Patent Description:
Rolled thread screw manufacturing methods include, for example, through-feed thread rolling and in-feed thread rolling. Application examples of through-feed thread rolling are described in, for example, Patent Literature <NUM> and Patent Literature <NUM>. Through-feed thread rolling is a thread rolling method in which pressure is applied to an outer circumferential portion of a long blank while the blank is moved in an axial direction of the blank between a pair of dies that are rotating. That is, through-feed thread rolling is a method in which thread rolling is performed on a continuous long blank to obtain a screw material therefrom, and the long screw material thus obtained by the thread rolling is cut into parts each of a certain length to obtain individual rolled thread screws therefrom. In contrast, in-feed thread rolling is a thread rolling method in which pressure is applied to an outer circumferential portion of a blank already cut into a length of an individual screw while the blank is placed between a pair of dies that are rotating. Thus, as compared with in-feed thread rolling, through-feed thread rolling tends to produce burrs and sharp edges on rolled thread screws when a post-process such as cutting is performed after thread rolling.

A rolled thread screw and a rolled thread screw manufacturing method that reduce burrs are desired when in-feed thread rolling is used.

The present disclosure has been made in view of the above inconvenience, and is directed to providing a rolled thread screw and a rolled thread screw manufacturing method that reduce burrs when in-feed thread rolling is used.

To achieve the above object, the invention provides a rolled thread screw comprising: a screw part having, on an outer circumferential part of the screw part, thread crests and thread grooves disposed alternately along an axial direction of the rolled thread screw; and a columnar part adjoining at least one of both sides of the screw part in the axial direction, wherein an axial end surface of the columnar part is provided with a first concave surface concave in the axial direction, a second concave surface positioned to an outer circumference of the first concave surface and being concave in the axial direction, and a salient part positioned on a boundary between the first concave surface and the second concave surface and projecting further than a bottom of the first concave surface and a bottom of the second concave surface, the salient part extending in an arc-like shape along a circumferential direction of the columnar part as viewed in the axial direction.

Salient excess-thickness portions, such as columnar parts and truncated cone parts, are provided on end parts of the blank, which is used in manufacturing the rolled thread screw. End-part wall surfaces of the screw part are thereby prevented from falling down into shapes extending in the axial direction, which become burrs. Parts of these excess-thickness portions are formed into the first concave surface, the second concave surface, and the salient part through thread rolling. The axial end surface of the columnar part is formed flatly because the first concave surface and the second concave surface extend along radial directions of the columnar part. As a result, the rolled thread screw that has prevented generation of burrs and that has the axial end surface of the columnar parts formed flatly can be obtained.

The invention further provides a rolled thread screw manufacturing method comprising: a preparation step of preparing a blank; and a processing step of plastically deforming the blank into a rolled thread screw, wherein the blank that is prepared at the preparation step is a columnar blank including a first truncated cone part having a first outer circumferential surface having an outer diameter that is larger toward one side of the rolled thread screw that faces in a first axial direction of the rolled thread screw, a second truncated cone part disposed adjoining one side of the first truncated cone part that faces in the first axial direction and having a second outer circumferential surface having an outer diameter that is larger toward the one side of the rolled thread screw, a first columnar part adjoining one side of the second truncated cone part that faces in the first axial direction, a second columnar part adjoining one side of the first columnar part that faces in the first axial direction and having a larger outer diameter than the first columnar part, an outer diameter of an end of the first truncated cone part that faces in the first axial direction is identical to an outer diameter of an end of the second truncated cone part that faces in a second axial direction that is opposite to the first axial direction, an outer diameter of an end of the second truncated cone part that faces in the first axial direction is equal to an outer diameter of an end of the first columnar part that faces in the second axial direction, and at the processing step, thread rolling in which pressure is applied to an outer circumferential part of the second columnar part inward in radial directions of the rolled thread screw is performed to form thread grooves, and the first outer circumferential surface of the first truncated cone part and the second outer circumferential surface of the second truncated cone part are thereby plastically deformed in such a manner as to span along the radial directions.

Salient excess-thickness portions, such as the first truncated cone part, the second truncated cone part, and the first columnar part, are provided to one side of the blank that is prepared at the preparation step of the present disclosure, the one side facing in the second axial direction. It is thereby made possible to prevent the end-part wall surfaces of the screw part from falling down into shapes extending in the axial directions, which become burrs. As a result, work of removing burrs such as buffing and deburring grinding is reduced. Quality checking work for checking whether there is any burr can also be reduced. In addition, the application of in-feed thread rolling to this case is advantageous in that a production yield from the blank is improved.

As a desirable embodiment of the rolled thread screw manufacturing method, at the processing step, the first outer circumferential surface is formed into a first concave surface spreading in the radial directions and being concave in the first axial direction, the second outer circumferential surface is formed into a second concave surface disposed to an outer circumference of the first concave surface, spreading in the radial directions, and being concave in the first axial direction, and a salient part projecting in the second axial direction is formed on a boundary between the first concave surface and the second concave surface, the salient part extending in an arc-like shape along a circumferential direction as viewed in the axial direction.

At the processing step, the first outer circumferential surface is deformed into the first concave surface, the second outer circumferential surface is deformed into the second concave surface, and the first concave surface and the second concave surface are deformed in such a manner as to span along the radial directions. As a result, an axial end surface of the screw part spreads in the radial directions to become flat. Thus, the thread grooves can be provided on the screw part including an end part thereof in the axial direction, whereby the effective length of the screw part can be set longer than otherwise. Advantageous Effects of Invention.

According to the present invention, a rolled thread screw and a rolled thread screw manufacturing method that reduce burrs are provided when in-feed thread rolling is used.

The following describes a mode for carrying out the invention (hereinafter "embodiment") in detail with reference to the drawings. The following embodiment is not intended to limit the present invention, which is defined in the appended claims. The constituent elements in the following embodiment include those easily conceivable by the skilled person, those substantially identical to each other, and those that fall within what is called the range of equivalents. The constituent elements described in the following embodiment can be combined as appropriate.

First, a rolled thread screw <NUM> according to the embodiment is described. <FIG> is a side view of a rolled thread screw according to an embodiment, as viewed from one side thereof. <FIG> is a view of <FIG> as viewed in an axial direction of the rolled thread screw. <FIG> is an enlarged side view of an end part of a screw part in <FIG> that faces in a second axial direction. <FIG> is an enlarged side view of another end part of the screw part in <FIG> that faces in a first axial direction. <FIG> is an enlarged schematic view of <FIG>. The axial directions herein mean directions along a shaft center Ax of the rolled thread screw <NUM>. A radial direction means a direction perpendicular to the shaft center Ax. Facing in the first axial direction means facing rightward in <FIG>, and facing in the second axial direction means facing leftward in <FIG>.

As illustrated in <FIG>, the rolled thread screw <NUM> includes a shaft part <NUM>, columnar parts <NUM> and <NUM>, and a screw part <NUM>. The shaft part <NUM> includes, in order along the axial directions, a narrow-diameter part <NUM>, a small-diameter part <NUM>, and a groove part <NUM>. The narrow-diameter part <NUM> and the small-diameter part <NUM> are columnar. The small-diameter part <NUM> adjoins one side that faces in the first axial direction (a rightward side in <FIG>) of the narrow-diameter part <NUM>. The small-diameter part <NUM> has a larger outer diameter than the narrow-diameter part <NUM>. The groove part <NUM> adjoins one side that faces in the first axial direction (a rightward side in <FIG>) of the small-diameter part <NUM>. The groove part <NUM> has an arc-like shape concave inward in the radial directions.

As illustrated in <FIG> and <FIG>, a large-diameter part <NUM> (a columnar part <NUM>) adjoins one side that faces in the first axial direction (a rightward side in <FIG>) of the groove part <NUM>. The large-diameter part <NUM> is referred to also as the columnar part <NUM>. The large-diameter part <NUM> has a larger outer diameter than the small-diameter part <NUM>. The large-diameter part <NUM> has an axial end surface <NUM> stretching in the radial directions. The axial end surface <NUM> is positioned at an end that faces in the second axial direction (a leftward side in <FIG>) of the large-diameter part <NUM>. The axial end surface <NUM> includes a first concave surface <NUM> and a second concave surface <NUM>. Preferably, the diameter of the large-diameter part <NUM> is, for example, +<NUM>% to -<NUM>% of a root part <NUM> of each thread groove <NUM>. The first concave surface <NUM> is provided on the axial end surface <NUM> of the large-diameter part <NUM> (the columnar part <NUM>) and is concave in the first axial direction. The second concave surface <NUM> is provided on the axial end surface <NUM> of the large-diameter part <NUM> (the columnar part <NUM>) and is concave in the first axial direction. The second concave surface <NUM> is positioned to the outer circumference of the first concave surface <NUM>.

Here, when the center of the screw part <NUM> in the axial direction is denoted as a first central part CL (see <FIG>), the first concave surface <NUM> is a concave surface that is concave toward the first central part CL. The second concave surface <NUM> is a concave surface that is concave toward the first central part CL. The first concave surface <NUM> and the second concave surface <NUM> have annular shapes centered at the shaft center Ax. A salient part <NUM> is provided on the boundary between the first concave surface <NUM> and the second concave surface <NUM>. The salient part <NUM> projects in the second axial direction (leftward in <FIG>). In other words, the salient part <NUM> projects in a direction going away from the first central part CL. Furthermore, as illustrated in <FIG>, the salient part <NUM> extends in an annular shape or an arc-like shape along a circumferential direction. The boundary between the second concave surface <NUM> and the outer circumferential surface of the large-diameter part <NUM> is an outer circumferential end <NUM> of the second concave surface <NUM>.

The screw part <NUM> has, on the outer circumferential part thereof, thread crests <NUM> and thread grooves <NUM> disposed alternately along the axial direction. The shape of a section of each of the thread grooves <NUM> is arc-like. The thread groove <NUM> has a root part <NUM>. The root part <NUM> is the innermost region of the thread groove <NUM> in the radial directions. An end-part wall surface <NUM> and an end-part wall surface <NUM> are provided to both ends of the screw part <NUM> in the axial direction. While the end-part wall surface <NUM> is an end surface of the screw part <NUM> that faces in the first axial direction, the end-part wall surface <NUM> is another end surface thereof that faces in the second axial direction. A swelling part <NUM> protruding in the second axial direction is provided on the end-part wall surface <NUM>. The leading end of the swelling part <NUM>, which faces in the second axial direction, is positioned further in the first axial direction than the axial end surface <NUM>. The salient part <NUM> described above is positioned further in the radial directions from the root part <NUM> of each of the thread grooves <NUM>.

A columnar part <NUM> adjoins one side of the end-part wall surface <NUM> that faces in the first axial direction. The columnar part <NUM> spreads outward in the radial directions with the shaft center Ax at the center thereof. The columnar part <NUM> includes an axial end surface <NUM>. The axial end surface <NUM> may have a QR code (registered trademark) or data for product management engraved thereon by laser processing or the like. The axial end surface <NUM> includes a first concave surface <NUM> and a second concave surface <NUM>. The first concave surface <NUM> is provided on the axial end surface <NUM> of the columnar part <NUM> and is concave in the second axial direction. The second concave surface <NUM> is provided on the axial end surface <NUM> of the columnar part <NUM> and is concave in the second axial direction. The second concave surface <NUM> is positioned to the outer circumference of the first concave surface <NUM>. In other words, the first concave surface <NUM> is a concave surface that is concave toward the first central part CL (see <FIG>), and the second concave surface <NUM> is a concave surface that is concave toward the first central part CL. The first concave surface <NUM> and the second concave surface <NUM> have annular shapes centered at the shaft center Ax. A salient part <NUM> is provided on the boundary between the first concave surface <NUM> and the second concave surface <NUM>. The boundary between the second concave surface <NUM> and the outer circumferential surface of the columnar part <NUM> is an outer circumferential end <NUM> of the second concave surface <NUM>.

Next, a rolled thread screw manufacturing method is described. <FIG> is a side view illustrating a blank for the rolled thread screw according to the embodiment. <FIG> is an enlarged side view of a part of <FIG>. <FIG> is a schematic view illustrating a state in which thread rolling is being performed on a blank.

First, the shape of a blank <NUM> for a rolled thread screw is described with reference to <FIG>. The blank <NUM> has a columnar shape including a shaft part <NUM>, a first truncated cone part <NUM>, a second truncated cone part <NUM>, a first columnar part <NUM>, a second columnar part <NUM>, and a third columnar part <NUM>. Preferably, corner parts of the blank <NUM> are chamfered.

The shaft part <NUM> includes a narrow-diameter part <NUM>, a small-diameter part <NUM>, and a groove part <NUM>. The narrow-diameter part <NUM> and the small-diameter part <NUM> are columnar. The small-diameter part <NUM> adjoins one side that faces in the first axial direction (a rightward side in <FIG>) of the narrow-diameter part <NUM>. The small-diameter part <NUM> has a larger outer diameter than the narrow-diameter part <NUM>. The groove part <NUM> adjoins one side that faces in the first axial direction (a rightward side in <FIG>) of the small-diameter part <NUM>. The groove part <NUM> has an arc-like shape concave inward in the radial directions.

As illustrated in <FIG>, the first truncated cone part <NUM> has a first outer circumferential surface <NUM>. The first outer circumferential surface <NUM> has a tapered shape the outer diameter of which is larger toward one side (the right side in <FIG>) thereof in the first axial direction. Here, as illustrated in <FIG>, the first outer circumferential surface <NUM> is tilted only by a first tilt angle <NUM> to a radial line <NUM> (indicated by a chain double-dashed line), which is perpendicular to the shaft center Ax. That is, the first outer circumferential surface <NUM> is tilted toward one side thereof in the first axial direction from the radial line <NUM> only by the first tilt angle θ0. Preferably, the first tilt angle <NUM> is, for example, larger than <NUM> degrees and less than or equal to <NUM> degrees. The second truncated cone part <NUM> has a second outer circumferential surface <NUM>. The second truncated cone part <NUM> adjoins one side of the first truncated cone part <NUM> that faces in the first axial direction. The second outer circumferential surface <NUM> has a tapered shape the outer diameter of which is larger toward the other side thereof in the first axial direction. The second outer circumferential surface <NUM> is tilted only by a second tilt angle Θ1 to the radial line <NUM>. That is, the second outer circumferential surface <NUM> is tilted toward one side thereof in the first axial direction from the radial line <NUM> only by the second tilt angle Θ1. The second tilt angle Θ1 is larger than the first tilt angle θ0. Preferably, the second tilt angle Θ1 is, for example, an angle in the range of <NUM>% to <NUM>% of the screw lead angle. A boundary part <NUM> is provided at the boundary between the first outer circumferential surface <NUM> and the second outer circumferential surface <NUM>. In other words, the boundary part <NUM> is an intersection between the first outer circumferential surface <NUM> and the second outer circumferential surface <NUM>.

The first columnar part <NUM> adjoins one side of the second outer circumferential surface <NUM> that faces in the first axial direction. The first columnar part <NUM> has a larger outer diameter than the small-diameter part <NUM>. An intersection between the outer circumferential surface of the first columnar part <NUM> and the second outer circumferential surface <NUM> is an outer circumferential end <NUM> of the second outer circumferential surface <NUM>. The second columnar part <NUM> adjoins one side of the first columnar part <NUM> that faces in the first axial direction. The second columnar part <NUM> has a larger outer diameter than the first columnar part <NUM>. An end surface of the second columnar part <NUM> that faces in the second axial direction is an end-part wall surface <NUM>. The outer diameter of an end of the first truncated cone part <NUM> that faces in the first axial direction is identical to the outer diameter of an end of the second truncated cone part <NUM> that faces in the second axial direction. The outer diameter of an end of the second truncated cone part <NUM> that faces in the first axial direction is equal to the outer diameter of an end of the first columnar part <NUM> that faces in the second axial direction.

Next, specifics of the rolled thread screw manufacturing method are described. As illustrated in <FIG>, the rolled thread screw <NUM> is formed by having threads thereof rolled using thread rolling dies <NUM>. In the present embodiment, a mode in which in-feed thread rolling is applied out of in-feed thread rolling and through-feed thread rolling is described. The thread rolling dies <NUM> include a first thread rolling die <NUM> and a second thread rolling die <NUM>. Respective outer circumferential parts of the first thread rolling die <NUM> and the second thread rolling die <NUM> are provided with annular projecting parts extending along the circumferential directions thereof. The first thread rolling die <NUM> and the second thread rolling die <NUM> can be driven to rotate by a drive source that is not illustrated. The first thread rolling die <NUM> and the second thread rolling die <NUM> can also be moved closer to and away from each other. The rotational axis of the first thread rolling die <NUM> and the rotational axis of the second thread rolling die <NUM> are disposed in parallel.

The first step in the manufacturing method is a preparation step of preparing the blank <NUM>. As described above, the blank <NUM> has a columnar shape including the shaft part <NUM>, the first truncated cone part <NUM>, the second truncated cone part <NUM>, the first columnar part <NUM>, the second columnar part <NUM>, and the third columnar part <NUM>.

Next, the second step in the manufacturing method is a processing step of plastically deforming the blank <NUM> into the rolled thread screw <NUM>. Specifically, the blank <NUM> is placed between the first thread rolling die <NUM> and the second thread rolling die <NUM>. The first thread rolling die <NUM> and the second thread rolling die <NUM> are set apart from each other at a distance larger than the diameter of the second columnar part <NUM> of the blank <NUM>. Thus, the blank <NUM> is placed between the first thread rolling die <NUM> and the second thread rolling die <NUM> in parallel to the respective rotational axes of the first thread rolling die <NUM> and the second thread rolling die <NUM>. The first thread rolling die <NUM> and the second thread rolling die <NUM> are then moved closer to each other while being rotated. In this manner, forming in which pressure is applied to the second columnar part <NUM> of the blank <NUM> inward in the radial directions while being sandwiched between the first thread rolling die <NUM> and the second thread rolling die <NUM> is performed. Here, as illustrated in <FIG>, pressure applied by the first thread rolling die <NUM> is F1, and pressure applied by the second thread rolling dies <NUM> is F2. The pressure F1 and the pressure F2 are of the same magnitude. After the entire screw part <NUM> (the second columnar part <NUM>) receives the pressure F1 and the pressure F2, the pressure F1 and the pressure F2 move toward both sides of the screw part <NUM> (both leftward and rightward in <FIG>) in the axial directions. In addition, when the pressure F1 and the pressure F2 are applied to the blank <NUM>, stress ST acts on the blank <NUM>. A range on which the stress ST acts is, as illustrated in <FIG>, a horizontally long elliptical range extending in the axial directions. The stress ST is larger in both end parts (end parts on the left side and the right side in <FIG>) of the screw part <NUM> (the second columnar part <NUM>) in the axial directions than in a central part thereof in the axial direction. Parts of the stress ST that act on the narrow-diameter part <NUM> and the small-diameter part <NUM> (the narrow-diameter part <NUM> and the small-diameter part <NUM>), which are disposed further left from the leftward end part of the screw part <NUM> (the second columnar part <NUM>) in <FIG>, are smaller than those of the stress ST that act on the end parts of the screw part <NUM> (the second columnar part <NUM>) in the axial directions.

This second step forms the thread crests <NUM> and the thread grooves <NUM> on the second columnar part <NUM> (see <FIG>). In addition, the first outer circumferential surface <NUM> of the first truncated cone part <NUM> illustrated in <FIG> deforms in such a manner as to become closer to the radial directions. That is, the first tilt angle <NUM> in <FIG> gradually becomes smaller, and the first outer circumferential surface <NUM> plastically deforms in such a manner as to move counterclockwise in <FIG>. The second outer circumferential surface <NUM> of the second truncated cone part <NUM> illustrated in <FIG> plastically deforms in such a manner as to become closer to the radial directions. That is, the second tilt angle Θ1 in <FIG> gradually becomes smaller, and the second outer circumferential surface <NUM> plastically deforms in such a manner as to move counterclockwise in <FIG>. As a result, the first outer circumferential surface <NUM> of the blank <NUM> illustrated in <FIG> is turned into the first concave surface <NUM> in <FIG>, and the second outer circumferential surface <NUM> in <FIG> is turned into the second concave surface <NUM> in <FIG>.

The boundary part <NUM> of the blank <NUM> is also turned into the salient part <NUM>. Furthermore, the swelling part <NUM> illustrated in <FIG> is formed by having pressure applied to the second columnar part <NUM> inward in the radial direction. Also when the columnar part <NUM> illustrated in <FIG> is formed, the axial end surface <NUM> that has the first concave surface <NUM> and the second concave surface <NUM> can be shaped by having, on the outer circumferential surface of the blank <NUM>, two truncated cone parts that have tilt angles of different degrees although illustrations thereof are omitted. Thus, the thread grooves <NUM> are formed by performing, on an outer circumferential part of the second columnar part <NUM>, thread rolling in which pressure is applied thereto inward in the radial directions. The first outer circumferential surface <NUM> of the first truncated cone part <NUM> and the second outer circumferential surface <NUM> of the second truncated cone part <NUM> can be thereby plastically deformed in such a manner as to span along the radial directions.

Next, one of the examples of utilization of the rolled thread screw <NUM> is described. <FIG> is a schematic view illustrating a state in which an inner ring of a bearing has been press-fit onto the rolled thread screw.

As illustrated in <FIG>, the small-diameter part <NUM> of the shaft part <NUM> is press-fit against an inner circumferential surface <NUM> of an inner ring <NUM> of the bearing. As a result, one end part of the rolled thread screw <NUM> that faces in a first axial direction thereof is rotatably supported by the inner ring <NUM> of the bearing. In addition, an axial end surface <NUM> of the inner ring <NUM> is in contact with the axial end surface <NUM> of the large-diameter part <NUM> of the rolled thread screw <NUM>.

Next, a comparative example is described. <FIG> is a side view illustrating a blank for a rolled thread screw according to a comparative example. <FIG> is a side view of the rolled thread screw according to the comparative example, which illustrates the state thereof before deburring thereof. <FIG> is a side view illustrating an end part of the rolled thread screw according to the comparative example that faces in a second axial direction thereof, which illustrates the state thereof after deburring thereof. <FIG> is a side view illustrating another end part of the rolled thread screw according to the comparative example that faces in a first axial direction thereof, which illustrates the state thereof after deburring thereof.

As compared with the blank <NUM> according to the embodiment illustrated in <FIG>, a blank 100A according to the comparative example illustrated in <FIG> is not provided with the first truncated cone part <NUM>, the second truncated cone part <NUM>, the first columnar part <NUM>, and the third columnar part <NUM>. Therefore, as illustrated in <FIG>, an intermediary screw body 1A obtained by performing thread rolling has burrs <NUM> and <NUM> on axial end surfaces of a screw part 3A that project in the axial directions. This necessitates work of planing end-part wall surfaces of the second columnar part <NUM> along the radial directions at a step after the thread rolling in order to remove the burrs <NUM> and <NUM> as illustrated in <FIG>. However, there is a possibility that planing the end-part wall surfaces may result in the formation of thread crests at sharp edges <NUM>, <NUM>, and <NUM> and in the formation of a sharp edge <NUM> at the end-part wall surfaces, as illustrated in <FIG>.

As described above, the rolled thread screw <NUM> according to the present embodiment includes the screw part <NUM> having the thread crests <NUM> and the thread grooves <NUM>, the columnar parts <NUM> and <NUM> provided to both sides of the screw part <NUM> in the axial directions. The axial end surfaces <NUM> and <NUM> are provided with the first concave surfaces <NUM> and <NUM>, the second concave surfaces <NUM> and <NUM>, and the salient parts <NUM> and <NUM> extending in arc-like shapes in the respective circumferential directions of the axial end surfaces <NUM> and <NUM>.

Generation of the burr <NUM> can be prevented by providing salient excess-thickness portions, such as the first truncated cone part <NUM> and the second truncated cone part <NUM>, in one side of the blank <NUM>, which is used in manufacturing the rolled thread screw <NUM>, the one side facing in the second axial direction. As a result, work such as buffing and deburring grinding can be reduced. Quality checking work for checking whether there is any burr can also be reduced. Furthermore, the axial end surfaces <NUM> and <NUM> of the columnar parts <NUM> and <NUM> are formed flatly because the first concave surfaces <NUM> and <NUM>, the second concave surfaces <NUM> and <NUM>, and the salient parts <NUM> and <NUM> are disposed along the radial directions. As a result, the rolled thread screw <NUM> that has prevented generation of burrs and that has the axial end surfaces <NUM> and <NUM> of the columnar parts <NUM> and <NUM> formed flatly can be obtained. Particularly when a QR code (registered trademark) or data for product management is engraved on the axial end surface <NUM> by laser processing or the like, it is desirable that the axial end surface <NUM> has a higher degree of flatness. Note that the swelling part <NUM> illustrated in <FIG> is formed by applying pressure inward in the radial directions to the second columnar part <NUM> including even the leading end thereof facing in the second axial direction. Thus, the thread grooves <NUM> can be provided on the screw part <NUM> including the leading end thereof facing in the second axial direction, whereby the effective length of the screw part <NUM> can be set longer than otherwise. In addition, the leading end of the swelling part <NUM>, which faces in the second axial direction, is positioned further in the first axial direction from the axial end surface <NUM>.

The rolled thread screw manufacturing method uses the blank <NUM>. The blank <NUM> includes the first truncated cone part <NUM> having the first outer circumferential surface <NUM>, the second truncated cone part <NUM> having the second outer circumferential surface <NUM>, the first columnar part <NUM>, and the second columnar part <NUM>. The thread grooves <NUM> are formed by performing, on the outer circumferential part of the second columnar part <NUM>, thread rolling in which pressure is applied thereto inward in the radial directions. The first outer circumferential surface <NUM> of the first truncated cone part <NUM> and the second outer circumferential surface <NUM> of the second truncated cone part <NUM> are thereby plastically deformed in such a manner as to span along the radial directions.

Unlike the blank <NUM> according to the embodiment, the blank 100A according to the comparative example is not provided with the first truncated cone part <NUM>, the second truncated cone part <NUM>, the first columnar part <NUM>, and the third columnar part <NUM>, as described for the comparative example. When thread rolling is performed using this blank 100A, the burrs <NUM> and <NUM> illustrated in <FIG> are formed, and thus work of removing the burrs <NUM> and <NUM> is necessitated. Furthermore, there is a possibility that the sharp edges <NUM>, <NUM>, <NUM>, and <NUM> may be formed on the thread crests or on the end-part wall surfaces when the end-part wall surfaces are planed for the purpose of removing the burrs <NUM> and <NUM>, as illustrated in <FIG>.

In contrast, the blank <NUM> according to the present embodiment is provided with the first truncated cone part <NUM>, the second truncated cone part <NUM>, and the first columnar part <NUM>, thereby being advantageous in that burrs and sharp edges that may be formed in the comparative example are less likely to be formed. Generation of the burr <NUM> in the comparative example illustrated in <FIG> can be prevented particularly by providing salient excess-thickness portions, such as the first truncated cone part <NUM> and the second truncated cone part <NUM>, in one side of the blank <NUM> that face in the second axial direction. As a result, work of removing burrs such as buffing and deburring grinding can be reduced. Quality checking work for checking whether there is any burr can also be reduced. Note that the application of in-feed thread rolling in the present embodiment is advantageous in that a production yield from the blank <NUM> is higher than when through-feed thread rolling is applied.

In addition, as illustrated in <FIG>, the large-diameter part <NUM> (the columnar part <NUM>) is provided, and the leading end of the swelling part <NUM> that faces in the second axial direction, positioned further in the first axial direction from the axial end surface <NUM>. Therefore, even when the swelling part <NUM> has been formed on the end-part wall surface <NUM> with pressure acting inward in the radial directions as a result of thread rolling, there is no possibility that the swelling part <NUM> projects in the second axial direction beyond the axial end surface <NUM>. As a result, as described with reference to <FIG>, the axial end surface <NUM> of the inner ring <NUM> reliably abuts on the axial end surface <NUM> of the large-diameter part <NUM> of the rolled thread screw <NUM>.

When the thread grooves <NUM> are formed by performing, on the outer circumferential part of the second columnar part <NUM>, thread rolling in which pressure is applied thereto inward in the radial directions, the first outer circumferential surface <NUM> is formed into the first concave surface <NUM>, the second outer circumferential surface <NUM> is formed into the second concave surface <NUM>, and the salient part <NUM> is formed on the boundary between the first concave surface <NUM> and the second concave surface <NUM>.

As illustrated in <FIG>, thread rolling using the blank 100A according to the comparative example may cause end-part wall surfaces of the screw part 3A to fall down into shapes extending in the axial directions, which become the burrs <NUM> and <NUM>. In contrast, salient excess-thickness portions, such as the first truncated cone part <NUM> and the second truncated cone part <NUM>, are provided to one side of the blank <NUM> according to the present embodiment that faces in the second axial direction. When such salient excess-thickness portions are provided, it is made possible to prevent the end-part wall surfaces of the screw part 3A from falling down into shapes extending in the axial directions, which become the burrs <NUM> and <NUM>, as in the case of the comparative example. Thus, in in-feed thread rolling, thread rolling using the blank <NUM> according to the present embodiment can prevent generation of burrs to a larger extent than thread rolling using the blank 100A according to the comparative example. That is, in-feed thread rolling according to the present embodiment generates a smaller amount of burrs than through-feed thread rolling according to a conventional example, and among in-feed thread rolling alternatives, thread rolling using the blank <NUM> according to the present embodiment, rather than the comparative example, can further reduce generation of burrs.

Next, a first modification is described. <FIG> is an enlarged schematic view of a part of a rolled thread screw according to the first modification. In the embodiment, as illustrated in <FIG>, the sectional shape of the bottom surface of the groove part <NUM> in a section that contains the shaft center Ax is arc-like. In contrast, as illustrated in <FIG>, the sectional shape of the bottom surface of a groove part 28A according to the modification is linear along the shaft center Ax. That is, the bottom surface of the groove part 28A has a cylindrical shape extending in a circumferential direction with the shaft center Ax at the center. The small-diameter part <NUM> of the shaft part <NUM> is press-fit against an inner circumferential surface <NUM> of an inner ring <NUM> of a bearing. As a result, a gap between the inner circumferential surface <NUM> of the inner ring <NUM> of the bearing and the bottom surface of the groove part 28A in the radial directions is uniform in size in the axial directions.

As described above, the sectional shape of the bottom surface of the groove part 28A according to the first modification is linear along the shaft center Ax. For this reason, the end-part wall surfaces of the screw part <NUM> are further less likely to fall down, due to pressure when thread rolling is performed, into shapes extending in the axial directions. Generation of the burrs <NUM> and <NUM> is thereby further prevented. Thus, an axial end surface <NUM> of the inner ring <NUM> of the bearing more reliably abuts on the axial end surface <NUM> of the large-diameter part of the rolled thread.

Next, a second modification is described. <FIG> is a schematic view illustrating a state in which an inner ring of a bearing is being press-fit onto a rolled thread screw according to the second modification. A rolled thread screw 1B according to the second modification further includes a shaft part 11B in the rolled thread screw <NUM> according to the embodiment. That is, the rolled thread screw 1B is provided with the shaft part <NUM> and the shaft part 11B on two opposite sides of a screw part 3B in the axial directions. The shaft part 11B includes, in order along the axial directions, a narrow-diameter part 10B, a small-diameter part 21B, and a groove part 28B. The narrow-diameter part 10B and the small-diameter part 21B are columnar. The small-diameter part 21B adjoins one side that faces in the second axial direction (a leftward side in <FIG>) of the narrow-diameter part 10B. The small-diameter part 21B has a larger outer diameter than the narrow-diameter part 10B. The groove part 28B adjoins one side that faces in the second axial direction (a leftward side in <FIG>) of the small-diameter part 21B. The groove part 28B has an arc-like shape concave inward in the radial directions. As illustrated in <FIG>, the small-diameter part <NUM> of the shaft part <NUM> is press-fit against an inner circumferential surface <NUM> of an inner ring <NUM> of a bearing. The small-diameter part 21B of the shaft part 11B is press-fit against an inner circumferential surface <NUM> of an inner ring <NUM> of a bearing.

Claim 1:
A rolled thread screw comprising:
a screw part (<NUM>) having, on an outer circumferential part of the screw part (<NUM>), thread crests and thread grooves disposed alternately along an axial direction of the rolled thread screw; and
a columnar part (<NUM>,<NUM>) adjoining at least one of both sides of the screw part (<NUM>) in the axial direction, wherein
an axial end surface (<NUM>,<NUM>) of the columnar part (<NUM>,<NUM>) is provided with
a first concave surface (<NUM>,<NUM>) concave in the axial direction,
a second concave surface (<NUM>,<NUM>) positioned to an outer circumference of the first concave surface (<NUM>,<NUM>) and being concave in the axial direction, and
a salient part (<NUM>,<NUM>) positioned on a boundary between the first concave surface (<NUM>,<NUM>) and the second concave surface (<NUM>,<NUM>) and projecting further than a bottom of the first concave surface (<NUM>,<NUM>) and a bottom of the second concave surface (<NUM>,<NUM>), the salient part (<NUM>,<NUM>) extending in an arc-like shape along a circumferential direction of the columnar part (<NUM>,<NUM>) as viewed in the axial direction.