JP4356526B2 - Ironing method - Google Patents

Description

  The present invention relates to an ironing method for plastically deforming an inner surface of a cup-shaped part into a shape along an outer surface of a mold by applying a pressing force from the outside to a workpiece having a cup-shaped part.

  Such ironing methods include, for example, fitting a workpiece such as a ball joint outer ring to the tip of the punch body and lowering the ironing die to apply pressure to the workpiece. There is a method of plastic working the inner peripheral surface of the material into a predetermined shape (see Patent Document 1).

  Next, the outer race of the ball type constant velocity joint according to the prior art and the processing method thereof will be described in detail with reference to FIGS. As shown mainly in FIG. 5, the outer race V is composed of a cup-shaped portion 1 and a shaft portion 19 which are integrally formed coaxially with each other, and a concave portion 15 inside the cup-shaped portion 1 is a spherical surface formed on the opening side. The guide surface 16 is formed, a relief portion 17 formed on the back side thereof, and a conical chamfered portion 18 formed on the inlet side of the guide surface 16. The guide surface 16 is a part of a belt-like spherical surface centered on the central axis of the outer race V, and an undercut portion 16d having a smaller diameter than the back side is formed on the opening side. Six ball grooves 16b are formed along a plane that is equally divided into six in the circumferential direction (see FIG. 4). The escape portion 17 includes a peripheral escape portion 17a that extends from the guide surface 16 to the back side, and an intermediate escape portion 17b and a central escape portion 17c that extend concentrically on the inside, and each of the escape portions 17a, 17b, and 17c is a guide surface. 16 is located outside the spherical surface 16c indicated by a two-dot chain line which is an extended surface of the sixteen. Six escape grooves 17a1 following the ball grooves 16b are formed in the peripheral escape portion 17a.

  A retainer (supported member) 7 having an annular shape and a spherical inner and outer surface is slidably fitted and supported on the guide surface 16 of the cup-shaped portion 1, and six inner surfaces of the retainer 7 correspond to the ball grooves 16 b. The inner race 8 in which the ball groove 8 a is formed is slidably fitted and supported, and the six balls 9 held by the retainer 7 are interposed between the cup-shaped portion 1 and the ball grooves 16 b and 8 a of the inner race 8. It is provided to roll freely. The outer race V, the retainer 7, the inner race 8 and the ball 9 constitute a ball type constant velocity joint. The escape groove 17a1 is formed to temporarily hold a part of the six balls 9 when the constant velocity joint is assembled or disassembled.

  Such an outer race V according to the prior art is formed by plastic working from an intermediate material VM as shown in FIG. The intermediate material VM is composed of a cup-shaped portion 1M and a shaft portion 19 which are integrally formed coaxially with each other, and the shape of the cup-shaped portion 1M of the intermediate material VM is slightly different from the cup-shaped portion 1 of the outer race V. The recess lower hole 15M of the cup-shaped portion 1M of the intermediate material VM corresponds to the chamfered portion 15a corresponding to the chamfered portion 18 of the concave portion 15, the strip-shaped portion 15b corresponding to the spherical portion 16a, and the peripheral relief portion 17a from the opening side. And a belt-like portion 15d and a central portion 15e corresponding to the intermediate and central relief portions 17b and 17c. The central portion 15e and the belt-like portion 15d of the concave hole 15M are substantially the same as the central relief portion 17c and the intermediate relief portion 17b of the concave portion 15, but the belt-like portion 15c is wider than the peripheral relief portion 17a as it goes to the outer periphery. The belt-like portion 15b further spreads out from the spherical portion 16a and has a cylindrical end portion, and the chamfered portion 15a has a conical shape with a larger apex angle than the chamfered portion 18. Each of the belt-like portions 15b and 15c corresponding to the guide surface 16 and the peripheral escape portion 17a is formed with six lower grooves 15f and escape portion lower grooves 15g corresponding to the ball groove 16b and the escape groove 17a1, respectively. The lower groove 15f is approximately the same depth as the ball groove 16b. However, the lower groove 15b is wider than the ball groove 16b according to the expansion of the band-shaped portion 15b, and the escape portion lower groove 15g is also approximately the same depth as the escape groove 17a1. According to the spread of the part 15c, it spreads more than the escape groove 17a1.

  The outer shape of the cup-shaped portion 1M of the intermediate material VM is as follows. The base portion 2 has a cylindrical surface formed coaxially and integrally with each other from the shaft portion 19 side, and the intermediate portion 3 and the skirt portion 4 have a diameter larger than that. have. The connecting surface 2a between the base portion 2 and the intermediate portion 3 has a planar shape orthogonal to the central axis of the intermediate material VM, and the shoulder portion 5 between the connecting surface 2a and the intermediate portion 3 has an inclination angle with respect to the central axis described later. A conical inclined surface 5a having the same inclination angle as the tapered surface 31a formed in the ironing hole 31 of the die 30 is formed. The connection surface 3a between the intermediate part 3 and the skirt part 4 is a loose inclined surface located in the axial direction intermediate part of the strip | belt-shaped part 15b. The diameter of the intermediate part 3 is larger than the outer diameter of the cup-shaped part 1 of the outer race V shown in FIG.

  As shown in FIG. 6, the molding die 20 used for molding the concave portion 15 inside the cup-shaped portion 1 is a rod as a whole, and is provided with a column portion 21 and a guide surface 16 of the outer race V provided at the tip thereof. The guide surface molding part 22 for the purpose and the relief molding part 23 provided continuously from the guide surface molding part 22 to the front end side to form the peripheral relief part 17a. The guide surface molding portion 22 is a part of a belt-like spherical surface centered on the central axis of the support column 21, and includes the central axis and extends to the guide surface 16 along a plane that divides the guide surface 16 into six equal parts in the circumferential direction. Six protrusions 22b for forming the ball groove 16b are formed (see FIG. 4). Each protrusion 22b extends to the column part 21, and the diameter of the column part 21 excluding the protrusion 22b is smaller than the diameter of the guide surface molding part 22. The relief forming part 23 is a rotating body centered on the central axis of the mold 20, and six protrusions 23a for forming relief grooves 17a1 in the peripheral escape part 17a are formed continuously on each protrusion 22b. ing. The molding die 20 is divided into a central core metal 25 and four partial molds 26a to 26d arranged around the central metal core 25. The core metal 25 abuts against the central relief portion 17c of the recess 15 and is an intermediate material of the outer race V. A top surface 24 for positioning and supporting VM is formed.

  As shown in FIG. 6, the die 30 used for the ironing process has an annular shape in which the ironing hole 31 having a conical tapered surface 31 a is formed, and the inclination angle of the tapered surface 31 a with respect to the central axis is the inclination of the shoulder portion 5. It is the same as the inclination angle of the surface 5a. The diameter of the ironing hole 31 is the same as the outer diameter of the cup-shaped portion 1 of the outer race V shown in FIG.

  Next, the processing method of the recessed part 15 of the outer race V of the prior art mentioned above is demonstrated in detail by FIG.6 and FIG.7. First, the guide surface molding part 22 and the relief molding part 23 at the tip of the molding die 20 fixed upright on the base 29 are inserted into the recess pilot hole 15M of the cup-like part 1M of the intermediate material VM of the outer race V, and the recess The central portion 15e of the pilot hole 15M is brought into contact with the top surface 24 of the mold 20 and positioned and supported. 6 and 7, the taper surface 31a of the die 30 is brought into contact with the inclined surface 5a of the shoulder portion 5 of the intermediate material VM, and the die 30 is pressed downward. The inclined surface 5a of the shoulder portion 5 of the intermediate material VM is pressed inward by the surface 31a, and the outer diameters of the intermediate portion 3 and the skirt portion 4 of the cup-shaped portion 1M of the intermediate material VM become the inner diameter of the ironing hole 31. The cup-shaped portion 1M is reduced in diameter, and the strip-shaped portion 15b, the strip-shaped portion 15c, the lower groove 15f, and the relief lower groove 15g of the recess lower hole 15M are formed by the spherical molding portion 22a, relief molding portion 23, and protrusions of the molding die 20, respectively. The outer race V is molded by being plastically deformed until it comes into contact with the outer surfaces of the b and the protrusions 23a.

As described above, since the inclination angle of the tapered surface 31a of the ironing hole 31 is the same as the inclination angle of the inclined surface 5a of the shoulder portion 5, from the start of the ironing process shown in FIGS. The inclined surface 5a of the 1M shoulder portion 5 abuts against the tapered surface 31a of the ironing hole 31 over the entire width, and the cup-shaped portion 1M is pressed in a direction perpendicular to the tapered surface 31a. 7, the taper surface 31a of the ironing hole 31 contacts the inclined surface 5a of the shoulder portion 5 of the intermediate material VM. The line K extending inward and orthogonally intersects the intermediate portion of the relief forming portion 23 that forms the peripheral relief portion 17a, so that the portion on the opening side of the intermediate portion of the recess lower hole 15M of the cup-like portion 1M However, it is plastically deformed until it has a shape that reliably contacts the outer surfaces of the guide surface molding portion 22 and the relief molding portion 23 of the mold 20.
JP 2001-219238 (paragraph [0003], FIG. 7, FIG. 8).

  The prior art outer race V formed as described above includes a portion indicated by a two-dot chain line 6 in FIG. 5, and this portion is not only unnecessary as the outer race V, but this results in excessive weight, Since it may interfere with other members, it is used after being removed by cutting. However, there is a problem that the material cost increases by this portion and the number of processing steps increases by this cutting. On the other hand, the peripheral escape portion 17a does not slidably guide and support the retainer (supported member) 7. Therefore, the peripheral escape portion 17a is plastically deformed until it has a shape that comes into contact with the escape molding portion 23 accurately. There is no need. The present invention solves the above-mentioned problems by minimizing the range in which the cup-shaped portion 10 is in a shape that comes into contact with the mold 20 accurately, and the manufacturing cost of a workpiece such as an outer race that is ironed. It aims at lowering.

To this end, the ironing method according to the present invention includes a guide surface molding portion for forming a guide surface for slidably guiding and supporting a supported member on the inner surface of a cup-shaped portion of a workpiece, and the guide surface molding portion. Provided with a relief forming part for forming a relief part on the inner surface of the cup-like part, which is provided continuously on the tip side of the cup and forms a gap with the supported member when the supported member slides. The intermediate material of the workpiece is inserted into the intermediate material cup-shaped part of the workpiece with the guide surface molding part and relief molding part at the tip of the molding die inserted inside the cup-shaped part of the intermediate material of the workpiece. From the top side, it passes through the ironing hole of the die and presses the cup-shaped part of the intermediate material inward by the tapered surface formed in the ironing hole so that the inner surface of the cup-shaped part contacts the outer surface of the guide surface molding part and the relief molding part. Ironing method to plastically deform until it comes into contact shape Among the positions where the tapered surface of the ironing hole abuts against the intermediate material of the workpiece, the most advanced ironing position that is the most distal side in the axial direction of the mold passes through the boundary position between the guide surface molding part and the relief molding part. it is characterized in that set on a line perpendicular to the tapered surface Te.

  In the ironing method according to claim 1, the guide surface is preferably a part of a belt-like spherical surface centered on one point on the central axis of the cup-shaped portion and centered on the central axis.

  The ironing method according to claim 2 includes forming a plurality of ball grooves along a plurality of planes including a central axis of the mold in a part of the spherical surface and equally dividing a part of the spherical surface in the circumferential direction. Is preferred.

As described above, according to the ironing method method of the first aspect, the ironing which is the most distal side in the axial direction of the forming die among the positions where the tapered surface of the ironing hole abuts against the intermediate material of the workpiece W. advanced position, since the set on a line perpendicular to the tapered surface through the boundary position of the guide surface molding portion and the relief forming portion, ironing advanced position close to the opening side of the cup-shaped portion than in the prior art ironing The processing length is reduced, and the guide surface of the cup-shaped portion of the workpiece W has an accurate shape that is surely brought into contact with the guide surface forming portion of the mold. Since the length of the cup-shaped part to be ironed is reduced in this way, the material cost is reduced, and the man-hours required for removing unnecessary parts are also reduced, thereby reducing the manufacturing cost and further reducing the work cost. Since the shape of the guide surface of the cup-shaped portion of the object is accurate, the accuracy of guide support of the supported member does not deteriorate.

  The guide surface is a part of a belt-like spherical surface centered at one point on the central axis of the cup-shaped portion, and according to the invention of claim 2, a swingable ball joint is obtained. be able to.

  The invention according to claim 3, wherein a plurality of ball grooves are formed along a plurality of planes including a central axis of the mold in a part of the spherical surface and equally dividing a part of the spherical surface in the circumferential direction. Thus, it is possible to obtain a ball joint that can swing and transmit a rotational force.

  The ball-type constant velocity joint is a fixed joint used as an outboard joint for a front-wheel drive vehicle (which allows a large axis crossing angle and does not slide), and is similar to the conventional one shown in FIG. Detailed description is omitted. Hereinafter, the best mode of the ironing method according to the present invention will be described with reference to FIGS. The outer race (workpiece) W to be ironed according to this embodiment is an outer race V which has been ironed according to the prior art shown in FIG. 5 except that the two-dot chain line portion removed by cutting after the ironing is small. Is substantially the same.

  Further, the intermediate material WM of the outer race W according to this embodiment is made of steel, and as shown in FIG. 1, it is composed of a cup-shaped portion 10M and a shaft portion 19 which are integrally formed coaxially with each other. The outer shape of the portion 10M has a cylindrical surface-shaped base portion 11 formed coaxially and integrally with each other from the shaft portion 19 side, and an intermediate portion 12 and a skirt portion 13 having a diameter larger than that. The connecting surface 11a between the base portion 11 and the intermediate portion 12 has a conical surface coaxial with the central axis of the intermediate material WM, and the shoulder portion 14 between the connecting surface 11a and the intermediate portion 12 has an iron hole in the die 30. A conical inclined surface 14 a having the same inclination angle as the inclination angle θ (see FIG. 2) with respect to the central axis of the tapered surface 31 a formed on 31 is formed. The connection surface 12a between the intermediate portion 12 and the skirt portion 13 is a loosely inclined surface located at the intermediate portion in the axial direction of the belt-like portion 15b inside the cup-like portion 10M, as in the prior art shown in FIG. Moreover, the diameter of the intermediate part 12 is larger than the outer diameter of the cup-shaped part 10 of the outer race W shown in FIG. The shape of the recess prepared hole 15M of the cup-shaped portion 10M of the intermediate material WM is the same as the recess prepared hole 15M of the prior art shown in FIG.

  The forming die 20 used for forming the concave portion 15 inside the cup-shaped portion 10 and the die 30 (FIG. 1) used for ironing are the same as the forming die 20 and the die 30 of the prior art shown in FIG.

  Next, a method for processing the recess 15 of the outer race W according to the above-described embodiment will be described with reference to FIGS. Similar to the prior art shown in FIG. 6, the guide surface molding portion 22 and the relief molding portion 23 at the tip of the molding die 20 standing and fixed on the base 29 are placed in the recess lower hole 15M of the cup-shaped portion 10M of the outer race W. The protrusion 22b and the lower groove 15f are inserted so that their phases coincide with each other, and the central portion 15e of the recessed lower hole 15M is brought into contact with the top surface 24 of the mold 20 to be positioned and supported. 1 and 2, the tapered surface 31a of the ironing hole 31 of the die 30 is brought into contact with the inclined surface 14a of the shoulder portion 14 of the intermediate material WM. As described above, since the inclination angle of the inclined surface 14a of the shoulder portion 14 is the same as the inclination angle θ of the tapered surface 31a of the ironing hole 31, the inclination of the shoulder portion 14 of the cup-shaped portion 10M at this contact point. The surface 14a abuts against the tapered surface 31a of the ironing hole 31 over the entire width. In FIG. 2, the position A, which is the upper end of the inclined surface 14a, is the position of the mold 20 out of the positions where the tapered surface 31a contacts the intermediate material WM of the outer race W. It is the position that is the most distal side in the axial direction (the most advanced position of ironing).

In this embodiment, a line L perpendicular to the taper surface 31a of the ironing hole 31 passes through the boundary position B between the guide surface molding portion 22 and the relief molding portion 23, which is the starting position of the mold shape transfer necessary range of the molding die 20. The outer shape of the intermediate material WM is set so that the foremost position A is located above. That is, in FIG. 2, the coordinate axes x and y are directed rightward and upward, respectively, the x and y coordinates of the ironing most advanced position A are (m, n), and the x and y coordinates of the boundary position B are (k, l). And the following equation: n−l = (m−k) tan θ
The outer shape of the intermediate material WM is set so that is established.

  As shown in FIGS. 1 and 2, if the taper surface 31a of the die 30 is brought into contact with the inclined surface 14a of the shoulder portion 14 of the intermediate material WM and the die 30 is pressed downward, the taper surface 31a of the ironing hole 31 The inclined surface 14a of the shoulder portion 14 of the intermediate material WM is pressed inward, and the outer diameter of the intermediate portion 12 and the bottom portion 13 of the cup-shaped portion 10M of the intermediate material WM is the inner diameter of the ironing hole 31, as shown in FIG. The cup-shaped portion 10M is reduced in diameter until it reaches the bottom, and the strip-shaped portion 15b, the strip-shaped portion 15c, the lower groove 15f, and the relief lower groove 15g of the recess lower hole 15M are formed into the spherical molded portion 22a and relief-formed portion of the molding die 20, respectively. 23, and is plastically deformed until it comes into contact with the outer surface of the ridge 22b and the ridge 23a, thereby forming a spherical surface portion 16a and a ball groove 16b of the guide surface 16, and a peripheral escape portion 17a and its escape groove 17a1, respectively. Recess 15 of the outer race W is molded Te. Of these, the guide surface 16 composed of the spherical surface portion 16a and the ball groove 16b is closer to the opening side of the cup-shaped portion 10M than the line L, and the pressing force applied to the inclined surface 14a from the tapered surface 31a of the die 30 is directly applied. The guide surface molding portion 22 composed of the spherical molding portion 22a and the protrusion 22b is securely brought into contact with the guide surface molding portion 22 and molded into an accurate shape. This ironing process is performed by cold working.

  Since the skirt portion 13 of the cup-shaped portion 10M has a larger diameter than the intermediate portion 12, the degree of diameter reduction of the ironing process by the die 30 is larger in the skirt portion 13 than in the intermediate portion 12, and thereby the guide surface 16 An undercut portion 16d on the opening side is formed. In the molding die 20, the cored bar 25 is first drawn down, the first and second partial molds 26 a and 26 b are drawn inward, and then the third and fourth partial molds 26 c and 26 d are drawn inward. That's fine. As shown in FIG. 3, the outer race W formed by ironing is cut into an unnecessary portion outside the two-dot chain line D to obtain a finished product. This excision part is smaller than the excision part in the prior art shown by the two-dot chain line 6 in FIG.

  In this embodiment, as described above, the foremost ironing position A is located on the line L perpendicular to the tapered surface 31a of the ironing hole 31 through the boundary position B between the guide surface molding part 22 and the relief molding part 23. In contrast, in the prior art shown in FIGS. 6 and 7, as described above, a line extending inwardly from the leading edge position C of the intermediate material WM is orthogonal to the tapered surface 31a. Since K intersects the intermediate portion of the relief forming portion 23, the line L in this embodiment is moved to the opening side of the cup-shaped portion 10M as compared to the line K of the prior art, and this line L has moved. The most advanced position A that is ironed by the distance approaches the opening side of the cup-shaped portion 10M. As a result, the length of the ironing process is reduced, so the ironing time is reduced, and the material can be reduced by reducing the outer diameter of the part other than the ironing process, which can reduce the material cost. At the same time, the processing man-hours required for removing unnecessary portions is reduced, so that the manufacturing cost can be reduced. In addition, the guide surface 16 composed of the spherical surface portion 16a and the ball groove 16b is surely brought into contact with the guide surface molded portion 22 composed of the spherical surface forming portion 22a and the protrusion 22b and is formed into an accurate shape. The accuracy of guiding and supporting the ball 9 is not lowered. It should be noted that the peripheral relief portion 17a including the relief groove 17a1 has sufficient accuracy required in the preceding process of ironing. Therefore, the peripheral relief portion 17a including the relief groove 17a1 does not necessarily need to be surely brought into contact with the relief molding portion 23 including the protrusion 23a. In the constant velocity joint shown in FIG. 5, it is not necessary to guide and support the retainer 7 with the peripheral escape portion 17a, and it is not necessary to support the ball 9 with the escape groove 17a1, so that the shape of the peripheral escape portion 17a including the escape groove 17a1 is obtained. A highly accurate shape by ironing is not required. In short, the relief groove 17a1 only needs to temporarily hold the ball 9 when the constant velocity joint is assembled or disassembled, and so high accuracy is not required.

  In the above-described embodiment, the inclination angle of the inclined surface 14a formed in the shoulder portion 14 of the material WM between the outer races W is the same as the inclination angle θ of the tapered surface 31a of the ironing hole 31 of the die 30. In this case, as shown in FIG. 2, the inclined surface 14a is in contact with the tapered surface 31a of the ironing hole 31 with the full width at the start of contact, and therefore the ironing cutting edge position A of the tapered surface 31a also exists first. However, the present invention is not limited to this, and the inclination angle of the inclined surface 14a of the shoulder portion 14 is made larger than the inclination angle θ of the tapered surface 31a of the ironing hole 31, and the tapered surface 31a is first inclined at the position on the opening side. As the die 30 is moved downwardly in contact with the surface 14a, the contact range extends to the upper side of the inclined surface 14a, and when reaching the upper edge of the inclined surface 14a, the ironing which is the most distal side in the axial direction of the mold 20 It is also possible to carry out by making contact at the most advanced position A. In this case, the ironing most advanced position A is generated at a position where the die 30 is moved downward to some extent. Alternatively, the shoulder portion 14 may be formed at a right angle without forming the inclined surface 14a.

  In the above-described embodiment, the present invention is applied to an outer race of a ball-type constant velocity joint, and a guide surface 16 formed on the inner surface of the cup-shaped portion 10 is set at one point on the central axis of the cup-shaped portion 10. A plurality of ball grooves are formed along a plurality of planes that are a part of a belt-like spherical surface that is centered on the same central axis and that divides the spherical surface in the circumferential direction. However, the present invention is not limited to this, and can be implemented without a plurality of ball grooves. In this case, an outer race for a swingable ball joint that does not transmit torque is obtained.

It is a longitudinal cross-sectional view in the time of the ironing process start of one Embodiment of the ironing process method by this invention. It is a partial expanded sectional view which shows the principal part of embodiment shown in FIG. It is a longitudinal cross-sectional view in the end of the ironing process of the embodiment shown in FIG. FIG. 4 is a sectional view taken along line 4-4 of FIG. 3. It is sectional drawing which shows an example of the workpiece processed by the ironing method. It is a longitudinal cross-sectional view equivalent to FIG. 1 of an example of the ironing method by a prior art. It is a partial expanded sectional view equivalent to FIG. 2 of the prior art shown in FIG.

Explanation of symbols

7 ... Supported member (retainer), 10 ... Cup-like part, 10M ... Cup-like part of intermediate material, 16 ... Guide surface, 16b ... Ball groove, 17 ... Escape part, 20 ... Mold, 22 ... Guide surface molding part , 23 ... relief forming part, 30 ... die, 31 ... ironing hole, 31a ... taper surface, A ... ironing cutting edge position, B ... boundary position, L ... line, W ... workpiece (outer race), WM ... intermediate material.

Claims (3)

A guide surface molding portion for forming a guide surface for slidably guiding and supporting the supported member on the inner surface of the cup-shaped portion of the workpiece, and the guide surface molding portion continuously provided on the distal end side of the guide surface molding portion. Using a rod-shaped mold comprising a relief molding part for forming a relief part on the inner surface of the cup-like part to form a clearance part between the support member and the supported member when sliding. In a state where the guide surface molding portion and the relief molding portion at the tip of the mold are inserted inside the cup-shaped portion of the intermediate material of the workpiece, the cup-shaped portion of the intermediate material of the workpiece is inserted into the die from the top side. A shape that passes through the ironing hole and presses the cup-shaped portion of the intermediate material inward by a tapered surface formed in the ironing hole, so that the inner surface of the cup-shaped portion comes into contact with the outer surface of the guide surface molding portion and the relief molding portion. To the ironing method of plastic deformation until The most advanced ironing position in the axial direction of the molding die among the positions where the tapered surface of the ironing hole abuts against the intermediate material of the workpiece is the boundary between the guide surface molding part and the relief molding part. ironing wherein the set to on line that is perpendicular to the tapered surface through the position. 2. The ironing method according to claim 1, wherein the guide surface is a part of a belt-like spherical surface centered on one point on the central axis of the cup-shaped portion and centered on the central axis. Processing method. 3. The ironing method according to claim 2, wherein a plurality of ball grooves are formed along a plurality of planes including a central axis of the molding die in a part of the spherical surface and equally dividing a part of the spherical surface in the circumferential direction. The ironing method characterized by having performed.

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