Angles of the Main Cutting Edge of the DrillThe rake angle near the core is about -30°, which is difficult to cut, and the main cutting edge at the outer circle is about +30. The large rake angle is sharp, but the edge strength is poor. The cutting speed is the highest, the heat concentration temperature is high, and it is easy to damage. It can be seen that the actual cutting conditions and conditions at each point on the main blade are different. In general, the bit guide only relies on two blades, and the guiding performance is insufficient, and the secondary clearance angle at the blade is “zero”, where the cutting speed is maximum, the cutting heat is high, and it is easy to wear. In addition, the spiral groove made for chip removal, although the wider and deeper the chip removal, the better, but in order to ensure the strength of the drill bit, a certain thickness of the drill core is necessary, so that a certain width of the transverse edge must be made on the top of the drill bit , The rake angle at the transverse edge is negative, even reaching a 63° to a 54°, it can not be cut, in fact, it can only forcibly scrape and squeeze the workpiece, and because of its existence, it is difficult to drill the centering bit and drill 60% to 70% of the axial resistance is caused by the cross-edged part, which also affects its processing quality and service life.
In response to the shortcomings of twist drills, people in the industry have been trying to improve. In the 1960s, Chinese workers repaired the front, back, and lateral edges of the main cutting edge of high-speed steel twist drills. Slotting and chip separation reduced the axial force torque during processing by 35% to 50% and 10%, respectively A 30%. The drilling efficiency is greatly improved, and there are many different grinding shapes for different materials to be processed. The grinding of such shapes is more difficult, so a CNC group drilling and grinding machine was also developed.
3 New Materials, New Coatings, New Properties
The progress of the times requires higher and higher production efficiency. The inherent structural shape of the high-speed steel twist drill and the performance of the material itself can no longer meet the needs. Carbide drills capable of high-speed cutting have been widely used. Because the hardness of cemented carbide (1 310 to 1 880HV) is much higher than that of high-speed steel (880 to 910HV), it is difficult to apply sharpening like group drilling, in order to eliminate twist drills The shortcomings of the major companies have carried out a variety of new structural design and constantly optimize the materials and coatings. First of all, in order to make up for the lack of toughness of cemented carbide and improve its comprehensive mechanical properties, the material is generally changed from general cemented carbide to ultrafine cemented carbide, and the coating is changed from general TIN to generally adopted (Ai, Ti) N series In recent years, due to the continuous development of scientific technology such as CAE and other computer technologies, it can accurately simulate the stress, strain and temperature distribution field during drilling, chip curling and cutting, etc., plus CAD/CAM technology, The rapid progress of mold manufacturing technology, powder metallurgy technology, five-axis linkage processing and sharpening technology has designed and manufactured reasonable spiral grooves, front and rear blade faces, main and auxiliary blades and other shapes and structures. For example, the Wstar series of MWE/MWS external cooling/internal cooling drills launched by Mitsubishi Corporation (see Figure 3).
Figure 3 Mitsubishi Wstar Series Drill BitsDue to the redesign of the spiral groove surface of this series of drills, the main cutting edge is changed from linear to concave and convex, and the one close to the drill core is changed to a concave blade, so that the negative rake angle originally formed becomes a positive rake angle. Cutting sharpness. The main cutting edge on the near outer circle side is designed to be convex. It turned out that the positive rake angle was too large, and the strength of the cutting edge was low, so that it was appropriately reduced, thereby increasing the strength of the cutting edge. The structure of the concave and convex edge also makes the wide chip segment easy to discharge, and the cutting force is also reduced. In addition, the cone surface of the top angle of the drill bit is x-shaped, that is, the triple back angle is ground, so that the width of the transverse blade is “zero”, and the axial force is greatly reduced. With the triple back angle, the centering is convenient.
MWE/MWS bits do not need pre-drilled pilot holes, they can be drilled directly. A new spiral groove shape is designed, which is composed of two parts of large and small curvature. The small radius of curvature makes the chips curl, and the large radius of curvature makes the chips easy to discharge. The strength of the new geometry is calculated according to CAE. Compared with the previous solid carbide drills, the bending rigidity is increased by 20% and the torsional rigidity is increased by 30%. The two internal coolant holes of the drill do not reduce the rigidity of these two aspects.
In addition to the use of particulate cemented carbide to improve toughness and strength, the Wstar series is also coated with (Al, Ti) N coating with Miracle technology, with a hardness of 2 800HV, an oxidation starting temperature of 840CC, and a total power reduction of 15% during cutting. It can not only drill steel, cast iron and alloy steel smoothly, but also drill difficult materials such as stainless steel, heat-resistant alloy and alloy. On this basis, further improvements were made to develop long drills with drill depths up to L=30D, indexable insert drills and drills with replaceable cutter heads. The MEGA-ECU-DRILL series of MAPALMILLER of Germany has also made similar improvements. The coating is made of (Al, Ti) N coating and MxF coating with excellent drilling performance. A drill bit with a drilling depth of L=30D has also been developed with a minimum diameter of only 1.0mm. Due to their reasonable structure and excellent coating, these drill bits quickly discharge the chips from the cutting area without collecting heat, and they can achieve dry cutting without cutting fluid. They have become green bits that are environmentally friendly.
In recent years, Mitsubishi has improved the W star series drill bit. The new grade is wei MV E/MVS. Its coating grade is changed to wear resistance and versatility. In addition to aluminum and titanium, it also contains chromium. The DPIO2O of the layered compound nitride, the hardness is increased to 2 900HV, the oxidation resistance temperature is increased to I 1OOC, the outer surface of the coating adopts Zero-ji surface treatment technology, so that the blade edge and other parts are very smooth, the friction force is greatly reduced, and prevent The film layer is cracked and stable processing is achieved. At the same time, the original wave edge of MWE/MWS was further optimized, and the rigidity of the edge was enhanced while maintaining the original chip handling. In addition, the original circular internal coolant hole is changed to a triangular hole using TRI-cooling technology. This original hole shape can increase the cutting fluid flow rate and quickly remove the cutting heat. Use it to match the original MZE/MZS and other Several brand-name drill bits are compared with carbon steel, alloy steel, stainless steel, cast iron for external cooling, internal cooling, and MQL lubrication processing under the same cutting conditions, and the life of MVElMVS has been improved to varying degrees.
4 Add Double Blade Belt to Improve the Balance Guide Smoothing Performance
Figure 4 shows the structure of the MVS-type drill head, from which we can also see that the new grinding method removes the lateral edge, which reduces the axial force and expands the space for chip removal in front. The newly designed groove shape also optimizes the core and the helix angle, the strength of the drill bit is improved, and further efficient deep hole machining can be realized. In addition, the number of blades of twist drills is generally small, and the balance guide smoothing performance is insufficient. MVS/MVE additionally has a double blade band, which improves the performance in this respect, and also improves the machining accuracy and surface quality.
Figure 4 MVS Bit Structure Characteristics- A ) Z-shaped transverse blade grinding: The new Z-shaped transverse blade grinding, large chip removal space, to achieve low axial force.
- B ) New wave edge: high cutting edge strength and chip handling performance, achieving high-efficiency deep hole machining.
- C ) Double blade belt: excellent balance performance and higher precision.
- D ) New groove design: optimized core thickness and helix angle, high bit strength.
Figure 5 shows several kinds of drills developed by MAPALMILLER. They also made innovative improvements to the spiral flute and front and rear flank cutting edges, and paid special attention to adding auxiliary cutting edges to the blade back. For example, the MEGA-QUADRO series drills have 4 bits As the cutting edge of the secondary blade, it improves the guiding, machining stability, straightness and roundness of the drill hole, and the machining accuracy reaches 1T8. The GIGA series of drills also made 4 blades, which increased the guidance and support, but the distribution was different, and 2 auxiliary spiral grooves were added to allow more and better chip evacuation, machining roughness and unevenness. Degree is only 1/3 of ordinary drilling. Its MEGA-DRILL-REANER drill reamer has 46 blades. As a smoothing blade, it also plays the role of reaming, and can even achieve 7-level accuracy in one feed.
Figure 5 Several Improved Drill Bits5 Three-Blade Drill Improves Guidance and Stability
MAPALMILLER not only made more blade bands on the back of the drill bit, but also designed and manufactured a three-edged drill. That is, not only the number of auxiliary blades is increased, but the main blade is also increased (see Figure 6). The new special blade shape of the main blade helps to reliably center and cut people, and the guidance and stability are improved. As the number of blades increases, the spiral grooves also increase, and the chips become finely crushed and easily discharged. More grooves can be made shallower to make the core thicker and more rigid. The feed rate can also be increased. Intermittent holes and holes with inclined exits can also be processed. The machining accuracy and surface quality are improved. In addition to the general-purpose type, its three-edged drill is also specially suitable for steel processing and high-speed steel. The high-speed steel three-blade drill has a feed rate that is 50% higher than that of a normal two-blade drill, and the life is increased by a factor of four. There are also economically applicable three-edged drill bits with interchangeable cutter heads.
Figure 6 Three-edged drill bit of MAPALMILLER companyFigure 7 OSG’s three-blade drillOSG’s three-blade drill (see Fig. 7), in addition to the blade designed as a wavy edge that can break the chip finely, in order to reduce the cutting temperature, accelerate the chip removal and increase the chip evacuation groove appropriately, and adopt nano-scale composite EgiAs coating, hardness It reaches 3 200HV, and the oxidation start temperature reaches 1 100°C. In domestic tests, it was used to drill 14mm blind holes on the cast iron HT250 cylinder block. When the cutting speed was 100m/min, the feed rate of the two-edge drill was 0.15 m/r, the feed rate of the three-edge drill was 0.25 mm/r, and the feed speed was 702mm. /min and 1 170mm/min. It can be seen that the efficiency has improved a lot. In actual processing, the service life has also been improved. The three-blade drill can process 5,600 holes, and the two-blade drill can only process 4,800 holes. After regrind, the number of workable three-blade drills is still higher than that of the two-blade drills.
6 Conclusion
The above introduces several typical examples of the development of universal twist drills in recent years. The major companies have their own characteristics. In addition to the continuous development of general-purpose drill bits, special drill bits that specialize in efficient processing of steel, iron, stainless steel, aluminum alloys, hardened steel and composite materials are also being developed. It also has the performance characteristics of the specified material, and performs various aspects of the coating, blade shape, blade number, groove shape, groove number, helix angle, apex angle, cutting fluid hole shape, hole number and flow direction control of hard drills. To improve the design, the special drill bits developed are constantly updated and developed continuously.
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