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简体中文 карбид drilling tools are key instruments for hole machining in mechanical manufacturing. With the development of the manufacturing industry, the demand for hole machining precision continues to rise in scenarios such as aerospace connection holes, medical bone drilling, and microholes in electronic information circuit boards. Tungsten carbide, known for its high hardness, heat resistance, and corrosion resistance, is widely used to produce carbide drilling tools. Among them, twist drills (a common type of carbide drilling tool) are prone to microdefects during grinding, while polishing can reduce their surface roughness and friction, decrease cutting force and heat, optimize chip evacuation, enhance edge sharpness, and extend the service life of the tool.
Currently, research on tungsten carbide tool polishing mainly focuses on simple-shaped inserts. Related technologies have achieved a surface roughness of Ra=7.60 nm, and a material removal rate model for chemical mechanical polishing has been established. However, twist drills (as a typical type of carbide drilling tool) feature complex curved surfaces, making it difficult for traditional polishing to achieve uniform and high-quality machining, and relevant research on such tool polishing is relatively scarce.
Shear Thickening Polishing (STP) is a novel fluid polishing technology that uses non-Newtonian power-law fluid with shear thickening effect as the polishing slurry. It boasts excellent adaptability to curved surfaces, low cost, and high efficiency, and has been successfully applied in the machining of ceramics, monocrystalline silicon, alloys, and tungsten carbide inserts. Nevertheless, there are no reports on its application in processing the complex curved surfaces of twist drills (a type of carbide drilling tool).
This study conducts polishing experiments on tungsten carbide twist drills (a key category of carbide drilling tools) using the STP method. It analyzes the polishing principle of such tools, and explores the influence of polishing tank speed and workpiece speed on their surface quality by testing surface roughness, material removal rate, and surface morphology.
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Shear Thickening Polishing Principle of Carbide Drilling Tools
Figure 1 shows the structure of the twist drill and the force diagram of the margin and flank of the twist drill. When the polishing slurry flows through the twist drill (a carbide drilling tool) at a certain velocity v, ithe tool’s surface is greatly affected by the hydrodynamic force FH under high shear rate, and the hydrodynamic force can be decomposed into normal force Fn and tangential force Ft. The magnitude of normal force and tangential force on all points of the tool varies, resulting in different degrees of material removal. Therefore, the toowist drill needs to rotate simultaneously during polishing to achieve uniform surface quality.
Fig.1 Configuration of the twist drill and force diagram of the margin and flank of the twist drill
STP machining principle at the margin and flank of the twist drill (carbide drilling tool): During machining, the toowist drill rotates in the flowing polishing slurry, and there is relative motion between the polishing slurry and the flank and margin of the tool. The spiral groove of the twist drill is a double-helix structure.
When the tool rotates against its own spiral direction, it not only drives the fluid to perform circular motion against friction but also pushes the fluid to move axially, causing the polishing slurry to scratch the surface of the spiral groove. At this time, the polishing fluid in contact with the tool’s surface undergoes shear thickening under shear action, forming a high-viscosity polishing film. The dispersed phase and abrasive particles aggregate into “particle clusters”, which enhance the clamping effect of the dispersed phase on the abrasive particles, thereby generating large shear force. This shear force, combined with hydrodynamic pressure, acts on the tool’s surface, resulting in micro-cutting of the peaks on the workpiece surface to form chips and material removal, ultimately achieving precision machining of the tool.
Polishing Experiment Design
Polishing Experiment Device
An STP machining experimental platform was built on an existing grinding machine, as shown in Figure 2. The workpiece (carbide drilling tootungsten carbide twist drill) is clamped on the spindle of the grinding machine; theits horizontal position of the tool is adjusted through the x and y axes, and the depth of the tool immersedimmersion depth in the polishing slurry is controlled through the z axis. The outer diameter of the polishing tank D1=400 mm, inner diameter D2=170 mm, height H=120 mm, and the maximum rotating speed n1 of the polishing tank can reach 200 r/min.
Fig.2 STP processing experimental platform
When clamping the workpiece, the part of the tool immersed in the polishing slurryimmersed part reaches half of its working part. During machining, ithe tool rotates at a speed of n2. The schematic diagram of the clamping position of the twist drill (carbide drilling tool) is shown in Figure 3, which is located 15 mm away from the side of the polishing tank and 5 mm away from the bottom.
Fig.3 Schematic diagram of twist drill (carbide drilling tool) clamping position
Experimental Scheme Design
The workpiece used in the experiment was a YK30F tungsten carbide twist drill (a type of carbide drilling tool) with a diameter D=10 mm. The positions of the surface roughness measurement points of the tool are shown in Figure 4: point PA is at the flank 20 mm axially away from the chisel edge, point PB is at the margin at the same height, and point PC is at the spiral groove near the cutting edge. A Wyko NT9100 optical surface profiler was used to measure the surface roughness, and the average value was taken from three measurements. The initial surface roughness values of points PA, PB, and PC on the tool surface were (310±30) nm, (450±50) nm, and (270±30) nm, respectively.
Fig.4 Position of measuring point for surface roughness of twist drill (carbide drilling tool)
This experiment explores the influence of polishing tank speed n1 and workpiece speed n2 on the surface quality of points PA, PB, and PC on the tool. The experimental process parameters are shown in Table 1. The polishing tank speed and workpiece speed affect the shear rate and shear thickening effect of the polishing slurry, thereby influencing the polishing effect of the tool.
Результаты и обсуждение
Surface Roughness Analysis
Flank Surface Roughness
Figure 5 shows the influence of polishing speed parameters on Ra at point PA. After 60 minutes of machining, the Ra of the tool in the 7 groups of experiments ranged from 11 to 229 nm. As shown in Figure 5(a), with the increase of polishing tank speed, the Ra at point PA first decreases and then increases, reaching the minimum value of 120 nm when n1=90 r/min.
When the polishing tank speed is low, according to Formula (1), the shear rate of the polishing slurry is low, the shear thickening effect is not obvious, and it cannot provide high hydrodynamic force of the polishing slurry, leading to an unremarkable polishing effect on the tool. When the polishing tank speed increases, the shear thickening effect is enhanced, and Ra gradually decreases. However, when the polishing tank speed is too high, due to excessive centrifugal force, the particles in the fluid are increasingly close to the wall of the polishing tank, and fewer abrasive particles participate in polishing, resulting in a decrease in the surface roughness change rate ΔRa of the tool.
Fig.5 The influence of polishing speed parameters on the Ra at point PA
It can be seen from Figure 5(b) that the surface roughness at point PA decreases with the increase of workpiece speed. When the workpiece speed increases to 3500 r/min, the shear rate γ˙ exceeds 205 s⁻¹, and the shear stress is large. At this time, the polishing slurry will produce an obvious shear thickening effect, and a relatively large number of abrasive particles participate in polishing. After 60 minutes of machining, the Ra of the tool can reach the nanometer level, basically stabilizing at about 12 nm, and the maximum surface roughness change rate ΔRa of the tool reaches 96.77%.
Margin Surface Roughness
Figure 6 shows the influence of polishing speed parameters on Ra at point PB. As shown in Figure 6(a), when the workpiece speed is constant, the Ra at point PB first decreases and then increases with the increase of polishing tank speed, reaching the minimum value of 323 nm when the polishing tank speed is 90 r/min. As shown in Figure 6(b), Ra first decreases and then increases with the increase of workpiece speed, reaching the minimum value of 309 nm when the workpiece speed is 3500 r/min. Overall, after 60 minutes of polishing, the ΔRa at point PB (margin of the tool) in all 7 groups of experiments is less than 35%, which is at a relatively low level. Analysis of the tool structure shows that both the margin and the flank are cylindrical outer surfaces with little difference in curvature and spatial position, indicating that the significant difference in surface roughness changes between points PB and PA has no relation to their shape and position. Observation shows that the surface tool marks on the flank of the tool are spiral along the tool’s surface, while the tool marks on the margin are horizontal, which is exactly consistent with the movement direction of the abrasive particles during polishing. When the abrasive particles remove the peak material on the surface of point PB (margin), they also remove the material in the depressions, resulting in a small ΔRa.
Fig.6 The influence of polishing speed parameters on the Ra at point PB
Spiral Groove Surface Roughness
Figure 7 shows the influence of polishing speed parameters on Ra at point PC. Compared with point PB, the overall surface roughness change rate of point PC is slightly lower, ranging from 1.85% to 26.67%. The variation trend of Ra with polishing tank speed and workpiece speed is basically consistent with that of point PB. When the polishing tank speed increases to 90 r/min, Ra reaches the minimum of 224 nm; when the workpiece speed increases to 3500 r/min, Ra reaches the minimum of 193 nm.
Fig.7 The influence of polishing speed parameters on the Ra at point PC
Point PB is located at the spiral groove of the tool. When the tool rotates against the polishing slurry, the spiral groove hinders the flow of the polishing slurry, resulting in a decrease in the speed of the polishing slurry, a reduction in shear stress, and a significant weakening of the shear thickening effect. Thus, there is not enough cutting force to remove the workpiece material, leading to a low ΔRa.
Material Removal Rate Analysis
Figure 8 shows the influence of polishing time on the material removal rate under different polishing tank speeds and workpiece speeds. As shown in Figure 8(a), when the polishing tank speed increases, the relative speed of the polishing slurry to the tool increases, and the shear thickening effect of the polishing slurry is more obvious. The high shear rate of the fluid leads to a larger dynamic pressure of the abrasive particles on the tool. According to Formula (3), the material removal rate of the tool also increases. Overall, the material removal rate of the tool ranges from 0.025 to 0.250 mg/min, fluctuates with time, and decreases in the last 15 minutes of the polishing process.
Fig.8 Effect of polishing time on material removal rate under different rotating speeds of polishing groove and workpiece
As shown in Figure 8(b), the material removal rate of the tool increases with the increase of workpiece speed. When the workpiece speed is low (500 r/min and 2000 r/min respectively), the material removal rate of the tool is also low and changes little with time; when the workpiece speed is high (3500 r/min and 5000 r/min respectively), the material removal rate of the tool is high, reaching a maximum of 1.03 mg/min. The overall trend with time is slight fluctuation in the early stage and decrease in the later stage. When the workpiece speed is high, the relative speed between the polishing slurry and the tool increases significantly, the shear thickening effect is enhanced, and the dynamic pressure of the “particle clusters” on the tool increases. According to Formula (3), the material removal rate of the tool increases significantly. In the first 45 minutes, the peaks on the surface of the tool are removed, the surface roughness is greatly reduced, and the material removal rate of the tool is also high; after 45 minutes, the surface of the tool has become relatively flat, and at this time, greater pressure is needed to generate material removal, leading to a decrease in the material removal rate of the tool.
Analysis of Optimized Process Parameter Experiment Results
Through the previous analysis, the optimal combination of polishing process parameters is determined: polishing tank speed 90 r/min, workpiece speed 3500 r/min. After 60 minutes of polishing, the Ra values of points PA, PB, and PC reach 10 nm, 243 nm, and 15 nm respectively.
Figure 9 shows the 3D surface profile of the flank of the tool. Before machining, the flank surface of the tool is covered with directional uneven grooves generated by grinding, accompanied by obvious pit defects. After 60 minutes of polishing, the flank surface of the tool has become very smooth, the surface roughness has decreased to 10 nm, and the surface quality of the tool has been greatly improved.
Fig.9 3D surface profile of the flank of the twist drill (carbide drilling tool)
Figure 10 shows the surface topography of the cutting edge of the spiral groove of the tool. It can be seen from Figure 10 that the defects at the cutting edge of the spiral groove are removed, and the smoothness consistency is good. Figure 11 shows the comparison of the tool surface before and after polishing. It can be seen from Figure 11 that the polished toowist drill becomes very bright, indicating that shear thickening polishing has a significant polishing effect on the carbide drilling tools.
Fig.10 Surface topography on the cutting edge of twist drills (carbide drilling tools)
Fig.11 The contrast of twist drill (carbide drilling tool) surface before (left) and after (right) polishing
Выводы
1) The surface roughness of the tool first decreases and then increases with the increase of polishing tank speed, and first decreases and then basically remains unchanged or slightly increases with the increase of workpiece speed.
2) The material removal rate of the tool during polishing increases with the increase of полирование tank speed, and also increases with the increase of workpiece speed; the maximum material removal rate of the tool can reach 1.03 mg/min when machined at a workpiece speed of 3500 r/min.
3) Optimized polishing process parameters: polishing tank speed 90 r/min, workpiece speed 3500 r/min. After 60 minutes of polishing, the Ra values of the flank, margin, and spiral groove of the tool decrease from the initial 310 nm, 450 nm, and 270 nm to 10 nm, 243 nm, and 15 nm respectively.
4) Using the STP solution configured in this paper, high-surface-quality polishing of the carbide drilling tool is achieved by optimizing the polishing tank speed and workpiece speed.
