Patent Publication Number: US-2022212244-A1

Title: Shaft member and method for manufacturing male shaft

Description:
TECHNICAL FIELD 
     The present invention relates to a shaft member and a method for manufacturing a male shaft. For example, the shaft member has, on an outer peripheral surface thereof, a processing target portion for forming a male-side engagement portion such as a male screw portion. For example, in the method for manufacturing a male shaft, a male shaft is manufactured by rolling-processing a shaft member. 
     BACKGROUND ART 
     In a machine tool or a position adjusting device of a steering wheel, a rotational motion of a drive source such as an electric motor is converted into a linear motion by a feed screw mechanism. As the feed screw mechanism, there are a slide screw type and a ball screw type. 
     A feed screw mechanism of a slide screw type includes a screw shaft and a nut. 
     An outer peripheral surface of the screw shaft has a male screw portion in which a male screw groove having a triangular or trapezoidal cross section is spirally formed. An inner peripheral surface of the nut has a female screw portion in which a female screw groove having a triangular or trapezoidal cross section is spirally formed. The feed screw mechanism is configured by screwing the male screw groove of the screw shaft and the female screw groove of the nut. 
     A feed screw mechanism of a ball screw type includes a screw shaft, a nut, and a plurality of balls. 
     An outer peripheral surface of the screw shaft has a male screw portion in which a male screw groove having an arc-shaped cross section is spirally formed. An inner peripheral surface of the nut has a female screw portion in which a female screw groove having an arc-shaped cross section is spirally formed. The plurality of balls are rollably disposed between the male screw groove of the screw shaft and the female screw groove of the nut. 
     Regardless of whether the feed screw mechanism is a feed screw mechanism of a slide screw type or a feed screw mechanism of a ball screw type, the male screw portion of the screw shaft configuring the feed screw mechanism can be formed by, for example, rolling processing. 
     JP-A-8-318340 (Patent Literature 1) describes a rolling machine for forming a male screw portion on an outer peripheral surface of a columnar metal material (workpiece material) by rolling processing. In the rolling machine, in a state where both axial end portions of the material are rotatably supported by jigs (center) each having a conical tip end portion, the material is sandwiched between a pair of round dies, the pair of round dies are pressed while being rotated, and thus the male screw portion is rolled on the outer peripheral surface of the material. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP-A-8-318340 
     SUMMARY OF INVENTION 
     Technical Problem 
     When the male screw portion is rolled on the outer peripheral surface of the material by using the rolling machine described in JP-A-8-318340, recessed holes each having a conical concave surface shape are formed in both axial end surfaces of the material. Then, it is necessary to position the material in a radial direction thereof by abutting conical tip end portions provided on the jig against inner peripheral surfaces of the recessed holes. 
     When the feed screw mechanism is incorporated into a structure in which another member is pressed by an axial end surface of the screw shaft, or when information for identifying a product is engraved on the axial end surface of the screw shaft, the axial end surface of the screw shaft is preferably a flat surface or a curved surface in which the recessed hole is not present. Therefore, a method of rolling a male screw portion on an outer peripheral surface of a columnar material in a state where tip end portions of a jig are abutted against inner peripheral surfaces of recessed holes formed in both axial end surfaces of the material to position the material in a radial direction thereof, and then cutting and removing an axial end portion of the material is considered. 
     However, when the axial end portion of the material is cut and removed, if the axial end portion of the material is cut in a state where the material is supported by gripping the male screw portion, the male screw portion may be deformed and the accuracy of the male screw portion may be reduced. That is, when the male screw portion of the screw shaft is gripped and a screw thread of the male screw portion is deformed so as to be recessed (a dent is formed), an inner surface of the screw groove is deformed so as to bulge (rise) accordingly. For example, when a feed screw mechanism of a ball screw type is configured by using a screw shaft in which such deformation occurs, the balls may ride on the bulging portion of the screw groove and the movement of the balls cannot be smoothly performed. 
     In order to prevent such deformation of the male screw portion, it is required to roll the male screw portion on the outer peripheral surface of the material in a state where the material is positioned in the radial direction thereof without forming recessed holes in the axial end surfaces of the material. Therefore, it is conceivable to support both axial end portions of the material by a jig having a recessed portion having a conical concave surface shape. 
     Specifically, as shown in  FIG. 14 , a pair of jigs  101 ,  101  each having a recessed portion  100  having a conical concave surface shape are supported on a frame of a processing apparatus (not shown). An inner peripheral surface of the recessed portion  100  of the jig  101  is abutted against a chamfered portion  103  of a columnar shaft member  102  that is formed at a connection portion between an axial end edge of an outer peripheral surface and an outer peripheral edge of an axial end surface of the shaft member  102 , and thus the shaft member  102  is positioned in a radial direction thereof In this state, when the outer peripheral surface of the shaft member  102  is pressed (sandwiched) from both radial sides by a pair of round dies rotating in the same direction, the shaft member  102  rotates while the chamfered portions  103  are in sliding contact with the inner peripheral surfaces of the recessed portions  100  of the jigs  101 . As a result, a male screw portion is rolled on the outer peripheral surface of the shaft member  102 .  FIG. 14  shows a state after the male screw portion is formed on a processing target portion on the outer peripheral surface of the shaft member  102 . 
     However, also in the above-described method, the following problem may occur. That is, in the method shown in  FIG. 14 , the chamfered portion  103  is a C-chamfered portion having a linear cross-sectional shape. Therefore, when the shaft member  102  is rotated in a state where the inner peripheral surface of the recessed portion  100  of the jig  101  is strongly pressed against the chamfered portion  103  in order to support both axial end portions of the shaft member  102 , burr or protrusion excess portions  104   a ,  104   b  may be formed to protrude in the radial direction from the axial end edge of the outer peripheral surface of the shaft member  102 , or may be formed to protrude in the axial direction from the outer peripheral edge of the axial end surface of the shaft member  102 , as exaggeratedly shown in  FIG. 15 . 
     As a result, after the male screw portion is formed, an operation (processing) for removing the excess portions  104   a ,  104   b  is required, and the manufacturing cost is increased. In the example of  FIG. 15 , an amount of the excess portion  104   a  protruding in the radial direction from the axial end edge of the outer peripheral surface of the shaft member  102  is larger than an amount of the excess portion  104   b  protruding in the axial direction from the outer peripheral edge of the axial end surface of the shaft member  102 . The amounts of the excess portions  104   a ,  104   b  vary depending on an outer diameter dimension of the shaft member  102 , an inclination angle of a generatrix of the chamfered portion  103  with respect to a central axis of the shaft member  102 , a surface area of the chamfered portion  103 , and the like, and in general, the amount of the excess portion  104   a  protruding in the radial direction is substantially the same as the amount of the excess portion  104   b  protruding in the axial direction. However, when the outer diameter dimension of the shaft member  102  is relatively small, the amount of the excess portion  104   b  protruding in the axial direction may be larger than the amount of the excess portion  104   a  protruding in the radial direction. 
     In view of the above-described circumstances, an object of the present invention is to provide a structure of a shaft member in which the shape accuracy of a male-side engagement portion is good and the manufacturing cost can be reduced. 
     Solution to Problem 
     The present invention can be configured, for example, as follows. 
     (1) A shaft member including:
         an outer peripheral surface;   at least one axial side surface facing an axial direction;   a chamfered portion formed at a connection portion between an axial end edge of the outer peripheral surface and an outer peripheral edge of the axial side surface; and   a processing target portion formed on at least a part of the outer peripheral surface, wherein   the chamfered portion includes at least one supported surface portion disposed at an axially intermediate portion of the chamfered portion and inclined radially outward as approaching the processing target portion in the axial direction, and   the axial end edge of the outer peripheral surface and the outer peripheral edge of the axial side surface are located radially inward than a tangent line at an axially intermediate portion of the supported surface portion, in a cross section in the axial direction.   (2) The shaft member according to (1), wherein the supported surface portion is a conical surface having a linear generatrix.   (3) The shaft member according to (1), wherein the supported surface portion is a convex curved surface having an arc-shaped generatrix.   (4) The shaft member according to any one of (1) to (3), wherein the chamfered portion further includes:   a first connection inclined surface portion that connects an axial end edge on a side axially close to the processing target portion in both axial end edges of the supported surface portion and the axial end edge of the outer peripheral surface, and that is inclined radially outward as approaching the processing target portion in the axial direction, or   a first step portion that has a circular ring-shaped flat surface portion bent radially inward from at least the axial end edge of the outer peripheral surface.   (5) The shaft member according to any one of (1) to (4), wherein the chamfered portion further includes:   a second connection inclined surface portion that connects an axial end edge on a side axially far from the processing target portion in the both axial end edges of the supported surface portion and the outer peripheral edge of the axial side surface, and that is inclined radially outward as approaching the processing target portion in the axial direction, or   a second step portion that has a cylindrical surface portion bent from at least the outer peripheral edge of the axial side surface in a direction approaching the processing target portion in the axial direction.   (6) The shaft member according to any one of (1) to (5), wherein the chamfered portion includes a pair of the supported surface portions, and the pair of supported surface portions are connected to each other by a third step portion including a circular ring-shaped flat surface portion and a cylindrical surface portion.   (7) The shaft member according to any one of (1) to (6), wherein the axial side surface is an axial end surface.   (8) The shaft member according to any one of (1) to (7), wherein a stepped cylindrical portion, in which a small-diameter cylindrical portion and a large-diameter cylindrical portion are connected to each other by a step surface facing the axial direction, is provided at a portion on the outer peripheral surface that is deviated from the processing target portion in the axial direction, and   the axial side surface is the step surface.   (9) The shaft member according to (8), wherein a recessed groove recessed radially inward is provided in an end portion of the small-diameter cylindrical portion that is on a side close to the large-diameter cylindrical portion in the axial direction.   (10) The shaft member according to any one of (1) to (9), wherein the shaft member is used as a screw shaft for a brake actuator.   (11) A method for manufacturing a male shaft including an outer peripheral surface, and a male-side engagement portion formed on at least a part of the outer peripheral surface, the method including:   a step of performing processing for forming the male-side engagement portion on the outer peripheral surface in a state where the shaft member according to any one of (1) to   (10) is positioned in a radial direction by pressing at least an axially intermediate portion of the supported surface portion of the shaft member against a tapered surface provided on a jig.   (12) A method for manufacturing a male shaft including an outer peripheral surface, and a male-side engagement portion formed on at least a part of the outer peripheral surface, the method including:   a step of performing processing for forming the male-side engagement portion on the outer peripheral surface in a state where at least an axially intermediate portion of the supported surface portion of the shaft member according to (10) is pressed against a tapered surface provided on a jig and grease is held in the recessed groove.   (13) The method for manufacturing a male shaft according to (11) or (12), wherein the processing for forming the male-side engagement portion is rolling processing.   (14) The method for manufacturing a male shaft according to any one of (11) to (13), wherein the male-side engagement portion is a male screw portion in which a male screw groove is spirally formed.   (15) The method for manufacturing a male shaft according to any one of (11) to (14) further including:   a heat treatment step of performing a heat treatment on the male shaft, wherein   the male shaft is not further processed after the heat treatment step.       

     The axial side surface facing the axial direction refers to an axial end surface or a step surface formed on an outer peripheral surface and connecting the small-diameter cylindrical portion and the large-diameter cylindrical portion. 
     Advantageous Effects of Invention 
     According to the present invention, deformation of the male-side engagement portion can be prevented, and the manufacturing cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a side view showing a shaft member according to a first embodiment of the present invention, and  FIG. 1B  is a male shaft manufactured by rolling a processing target portion of the shaft member. 
         FIG. 2  is an enlarged view of a portion A in  FIG. 1(A) , and showing the shaft member that is taken out. 
         FIG. 3  is an enlarged view of a portion B in  FIG. 1(A) , and showing the shaft member that is taken out. 
         FIG. 4A  to  FIG. 4E  are side views showing a method for manufacturing the shaft member according to the first embodiment of the present invention. 
         FIG. 5  is a cross-sectional view taken along a line C-C of  FIG. 4(C) . 
         FIG. 6A  and  FIG. 6B  are cross-sectional views showing another example of a method of forming a male screw portion by rolling processing. 
         FIG. 7  is a side view showing a method of measuring an amount of runout of the shaft member. 
         FIG. 8  is a view corresponding to  FIG. 2  and showing a shaft member according to a second embodiment of the present invention. 
         FIG. 9  is a view corresponding to  FIG. 2  and showing a shaft member according to a third embodiment of the present invention. 
         FIG. 10  is a view corresponding to  FIG. 2  and showing a shaft member according to a fourth embodiment of the present invention. 
         FIG. 11  is a side view showing a shaft member according to a fifth embodiment of the present invention. 
         FIG. 12  is an enlarged view of a portion D in  FIG. 11 . 
         FIG. 13  is a side view showing a shaft member according to a sixth embodiment of the present invention. 
         FIG. 14  is a side view illustrating a method of rolling a male screw portion on an outer peripheral surface of a columnar material. 
         FIG. 15  is a partially enlarged view illustrating a problem of the method of rolling a male screw portion on an outer peripheral surface of a columnar material. 
         FIG. 16  is a cross-sectional view of a main part for a brake actuator including the shaft member of the first embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIGS. 1A to 5  show a first embodiment of the present invention. A male shaft  11  serving as a target of the present embodiment is a ball screw shaft configuring a feed screw mechanism of a ball screw type. The male shaft  11  includes an outer peripheral surface  2 , first and second end surfaces  3   a  and  3   b  on both axial sides, and a male screw portion  4  that is a male-side engagement portion. 
     Both of the first and second end surfaces  3   a ,  3   b  on both axial sides do not have recessed holes that is opened in the first and second end surfaces  3   a ,  3   b.    
     In the present embodiment, the first end surface  3   a  on one axial side (a left side in  FIGS. 1A and 1B ) is configured by a single convex curved surface having a partially spherical shape. 
     The second end surface  3   b  on the other axial side (a right side in  FIGS. 1A and 1B ) is configured by a flat surface orthogonal to a central axis of the male shaft  11 . 
     In the male screw portion  4 , a male screw groove  8  having an arc-shaped cross section is formed spirally in an axially intermediate portion of the outer peripheral surface  2  of the male shaft  11 . 
     The feed screw mechanism of a ball screw type is configured by assembling the male shaft  11 , a ball nut (not shown) having a female screw portion in which a female screw groove having an arc-shaped cross section is formed spirally on an inner peripheral surface of the ball nut, and a plurality of balls (not shown). That is, the ball nut is disposed around the male screw portion  4  of the male shaft  11 , and the balls are rollably disposed between the male screw groove  8  and the female screw groove of the ball nut, thereby configuring the feed screw mechanism. 
     The male shaft  11  is obtained by rolling a processing target portion  12  of the shaft member  1  as shown in  FIG. 1A . The shaft member  1  includes the outer peripheral surface  2 , first and second end surfaces  3   a  and  3   b  on both axial sides, the processing target portion  12 , and first and second chamfered portions  7   a ,  7   b . The first and second chamfered portions  7   a ,  7   b  are respectively formed at connection portions between first and second end edges  5   a  and  5   b  on both axial sides of the outer peripheral surface  2  and first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  on both axial sides. 
     The processing target portion  12  is disposed at the axially intermediate portion of the outer peripheral surface  2 . An outer diameter dimension of the processing target portion  12  is larger than outer diameter dimensions of portions adjacent thereto on both axial sides. 
     Each of the first and second chamfered portions  7   a ,  7   b  is a composite surface formed by combining three or more surfaces. In the composite surface, axially adjacent surfaces are different in an inclination angle or a curvature radius of a generatrix of the surface with respect to a central axis O of the shaft member  1 . 
     The first and second chamfered portions  7   a ,  7   b  respectively include first and second supported surface portions  9   a ,  9   b  which are respectively disposed at axially intermediate portions of the first and second chamfered portions  7   a ,  7   b  and are each inclined radially toward as approaching the male screw portion  4  in the axial direction. In the present embodiment, each of the first and second supported surface portions  9   a ,  9   b  is a conical surface having a linear generatrix. In addition, in a cross section in the axial direction (a cross section including the central axis O of the shaft member  1 ), the first and second end edges  5   a ,  5   b  of the outer peripheral surface  2  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  on both axial sides are located radially inward than generatrices (including an extension line of the generatrix) α, β of the first and second supported surface portions  9   a ,  9   b . That is, in the cross section in the axial direction (the cross section including the central axis O of the shaft member  1 ), the first and second end edges  5   a ,  5   b  of the outer peripheral surface  2  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  on both axial sides are located radially inward than tangent lines at axially intermediate portions of the first and second supported surface portions  9   a ,  9   b.    
     For this reason, in the present embodiment, in the first and second chamfered portions  7   a ,  7   b , the first chamfered portion  7   a  on the one axial side which is formed at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, includes the first supported surface portion  9   a  and a pair of connection inclined surface portions  10   a   1 ,  10   a   2 . 
     The first supported surface portion  9   a  is a conical surface that is disposed at an axially intermediate portion of the first chamfered portion  7   a  on the one axial side and has a linear generatrix inclined radially outward as approaching the other axial side. An inclination angle θa of the generatrix of the first supported surface portion  9   a  with respect to the central axis O of the shaft member  1  is preferably 20° or more and 60° or less, and is about 30° in the illustrated example. 
     In the pair of connection inclined surface portions  10   a   1 ,  10   a   2 , the connection inclined surface portion  10   a   1  on the one axial side, that connects the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side and an end edge on the one axial side of the first supported surface portion  9   a , is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the other axial side. 
     In the pair of connection inclined surface portions  10   a   1 ,  10   a   2 , the connection inclined surface portion  10   a   2  on the other axial side, that connects an end edge on the other axial side of the first supported surface portion  9   a  and the first end edge  5   a  on the one axial side of the outer peripheral surface  2 , is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the other axial side. 
     Each of axial dimensions d 10   a   1 , d 10   a   2  of the connection inclined surface portions  10   a   1 ,  10   a   2  is preferably 10% or more and 35% or less of an axial dimension La of the entire first chamfered portion  7   a . Each of curvature radiuses of the generatrices of the connection inclined surface portions  10   a   1 ,  10   a   2  is preferably 1% or more and 30% or less of an outer diameter dimension of a portion of the shaft member  1  adjacent to the other axial side of the connection inclined surface portion  10   a   2  (an outer diameter dimension of the first end edge  5   a  on the one axial side of the outer peripheral surface  2 ). 
     In the first and second chamfered portions  7   a ,  7   b , the second chamfered portion  7   b  on the other axial side, that connects the second end edge  5   b  on the other axial side of the outer peripheral surface  2  and the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side, includes the second supported surface portion  9   b  and a pair of connection inclined surface portions  10   b   1 ,  10   b   2 . 
     The second supported surface portion  9   b  is a conical surface that is disposed at an axially intermediate portion of the second chamfered portion  7   b  on the other axial side and has a linear generatrix inclined radially outward as approaching the one axial side. An inclination angle θb of the generatrix of the second supported surface portion  9   b  with respect to the central axis O of the shaft member  1  is preferably 20° or more and 60° or less, and is about 30° in the illustrated example. 
     In the pair of connection inclined surface portions  10   a   1 ,  10   a   2 , the connection inclined surface portion  10   b   1  on the other axial side, that connects the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side and an end edge on the other axial side of the second supported surface portion  9   b , is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the one axial side. In the pair of connection inclined surface portions  10   b   1 ,  10   b   2 , the connection inclined surface portion  10   b   2  on the one axial side, that connects an end edge on the one axial side of the second supported surface portion  9   b  and the second end edge  5   b  on the other axial side of the outer peripheral surface  2 , is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the one axial side. Each of axial dimensions d 10   b   1 , d 10   b   2  of the connection inclined surface portions  10   b   1 ,  10   b   2  is preferably 10% or more and 35% or less of an axial dimension Lb of the entire second chamfered portion  7   b . Each of curvature radiuses of the generatrices of the connection inclined surface portions  10   a   1 ,  10   a   2  is preferably 1% or more and 30% or less of an outer diameter dimension of a portion of the shaft member  1  adjacent to the one axial side of the connection inclined surface portion  10   b   2  (an outer diameter dimension of the second end edge  5   a  on the other axial side of the outer peripheral surface  2 ). 
     In this way, the pair of connection inclined surface portions  10   a   1 ,  10   a   2  of the first end surface  3   a  and the pair of connection inclined surface portions  10   b   1 ,  10   b   2  of the second end surface  3   b  are each set into a gentle R shape, and thus elongation is absorbed by the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  at the time of the rolling processing, and the male shaft  11  can be prevented from extending in the axial direction. The first end surface  3   a  and the second end surface  3   b  may have any shape, and can have a shape capable of absorbing the axial elongation of the male shaft  11  at the time of the rolling processing. 
     As to be described later, before the rolling processing ( FIG. 4B  to  FIG. 4D ) is performed, cutting processing such as cutting or grinding is performed on the columnar material, and the shaft member  1  as shown in  FIG. 4A  is obtained. When the columnar material is processed into the shaft member  1 , the first and second chamfered portions  7   a ,  7   b  are formed. Therefore, since the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  are formed on the first end surface  3   a  and the second end surface  3   b  in advance before the rolling processing, elongation can be absorbed by the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  at the time of the rolling processing performed thereafter. 
     Next, a method for manufacturing the male shaft  11  from the shaft member  1  of the present embodiment will be described with reference to  FIGS. 4A to 5 . 
     First, the columnar material is obtained by cutting a bar-shaped material, which is made of an iron-based metal such as carbon steel or chromium-molybdenum steel and has a circular cross-sectional shape, into a predetermined length. Next, cutting processing such as cutting and grinding is performed on the material, and the shaft member  1  as shown in  FIG. 4A  is obtained. When the columnar material is processed into the shaft member  1 , the first and second chamfered portions  7   a ,  7   b  are formed at the connection portions between the outer peripheral surface  2  and the first and second end surfaces  3   a ,  3   b  on both axial sides. Thereafter, as shown in  FIGS. 4B to 4D , during the rolling processing for forming the male screw portion  4 , the outer peripheral surface  2  and the first and second end surfaces  3   a ,  3   b  are finished in a state where the shaft member  1  is positioned in the radial direction by pressing tapered surfaces provided on the inner peripheral surfaces of the recessed portions  14  of the jigs  13  against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b.    
     Next, the male shaft  11  is to be obtained by performing the rolling processing on the processing target portion  12  of the shaft member  1  and forming the male screw portion  4 . For this purpose, first, as shown in  FIG. 4B , both axial end portions of the shaft member  1  are supported by the pair of jigs  13  each having the recessed portion  14 . A generatrix of a tapered surface  35  provided on the inner peripheral surface of each recessed portion  14  of the jig  13  is linearly inclined in a direction in which an inner diameter dimension of the tapered surface  35  increases as approaching an opening of the recessed portion  14 . An inclination angle φ of a generatrix of the tapered surface  35  with respect to the central axis O is the same as the inclination angles θa, θb of the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b  of the shaft member  1 . Therefore, in a state where the tapered surfaces  35  are abutted against (pressed against) the first and second supported surface portions  9   a ,  9   b , the first and second end edges  5   a ,  5   b  on both axial sides of the outer peripheral surface  2  of the shaft member  1  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  are not in contact with the tapered surfaces  35 . 
     The jig  13  may be made of, for example, an iron-based metal such as high-speed steel. Alternatively, (i) a portion including at least the tapered surface  35  of the jig  13  is made of cemented carbide or polycrystalline diamond, and/or (ii) mirror finishing is performed on the tapered surface  35 , thus frictional resistance with respect to the first and second supported surface portions  9   a ,  9   b  can be reduced, and wear of the tapered surface  35  can be prevented. In addition, as a method of preventing the wear of the tapered surface  35  while reducing the cost, a coating layer can also be formed on the tapered surface  35  by a chemical vapor deposition method (CVD), a high-temperature salt bath treatment method (TD method, TD-VC method), or the like. During forming of the male screw portion  4 , it is preferable to supply a lubricant or air for lubrication and cooling to sliding contact portions between the tapered surfaces  35  and the first and second supported surface portions  9   a ,  9   b . When a main body portion of the jig  13  and a portion including the tapered surface  35  are made of different materials, adhesiveness between a material forming the main body portion of the jig  13  and a material forming the portion including the tapered surface  35  can be improved by forming a vent hole in the jig  13 . 
     As described above, in a state where both axial end portions of the shaft member  1  are supported by the pair of jigs  13 , the pair of round dies  15  approach the shaft member  1  from both radial sides while being rotated. Each of the pair of round dies  15  has screw cutting teeth (not shown) formed spirally on an outer peripheral surface thereof. Therefore, as shown in  FIGS. 4C and 5 , when the shaft member  1  is pressed between the pair of round dies  15  from both radial sides, the shaft member  1  rotates while the first and second supported surface portions  9   a ,  9   b  are in sliding contact with the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the jigs  13 . As a result, as shown in  FIG. 4D , the male screw portion  4  formed by spirally forming the male screw groove  8  is rolled on the processing target portion  12  of the shaft member  1 . Thereafter, as shown in  FIG. 4E , the male shaft  11  including the male screw portion  4  is taken out by retracting the pair of round dies  15  in the radial direction and further retracting the pair of jigs  13  in the axial direction. Washing, finishing, a heat treatment, and the like are performed on the male shaft  11  obtained in this way as necessary so as to make the male shaft  11  into a finished shape. That is, in a rolling step in the present embodiment, since burr or protrusion excess portion  104   a ,  104   b  (to be described later) with a small thickness can be prevented from being formed on the shaft member  1 , it is not necessary to further perform processing (polishing processing or the like) after the heat treatment, and the heat treatment can be set as a final step in the method for manufacturing the male shaft  11 . For example, when polishing processing or the like is performed on the male shaft  11  after heat treatment as in the related art, a very fine scratch may be generated on a ball rolling surface. However, since the heat treatment is the final step in the present embodiment, such a disadvantage can be solved. 
     In the present embodiment, when the male screw portion  4  is formed by the rolling processing, the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the pair of jigs  13  are pressed against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b  so as to position the shaft member  1  in the radial direction. That is, according to the method for manufacturing the shaft member  1  of the present embodiment, unlike the case of rolling the male screw portion on the outer peripheral surface of the material using a rolling machine described in JP-A-8-318340, it is not necessary to form the recessed holes for abutting the conical tip end portions provided in the jigs against the end surfaces on both axial sides of the material. Therefore, according to the manufacturing method of the present embodiment, after the male screw portion is formed on the outer peripheral surface of the material, it is not necessary to cut and remove the axial end portions of the material in a state where the material is supported by gripping the male screw portion. In short, according to the manufacturing method of the present embodiment, the male screw portion  4  can be prevented from being deformed, and the shape accuracy of the male screw portion  4  can be satisfactorily secured. 
     Further, in the present embodiment, the both axial end portions of the shaft member  1  are supported by pressing the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the pair of jigs  13  against the first and second chamfered portions  7   a ,  7   b  respectively formed at the connection portions between the first and second end edges  5   a ,  5   b  on both axial sides of the outer peripheral surface  2  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  on both axial sides. Therefore, deformation in a bending direction can be less likely to occur (buckling deformation can be less likely to occur) even when a force in the axial direction is applied, as compared with a structure in which both axial end portions of the material are supported by abutting the conical tip end portions provided in the jigs against the inner peripheral surfaces of the recessed holes formed in the both axial end surfaces of the material, as in the method described in JP-A-8-318340. From this viewpoint as well, the shape accuracy of the male screw portion  4  can be satisfactorily secured. 
     In the shaft member  1  of the present embodiment, the first and second end edges  5   a ,  5   b  of the outer peripheral surface  2  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  are located radially inward than the generatrices α, β of the first and second supported surface portions  9   a ,  9   b  in the cross section in the axial direction. Therefore, in a state where the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the pair of jigs  13  are pressed against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b , the first and second end edges  5   a ,  5   b  of the outer peripheral surface  2  of the shaft member  1  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  are not in contact with the tapered surfaces  35 . Therefore, even if the shaft member  1  is deformed as the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the jigs  13  are strongly pressed against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b  when the male screw portion  4  is rolled, the deformation can be retained in the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  adjacent to the supported surface portions  9   a ,  9   b  in the axial direction. In other words, the deformation of the shaft member  1  can be reduced to such an extent that the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  slightly bulge outward in the radial direction. In short, according to the manufacturing method of the present embodiment, the burr or protrusion excess portions  104   a ,  104   b  with a small thickness as shown in  FIG. 15  can be prevented from being formed. In particular, in the present embodiment, since the connection inclined surface portions  10   a   1 ,  10   a   2 ,  10   b   1 ,  10   b   2  are disposed on both axial sides of the first and second supported surface portions  9   a ,  9   b , it is possible to prevent both the formation of the excess portion  104   a  protruding in the radial direction from the axial end edge of the outer peripheral surface and the formation of the excess portion  104   b  protruding in the axial direction from the outer peripheral edge of the axial end surface. Therefore, it is not necessary to perform an operation (processing) of removing the excess portions  104   a ,  104   b  after the male screw portion  4  is formed, and the manufacturing cost of the male shaft  11  can be reduced. 
     In the present embodiment, the male screw portion  4  is rolled by pressing the outer peripheral surface of the shaft member  1  from both radial sides between the pair of round dies  15  having the spiral screw cutting teeth  16  (not shown in  FIGS. 4B to 4E , and shown in  FIG. 5  only). However, the method of forming the male screw portion  4  is not particularly limited as long as the shaft member  1  is positioned in the radial direction by pressing the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the jigs  13  against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b . For example, as shown in  FIGS. 6A and 6B , the male screw portion  4  can be rolled by moving a pair of flat dies  17 , that are disposed to face each other, toward or away from each other. Alternatively, the male screw portion  4  may be formed by processing other than the rolling processing (for example, cutting processing or the like). However, in this case, one end portion of the shaft member  1  needs to be gripped (chucked) by a rotation drive mechanism. 
     Further, the case of carrying out the method for manufacturing the male shaft of the present invention can be configured such that, as shown in  FIG. 7 , a through hole  32  is formed in the jig  13   a  to open a bottom surface of the recessed portion  14   a , and during the processing of the male screw portion  4 , center runout of the shaft member  1  is measured by abutting a tip end portion of a probe  34  of a dial gauge  33  against the first end surface  3   a  on the one axial side of the shaft member  1  through the through hole  32 .  FIG. 7  shows the shaft member  1  in a state after the male screw portion  4  is formed. The center runout of the shaft member  1  may be configured to be measured by abutting the tip end portion of the probe  34  of the dial gauge  33  against the second end surface  3   b  on the other axial side of the shaft member  1 . 
     Further, in the present embodiment, the male shaft  11  is used as a ball screw shaft configuring a feed screw mechanism of a ball screw type, and the male screw portion  4  in which the male screw groove  8  having an arc-shaped cross-sectional shape is spirally formed is formed on the outer peripheral surface  2 , but the present invention is not limited to such a structure. Specifically, for example, the male shaft to which the present invention is applied can be applied to a screw shaft configuring a feed screw mechanism of a slide screw type. In this case, the male-side engagement portion provided on the outer peripheral surface is a male screw portion in which a male screw groove having a substantially trapezoidal or triangular cross-sectional shape is formed spirally. Alternatively, the male shaft to which the present invention is applied can be an inner shaft, and the male-side engagement portion can be a male spline portion or a male serration portion formed on the outer peripheral surface. 
     Second Embodiment 
       FIG. 8  shows a second embodiment of the present invention. A shaft member la of the present embodiment includes a chamfered portion  7   c  at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side. In a cross section in the axial direction, the first end edge  5   a  of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  are located radially inward than the generatrix (including an extension line of the generatrix) a of the first supported surface portion  9   a  disposed at an axially intermediate portion of the chamfered portion  7   c . Therefore, the chamfered portion  7   c  includes a first supported surface portion  9   a  and a pair of step portions  18   a ,  18   b.    
     The first supported surface portion  9   a  is a conical surface that is disposed at an axially intermediate portion of the first chamfered portion  7   c  and has a linear generatrix inclined radially outward as approaching the other axial side. 
     In the pair of step portions  18   a ,  18   b , the step portion  18   a  on the one axial side connects the first outer peripheral edge  6   a  of the first end surface  3   a  and an end edge on the one axial side of the first supported surface portion  9   a . The step portion  18   a  includes a circular ring-shaped flat surface portion  19   a  that is bent radially inward from the end edge on the one axial side of the first supported surface portion  9   a , and a cylindrical surface portion  20   a  which is bent toward the one axial side from an inner peripheral edge of the flat surface portion  19   a , and whose end edge on the one axial side is connected to the first outer peripheral edge  6   a  of the first end surface  3   a.    
     In the pair of step portions  18   a ,  18   b , the step portion  18   b  on the other axial side connects an end edge on the other axial side of the first supported surface portion  9   a  and the first end edge  5   a  on the one axial side of the outer peripheral surface  2 . The step portion  18   b  includes a circular ring-shaped flat surface portion  19   b  that is bent radially inward from the first end edge  5   a  on the one axial side of the outer peripheral surface  2 , and a cylindrical surface portion  20   b  which is bent toward the one axial side from an inner peripheral edge of the flat surface portion  19   b , and whose end edge on the one axial side is connected to the end edge on the other axial side of the first supported surface portion  9   a.    
     In the present embodiment, the first end edge  5   a  on the one axial side of the outer peripheral surface  2  of the shaft member  1   a  and the first outer peripheral edge  6   a  of the first end surface  3   a  are also not in contact with the tapered surface  35  in a state where the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  (see  FIGS. 1A and 1B ) is pressed against the first supported surface portion  9   a  of the chamfered portion  7   c . Therefore, even if the shaft member  1  is deformed as the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  is strongly pressed against the first supported surface portion  9   a  of the first chamfered portion  7   a  when the male screw portion  4  is rolled, the deformation can be retained in the step portions  18   a ,  18   b  that are axially adjacent to the first supported surface portion  9   a . In short, the burr or protrusion excess portions  104   a ,  104   b  with a small thickness as shown in  FIG. 15  can be prevented from being formed. The configuration and the operation and effect of other portions are the same as those of the first embodiment. 
     Although the chamfered portion  7   c  of the present embodiment is provided at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, the chamfered portion  7   c  may be provided at the connection portion between the second end edge  5   b  on the other axial side of the outer peripheral surface  2  and the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side. 
     Third Embodiment 
       FIG. 9  shows a third embodiment of the present invention. A shaft member  1   b  of the present embodiment includes a chamfered portion  7   d  at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side. In the cross section in the axial direction, the first end edge  5   a  of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  are located radially inward than a tangent line γ at an axially intermediate portion of a supported surface portion  9   c , that is disposed at an axially intermediate portion of a chamfered portion  7   d . Therefore, the chamfered portion  7   d  includes a supported surface portion  9   c  and a pair of step portions  18   c ,  18   d.    
     The supported surface portion  9   c  is disposed at an axially intermediate portion of the chamfered portion  7   d , and is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the other axial side. 
     In the pair of step portions  18   c ,  18   d , the step portion  18   c  on the one axial side connects the first outer peripheral edge  6   a  of the first end surface  3   a  and an end edge on the one axial side of the first supported surface portion  9   c . The step portion  18   c  includes a cylindrical surface portion  20   c  that is bent toward the other axial side from the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, and whose end edge on the other axial side is connected to an end edge on the one axial side of the supported surface portion  9   c.    
     In the pair of step portions  18   c ,  18   d , the step portion  18   d  on the other axial side connects an end edge on the other axial side of the supported surface portion  9   c  and the first end edge  5   a  on the one axial side of the outer peripheral surface  2 . The step portion  18   d  on the other axial side includes a circular ring-shaped flat surface portion  19   c  which is bent radially inward from the first end edge  5   a  on the one axial side of the outer peripheral surface  2 , and whose inner peripheral edge is connected to an end edge on the other axial side of the supported surface portion  9   c.    
     In the present embodiment, the first end edge  5   a  on the one axial side of the outer peripheral surface  2  of the shaft member la and the first outer peripheral edge  6   a  of the first end surface  3   a  are also not in contact with the tapered surface  35  in a state where the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  is pressed against the axially intermediate portion of the supported surface portion  9   c  of the chamfered portion  7   d . Therefore, when the male screw portion  4  is rolled, deformation caused by strongly pressing the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  against the axially intermediate portion of the supported surface portion  9   c  of the chamfered portion  7   d  can be retained in the step portions  18   c ,  18   d . The configuration and the operation and effect of other portions are the same as those of the first embodiment and the second embodiment. 
     Although the chamfered portion  7   d  of the present embodiment is provided at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, the chamfered portion  7   d  may be provided at the connection portion between the second end edge  5   b  on the other axial side of the outer peripheral surface  2  and the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side. 
     Fourth Embodiment 
       FIG. 10  shows a fourth embodiment of the present invention. A shaft member  1   c  of the present embodiment includes a chamfered portion  7   e  at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side. The chamfered portion  7   e  has a two-stage structure including a pair of supported surface portions  9   d ,  9   e  and three step portions  18   e  to  18   g.    
     The pair of supported surface portions  9   d ,  9   e  are disposed at two positions on an axially intermediate portion of the chamfered portion  7   e  and each are a conical surface having a linear generatrix inclined radially outward as approaching the other axial side. The pair of supported surface portions  9   d  and  9   e  exist on the same conical surface. In a cross section in the axial direction, the first end edge  5   a  of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  are located radially inward than a generatrix δ of the pair of supported surface portions  9   d ,  9   e  disposed at two positions on the axially intermediate portion of the chamfered portion  7   e.    
     Among the three step portions  18   e  to  18   g , the step portion  18   e  connects the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side and an one axial side edge of the supported surface portion  9   d  on the one axial side. The step portion  18   e  includes a circular ring-shaped flat surface portion  19   d  that is bent radially inward from the one axial side edge of the supported surface portion  9   d  on the one axial side, and a cylindrical surface portion  20   d  which is bent toward the one axial side from an inner peripheral edge of the flat surface portion  19   d  and whose one axial side edge is connected to the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side. 
     Among the three step portions  18   e  to  18   g , the step portion  18   f  connects the other axial side edge of the supported surface portion  9   d  on the one axial side and an one axial side edge of the supported surface portion  9   e  on the other axial side. The step portion  18   f  includes a circular ring-shaped flat surface portion  19   e  that is bent radially inward from the one axial side edge of the supported surface portion  9   e  on the other axial side, and a cylindrical surface portion  20   e  which is bent toward the one axial side from the flat surface portion  19   e  and whose one axial side edge is connected to the other axial side edge of the supported surface portion  9   d  on the one axial side. 
     Among the three step portions  18   e  to  18   g , the step portion  18   g  connects the other axial side edge of the supported surface portion  9   e  on the other axial side and the first end edge  5   a  on the one axial side of the outer peripheral surface  2 . The step portion  18   g  includes a circular ring-shaped flat surface portion  19   f  that is bent radially inward from the first end edge  5   a  on the one axial side of the outer peripheral surface  2 , and a cylindrical surface portion  20   f  which is bent toward the one axial side from the flat surface portion  19   f  and whose one axial side edge is connected to the other axial side edge of the supported surface portion  9   e  on the other axial side. 
     According to the present embodiment, when the male screw portion  4  is formed by rolling processing, a gap can be present over an entire periphery between the tapered surface  35  and the middle step portion  18   f  among the three step portions  18   e  to  18   g  in a state where the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  is pressed against the pair of supported surface portions  9   d ,  9   e  of the chamfered portion  7   e . In present embodiment, grease can be held in such a gap, and when the male screw portion  4  is formed, the chamfered portion  7   e  of the shaft member  1   c  can be smoothly slid with respect to the tapered surface  35 . Further, when the male screw portion  4  is rolled, deformation caused by strongly pressing the tapered surface  35  against the pair of supported surface portions  9   d ,  9   e  of the chamfered portion  7   e  can be absorbed by the middle step portion  18   f  to some extent, and an amount of deformation of the step portions  18   e ,  18   g  on both axial sides can be reduced. The configuration and the operation and effect of other portions are the same as those of the first embodiment and the second embodiment. 
     Although the chamfered portion  7   e  of the present embodiment is provided at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, the chamfered portion  7   e  may be provided at a connection portion between the second end edge  5   b  on the other axial side of the outer peripheral surface  2  and the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side. 
     Fifth Embodiment 
       FIGS. 11 and 12  show a fifth embodiment of the present invention. A shaft member  1   d  of the present embodiment includes a chamfered portion  7   f  at an axially intermediate portion of the shaft member  1   d  in addition to the first and second chamfered portions  7   a ,  7   b  respectively formed at connection portions between the first and second end edges  5   a ,  5   b  on both axial sides of an outer peripheral surface  2   a  and the first and second outer peripheral edges  6   a ,  6   b  of the first and second end surfaces  3   a ,  3   b  on both axial sides. Therefore, the shaft member  1   d  of the present embodiment includes a stepped cylindrical portion  22  at a portion of an outer peripheral surface  2   a  which is deviated to the one axial side from a portion where the male screw portion  4  is formed. In the stepped cylindrical portion  22 , a small-diameter cylindrical portion  23  on the one axial side and a large-diameter cylindrical portion  24  on the other axial side are connected by a step surface  25  facing the one axial side. 
     The chamfered portion  7   f  is formed at a connection portion between an end edge  5   c  on the one axial side of the large-diameter cylindrical portion  24  and an outer peripheral edge  6   c  of the step surface  25 . The chamfered portion  7   f  includes a supported surface portion  9   f  and a pair of connection inclined surface portions  10   c   1 ,  10   c   2 . In the present embodiment, the supported surface portion  9   f  is a conical surface having a linear generatrix. Further, in a cross section in the axial direction (a cross section including the central axis O of the shaft member  1   d ), the end edge  5   c  on the one axial side of the large-diameter cylindrical portion  24  and the outer peripheral edge  6   c  of the step surface  25  are located radially inward than a generatrix (including an extension line of the generatrix) of the supported surface portion  9   f .  FIG. 11  shows the shaft member  1   d  in a state after the male screw portion  4  is formed. 
     The supported surface portion  9   f  is a conical surface that is disposed at an axially intermediate portion of the chamfered portion  7   f  and has a linear generatrix inclined radially outward as approaching the other axial side. 
     In the pair of connection inclined surface portions  10   c   1 ,  10   c   2 , the connection inclined surface portion  10   c   1  on the one axial side connects the outer peripheral edge  6   c  of the step surface  25  and an end edge on the one axial side of the supported surface portion  9   f  The connection inclined surface portion  10   c   1  is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the other axial side. 
     In the pair of connection inclined surface portions  10   c   1 ,  10   c   2 , the connection inclined surface portion  10   c   2  on the other axial side connects an end edge on the other axial side of the supported surface portion  9   f  and the end edge  5   c  on the one axial side of the large-diameter cylindrical portion  24 . 
     The connection inclined surface portion  10   c   2  is a convex curved surface having an arc-shaped generatrix inclined radially outward as approaching the other axial side. 
     Further, the shaft member  1   d  is provided with a recessed groove  26 , which is recessed radially inward, at an end portion on the other axial side of the small-diameter cylindrical portion  23  over an entire periphery thereof. 
     In the present embodiment, when the male screw portion  4  is formed by the rolling processing, as shown in  FIG. 11 , the shaft member  1   d  is supported by pressing the tapered surfaces  35  provided on the inner peripheral surfaces of the recessed portions  14  of the pair of jigs  13  against the first and second supported surface portions  9   a ,  9   b  of the first and second chamfered portions  7   a ,  7   b  formed on both axial end portions of the shaft member  1   d , and pressing a tapered surface  28  provided on another jig  27  against the supported surface portion  9   f  of the chamfered portion  7   f  formed at the axially intermediate portion of the shaft member  1   d . The jig  27  has a circular hole  29  penetrating in the axial direction, and the tapered surface  28 , inclined in a direction in which an inner diameter thereof increases as approaching the other axial side, at a portion on the other axial side of an inner peripheral surface of the circular hole  29 . The circular hole  29  has an inner diameter larger than an outer diameter of the small-diameter cylindrical portion  23  of the shaft member  1   d  (a portion of the small-diameter cylindrical portion  23  that is deviated in the axial direction from a portion where the recessed groove  26  is present). 
     When the male screw portion  4  is formed, first, in a state where an end surface on the one axial side of the jig  27  overlaps a tip end surface (an end surface on the other axial side) of the jig  13  on the one axial side in the pair of jigs  13 , as shown by a two-dot chain line in  FIG. 11 , the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  on the one axial side is pressed against the first support surface portion  9   a  of the first chamfered portion  7   a  on the one axial side. Next, the jig  27  is slid toward the other axial side, and the tapered surface  28  is pressed against the supported surface portion  9   f  of the chamfered portion  7   f  at the middle in the axial direction. 
     In the present embodiment, in a state where the tapered surface  28  of the jig  27  is pressed against the supported surface portion  9   f  of the chamfered portion  7   f , the end edge  5   c  on the one axial side of the large-diameter cylindrical portion  24  of the shaft member  1   d  and the outer peripheral edge  6   c  of the step surface  25  are not in contact with the tapered surface  28 . Therefore, even if the shaft member  1   d  is deformed by strongly pressing the tapered surface  28  of the jig  27  against the supported surface portion  9   f  of the chamfered portion  7   f  when the male screw portion  4  is rolled, the deformation can be retained in the connection inclined surface portions  10   c   1 ,  10   c   2  axially adjacent to the supported surface portion  9   f    
     According to the present embodiment, since the axially intermediate portion can be supported in addition to the both axial end portions of the shaft member  1   d , even when an axial dimension of a portion of the outer peripheral surface  2  of the shaft member  1   d  that is deviated from the male screw portion  4  is long to some extent, the shaft member  1   d  can be prevented from being bent and deformed (bucked). 
     Further, in the present embodiment, grease can be held in the recessed groove  26  provided in the end portion on the other axial side of the small-diameter cylindrical portion  23  of the shaft member  1   d , and therefore, the chamfered portion  7   f  of the shaft member  1   d  can be smoothly slid with respect to the tapered surface  28  of the jig  27  when the male screw portion  4  is formed. The configuration and the operation and effect of other portions are the same as those of the first embodiment. 
     The stepped cylindrical portion  22  of the present embodiment is provided in a portion of the outer peripheral surface  2   a  that is deviated to the one axial side from a portion where the male screw portion  4  is formed, but the stepped cylindrical portion  22  be provided in a portion of the outer peripheral surface  2   a  that is deviated to the other axial side from the portion where the male screw portion  4  is formed. 
     Sixth Embodiment 
       FIG. 13  shows a sixth embodiment of the present invention. A shaft member le of the present embodiment includes the male screw portion  4  in a range from an axially intermediate portion to an end edge on the other axial side on an outer peripheral surface  2   b . That is, the shaft member  1   e  of the present embodiment includes only the first chamfered portion  7   a  at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2   b  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side, and includes a recessed hole  30  having a conical concave surface shape at a central portion of the second end surface  3   b  on the other axial side.  FIG. 13  shows the shaft member  1   e  in a state after the male screw portion  4  is formed. 
     In the present embodiment, when the male screw portion  4  is formed by the rolling processing, the shaft member  1   e  is supported by pressing the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  against the first supported surface portion  9   a  of the first chamfered portion  7   a  formed on one axial end portion of the shaft member  1   e , and abutting an outer peripheral surface of the tip end portion  31  provided on the jig  13   b  against an inner peripheral surface of the recessed hole  30  formed in the end surface  3   b  on the other axial side of the shaft member  1   e . The configuration and the operation and effect of other portions are the same as those of the first embodiment. 
     The shaft member  1   e  of the present embodiment includes the male screw portion  4  in a range from the axially intermediate portion to the end edge on the other axial side on the outer peripheral surface  2   b , but may include the male screw portion  4  in a range from the axially intermediate portion to the one axial side on the outer peripheral surface  2   b . In this case, the shaft member  1   e  includes only the second chamfered portion  7   b  at the connection portion between the second end edge  5   b  on the other axial side of the outer peripheral surface  2   b  and the second outer peripheral edge  6   b  of the second end surface  3   b  on the other axial side, and includes the recessed hole  30  having a conical concave surface shape at a central portion of the first end surface  3   a  on the one axial side. 
     The structures of the first to sixth embodiments described above can be implemented in appropriate combinations as long as the embodiments do not cause a contradiction. 
     The shaft member of the present invention is not limited to the first to sixth embodiments described above. That is, the shaft member of the present invention can include a chamfered portion including at least one supported surface portion disposed at an axially intermediate portion thereof on at least one connection portion in connection portions between an outer peripheral surface and an axial side surface that is an axial end surface or a step surface, and can have a structure in which in a cross section in the axial direction, an axial end edge of the outer peripheral surface and an outer peripheral edge of the axial side surface are located radially inward than a tangent line at an axially intermediate portion of the supported surface portion. Specifically, for example, the shaft member of the present invention can have a structure having only the chamfered portion  7   f  formed at the connection portion between the large-diameter cylindrical portion  24  and the step surface  25  as in the fifth embodiment shown in  FIG. 11 , or can have a structure having only the first chamfered portion  7   a  at the connection portion between the first end edge  5   a  on the one axial side of the outer peripheral surface  2   b  and the first outer peripheral edge  6   a  of the first end surface  3   a  on the one axial side as in the sixth embodiment shown in  FIG. 13 . 
     The method for manufacturing a male shaft of the present invention is not limited to the first embodiment to the sixth embodiment described above. That is, the method for manufacturing a male shaft of the present invention can includes a step of performing processing for forming the male-side engagement portion on the outer peripheral surface of the shaft member in a state where the shaft member is positioned in the radial direction by pressing at least the axially intermediate portion of the supported surface portion of the at least one chamfered portion provided on the shaft member against the tapered surface provided on the jig. Specifically, for example, the method for manufacturing the male shaft of the present invention can include a step of performing processing for forming the male-side engagement portion in a state where the axially intermediate portion of the shaft member is supported by pressing the tapered surface  28  of the jig  27  against only the chamfered portion  7   f  formed at the connection portion between the large-diameter cylindrical portion  24  and the step surface  25 , as in the fifth embodiment shown in  FIG. 11 . In this case, recessed holes each having a conical concave surface shape are formed in the end surfaces on both axial sides of the shaft member and both axial end portions of the shaft member are supported by abutting a conical outer peripheral surface of a tip end portion provided on the jig against an inner peripheral surface of the recessed hole. Further, the shaft member  1   e  can also be supported by pressing the tapered surface  35  provided on the inner peripheral surface of the recessed portion  14  of the jig  13  against the first support surface portion  9   a  of the first chamfered portion  7   a  formed on the one axial end portion of the shaft member  1   e , and abutting the outer peripheral surface of the conical tip end portion  31  provided on the jig  13   b  against the recessed hole  30  formed on the end surface  3   b  on the other axial side of the shaft member  1   e , as in the sixth embodiment shown in  FIG. 13 . 
     In the shaft member of the present invention, for example, the chamfered portion may include a supported surface portion having an arc-shaped cross-sectional shape and a pair of connection inclined surface portions each having an arc-shaped cross-sectional shape. In this case, a curvature radius of the supported surface portion is set to be larger than a curvature radius of each of the connection inclined surface portions. 
     The male shaft  11  of the present invention can be applied as a screw shaft for a brake actuator.  FIG. 16  shows a cross-sectional view of a main part of a brake actuator  200  in a case where the male shaft  11  of the first embodiment is applied as a screw shaft for a brake actuator. 
     The brake actuator  200  is used for a vehicle, and is fixed to a suspension device of a vehicle body via a bracket, a housing, or the like. The brake actuator  200  includes a ball screw  210  that is used for nut rotation, a bearing  203  that rotatably supports the ball screw  210  in a housing (not shown), a belt  205  that is connected to a motor or the like and drives a nut  211  (to be described later) of the ball screw  210 , and a pressed member  207  that is pressed by the male shaft  11  of the ball screw  210 . The nut  211  is not limited to being driven by the belt  205 , and may be driven by a gear or the like. 
     The ball screw  210  includes the male shaft  11 , the nut  211 , a rolling path  215  formed between the male screw groove  8  on the outer peripheral surface of the male shaft  11  and a female screw groove  213  on an inner peripheral surface of the nut  211 , and a plurality of balls  221  disposed in the rolling path  215 . A ball circulation method of the ball screw  210  is not particularly limited, and an end cap method, a flop over method, a return tube method, a guide plate method, and the like are appropriately applied. 
     A recessed portion  209  is formed in a side surface of the pressed member  207 . A shape of the recessed portion  209  matches the shape of the first end surface  3   a  of the male shaft  11 . Further, the first end surface  3   a  of the male shaft  11  is fitted into the recessed portion  209  of the pressed member  207 , and thus the pressed member  207  is fixed to the male shaft  11 . 
     In the brake actuator  200 , the nut  211  is rotationally driven by a motor or the like via the belt  205 , the male shaft  11  advances toward one axial side, and the pressed member  207  is pressed toward the one axial side. Accordingly, the pressed member  207  presses a brake pad against a brake disc, and a braking force is generated on the brake disc. 
     As described above, the male shaft  11  of the present application can be used as a screw shaft for a brake disc actuator. In particular, the male shaft  11  of the present application is preferable as a screw shaft of a brake actuator because of good shape accuracy. 
     The present application is based on Japanese Patent Application No. 2019-092097 filed on May 15, 2019, and the contents thereof are incorporated herein as reference.