In the bearing industry, bearing outer rings are generally thin-walled parts. Generally, CNC lathes use self-centering or multi-jaw chuck clamping methods when hard turning bearing outer rings. In view of the deformation problem caused by ordinary hydraulic self-centering or multi-jaw chuck clamping on the outer ring of hard turning bearings, a method of using electro-permanent magnetic chucks to clamp the outer rings of hard turning thin-walled bearings was proposed. At the same time, in view of the problem that the positive taper phenomenon occurs in the inner hole of the outer ring of a thin-walled bearing during hard turning, which leads to the excessive roundness of the inner hole, it is proposed to improve the positive taper phenomenon in the inner hole by changing the cutting depth and cutting path to ensure the hard turning of thin-walled bearings. The roundness of the inner hole of the bearing outer ring proves the feasibility of using electro-permanent magnetic chucks to clamp the outer ring of hard-turned thin-walled bearings.
01
Preface
As one of the weakest links in hard turning, the bearing clamping and positioning method has always been a bottleneck restricting the widespread application of hard turning technology in the bearing industry. The clamping method and clamping accuracy directly affect the machining accuracy. To achieve the accuracy of grinding processing in the hard turning process, it is very important to choose the appropriate clamping method and how to maximize the clamping accuracy [1].
Self-centering or multi-jaw chucks have become the fixture of choice for turning operations due to their broad versatility, clamping reliability and centering accuracy. For the clamping method of self-centering or multi-jaw chucks, by changing the structure of the jaws, improving the accuracy of the jaws, and improving the layout of the jaws, the amount of hard turning deformation of the bearing can be limited and the clamping accuracy can be improved. Ren Minjie et al. [2] improved the structure of the claws of the hydraulic self-centering chuck of CNC lathes, which significantly reduced the deformation of the bearing hard turning outer ring and solved the problem of large deformation of the outer ring held by the three claws. Dr. Jeongmin Byun [3] of Purdue University in the United States systematically analyzed the self-centering chuck clamping. He not only found out the main factors affecting the clamping error of hard turning cylindrical parts, but also proposed a method to eliminate the tilt of the workpiece and reduce the clamping error. Methods to gradually improve clamping accuracy include maintaining redundancy, improving the machining accuracy of the jaws, and improving the accuracy of the jaw arrangement. The research results show that by processing cylindrical roller bearing rings on the basis of improving the clamping accuracy, the processing accuracy can reach the level of grinding processing, proving the feasibility of "replacing grinding with turning machines". At present, there are few cases of using electro-permanent magnetic chucks to clamp hard-turned bearings. This clamping method has obvious advantages and can avoid the deformation of thin-walled bearings caused by claw clamping. However, the disadvantages are not yet clear and need to be verified by experiments. J.M. Zhou et al. [4] from Lund University in Sweden found that when cutting rolling bearings, using a self-centering chuck for clamping can cause deformation of up to 20 μm, and recommended the use of a six-jaw chuck or electromagnetic chuck for clamping. Through experimental research on hard turning 100Cr6 ring parts with a hardness of 60 to 62HRC, they found that when using a self-centering chuck for clamping, the out-of-roundness error of the ring parts exceeded 10 μm; when using a six-claw chuck for clamping, the roundness error of the ring parts exceeded 10 μm. The out-of-roundness error of the ring parts is about 9 μm; when using electromagnetic chuck clamping, the out-of-roundness error of the ring parts is less than 4 μm.
Although self-centering or multi-jaw chucks can improve the deformation of thin-walled bearing outer rings during hard turning by optimizing the jaws, they cannot completely solve the problem of hard turning deformation of thin-walled bearing outer rings. Therefore, using Electro-permanent magnetic chuck clamping has become the best method for hard turning thin-walled bearing outer rings. This article conducts experimental research for the first time using an electro-permanent magnetic chuck to clamp the outer ring of a hard-turned thin-walled bearing. It verifies the feasibility of the electro-permanent magnetic chuck to clamp the outer ring of a hard-turned thin-walled bearing. It also analyzes the inner hole of the outer ring of a hard-turned thin-walled bearing. When the positive taper phenomenon occurs, the roundness of the inner hole is out of tolerance. It is proposed to improve the positive taper phenomenon of the inner hole by changing the cutting depth and cutting path.
02
Introduction to electro-permanent magnetic chuck
The electro-permanent magnetic chuck is made of permanent magnets instead of electromagnets, and the magnetically permeable blocks are usually permanent magnets. When it starts working, the electro-permanent magnetic chuck is energized and magnetized. When the set magnetic strength is reached, the power is automatically cut off to maintain the magnetic force. Because there is no need for continuous power supply during the working process, it will not generate heat during continuous operation, preventing the workpiece from being deformed by heat.
The electromagnetic chuck is made based on the principle of the magnetic effect of electricity. The magnetic force depends on a continuous electrical current. The disadvantage of this type of suction cup is that when the current stops, it will release the workpiece. If this happens during work, it will cause flying parts and put the operator at risk of injury. At the same time, after the electromagnetic chuck has been running for a period of time, the continuous flow of current will generate heat, causing the workpiece to be heated and deformed, and the machining accuracy cannot be guaranteed.
This test uses an electric permanent magnet chuck, model X61-500. The controller model is LMSDVPL2VH301, as shown in Figure 1.
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a) Electro-permanent magnetic chuck b) Controller
Figure 1 Electric permanent magnet chuck and controller
The electro-permanent magnetic chuck can absorb and clamp the outer ring of thin-walled bearings through magnetic force. It has no radial clamping force on the workpiece and avoids clamping deformation. This electro-permanent magnetic chuck has a total of 16 levels of magnetic force, and the magnetic force is controlled by adjusting its current.
03
Experiment on the outer ring of thin-walled bearings clamped by electro-permanent magnetic chucks
3.1 Machine tool equipment selection
The CNC vertical lathe T6-85H independently developed by General Technology Group Shenyang Machine Tool Co., Ltd. was selected for the test. The workpiece is clamped vertically so that the center of gravity of the workpiece coincides with the center of gravity of the spindle to avoid roundness errors caused by gravity during horizontal clamping and ensure the roundness of the workpiece [5]. At the same time, the self-weight of the workpiece makes the contact with the fixture datum surface accurate and close, thereby obtaining high positioning accuracy and stable processing accuracy.
3.2 Workpiece selection
The hard turning test selects the outer ring of the cylindrical roller bearing as the test object. The wall thickness is 6.5mm, which is a thin-walled bearing [6]. The material is GCr15, and the hardness after quenching is 60~64HRC. The end face and outer circle of the sample are both rough grinding surfaces, with good dimensional consistency and surface quality. Since rough grinding of the inner hole is inefficient and prone to burns, this test only hard-turned the inner hole, using hard turning instead of rough grinding. After hard turning, there is also a fine grinding process and a super-finishing process. The test piece requires that the inner hole size after hard turning is 136.82~136.86mm, the inner hole roundness is 0.011mm, the inner hole cylindricity is 0.011mm, the coaxiality between the inner hole and the outer circle is 0.02mm, the verticality between the inner hole and the end face is 0.011mm, and the inner hole is 0.011mm deep. The hole surface roughness value Ra=1μm, as shown in Figure 2.
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a) Pattern
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b) Blank
Figure 2 Cylindrical roller bearing outer ring
3.3 Process plan
The process plan consists of two tools for hard turning the inner hole of the outer ring of the cylindrical roller bearing. The cutting parameters are: rotation speed 250r/min, feed rate 0.1mm/r, single-sided cutting depth of the first tool 0.08mm, and single-sided cutting depth of the second tool 0.07 mm. CBN inserts are used, and the tool tip radius is 0.8mm, as shown in Figure 3.
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a) Blade b) Tool
Figure 3 CBN blades and cutting tools
04
Experimental procedure
The rated magnetic force of the electro-permanent magnet chuck used in the test is 160N/cm2, and the axial adsorption force on the outer ring of the cylindrical roller bearing is 2432N. According to the contact surface principle, the friction coefficient between the workpiece and the magnetic positioning block of the electro-permanent magnet chuck is 0.15, the friction force is 364.8N, and the combined force of the main cutting force and the radial cutting force of the workpiece in hard turning is about 120N. Therefore, the electro-permanent magnetic chuck can fully adapt to the clamping needs of hard turning.
Since the outer ring of the bearing is positioned by a magnetic chuck on the end face, there is no centering measure on the circumference. In fact, it is very difficult to center the outer ring with a circumference meter. Therefore, an arc step with a depth of 5 mm is turned on the magnetic positioning block to make the step circular. The gap between the arc and the bearing outer ring should be as small as possible. The gap is about 0.01mm, as shown in Figure 4. At the same time, when self-turning the step surface, it is necessary to ensure that the surface quality of the contact surface between the magnetic conductive block and the bearing outer ring is good. After the parts are processed, the magnetic disassembly will be very fast. Easily, it only takes 5 seconds to load and unload parts.
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Figure 4 Schematic diagram of hard turning and clamping of the outer ring of cylindrical roller bearings
Parts No. 1 to 5 were hard turned according to the above clamping method and cutting parameters. After turning, the parts were inspected by three-dimensional coordinates, as shown in Figure 5, and the surface roughness was inspected by a roughness meter, as shown in Figure 6. The test results are shown in Table 1, and the status of the parts after hard turning is shown in Figure 7.
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Figure 5 Three-coordinate detection
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Figure 6 Surface roughness detection
Table 1 Cylindrical roller bearing (No. 1~5) outer ring inner hole inspection results (unit: μｍ) picture
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Figure 7: The state after hard turning
It can be seen from the data in Table 1 that after the outer ring of the cylindrical roller bearing is clamped by the electro-permanent magnetic chuck and the inner hole is hard-turned, the roundness of the inner hole, the coaxiality between the inner hole and the outer circle, the perpendicularity between the inner hole and the end face, and the surface roughness of the inner hole The degree can all meet the requirements of the drawing, and the cylindricity of the inner hole mostly meets the requirements, but it is also close to the limit tolerance required by the drawing. A slight accumulation of other errors will exceed the tolerance and cause the part to be unqualified. The reason is that the difference between the upper and lower circles of the inner hole is large, causing a forward taper phenomenon in the inner hole.
For this positive cone phenomenon, as shown in Figure 4, the analysis reason is that under the axial clamping of the electro-permanent magnetic chuck, when the height of the self-turning arc step can contact the outer circle of the bearing ring, the vertical lathe starts from the top. During the hard turning process of the inner hole of the thin-walled bearing outer ring, the insufficient rigidity of the bearing outer ring causes radial deformation of the tool during the cutting process. The upper circular part of the inner hole of the bearing outer ring undergoes elastic deformation due to the cutting force, and the cutting amount becomes smaller, resulting in actual The cutting depth is inconsistent with the nominal cutting depth. The deformation of the lower circular part of the inner hole is small due to the bottom surface of the magnetic block, the suction force of the side steps and the pressure of the side steps. The cutting amount is larger than that of the upper circular part of the inner hole. The outer ring of the bearing has a positive taper phenomenon in the inner hole.
Theoretically, this taper phenomenon can be reduced or eliminated by two methods. The first is to change the cutting depth to reduce the radial cutting force, such as reducing the cutting depth when finishing the inner hole of the outer ring of a thin-walled bearing; the second is to change the cutting path, such as when finishing the inner hole of the outer ring of a thin-walled bearing, according to the positive taper size, use a certain reverse taper to cut the inner hole, so that the cutting amount of the lower circle of the inner hole ≤ the cutting amount of the upper circle of the inner hole, reducing or eliminating the forward taper phenomenon of the inner hole.
05
Experimental verification
5.1 Verify the effect of changing the cutting depth on the forward taper phenomenon
The process plan consists of three tools for hard turning the inner hole of the outer ring of the cylindrical roller bearing. The cutting parameters are: rotation speed 250r/min, feed rate 0.1mm/r, single-sided cutting depth of the first tool 0.08mm, and single-sided cutting depth of the second tool 0.05 mm, the cutting depth of the third knife on one side is 0.02mm. The clamping method remains unchanged, and the hard-turned parts No. 6 to 10 are tested by three-dimensional coordinate inspection and surface roughness meter after turning. The test results are shown in Table 2.
Table 2 Cylindrical roller bearing (No. 6 to No. 10) outer ring inner hole inspection results (unit: μｍ) pictures
It can be seen from the data in Table 2 that by changing the cutting depth, the finishing cutting amount changed from 0.07mm to 0.02mm on one side. After hard turning the inner hole of the outer ring of the cylindrical roller bearing, all geometric tolerances and surface roughness were tested to meet the drawings. Require.
5.2 Verify the effect of changing the cutting path on the forward taper phenomenon
The process plan consists of two tools for hard turning the inner hole of the outer ring of the cylindrical roller bearing. The cutting parameters are: rotation speed 250r/min, feed rate 0.1mm/r, single-sided cutting depth of the first tool 0.08mm, and single-sided cutting depth of the second tool 0.07 mm. According to the data in Table 1, the average difference between the upper and lower circles of the bearing inner hole and the width of the bearing, the taper ratio is 1:2.6493. When turning the second tool, the cutting depth remains unchanged, taper compensation is performed, and the cutting path is the path corresponding to the inverted taper ratio. The clamping method remains unchanged, and the hard-turned parts No. 11 to 15 are tested. After turning, the parts are tested by three-dimensional coordinate inspection and surface roughness meter. The test results are shown in Table 3.
Table 3 Cylindrical roller bearing (No. 11-15) outer ring inner hole inspection results (unit: μｍ) pictures
It can be seen from the data in Table 3 that by changing the cutting path, the taper ratio is obtained based on the known forward taper phenomenon. The cutting depth remains unchanged during cutting, and the cutting path is the corresponding inverted taper ratio path. After hard turning the inner hole of the outer ring of the cylindrical roller bearing, Check that all geometric tolerances and surface roughness meet the drawing requirements.
06
Conclusion
Aiming at the problem that self-centering or multi-jaw chucks will cause deformation of the outer ring of thin-walled bearings when clamping hard-turned thin-walled bearings, this paper proposes the use of electro-permanent magnetic chucks to clamp the outer rings of hard-turned thin-walled bearings, and proves that the electro-permanent magnetic chuck is used to clamp the outer ring of hard-turned thin-walled bearings The feasibility of using magnetic chuck to clamp the outer ring of hard-turned thin-walled bearings.
For the problem of forward taper phenomenon in the outer ring of hard turning thin-walled bearings, two methods are proposed for optimization. One is to change the cutting depth, and the other is to change the cutting path. Through experimental verification, it is found that changing the cutting depth is better than changing the cutting path. .
