A method of ball cage outer race grinding

Abstract

The application relates to the technical field of driving shafts, in particular to a ball cage shaft outer circle grinding method. The ball cage shaft outer circle grinding method comprises the following steps: positioning a ball cage shaft in a positioning assembly; based on the positioning of the ball cage shaft being completed, rotating the ball cage shaft; based on the ball cage shaft being in a rotating state, rotating a grinding wheel; based on the grinding wheel being in a rotating state, moving the grinding wheel to a machining position, so that a first grinding surface on the grinding wheel abuts against a first outer circumferential surface, and a second grinding surface abuts against a second outer circumferential surface; based on the first outer circumferential surface and the second outer circumferential surface being ground, moving the grinding wheel until the grinding wheel abuts against a third outer circumferential surface; and based on the third outer circumferential surface being machined, completing the ball cage shaft outer circle grinding machining. Thus, the problem that the ball cage shaft outer circle is ground for a long time is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of drive shafts, in particular to a ball cage shaft outer circle grinding method. BACKGROUND

[0002] The drive shaft is a rigid shaft part connecting the differential and the wheel in the automobile, and its core function is to transmit torque, directly delivering the power of the engine after speed reduction and torque increase by the gearbox to the drive wheel. The universal joint is a key component in the automobile transmission system, which is a component that realizes the transmission of power between rotating shafts with variable angles. It is located at the end of the transmission shaft and plays the role of connecting the transmission shaft and the drive axle, half shaft and other machine parts. Because the wheels will change in angle and distance due to steering and suspension compression when the vehicle is running, the ordinary rigid shaft cannot adapt to this dynamic misalignment, so the drive shaft must be matched with a universal joint at both ends to realize the dual functions of torque transmission and angle compensation. The ball cage universal joint is a kind of universal joint, which is widely used in the drive shaft of the automobile due to its high transmission efficiency, large allowable angle range and stable performance at high speed. The ball cage universal joint includes a ball cage shell, a bell-shaped shell, steel balls, a retainer, a dust cover and other components.

[0003] The ball cage shaft is an integrated shaft structure with the ball cage shell, which is an extension part for transmitting power in the ball cage universal joint. One end is connected to the core structure of the universal joint, and the other end can be connected to the transmission shaft or other transmission components, playing the role of further outputting the power transmitted by the universal joint. During the processing of the ball cage shaft, multiple outer circumferential surfaces with different diameters need to be ground, and multiple outer circumferential surfaces need to be ground multiple times, which is low in processing efficiency. How to improve the grinding efficiency is a problem to be solved at present. SUMMARY

[0004] To solve the problem of long time consumption of multiple grinding of the outer circle of the ball cage shaft and long time consumption of three grinding of the outer circle of the ball cage shaft, the present application provides a ball cage shaft outer circle grinding method, which comprises:

[0005] Positioning the ball cage shaft in the positioning assembly; the ball cage shaft comprises an integrally formed ball cage shell and a shaft rod; the shaft rod has a first outer circumferential surface, a second outer circumferential surface and a third outer circumferential surface; the second outer circumferential surface is located between the first outer circumferential surface and the third outer circumferential surface; the shaft rod coincides with the positioning axis in the state of positioning completion of the ball cage shaft in the positioning assembly;

[0006] Based on the positioning completion of the ball cage shaft, the ball cage shaft is controlled to rotate;

[0007] control the grinding wheel to rotate based on the fact that the ball cage shaft is in a rotating state; the grinding wheel has a first grinding surface and a second grinding surface; the first grinding surface is a first distance from the positioning axis; the second grinding surface is a second distance from the positioning axis; the first distance and the second distance have a difference;

[0008] move the grinding wheel to a machining position based on the fact that the grinding wheel is in a rotating state, so that the first grinding surface on the grinding wheel abuts the first outer circumferential surface and the second grinding surface abuts the second outer circumferential surface;

[0009] control the grinding wheel to move based on the fact that the first outer circumferential surface and the second outer circumferential surface are ground to completion, until the grinding wheel abuts the third outer circumferential surface;

[0010] based on the fact that the third outer circumferential surface is machined to completion, the ball cage shaft external circle grinding machining is completed.

[0011] In some embodiments, the first outer circumferential surface is located on the side of the second outer circumferential surface close to the ball cage shell.

[0012] In some embodiments, the first distance is greater than the second distance;

[0013] The control of the grinding wheel to move based on the fact that the first outer circumferential surface and the second outer circumferential surface are ground to completion, until the grinding wheel abuts the third outer circumferential surface, comprises:

[0014] control the grinding wheel to move based on the fact that the first outer circumferential surface and the second outer circumferential surface are ground to completion, until the second grinding surface abuts the third outer circumferential surface.

[0015] In some embodiments, the ball cage shaft external circle grinding method further comprises:

[0016] accumulate the number of machined parts based on the fact that the ball cage shaft external circle grinding machining is completed;

[0017] based on the fact that the number of machined parts is less than a first preset number of parts, return to execute the step of positioning the ball cage shaft in the positioning assembly for the next ball cage shaft to be ground.

[0018] In some embodiments, the ball cage shaft external circle grinding method further comprises:

[0019] based on the fact that the number of machined parts reaches the first preset number of parts, the second grinding surface is dressed;

[0020] based on the fact that the second grinding surface is dressed to completion, return to execute the step of positioning the ball cage shaft in the positioning assembly for the next ball cage shaft to be ground.

[0021] In some embodiments, the ball cage shaft external circle grinding method further comprises:

[0022] based on the machining piece number reaching a second preset piece number, simultaneously trimming the first grinding surface and the second grinding surface; the second preset piece number is greater than the first preset piece number;

[0023] based on the first grinding surface trimming being completed, clearing the machining piece number, and returning to perform the step of positioning the ball cage shaft in the positioning assembly for a next ball cage shaft to be ground.

[0024] In some embodiments, in a state where the first grinding surface and the second grinding surface are simultaneously trimmed, a difference between an upper limit of a tolerance range of the first distance and an upper limit of a tolerance range of the first outer circumferential surface is less than a first threshold value.

[0025] In some embodiments, in a state where the first grinding surface and the second grinding surface are simultaneously trimmed, a difference between a lower limit of a tolerance range of the second distance and a lower limit of a tolerance range of the second outer circumferential surface is less than a second threshold value.

[0026] In some embodiments, an axial length of the first outer circumferential surface is less than an axial length of the second outer circumferential surface.

[0027] A length of the first grinding surface along the positioning axis is less than a length of the second grinding surface along the positioning axis.

[0028] In some embodiments, the first grinding surface and the second grinding surface are both conical surfaces; an included angle between an axis of the grinding wheel and the positioning axis is an acute angle.

[0029] To solve the problem of long time consumption of multiple grinding of the outer circle of the ball cage shaft, the present application has the following advantages:

[0030] By making the first grinding surface on the grinding wheel abut against the first outer circumferential surface and the second grinding surface abut against the second outer circumferential surface, the first outer circumferential surface and the second outer circumferential surface can be simultaneously ground, and only one retreat of the tool is needed to enter the next grinding step after the first outer circumferential surface and the second outer circumferential surface are processed. Compared with grinding the first outer circumferential surface and the second outer circumferential surface respectively and performing two retreats of the tool after grinding is completed, one grinding and one retreat of the tool are saved, and the efficiency of the grinding process is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 A ball cage shaft outer circle grinding method flowchart of an embodiment is shown;

[0032] Figure 2 A structural diagram of a device to which the ball cage shaft outer circle grinding method in Figure 1 is applied is shown;

[0033] Figure 3 a partial cross-sectional view of the ball cage shaft is shown. Figure 2 a partial cross-sectional view of the ball cage shaft is shown.

[0034] Reference numerals: 10 grinding assembly; 11 grinding wheel; 12 first driving part; 13 first grinding surface; 14 second grinding surface; 20 positioning assembly; 30 ball cage shaft; 31 ball cage housing; 32 shaft rod; 33 first outer peripheral surface; 34 second outer peripheral surface; 35 third outer peripheral surface. DETAILED DESCRIPTION

[0035] The present disclosure will now be discussed with reference to several exemplary embodiments. It should be appreciated that these embodiments are discussed only with the intent that one of ordinary skill in the art would be better able to understand and thus implement the present disclosure, and not with the intent to limit the scope of the disclosure.

[0036] As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having," "contains," "containing," or any other variation thereof, are intended to be construed as meaning "including, but not limited to," unless expressly specified otherwise. The term "based on" is intended to be construed as "based at least in part on." The terms "one embodiment" and "an embodiment" are intended to be construed as "at least one embodiment." The term "another embodiment" is intended to be construed as "at least one other embodiment." The terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "vertical," "horizontal," "lateral," "longitudinal," and similar terms are used for description only and not intended to convey any constraint on the apparatus, element, or component being described. These terms are merely used to describe particular embodiments and are in no way meant to limit the scope of the application or its embodiments. Also, the terms "mount," "set," "provided with," "connected," "linked," should be given a broadest possible interpretation. For example, it can be fixed connection, detachable connection, or integral structure; it can be mechanical connection, or electrical connection; it can be direct connection, or indirect connection via an intermediate medium; or it can be internal connection between two devices, elements, or components. For one of ordinary skill in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances. In addition, the terms "first," "second," and the like are mainly used to distinguish different devices, elements, or components (the specific type and structure can be the same or different), and are not intended to indicate or imply relative importance or quantity of the indicated devices, elements, or components. Unless otherwise stated, the meaning of "multiple" is two or more.

[0037] Embodiment One:

[0038] Drive shaft is a rigid shaft part in automobile connecting differential and wheel, the core function is to transmit torque, the power of engine through gearbox deceleration and torque increase is directly delivered to the drive wheel. The universal joint is a key component in the automobile transmission system, it is a component that realizes the angle change transmission between the rotating shaft. Because the wheels will change angle and distance when the vehicle is running due to steering and suspension compression, the ordinary rigid shaft cannot adapt to this dynamic misalignment, therefore the drive shaft must be matched with the universal joint at both ends, so as to realize the dual function of torque transmission and angle compensation. The ball cage universal joint is a kind of universal joint, which includes ball cage shell 31, bell-shaped shell, steel ball, retainer, dust cover and other components.

[0039] The ball cage shaft 30 is an integral structure of the shaft rod 32 with the ball cage shell 31. The ball cage shaft 30 has multiple outer circumferential surfaces that need to be ground, and multiple outer circumferential surfaces grinding needs multiple times of tool withdrawal, which is low in processing efficiency. How to improve the grinding efficiency is a problem to be solved at present.

[0040] In the embodiment, as shown in Figure 1 A ball cage shaft 30 external grinding method includes steps S10-S60, which are specifically described as follows:

[0041] Step S10: as shown in Figure 2 The ball cage shaft 30 is positioned in the positioning assembly 20; as shown in Figure 3 The ball cage shaft 30 includes an integral ball cage shell 31 and a shaft rod 32, and the shaft rod 32 has a first outer circumferential surface 33, a second outer circumferential surface 34 and a third outer circumferential surface 35. The second outer circumferential surface 34 is located between the first outer circumferential surface 33 and the third outer circumferential surface 35; thus, the first outer circumferential surface 33 and the second outer circumferential surface 34 are adjacent, which can reduce the distance of the first grinding surface 13 and the second grinding surface 14 of the grinding wheel 11 extending along the positioning axis, that is, reduce the volume of the grinding wheel 11, and reduce the equipment cost. When the ball cage shaft 30 is positioned in the positioning assembly 20, the shaft rod 32 coincides with the positioning axis;

[0042] Step S20: based on the positioning of the ball cage shaft 30, the ball cage shaft 30 is controlled to rotate; in this way, the ball cage shaft 30 can rotate around the axis of its own shaft rod 32, that is, around the positioning axis after positioning. Instead of being in a stationary state after starting grinding, which reduces the difficulty and time required for grinding.

[0043] Step S30: based on the rotating state of the ball cage shaft 30, the grinding wheel 11 is controlled to rotate; as shown in Figure 2As shown, the grinding assembly 10 comprises a grinding wheel 11, a first driving part 12, and the grinding wheel 11 has a first grinding surface 13 and a second grinding surface 14. The first grinding surface 13 and the second grinding surface 14 form a step shape. The distance from the first grinding surface 13 to the positioning axis is a first distance; the distance from the second grinding surface 14 to the positioning axis is a second distance; the first distance and the second distance are respectively arranged corresponding to the radii of the first outer circumferential surface 33 and the second outer circumferential surface 34. The first distance and the second distance have a difference, so as to adapt to the difference between the radii of the first outer circumferential surface 33 and the second outer circumferential surface 34.

[0044] Step S40: based on the grinding wheel 11 being in a rotating state, moving the grinding wheel 11 to a machining position, so that the first grinding surface 13 on the grinding wheel 11 abuts against the first outer circumferential surface 33, and the second grinding surface 14 abuts against the second outer circumferential surface 34; in this way, the first outer circumferential surface 33 and the second outer circumferential surface 34 can be machined at the same time, and after machining is completed, only one retreat is needed to enter the next grinding step. Compared with machining the first outer circumferential surface 33 and the second outer circumferential surface 34 respectively and performing two retreats after machining is completed, one grinding and one retreat are saved, and the efficiency of grinding machining is greatly improved.

[0045] Step S50: based on the first outer circumferential surface 33 and the second outer circumferential surface 34 being machined, controlling the grinding wheel 11 to move until the grinding wheel 11 abuts against the third outer circumferential surface 35; since the ball cage shaft 30 and the grinding wheel 11 are always in a rotating state, in this way, the third outer circumferential surface 35 can be machined after abutting.

[0046] Step S60: based on the third outer circumferential surface 35 being machined, the ball cage shaft 30 is machined.

[0047] Further, as shown in the figure, Figure 3 The first outer circumferential surface 33 is located on the side of the second outer circumferential surface 34 close to the ball cage shell 31. Since there are two abutments between the ball cage shaft 30 and the grinding wheel 11 when the first outer circumferential surface 33 and the second outer circumferential surface 34 are machined at the same time, the abutment area is large, so the abutment force is large, and this arrangement can shorten the distance from the action point of the abutment force on the first outer circumferential surface 33 and the second outer circumferential surface 34 to the center of gravity of the ball cage shaft 30, thereby reducing the moment of the abutment force, and thus ensuring the stability of the grinding process.

[0048] Further, as shown in the figure, Figure 2 The first distance is greater than the second distance.

[0049] Based on the first outer circumferential surface 33 and the second outer circumferential surface 34 being machined, the grinding wheel 11 is controlled to move until the grinding wheel 11 abuts against the third outer circumferential surface 35, comprising:

[0050] After the grinding of the first outer circumferential surface 33 and the second outer circumferential surface 34 is completed, the grinding wheel 11 is controlled to move until the second grinding surface 14 abuts against the third outer circumferential surface 35. Since the second outer circumferential surface 34 is located between the first outer circumferential surface 33 and the third outer circumferential surface 35, and the second distance is smaller than the first distance, the third outer circumferential surface 35 is ground by the second grinding surface 14, so that the possibility of interference between the first grinding surface 13 and other parts of the shaft 32 during the movement of the grinding wheel 11 is small, and the rationality of the structural design is improved.

[0051] Further, the outer circle grinding method of the ball cage shaft 30 further comprises a step S70; the step S70 comprises a step S71 and a step S72:

[0052] The step S71 comprises: based on the completion of the outer circle grinding of the ball cage shaft 30, accumulating the number of workpieces;

[0053] The step S72 comprises: based on the number of workpieces being less than the first preset number of workpieces, returning to the step of positioning the ball cage shaft 30 in the positioning assembly 20 for the next ball cage shaft 30 to be ground.

[0054] In this way, when the number of workpieces does not reach the first preset number of workpieces, the outer surface machining of one ball cage shaft 30 is completed, and the machining of the next ball cage shaft 30 can be automatically started, so that the above-mentioned outer circle grinding process of the ball cage shaft 30 is realized.

[0055] Further, the outer circle grinding method of the ball cage shaft 30 further comprises a step S80; the step S80 comprises a step S81 and a step S82:

[0056] The step S81 comprises: based on the number of workpieces reaching the first preset number of workpieces, trimming the second grinding surface 14;

[0057] The step S82 comprises: based on the completion of the trimming of the second grinding surface 14, returning to the step of positioning the ball cage shaft 30 in the positioning assembly 20 for the next ball cage shaft 30 to be ground.

[0058] In the above-mentioned outer circle grinding method of the ball cage shaft 30, the number of uses of the second grinding surface 14 is more than that of the first grinding surface 13, so the number of uses of the second grinding surface 14 will reach the threshold value of trimming first. This setting makes the above-mentioned outer circle grinding process of the ball cage shaft 30 stop accurately when the number of workpieces reaches the first preset number of workpieces, so that the second grinding surface 14 is trimmed, and the machining quality of the subsequent ball cage shaft 30 is ensured.

[0059] Further, the outer circle grinding method of the ball cage shaft 30 further comprises a step S90; the step S90 comprises a step S91 and a step S92:

[0060] Step S91: Based on the number of processed parts reaching the second preset number, the first grinding surface 13 and the second grinding surface 14 are simultaneously trimmed; the second preset number of parts is greater than the first preset number of parts;

[0061] Step S92: Based on the completion of the first grinding surface 13, the number of processed parts is cleared to zero. For the next ball cage shaft 30 to be ground, the step of positioning the ball cage shaft 30 in the positioning assembly 20 is returned to be executed.

[0062] This allows the first grinding surface 13 and the second grinding surface 14 to be simultaneously trimmed after the number of times the first grinding surface 13 is used, i.e., the number of preset parts, reaches the threshold, reducing the number of downtimes caused by trimming and improving processing efficiency.

[0063] In other embodiments, the second preset number of parts can be approximately twice the first preset number of parts. Since the second grinding surface 14 is used twice as many times as the first grinding surface 13 after any number of complete cycles of the above-described ball cage shaft 30 outer cylindrical grinding method, setting the second preset number of parts to approximately twice the first preset number based on this quantitative relationship minimizes the number of times the machine needs to be stopped for maintenance, thereby improving processing efficiency.

[0064] Furthermore, such as Figure 2 As shown, when the first grinding surface 13 and the second grinding surface 14 are simultaneously finished, the difference between the upper limit of the tolerance range of the first distance and the upper limit of the tolerance range of the first outer peripheral surface 33 is less than the first threshold.

[0065] Because the second grinding surface 14 is used twice as many times as the first grinding surface 13 after any number of complete cycles of the above-mentioned ball cage shaft 30 external cylindrical grinding method, the wear of the second grinding surface 14 of the grinding wheel 11 is greater than that of the first grinding surface 13. As the number of grinding operations and dressing operations gradually increases, the difference between the second distance and the first distance will gradually decrease. This results in the difference between the second distance and the first distance becoming too small after a certain number of dressing operations. In this case, a significant adjustment to the depth of the first grinding surface 13 is required to ensure the step depth formed by the first grinding surface 13 and the second grinding surface 14. Furthermore, adjusting the step depth requires significantly more time than cleaning the outer surface.

[0066] By setting the upper limit of the tolerance range of the first distance to be less than the upper limit of the tolerance range of the first outer peripheral surface 33, the first distance is kept within a large range after adjustment, so that the design depth of the step is as large as possible. This reduces the number of times the step depth is adjusted, and more adjustment times are used to clean foreign objects from the surfaces of the first grinding surface 13 and the second grinding surface 14, rather than to adjust the step depth. This allows the present invention to save adjustment time.

[0067] Furthermore, such as Figure 2As shown, when the first grinding surface 13 and the second grinding surface 14 are simultaneously finished, the difference between the lower limit of the tolerance range of the second distance and the lower limit of the tolerance range of the second outer peripheral surface 34 is less than the second threshold.

[0068] By setting the difference between the lower limit of the tolerance range of the second distance and the lower limit of the tolerance range of the second outer circumference 34 to be less than the second threshold, the second distance is kept within a small range after adjustment, so that the design depth of the step is as large as possible, and similarly, a lot of adjustment time is saved.

[0069] Furthermore, such as Figure 3 As shown, the axial length of the first outer peripheral surface 33 is less than the axial length of the second outer peripheral surface 34; this reduces the thickness of the grinding wheel 11. The length of the first grinding surface 13 along the positioning axis is less than the length of the second grinding surface 14 along the positioning axis, making the second grinding surface 14 longer along the positioning axis. This allows the second grinding surface 14 to grind the second outer peripheral surface 34 and the third outer peripheral surface 35 with different axial lengths, improving adaptability.

[0070] Furthermore, such as Figure 2 As shown, both the first grinding surface 13 and the second grinding surface 14 are conical surfaces. While maintaining the same contact area between the grinding wheel 11 and the ball cage shaft 30, this design reduces the thickness of the grinding wheel 11 compared to a cylindrical outer circumference, thus lowering costs. The angle between the axis of the grinding wheel 11 and the positioning axis is acute, which shortens the lever arm of the contact force from the ball cage shaft 30 on the grinding wheel 11, thereby reducing the torque on the grinding wheel 11 and consequently reducing damage to the grinding wheel 11.

[0071] Those skilled in the art will understand that the above embodiments are specific examples of implementing this disclosure, and in practical applications, various changes can be made in form and detail without departing from the scope of this disclosure.

Claims

1. A method of outer race grinding of a ball cage, characterized by, The ball cage shaft outer circle grinding method comprises: Positioning the ball cage shaft in a positioning assembly; the ball cage shaft comprises an integrally formed ball cage housing and a shaft rod; the shaft rod has a first outer peripheral surface, a second outer peripheral surface and a third outer peripheral surface; the second outer peripheral surface is located between the first outer peripheral surface and the third outer peripheral surface; in the state that the ball cage shaft is positioned in the positioning assembly, the shaft rod coincides with a positioning axis; Based on the positioning of the ball cage shaft being completed, the ball cage shaft is controlled to rotate; Based on the ball cage shaft being in a rotating state, a grinding wheel is controlled to rotate; the grinding wheel has a first grinding surface and a second grinding surface; the distance from the first grinding surface to the positioning axis is a first distance; the distance from the second grinding surface to the positioning axis is a second distance; the first distance and the second distance have a difference; Based on the grinding wheel being in a rotating state, the grinding wheel is moved to a machining position, so that the first grinding surface on the grinding wheel abuts against the first outer peripheral surface, and the second grinding surface abuts against the second outer peripheral surface; Based on the first outer peripheral surface and the second outer peripheral surface being ground, the grinding wheel is controlled to move until the grinding wheel abuts against the third outer peripheral surface; The first distance is greater than the second distance; Based on the first outer peripheral surface and the second outer peripheral surface being ground, the grinding wheel is controlled to move until the grinding wheel abuts against the third outer peripheral surface, which comprises: Based on the first outer peripheral surface and the second outer peripheral surface being ground, the grinding wheel is controlled to move until the second grinding surface abuts against the third outer peripheral surface; Based on the third outer peripheral surface being machined, the ball cage shaft outer circle grinding machining is completed; The ball cage shaft outer circle grinding method further comprises: Based on the ball cage shaft outer circle grinding machining being completed, the number of machined pieces is accumulated; Based on the number of machined pieces being less than a first preset number of pieces, for the next ball cage shaft to be ground, the step of positioning the ball cage shaft in the positioning assembly is executed again; The ball cage shaft outer circle grinding method further comprises: Based on the number of machined pieces reaching the first preset number of pieces, the second grinding surface is dressed; Based on the second grinding surface being dressed, for the next ball cage shaft to be ground, the step of positioning the ball cage shaft in the positioning assembly is executed again; The ball cage shaft outer circle grinding method further comprises: Based on the number of machined pieces reaching a second preset number of pieces, the first grinding surface and the second grinding surface are simultaneously dressed; the second preset number of pieces is greater than the first preset number of pieces; Based on the first grinding surface being dressed, the number of machined pieces is cleared, and for the next ball cage shaft to be ground, the step of positioning the ball cage shaft in the positioning assembly is executed again.

2. The ball cage shaft outer circle grinding method according to claim 1, wherein The first outer peripheral surface is located on the side of the second outer peripheral surface close to the ball cage housing.

3. The ball cage shaft outer circle grinding method according to claim 2, wherein In the state that the first grinding surface and the second grinding surface are simultaneously dressed, the difference between the upper limit of the tolerance range of the first distance and the upper limit of the tolerance range of the first outer peripheral surface is less than a first threshold value.

4. The method of claim 3, wherein, in a state in which the first grinding surface and the second grinding surface are simultaneously dressed, a difference between a lower limit of a tolerance range of the second distance and a lower limit of a tolerance range of the second outer circumferential surface is less than a second threshold value.

5. The method of claim 1, wherein, in the state in which the first grinding surface and the second grinding surface are simultaneously dressed, an axial length of the first outer circumferential surface is less than an axial length of the second outer circumferential surface; and a length of the first grinding surface along the positioning axis is less than a length of the second grinding surface along the positioning axis.

6. The method of claim 1, wherein, in the state in which the first grinding surface and the second grinding surface are simultaneously dressed, the first grinding surface and the second grinding surface are both conical surfaces; and an angle between an axis of the grinding wheel and the positioning axis is an acute angle. ​ ​ ​ ​

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