A method and fixture for mirror polishing of large-diameter straight tubes using abrasive flow

Abstract

The present invention belongs to the technical field of abrasive flow precision machining, and specifically relates to a method and fixture for abrasive flow mirror polishing of large-diameter straight tubes. The fixture comprises an upper flow guide assembly, a lower flow guide assembly, and a contoured core mold that matches the inner surface structure of the large-diameter straight tube. The large-diameter straight tube is sleeved on the outer periphery of the contoured core mold, and the two ends of the large-diameter straight tube are respectively nested in the groove-shaped step rings of the upper flow guide assembly and the lower flow guide assembly. The two ends of the contoured core mold are respectively fixedly connected to the upper flow guide assembly and the lower flow guide assembly via threads. Abrasive flow machining using the method of the present invention will not scratch the inner wall of the large-diameter straight tube or cause the coating to fall off. In addition, because an abrasive flow channel is formed between the outer surface of the contoured core mold and the inner cavity wall of the large-diameter straight tube, the abrasive flow area is reduced. When the abrasive passes through the channel, the pressure and flow rate increase due to the reduction in the flow area, and the shear stress increases, thereby achieving mirror polishing of the inner wall of the large-diameter straight tube by the abrasive flow.

Description

Technical Field

[0001] The invention belongs to the technical field of abrasive flow precision machining, and in particular relates to a method and a fixture for mirror polishing of a large-diameter straight tube with abrasive flow. Background Art

[0002] At present, the processing technology that can make the surface quality of parts Ra to 0.05μm ~ 0.8μm is called mirror polishing; mirror polishing can make the surface performance of parts more reliable and excellent. In the fields of defense industry, aerospace industry, microelectronics industry, biomedical engineering and household appliances, parts are expected to achieve mirror polishing.

[0003] With the advancement of science and technology and the manufacturing industry, there is an increasing demand for straight tubes with diameters greater than 60 mm, such as reactor tubes for chemical reactions, piston tubes for hydraulics, and waveguide tubes for communications. These tubes typically have aspect ratios greater than 10:1, and their inner cavities often contain micro-grooves and bosses. These straight tubes typically require a mirror finish with a surface roughness Ra of 0.05μm to 0.8μm to fully guarantee their performance. Furthermore, these straight tubes with diameters greater than 60 mm must also possess the corrosion resistance, high temperature, high pressure, oxidation resistance, radiation protection, insulation, and friction resistance required by the operating environment. Therefore, the inner surface of the straight tubes is sometimes treated with a special coating. The coating thickness and surface roughness are key factors affecting the performance of the part, and they also require a mirror finish with a roughness Ra of 0.05μm to 0.8μm.

[0004] Traditional polishing processes for large-diameter straight tubes primarily rely on mechanical grinding, such as honing and lapping. Mechanical grinding is characterized by micro-cutting and polishing between the tool and the workpiece surface, resulting in knife marks on the workpiece surface. The resulting roughness (Ra) is generally 0.2μm to 0.8μm, which cannot meet the surface roughness requirements of 0.05μm to 0.2μm under demanding conditions. Furthermore, for straight tubes coated with special properties, mechanical grinding tools can cause scratches on the coating on the tube walls, reducing the thickness and uniformity of the coating and even partially peeling the coating from the substrate, thereby affecting the coating's performance. Furthermore, large-diameter straight tubes containing micro-grooves and step structures cannot be machined by mechanical grinding tools due to limited tool accessibility. Other common specialty machining methods include chemical / electrochemical polishing, ultrasonic polishing, and magnetic fluid polishing. The polishing fluid used in chemical / electrochemical polishing can corrode parts and have adverse effects on the environment. It is also selective with materials, making most materials inaccessible to chemical / electrochemical polishing techniques. Due to the low hardness of the magnetic needle used in magnetic fluid polishing, it cannot polish materials harder than the needle. This technique is particularly useful for polishing straight tubes made of different materials, polishing tubes with coatings on the surface, and polishing large-diameter straight tubes with micro-grooves and step structures inside. The aforementioned polishing techniques have significant limitations when achieving a mirror-finished tube surface roughness Ra of 0.05μm to 0.8μm.

[0005] Abrasive flow polishing technology utilizes a viscoelastic creep fluid in a semi-flowing state to flow back and forth across the machined surface of a part under a certain pressure, and utilizes the extrusion and grinding effect of its abrasive particles to polish the material surface. It is a flexible micro-cutting precision polishing technology that can meet the polishing requirements of a surface roughness Ra of 0.05μm to 0.8μm for polished parts. Abrasive flow technology belongs to physical grinding and is therefore basically not restricted by the material to be processed. Abrasive flow polishing technology can ensure polishing efficiency and economy, and the processing process is automated, quantitative, and controllable. Because its processing medium is fluid abrasive, it can polish the inner surface of special-shaped cavities and is now widely used for polishing the inner cavities of pipes. Abrasive flow polishing technology can also select different abrasive media for polishing according to the material of the pipe, thereby obtaining better polishing effects.

[0006] However, the following problems exist when using abrasive flow machining technology to mirror-polish the inner wall of large-diameter straight pipes. The abrasive flow polishing equipment uses two plunger pumps to push the abrasive medium in the material cylinder to flow back and forth through the inner wall of the pipe at a certain pressure and speed to achieve polishing. In order for the abrasive particles to produce a micro-cutting effect on the pipe wall when flowing through the pipe, the pressure and speed of the fluid inside the pipe must be in a high-pressure and high-speed state. However, for straight pipe parts with a diameter greater than 60mm, after the fluid enters the pipe, due to the large flow area, the flow rate and pressure attenuation are very large, so the abrasive particles in the fluid can hardly produce a micro-cutting polishing effect on the inner wall surface [CN202211343679.5]. If abrasive flow technology is to achieve mirror polishing of Ra=0.05μm~0.8μm for straight pipe parts with a diameter greater than 60mm, it is necessary to increase the thrust of the plunger pump and thus increase the pressure and flow rate of the polishing medium fluid. Currently, the maximum plunger pressure can reach 35MPa [CN115519466A]. For straight pipes with diameters greater than 60mm, increasing the flow rate and pressure of the fluid and abrasive to the critical values ​​for micro-cutting is difficult and uneconomical. Another issue is that as the abrasive flow polishing medium moves through the pipe, especially when the pipe aspect ratio is greater than 10:1, the pressure decays along the pipe's length. The polishing medium exerts high extrusion and chipping forces at the pipe's ends, while the extrusion and chipping forces in the middle of the pipe are lower, resulting in inconsistent polishing results between the pipe's interior and ends, and even deformation of the pipe's ends. For large-diameter straight pipes with internal microgrooves and bosses, the movement of the abrasive flow polishing medium within the pipe causes changes in the pipe's cross-sectional area and flow area due to the microgrooves and bosses, ultimately leading to inconsistent polishing results for the grooves and bosses. A final issue is that the abrasive particles in the abrasive flow exert a strong extrusion and cutting force on the pipe wall. When a coating is attached to the pipe wall, excessive abrasive pressure and flow rate can cause some scratching of the coating on the straight pipe wall, even partially peeling the coating from the substrate. Therefore, developing a method and fixture for mirror polishing large-diameter straight pipes with abrasive flow is of great significance. Summary of the Invention

[0007] The purpose of the present invention is to provide a method and a fixture for mirror polishing of a large-diameter straight tube abrasive flow.

[0008] The implementation process of the present invention is as follows:

[0009] A fixture for abrasive flow mirror polishing of a large-diameter straight tube comprises an upper flow guide assembly, a lower flow guide assembly, and a profiling core mold matching the inner surface structure of the large-diameter straight tube; the centers of the upper flow guide assembly and the lower flow guide assembly are both provided with internal threaded holes for fixing the profiling core mold; the planes of the upper flow guide assembly and the lower flow guide assembly are provided with annular feed through holes, the annular feed through holes of the upper flow guide assembly and the annular feed through holes of the lower flow guide assembly are positioned correspondingly and are on the same axis; the lower surface of the upper flow guide assembly and the upper surface of the lower flow guide assembly are both provided with groove-shaped step rings for fixing the large-diameter straight tube; the profiling core mold is a smooth profiling columnar structure, the height of the profiling core mold is higher than the height of the large-diameter straight tube, and both ends of the profiling core mold are provided with external thread structures matching the internal threaded holes;

[0010] The large-diameter straight pipe is sleeved on the outer circumference of the profiling core mold, and the two ends of the large-diameter straight pipe are respectively nested in the groove-shaped step rings of the upper guide component and the lower guide component. The profiling core mold and the large-diameter straight pipe are located on the same axis, and the two ends of the profiling core mold are respectively fixedly connected to the upper guide component and the lower guide component by threads; a polymer nylon material gasket is provided on the contact surface of the step ring with the large-diameter straight pipe.

[0011] Furthermore, the height of the profiling core mold is 10 to 20 mm higher than that of the large-diameter straight tube.

[0012] Furthermore, the material of the profiling core mold is 45 steel, and a polymer nylon material layer is provided on the periphery of the profiling core mold.

[0013] Furthermore, the upper guide assembly and the lower guide assembly are both made of 45 steel.

[0014] The method for abrasive flow mirror polishing of a large-diameter straight tube using the above-mentioned fixture comprises the following steps:

[0015] (S1) Structural assembly;

[0016] A profiling core mold is machined to match the inner surface structure of the large-diameter straight pipe. The large-diameter straight pipe is sleeved on the outer circumference of the profiling core mold, and an abrasive flow channel is formed between the outer surface of the profiling core mold and the inner wall of the large-diameter straight pipe. The two ends of the large-diameter straight pipe are respectively embedded in the groove-shaped step rings of the upper guide assembly and the lower guide assembly. The profiling core mold and the large-diameter straight pipe are located at the same axis, and the two ends of the profiling core mold are respectively fixedly connected to the upper guide assembly and the lower guide assembly through threads.

[0017] (S2) Maximum gap d between large diameter straight tube and contoured core die max ;

[0018] Assume that the inner radius of the large diameter straight tube is R, the radius of the profiling core die is r, and the gap d = Rr has an upper limit d max , when d>d maxWhen the fluid enters the straight pipe with a diameter greater than 60 mm, due to the large flow area, the flow velocity and pressure attenuation are large, so the abrasive particles in the fluid cannot produce micro-cutting polishing on the inner wall surface, and the polishing requirement of the inner wall roughness Ra of the polished pipe cannot be achieved to be 0.05μm to 0.2μm. It is necessary to find the maximum gap value d that meets the requirement of the part surface Ra of 0.05μm to 0.2μm. max , d max Through engineering tests, it was found that when the abrasive particles are 100-400 mesh, d max =0.4R; when the abrasive is 400-1000 mesh, d max =0.35R; when the abrasive is 1000-2000 mesh, d max =0.3R; when the abrasive mesh number is higher and the particle size is smaller, the passability is stronger in the same flow area, the abrasive pressure is lower, the grinding force is weaker, and the gap value d max It needs to be reduced accordingly to reduce the flow area and maintain sufficient abrasive pressure;

[0019] (S3) Minimum gap d between large diameter straight tube and profiled core mold min ;

[0020] Assume that the inner radius of the large diameter straight pipe is R, the outer diameter of the profiling core is r, and the gap d = Rr has a lower limit d min , when d <d min When the fluid enters the straight pipe with a diameter greater than 60 mm, the flow area is too small, and the abrasive flow velocity and pressure are very high. This will cause excessive grinding and polishing of the inner wall of the pipe, resulting in scratches on the inner wall of the pipe, and the polishing requirement of the inner wall roughness Ra of 0.05μm to 0.2μm after polishing cannot be achieved. At the same time, the abrasive has a strong extrusion and cutting force on the inner wall of the pipe. When the coating adheres to the inner wall of the pipe, the high abrasive pressure and flow velocity will cause scratches or partial peeling of the coating on the inner wall of the pipe. It is necessary to find the minimum gap d that will not cause scratches. min； d min Through engineering tests, it was found that when the abrasive particles are 150-500 mesh, d min =0.15R; when the abrasive is 500-1200 mesh, d min =0.1R; when the abrasive is 1200-1800 mesh, d min =0.05R; when the abrasive mesh number is higher and the particle size is smaller, the flow rate is stronger under the same flow area, and the gap value d min It needs to be reduced accordingly to maintain sufficient abrasive pressure;

[0021] (S4) Abrasive particles enter from the annular abrasive particle through hole of the upper guide assembly, flow through the outer surface of the profiling core mold and the inner wall of the large-diameter straight tube to form an abrasive flow channel, and then flow out from the annular abrasive particle through hole of the lower guide assembly to the lower abrasive cylinder of the abrasive flow machine tool to complete the downstroke; similarly, the abrasive particles enter the upper abrasive cylinder from bottom to top to complete the upstroke, thereby realizing reciprocating processing.

[0022] Furthermore, the preferred material of the abrasive is aluminum oxide or silicon carbide. The purpose is that cutting can only occur when the hardness of the abrasive is higher than that of the workpiece, but if the hardness is too high, it is easy to scratch the surface of the pipe. Therefore, the selection of abrasive materials should follow: the hardness of the abrasive should be greater than the hardness of the workpiece, and less than the hardness of the material corresponding to the scratching of the inner wall of the pipe. The material of the large-diameter pipe fitting is titanium alloy (TC4) with a Mohs hardness of 7. Among the four alternative abrasives, the Mohs hardness of silicon oxide is 7, that of aluminum oxide is 8.8, that of silicon carbide is 9.2, and that of cubic boron nitride is 10. Therefore, the polishing efficiency is low when silicon oxide is used as an abrasive. It is also known through engineering tests that under the same predetermined feed pressure, cubic boron nitride will scratch the inner wall of the pipe when used as an abrasive. Therefore, aluminum oxide and silicon carbide are preferred materials for achieving mirror polishing of large-diameter pipe fittings.

[0023] Furthermore, different mirror polishing requires different abrasive polishing sequence adjustments, and the specific polishing sequence is selected according to the Ra requirement; when achieving mirror polishing of Ra = 0.05μm to 0.2μm, it is necessary to first polish to Ra = 0.8μm, and then continue to polish step by step from Ra = 0.8μm to achieve a higher level mirror polishing of Ra = 0.05μm; the specific step-by-step polishing sequence is first using abrasives with a mesh size of 100 mesh to 400 mesh to achieve Ra = 0.2μm after polishing, then using abrasives with a mesh size of 400 mesh to 1000 mesh to achieve Ra = 0.1μm to 0.07μm after polishing, and finally using abrasives with a mesh size of 1000 mesh to 2000 mesh to achieve Ra = 0.05μm to 0.07μm after polishing.

[0024] Furthermore, the viscosity Cp of the abrasive particles is in the range of 500 ≤ Cp ≤ 1000. Too high a viscosity may cause the abrasive flow to clog the gap and fail to polish, while too low a viscosity may cause the abrasive flow to flow freely through the gap without exerting significant pressure on the wall, thus failing to produce effective polishing.

[0025] Furthermore, for large-diameter straight tubes with internal microgrooves and bosses, the profiling core mold should also retain corresponding bosses and grooves to maintain a consistent gap d. This is intended to prevent the abrasive flow polishing medium from moving through the tube, causing the grinding force to vary with changes in the tube's cross-sectional area (flow area). If the profiling core mold is a straight rod, the gap d will inevitably change axially with the concave-convex structure of the large-diameter straight tube, resulting in inconsistent internal roughness after polishing.

[0026] Furthermore, for large-diameter straight pipes with an aspect ratio greater than 10, the diameter of the profiling core should vary linearly to ensure that the gap d is large at the ends of the large-diameter straight pipe and small in the middle. This variation is intended to ensure that the abrasive flow polishing medium, as it moves through the pipe, is continuously attenuated by the resistance along the pipe. This results in a high extrusion and chipping force at the pipe ends, while a low extrusion and chipping force in the middle of the pipe leads to a weaker polishing effect in the middle compared to the pipe ends. Therefore, at a predetermined feed pressure of P, a large gap at the pipe ends slightly reduces the abrasive pressure and flow rate, while a small gap in the middle slightly increases the abrasive pressure and flow rate, thereby achieving a consistent polishing effect at both the pipe ends and the middle.

[0027] The basic design idea is: insert a profiling core mold with a radius of r into a large-diameter straight pipe with a radius of R, and the gap between the profiling core mold and the inner wall of the part is d = Rr. If d is too large, the pressure and flow rate of the abrasive particles are too small, and the mirror polishing requirement of the inner wall roughness of the polished pipe fitting Ra = 0.05μm ~ 0.2μm cannot be achieved; if d is too small, the pressure and flow rate of the abrasive particles are too large, which will cause scratches on the inner wall of the pipe or peeling of the coating, resulting in a decrease in the dimensional accuracy of the part. Therefore, it is necessary to find a suitable gap value d so that after the pipe fitting and the profiling core mold are processed under this gap value, they can meet the inner wall roughness Ra = 0.05μm ~ 0.2μm without causing scratches on the inner wall or peeling of the coating. The upper limit of the gap d is d max That is, the gap corresponding to the inner wall roughness Ra≤0.2 of the pipe fitting, and the lower limit of the gap d min The gap is just enough to avoid scratching the inner wall or peeling off the coating. min <d<d max , the mirror polishing of the inner wall of the pipe by abrasive particles can be achieved.

[0028] Positive effects of the present invention:

[0029] The method of the present invention requires processing a profiling core mold that matches the structure of a large-diameter straight tube, and forming an abrasive flow channel between the outer surface of the profiling core mold and the inner cavity wall of the large-diameter straight tube to reduce the abrasive flow area. When the abrasive particles pass through the channel, the pressure and flow rate increase due to the reduction in the flow area, and the shear stress increases, thereby achieving mirror polishing of the inner wall of the large-diameter straight tube by the abrasive flow. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Exploded view of the assembly for abrasive flow polishing of large diameter straight pipes;

[0031] Figure 2 This is a schematic cross-sectional view of the abrasive flow polishing process for a large-diameter straight pipe;

[0032] Figure 3 is a perspective view of the upper guide assembly;

[0033] Figure 4 is a three-dimensional diagram of the lower guide assembly;

[0034] Figure 5 It is a three-dimensional diagram of the profiling core mold;

[0035] Figure 6 It is a stereogram of a large-diameter straight pipe;

[0036] Figure 7 The microscope image of the inner wall of the straight tube after polishing when d = 8 mm, the left is the original image, and the right is a partial image magnified 50 times;

[0037] Figure 8 The microscope image of the inner wall of the straight tube after polishing when d = 18 mm, the left is the original image, and the right is a partial image magnified 50 times;

[0038] Among them, 1 is an upper guide assembly, 2 is a polymer nylon material gasket, 3 is a contoured core mold, 4 is a large-diameter straight pipe, 5 is a lower guide assembly, 6 is an external thread structure, 7 is abrasive, 8 is an annular feed hole, 9 is an internal thread hole, and 10 is a step ring. DETAILED DESCRIPTION

[0039] The present invention will be further described below with reference to the embodiments.

[0040] The method of the present invention ensures that the machining gap between the large-diameter straight tube and the profiling mandrel is within a certain suitable range. Within this range, abrasive flow machining will not scratch the inner wall of the large-diameter straight tube or cause the coating to fall off. In addition, an abrasive flow channel is formed between the outer surface of the profiling mandrel and the inner wall of the large-diameter straight tube, thereby reducing the flow area of ​​the abrasive particles. When the abrasive particles pass through this channel, the pressure and flow rate increase due to the reduced flow area, and the shear stress increases, thereby achieving a mirror polishing of the inner wall of the large-diameter straight tube by the abrasive flow.

[0041] Example 1 Abrasive Flow Mirror Polishing Fixture

[0042] A fixture for abrasive flow mirror polishing of large diameter straight tubes, see Figures 1 to 6 , including an upper guide component 1, a lower guide component 5, and a profiling core mold 3 that matches the inner surface structure of a large-diameter straight pipe 4; the upper guide component 1 and the lower guide component 5 are both made of 45 steel; the centers of the upper guide component 1 and the lower guide component 5 are each provided with an internal threaded hole 9 for fixing the profiling core mold 3, and an annular feed through hole 8 is provided on the plane of the upper guide component 1 and the lower guide component 5. The annular feed through hole 8 of the upper guide component 1 and the annular feed through hole 8 of the lower guide component 5 correspond to each other and are on the same axis. The lower surface of the upper guide assembly 1 and the upper surface of the lower guide assembly 5 are both provided with a groove-shaped step ring 10 for fixing the large-diameter straight pipe 4; the profiling core mold 3 is a smooth profiling columnar structure, the height of the profiling core mold 3 is 10 to 20 mm higher than the height of the large-diameter straight pipe 4, the profiling core mold 3 is made of 45 steel, and the periphery of the profiling core mold 3 is provided with a polymer nylon material layer to prevent the large-diameter straight pipe 4 from being bumped during the clamping process. Both ends of the profiling core mold 3 are provided with external thread structures 6 that match the internal thread holes 9;

[0043] The large-diameter straight tube 4 is sleeved on the outer circumference of the profiling core mold 3, and the two ends of the large-diameter straight tube 4 are respectively nested in the groove-shaped step rings 10 of the upper guide component 1 and the lower guide component 5. The profiling core mold 3 and the large-diameter straight tube 4 are located on the same axis, and the two ends of the profiling core mold 3 are respectively fixedly connected to the upper guide component 1 and the lower guide component 5 by threads; the step ring 10 is provided with a polymer nylon material gasket 2 on the contact surface with the large-diameter straight tube 4 to prevent the clamp from causing pressure damage and scratches to the large-diameter straight tube 4.

[0044] The height of the profiling core mold 3 is 10 to 20 mm higher than that of the large-diameter straight pipe 4. This is because when the abrasive flow polishing medium moves in the pipe, the pressure will continuously decrease with the resistance along the pipe. The polishing medium has the greatest extrusion force and cutting force at the pipe inlet, which can cause over-polishing or even deformation at the pipe end. Therefore, a 10 to 20 mm buffer zone is required when the polishing medium flows into the large-diameter straight pipe. The profiling core mold 3 is made of 45 steel, and a polymer nylon material layer is provided on the periphery of the profiling core mold 3. This is to prevent scratches and dents on the pipe wall caused by collision and sliding between the pipe and the profiling core mold during pipe disassembly. The upper guide assembly 1 and the lower guide assembly 5 are both made of 45 steel. This is because 45 steel is easy to machine.

[0045] Example 2 Abrasive Flow Mirror Polishing Method

[0046] The method for mirror polishing a large-diameter straight tube using the above-mentioned fixture comprises the following steps:

[0047] (S1) Structural assembly;

[0048] A profiling core mold 3 is machined to match the inner surface structure of the large-diameter straight tube 4. The large-diameter straight tube 4 is sleeved on the outer circumference of the profiling core mold 3, and an abrasive flow channel is formed between the outer surface of the profiling core mold 3 and the inner wall of the large-diameter straight tube 4. The two ends of the large-diameter straight tube 4 are respectively embedded in the groove-shaped step rings 10 of the upper guide component 1 and the lower guide component 5. The profiling core mold 3 and the large-diameter straight tube 4 are located on the same axis. The two ends of the profiling core mold 3 are respectively fixedly connected to the upper guide component 1 and the lower guide component 5 by threads.

[0049] (S2) Maximum gap d between the large-diameter straight tube 4 and the contoured core mold 3 max ;

[0050] Assume that the inner radius of the large diameter straight tube is R, the radius of the profiling core die is r, and the gap d = Rr has an upper limit d max , when d>d max When the fluid enters the straight pipe with a diameter greater than 60 mm, due to the large flow area, the flow velocity and pressure attenuation are large, so the abrasive particles in the fluid cannot produce micro-cutting polishing on the inner wall surface, and the polishing requirement of the inner wall roughness Ra of the polished pipe cannot be achieved to be 0.05μm to 0.2μm. It is necessary to find the maximum gap value d that meets the requirement of the part surface Ra of 0.05μm to 0.2μm. max , d max Through engineering tests, it was found that when the abrasive particles are 100-400 mesh, d max =0.4R; when the abrasive is 400-1000 mesh, d max =0.35R; when the abrasive is 1000-2000 mesh, d max =0.3R; when the abrasive mesh number is higher and the particle size is smaller, the passability is stronger in the same flow area, the abrasive pressure is lower, the grinding force is weaker, and the gap value d max It needs to be reduced accordingly to reduce the flow area and maintain sufficient abrasive pressure.

[0051] The critical mesh size of abrasive particles is obtained by the following method: first, the mesh size of abrasive particles is roughly set into 3 levels, large mesh, medium mesh, and small mesh. When the large mesh value is 200 mesh, a d max , then keep d max The grinding and polishing results are tested at around 200 meshes through engineering tests. When the grinding and polishing result Ra corresponding to the selected mesh number is not within the range of 0.05μm to 0.2μm, the corresponding mesh number is the critical mesh number. The method for obtaining the critical mesh value in the medium and small mesh numbers is the same.

[0052] d maxThe result is obtained by the following engineering test method: when the feed pressure is P and the abrasive mesh size is 200 mesh, an initial gap value d is given, and the roughness Ra of the part is tested. If Ra≤0.2, the given initial gap value is d max If Ra>0.2, then reduce the gap step by step until Ra≤0.2, then the gap value used this time is d max Assuming k = r / R, the engineering test data are as follows:

[0053] k 0.3 0.4 0.5 0.6 Ra(μm) 0.92 0.67 0.25 0.08

[0054] When k is 0.6, r=0.6R, that is, d=Rr=0.4R, Ra≤0.2, then d max =0.4R.

[0055] Then, the gap between the large-diameter straight tube and the profiling die core is kept at 0.4R, and the abrasive mesh is gradually increased or decreased at about 200 meshes. The roughness Ra of the inner wall of the large-diameter straight tube after grinding and polishing is measured, and d is obtained. max =0.4R, the critical value of the abrasive mesh size is 100 mesh to 400 mesh, and the engineering test data are as follows:

[0056] Abrasive grain size 50 100 150 200 300 350 400 450 Ra(μm) 0.26 0.13 0.097 0.08 0.062 0.096 0.16 0.23

[0057] (S3) Minimum gap d between large diameter straight tube 4 and profiling core mold 3 min ;

[0058] Assume that the inner radius of the large diameter straight pipe is R, the outer diameter of the profiling core is r, and the gap d = Rr has a lower limit d min , when d <d min When the fluid enters the straight pipe with a diameter greater than 60 mm, the flow area is too small, and the abrasive flow velocity and pressure are very high. This will cause excessive grinding and polishing of the inner wall of the pipe, resulting in scratches on the inner wall of the pipe, and the polishing requirement of the inner wall roughness Ra of 0.05μm to 0.2μm after polishing cannot be achieved. At the same time, the abrasive has a strong extrusion and cutting force on the inner wall of the pipe. When the coating adheres to the inner wall of the pipe, the high abrasive pressure and flow velocity will cause scratches or partial peeling of the coating on the inner wall of the pipe. It is necessary to find the minimum gap d that will not cause scratches. min； d min Through engineering tests, it was found that when the abrasive particles are 150-500 mesh, d min =0.15R; when the abrasive is 500-1200 mesh, d min =0.1R; when the abrasive is 1200-1800 mesh, d min =0.05R; when the abrasive mesh number is higher and the particle size is smaller, the flow rate is stronger under the same flow area, and the gap value d min It needs to be increased accordingly to maintain sufficient abrasive pressure;

[0059] d min The following engineering test method is used to obtain: when the feed pressure is predetermined to be P and the abrasive mesh size is 200 meshes, at the maximum gap value d max =0.4R, gradually reduce the gap value until scratches appear on the surface of the part, then the previous gap value is the minimum gap value d min , the engineering test data are as follows:

[0060] k 0.7 0.75 0.8 0.85 0.9 Are there scratches? No scratches No scratches No scratches No scratches Scratches appear

[0061] When k is 0.85, r = 0.85R, that is, d = Rr = 0.15R, which is the minimum gap that will not scratch the inner wall. Then d min =0.15R.

[0062] Then, the gap between the large-diameter straight tube and the profiling die core is maintained at 0.15R, and the abrasive mesh is gradually increased or decreased at about 200 meshes. The roughness of the inner wall surface of the large-diameter straight tube after grinding and polishing and whether scratches appear are measured to obtain d min =0.15R, the critical value of the abrasive mesh size is 150 mesh to 500 mesh, and the engineering test data are as follows:

[0063] Abrasive grain size 100 150 200 300 400 500 550 Are there scratches? Scratches appear No scratches No scratches No scratches No scratches No scratches No scratches Ra(μm) 0.27 0.12 0.067 0.083 0.105 0.16 0.24

[0064] Commonly used abrasives are silicon oxide, aluminum oxide, silicon carbide or cubic boron nitride.

[0065] In order to achieve the best effect of abrasive flow mirror polishing of the inner wall of the large-diameter straight pipe 4, it is necessary to screen the type of abrasive particles, the particle size of the abrasive particles and the viscosity of the abrasive particles.

[0066] (I) Preferred types of abrasive grains

[0067] First, the abrasive types are compared and selected. The hardness of the abrasive must be higher than that of the workpiece in order for cutting to occur, but excessive hardness can easily scratch the surface of the pipe. Therefore, the selection of abrasive materials should follow the following principles: the abrasive hardness must be greater than the hardness of the workpiece, and less than the material hardness corresponding to the scratches on the inner wall of the pipe. The large-diameter pipe fitting material described in the present invention is titanium alloy (TC4), with a Mohs hardness of 7. As shown in Table 1, the Mohs hardness of silicon oxide is also 7, so the polishing efficiency is low when silicon oxide is used as an abrasive. Through engineering tests, it is known that under the same predetermined feed pressure, cubic boron nitride will scratch the inner wall of the pipe when used as an abrasive. Therefore, aluminum oxide and silicon carbide are the preferred materials for achieving mirror polishing of large-diameter pipe fittings.

[0068] Table 1 Comparison of hardness of four abrasive materials

[0069] Abrasive materials Silicon oxide Alumina Silicon carbide Cubic boron nitride Mohs hardness 7 8.8 9.2 10

[0070] (II) Preferred particle size of abrasive grains

[0071] If the abrasive particle size is too small, the cutting ability is poor and it is impossible to achieve mirror polishing of Ra=0.05μm~0.2μm. If the abrasive particle size is too large, it will block the flow channel and reduce the polishing efficiency. As Ra becomes larger, it is also impossible to achieve mirror polishing. Therefore, there is an optimal range for the particle size of the abrasive particles. Within this range, a mirror polishing effect of Ra=0.05~0.2 can be achieved. Through engineering tests, it is concluded that: under the conditions of a predetermined feed pressure P and a processing gap d (d is obtained by the method described in steps (S2) and (S3)), an initial particle size Φ is given, and the surface roughness Ra of the workpiece is measured. If Ra<0.2, the particle size is gradually reduced until Ra>0.2. The previous particle size value is the minimum particle size Φ for achieving mirror polishing of the workpiece. min Similarly, Φ max Also obtained by the above method, given an initial particle size Φ, the surface roughness Ra of the workpiece is measured. If Ra<0.2, the particle size is increased step by step until Ra>0.2. The previous particle size value is the maximum particle size Φ for achieving mirror polishing of the workpiece. max , engineering experiment data are shown in Table 2:

[0072] Table 2 Roughness of different particle size abrasives after processing

[0073] Mesh 80 150 200 230 270 325 400 460 Particle size (μm) 180 106 75 63 53 44 38 20 Ra(μm) 0.22 0.15 0.13 0.08 0.12 0.15 0.17 0.23

[0074] Table 2 shows the engineering data when d is in the range of [0.15R~0.4R]. Therefore, when d is in the range of [0.15R~0.4R], to achieve mirror polishing of Ra=0.05μm~0.2μm, the optimal range of abrasive particle size is 38μm~

[0075] 100μm, that is, 150 mesh to 400 mesh.

[0076] (III) Preferred viscosity of abrasive grains

[0077] The choice of abrasive viscosity is mainly based on the processing purpose and the size of the processed parts. When the viscosity is too low, the cutting efficiency is low and Ra≤0.2μm cannot be met; when the viscosity is too high, it will cause excessive extrusion force, scratching the inner wall of the pipe, thereby reducing the dimensional accuracy of the parts and having a greater destructive effect on the parts. The preferred viscosity is obtained through experimental data: under the conditions of a predetermined feed pressure P and a gap d between the large-diameter straight pipe (4) and the profiling core mold (3) (d is obtained by the method described in steps (S2) and (S3)), an initial viscosity Cp is given, and the surface roughness Ra of the workpiece is measured. If Ra<0.2μm, further check whether the inner wall of the large-diameter straight pipe has scratches. If not, the viscosity value is increased step by step until scratches are produced on the inner wall. The viscosity value used in the previous test is the maximum viscosity Cp.max If Ra>0.2μm, then increase the viscosity value step by step until Ra<0.2μm, then the viscosity value of the test is the minimum viscosity value Cp min The experimental data are shown in Table 3 and Table 4:

[0078] Table 3 Roughness of different viscosity abrasives after processing

[0079] Cp (Pa·s) 100 200 300 400 500 Ra(μm) 0.66 0.51 0.39 0.27 0.14

[0080] Table 4 Inner wall scratches after processing with different viscosity abrasives

[0081] Cp (Pa·s) 600 700 800 900 1000 1100 Surface condition No scratches No scratches No scratches No scratches No scratches Scratches appear

[0082] The preferred range of the abrasive viscosity Cp for achieving mirror polishing of the inner wall of a large-diameter pipe fitting in the present invention is 500≤Cp≤1000.

[0083] (S4) Abrasive particles (7) enter from the annular feed hole (8) of the upper guide assembly (1), flow through the outer surface of the contoured core mold (3) and the inner wall of the large-diameter straight tube (4) to form an abrasive flow channel, and then flow out from the annular feed hole (8) of the lower guide assembly (5) to the lower abrasive cylinder of the abrasive flow machine tool (model NXLT-A-01, manufacturer: Suzhou Yilaierte Machinery Co., Ltd.), completing the downstroke; similarly, the abrasive particles enter the upper abrasive cylinder from bottom to top, completing the upstroke, thereby realizing reciprocating processing.

[0084] Example 3 Application Example

[0085] During implementation, the material of the large-diameter straight pipe (4) is titanium alloy (TC4), with a Mohs hardness of 7, a pipe radius of 32 mm, and a pipe length of 18 cm. According to the method of step (S2) and step (S3) in Example 2, it can be concluded that when the value range of the gap d is 0.15R≤d≤0.4R, that is, 4.8mm≤d≤12.8mm, the particle size must meet 150 mesh to 400 mesh. In this embodiment, d=8mm (between the upper limit and the lower limit) and d=18mm (greater than the upper limit) are selected respectively to test the different effects after polishing. The abrasive material is preferably silicon carbide, the particle size must meet 150 mesh to 400 mesh, and the viscosity Cp must meet 500Pa·s≤Cp≤1000Pa·s. In this embodiment, the particle size is selected as 200 mesh and the viscosity is selected as Cp=700Pa·s. The abrasive mobile phase is dimethyl silicone oil, the mass concentration of silicon carbide abrasive particles is 60%, the feed pressure P of the abrasive flow machine is 10 MPa, and the processing time is 30 min.

[0086] When the gap is selected as d = 8 mm (between the upper limit and the lower limit), the effect of the large diameter straight pipe (4) after polishing is as shown in the figure Figure 7When the gap is selected as d = 18 mm (greater than the upper limit), the effect of the large-diameter straight tube (4) after polishing is as shown in FIG. Figure 8 As shown in the figure, it can be seen that when the value of d is between the upper limit and the lower limit, the corresponding polishing effect is much better than the corresponding polishing effect when d is greater than the upper limit.

[0087] For large-diameter straight tubes with internal microgrooves and bosses, the profiling core mold should also retain corresponding bosses and grooves to maintain a consistent gap d. As the abrasive flow polishing medium moves through the pipe, the grinding force changes as the cross-sectional area of ​​the pipe (the flow area) changes. If the profiling core mold for the large-diameter straight tube with microgrooves and bosses is a straight rod, the gap d will inevitably change axially with the concave-convex structure of the large-diameter straight tube, resulting in inconsistent internal roughness after polishing.

[0088] For large-diameter straight pipes with an aspect ratio greater than 10, the diameter of the profiling mandrel should vary linearly to ensure that the gap d is large at the ends and small in the middle. This variation is intended to ensure that the abrasive flow polishing medium, as it moves through the pipe, is continuously attenuated by the resistance along the pipe. This results in a higher extrusion pressure and greater chipping force at the pipe ends, while a lower extrusion pressure and lower chipping force in the middle, resulting in a weaker polishing effect in the middle compared to the pipe ends. Therefore, at a predetermined feed pressure of P, a larger gap at the pipe ends slightly reduces the abrasive pressure and flow rate, while a smaller gap in the middle slightly increases the abrasive pressure and flow rate, thereby achieving a consistent polishing effect at both the pipe ends and the middle.

[0089] The above is a further detailed description of the present invention in conjunction with specific preferred embodiments, and the specific implementation of the present invention should not be considered to be limited to these descriptions. For those skilled in the art to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all of these should be considered to fall within the scope of protection of the present invention.

Claims

1. A method for mirror polishing of a large-diameter straight tube abrasive flow, characterized in that: The steps include: (S1) Structural assembly; A profiling core mold (3) is processed to match the inner surface structure of the large-diameter straight tube (4); the large-diameter straight tube (4) is sleeved on the outer periphery of the profiling core mold (3); an abrasive flow channel is formed between the outer surface of the profiling core mold (3) and the inner wall of the large-diameter straight tube (4); the two ends of the large-diameter straight tube (4) are respectively embedded in the groove-shaped step rings (10) of the upper guide component (1) and the lower guide component (5); the profiling core mold (3) and the large-diameter straight tube (4) are located at the same axis, and the two ends of the profiling core mold (3) are respectively fixedly connected to the upper guide component (1) and the lower guide component (5) through threads; (S2) Maximum gap between the large diameter straight tube (4) and the contoured core mold (3) d max ; Set the inner radius of the large diameter straight pipe to R , the radius of the profiling core die is r ,gap d = R - r There is an upper limit d max ,when d > d max When the fluid enters the straight pipe with a diameter greater than 60 mm, the flow rate and pressure attenuation are large due to the large flow area, so the abrasive particles in the fluid cannot produce micro-cutting polishing on the inner wall surface, and the roughness of the inner wall of the pipe after polishing cannot be achieved. R a is the polishing requirement of 0.05μm~0.2μm; it is necessary to find a polishing surface that meets the requirements of the part. R The maximum gap value corresponding to a is 0.05μm~0.2μm d max , d max Through engineering tests, it was found that when the abrasive particles are 100-400 mesh, d max =0.4 R ; When the abrasive is 400-1000 mesh, d max =0.35 R ; When the abrasive is 1000-2000 mesh, d max =0.3 R When the abrasive mesh number is higher and the particle size is smaller, the passability is stronger in the same flow area, the abrasive pressure is lower, the grinding force is weaker, and the gap value is d max It needs to be reduced accordingly to reduce the flow area and maintain sufficient abrasive pressure; (S3) Minimum gap between large diameter straight tube (4) and profile core mold (3) d min ; Set the inner radius of the large diameter straight pipe to R , the outer diameter of the profiling core mold is r ,gap d = R - r There is a lower limit d min ,when d< d min When the fluid enters the straight pipe with a diameter greater than 60mm, the flow area is too small, the abrasive velocity and pressure are too high, which will cause excessive grinding and polishing of the inner wall of the pipe, resulting in scratches on the inner wall of the pipe, and the roughness of the inner wall of the pipe cannot be achieved after polishing. R a is the polishing requirement of 0.05μm to 0.2μm. At the same time, the abrasive has a strong extrusion and cutting force on the inner wall of the pipe. When the inner wall of the pipe is coated, the abrasive pressure and flow rate are too high, which will cause scratches or partial peeling of the coating on the inner wall of the pipe. It is necessary to find the minimum gap that will not cause scratches. d min； d min Through engineering tests, it was found that when the abrasive particles are 150-500 mesh, d min =0.15 R ; When the abrasive is 500-1200 mesh, d min =0.1 R ; When the abrasive is 1200-1800 mesh, d min =0.05 R ; When the abrasive mesh number is higher and the particle size is smaller, the passability is stronger in the same flow area, and the gap value is d min It needs to be reduced accordingly to maintain sufficient abrasive pressure; (S4) Abrasive particles (7) enter from the annular abrasive particle through hole (8) of the upper flow guide component (1), flow through the outer surface of the profiling core mold (3) and the inner wall of the large-diameter straight pipe (4) to form an abrasive particle flow channel, and then flow out from the annular abrasive particle through hole (8) of the lower flow guide component (5) to the lower abrasive cylinder of the abrasive flow machine tool, completing the downstroke; similarly, the abrasive particles enter the upper abrasive cylinder from bottom to top, completing the upstroke, thereby realizing reciprocating processing; A fixture for abrasive flow mirror polishing of a large-diameter straight tube comprises an upper flow guide component (1), a lower flow guide component (5), and a profile core mold (3) matching the inner surface structure of the large-diameter straight tube (4); the centers of the upper flow guide component (1) and the lower flow guide component (5) are both provided with an internal threaded hole (9) for fixing the profile core mold (3); the planes of the upper flow guide component (1) and the lower flow guide component (5) are provided with an annular feed through hole (8); the annular feed through hole (8) of the upper flow guide component (1) and The annular feed through hole (8) of the lower guide assembly (5) is positioned correspondingly and on the same axis. The lower surface of the upper guide assembly (1) and the upper surface of the lower guide assembly (5) are both provided with a groove-shaped step ring (10) for fixing the large-diameter straight pipe (4). The profiling core mold (3) is a smooth profiling columnar structure. The height of the profiling core mold (3) is higher than the height of the large-diameter straight pipe (4). Both ends of the profiling core mold (3) are provided with external thread structures (6) matching the internal thread holes (9). The large-diameter straight pipe (4) is sleeved on the outer periphery of the profiling core mold (3), and the two ends of the large-diameter straight pipe (4) are respectively embedded in the groove-shaped step rings (10) of the upper guide assembly (1) and the lower guide assembly (5). The profiling core mold (3) and the large-diameter straight pipe (4) are located at the same axis, and the two ends of the profiling core mold (3) are respectively fixedly connected to the upper guide assembly (1) and the lower guide assembly (5) through threads; the step ring (10) is provided with a polymer nylon material gasket (2) on the contact surface with the large-diameter straight pipe (4); the height of the profiling core mold (3) is 10 to 20 mm higher than the height of the large-diameter straight pipe (4); the material of the profiling core mold (3) is 45 steel, and a polymer nylon material layer is provided on the outer periphery of the profiling core mold (3); the material of the upper guide assembly (1) and the lower guide assembly (5) are both 45 steel.

2. The method according to claim 1, wherein: The abrasive material is aluminum oxide or silicon carbide.

3. The method according to claim 1, wherein: Different mirror polishing requires different abrasive polishing order adjustment, according to R a requires the selection of a specific polishing sequence; when R When polishing a=0.05μm~0.2μm mirror, it is necessary to polish to R a=0.8μm, then from R a=0.8μm continue polishing step by step to achieve R Higher-grade mirror polishing with a=0.05μm; The specific step-by-step polishing sequence is to first use abrasive particles with a mesh size of 100 to 400 to achieve polishing. R a=0.2μm, and then polished with abrasive grains of 400-1000 mesh. R a=0.1μm~0.07μm, and finally polished with abrasive grains of 1000-2000 mesh. R a=0.05μm~0.07μm.

4. The method according to claim 1, wherein: Viscosity of abrasive particles C p range is 500≤ C p≤1000.

5. The method according to claim 1, wherein: For large diameter straight tubes with fine grooves and boss structures inside, the profiling core mold should also retain bosses and grooves to make the gap d Stay consistent.

6. The method according to claim 1, wherein: For large diameter straight pipes with an aspect ratio greater than 10, the diameter of the profiling core mold should change linearly to ensure the clearance at the port of the large diameter straight pipe. d Large, gap in the middle d Small and linear change.

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