JPS62110831A - Forming method for inner wheel of uniform joint - Google Patents

Abstract

PURPOSE:To obtain the product having smooth bond on an inflection face by performing the optimum distribution of the ironing allowance for the adjustment of the relative positional relation of the inflection face for both end faces and the 2nd forging stage in case of forming an intermediate forming item in the 1st forging stage. CONSTITUTION:The intermediate forming item 42b that the distance from the 1st end face 45 upto the position of an inflection face 47 is longer than that of a completed item 42c and that from the inflection face 47 upto the 2nd end face 46 is shorter than that of the completed item 42c, is formed in the 1st stage. The ironing allowance t' for the 2nd stage is made thickest at the bottom part of each guide groove part 43 and thinner gradually from the bottom part toward the outmost peripheral part 44. In the 2nd stage the completed item 42c is subjected to a die forming by a die push-in forging in the stage of giving the relief gap of the material accompanied by the material deformation to each the outmost peripheral part 44 and 2nd end face 46 part. In this way the inner wheel of the uniform joint having smooth bond on the inflection face can be forged.

Description

Detailed Description of the Invention A6 Object of the Invention +11 Industrial Field of Application The present invention relates to a method of forming an inner ring of a constant velocity joint, and particularly relates to a method for forming an inner ring of a constant velocity joint. A plurality of guide grooves for torque transmission balls are formed along a plane, and the locus line of the center of the ball rolling along the guide groove smoothly continues to a straight line parallel to the center line and to this straight line. The present invention relates to a method for forging an inner ring of a constant velocity joint, which comprises a circular arc portion.

(2) Prior Art First, a constant velocity joint to which the present invention is directed will be explained with reference to the drawings.

In FIG. 1, on the inner peripheral surface of the outer ring 2 of the constant velocity joint, which is attached to the end of the rotating shaft 1 so as to be able to rotate integrally with the rotating shaft 1, outer rings 2 are arranged at equal intervals in the circumferential direction. A plurality of guide grooves 3 are formed along the longitudinal section of the rotary shaft 1, and the rotary shaft 1 is attached to an end of the rotary shaft 4 facing the rotary shaft 1 in the axial direction so as to be able to rotate integrally with the rotary shaft 4. On the outer peripheral surface of the inner ring 5 of the constant velocity joint, a plurality of guide grooves 6 are formed along the vertical cross section of the inner ring 5, facing each guide groove 3, and the guide grooves 3 and guide grooves facing each other are formed on the outer peripheral surface of the inner ring 5 of the constant velocity joint. Groove 6
A torque transmitting ball 7 held by a ball holding ring 8 is interposed between the two. A dustproof boot 9 is installed between the outer circumference of the outer ring 2 and the outer circumference of the rotating shaft 4 to protect the internal mechanism of the constant velocity joint.

(Thus, torque can be freely transmitted between the rotating shaft 1 and the rotating shaft 4 not only when the center line of the rotating shaft 1 and the center line of the rotating shaft 4 are in a straight line, but also when they cross each other. will be held.

In FIGS. 2 and 3, the inner ring 5 has first end surfaces 1 perpendicular to its center line C and spaced apart from each other in the axial direction.
0 and a second end surface 11, and the contact points 12, 13 between each guide groove 6 and the ball 7 are mutually located from the center line C of the inner ring 5 on any cross section perpendicular to the center line C of the inner ring 5. The center of each ball 7 is on the circle 14. The outermost circumferential surface 15 of the inner ring 5 is on the same circumference with each guide groove 6 in between, but after forging, it is machined and polished into a spherical shape, so the accuracy of the forged material is the same as that of each guide groove 6. It's not as demanding as it seems on the surface.

The inner ring 5 of the constant velocity joint, which is the object of the present invention, is particularly characterized by its vertical cross-sectional shape. It is a straight line up to the virtual inflection surface 16 perpendicular to the center line C, and the line from the inflection surface 16 to the second end surface 11 smoothly continues to the straight line from the first end surface 10 to the inflection surface 16. The longitudinal cross-sectional shape of the inner ring 5 is set so as to draw a circular arc with a radius R that gradually approaches the center line C toward the end face 11.

The shape of the inner ring 5 as described above is such that it can be easily released from the mold even if it is forged using a mold, and the trajectory line 17 of the center of the ball 7 rolling along the guide groove 6 is a shape that allows the inner ring 5 to be easily released from the mold. It is difficult to process a molded body with a composite shape, which consists of a straight part and a circular arc part, using a normal polishing machine, and if the post-forging processing of the guide groove 6 is not required, productivity will be improved. For these reasons, conventional attempts have been made to form the inner ring 5 by forging with a die.

FIG. 4 shows a conventional forging process for forming the inner ring 5. First, in FIG. 4(a), the forging die consists of a concave die 18 and a convex die 19, and the concave die 18 defines the protrusions 20 for forming each guide groove 6 of the inner ring 5 and the second end surface 11. each protrusion 20 has a bottom surface 21 for
The side of the opening end of the concave mold 18 from the inflection surface 22 corresponding to the inflection surface 16 (FIG. 3) has a linear longitudinal cross-sectional shape parallel to the center line of the concave mold 18, and from the inflection surface 22 to the bottom surface 21. It has an arcuate vertical cross-sectional shape that smoothly continues to the linear vertical cross-sectional shape up to and approaches the center line of the concave mold 18 as it goes toward the bottom surface 21 . In contrast, the convex die 19 has an axial groove 23 that aligns with each protrusion 20 of the concave die 18 and a forging surface 23a.

In forming the inner ring 5, the round bar material 25 is positioned in the concave mold 18 as shown in FIG. 4(a), and then the convex mold 19 is placed in the concave mold 18 as shown in FIG. 4(b). When press-fitted, the round bar material 25 is forged and deformed along the shape of the concave die 18.

Then, the convex mold 19 is retreated from the concave mold 18, and the molded product 25'
is released from the concave mold 18 as shown in FIG. 4(C), and the first end surface 26, second end surface 27 and curved surface 28 are
A molded article 25' is obtained.

(3) Problems to be Solved by the Invention In the conventional forming method as described above, the amount of material deformation is large and the forging load during forming is also large, resulting in distortion of the mold during forging and There were many variations in dimensions due to the elastic return of the molded product after forging around the circular arc portion shown by the arrow 29 in C), and even if the precision of the mold was improved, there was a limit to the improvement of the precision of the molded product. In particular, the elongation rate of the material in the area forming the guide groove 6 is high and the contact pressure against the mold is high, so the wear of the mold is significant around the part indicated by the arrow 24 in FIG. 4(a), and the same When manufacturing a large number of inner rings 5 using a mold of The dimensional deviation of product 25' after heat treatment was large, and it could not be used as the inner ring of a constant velocity joint as it was.

In addition, in order to lower the deformation resistance of the material, the material 25
If the material 25 is heated, problems such as variations in the temperature of the material 25, changes in dimensions due to thermal distortion and shrinkage of the molded product 25', softening of the mold and variations in lubrication performance may occur, causing the mold and the molded product 25' to be heated.
Maintaining the accuracy of ′ has become even more difficult.

Therefore, the forging process for forming the inner ring 5 is divided into two processes, the first process and the second process, and in the first process, as shown in FIG. The intermediate molded product 30, which is relatively easily molded, is forged, taking into consideration a certain ironing allowance t, and in the second step, the intermediate molded product 30 obtained in the first step is forged finished. It is possible to perform processing. Figure 6 shows the first
The second step performed on the intermediate molded product 30 shown in FIG. 5 obtained by the step is shown.

As shown in FIG. 6(a), the intermediate molded product 30 is placed in the concave mold 3.
Position the convex mold 3 at the opening 1 as shown in Fig. 6(b).
After press-fitting the intermediate molded product 30 into the concave mold 31 in step 2, the intermediate molded product 30 is further pressed until it becomes the final molded product 30' as shown in FIG. 6(d). As shown, a final molded article 30' having a first end surface 35, a second end surface 36, an inflection surface 37 and a guide groove 39 is obtained.

However, in this case, although the part from the first end surface 35 to the inflection surface 37 of the final molded product 30' is ironed to improve surface accuracy, the part above the inflection surface is mainly ironed. In addition, the lower part is mainly squeezed and crushed, and the deformation style and degree of forming are different between the upper part and the lower part, and in the case of Fig. 6, the degree of forming of the lower part is larger and the difference in level occurs. . Similarly, although not shown, in the first step, a material having approximately the same axial length and arc length I as the finished product is formed, and in the second step, the groove portion is iron-formed, the portion below the inflection surface. In this case, the material is mainly pressed against the mold and undergoes almost no deformation, so it is not sufficiently stretched and the surface roughness does not improve.

Furthermore, the degree of forming is also small, resulting in an opposite level difference. As described above, in forming grooves in the inner ring of a constant velocity joint having a composite shape of a straight line portion and a circular arc portion, the quality of the connection at the curved surface is a major issue. In particular, as shown in FIG. 6(b), excessive pressure is applied to the curved surface 33 of the concave mold 31, causing abnormal wear, and as shown in FIG. 6(d), the final molded product 33 The amount of elastic return is large in the portion indicated by the arrow 38 between the curved surface 37 and the second end surface 36. Regarding the guide groove of a molded product, the ironing direction changes from the radial direction to the circumferential direction as it goes radially outward, so if it is simply ironed under a uniform ironing width t, the radial direction On the outside, the ironing rate of the grooves increases, making it difficult to squeeze in the circumferential direction, which tends to cause galling against the mold, and there is a limit to the improvement of precision. In addition, in the second step, if the outermost periphery and the second end surface are restrained by the mold, the free flow of the material in one direction during ironing of the groove and drawing of the arc portion will be inhibited, causing interference and resulting in poor performance. Don't follow the pattern.

As a result of the experiment, as shown in FIG. 7, in each guide groove 39 of the final molded product 30', a ball and a straight part of the longitudinal section on the first end surface 35 side, with the curved surface 37 as a boundary, were found. A contact line 40.40', which is a contact point locus, is generated, whereas a contact line 41 is formed on the arcuate longitudinal section on the second end surface 36 side.
．． 41' occurs, and the contact line 40 and the contact line 41 and the contact line 40' and the contact line 41' do not continue smoothly, respectively.
This means that it cannot be used as a ball transfer surface for constant-velocity gravity.

In view of the above-mentioned circumstances, the main object of the present invention is to provide a method for forming the inner ring of a constant velocity joint using only a forging process with high precision and which can be put to practical use.

B0 Configuration of the Invention (1) Means for Solving the Problems According to the present invention, the invention has a first end face and a second end face perpendicular to the center line, and the first end face and the second end face are perpendicular to the center line. A plurality of guide grooves for torque transmission balls are formed along a longitudinal plane passing through the center line at equal intervals in the circumferential direction on the side circumferential surface between the end face of the
The locus lines of the centers of the balls rolling along each of the guide grooves are equidistant from the center line on any cross section perpendicular to the center line, and extend from the first end surface to the center line. A straight line is parallel to the center line up to an imaginary inflection surface perpendicular to , a straight line is connected smoothly to the straight line from the inflection surface to the second end surface, and the center line gradually extends toward the second end surface. 1. A method for forming an inner ring of a constant velocity joint to have an arc shape approaching .
and a second forging step, and in the first forging step, the distance from the first end surface to the position of the inflection surface is the axial length of the finished product from the first end surface to the inflection surface. The length from the curved surface to the second end surface is shorter than the length in the axial direction from the curved surface to the second end surface of the finished product, and the amount is determined in the second step. The ironing allowance for the second step is set so that the degree of forming above the curved surface and the degree of forming below the curved surface are well balanced. It is formed so that it becomes gradually thinner from the bottom toward the outermost periphery, and in the second forging step, an escape gap for the material is provided at each outermost periphery and the second end surface as the material deforms. A method for forming an inner ring of a constant velocity joint is obtained, which is characterized in that die forming is performed by die forging in a state in which the inner ring of a constant velocity joint is formed.

(2) Function The purpose of the first forging process is to form an intermediate molded product, and particularly high precision is not required. For this reason, for example, by conventional die forging from a round bar material, the first
The distance from the end surface to the inflection surface is longer than the axial length from the first end surface to the inflection surface of the finished product, and the length from the inflection surface to the second end surface is longer than the axial length from the inflection surface to the second end surface of the finished product. An intermediate molded product of the inner ring is obtained, which is shorter than the length in the direction and has a shape in which the ironing allowance is thickest at the bottom of each guide groove and gradually becomes thinner from the bottom of each guide groove toward the outermost periphery.

In the second forging step, the bottom of each guide groove of the intermediate molded product receives the most radially inward forging force,
By receiving circumferential forging force from the bottom of each guide groove toward the outermost periphery, the material stretches in the axial direction and at the same time flows from the bottom of each guide groove to the outermost periphery. is forced radially outward toward the relief gap. As a result, important points in each guide groove are squeezed intensively, and the flow of material is not hindered and can easily follow the mold.
Also, by varying the ironing distance, the forging load can be reduced compared to the conventional method, and the entire guide groove can be squeezed in a well-balanced manner, improving accuracy.

In the second forging process, the straight longitudinal section of the intermediate molded product is squeezed first, then the material flows into the arcuate longitudinal section, and the second end surface is pushed out toward the concave escape gap. . As a result, at the end of forging, the distance from the curved surface of the intermediate molded product to the second end surface increases to reach the distance from the curved surface to the second end surface of the finished product. At this time, the second end face portion is not restrained by the mold, so the flow is not hindered, and the second end face portion is easily imitated to the mold, improving accuracy. At the same time, the degree of molding above the curved surface and below the curved surface is balanced, and a product with no dimensional difference between the top and bottom, and therefore a smooth connection at the curved surface can be obtained.

(3) Examples An embodiment of the present invention will be described below with reference to the drawings.

In Fig. 8, first, the first forging process is performed as shown in Fig. 8 (a).
) An intermediate molded product 42b as shown in FIG. 8(b) is obtained from a round bar material 42a as shown in FIG. 8(b) by, for example, a conventional forging method. Hereinafter, the intermediate molded product 42b will be explained in detail according to FIG. 9 and FIG. 1O. Intermediate molded product 42
b has a guide groove portion 43, an outermost peripheral portion 44, a first end surface 45 and a second end surface 46 perpendicular to the center line C', and has an imaginary change perpendicular to the center line C'. Curved surface 47
has. The length from the first end surface 45 to the curved surface 47 is longer than the length in the axial direction from the first end surface to the curved surface of the finished product;
The length in the axial direction from the curved surface 47 to the second end surface 46 is
The distance from the curved surface of the finished product to the second end surface, that is, the reference surface 48, is shorter by the reduced length p. Regarding the ironing allowance t', the ironing allowance t'max is the thickest at the bottom of each guide groove part 43, and the ironing allowance t' gradually becomes thinner as it goes from the bottom of each guide groove part 43 toward the outermost peripheral part 44. .

The intermediate molded product 42b is subjected to intermediate treatments such as annealing, shot blasting, and lubrication, and then undergoes a second forging process to transform the intermediate molded product shown in FIG. 8(b) into a shape shown in FIG.
) is obtained as a finished product 42C. The second forging process will be described in detail below.

In FIG. 11(a), the mold consists of a concave mold 49 and a punch or convex mold 50, and the concave mold 49 has a protrusion 51 for squeezing the guide groove 43 of the intermediate molded product 42b and an imaginary curved surface 52. , a virtual reference plane 53, and a knockout member 55 is disposed opposite the reference plane 53, and a material escape gap 54 is formed at the position of the reference plane 53. Similarly, as shown in FIG. 12, a material escape gap 57 is formed in each groove bottom 56 of the concave mold 49 facing each outermost peripheral portion 44 of the intermediate molded product 42b. On the other hand, grooves 58 are formed on the outer periphery of the convex mold 50 to match the respective protrusions 51 of the concave mold 49.

As shown in FIG. 11<a>, when the intermediate molded product 42b is positioned at the opening of the concave mold 49 and then pressed by the convex mold 50, the guide groove 43 is first pressed against the guide groove 43 at the opening of the concave mold 49 as indicated by the arrow 59. is started. As the ironing against the guide groove portion 43 progresses, the material moves in the axial direction and simultaneously flows toward the outermost peripheral portion 44 , and the outermost peripheral portion 44 is pushed out toward the escape gap 57 . In this way, the outermost periphery 44 does not lose its escape and is sealed, so the forging load can be reduced compared to the conventional method, and since the ironing allowance changes depending on the material, the entire guide groove 43 is It is well-balanced and improves accuracy.

When the intermediate molded product 42b continues to be pushed further, the arcuate longitudinal section of the intermediate molded product 42b is forged by the arcuate longitudinal section of the concave die 49, and the material near the second end face 46 has no place to escape and is trapped. 11(b), it is pushed out toward the escape gap 54. At this time, the amount by which the material near the second end surface 46 is pushed out in the axial direction is set in advance to be the reduced length l.

When the molded product thus obtained is released from the concave mold 49 and taken out, a finished product 42C is obtained as shown in FIG. 8(c).

In this finished product 42C, the distance from the first end surface 45 to the curved surface 47 is equal to the distance in the axial direction from the first end surface 10 to the curved surface 16 of the target inner ring 5 in FIG.
Also, the distance from the curved surface 47 to the second end surface 46' is equal to that of the inner ring 5.
is equal to the axial distance from the inflection surface 16 to the second end surface 11. □At this time, the setting of the reduced length l is particularly important. If the reduction length 1 is too small with respect to the constant ironing allowance and the arc length of the finished product, the degree of forming of the part below the curved surface will be insufficient compared to the part above it, resulting in (
PCR-PCD) is in the negative direction, and the surface roughness of the arc portion is not improved. On the other hand, if the reduction length l is too large, the degree of forming of the lower part of the curved surface will be greater than that of the upper part, and although the surface roughness of the circular arc part will improve, -PCD) is in the positive direction, and if the setting of 1 is not appropriate, a dimensional difference, that is, a step difference will occur in the curved surface in any of the above cases, making it unusable as a ball transfer surface. The amount of reduction length l varies depending on the ironing allowance t'wax and the arc length L of the finished product, but it should be set so that the degree of forming, that is, the amount of elastic return, of the part above and below the inflection surface is equal. . Figure 14 shows a graph obtained from the experiment. Diameter t'max: Ironing allowance t'max/ at the bottom of the guide groove
D: Maximum ironing rate at the bottom of the guide groove C: As a coefficient determined by experiment, the horizontal axis shows the reduction length β as the arc length and the ironing rate t'
max/D, and the vertical axis shows the forming load p (ton), cylindricity cy (μ), and dimensional difference P in the second step.
The absolute value of CR-PCD is taken, and the scrubbing rate t'/
D, there is an optimal reduction length l for the arc length L, and the ball has small dimensional differences near the value 1.0 on the horizontal axis, where the molding ratio is equal in the part above and below the curved surface. Indicates that it is within usage range A as a transfer aspect.

As a result of the test, the stroke rate (t' 1nax /DX 1
00%) was 1.3 to 1.5%, and a sufficiently satisfactory accuracy was obtained, but it is not limited to this.

The reduced length l may be determined according to the finished product arc length L from the above explanation. In FIG. 9, the guide groove 43' shown by the dotted line after molding is formed while the material flows smoothly, so the guide groove 43' from the first end surface 45 to the inflection surface 47 is formed. The guide groove from the curved surface 47 to the second end surface 46' continues smoothly, forming a practical ball transfer surface that does not require post-processing.

FIG. 13 shows an example of a process for forming the shaft hole of the inner ring 5 of FIG. 1 through a first forging process and a second forging process. In the first forging step based on the round bar material 42a as shown in FIG. 13(a),
b) a first end face 45 and a second end face 4;
Recesses 60 and 61 are formed by forging on both end surfaces of 6, respectively, to obtain an intermediate molded product 42b. Subsequently, as shown in FIG. 13(C), an intermediate molded product 42b' is obtained by punching out the inside between each recessed part 60 and 61 to form a through hole 62, and then this intermediate molded product 42b' After performing intermediate treatments such as annealing, shot blasting, and lubrication, the inner circumferential surface of the through hole 62 is restrained in the second forging step, as shown in FIG. 13(d). An insertion hole 63 is formed to obtain a finished product 42c having a first end surface 45 and a second end surface 46'.

C9 Effects of the Invention As described above, according to the present invention, the intermediate product is formed in the first forging process, and the finished product is formed in the second forging process, so that the forging load in each process is simple. A smaller forging load is required compared to the case where a finished product is obtained in one step, and a finished product with high precision can be easily obtained in the second forging step. and,
In the first forging process, the distance from the first end surface to the position of the inflection surface is longer than the axial length from the first end surface to the inflection surface of the finished product, and the length from the inflection surface to the second end surface of the finished product is Since the intermediate molded product is formed so that the length in the axial direction from the inflection surface to the second end surface is shorter, the forming of the part above the inflection surface and the part below the inflection surface in the second forging process is It is possible to balance the degree, and it is possible to obtain a smooth guide groove with no difference in dimension between the top and bottom of the curved surface, as well as the direction of accuracy and surface roughness.

The ironing allowance for the process of text box 2 is thickest at the bottom of each guide groove of the intermediate molded product, and gradually becomes thinner from the bottom of each guide groove toward the outermost periphery. By effectively utilizing the fluidity of the material in the material and squeezing each guide groove portion in a well-balanced manner according to the width of the material, it is possible to form a guide groove with high precision without difficulty.

Furthermore, in the second forging process, a material escape gap is provided at each outermost peripheral part and second end face part as the material deforms, so that the material that is deformed and is about to be extruded is contained and free flow is prevented. It is possible to carry out forging without any hindrance. In addition, the forging load can be relatively small, the elastic return of the finished product is small, a smoothly continuous and highly accurate guide groove can be formed, and wear of the mold can be reduced.

[Brief explanation of drawings]

Fig. 1 is a longitudinal cross-sectional view showing an example of a constant velocity joint, Fig. 2 is a cross-sectional view of the forged material of the inner ring of the constant velocity joint shown in Fig. 1, and Fig. 3 is a longitudinal cross-section of the main part of the inner ring of Fig. 2. Figure 4 shows the conventional forging process of the inner ring, Figure 4 (a) is a longitudinal cross-sectional view of the mold and material showing the state immediately before forging, and Figure 4 (b) shows the mold at the end of forging. 4(C) is a vertical sectional view of the molded product taken out from the mold, FIG. FIG. 6 shows the forging process for obtaining a finished product from the intermediate molded product shown in FIG. 5, and FIG. Figure (b) is a vertical sectional view of the main parts of the mold and intermediate molded product showing the state during the forging process, No. 6
Figure (C) is a vertical cross-sectional view of the main parts of the mold and finished product after forging is completed, Figure 6 (d) is a vertical cross-sectional view of the finished product taken out from the mold, and Figure 7 is a vertical cross-sectional view of the finished product taken out of the mold. FIG. 8 is a side view for explaining the discontinuity of the guide groove portion of the obtained finished product, and FIG. ) is a vertical cross-sectional view of the material, Figure 8 (
b) is a vertical cross-sectional view of the intermediate molded product, Figure 8 (C) is a vertical cross-sectional view of the finished product, Figure 9 is a cross-sectional view of the main part for explaining the ironing allowance of the intermediate molded product, and Figure 1θ is a vertical cross-sectional view of the intermediate molded product. FIG. 11 is a longitudinal sectional view of the main part of the intermediate molded product, showing the second forging process based on an embodiment of the present invention, and FIG. Fig. 11(b) is a longitudinal cross-sectional view of the main parts of the mold and the finished product showing the state at the end of forging, and Fig. 12 is a cross-sectional view of the main parts of the mold and intermediate molded product during the forging process. A plan view, FIG. 13 shows an example of forming the shaft hole of the inner ring through a forging process based on an embodiment of the present invention, FIG. 13(a) is a longitudinal cross-sectional view of the material, and FIG. 13(b) 13(C) is a longitudinal sectional view of the main part when recesses are formed on both axial end surfaces of the intermediate molded product in the first forging process, and FIG. Sectional longitudinal sectional view, 1st
FIG. 3(d) is a vertical cross-sectional view of the main part of the finished product obtained by the second forging process, and FIG. 14 is a graph based on the experimental results. 7... Torque transmission ball, 43... Guide groove portion, 4
3'...Guide groove, 44...Outermost periphery, 45...First end surface, 46.46'...Second end surface, 47...
Inflection surface, 54.57... Evacuation gap, C'... Center line, t'... Straightening width Fig. 8 (a) /υ Fig. 10 (C) Fig. 9

Claims (1)

[Claims] The first end surface (45) has a first end surface (45) and a second end surface (46') perpendicular to the center line (C');
and the second end surface (46'), a plurality of torque transmission balls (7 ) guide groove (43
′) is formed, and the locus line of the center of the ball (7) rolling along each guide groove (43′) is the center line (
The center line (C') on any cross section perpendicular to the center line (C')
') and are equidistant from each other from the first end surface (45);
A virtual inflection surface (47) perpendicular to the center line (C') from
The section from the curved surface (47) to the second end surface (46') is a straight line parallel to the center line (C'), and the section from the curved surface (47) to the second end surface (46') smoothly connects with the straight section, and the section from the curved surface (47) to the second end surface (46') ) A method for forming an inner ring of a constant velocity joint into an arc shape that gradually approaches the center line (C'), the method comprising a first forging process and a second forging process. , in the first forging step, the first end surface (45
) to the position of the inflection surface (47) is longer than the axial length from the first end surface (45) to the inflection surface (47) of the finished product. From the curved surface (47) to the second end surface (4) of the finished product
6'), and the ironing allowance (t') for the second step is thickest at the bottom of each guide groove part (43), and is thickest at the bottom of each guide groove part (43). (43) so that it becomes gradually thinner as it goes from the bottom to the outermost peripheral part (44), and in the second forging process, each of the outermost peripheral parts (44) and the second end face (46) The material escape gap (5
4, 57) A method for forming an inner ring of a constant velocity joint, characterized in that die forming is performed by die forging in a state where the inner ring of a constant velocity joint is formed.