Method for manufacturing ball screw component

Abstract

To provide a method for manufacturing a ball screw component which reduces processing cost.SOLUTION: A circulation groove processing step subjects an intermediate base material 7, which has conductivity and is made of metal, and has an annular machined surface 8 facing a radial direction, to engraving electrical discharge machining, and forms a circulation groove 4 having a substantially S-shape on the machined surface 8.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a ball screw component that constitutes a ball screw device, which is a mechanical element for converting linear motion into rotational motion or rotational motion into linear motion. [Background technology]

[0002] Ball screw devices use balls to roll between a screw shaft and a nut, which allows for higher efficiency than sliding screw devices, which have direct contact between the screw shaft and the nut. For this reason, ball screw devices are incorporated into various types of machinery, such as electric brake systems and automatic manual transmissions (AMTs) for automobiles, and positioning devices for machine tools, to convert the rotational motion of a drive source, such as an electric motor, into linear motion.

[0003] The ball screw device comprises a screw shaft having a shaft-side spiral groove on its outer surface, a nut having a nut-side spiral groove on its inner surface, and a plurality of balls arranged to roll freely between the shaft-side spiral groove and the nut-side spiral groove.

[0004] The shaft-side spiral groove and the nut-side spiral groove are arranged radially opposite each other to form a spiral load path. The start and end points of the load path are connected by a circulation means. The circulation means returns balls that have reached the end point of the load path to the start point of the load path, allowing the balls to circulate infinitely. The start and end points of the load path are interchanged depending on the direction of relative displacement (relative rotation direction) between the screw shaft and the nut in the axial direction.

[0005] Depending on the application, the ball screw device uses one of the screw shaft and the nut as a rotational motion element, and the other of the screw shaft and the nut as a linear motion element.

[0006] In ball screw devices, circulation parts such as bearings, tubes, and end deflectors have traditionally been used as circulation means. However, in recent years, in order to reduce the size and cost of ball screw devices, the circulation parts have been omitted, and instead, an approximately S-shaped circulation groove has been formed directly on the inner surface of the nut or the outer surface of the screw shaft as the circulation means.

[0007] Japanese Patent Application Laid-Open Publication No. 2008-281063 discloses the structure of a ball screw device in which a substantially S-shaped circulation groove is formed directly on the inner peripheral surface of a nut by forging.

[0008] Japanese Patent Application Laid-Open Publication No. 2005-282855 discloses the structure of a ball screw device in which a substantially S-shaped circulation groove is formed directly on the outer circumferential surface of the screw shaft by forging. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-281063 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-282855 Summary of the Invention [Problem to be solved by the invention]

[0010] For example, forging can be used as a method for forming a substantially S-shaped circulation groove in a nut or a screw shaft. The molds used in forging are expensive because they are made from high-hardness materials such as cemented carbide steel. Furthermore, forming the circulation groove by forging requires multiple parts, such as a mold for processing the circulation groove and a jig for holding the ball screw component. Therefore, when the number of ball screw components (nuts or screw shafts) manufactured by forging is small, the manufacturing cost of the ball screw component increases, resulting in a problem of increased manufacturing costs.

[0011] An object of the present disclosure is to provide a method for manufacturing a ball screw component that can reduce manufacturing costs. [Means for solving the problem]

[0012] A manufacturing method of a ball screw component according to a first aspect of the present disclosure includes a circulation groove machining step of performing die-sinking electrical discharge machining on an intermediate material made of a conductive metal and having a radially oriented annular workpiece surface to form an approximately S-shaped circulation groove on the workpiece surface.

[0013] A manufacturing method for a ball screw component of a second aspect of the present disclosure is the manufacturing method for a ball screw component of the first aspect of the present disclosure, in which the intermediate material has a flat phase reference surface facing in the circumferential direction, and in the circulation groove machining process, the phase reference surface is used to position the intermediate material in the circumferential direction.

[0014] A manufacturing method of a ball screw component of a third aspect of the present disclosure is the manufacturing method of a ball screw component of the second aspect of the present disclosure, wherein the intermediate material has a cylindrical or columnar main body portion having the workpiece surface and a protrusion portion protruding axially or radially from a circumferential portion of the main body portion, and the phase reference surface is formed by the circumferential side surface of the protrusion portion.

[0015] A fourth aspect of the present disclosure is a method for manufacturing a ball screw component, wherein in the method for manufacturing a ball screw component of any of the first to third aspects of the present disclosure, the intermediate material has an axial position reference surface in the form of a flat surface facing the axial direction, and in the circulation groove machining process, the axial position reference surface is used to position the intermediate material in the axial direction.

[0016] A manufacturing method for a ball screw component according to a fifth aspect of the present disclosure is the manufacturing method for a ball screw component according to any one of the first to fourth aspects of the present disclosure, further comprising a spiral groove machining step of forming a spiral groove on the machined surface.

[0017] A manufacturing method for a ball screw component according to a sixth aspect of the present disclosure is the manufacturing method for a ball screw component according to the fifth aspect of the present disclosure, wherein the spiral groove machining step is performed after the circulation groove machining step.

[0018] A manufacturing method for a ball screw component according to a seventh aspect of the present disclosure is the manufacturing method for a ball screw component according to the fifth aspect of the present disclosure, wherein the circulation groove machining step is performed after the spiral groove machining step.

[0019] The eighth aspect of the present disclosure is a method for manufacturing a ball screw component according to any one of the fifth to seventh aspects of the present disclosure, in which a heat treatment step is performed after the circulation groove machining step and the spiral groove machining step are performed.

[0020] A manufacturing method for a ball screw component according to a ninth aspect of the present disclosure is the manufacturing method for a ball screw component according to any one of the first to eighth aspects of the present disclosure, wherein the ball screw component is a nut.

[0021] A manufacturing method for a ball screw component according to a tenth aspect of the present disclosure is the manufacturing method for a ball screw component according to any one of the first to eighth aspects of the present disclosure, wherein the ball screw component is a screw shaft. [Effects of the Invention]

[0022] According to the method for manufacturing a ball screw component according to one aspect of the present disclosure, manufacturing costs can be reduced. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a cross-sectional schematic view showing a state in which a circulation groove machining process is being carried out on an intermediate material for manufacturing a nut, which is a ball screw component, in accordance with a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a perspective view showing an electrode used in the circulation groove machining step of the first example. [Figure 3] FIG. 3 is a cross-sectional view showing the intermediate material before the circulating groove processing step is carried out in the first example. [Figure 4]FIG. 4 is a perspective view showing the intermediate material before the circulating groove processing step is carried out in the first example. [Figure 5] FIG. 5 is a cross-sectional view showing the intermediate material after the circulating groove processing step has been carried out in the first example. [Figure 6] FIG. 6 is a perspective view showing the intermediate material after the circulating groove machining step has been carried out in the first example. [Figure 7] FIG. 7 is a cross-sectional view showing the intermediate material after the circulating groove machining step and the spiral groove machining step have been performed in the first example. [Figure 8] FIG. 8 is a perspective view showing the intermediate material after the circulating groove machining step and the spiral groove machining step have been performed in the first example. [Figure 9] FIG. 9 is a cross-sectional view showing an example of a ball screw device including a nut manufactured by the manufacturing method of the first example. [Figure 10] FIG. 10 is a cross-sectional schematic view showing a state in which a circulation groove machining process is being carried out on an intermediate material for manufacturing a screw shaft, which is a ball screw component, in a second example of an embodiment of the present disclosure. [Figure 11] FIG. 11 is a plan view showing the intermediate material before the circulating groove processing step is carried out in the second example. [Figure 12] FIG. 12 is a perspective view showing the intermediate material before the circulating groove processing step is carried out in the second example. [Figure 13] FIG. 13 is a plan view showing the intermediate material after the circulating groove machining step has been carried out in the second example. [Figure 14] FIG. 14 is a perspective view showing the intermediate material after the circulating groove machining step has been carried out in the second example. [Figure 15] FIG. 15 is a plan view showing the intermediate material after the circulating groove machining step and the spiral groove machining step have been performed in the second example. [Figure 16] FIG. 16 is a perspective view showing the intermediate material after the circulating groove machining step and the spiral groove machining step have been performed in the second example. [Figure 17]FIG. 17 is a cross-sectional view showing an example of a ball screw device including a screw shaft manufactured by the manufacturing method of the second example. DETAILED DESCRIPTION OF THE INVENTION

[0024] [Example 1] A first example of an embodiment of the present disclosure will be described with reference to FIGS.

[0025] In the following description, unless otherwise specified, the axial direction, radial direction, and circumferential direction refer to the axial direction, radial direction, and circumferential direction with respect to the intermediate material 7 used to manufacture the ball screw component 1 that is the target of manufacture by the manufacturing method of the present disclosure.

[0026] The ball screw component 1 to be manufactured by the manufacturing method of the present disclosure refers to both a nut 2 (see Figure 9) having a circulation groove 4 on its inner surface and a screw shaft 3 (see Figure 17) having a circulation groove 4 on its outer surface.

[0027] As shown in Fig. 9, a nut 2 having a circulation groove 4 on its inner peripheral surface is combined with a screw shaft 3a having no circulation groove on its outer peripheral surface and a plurality of balls 5 to form a ball screw device 6. As shown in Fig. 17, a screw shaft 3 having a circulation groove 4 on its outer peripheral surface is combined with a nut 2a having no circulation groove on its inner peripheral surface and a plurality of balls 5 to form a ball screw device 6.

[0028] The ball screw device 6 including the ball screw component 1 manufactured by the manufacturing method of the present disclosure is widely applicable to applications for converting linear motion into rotational motion or rotational motion into linear motion, and its applications are not limited. The ball screw device 6 can be incorporated into mechanical devices such as electric brake devices (EMB), electric brake booster devices (EHB), and gear ratio change devices, and can be used to convert rotational motion into linear motion.

[0029] The manufacturing method of the ball screw component of the present disclosure includes a circulation groove machining step.

[0030] The manufacturing method of the ball screw component of the present disclosure may optionally and additionally include other processes such as a molding process, a spiral groove machining process, a heat treatment process, a removal process, a grinding process, etc. In the manufacturing method of the ball screw component of the present disclosure, one or more of the other processes may be performed before the circulation groove machining process, or one or more of the other processes may be performed after the circulation groove machining process.

[0031] The circulation groove machining process is a process in which die-sinking electrical discharge machining is performed on an intermediate material 7 made of a conductive metal and having a radially facing annular workpiece surface 8, to form an approximately S-shaped circulation groove 4 on the workpiece surface 8.

[0032] The intermediate material 7, which is the workpiece for die-sinking electrical discharge machining, is made of a conductive metal and has an annular machining surface 8 facing in the radial direction, so long as the other structure is arbitrary.

[0033] The annular work surface 8 facing in the radial direction refers to an inner peripheral surface facing inward in the radial direction or an outer peripheral surface facing outward in the radial direction.

[0034] The intermediate material 7 can be composed of only a cylindrical or columnar main body 9 having a work surface 8, or can be composed of the main body 9 and other parts such as a protrusion 10 protruding in the axial or radial direction from a circumferential portion of the main body 9, a coaxial part 11 arranged coaxially with the main body 9, or a flange part protruding in the radial direction from the main body 9. When the intermediate material 7 has other parts such as the protrusion 10, coaxial part 11, or flange part in addition to the main body 9, the use and function of the other parts are arbitrary.

[0035] The intermediate material 7 may have a surface other than the work surface 8. Specifically, the intermediate material 7 may have, in addition to the work surface 8, flat reference surfaces 12, 13 that can be used to position the intermediate material 7. Specifically, the intermediate material 7 may have a flat phase reference surface 12 facing the circumferential direction that can be used to position the intermediate material 7, and a flat axial position reference surface 13 facing the axial direction that can be used to position the intermediate material 7. If the intermediate material 7 has a phase reference surface 12, the phase reference surface 12 may be formed, for example, by the circumferential side surface of the protrusion 10 that constitutes the intermediate material 7. Furthermore, the phase reference surface 12 may be formed by the circumferential surface of a groove, such as a keyway, formed on the end face or side face of the intermediate material 7. If the intermediate material 7 has an axial position reference surface 13, the axial position reference surface 13 may be formed, for example, by the axial end face of the intermediate material 7.

[0036] Any processing method may be used to obtain the intermediate material 7. For example, the intermediate material 7 may be formed by forging or cutting a conductive metal material (billet). When the intermediate material 7 is formed by forging a material, the type of forging may be any, and for example, hot forging or cold forging may be selected.

[0037] There are no limitations on the type of material for the intermediate material 7 as long as it is made of a conductive metal. The material for the intermediate material 7 can be determined appropriately depending on the application of the ball screw device 6 that includes the ball screw component 1 formed by applying die-sinking electrical discharge machining to the intermediate material 7, and for example, carbon steel for mechanical structures, alloy steel for mechanical structures, high-carbon chromium bearing steel, etc. can be used.

[0038] In the circulation groove machining process, die-sinking electrical discharge machining is performed using an electrode 14 that is roughly S-shaped and convex, which is the inverse of the circulation groove 4 formed on the workpiece surface 8 of the intermediate material 7, as shown in Figure 2. Specifically, as shown in Figure 1, the intermediate material 7 is set in an insulating machining fluid 15, the electrode 14 is brought close to and faces the workpiece surface 8 of the intermediate material 7, and a pulse current is passed through the electrode 14. This causes a dielectric breakdown in the machining fluid 15 present between the electrode 14 and the workpiece surface 8, generating a high-density electrical discharge phenomenon known as an arc column between the electrode 14 and the workpiece surface 8, which melts the workpiece surface 8.

[0039] The molten metal is blown away by the machining fluid 15 around the arc column, which vaporizes and expands rapidly due to the heat of the arc column. Furthermore, the molten metal adhering to the workpiece surface 8 is cooled and washed away by the machining fluid 15 that flows between the electrode 14 and the workpiece surface 8 after the machining fluid 15 expands.

[0040] The surface to be machined 8 is repeatedly melted by the discharge phenomenon and the dissolved metal is removed for each pulse voltage, so that the shape of the electrode 14 is transferred to the surface to be machined 8, forming a circulation groove 4 having a substantially S-shape.

[0041] The material of the electrode 14 can be determined arbitrarily depending on factors such as the cutting ability and the surface roughness required for the circulation groove 4. Specifically, copper, graphite, etc. can be selected. Regardless of the material selected for the electrode 14 used in die-sinking electrical discharge machining, the hardness will be sufficiently lower than that of cemented carbide steel, etc., used for dies in forging processes.

[0042] Any type of machining fluid 15 can be used as long as it is an insulating liquid. For example, oil or water can be used as machining fluid 15. During the circulation groove machining process, machining fluid 15 can be circulated to filter out impurities, or it can be cooled by a cooling device to keep the temperature constant.

[0043] In the manufacturing method of the ball screw component disclosed herein, the circulation groove machining step involves forming the circulation groove 4 on the workpiece surface 8 of the intermediate material 7 by die-sinking electrical discharge machining (DEEEM), rather than the conventional forging process. The electrode 14 used in DEEEM can be made of a material with low hardness and easy machining, such as copper or graphite, which allows the electrode 14 to be manufactured more inexpensively than the mold used in forging. Furthermore, forming the circulation groove 4 by forging requires multiple components, such as a jig, in addition to the mold for machining the circulation groove 4. However, forming the circulation groove 4 by DEEEM requires only the electrode 14. Therefore, even when the number of ball screw components 1 manufactured by DEEEM is small, the increase in manufacturing costs of the ball screw component 1 can be sufficiently suppressed. As a result, the manufacturing cost of the ball screw component 1 can be reduced.

[0044] Because the circulation groove 4 is formed on the workpiece surface 8 by die-sinking electrical discharge machining, sagging between the circulation groove 4 and the portion of the workpiece surface 8 outside the circulation groove 4 can be suppressed compared to when the circulation groove 4 is formed by forging. This suppresses the smooth movement of the ball 5 passing inside the circulation groove 4 from being impeded by sagging. Furthermore, a sagging removal process after the circulation groove machining process is unnecessary, or the labor required for removing the sagging can be reduced, thereby also reducing manufacturing costs. Furthermore, because sagging is suppressed, the nut-side spiral groove 25 or the shaft-side spiral groove 26 can be machined using the circulation groove 4 as a machining reference. This allows the positional relationship between the circulation groove 4 and the nut-side spiral groove 25 or the shaft-side spiral groove 26 to be precisely controlled. Even if sagging occurs between the circulation groove 4 and the portion of the workpiece surface 8 outside the circulation groove 4, the sagging can be easily removed by performing cutting or grinding in the removal process.

[0045] When performing the circulation groove machining process, a portion of the intermediate material 7 can be used to position the intermediate material 7. For example, if the intermediate material 7 has a flat phase reference surface 12 facing the circumferential direction, the phase reference surface 12 can be used to position the intermediate material 7 in the circumferential direction. If the phase reference surface 12 is formed by the circumferential side surface of a protrusion 10 constituting the intermediate material 7, the protrusion 10 can be removed by performing a removal process after the circulation groove machining process, or it can be used to regulate the stroke end of the ball screw component 1 without being removed after the circulation groove machining process. If the intermediate material 7 has a flat axial position reference surface 13 facing the axial direction, the axial position reference surface 13 can be used to position the intermediate material 7 in the axial direction. In this way, by using a portion of the intermediate material 7 for positioning, the number of steps can be reduced and the machining accuracy of the circulation groove 4 can be improved.

[0046] The circulation groove 4 formed by die-sinking electrical discharge machining has a rougher surface than the circulation groove 4 formed by forging. For this reason, by using the circulation groove 4 as it is without grinding, it is possible to give the circulation groove 4 the function of retaining grease.

[0047] When manufacturing a large ball screw part 1 with a deep circulation groove 4, forming the circulation groove 4 by forging can easily lead to problems such as the equipment becoming larger and the mold being damaged. However, when forming the circulation groove 4 by die-sinking electrical discharge machining, it is only necessary to prepare an electrode 14 with a large protrusion, and the problems that arise with forging do not occur.

[0048] Furthermore, when manufacturing a small diameter nut 2, if the circulation groove 4 is formed by forging, it becomes difficult to ensure the strength of the mold placed inside the intermediate material 7, which can easily lead to problems such as damage to the mold. However, with die-sinking electrical discharge machining, the electrode 14 is not subjected to as large a force as a mold, so the problems that occur with forging do not occur.

[0049] Next, an example of a case where the manufacturing method of the ball screw component of the present disclosure is applied to a manufacturing method of the nut 2 will be described.

[0050] In this example, in addition to the circulation groove machining step, optional and additional steps include a molding step, a spiral groove machining step, a heat treatment step, and a grinding step. Of these steps, the molding step is performed before the circulation groove machining step, and the other steps are performed after the circulation groove machining step. However, the spiral groove machining step can also be performed before the circulation groove machining step.

[0051] <Molding process> The molding step is a step for obtaining an intermediate material 7a made of a conductive metal and having an annular work surface 8a facing radially inward.

[0052] In this example, a conductive metal material (billet) is hot forged in the forming step to obtain an intermediate material 7a made of conductive metal and having an annular work surface 8a facing radially inward, as shown in Figures 3 and 4.

[0053] The intermediate material 7a has a cylindrical shape as a whole. The intermediate material 7a has a cylindrical main body 9a having a cylindrical work surface 8a on its inner circumferential surface, as well as a protrusion 10a and a coaxial portion 11a as optional additional elements. The protrusion 10a and the coaxial portion 11a are each provided integrally with the main body 9a.

[0054] The protrusion 10a is provided on a portion of the circumferential direction of the end face on one axial side (the right side in Figures 3 and 4) of the main body 9a, and protrudes toward one axial side. The protrusion 10a has flat circumferential side surfaces facing the circumferential direction on both circumferential sides. In this example, the protrusion 10a is not removed in the removal process, but is used to regulate the stroke end of the nut 2 after the nut 2 manufactured from the intermediate material 7a is assembled into the ball screw device 6. However, the protrusion 10a can also be removed by performing a removal process after the circulation groove machining process.

[0055] The coaxial portion 11a has a cylindrical shape and is disposed coaxially with the main body portion 9a on the other axial side of the main body portion 9a (the left side in FIGS. 3 and 4). The coaxial portion 11a has the same outer diameter as the main body portion 9a and a larger inner diameter than the workpiece surface 8a. In this example, the coaxial portion 11a is used to connect the nut 2 manufactured from the intermediate material 7a to another member.

[0056] In this example, in the forming process, after forging, the intermediate material 7a is subjected to cutting as an optional additional process. Specifically, cutting is performed on the outer peripheral surface and both axial end faces of the intermediate material 7a, both circumferential side faces of the protrusion 10a, and the inner peripheral surface of the coaxial portion 11a. This removes oxide coatings from the outer peripheral surface and both axial end faces of the intermediate material 7a, both circumferential side faces of the protrusion 10a, and the inner peripheral surface of the coaxial portion 11a, and also adjusts the shape of each part.

[0057] <Circulation groove machining process> Next, in the circulation groove machining step, the intermediate material 7a is subjected to die-sinking electrical discharge machining to form a substantially S-shaped circulation groove 4 on the work surface 8a of the intermediate material 7a.

[0058] In the circulating groove machining process, a circulating groove 4 is formed in a workpiece surface 8a using a die-sinking electric discharge machine 17 as shown in Fig. 1. Note that Fig. 1 shows an example of a case where the circulating groove machining process is performed in a state where the central axis of the intermediate material 7a, which is the workpiece, is positioned horizontally, but the circulating groove machining process can also be performed in a state where the central axis of the intermediate material 7a is positioned vertically, for example.

[0059] The die-sinking electrical discharge machine 17 includes an electrode 14, a machining tank 18, a spindle 19, a spindle feed mechanism 20, a high-frequency power supply 21, a table 22, and a control device 23. In the die-sinking electrical discharge machine 17, the X-axis direction, the Y-axis direction, and the Z-axis direction correspond to the left-right direction, the depth direction, and the up-down direction, respectively, when the die-sinking electrical discharge machine 17 is viewed from the front.

[0060] The electrode 14 is made of a conductive electrode material such as copper or graphite, and has a generally S-shaped, convex shape that is the inverse of the circulation groove 4 to be formed on the work surface 8a of the intermediate material 7a. In this example, since the circulation groove 4 needs to be machined on the work surface 8a formed on the inner circumferential surface of the intermediate material 7a, the electrode 14 has a size that allows it to be inserted inside the intermediate material 7a.

[0061] The machining tank 18 is a container capable of storing the machining fluid 15, and has a size large enough to accommodate the intermediate material 7a and the electrode 14. Additionally and optionally, the machining tank 18 may be equipped with a circulation device for circulating the machining fluid 15 and a cooling device for cooling the machining fluid 15.

[0062] The main shaft 19 is disposed with its central axis directed in the Z-axis direction (the vertical direction in FIG. 1), and the electrode 14 can be detachably attached to the lower end thereof.

[0063] The spindle feed mechanism 20 moves the spindle 19 in the Z-axis direction. The spindle feed mechanism 20 includes, for example, but is not limited to, a servo motor and a feed screw mechanism.

[0064] The high frequency power supply 21 sends a pulse current to the electrode 14, and repeatedly generates a state in which a voltage is applied and a state in which no voltage is applied between the electrode 14 and the work surface 8a.

[0065] The table 22 is capable of placing the processing tank 18 thereon and is movable in the X-axis direction (left and right direction in FIG. 1) and the Y-axis direction (front and back direction in FIG. 1).

[0066] The control device 23 controls each of the spindle feed mechanism 20, the high-frequency power supply 21, and the table 22. By controlling the spindle feed mechanism 20, the control device 23 controls the position of the electrode 14 in the Z-axis direction and the moving speed of the electrode 14 in the Z-axis direction. By controlling the high-frequency power supply 21, the control device 23 controls the magnitude and frequency of the pulse voltage applied between the electrode 14 and the workpiece surface 8a. By controlling the table 22, the control device 23 controls each of the positions of the intermediate material 7a in the X-axis direction and the Y-axis direction.

[0067] In this example, in order to perform die-sinking electrical discharge machining on the intermediate material 7a using the die-sinking electrical discharge machine 17, first the intermediate material 7a is set inside a machining tank 18 that stores an insulating machining fluid 15. Specifically, a jig 24 fixed to the machining tank 18 is used to immobilize the intermediate material 7a inside the machining tank 18. In this example, the intermediate material 7a is set in the machining tank 18 with the central axis of the intermediate material 7a facing the X-axis direction.

[0068] When the intermediate material 7a is set in the processing tank 18, the intermediate material 7a is positioned in the circumferential direction relative to the processing tank 18 using a phase reference surface 12a that is a flat surface facing in the circumferential direction and is formed by the circumferential side surface of the protrusion 10. For example, the intermediate material 7a is positioned in the circumferential direction using the phase reference surface 12a so that the portion of the processing surface 8a that forms the circulation groove 4 is positioned vertically downward. In addition, the intermediate material 7a is positioned in the axial direction relative to the processing tank 18 using an axial position reference surface 13a that is a flat surface facing in the axial direction and is formed by the axial end surface of the intermediate material 7a.

[0069] After the intermediate material 7a is set in the machining tank 18, the table 22 is moved to insert the electrode 14 inside the intermediate material 7a. Then, the main shaft 19 is moved in the Z-axis direction to bring the electrode 14 close to and opposite the portion of the workpiece surface 8a where the circulation groove 4 is to be formed. The size of the gap between the electrode 14 and the workpiece surface 8a is set to, for example, about several μm, although it is not limited to this.

[0070] The high-frequency power supply 21 applies a pulse current to the electrode 14 while the electrode 14 is positioned closely opposite the workpiece surface 8a. The pulse voltage applied to the electrode 14 is not limited to this, but can have a magnitude of several tens of volts to several hundreds of volts and a frequency of several tens of Hz to 1 MHz, for example.

[0071] This causes a dielectric breakdown in the machining fluid 15 present between the electrode 14 and the work surface 8a, generating a high-density discharge phenomenon known as an arc column between the electrode 14 and the work surface 8a. Then, while the discharge continues (several μs to several thousand μs), the work surface 8a is locally heated, melting the work surface 8a.

[0072] The molten metal is blown away by the machining fluid 15 around the arc column, which vaporizes and expands rapidly due to the heat of the arc column. The molten metal adhering to the workpiece surface 8a is cooled and washed away by the machining fluid 15 that flows between the electrode 14 and the workpiece surface 8a after the machining fluid 15 expands. In this example, the spindle feed mechanism 20 reciprocates the spindle 19 in the Z-axis direction at high speed, repeatedly flowing machining fluid 15 between the electrode 14 and the workpiece surface 8a, improving the dischargeability of the molten metal.

[0073] To prevent the gap between the electrode 14 and the work surface 8a from widening due to the melting of the surface of the work surface 8a, the spindle 19 is gradually lowered by the spindle feed mechanism 20 to keep the size of the gap between the electrode 14 and the work surface 8a constant.

[0074] The surface to be processed 8a is repeatedly melted by the discharge phenomenon and the dissolved metal is removed for each pulse voltage, so that the shape of the electrode 14 is transferred to the surface to be processed 8a, forming a circulation groove 4 having a substantially S-shape.

[0075] When forming multiple circulation grooves 4 on the workpiece surface 8a, after machining of one circulation groove 4 is completed, the intermediate material 7a is temporarily removed from the machining tank 18. Then, using the phase reference surface 12a or the previously formed circulation groove 4 and the axial position reference surface 13a, the phase and axial position of the intermediate material 7a relative to the machining tank 18 are changed, and the intermediate material 7a is reset in the machining tank 18. The next circulation groove 4 is then machined using the procedure described above. This process is repeated until machining of all circulation grooves 4 is completed. As a result, multiple circulation grooves 4 are formed on the workpiece surface 8a, as shown in Figures 5 and 6.

[0076] <Spiral groove machining process> Next, cutting processing such as cutting tapping is performed on the work surface 8a of the intermediate material 7a to form the nut side spiral groove 25. As a result, as shown in Figures 7 and 8, a plurality of circulation grooves 4 and a nut side spiral groove 25 are formed on the work surface 8a of the intermediate material 7a. In this example, when machining the nut side spiral groove 25, the phase reference surface 12a formed by the circumferential side surface of the protrusion 10a is used to position the intermediate material 7 in the circumferential direction relative to the cutting machine. This allows the phase between the nut side spiral groove 25 and the circulation groove 4 to be controlled with high precision.

[0077] <Heat treatment process> Next, the intermediate material 7a is subjected to heat treatment such as carburizing and quenching or induction hardening, and tempering. This increases the surface hardness of the intermediate material 7a and improves its wear resistance. Note that, in addition to induction hardening, through hardening, carburizing and quenching can also be used as the hardening treatment.

[0078] <Grinding process> Thereafter, if necessary, grinding is performed on both axial end faces of the intermediate material 7a and on the nut-side spiral groove 25 to obtain the finished nut 2. If necessary, grinding can also be performed on the inner peripheral surface of the intermediate material 7a to remove heat treatment deformation and sagging. In this process, grinding is not performed on the circulation groove 4. In other words, the circulation groove 4 is used as a non-ground surface. This allows the circulation groove 4 to have a mechanism for retaining grease. The grinding process can be performed as necessary or omitted.

[0079] When manufacturing the nut 2, other steps may be inserted between the steps described above.

[0080] The nut 2 having a circulation groove 4 on its inner surface manufactured by the manufacturing method of this example is combined with a screw shaft 3a having no circulation groove 4 on its outer surface and a plurality of balls 5 in the assembly process to form a ball screw device 6 as shown in Figure 9.

[0081] The balls 5 are arranged to roll freely between a nut-side spiral groove 25 formed on the inner peripheral surface of the nut 2 and a shaft-side spiral groove 26 formed on the outer peripheral surface of the screw shaft 3a.

[0082] The circulation groove 4 formed on the inner peripheral surface of the nut 2 connects the start point and end point of a spiral load path 27 formed by the nut-side spiral groove 25 and the shaft-side spiral groove 26. Specifically, one end of the circulation groove 4 in the length direction is connected to the end point of the load path 27, and the other end of the circulation groove 4 in the length direction is connected to the start point of the load path 27. As a result, the circulation groove 4 returns the balls 5 that have reached the end point of the load path 27 to the start point of the load path 27, allowing the balls to circulate infinitely.

[0083] Either the nut 2 or the screw shaft 3a is used as a rotational motion element, and the other is used as a linear motion element. Which of the nut 2 and the screw shaft 3a is used as a rotational motion element and which is used as a linear motion element can be determined appropriately depending on the application of the ball screw device.

[0084] [Example 2] A second example of the embodiment of the present disclosure will be described with reference to FIGS.

[0085] In this example, an example in which the manufacturing method of the ball screw component of the present disclosure is applied to a manufacturing method of the screw shaft 3 will be described.

[0086] In this example, in addition to the circulation groove machining step, optional and additional steps include a molding step, a spiral groove machining step, a heat treatment step, and a grinding step. Of these steps, the molding step is performed before the circulation groove machining step, and the other steps are performed after the circulation groove machining step. However, the spiral groove machining step can also be performed before the circulation groove machining step.

[0087] <Molding process> The molding step is a step for obtaining an intermediate material 7b made of a conductive metal and having an annular work surface 8b facing radially outward.

[0088] In this example, a conductive metal material (billet) is hot forged in the forming step to obtain an intermediate material 7b made of conductive metal and having an annular processed surface 8b facing radially outward on its outer circumferential surface, as shown in Figures 11 and 12.

[0089] The intermediate material 7b has a stepped cylindrical shape. The intermediate material 7b has a cylindrical main body 9b having a cylindrical work surface 8b on its outer circumferential surface, and an optional and additional element, a coaxial portion 11b. The coaxial portion 11b is provided integrally with the main body 9b.

[0090] The coaxial portion 11b has a cylindrical shape and is disposed coaxially with the main body portion 9b on one axial side of the main body portion 9b (the right side in FIGS. 11 and 12). The coaxial portion 11b has a smaller outer diameter than the main body portion 9b. In this example, the coaxial portion 11b is used to connect the screw shaft 3 to another member.

[0091] In this example, after forging in the forming process, cutting is performed on the intermediate material 7b as an optional additional process. Specifically, cutting is performed on the outer peripheral surface and both axial end faces of the main body 9b. This removes oxide coatings from the outer peripheral surface and both axial end faces of the main body 9b and adjusts the shape of each part.

[0092] <Circulation groove machining process> Next, in the circulation groove machining step, the intermediate material 7b is subjected to die-sinking electrical discharge machining to form a substantially S-shaped circulation groove 4 on the workpiece surface 8b of the intermediate material 7b.

[0093] In the circulation groove machining process, the same die-sinking electric discharge machine 17 as in the first example is used.

[0094] In this example, when the intermediate material 7b is set in the processing tank 18, the axial positioning reference surface 13b formed by the axial end face of the main body 9b is used to position the intermediate material 7b in the axial direction relative to the processing tank 18. Also in this example, the phase reference surface provided on the intermediate material 7b can be used to position the intermediate material 7b in the circumferential direction relative to the processing tank 18. The phase reference surface may be removed after the circulation groove processing step.

[0095] After the intermediate material 7b is set in the processing tank 18, the table 22 is moved so that the processing surface 8b is positioned vertically below the electrode 14. Then, with the electrode 14 positioned closely facing the processing surface 8b, a pulse current is passed through the electrode 14, and a substantially S-shaped circulation groove 4 is formed in the processing surface 8b, as in the first example.

[0096] When forming multiple circulation grooves 4 on the workpiece surface 8b, after machining of one circulation groove 4 is completed, the intermediate material 7b is temporarily removed from the machining tank 18. Then, using the previously formed circulation groove 4 and the axial position reference surface 13b, the phase and axial position of the intermediate material 7b relative to the machining tank 18 are changed, and the intermediate material 7b is reset in the machining tank 18. Then, the same operations as in the first example are repeated until machining of all circulation grooves 4 is completed. In this way, multiple circulation grooves 4 are formed on the workpiece surface 8b of the intermediate material 7b, as shown in Figures 13 and 14.

[0097] <Spiral groove machining process> Next, cutting is performed on the work surface 8b of the intermediate material 7b to form the shaft side spiral groove 26. As a result, as shown in Figures 15 and 16, a plurality of circulation grooves 4 and the shaft side spiral groove 26 are formed on the work surface 8b of the intermediate material 7b. When the circulation groove machining step is performed after the spiral groove machining step, the shaft side spiral groove 26 can be formed not only by cutting but also by through-feed or in-feed rolling.

[0098] <Heat treatment process> Next, the outer peripheral surface of the main body 9b is subjected to heat treatment such as induction hardening and tempering. This increases the surface hardness and wear resistance of the outer peripheral surface of the main body 9b. Note that, in addition to induction hardening, through hardening, carburizing hardening, etc. can also be used as the hardening treatment.

[0099] <Grinding process> Thereafter, if necessary, the shaft-side spiral groove 26 is ground to obtain the finished product, the screw shaft 3. Also, if necessary, the outer peripheral surface of the intermediate material 8b can be ground to remove heat treatment deformation and sagging. In this process, the circulation groove 4 is not ground. The grinding process can be performed if necessary, or can be omitted.

[0100] When manufacturing the screw shaft 3, other steps may be inserted between the steps described above.

[0101] The screw shaft 3 having a circulation groove 4 on its outer surface manufactured by the manufacturing method of this example is combined with a nut 2a having no circulation groove 4 on its inner surface and a plurality of balls 5 in the assembly process to form a ball screw device 6 as shown in Figure 17.

[0102] Other configurations and effects of the second example are the same as those of the first example. [Explanation of symbols]

[0103] 1 Ball screw parts 2, 2a Nut 3, 3a screw shaft 4 Circulation groove 5 balls 6 Ball screw device 7, 7a, 7b intermediate material 8, 8a, 8b Processed surface 9, 9a, 9b Main body 10, 10a protrusion 11, 11a, 11b coaxial section 12, 12a Phase reference plane 13, 13a, 13b Axial position reference plane 14 electrodes 15 Processing fluid 17 Die-sinking EDM machine 18 Processing tank 19 Spindle 20 Spindle feed mechanism 21 High frequency power supply 22 tables 23 Control device 24 Jig 25 Spiral groove on nut side 26 Shaft side spiral groove 27 Load path

Claims

1. A manufacturing method for a ball screw component, comprising a circulation groove machining step of performing die-sinking electrical discharge machining on an intermediate material made of a conductive metal and having a radially facing annular workpiece surface to form a substantially S-shaped circulation groove on the workpiece surface.

2. the intermediate material has a phase reference surface in the form of a flat surface facing in the circumferential direction, In the circulating groove machining step, the intermediate material is positioned in the circumferential direction by utilizing the phase reference surface. The method for manufacturing the ball screw component according to claim 1.

3. the intermediate material has a cylindrical or columnar main body having the work surface, and a protrusion protruding in an axial or radial direction from a circumferential portion of the main body, the phase reference surface is formed by a circumferential side surface of the protrusion, The method for manufacturing the ball screw component according to claim 2.

4. the intermediate material has an axial position reference surface in the form of a flat surface facing the axial direction, In the circulation groove machining step, the intermediate material is positioned in the axial direction by utilizing the axial position reference surface. The method for manufacturing the ball screw component according to claim 1.

5. The method for manufacturing a ball screw component according to claim 1, further comprising a spiral groove machining step of forming a spiral groove on the machined surface.

6. The method for manufacturing a ball screw component according to claim 5, wherein the spiral groove machining step is performed after the circulation groove machining step.

7. The method for manufacturing a ball screw component according to claim 5, wherein the circulation groove machining step is performed after the spiral groove machining step.

8. The method for manufacturing a ball screw component according to claim 5, further comprising the steps of: performing a heat treatment after the circulation groove machining step and the spiral groove machining step are performed;

9. The method for manufacturing a ball screw component according to claim 1, wherein the ball screw component is a nut.

10. The method for manufacturing a ball screw component according to claim 1 , wherein the ball screw component is a screw shaft.

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