Rack, method for manufacturing rack, and device for manufacturing rack

Abstract

Provided are a manufacturing method and manufacturing device that allow the depth of rack teeth to be adequately maintained across the axial direction. A stepped surface (47) is provided on one surface in the axial direction of a pressure punch (46) that moves together with a teeth-forming punch (32). The teeth-forming punch (32) is displaced downward and rack teeth are formed on the upper surface of an intermediate material (23). At the same time, the surface on the end in the axial direction of the intermediate material (23) is pressed in the axial direction by a movable die (41a) due to the engagement between the stepped surface (47) and the surface on the other side in the axial direction of the movable die (41a).

Claims

1. A manufacturing method for a rack comprising a step of: forming rack teeth on a surface on one side in a radial direction of a raw material made of metal by pressing a teeth-forming punch that is provided with rack-shaped uneven processing teeth against a part in an axial direction of the surface of the raw material made of metal, and causing the surface to plastically deform, while pressing the raw material toward an inside in the axial direction, wherein a pressure punch that raises and lowers together with the teeth-forming punch, and comprises a surface on one side in the axial direction that is a stepped surface; and a movable die that comprises a surface on one side in the axial direction that faces an end surface in the axial direction of the raw material, and a surface on another side in the axial direction that faces the stepped surface of the pressure punch when the teeth-forming punch is displaced toward the raw material, and to which an elastic force is applied toward the pressure punch; are used, and the stepped surface of the pressure punch is engaged with the surface on the other side in the axial direction of the movable die as the teeth-forming punch moves toward the raw material, causing the movable die to move in the axial direction, and pressing the surface on the end surface in the axial direction of the raw material in the axial direction by way of the movable die, and wherein the raw material is pressed in the axial direction by a constant force while the surface on the one side in the radial direction is being plastically deformed.

2. The manufacturing method according to claim 1, wherein the rack teeth are formed so that when the length in the axial direction of the portion of the raw material where the rack teeth are formed is taken to be LR, and the length in the axial direction of the remaining portion of the raw material that is separated in the axial direction from the portion where the rack teeth are formed is taken to be L, the relationship LL.sub.R/4 is satisfied.

3. A rack that is manufactured using the manufacturing method according to claim 1, wherein the raw material is provided by a metal rod, and the rack teeth are formed on a part in the axial direction of the surface on one side in the radial direction of the rod, wherein the tooth depth of the rack teeth at an end section in the axial direction of the part of the surface of the rod is the same size as the tooth depth of the rack teeth in a middle section in the axial direction of the part of the surface of the rod, wherein the rod has a portion that is adjacent in the axial direction to the part of the surface where the rack teeth are formed, and the portion that is adjacent in the axial direction to the part of the surface where the rack teeth are formed has a fiber flow having a slight upward convex shape.

4. A manufacturing method for a rack comprising a step of: forming rack teeth on a surface on one side in a radial direction of a raw material made of metal by pressing a teeth-forming punch that is provided with rack-shaped uneven processing teeth against a part in axial direction of the surface of the raw material made of metal, and causing the surface to plastically deform, while pressing the raw material toward an inside in the axial direction, wherein a pressure punch that raises and lowers together with the teeth-forming punch, and comprises a surface on one side in the axial direction that is an inclined surface; and a movable die that comprises a surface on one side in the axial direction that faces an end surface in the axial direction of the raw material, and a surface on another side in the axial direction that faces the inclined surface of the pressure punch when the teeth-forming punch is displaced toward the raw material, and to which an elastic force is applied toward the pressure punch; are used, and the inclined surface of the pressure punch is engaged with the surface on the other side in the axial direction of the movable die as the teeth-forming punch moves toward the raw material, causing the movable die to move in the axial direction, and pressing the surface on the end surface in the axial direction of the raw material in the axial direction by way of the movable die, wherein the raw material is pressed in the axial direction while the force of pressing the raw material in the axial direction is increased as the surface on one side in the radial direction is plastically deformed.

5. The manufacturing method according to claim 4, wherein the rack teeth are formed so that when the length in the axial direction of the portion of the raw material where the rack teeth are formed is taken to be LR, and the length in the axial direction of the portion of the raw material that is separated in the axial direction from the portion where the rack teeth are formed is taken to be L, the relationship LL.sub.R is satisfied.

6. A rack that is manufactured using the manufacturing method according to claim 4, wherein the raw material is provided by a metal rod, and the rack teeth are formed on a part in the axial direction of the surface on one side in the radial direction of the rod, wherein the tooth depth of the rack teeth at an end section in the axial direction of the part of the surface of the rod is the same size as the tooth depth of the rack teeth in a middle section in the axial direction of the part of the surface of the rod, wherein the rod has a portion that is adjacent in the axial direction to the part of the surface where the rack teeth are formed, and the portion that is adjacent in the axial direction to the part of the surface where the rack teeth are formed has a fiber flow having a slight upward convex shape.

7. A manufacturing device for a rack, comprising: a teeth-forming punch provided with rack-shaped uneven processing teeth; a pressure punch raising and lowering together with the teeth-forming punch and comprising a surface on one side in an axial direction having a stepped surface or inclined surface; and a movable die comprising a surface on one side in the axial direction that faces the surface on an end in the axial direction of a raw material made of metal, and a surface on another side in the axial direction that faces the stepped surface or inclined surface of the pressure punch when the teeth-forming punch moves toward the raw material, and to which an elastic force is applied toward the pressure punch; wherein the manufacturing device for a rack is constructed so that when forming rack teeth on the surface of the one side in a radial direction of part in the axial direction of the raw material made of metal by pressing the teeth-forming punch against the surface on the one side in the radial direction, as the teeth-forming punch moves toward the raw material, the surface on the other side in the axial direction of the movable die engages with the stepped surface or inclined surface of the pressure punch, the movable die moves in the axial direction, and the movable die presses the surface on the end in the axial direction of the raw material.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a cross-sectional view of a first example of an embodiment of the present invention, and illustrates a state in which a ram is positioned at the top dead center.

(2) FIG. 2 is a cross-sectional view of the first example, and illustrates a state in which the ram has been displaced downward.

(3) FIG. 3 is a cross-sectional view of the first example, and illustrates a state in which the ram has been displaced downward further than the state illustrated in FIG. 2.

(4) FIG. 4 is a cross-sectional view of the first example, and illustrates a state in which the ram has been lowered to the bottom dead center.

(5) FIG. 5 is an enlarged view of the center section of FIG. 3.

(6) FIG. 6 is an enlarged view of the center section of FIG. 4.

(7) FIG. 7A is an enlarged view of section a in FIG. 2; and FIG. 7B is an enlarged view of section b in FIG. 4.

(8) FIG. 8 is a side view for explaining the dimensions of each of the parts of a rack.

(9) FIGS. 9A and 9B are views similar to FIGS. 7A and 7B, and illustrate an example of a different shape pressure punch and one moving die.

(10) FIG. 10 is a view for explaining the effect of the invention, and corresponds to an enlarged view of the right half of FIG. 6

(11) FIG. 11A illustrates the fiber flow of a rack that is obtained by a conventional manufacturing method for a rack, and FIG. 11B illustrates the fiber flow of a rack that is obtained by the manufacturing method for a rack of the present invention.

(12) FIGS. 12A to 12G are cross-sectional views of the second example of an embodiment of the present invention, and illustrate the processing order of the manufacturing process for a rack.

(13) FIG. 13 is an enlarged cross-sectional view of section c-c in FIG. 12F.

(14) FIG. 14 is a partial cross-sectional view illustrating a first example of a conventional steering apparatus for an automobile.

(15) FIG. 15 is a partial cross-sectional view illustrating a second example of a conventional steering apparatus for an automobile.

(16) FIG. 16 is a perspective view illustrating an example of a conventional rack.

(17) FIG. 17 is a view as seen in the direction d in FIG. 16.

(18) FIG. 18 is a view as seen in the direction e in FIG. 16.

(19) FIG. 19 is an enlarged cross-sectional view of section f-f in FIG. 18.

(20) FIGS. 20A to 20F are cross-sectional views as seen from the same direction as in FIG. 19, and illustrates a first example of the processing order of a conventional manufacturing method for a rack.

(21) FIGS. 21A and 21B are partial perspective views illustrating the shape of rack teeth before and after sizing in the first example of a conventional manufacturing method for a rack.

(22) FIG. 22 is a cross-sectional view for explaining the problems of a conventional manufacturing method and manufacturing device for a rack.

(23) FIG. 23 is a cross-sectional view illustrating a second example of a conventional manufacturing method for a rack.

MODES FOR CARRYING OUT INVENTION

(24) [First Example]

(25) FIG. 1 to FIG. 11 illustrate a first example of an embodiment of the present invention. In this example as well, similar to the conventional technology, this manufacturing method for a rack has steps of: (1) performing an upsetting process to squash the portion of a circular rod shaped raw material 19 where rack teeth are to be formed and expand the width dimension in the horizontal direction to obtain an intermediate material 23 (see FIG. 20A and FIG. 20B); (2) inserting and placing the intermediate material 23 into a support hole 28 that is provided in a die 27, then further inserting a teeth-forming punch 32 inside the support hole 28, and using the teeth-forming punch 32 to strongly press the intermediate material 23 inside the support hole 28 to obtain a raw rack 33 (see FIG. 20C and FIG. 20D); and (3) using a sizing die 35 to complete the rack teeth 16, or in other words to make the shape and dimensions of the rack teeth 16 proper, and to provide a chamfer on the edges of the ends of each of the teeth (see FIG. 20E and FIG. 20F). A feature of the manufacturing method and manufacturing device for a rack of this invention, including this example, is to suppress movement of the metal material of the rack 11c in the axial direction when forming rack teeth 16 on the surface of one side in the radial direction of part in the axial direction of the rod section 15 of the rack 11c (process corresponding to FIG. 20C and FIG. 20D), and prevent the tooth depth of the rack teeth 16 from becoming small on the end sections in the axial direction. The construction and functions of the other parts are the same as those of the conventional manufacturing method and manufacturing device for a rack. In FIG. 1 to FIG. 6, the rack teeth 16 and the unevenness of the processing surface of the teeth-forming punch 32 are omitted.

(26) In this example, the manufacturing device for a rack has a die 27a, a teeth-forming punch 32, and a pressure punch 46. The die 27a is supported by a support base 43, and has a stationary die 40, and a pair of movable dies 41a, 41b. The stationary die 40 has a support hole 28. Moreover, an elastic force is applied in an upward direction (direction going away from the stationary die 40) to the pair of movable dies 41a, 41b by an elastic member 42 such as coil springs, air cylinders or the like. An elastic force outward in the axial direction (to the right in FIG. 1) is applied by an elastic member such as a spring not illustrated in the figure to one of the movable dies 41a (die on the right in FIG. 1) of the pair of movable dies 41a, 41b. On the other hand, the other movable die 41b (left side in FIG. 1) is able to move only in an up or down direction along the inside surface of a concave section 44 that is formed in the support base 43.

(27) Moreover, the teeth-forming punch 32 and the pressure punch 46 are supported by a rack 45 that is able to move up or down. Rack-shaped uneven teeth are provided on the teeth-forming punch 32. The pressure punch 46 moves up or down together with the teeth-forming punch 32, and has a surface on one side in the axial direction (inside surface; surface on the left side in FIG. 7). The surface on one side in the axial direction of the pressure punch 46 is a stepped surface 47 such that upper half is closer to the center in the axial direction than the lower half.

(28) As illustrated in FIG. 1, the intermediate material 23 that is obtained by performing an upsetting process on the raw material 19 is set into the die 27a. In this state, the surface on one side in the axial direction of one movable die 41a faces the surface on one end in the axial direction of the intermediate material 23. In this state, there is a space between the surface on the other side in radial direction of the intermediate material 23, which will become the rear surface portion of the completed rack 11c, and the bottom section 29 of the support hole 28.

(29) When the ram 45 is lowered, the stepped surface 47 of the surface on one side in the axial direction of the pressure punch 46 faces the surface on the other side (outside surface; surface on the right side in FIG. 7) in the axial direction of one of the movable dies 41a. In other words, as illustrated in FIG. 1 to FIG. 3, when the ram 45 is displaced downward and the processing surface (bottom surface) of the teeth-forming punch 32 and the surface on one side in the radial direction of the intermediate material 23 come in contact, then as illustrated in FIG. 7A, the surface on one side in the axial direction of one of the movable dies 41a faces the lower half of the stepped surface 47 by way of a space. In this state, the intermediate material 23 is elastically held between the processing surface of the teeth-forming punch 32 and the pair of movable dies 41a, 41b, which keeps the intermediate material 23 in the proper position. Then, in this state, as illustrated in FIG. 4, when the ram 45 is lowered to the bottom dead center, the pair of movable dies 41a, 41b move downward against the elastic force of the elastic member 42, and the intermediate material 23 is strongly pressed inside the support hole 28 of the stationary die 40. In this way, at the same time or just before the rack teeth 16 begin to be formed on the surface on one side in the radial direction of the intermediate material 23, one movable die 41a is pressed inward in the radial direction (to the left in FIG. 1 to FIG. 7B) due to the engagement between the surface on one side in the axial direction of the one movable die 41a and the upper half of the stepped surface of the pressure punch 46, and the surface on one end in the axial direction of the intermediate material (surface on the right end in FIG. 4 and FIG. 6) is pressed by the constant pressure force toward the inside in the axial direction.

(30) The size of the force by which the pressure punch 46 presses the one movable die 41a (amount of movement in the axial direction of the movable die 41a), and the size of the force by which the processing surface of the teeth-forming punch 32 presses the surface on one side in the radial direction of the intermediate material 23 are regulated by the dimensions in the axial direction of each of the component parts of the completed rack 11c including whether the sizes of these forces are constant or are changed while the rack teeth 16 are being formed.

(31) First, the case will be explained in which, when the length in the axial direction of the portion where the rack teeth 16 are formed on part in the axial direction of the rod section 15 of the completed rack 11c (see FIG. 8) is taken to be L.sub.16, and the length in the axial direction of the circular rod section 18, which is the remaining portion in the axial direction of the rod section 15 is taken to be L.sub.18, these lengths satisfy the relationship L.sub.18L.sub.16/4. In this case, when pressing with the teeth-forming punch 32 when forming the rack teeth 16, the amount of metal material of the intermediate material 23 that moves from the end section in the axial direction of the portion where the rack teeth are to be formed 16 is relatively small (when compared with when the length L.sub.18 in the axial direction of the circular rod section 18 is long). Therefore, by pressing the end surface in the axial direction of the intermediate material 23 with a constant pressure, it is possible to suppress the movement of the metal material of the intermediate material 23. The size of the pressure that the end surface in the axial direction of the intermediate material 23 is pressed is appropriately set based on the kind of the metal material of the intermediate material 23 and the dimensions of the component parts.

(32) On the other hand, when the length L.sub.16 in the axial direction of the portion where the rack teeth 16 are formed and the length L.sub.18 in the axial direction of the circular rod section 18 satisfy the relationship L.sub.18L.sub.16, the amount of metal material of the intermediate material 23 that moves from the end section in the axial direction of the portion where the rack teeth 16 are to be formed becomes large. When this kind of relationship is satisfied, as illustrated in FIGS. 9A and 9B, the upper half of the surface on the other side in the axial direction of the pressure punch 46 is taken to be an inclined surface 48 that is inclined toward the inside in the axial direction when going in an upward direction. Moreover, a separate inclined surface 49 that engages with the inclined surface 48 is formed on the surface on one side in the axial direction of the one movable die 41a. As a result, as ram 45 (see FIG. 1 to FIG. 4) moves downward, it is possible to increase the pressure by which the one movable die 41a presses the surface on one end in the axial direction of the intermediate material 23. The angle of inclination of the inclined surface 48 and the other inclined surface 49 are also appropriately determined based on the kind of the metal material of the intermediate material 23 and the dimensions of the component parts.

(33) When the lengths L.sub.16, L.sub.18 in the axial direction satisfy the relationship L.sub.16/4<L.sub.18<L.sub.16, the shape of the pressure punch 46, including whether to make the surface on the other side in the axial direction of the pressure punch 46 a stepped surface such as illustrated in FIGS. 7A, 7B, or an inclined surface 48 such as illustrated in FIGS. 9A, 9B, is appropriately regulated based on the metal material of the intermediate material 23 and the dimensions of the component parts.

(34) Moreover, in this example, in order to adjust the up-down position of the pressure punch 46 with respect to the ram 45, the pressure punch 46 is provided on the bottom surface of the ram 45 by way of a spacer 50. However, construction is possible in which instead of the spacer 50, the amount of displacement in the up-down direction of the ram 45 is maintained by providing an elastic member that contracts in the up-down direction when the one movable die 41a is strongly pressing the surface on one end in the axial direction of the intermediate material 23. Furthermore, the position in the axial direction of the pressure punch 46 with respect to the teeth-forming punch 32 can be adjusted by changing the thickness of a shim plate 51 that is sandwiched between the teeth-forming punch 32 and the pressure punch 46.

(35) In either case, as the teeth-forming punch 32 is lowered, the flat surface section 25 (see FIG. 20C) of the intermediate material 23 is plastically deformed to conform to the wave-shaped unevenness for molding that is provided on the processing surface of the teeth-forming punch 32, and the rack teeth 16 are formed. When doing this, the surface on the one end in the axial direction of the intermediate material 23 (surface on the right end in FIG. 4) is pressed by the one movable die 41a toward the inside in the axial direction, and the intermediate material 23 is strongly held in the axial direction between the pair of movable dies 41a, 41b, and pressed in the axial direction. After that, when the ram 45 is caused to move upward, the elastic force of the elastic member 42 causes the pair of movable dies 41a, 41b to move upward with the rack teeth 16 of the raw rack 33a in contact as is with the processing surface of the teeth-forming punch 32. In the process of raising the ram 45 to the top dead center, the raw rack 33a becomes separated from the teeth-forming punch 32, so the raw rack 33a is removed. The completed rack 11c is then obtained by performing a sizing process as in the conventional technology on the raw rack 33a that was processed in this way.

(36) With the manufacturing method and manufacturing device for a rack of this example, it is possible to maintain the tooth depth of the rack teeth 16 over the entire length in the axial direction. In other words, when plastically deforming the surface on one side in the radial direction of the intermediate material 23 and forming the rack teeth 16, the intermediate material 23 is pressed in the axial direction. Therefore, as the teeth-forming punch 32 presses, the metal material of the intermediate material 23 is kept from moving toward the outside in the radial direction on the end section in the axial direction of the portion where the rack teeth 16 are to be formed. As a result, as illustrated in FIG. 10 and FIG. 11B, it is possible to prevent the tooth depth of the rack teeth 16 on the end section in the axial direction of the rack teeth 16 from becoming small.

(37) More specifically, the rack of the present invention can be distinguished from the rack according to conventional technology by the following points. FIG. 11A and FIG. 11B illustrate the state in which rack teeth 16 have been formed on an intermediate material 23 where only one dummy tooth 39a is provided by using the conventional technology and the present invention. When the rack teeth 16 are formed without pressing the intermediate material 23 in the axial direction, then as illustrated in FIG. 11A, at the end section in the axial direction of the rack teeth 16, the tooth depth of the rack teeth 16 becomes smaller going toward the outside in the axial direction; the end section of the portion that is adjacent in the axial direction with the portion where the rack teeth 16 are formed does not come in contact with the teeth-forming punch 32; and, in this end section, the fiber flow that is a flow of the fibrous structure of the metal material of the intermediate material 23 is formed so as to rise monotonously as separated from the portion where the rack teeth 16 are to be formed. On the other hand, with the present invention, when the rack teeth 16 are formed by pressing the intermediate material 23 in the axial direction, even when only one dummy tooth 29a is provided or the dummy tooth is omitted, the tooth depth of the rack teeth 16 is maintained at the same size as the tooth depth in the middle section in the axial direction even at the end section in the axial direction of the rack teeth 16. Moreover, the end section of the portion that is adjacent in the axial direction to the portion where the rack teeth 16 are formed comes in contact with the teeth-forming punch 32 and has sufficient thickness, and the fiber flow in this portion has a slight upward convex shape. Having this kind of structure, in the case of the present invention, when the completed rack 11c is assembled in the steering gear 5, it is possible to maintain proper tooth engagement on the end section in the axial direction of the rack teeth 16 between the rack teeth 16 and the pinion that is formed around outer-circumferential surface of the input shaft 6, and even when a screw hole 57 for screwing a male screw of a ball joint into is formed nearby, for example, it is easy to maintain the strength of the completed rack 11c in this portion.

(38) Furthermore, in the case of this example, even when dummy teeth 39, 39a (see FIG. 23) are not provided or are temporarily provided in the portion adjacent in the axial direction to the rack teeth 16, it is possible to keep the number to a minimum. Therefore, it is possible to prevent the length in the axial direction of the processing tools (punch and dies) from increasing, and in the case in which a screw hole 57 (see FIG. 23) is formed in the end section of the rack 11c, it is possible to prevent the thickness of the end section from becoming small.

(39) [Second Example]

(40) FIGS. 12A to 13 illustrate a second example of an embodiment of the present invention. This example is an example of manufacturing a rack 11 (see FIG. 15) that is to be assembled in a dual-pinion type electric-powered power-steering apparatus as illustrated in FIG. 15. Therefore, as illustrated in FIG. 12A to FIG. 12D, rack teeth 16 are formed in one half in the axial direction of a raw material 19a in the same way as in the first example. However, in this example, a sizing process is performed on the raw rack 33 by pressing with a sizing punch 52 with the raw rack 33 set as is in the die 27.

(41) In this example, by independently providing rack teeth 16, 16a having different torsion angles at two locations in the axial direction of the rack 11, the angle between the center axis of the first input shaft 6 and the center axis of the rack 11, and the angle between the center axis of the second input shaft 12 and the center axis of the rack 11 are made to be different (see FIG. 13). Therefore, different rack teeth 16a having a different torsion angle than the rack teeth 16 are formed on the other half section in the axial direction of the obtained intermediate material 53. These different rack teeth 16a are also basically formed by the same procedure as the rack teeth 16. However, in this example, the shape of the tip-end surface of the punch 22a that presses the portion where the rack teeth 16a are to be formed is a curved surface having a radius of curvature R.sub.22 that is larger than the radius of curvature r.sub.21 of the concave groove section 21 that is provided in the receiving mold 20. Moreover, the finishing die that is used in the process (FIG. 12F) of forming rack teeth 16a by pressing the intermediate material 52 with the punch 32a for forming teeth, and the process (FIG. 12G) of using a sizing punch 52a to perform a sizing process on the second intermediate material that was obtained is the same as that used between the process of forming the rack teeth 16a and the sizing process. Therefore, it is possible to keep down an increase in management cost for managing the finishing die, and thus it is possible to reduce the manufacturing cost of the rack 11. Furthermore, both inside surfaces in the width direction of the finishing die are flat surfaces that are parallel with each other. As a result, both outside surfaces in the width direction of the portion of the raw rack 58 where the rack teeth 16a are formed are flat surface sections 55 that are parallel with each other, and the connecting sections between the flat surface sections 55 and the portion where the rack teeth 16a are formed (portion surrounded by the chain line in FIG. 13) are a pair of curved surface sections 56 having a comparatively small radius of curvature. The rack 11 is obtained by performing finishing such as heat treatment and polishing on this kind of raw rack 58.

(42) Table 1 is for the case of manufacturing a rack 11 that is to be assembled in a dual-pinion type electric-powered power-steering apparatus, and illustrates examples of dimensions of each of the parts used when forming rack teeth 16a that engage with a pinion that is formed around the outer-circumferential surface of the input shaft 6.

(43) TABLE-US-00001 TABLE 1 Pressure punch 22a Punch 32a for forming teeth Sizing punch 52a Rack 11 and Receiving mold 20 and finishing die and finishing die Width W of flat surface sections Radius of curvature Pressure angle of rack teeth 16a: 10 degrees Pressure angle of rack teeth 16a: 20 degrees 55: 29[mm] R.sub.22: 31.5[mm] Radius of curvature of the bottom section Radius of curvature of the bottom section Radius of curvature R.sub.17 of the rear Radius of curvature of the finishing die: 13.5[mm] of the finishing die: 13.5[mm] surface portion: 13.5[mm] r.sub.21: 13.5[mm] Width between inside surfaces: 29[mm] Width between inside surfaces: 29[mm] Width W of flat surface sections Radius of curvature Pressure angle of rack teeth 16a: 10 degrees Pressure angle of rack teeth 16a: 20 degrees 55: 28[mm] R.sub.22: 31.5[mm] Radius of curvature of the bottom section Radius of curvature of the bottom section Radius of curvature R.sub.17 of the rear Radius of curvature of the finishing die: 15[mm] of the finishing die: 15[mm] surface portion: 15[mm] r.sub.21: 15[mm] Width between inside surfaces: 28[mm] Width between inside surfaces: 28[mm] Width W of flat surface sections Radius of curvature Pressure angle of rack teeth 16a: 10 degrees Pressure angle of rack teeth 16a: 20 degrees 55: 26.5[mm] R.sub.22: 31.5[mm] Radius of curvature of the bottom section Radius of curvature of the bottom section Radius of curvature R.sub.17 of the rear Radius of curvature of the finishing die: 19[mm] of the finishing die: 19[mm] surface portion: 19[mm] r.sub.21: 19[mm] Width between inside surfaces: 26.5[mm] Width between inside surfaces: 26.5[mm]

(44) In the examples in Table 1, the radius of curvature R.sub.22 of the pressure punch 22a is 31.5 mm in all examples. By using the same pressure punch 22a in this way regardless of the shape of the completed rack teeth 16a, it is possible to reduce the manufacturing cost of the rack 11. However, the radius of curvature R.sub.22 can be selected from the range 10 mm to 250 mm to correspond to the shape of the rack teeth 16a to be formed. Moreover, by making the radius of curvature r.sub.21 of the receiving mold 20 the same as the radius of curvature of the bottom section of the finishing die, sliding of the metal material of the raw material 19 is kept small, and wear of the finishing die is suppressed. Furthermore, in this example, when forming the rack teeth 16, 16a, the rod section 15 is pressed by the pressure punch 46 and the movable die 41a illustrated in FIG. 9 in the axial direction within the range of 3.5 mm or less (0 mm to 3.5 mm). The construction and functions of the other parts are the same as in the first example of an embodiment.

EXPLANATION OF REFERENCE NUMBERS

(45) 1 Steering wheel 2 Steering shaft 3 Universal joint 4 Intermediate shaft 5 Steering gear 6 Input shaft 7 Tie rod 8 Steering column 9 Gear housing 10, 10a Electric motor 11, 11a to 11c Rack 12 Second input shaft 13 Housing 14 Reduction gear 15, 15a Rod section 16, 16a Rack teeth 17 Rear surface portion 18 Circular rod section 19 Raw material 20 Receiving mold 21 Concave groove section 22 Pressure punch 23 Intermediate material 24 Partial cylindrical surface section 25 Flat surface section 26 Curved surface section 27, 27a Die 28 Support hole 29 Bottom section 30 Inside surface 31 Inclined guide surface 32 Teeth-forming punch 33, 33a Raw rack 34 Flat flank surface 35 Sizing die 36 Uneven sizing surface 37 Stamp 38 Concave pressing groove 39, 39a Dummy teeth (excess material section) 40 Stationary die 41a, 41b Movable die 42 Elastic member 43 Support base 44 Concave section 45 Ram 46, 46 Pressure punch 47 Stepped surface 48 Inclined surface 49 Inclined surface 50 Spacer 51 Shim plate 52, 52a Sizing punch 53 Intermediate material 54 Second intermediate material 55 Flat surface section 56 Curved surface section 57 Screw hole 58 Raw rack