Method for manufacturing ball screw nuts

Abstract

This enables the efficient and precise formation of ball circulation grooves on the inner surface of ball screw nuts. [Solution] A method for manufacturing a ball screw nut 4 having an internal screw groove 5 and a ball circulation path 9 (circulation groove 11) on its inner circumferential surface, comprising a circulation groove forming step in which the circulation groove 11 is formed on the inner circumferential surface Ma of a blank M, which is positioned radially outside a forming punch 21 (split punch 26) having a circulation groove forming section 27, by drawing the outer circumferential surface Mb of the blank M in the axial direction with a drawing die 23. In this step, the outer circumferential surface of the forming punch 21 is restrained by a punch holder 23 as a stationary restraining member and an upper restraining die 24 as a movable restraining member, which are positioned on both sides of the blank M in the axial direction, and the outer circumferential surface 24e of the upper restraining die 24 is restrained by the inner circumferential surface 22a of the drawing die 22, while the outer circumferential surface Mb of the blank M is drawn in the axial direction with the drawing die 22.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a nut for a ball screw.

Background Art

[0002] A ball screw, which is a type of motion conversion mechanism that converts rotational motion into linear motion or vice versa, includes a screw shaft and a nut that rotate relative to each other via a plurality of balls. When the screw shaft and the nut rotate relative to each other, a plurality of balls intervening in the ball rolling path between the male screw groove formed on the outer peripheral surface of the screw shaft and the female screw groove formed on the inner peripheral surface of the nut roll along both screw grooves. A ball screw having such a structure is incorporated into a drive mechanism that requires high operating accuracy, for example, in machine tools, conveying machines, industrial robots, automobiles, motorcycles, ships, and the like.

[0003] It is essential for a ball screw to have a ball circulation path for returning the balls that reach the end point of the ball rolling path to the start point of the ball rolling path (circulating the balls in the nut) as the screw shaft and the nut rotate relative to each other. In addition to the female screw groove, a recess (circulation groove) that functions as a ball circulation path may be provided on the inner peripheral surface of the nut. A method for manufacturing a nut having such a configuration is described in, for example, Japanese Patent No. 6015444 (Patent Document 1). In Patent Document 1, with a blank, which is a cylindrical member made of steel, held by a holding member, a circulation groove forming portion provided on a forming punch (referred to as a "processing head" in Patent Document 1) is pressed against the inner peripheral surface of the blank to plastically deform (partially) the inner peripheral surface of the blank, thereby forming a circulation groove on the inner peripheral surface of the blank. Then, a female screw groove is formed by cutting the inner peripheral surface of the blank. <00,00013>

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] In the method described in Patent Document 1, the circulation groove is formed by pressing the circulation groove forming part (forming punch) against the inner circumferential surface of the blank, which is restrained by a holding member on its outer circumferential surface (entire surface). Although this suppresses or prevents changes in the blank's orientation (tilting with respect to the axial direction) during the formation of the circulation groove, Patent Document 1 does not mention any specific technical means for suppressing or preventing changes in the orientation of the forming punch. If changes in the orientation (tilting) of the forming punch occur during the formation of the circulation groove, the groove depth of the circulation groove becomes uneven, making it impossible to obtain a circulation groove with a predetermined accuracy.

[0006] Furthermore, since ball screw nuts are mass-produced parts, it is preferable that the circulation grooves can be formed efficiently.

[0007] In view of these circumstances, the present invention aims to provide a technical means that enables the accurate and efficient molding of the ball circulation groove to be provided on the inner circumferential surface of a ball screw nut, thereby contributing to a reduction in the manufacturing cost of ball screw nuts and, consequently, ball screws. [Means for solving the problem]

[0008] The present invention, devised to achieve the above objectives, A method for manufacturing a ball screw nut, comprising: an internal screw groove that forms a spiral ball rolling path together with an external screw groove provided on the outer circumference of the screw shaft for the rolling of balls; and a circulation groove provided on the inner circumference for returning the balls from the end point to the starting point of the ball rolling path, The process includes a circulation groove forming step in which, as the outer surface of a cylindrical blank positioned radially outside the forming punch is squeezed axially by the inner surface of a drawing die, a circulation groove forming section provided on the forming punch is forced into the inner surface of the blank, thereby forming a circulation groove on the inner surface of the blank. In the circulation groove forming process, the outer surface of the forming punch is restrained by stationary and movable restraining members positioned on both sides of the blank in the axial direction, and the outer surface of the movable restraining member is restrained by the inner surface of the drawing die, while the outer surface of the blank is being squeezed in the axial direction by the inner surface of the drawing die.

[0009] In this invention, a circulation groove is formed on the inner surface of a cylindrical blank by a so-called drawing process, in which the outer surface of the cylindrical blank is drawn axially on the inner surface of a (cylindrical) drawing die, gradually reducing the diameter of the blank. At this time, the outer surface of the forming punch is restrained at two points spaced apart in the axial direction by stationary and movable restraining members positioned on both sides of the blank in the axial direction, so that unintended changes in the posture of the forming punch when drawing the outer surface of the blank in the axial direction on the inner surface of the drawing die can be prevented as much as possible. Furthermore, since the drawing of the outer surface of the blank by the inner surface of the drawing die is performed with the outer surface of the movable restraining member restraining the outer surface of the forming punch restrained by the inner surface of the drawing die, the drawing process can be performed with the forming punch, blank, and drawing die centered. These effects combined enable the formation of a circulation groove on the inner surface of the blank with high precision and efficiency.

[0010] In the circulation groove forming process, it is preferable to restrain one end face of the blank with a stationary restraining member and the other end face of the blank with a movable restraining member, and then axially scrape the outer surface of the blank with the inner surface of a drawing die. In particular, by restraining the inner diameter ends of one end face and the other end face of the blank, it is possible to prevent axial stretching deformation of the inner surface of the blank during scraping as much as possible, thereby preventing a decrease in the accuracy of the circulation groove formed on the inner surface of the blank.

[0011] In the circulation groove forming process, it is preferable to guide the outer surface of the drawing die with the inner surface of the drawing die, which is positioned radially outside the drawing die, while simultaneously working the outer surface of the blank axially with the inner surface of the drawing die. This improves the accuracy of the drawing die's movement relative to the blank, thereby further improving the accuracy of the circulation groove formation.

[0012] As a forming punch, one can be used that comprises multiple divided punches, each having a circulation groove forming section on its outer diameter surface, and a punch base that holds these multiple divided punches at intervals in the circumferential direction, wherein the multiple divided punches move relative to the punch base so that the circumscribed circle diameter of the circulation groove forming section changes. In this case, for example, if the multiple divided punches can move relative to the punch base so that the circumscribed circle diameter of the circulation groove forming section becomes smaller than the inner diameter of the blank when the outer circumferential surface is constrained by the inner circumferential surface of the drawing die, then the forming punches (multiple divided punches) can be smoothly removed from the blank after the ironing process is complete, without resorting to forced removal. This prevents a decrease in the accuracy of the inner circumferential surface shape of the blank, i.e., a decrease in the accuracy of the circulation grooves formed on the inner circumferential surface of the blank. [Effects of the Invention]

[0013] As described above, the present invention makes it possible to accurately and efficiently form the ball circulation groove to be provided on the inner circumferential surface of a ball screw nut. This contributes to reducing the manufacturing cost of ball screw nuts and, consequently, ball screws. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic longitudinal cross-sectional view of a ball screw whose constituent component is a nut obtained by applying a method for manufacturing a ball screw nut according to an embodiment of the present invention. [Figure 2] This is a magnified section of Figure 1. [Figure 3] This is a simplified flowchart illustrating the manufacturing process for ball screw nuts. [Figure 4] This is a conceptual longitudinal cross-sectional view showing the main part of a press machine used in the circulation groove forming process, with a blank set in place. [Figure 5] (A) is a front view of the split punch, (B) is a side view of the split punch, and (C) is a cross-sectional view taken along the line Z1-Z1 in (A). [Figure 6](A) figure is the bottom view of the punch holder, (B) figure is the side view of the punch holder, (C) figure is the top view of the punch holder, and (D) figure is the sectional view taken along the line Z2-Z2 of (B) figure. [Figure 7] It is a longitudinal sectional view of the press device in a state where the outer peripheral surface of the blank is squeezed by a drawing die. [Figure 8] It is a longitudinal sectional view of the press device in a state where the blank is moved upward together with the drawing die and the split punch. [Figure 9] It is a longitudinal sectional view showing a state where the blank (primary intermediate product) formed with the circulation groove and the phase reference is taken out from the press device. [Figure 10] (A) figure is the plan view and longitudinal sectional view of the primary intermediate product shown in Fig. 9, and (B) figure is the plan view and longitudinal sectional view of the primary intermediate product after turning. [Figure 11] (A) figure is a schematic longitudinal sectional view of the rolling groove forming process, and (B) figure is a schematic plan view of the rolling groove forming process.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments of the present invention will be described based on the drawings.

[0016] First, based on Figs. 1 and 2, a ball screw using a ball screw nut obtained by applying the manufacturing method of the ball screw nut according to the embodiment of the present invention (hereinafter simply referred to as "nut") as a constituent member will be described. Fig. 1 is a schematic longitudinal sectional view of the ball screw, and Fig. 2 is a partial enlarged view of Fig. 1, which is an enlarged view of the ball circulation path. In the following description, the "axial direction" refers to the direction along the central axis X of the nut (and the blank processed on the nut). The "radial direction" is the radial direction of the circle centered on the central axis X, and the "circumferential direction" is the circumferential direction of the circle centered on the central axis X.

[0017] The ball screw 1 comprises a screw shaft 2 with a helical male screw groove 3 on its outer circumference, a nut 4 with a helical female screw groove 5 on its inner circumference opposite to the male screw groove 3, a plurality of balls 7 arranged to roll freely in a helical ball rolling path 8 formed between the two screw grooves 3 and 5, and a series of (endless) ball passages 10 consisting of the ball rolling path 8 and the ball circulation path 9. The ball circulation path 9 is composed of a concave circulation groove 11 formed on the inner circumference of the nut 4. Therefore, this ball screw 1 does not have a separate component such as a return tube or ball bearings to form the ball circulation path 9. In this embodiment, two circulation grooves 11 are provided at two locations on the inner circumference of the nut 4 that are radially opposite each other with respect to the central axis X of the nut 4.

[0018] In a ball screw 1 having the above configuration, when the nut 4 fitted to the outer circumference of the screw shaft 2 via the balls 7 rotates relative to the screw shaft 2, the balls 7 roll within the ball rolling path 8, causing the screw shaft 2 and the nut 4 to move relative to each other in the axial direction. At this time, when the balls 7 rolling within the ball rolling path 8 reach the end of the ball rolling path 8, the balls 7 are scooped up into the ball circulation path 9 from one end. As shown in Figure 2, the scooped-up balls 7 move within the ball circulation path 9, overcoming the threads of the male screw groove 3, and are returned to the starting point of the ball rolling path 8 from the other end of the ball circulation path 9. In this way, when the screw shaft 2 and the nut 4 rotate relative to each other, the balls 7 can circulate infinitely in the endless ball passage 10, allowing the screw shaft 2 and the nut 4 to move relative to each other continuously. Although not shown in the diagram, the ball screw 1 (in the product in which it is incorporated) is typically equipped with a stopper structure to define the limit of the forward and backward movement of the linear-moving component (in this case, the screw shaft 2) of the screw shaft 2 and nut 4, and a rotation restricting mechanism to prevent the screw shaft 2 and nut 4 from rotating together.

[0019] The nut 4 of the ball screw 1 having the above configuration is shown simply in Figure 3, • A blank manufacturing process for producing a cylindrical blank (a blank having a cylindrical inner surface Ma and an outer surface Mb) M from steel material to be used as a nut material. - A circulation groove forming process is performed on a blank M to create a primary intermediate product M' in which a circulation groove 11 that functions as a ball circulation path 9 is formed. A machining process to produce a secondary intermediate product in which a female screw groove 5 is formed on the inner surface by machining the primary intermediate product M'. • A heat treatment process for applying heat treatment to secondary intermediate products, and • A finishing process to shape the heat-treated secondary intermediate product M into the final product shape. The nut 4 is obtained through the following steps. The method for manufacturing the nut 4 according to the present invention is mainly characterized by the embodiment of the circulation groove forming process, so the circulation groove forming process will be described in detail below.

[0020] Figure 4 is a conceptual vertical cross-sectional view showing the main parts of the press device 20 used in the above-described circulation groove forming process, and shows the blank M as the workpiece set in place. The blank M is set in the press device 20 in a vertical position with its central axis X aligned with the vertical direction. The press device 20 is equipped with a forming punch 21, a drawing die 22, a punch holder 23, an upper surface restraint die 24, and a die guide 25, all of which are made of a highly rigid metal material such as stainless steel and are arranged coaxially. In this press device 20, the forming punch 21 (part of the forming punch 21; details will be described later), the drawing die 22, and the upper surface restraint die 24 constitute the movable side, which moves up and down, while the punch holder 23 and die guide 25 are fixed to the structure of the press device 20, which does not move up and down, and constitute the stationary side.

[0021] The forming punch 21 comprises two divided punches 26, each having a circulation groove forming section 27 on its outer diameter surface, and a punch base 28 that holds the two divided punches 26 so as to be able to move up and down. The two divided punches 26 can move up and down synchronously, and the punch base 28 is fixed to the structure of the press device 20. As shown in Figures 5(A) to (C), each divided punch 26 has a convex circulation groove forming section 27 corresponding to the circulation groove 11 to be formed on the inner circumferential surface Ma of the blank M, a first guided surface 26a, and a second guided surface 26b. The first guided surface 26a and the second guided surface 26b are surfaces that are slidably guided by the punch holder 23 and the punch base 28 when the divided punch 26 moves up and down, respectively. In this embodiment, since the circulation groove 11 is formed at two locations spaced apart in the circumferential direction on the inner circumferential surface Ma of the blank M, one circulation groove forming section 27 is provided for each of the two divided punches 26.

[0022] The punch base 28 is a rod-shaped member that holds two divided punches 26 so as to be able to move up and down. As shown in Figures 6(A) to (C), two punch housing grooves 28a capable of individually accommodating the two divided punches 26 are formed on its outer circumference so as to face each other radially. The punch housing groove 28a in the illustrated example has a substantially V-shaped cross-section formed by a pair of guide surfaces 28b that are inclined in opposite directions to widen the groove width of the punch housing groove 28a toward the outer diameter, and a groove bottom surface 28c that connects the inner diameter ends of the pair of guide surfaces 28b. The (pair) guide surfaces 22b face the second guided surface 26b of the divided punch 26 housed in the punch housing groove 28a and slide-guide the second guided surface 26b when the divided punch 26 moves up and down.

[0023] As shown in Figure 6(D), the guide surface 28b and groove bottom surface 28c defining the punch housing groove 28a are not planes parallel to the central axis (axial direction X) of the punch base 28, but are formed as inclined surfaces that gradually shift toward the central axis of the punch base 28 as they move vertically upward (to the right side of the paper in the figure). Therefore, when the divided punch 26 housed in the punch housing groove 28a moves up and down as it is slidably guided by the guide surface 28b of the punch housing groove 28a, the circumscribed circle diameter D (see Figure 4) of the circulation groove forming section 27 decreases or expands accordingly. The circumscribed circle diameter D of the circulation groove forming section 27 is the diameter of the circle that is centered on the central axis of the forming punch 21 and passes through the outer diameter ends of the two circulation groove forming sections 27.

[0024] The two vertically moving split punches 26 are capable of moving vertically between two positions: a first position where the circumscribed circle diameter D of the circulation groove forming section 27 is less than or equal to the inner diameter of the blank M in a free state where the outer surface is not constrained, and is a predetermined amount larger than the inner diameter of the blank M in a state where the outer surface Mb is constrained by the drawing die 23; and a second position where the diameter D is smaller than the inner diameter of the blank M in a state where the outer surface Mb is constrained by the drawing die 23. Here, the lower limit of the split punches 26 shown in Figures 4 and 7 corresponds to the first position, and the upper limit of the split punches 26 shown in Figure 8 corresponds to the second position.

[0025] The drawing die 22 reduces the diameter of the blank M by drawing the outer circumferential surface Mb of the blank M, which is positioned vertically on the punch holder 24, in the axial direction. It has a cylindrical first inner circumferential surface 22a and a second inner circumferential surface 22b that is tapered and gradually widens towards the bottom, with its upper end connected to the lower end of the first inner circumferential surface 22a. Since the drawing die 22 draws the outer circumferential surface Mb of the blank M, the inner diameter of at least the first inner circumferential surface 22a is set to be smaller by a predetermined amount than the outer diameter of the blank M in its free state, over its entire axial range.

[0026] The punch holder 23 is composed of a cylindrical body in which the forming punch 21 (split punch 26 and punch base 28) is arranged on the inner circumference. It contacts and supports the blank M arranged in a vertical posture on its upper end surface 23b from the lower side, and receives the axial load acting on the blank M when the outer peripheral surface Mb of the blank M is axially squeezed by the drawing die 22. Further, the punch holder 23 has a tapered shape that gradually decreases in diameter upward, and has a tapered inner peripheral surface 23a that slidably guides the first guided surface 26a (outer peripheral surface of the forming punch 21) of the split punch 26 when the split punch 26 moves up and down. This tapered inner peripheral surface 23a also functions as a regulating surface for regulating the displacement of the split punch 26 when the outer peripheral surface Mb of the blank M is squeezed. The punch holder 23 in the illustrated example integrally has a flange portion that projects outward in the radial direction and whose outer diameter end abuts against the inner peripheral surface of the die guide 25. Thereby, the punch holder 23 and further the punch base 28 arranged on the inner circumference of the punch holder 23 are positioned in the radial direction. Note that the above flange portion may be constituted by a member different from the punch holder 23.

[0027] As shown in FIG. 4 and the like, a recess 23c having a fan shape in plan view that retreats downward is provided on the upper end surface 23b of the punch holder 23. Although details will be described later, this recess 23c functions as a forming portion for forming a convex phase reference Md for forming a female screw groove 5 on the lower end surface Mc of the blank M.

[0028] The upper surface restraint type 24 is arranged so as to be able to move up and down independently of the drawing die 22 inside the drawing die 22 in the radial direction and above the forming punch 21 (blank M), and has a reverse concave cross-sectional shape having a disk-shaped plate portion 24a and an annular portion 24b provided one above the other. The outer diameter (maximum outer diameter) Da of this upper surface restraint type 24 is set to be slightly smaller than the inner diameter Dc of the cylindrical inner peripheral surface 22a of the drawing die 22 (Da < Dc), and the inner diameter Db of the annular portion 24b is set to be slightly larger than the outer diameter Dd of the circumscribed circle of the upper end portions of the two split punches 26 (see FIG. 4) located at the lower limit (first position) (Db > Dd). [[ID=??]]

[0029] As shown in Figure 4, when a blank M in a vertical position is placed on the upper end surface 23a of the punch holder 23, the upper ends of the two divided punches 26 located at the lower limit protrude a predetermined amount above the blank M. Therefore, when the upper restraint type 24 is moved downward, the upper ends of the outer diameter surfaces of the two divided punches 26 (including the outer peripheral surface of the forming punch 21) are restrained by the annular portion 24b of the upper restraint type 24 (see Figure 7). In addition, the tapered inner peripheral surface 23a of the punch holder 23, which is positioned below the blank M, slides and guides the first guided surface 26a of the moving divided punches 26 (including the outer peripheral surface of the forming punch 21), thus restraining the outer peripheral surface of the forming punch 21 below the blank M. Accordingly, the punch holder 23 and the upper restraint type 24 correspond to the "static restraint member" and the "movable restraint member" as defined in the present invention.

[0030] The die guide 25 is a cylindrical member that slides and guides the outer circumferential surface 22c of the vertically moving drawing die 22. As mentioned above, the die guide 25 also functions as a positioning member that radially positions the punch holder 23 and the punch base 28 of the forming punch 21 (restricting the radial movement of the punch holder 23 and the punch base 28).

[0031] The press device 20 has generally the above configuration and forms the circulation groove 11 and the phase reference Md on the blank M as follows.

[0032] First, as shown in Figure 4, the blank M in a vertical position is fitted onto the outer circumference of the forming punch 21 (which has two divided punches 26) located at the lower limit, and the blank M is placed on the upper end surface 23b of the punch holder 23. At this time, since the drawing die 22 and the upper surface restraint die 24 are retracted upward relative to the blank M, the blank M is in a free state with its outer circumference Mb not restrained. Since the inner diameter of the free blank M is greater than or equal to the circumscribed circle diameter D of the circulation groove forming section 27 provided on the divided punch 26 located at the lower limit, the blank M is smoothly fitted onto the outer circumference of the forming punch 21.

[0033] Next, as shown in Figure 7, the upper restraint type 24 is moved downward until the lower end surface 24d of its annular portion 24b contacts the upper end surface of the blank M. As mentioned above, the inner diameter Db of the annular portion 24b of the upper restraint type 24 is set to be slightly larger than the circumscribing circle diameter Dd of the upper ends of the two divided punches 26 located at the lower limit (Db > Dd), and the upper ends of the two divided punches 26 protrude above the blank M. Therefore, when the upper restraint type 24 is moved downward in the above manner, the upper end of the forming punch 21 is accommodated within the inner circumference of the annular portion 24b of the upper restraint type 24. As a result, the outer circumferential surface of the forming punch 21, which includes the divided punches 26 as constituent members, is restrained on both axial sides (upper and lower) of the blank M by the upper restraint type 24 as a movable restraint member and the punch holder 23 as a stationary restraint member. In addition, the blank M is clamped from both the upper and lower sides by the upper restraint type 24 and the punch holder 23.

[0034] At this time, in order to avoid interference between the drawing die 22 and the upper restraint type 24 and the punch holder 23 when the drawing die 22 moves downward (to ensure that the drawing allowance by the drawing die 22 is secured on the outer circumference of the blank M), the outer diameter region of the upper and lower end faces Mc of the blank M, including their outer diameter ends, is not restrained by the upper restraint type 24 and the punch holder 23, respectively. On the other hand, the inner diameter ends of the upper and lower end faces Mc of the blank M are restrained by the upper restraint type 24 and the punch holder 23, respectively.

[0035] Then, as shown in Figure 7, the drawing die 22 is moved downward, and the outer surface Mb of the blank M is squeezed axially by the inner surfaces 22b and 22a of the drawing die 22. As a result, the blank M is drawn inward in the radial direction and its diameter is reduced, and the circulation groove forming section 27 provided on the split punch 26 bites into its inner surface Ma. This forms a circulation groove 11 on the inner surface Ma of the blank M.

[0036] When the outer circumferential surface Mb of the blank M is squeezed in the axial direction, a portion of the material of the blank M undergoes plastic flow downward. Consequently, the material of the lower outer circumferential portion of the blank M that is not constrained by the punch holder 23 stretches downward, forming a cylindrical stretched portion Me (see Figure 9). At the same time, a portion of the material of the blank M flows into a recess 23c provided on the upper end surface 23b of the punch holder 23, forming a convex phase reference Md (see Figure 9) on the lower end surface Mc of the blank M.

[0037] As described above, circulation grooves 11 and convex phase references Md are formed on the inner circumferential surface Ma and the lower end surface Mc of the blank M, respectively.

[0038] As described above, in this embodiment, the outer circumferential surface Mb of the cylindrical blank M is squeezed axially by the inner circumferential surfaces 22b and 22a of the drawing die 22, thereby (gradually) reducing the diameter of the blank M, and a circulation groove 11 is formed on the inner circumferential surface Ma of the blank M. This makes it possible to efficiently form the circulation groove 11 on the inner circumferential surface Ma of the blank M.

[0039] As shown in Figure 7, the ironing of the outer circumferential surface Mb of the blank M proceeds with the outer circumferential surface of the forming punch 21 constrained by the punch holder 23, which acts as a stationary restraining member, and the upper restraining die 24, which acts as a movable restraining member, located on both sides of the axial direction of the blank M. That is, two points on the outer circumferential surface of the forming punch 21 that are spaced apart in the axial direction are restrained. This makes it possible to prevent unintended changes in the posture of the forming punch 21 (or the divided punches 26 that make up the forming punch) as much as possible when ironing the outer circumferential surface Mb of the blank M. Furthermore, the ironing of the outer circumferential surface Mb of the blank M proceeds as the drawing die 22 moves downward so that the first inner circumferential surface 22a of the drawing die 22 is guided by the outer circumferential surface 24e of the upper restraining die 24. For this reason, the ironing of the blank M is performed with the outer circumferential surface 24e of the upper restraining die 24 constrained by the first inner circumferential surface 22a of the drawing die 22, that is, with the forming punch 21, blank M, and drawing die 22 centered. The combined effects of the above mechanisms enable the circulation groove 11 to be accurately formed on the inner circumferential surface Ma of the blank M.

[0040] Furthermore, in this embodiment, a die guide 25 is positioned radially outside the drawing die 22, and the outer surface 22c of the drawing die 22 is guided by the inner surface of the die guide 25, while the outer surface Mb of the blank M is squeezed axially by the inner surfaces 22b and 22a of the drawing die 22. This improves the accuracy of the movement of the drawing die 22 relative to the blank M. As a result, the molding accuracy of the circulation groove 11 can be further improved.

[0041] When the outer circumferential surface Mb of the blank M is squeezed axially with the drawing die 22, plastic flow of the material occurs within the blank M, as described above. If this plastic flow occurs in a way that causes the inner circumferential surface Ma of the blank M to elongate in the axial direction, there is a risk that the shape accuracy of the circulation groove 11 formed on the inner circumferential surface Ma of the blank M will decrease. In this embodiment, as shown in Figure 7, the inner diameter ends of the upper and lower end surfaces Mc of the blank M are constrained by the upper surface constraining die 24 and the punch holder 23, respectively, that is, the outer circumferential surface Mb of the blank M is squeezed while restricting the axial elongation deformation of the inner circumferential surface Ma of the blank M. This prevents the occurrence of the above-mentioned problem and further improves the molding accuracy of the circulation groove 11.

[0042] When the circulation groove 11 and phase reference Md are formed in the blank M, the drawing die 22 and the upper surface restraint die 24 are moved upward as shown in Figure 8. At this time, the outer circumferential surface Mb of the blank M is restrained by the drawing die 22, and the circulation groove forming portion 27 of the split punch 26 is engaged with the inner circumferential surface Ma of the blank M. Therefore, when the drawing die 22 and the upper surface restraint die 24 are moved upward, the blank M and the split punch 26 also move upward. The split punch 26 moves upward in an inclined direction with respect to the axial direction so as to reduce the circumferential circle diameter D of the circulation groove forming portion 27. The split punch 26 moves upward until the circumferential circle diameter D of the circulation groove forming portion 27 becomes smaller than the inner diameter of the blank M whose outer circumferential surface is restrained by the drawing die 23 (here the split punch 26 moves to its upper limit), and the engagement state in the axial direction X between the blank M (the circulation groove 11 formed in it) and the split punch 26 (the circulation groove forming portion 27 provided on it) is released.

[0043] Then, the drawing die 22 and the upper surface restraint die 24, which are restraining the outer surface Mb of the blank M, are moved further upward to extract the dividing punch 26 from the inner circumference of the blank M. After the dividing punch 26 is returned to its origin (moved downward until it is at its lower limit), the upper surface restraint die 24 is moved downward as shown in Figure 9 to discharge the blank M to the lower side of the drawing die 22. As a result, a primary intermediate product M' is produced, which has a cylindrical stretched portion Me and a circulation groove 11 and a phase reference Md, as shown in Figure 10(A).

[0044] This primary intermediate product M' is transferred to the machining process, where it undergoes predetermined machining. This produces a secondary intermediate product having a shape similar to the final product, the nut 4 (see Figure 1). The predetermined machining described above refers to turning and cutting.

[0045] In the turning process, the outer circumference of the primary intermediate product M' is turned to remove the stretched portion Me and finish it to a predetermined outer diameter. Therefore, the primary intermediate product M' after turning has a circulation groove 11 and a phase reference Md, as shown in Figure 10(B).

[0046] In the cutting process, a secondary intermediate product is obtained in which an internal screw groove 5 is formed on the inner surface of the primary intermediate product M' by cutting the inner surface of the primary intermediate product M'. Here, the inner surface of the primary intermediate product M' is cut using a phase reference Md. The outline of the cutting process will be briefly explained based on the longitudinal section view and plan view shown in Figures 11(A) and (B), respectively.

[0047] First, with the chuck 50 released, the primary intermediate product M' is placed on the spindle head 51 of the lathe. Then, as shown in Figure 11(B), the reference receiving portion 52, which positions the phase reference Md formed on the primary intermediate product M', is locked onto the phase reference Md, and the chuck 50 is tightened. Since the reference receiving portion 52, which receives the force in the direction of spindle rotation, is locked (in contact) with the phase reference Md, no phase shift occurs due to gaps. The spindle is rotated, and the cutting tool 53 is pressed against the inner circumferential surface of the primary intermediate product M' to cut the female screw groove 5. The cutting tool 53 is fed axially in synchronization with the rotation of the spindle, and the lead of the female screw groove 5 is formed. In this way, a secondary intermediate product with a female screw groove 5 formed on its inner circumferential surface is obtained.

[0048] Since the phase reference Md was formed simultaneously with the circulation groove 11 in the circulation groove forming process preceding this machining process, the female screw groove 5 is formed in an accurate position relative to the circulation groove 11, as shown in Figure 11(A). That is, there is no misalignment between the end of the circulation groove 11 and the end of the female screw groove 5, and there is almost no step at the connection portion between the circulation groove 11 and the female screw groove 5.

[0049] The secondary intermediate product obtained in the manner described above is subjected to heat treatment in a heat treatment process, and then finished into a finished product in a subsequent finishing process. In the finishing process, finishing can be performed using the phase reference Md formed on the primary intermediate product M'.

[0050] Although a method for manufacturing the nut 4 according to an embodiment of the present invention has been described above, the embodiments of the present invention are not limited thereto.

[0051] For example, each divided punch 26 can be provided with multiple circulation groove forming sections 27 corresponding to the circulation groove 11. In other words, the number and shape of the circulation groove forming sections 27 provided on each divided punch 26 can be appropriately changed according to the specifications of the ball screw 1 (and its nut 4).

[0052] The present invention is not limited in any way to the embodiments described above, and can be implemented in various other forms without departing from the spirit of the invention. The scope of the present invention is indicated by the claims, and includes all modifications within the meaning and scope of equivalents set forth in the claims. [Explanation of symbols]

[0053] 1 Ball screw 2 Screw shaft 3 Male screw groove 4. Ball screw nuts (nuts) 5 Female thread groove 7 Ball 8 Ball Rolling Path 9 Ball Circulation Route 11 Circulation groove 21 Forming punch 22 Die Set 23. Punch holder (static restraint member) 24. Top-mounted restraint type (movable restraint member) 25 Dice Guide 26-part punch 27 Circulation groove molding part M Blank Ma inner surface Mb outer surface X center axis

Claims

1. A method for manufacturing a ball screw nut, comprising a female screw groove that forms a spiral ball rolling path together with a male screw groove provided on the outer circumference of the screw shaft, and a circulation groove provided on the inner circumference that returns the ball from the end point to the starting point of the ball rolling path, The process includes a circulation groove forming step in which, as the outer surface of a cylindrical blank positioned radially outside the forming punch is squeezed axially by the inner surface of a drawing die, a circulation groove forming section provided on the forming punch is driven into the inner surface of the blank, thereby forming the circulation groove on the inner surface of the blank. A method for manufacturing a ball screw nut, characterized in that, in the circulation groove forming step, the outer surface of the forming punch is restrained by stationary restraining members and movable restraining members arranged on both sides of the blank in the axial direction, and the outer surface of the movable restraining member is restrained by the inner surface of the drawing die, while the outer surface of the blank is drawn axially by the drawing die.

2. The method for manufacturing a ball screw nut according to claim 1, wherein one end face of the blank is restrained by the stationary restraining member, and the other end face of the blank is restrained by the movable restraining member, and the outer circumferential surface of the blank is squeezed in the axial direction by the inner circumferential surface of the drawing die.

3. A method for manufacturing a ball screw nut according to claim 1, wherein the outer surface of the blank is squeezed in the axial direction by the drawing die while the outer surface of the drawing die is guided by the inner surface of the drawing die guide which is positioned radially outside the drawing die.

4. A method for manufacturing a ball screw nut according to any one of claims 1 to 3, comprising: a plurality of divided punches having the circulation groove forming portion on their outer diameter surface; a punch base that holds the plurality of divided punches spaced apart in the circumferential direction; and using a forming punch in which the plurality of divided punches move relative to the punch base such that the circumscribed circle diameter of the circulation groove forming portion changes.

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