Method for manufacturing screw shafts for ball screws

Abstract

A method for manufacturing a screw shaft for a ball screw, having a helical groove on which multiple rolling elements roll, and a circulation groove that returns the rolling elements from one end of the helical groove to the other, comprising a circulation groove forming step and a helical groove forming step. In the circulation groove forming step, a punch (31) having a convex portion (32) corresponding to the concave portion of the circulation groove is moved radially inward relative to the blank (8) without moving it axially, thereby pressing the convex portion (32) against the outer circumferential surface of the blank (8) and forming a circulation groove on the outer circumferential surface of the blank (8).

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a screw shaft for a ball screw. This application claims priority based on Japanese Patent Application No. 2024-185745 filed on October 22, 2024, and incorporates its content herein.

Background Art

[0002] Conventionally, in a ball screw having a nut, a screw shaft, and a plurality of rolling elements, as one of the circuit structures for circulating the plurality of rolling elements, a structure is known that includes a circulation groove for a return path (return orbit) provided on the outer peripheral surface of the screw shaft separately from the groove for the main path (main orbit). In such a ball screw, various methods for directly forming a circulation groove on the outer peripheral surface of the screw shaft have been proposed.

[0003] For example, Patent Document 1 discloses a method for manufacturing a screw shaft for a ball screw having a spiral groove in which a plurality of rolling elements roll and a circulation groove for returning the rolling elements from the end point to the start point of the spiral groove and circulating them, and a configuration in which a circulation groove is formed on the outer peripheral surface of the screw shaft by cutting using an end mill.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, in the technology described in Patent Document 1, multiple circulation grooves are formed by cutting, which tends to increase processing time. As a result, there is a risk that the cost of manufacturing the screw shaft will increase. Furthermore, since cutting is performed using an end mill, if the processing position coincides with the rotation axis of the end mill, the rotation speed at the center of rotation of the end mill becomes low, reducing cutting performance. This may reduce the machinability and machining accuracy of the screw shaft.

[0006] Therefore, in the prior art described in Patent Document 1, etc., there was room for improvement in terms of improving machinability and machining accuracy in the method for manufacturing a screw shaft for a ball screw in which a circulation groove is integrally formed on the screw shaft.

[0007] Therefore, the present invention aims to provide a method for manufacturing a screw shaft for a ball screw that can improve the machinability and machining accuracy when forming a circulation groove on the screw shaft compared to the conventional technology. [Means for solving the problem]

[0008] To solve the above problems, this invention proposes the following means. A method for manufacturing a screw shaft for a ball screw according to a first aspect of the present invention is a method for manufacturing a screw shaft for a ball screw having a helical groove on which a plurality of rolling elements roll, and a circulation groove that returns the rolling elements from one end to the other end of the helical groove, comprising: a circulation groove forming step of forming the circulation groove on the outer circumferential surface of the blank by moving a first mold, which has protrusions corresponding to the recesses of the circulation groove, from the radially outside to the inside relative to the blank of the screw shaft; and a helical groove forming step of forming the helical groove on the outer circumferential surface of the blank by a removal process, wherein in the circulation groove forming step, the outer circumferential surface of the blank is pressed by moving the first mold radially inward without moving it axially, and the circulation groove is formed by pressing the protrusions against the outer circumferential surface of the blank. [Effects of the Invention]

[0009] The present invention provides a method for manufacturing a ball screw shaft that improves machinability and machining accuracy when forming a circulation groove on the screw shaft compared to the conventional technology. [Brief explanation of the drawing]

[0010] [Figure 1] A schematic cross-sectional view showing a ball screw according to the first embodiment. [Figure 2] A flowchart showing the process for manufacturing a screw shaft for a ball screw according to the first embodiment. [Figure 3] Cross-sectional view of a blank screw shaft according to the first embodiment. [Figure 4] A cross-sectional view showing an overview of the circulation groove formation process in the manufacturing method of screw shafts. [Figure 5] A cross-sectional view showing an overview of the circulation groove formation process in the manufacturing method of screw shafts. [Figure 6] A perspective view illustrating the outline of the circulation groove formation process in the manufacturing method of screw shafts. [Figure 7] A cross-sectional view showing an overview of the mold removal process in the manufacturing method of screw shafts. [Figure 8] A schematic cross-sectional view showing an overview of the circulation groove formation process in the manufacturing method of a screw shaft according to the second embodiment. [Modes for carrying out the invention]

[0011] Embodiments of the present invention will be described with reference to the drawings. In one embodiment, the ball screw 10 is incorporated into various mechanical devices such as electric brake systems and automated manual transmissions (AMTs) of vehicles, and positioning devices for machine tools, and is used to convert the rotational motion of a drive source such as an electric motor into linear motion to operate a driven part (operating part). Various types of electric brake systems are applicable, such as EMB (Electro-Mechanical Brake) which applies braking force via a ball screw driven by a motor, and EHB (Electro-Hydraulic Brake) which controls the hydraulic pressure of a hydraulic brake via a ball screw driven by a motor. The ball screw 10 can also be applied to mechanical devices other than those mentioned above.

[0012] In the following description, unless otherwise specified, axial, radial, and circumferential directions refer to the direction along the central axis C of the ball screw 10 (screw shaft 2), the radial direction of the ball screw 10 (screw shaft 2), and the direction around the central axis C of the ball screw 10 (screw shaft 2), respectively.

[0013] (First Embodiment) (Ball screw) Figure 1 is a schematic cross-sectional view showing a ball screw 10 according to the first embodiment. The ball screw 10 is a device that converts rotational motion into linear motion. As shown in Figure 1, the ball screw 10 comprises a nut 1, a screw shaft 2 (a screw shaft for a ball screw according to the claim), and a plurality of rolling elements 3 arranged between the nut 1 and the screw shaft 2.

[0014] The nut 1 is formed in a cylindrical shape with a central axis C as its axis. A helical screw groove (internal helical groove 15) is formed on the inner circumferential surface of the nut 1. In one example, at least a part of the nut 1 is made of metal. The internal helical groove 15 is formed by machining or plastic deformation on the inner circumferential surface of the nut 1. Grinding can be additionally performed in the formation of the internal helical groove 15. The cross-sectional shape of the internal helical groove 15 is, for example, a Gothic arch or a circular arc containing two arcs.

[0015] The screw shaft 2 has a shaft main body 21 and a small-diameter portion 22 provided side by side in the axial direction of the shaft main body 21. The shaft main body 21 is formed in a cylindrical or columnar shape centered on the central axis C. The shaft main body 21 of the present embodiment is a bottomed cylindrical shape with one end closed (see also FIG. 3). A spiral screw groove (outer peripheral spiral groove 24 (the spiral groove in the claims)) is formed on the outer peripheral surface of the shaft main body 21. The small-diameter portion 22 is connected to the end portion on the side where the bottom is provided in the axial direction of the shaft main body 21. The small-diameter portion 22 is formed in a columnar shape. The outer diameter dimension of the small-diameter portion 22 is smaller than the outer diameter dimension of the shaft main body 21. The screw shaft 2 is integrally formed with the shaft main body 21 and the small-diameter portion 22. In one example, at least a part of the screw shaft 2 is made of metal. The outer peripheral spiral groove 24 is formed by performing cutting or plastic working or the like on the outer peripheral surface of the shaft main body 21. In the formation of the outer peripheral spiral groove 24, grinding can be additionally performed. The cross-sectional shape of the outer peripheral spiral groove 24 corresponds to the shape of the inner peripheral spiral groove 15 of the nut 1, and is, for example, a Gothic arch or a circular arc.

[0016] The plurality of rolling elements 3 are arranged between the nut 1 and the screw shaft 2. The rolling elements 3 are, for example, balls. In one example, the plurality of rolling elements 3 are made of metal (such as steel) or ceramics. In a state where the nut 1 and the screw shaft 2 are combined, a spiral rolling path 25 is formed by the inner peripheral spiral groove 15 formed in the nut 1 and the outer peripheral spiral groove 24 formed in the screw shaft 2 being arranged opposite to each other. The rolling elements 3 move in this rolling path 25 as the nut 1 and the screw shaft 2 rotate relative to each other.

[0017] On the outer peripheral surface of the screw shaft 2, in addition to the outer peripheral spiral groove 24, a plurality (four in this embodiment) of circulation grooves 26 are formed. The circulation grooves 26 are integrally formed on the outer peripheral surface of the screw shaft 2 by a manufacturing method to be described in detail later. The circulation grooves 26 connect one end and the other end of the spiral rolling path 25 formed by the nut 1 and the screw shaft 2 to constitute an infinite circulation circuit. Then, a plurality of rolling elements 3 are filled in this infinite circulation circuit, and the rolling elements 3 circulate infinitely in the infinite circulation circuit. When viewed from the radially outer side of the screw shaft 2, the circulation grooves 26 are formed, for example, in a curved S shape. The circulation grooves 26 are formed in a U-shaped cross section corresponding to the spherical surface of the rolling element 3 in a cross-sectional view perpendicular to the traveling direction of the rolling element 3. The maximum depth of the circulation grooves 26 is set larger than the maximum depth of the outer peripheral spiral groove 24. For example, the depth of the circulation grooves 26 is set such that the rolling element 3 can overcome the thread of the nut 1 in a state where the nut 1 and the screw shaft 2 are combined.

[0018] The rolling element 3 moves while turning around the screw shaft 2 within the rolling path 25 and reaches one end (end point) of the rolling path 25, where it is guided into the circulation groove 26 from one end of the circulation groove 26. The rolling element 3 guided by the circulation groove 26 is pushed by the subsequent rolling element 3 and moves to the other end of the circulation groove 26. Then, the rolling element 3 is returned from the other end of the circulation groove 26 to the other end (starting point) of the rolling path 25. The starting point and the end point of the rolling path 25 are interchanged according to the relative rotation direction of the nut 1 and the screw shaft 2. Thus, as the nut 1 and the screw shaft 2 rotate relative to each other, the rolling element 3 can continuously circulate through the circulation path (circulation circuit) including the rolling path 25 and the circulation groove 26.

[0019] (Manufacturing Method of Screw Shaft for Ball Screw) Next, an example of the manufacturing method of the screw shaft 2 for the above-described ball screw 10 will be described with reference to FIGS. 2 to 7. FIG. 2 is a flowchart showing the flow of the manufacturing method of the screw shaft 2 for the ball screw according to the first embodiment. As shown in FIG. 2, the manufacturing method of the screw shaft 2 includes a circulation groove forming step ST01, a removing step ST02, and a spiral groove forming step ST03. The circulation groove forming step ST01 includes a forging step ST12 and a die removing step ST13.

[0020] Figure 3 is a cross-sectional view of the blank 8 of the screw shaft 2 according to the first embodiment. Figures 4 and 5 are cross-sectional views showing an overview of the circulation groove formation process ST01 in the manufacturing method of the screw shaft 2. Figures 4 and 5 show an overview of the forging process ST12 within the circulation groove formation process ST01. Figure 6 is a perspective view showing an overview of the circulation groove formation process ST01 in the manufacturing method of the screw shaft 2. Figure 7 is a cross-sectional view showing an overview of the mold removal process ST13 in the manufacturing method of the screw shaft 2.

[0021] As shown in Figure 3, first, a blank 8 of the screw shaft 2 (the screw shaft 2 before the outer helical groove 24 and circulation groove 26 are formed) is prepared, in which only the outer shape of the shaft body 21 and the small diameter portion 22 is formed. In this embodiment, the blank 8 is formed in a cylindrical shape with at least a part in the axial direction (the part corresponding to the shaft body 21) being hollow. Although the screw shaft 2 and the blank 8 described above are substantially the same part, for the sake of explanation, in the following description, the blank 8 will be used to refer to the part before the circulation groove 26 and outer helical groove 24 are formed on the outer surface, and the screw shaft 2 will be used to refer to the part after the circulation groove 26 and outer helical groove 24 are formed.

[0022] Next, the forging process ST12 is carried out. In the forging process ST12, first, the blank 8 is placed in a predetermined mold device. As shown in Figures 4 and 6, in this embodiment, the mold device used in the circulation groove forming process ST01 (forging process ST12) includes a heel block 33, a plurality of punches 31 (first mold according to the claim), and a mandrel 36, which are arranged radially outward of the blank 8, and a second mold 40 which are arranged radially inward of the blank 8.

[0023] The heel block 33 is formed in a substantially cylindrical shape with the central axis C as its axis, and the blank 8 is positioned radially inside the heel block 33. The heel block 33 has a plurality of radial through holes 51 (four in this embodiment) that penetrate the heel block 33 radially, and a plurality of axial through holes 52 (four in this embodiment) that penetrate the heel block 33 axially. The number of radial through holes 51 is the same as the number of circulation grooves 26 formed in the screw shaft 2 (four in this embodiment). Each radial through hole 51 is positioned at a different location in the circumferential and axial directions. Specifically, each radial through hole 51 is positioned at a location corresponding to each circulation groove 26 that may be formed in the blank 8 when the blank 8 is positioned radially inside the heel block 33. Each axial through hole 52 is positioned at a location corresponding to each radial through hole 51 in the circumferential direction. Therefore, as shown in Figure 4, the corresponding radial through-hole 51 and axial through-hole 52 intersect inside the heel block 33.

[0024] Multiple punches 31 are inserted into multiple radial through holes 51 from the radial outside. A protrusion 32 corresponding to the shape of the circulation groove 26 is formed on the radially inward-facing end of the punch 31. The circulation groove 26 is formed on the blank 8 by pressing this protrusion 32 against the outer surface of the blank 8 and transferring its shape. A tapered surface 31a is formed on the radially outward-facing end of the punch 31, which engages with the mandrel 36, which will be described in more detail later.

[0025] The mandrel 36 is inserted into each of the multiple axial through holes 52 from one side in the axial direction (from above in Figure 4). The mandrel 36 extends in the axial direction. A planar tapered surface 36a is formed on the end of the mandrel 36 facing downward in the axial direction (towards the direction of travel in the insertion direction). The tapered surface 36a of the mandrel 36 is formed to have the same inclination angle as the tapered surface 31a of the punch 31. In this embodiment, a cam mechanism 7 is formed by the tapered surface 36a of the mandrel 36 and the tapered surface 31a of the punch 31. The cam mechanism 7 allows the punch 31 to be moved radially inward by inserting the mandrel 36 into the axial through hole 52 of the heel block 33 (see arrow in Figure 4).

[0026] The second mold 40 has a plurality of (four in this embodiment) divided dies 41 and a die base 44. The number of divided dies 41 is the same as the number of circulation grooves 26 formed in the screw shaft 2, and they are arranged to correspond to each circulation groove 26. On the outer circumferential surface of each divided die 41, a relief portion 42 is formed that is recessed radially inward. Therefore, the position of the relief portion 42 differs for each divided die 41. Each divided die 41 is arranged so that its outer circumferential surface contacts the inner circumferential surface of the blank 8. The relief portion 42 is formed in a rectangular shape that is slightly larger than the circulation groove 26 when viewed radially. The relief portion 42 may be formed in a shape equivalent to the circulation groove 26 (for example, S-shaped). The size of the relief portion 42 is not limited to the size described above. For example, the relief portion 42 may be slightly smaller or larger than the circulation groove 26. Alternatively, the relief portion 42 may be formed in a size equivalent to the circulation groove 26. The die base 44 is positioned radially inward from the multiple segmented dies 41. The die base 44 regulates the position of each segmented die 41 and also receives the load acting on the segmented dies 41.

[0027] As shown in Figure 4, the blank 8 is positioned axially by being held axially by a holder 81 and a pressing part 80, which are part of the mold apparatus. The holder 81 is located at one end of the blank 8 in the axial direction (the lower end in Figure 4) and supports the blank 8. The pressing part 80 is located at the other end of the blank 8 in the axial direction (the upper end in Figure 4). The pressing part 80 restricts the axial movement of the blank 8 by pressing the blank 8 downward in the axial direction. The pressing part 80 may be formed including an elastic member such as rubber or a spring. For example, the pressing part 80 may have a rigid body portion that contacts the end face of the blank 8 and an elastic member (such as a spring) that presses this rigid body portion toward the blank 8. In this case, the pressing part 80 may be configured to press the blank 8 axially by the elastic force of the elastic member. Alternatively, the pressing part 80 may press the blank 8 axially by using a hydraulic mechanism instead of an elastic member. In this case, a hydraulic cushion or the like may be used to cause the pressing part 80 to perform an action that simulates elastic force.

[0028] With the heel block 33, multiple punches 31, mandrel 36, and second die 40 positioned in their respective locations relative to the blank 8, the mandrel 36 is then inserted into the heel block 33 from one axial side (the upper side in Figure 4). As described above, the cam mechanism 7 is formed by the tapered surface 36a of the mandrel 36 and the tapered surface 31a of the punch 31. Therefore, by inserting the mandrel 36 into the heel block 33 from the axial direction, the punch 31 can be moved radially inward.

[0029] As shown in Figure 5, when the tapered surface 36a of the mandrel 36 and the tapered surface 31a of the punch 31 are in contact, and the mandrel 36 is further inserted downward in the axial direction, the punch 31 inserted into the radial through hole 51 moves further inward in the radial direction while deforming the blank 8. The punch 31 presses against the outer circumferential surface of the blank 8 by moving radially inward relative to the blank 8 without moving relative to the blank 8 in the axial direction. Then, by pressing the convex portion 32 of the punch 31 against the outer circumferential surface of the blank 8, a circulation groove 26 with a shape transferred to the shape of the convex portion 32 is formed on the outer circumferential surface of the blank 8. In other words, in the forging process ST12, the circulation groove 26 is formed on the outer circumferential surface of the blank 8 by moving the punch 31, which has a convex portion 32 formed on it that corresponds to the concave portion of the circulation groove 26, from the radial outside to the inside relative to the blank 8.

[0030] Furthermore, in the forging process ST12, multiple mandrels 36 are inserted simultaneously in the axial direction. This makes it possible to simultaneously form multiple circulation grooves 26 on the outer surface of the blank 8 in a single forging. A portion of the blank 8 material that has flowed due to the formation of the circulation grooves 26 flows toward the relief portion 42 of the split die 41, forming a raised portion 48 that protrudes toward the inner circumference of the shaft body 21 of the screw shaft 2. The raised portion 48 is removed by a removal process described later.

[0031] Next, the mold removal process ST13 is performed. In the mold removal process ST13, first, the mandrel 36 is moved to the opposite side from the punch 31 to release the engagement of the cam mechanism 7. Then, the punch 31 is moved radially outward to remove it from the blank 8 (moved in the opposite direction to the arrow in Figure 4). Next, as shown in Figure 7, the die base 44 of the second mold 40 is pulled out axially to remove it from the blank 8. This allows the divided die 41 to move radially inward, so the divided die 41 is tilted toward the inner diameter to separate the relief portion 42 from the raised portion 48, and the divided die 41 is also removed by pulling it out axially. Multiple divided dies 41 are removed in the same manner. As a result, the second mold 40 is removed from the inner circumference of the blank 8. When the mold removal process ST13 is completed, a screw shaft 2 with multiple circulation grooves 26 formed on its outer surface is formed.

[0032] Once the circulation groove formation process ST01 is completed, the removal process ST02 is then performed. In the removal process ST02, the inner circumferential surface of the shaft body 21 of the screw shaft 2 is subjected to removal processing. In this embodiment, cutting is performed on the inner circumferential surface of the shaft body 21 for the purpose of removing irregularities such as the raised portion 48 (see Figure 7) that occurred in the circulation groove formation process ST01 described above. In addition, cutting or other processing may be performed on the outer circumferential surface of the shaft body 21 in the removal process ST02. Furthermore, removal processing other than cutting, such as polishing, may also be performed. Furthermore, the raised portion 48 may be left in place if no product or functional problems arise. In other words, the raised portion 48 does not need to be removed in the removal process ST02.

[0033] Once the removal process ST02 is completed, the spiral groove forming process ST03 is performed. In the spiral groove forming process ST03, an outer spiral groove 24 is formed on the outer surface of the shaft body 21 by removal processes such as cutting. At this time, the outer spiral groove 24 is formed so that its inner shape is smaller than that of the circulation groove 26. The inner shape refers to the size of the cross-sectional shape of the groove (groove depth, cross-sectional area, etc.) in a cross section perpendicular to the direction of travel of the rolling elements 3 that roll in the circulation groove 26 or the outer spiral groove 24. In other words, the cross-sectional shape of the outer spiral groove 24 is smaller than that of the circulation groove 26. As a result, for example, one end of the circulation groove 26 is connected to a part of the outer spiral groove 24, and the other end of the circulation groove 26 is connected to another part of the outer spiral groove 24.

[0034] By going through the above steps, a screw shaft 2 having an outer helical groove 24 and a circulation groove 26 on its outer surface is formed. Although not explained in the above embodiment, for example, a rough forming step to obtain a blank 8 from the base material may be included before the circulation groove formation step ST01. In addition, an outer surface finishing step may be included separately to form gears, flanges, keyways, spline grooves, etc. on the outer surface of the small diameter portion 22 of the screw shaft 2 by performing plastic working or removal work on the outer surface of the small diameter portion 22. A chamfering step may be included separately to chamfer the boundary between the circulation groove 26 and the outer helical groove 24. After the helical groove formation step ST03 and the chamfering step, a heat treatment step such as carburizing, carbonitriding, quenching, tempering, or high-frequency induction hardening may be included separately.

[0035] (Effect, Action) In the manufacturing method for a ball screw shaft of this embodiment, a punch 31 having a convex portion 32 corresponding to a recess of the circulation groove 26 is moved radially inward without axial movement, thereby pressing the convex portion 32 against the outer circumferential surface of the blank 8 of the screw shaft 2, and forming the circulation groove 26 on the outer circumferential surface of the blank 8. Since the circulation groove 26 is formed by plastic deformation (forging) in this way, the processing time can be shortened and the machinability of the screw shaft 2 can be improved compared to the conventional technique of forming the circulation groove 26 by cutting with an end mill. This makes it possible to keep manufacturing costs low. Furthermore, in the conventional technique using an end mill, for example, if the processing position and the rotation center of the end mill coincide, the rotation speed may become low and the machinability may decrease. As a result, the processing accuracy of the circulation groove 26 may decrease. In contrast, with the manufacturing method of this embodiment, the circulation groove 26 can be formed with high precision regardless of the shape of the circulation groove 26. Furthermore, in the circulation groove formation process ST01, forging is performed by moving the punch 31 radially inward without moving it axially. This suppresses the axial flow of the material in the blank 8. Therefore, deformation of the shape of the circulation groove 26 and a decrease in the positional accuracy of the circulation groove 26 caused by axial flow of the material can be suppressed. Thus, the circulation groove 26 can be formed with high precision. In addition, since no shear force is generated along the axial direction on the protrusion 32 of the punch 31, the life of the mold can be extended. Therefore, compared to conventional techniques, this method provides a method for manufacturing a screw shaft for a ball screw that improves the machinability and machining accuracy when forming a circulation groove 26 on the screw shaft 2.

[0036] A portion of the blank 8 (shaft body 21) is formed in a hollow cylindrical shape. In the circulation groove formation process ST01, the second die 40 is positioned inside the shaft body 21 in the radial direction, and the punch 31 is moved inward in the radial direction to form the circulation groove 26. This increases the versatility of the screw shaft 2. Furthermore, by positioning the second die 40 inside the blank 8 in the radial direction, the load from the punch 31 during the formation of the circulation groove 26 can be received by the second die 40. Therefore, the machining accuracy of the circulation groove 26 can be improved.

[0037] On the outer surface of the second mold 40, a relief portion 42 is formed at a position corresponding to the circulation groove 26, which is recessed radially inward. As a result, the material of the blank 8 extruded by the flow during the formation of the circulation groove 26 protrudes toward the relief portion 42 (see the raised portion 48 in Figure 5). This suppresses the axial flow of the material of the blank 8 and prevents the shape of the circulation groove 26 from collapsing. Therefore, the circulation groove 26 can be formed with high precision.

[0038] The punch 31 is moved radially inward by a cam mechanism 7 formed by the punch 31 and the mandrel 36. This allows for easy and highly accurate formation of the circulation groove 26.

[0039] In the circulation groove formation process ST01, the circulation groove 26 is formed with the pressing portion 80 positioned at the end of the blank 8. The pressing portion 80 presses the blank 8 in the axial direction, restricting its movement in the axial direction. Therefore, axial deformation and movement of the blank 8 are suppressed, and the circulation groove 26 can be formed with high precision.

[0040] The screw shaft 2 has multiple circulation grooves 26, and in the circulation groove forming step ST01, multiple circulation grooves 26 are formed simultaneously. For example, in the above embodiment, multiple mandrels 36 are simultaneously inserted into the heel block 33 along the axial direction, thereby moving multiple punches 31 radially inward simultaneously and pressing them against the blank 8. This makes it possible to form the circulation grooves 26 easily and quickly, even when the screw shaft 2 has multiple circulation grooves 26. In addition, the positional accuracy of the multiple circulation grooves 26 can be improved.

[0041] (Second Embodiment) Next, a second embodiment of the present invention will be described. In the description of the second embodiment, components similar to those in the first embodiment described above will be denoted by the same reference numerals and their descriptions will be omitted as appropriate. Note that the specific configurations are not limited to these embodiments and can be modified as appropriate without departing from the spirit of the present invention. Figure 8 is a schematic cross-sectional view showing an overview of the circulation groove formation process ST01 in the manufacturing method of the screw shaft 2 according to the second embodiment. The second embodiment differs from the first embodiment described above in that a plurality of circulation grooves 26 are formed sequentially using a single punch 231.

[0042] As shown in Figure 8, in the second embodiment, in the circulation groove forming step ST01, the blank 8 is positioned so that its axial direction (central axis C) coincides with the horizontal direction. The second mold 240 is positioned radially inside the blank 8. The second mold 240 abuts against the inner circumferential surface of the blank 8 (shaft body 21) at a position corresponding to the location where the circulation groove 26 is formed by the punch 231. Similar to the first embodiment, the second mold 240 has a relief portion 242 formed at a position corresponding to the circulation groove 26 that is formed.

[0043] Holders 81 and pressing portions 80 are provided on both sides of the shaft body 21 in the axial direction. The configuration of the holders 81 and pressing portions 80 may be the same as that of the holders 81 and pressing portions 80 in the first embodiment. For example, the pressing portion 80 may be formed including an elastic member such as rubber or a spring. The blank 8 is clamped in the axial direction by the holders 81 and pressing portions 80, thereby restricting the axial movement of the blank 8.

[0044] A punch 231 is positioned on the radially outer side of the blank 8. A protrusion 232 is formed at the lower end of the punch 231. The punch 231 moves, for example, from top to bottom along the vertical direction. This moves the punch 231 radially inward without moving it axially along the blank 8, and the protrusion 232 is pressed against the outer surface of the blank 8 to form a circulation groove 26. Once the first forging is complete, the blank 8 is rotated a predetermined number of times around the central axis C and moved a predetermined distance axially. Then, the punch 231 is pressed against the blank 8 in the same way as the first time to form a second circulation groove 26. This is repeated multiple times (four times in this embodiment) to form multiple (four in this embodiment) circulation grooves 26 one by one in sequence.

[0045] The manufacturing method of the second embodiment can achieve the same effects as the first embodiment. Specifically, compared to the conventional technique of forming the circulation groove 26 by cutting with an end mill, the processing time can be shortened and the machinability and processing accuracy of the circulation groove 26 can be improved. Furthermore, since forging is performed by moving the punch 231 radially inward without moving it axially, the flow of the blank 8 material in the axial direction can be suppressed. Therefore, the circulation groove 26 can be formed with high precision. In addition, the life of the mold can be extended. Therefore, compared to conventional techniques, this method provides a method for manufacturing a screw shaft for a ball screw that improves the machinability and machining accuracy when forming a circulation groove 26 on the screw shaft 2. Furthermore, according to the manufacturing method of the second embodiment, multiple circulation grooves 26 can be formed using a single punch 231. Therefore, the number of parts in the mold apparatus can be reduced, and the size of the apparatus in the circulation groove formation process ST01 can be suppressed. This further reduces processing costs. In the second embodiment, the orientation in which the blank 8 is positioned is not limited to the orientation described above (i.e., the orientation in which the axial direction (central axis C) of the blank 8 coincides with the horizontal direction). For example, the blank 8 may be positioned so that its axial direction (central axis C) coincides with the vertical direction.

[0046] It should be noted that the technical scope of the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. For example, in the first embodiment described above, an example was described in which cutting is performed on the inner and outer surfaces of the screw shaft 2 (shaft body 21) in the removal step ST02, but the invention is not limited to this. In the removal step ST02, removal may be performed on at least one of the inner and outer surfaces of the screw shaft 2.

[0047] The number of punches 31 is not limited to four. Depending on the number of circulation grooves 26 formed on the screw shaft 2, the number of punches 31 may be two, three, or five or more. Similarly, the number of mandrels 36 and the number of segmented dies 41 may be two, three, or five or more. The number of mandrels 36 and punches 31 may differ from the number of segmented dies 41.

[0048] In the embodiments described above, a method for manufacturing a screw shaft 2 having multiple circulation grooves 26 has been described, but the present invention may also be applied to the manufacture of a screw shaft 2 having only one circulation groove 26. In this case, for example, only one punch 31 or one mandrel 36 may be provided. However, considering the removal of the mold, it is necessary to use at least two or more split dies 41. For example, two split dies 41 may be prepared, with one split die 41 having a relief portion 42 and the other split die 41 not having a relief portion 42.

[0049] A method other than the cam mechanism 7 may be used to move the punch 31 radially inward. For example, a method using hydraulic pressure or air pressure, a method using a motor and ball screw, or a method using the elastic force of an elastic body may be used as a substitute for the cam mechanism 7. If the blank 8 is thin-walled, bulge forming (hydroforming) may be used.

[0050] The shaft body 21 of the screw shaft 2 may be solid. In this case, the second mold 40 does not need to be provided in the circulation groove forming process ST01. When the blank 8 is solid, a raised portion may be formed that protrudes radially outward. Therefore, it is preferable to provide a gap of an appropriate size between the inner circumferential surface of the heel block 33 and the outer circumferential surface of the blank 8. Furthermore, in the area where the circulation groove 26 is not provided in the circumferential direction, the gap between the inner circumferential surface of the heel block 33 and the outer circumferential surface of the blank 8 may be reduced. This makes it possible to achieve coaxiality between the heel block 33 and the blank 8 regardless of the presence or absence of the circulation groove 26. The shape of the relief section 42 is not limited to rectangular or S-shaped. However, if the shape of the relief section 42 is formed to be the same as the shape of the circulation groove 26 (for example, S-shaped), it has the advantage of preventing the material from sagging during the formation of the circulation groove 26 and improving the transferability of the circulation groove 26.

[0051] Furthermore, this disclosure may also be a combination of the following configurations. (1) A method for manufacturing a screw shaft for a ball screw, having a helical groove on which multiple rolling elements roll, and a circulation groove that returns the rolling elements from one end to the other of the helical groove, A circulation groove forming step is performed by moving a first mold, which has a convex portion corresponding to the concave portion of the circulation groove, from the radially outer side to the inner side relative to the blank of the screw shaft, thereby forming the circulation groove on the outer circumferential surface of the blank. The process includes a helical groove forming step, in which the helical groove is formed on the outer circumferential surface of the blank by a removal process, In the circulation groove forming step, the circulation groove is formed by pressing the outer circumferential surface of the blank by moving the first mold radially inward without moving it axially, and pressing the protrusions against the outer circumferential surface of the blank. A method for manufacturing a screw shaft for a ball screw. (2) The blank is formed in a cylindrical shape having at least a portion of its axial direction as hollow, In the circulation groove forming step, the first mold is moved radially inward while the second mold is positioned radially inward of the blank. (1) A method for manufacturing a screw shaft for a ball screw as described in (1). (3) A relief portion is formed on the outer circumferential surface of the second mold at a position corresponding to the circulation groove, which is recessed radially inward. (2) A method for manufacturing a screw shaft for a ball screw as described in (2). (4) In the circulation groove forming step, the first mold is moved radially inward by a cam mechanism formed by the first mold and a third mold inserted along the axial direction of the blank. A method for manufacturing a screw shaft for a ball screw, as described in any one of (1) to (3). (5) In the circulation groove forming step, the circulation groove is formed with a pressing portion positioned to restrict the axial movement of the blank by pressing the blank from the axial direction. A method for manufacturing a screw shaft for a ball screw, as described in any one of (1) to (4). (6) The screw shaft has a plurality of circulation grooves, In the circulation groove formation step, multiple circulation grooves are formed simultaneously. A method for manufacturing a ball screw nut as described in any one of (1) to (5). (7) In the circulation groove formation step, one circulation groove is formed by moving the first mold from the radial outside to the inside of the blank, and then another circulation groove is formed by changing the relative position between the blank and the first mold with respect to at least one of the axial and circumferential directions of the blank, thereby forming multiple circulation grooves one by one. A method for manufacturing a screw shaft for a ball screw, as described in any one of (1) to (5). (8) The process further includes a removal step, performed after the circulation groove formation step, on removing material from at least one of the inner and outer surfaces of the screw shaft. A method for manufacturing a screw shaft for a ball screw, as described in any one of (1) to (7). [Explanation of symbols]

[0052] 2 Screw shaft 3 Rolling element 7 Cam mechanism 8 Blank 10 Ball screws 24. Outer circumference spiral groove (spiral groove) 26 Circulation groove 31,231 Punch (First Die) 32 Convex part 36 Mandrel (Third mold) 40,240 Second mold 42,242 Escape Club 80 Pressing part ST01 Circulation groove formation process ST02 Removal process ST03 Spiral groove formation process

Claims

1. A method for manufacturing a screw shaft for a ball screw, having a helical groove on which multiple rolling elements roll, and a circulation groove that returns the rolling elements from one end to the other of the helical groove, A circulation groove forming step is performed by moving a first mold, which has a convex portion corresponding to the concave portion of the circulation groove, from the radially outer side to the inner side relative to the blank of the screw shaft, thereby forming the circulation groove on the outer circumferential surface of the blank. A helical groove forming step, in which the helical groove is formed on the outer surface of the blank by removal processing, Equipped with, In the circulation groove forming step, the circulation groove is formed by pressing the outer circumferential surface of the blank by moving the first mold radially inward without moving it axially, and pressing the protrusions against the outer circumferential surface of the blank. The blank is formed in a cylindrical shape having at least a portion of its axial direction as hollow, In the circulation groove forming step, with the second mold positioned radially inward of the blank, the first mold is moved radially inward. A relief portion is formed on the outer surface of the second mold at a position corresponding to the circulation groove, which is recessed radially inward. The shape of the relief portion is formed to be equivalent to the shape of the circulation groove. A method for manufacturing a screw shaft for a ball screw.

2. A method for manufacturing a screw shaft for a ball screw, having a helical groove on which a plurality of rolling elements roll, and a circulation groove that returns the rolling elements from one end to the other end of the helical groove, A circulation groove forming step is performed by moving a first mold, which has a convex portion corresponding to the concave portion of the circulation groove, from the radially outer side to the inner side relative to the blank of the screw shaft, thereby forming the circulation groove on the outer circumferential surface of the blank. A helical groove forming step, in which the helical groove is formed on the outer surface of the blank by removal processing, Equipped with, In the circulation groove forming step, the circulation groove is formed by pressing the outer circumferential surface of the blank by moving the first mold radially inward without moving it axially, and pressing the protrusions against the outer circumferential surface of the blank. The blank is formed in a cylindrical shape having at least a portion of its axial direction as hollow, In the circulation groove forming step, with the second mold positioned radially inward of the blank, the first mold is moved radially inward. The second mold comprises a die base provided at the axial center and a plurality of segmented dies arranged on the outer circumference of the die base. In the circulation groove formation step, multiple circulation grooves located at different locations in the axial and circumferential directions are formed simultaneously in a single step. A method for manufacturing a screw shaft for a ball screw.

3. A method for manufacturing a screw shaft for a ball screw, having a helical groove on which a plurality of rolling elements roll, and a circulation groove that returns the rolling elements from one end to the other end of the helical groove, A circulation groove forming step is performed by moving a first mold, which has a convex portion corresponding to the concave portion of the circulation groove, from the radially outer side to the inner side relative to the blank of the screw shaft, thereby forming the circulation groove on the outer circumferential surface of the blank. A helical groove forming step, in which the helical groove is formed on the outer surface of the blank by removal processing, Equipped with, In the circulation groove forming step, the circulation groove is formed by pressing the outer circumferential surface of the blank by moving the first mold radially inward without moving it axially, and pressing the protrusions against the outer circumferential surface of the blank. In the circulation groove forming step, the first mold is moved radially inward by inserting the third mold axially into the heel block located radially outward of the blank. The heel block has a radial through hole for guiding the first mold radially and an axial through hole for guiding the third mold radially. A method for manufacturing a screw shaft for a ball screw.

4. The blank is formed in a cylindrical shape having at least a portion of its axial direction as hollow, In the circulation groove forming step, the first mold is moved radially inward while the second mold is positioned radially inward of the blank. A method for manufacturing a screw shaft for a ball screw according to claim 3.

5. A relief portion is formed on the outer circumferential surface of the second mold at a position corresponding to the circulation groove, which is recessed radially inward. A method for manufacturing a screw shaft for a ball screw according to claim 2 or claim 4.

6. The shape of the relief portion is formed to be equivalent to the shape of the circulation groove. A method for manufacturing a screw shaft for a ball screw according to claim 5.

7. The second mold comprises a die base provided at the axial center and a plurality of segmented dies arranged on the outer circumference of the die base. In the circulation groove formation step, multiple circulation grooves located at different locations in the axial and circumferential directions are formed simultaneously in a single step. A method for manufacturing a screw shaft for a ball screw according to claim 1.

8. The process further includes a mold removal step of removing the second mold from the blank, In the mold removal process, the die base is pulled out in the axial direction, and then the second mold is removed by tilting the divided die and pulling it out in the axial direction. A method for manufacturing a screw shaft for a ball screw according to claim 2 or claim 7.

9. In the circulation groove forming step, the first mold is moved radially inward by a cam mechanism formed by the first mold and a third mold inserted along the axial direction of the blank. A method for manufacturing a screw shaft for a ball screw according to any one of claims 1 to 3.

10. In the circulation groove forming step, the circulation groove is formed with a pressing portion positioned to restrict the axial movement of the blank by pressing the blank from the axial direction. A method for manufacturing a screw shaft for a ball screw according to any one of claims 1 to 3.

11. The pressing portion includes at least one of an elastic member and a hydraulic cushion provided by a hydraulic mechanism. A method for manufacturing a screw shaft for a ball screw according to claim 10.

12. The screw shaft has a plurality of circulation grooves, In the circulation groove formation step, multiple circulation grooves are formed simultaneously. A method for manufacturing a screw shaft for a ball screw according to any one of claims 1 to 3.

13. In the circulation groove forming step, one circulation groove is formed by moving the first mold from the radial outside to the inside of the blank, and then another circulation groove is formed by changing the relative position between the blank and the first mold with respect to at least one of the axial and circumferential directions of the blank, thereby forming multiple circulation grooves one by one. A method for manufacturing a screw shaft for a ball screw according to claim 1 or claim 3.

14. The process further includes a removal step, performed after the circulation groove formation step, on removing material from at least one of the inner and outer surfaces of the screw shaft. A method for manufacturing a screw shaft for a ball screw according to any one of claims 1 to 3.

15. In the circulation groove forming step, the first mold is moved radially inward by inserting the third mold axially into the heel block located radially outward of the blank. The heel block has a radial through hole for guiding the first mold radially and an axial through hole for guiding the third mold radially. A method for manufacturing a screw shaft for a ball screw according to claim 1.

16. A gap is provided between the heel block and the blank. The size of the gap in the region where the circulation groove is not provided is smaller than the size of the gap in the region where the circulation groove is provided. A method for manufacturing a screw shaft for a ball screw according to claim 3 or claim 15.

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