JP7629316B2 - How to polish the rack bar - Google Patents

Description

The present invention relates to a method for polishing a rack bar.

The rack bar is used as a component of a rack-and-pinion steering device mounted on, for example, an automobile, and has a rack portion formed with a rack having multiple teeth that mesh with the pinion.

In the manufacture of rack bars used in such rack-and-pinion steering devices, the rack is usually formed by pressing a die against a cylindrical or columnar steel material, and then heat treatment (hardening) is performed to increase the strength of the rack (for example, Patent Document 1).

JP 2020-66020 A

However, in the method described in Patent Document 1, when the rack is heat-treated (quenched), an oxide film is formed on the surface of the rack bar, which causes the surface of the rack bar to become uneven.

In a steering device, the rack bar is usually supported by bearing members such as a support yoke that supports the opposite side in the thickness direction of the rack section (described later) and rack bushes that support both ends. When the pinion rotates by operating the steering shaft connected to the pinion, the rack bar moves along its axial direction. At this time, the rack bar moves while sliding against the bearing members (hereinafter, the surface of the rack bar that is supported by the bearing members is referred to as the "sliding surface").

Due to the drive mechanism of the steering device as described above, the bearing member may be worn due to sliding contact with the sliding surface of the rack bar. Therefore, it is preferable that the friction resistance between the sliding surface of the rack bar and the bearing member is small.

In view of the above circumstances, the present invention aims to provide a method for polishing a rack bar that can reduce frictional resistance with bearing members in a steering device.

In the method for polishing a rack bar according to the present invention,
the rack bar is slidable relative to a bearing member that supports the rack bar when the rack bar is incorporated into the steering device, and includes a shaft member, the shaft member having a rack portion on which a rack that meshes with a pinion is formed, and a sliding contact surface that is in sliding contact with the bearing member,
The sliding surface is polished with an abrasive so that the polishing direction is along the axial direction of the rack bar.

With this configuration, due to the grinding direction as described above, grinding marks are formed on the surface of the sliding surface along the axial direction of the rack bar. In other words, the direction of the grinding marks is along the direction of movement of the rack bar. This makes it possible to reduce frictional resistance between the sliding surface of the rack bar and the bearing member.

In addition, in the method for grinding the rack bar according to the present invention, it is preferable to grind the sliding surface located on the opposite side facing the thickness direction of the rack portion.

In the steering device, the rack bar is supported by a bearing member called a support yoke at the sliding surface located on the opposite side in the thickness direction of the rack section. This support yoke is usually biased toward the rack bar by a spring member or the like. For this reason, frictional resistance is relatively likely to occur between the support yoke and the sliding surface. In contrast, with the above-mentioned polishing method, the direction of the polishing marks formed on the surface of the sliding surface that slides against the support yoke is along the direction of movement of the rack bar, so frictional resistance can be reduced more effectively.

In addition, in the method for polishing a rack bar according to the present invention,
As the abrasive material, a rotating grindstone that is disk-shaped and can rotate in a circumferential direction and has abrasive grains on the rotating outer peripheral surface is used,
It is preferable to rotate the rotating grindstone so that the outer peripheral surface is aligned along the axial direction of the rack bar, and to grind the sliding surface with the outer peripheral surface while moving at least one of the rotating grindstone or the rack bar along the axial direction.

According to this configuration, the outer peripheral surface of the grinding wheel having abrasive grains (the grinding surface of the grinding wheel) is rotated along the axial direction of the rack bar, and the sliding surface is polished with the outer peripheral surface while at least one of the grinding wheel or the rack bar is moved along the axial direction. This means that both the direction of rotation (the direction in which the grinding wheel rotates and polishes) of the outer peripheral surface of the grinding wheel and the direction of movement (the direction in which the grinding wheel moves and polishes) are along the direction of movement of the rack bar. Therefore, grinding marks are more likely to be formed on the surface of the sliding surface along the axial direction of the rack bar. This makes it possible to further reduce the frictional resistance between the sliding surface of the rack bar and the bearing member.

As described above, the present invention provides a method for polishing a rack bar that can reduce frictional resistance with bearing members in a steering device.

FIG. 1 is a schematic diagram showing a steering device. FIG. 2 is a schematic side view of the rack bar. FIG. 3 is a cross-sectional view taken along line III-III in FIG. FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. FIG. 5 is a diagram showing a surface requiring polishing located on the opposite side in the thickness direction of the rack portion. FIG. 6 is a schematic diagram showing the outer circumferential surface of a grinding wheel. FIG. 7 is a diagram showing the positional relationship between the rack bar and the abrasive in the rack bar abrasive grinding method of one embodiment, as viewed from a direction perpendicular to the axial direction of the rack bar. FIG. 8 is a view of FIG. 7 as viewed from one axial side of the rack bar.

Below, a method for polishing a rack bar according to one embodiment of the present invention will be described with reference to the drawings. Note that below, the axial direction of the rack bar may be referred to as the axial direction X (longitudinal direction). The horizontal direction perpendicular to the axial direction X may be referred to as the width direction Y (first short direction). The vertical direction perpendicular to each of the axial direction X and the width direction Y may be referred to as the thickness direction Z (second short direction). The cross section (Y-Z plane) of the rack bar perpendicular to the axial direction X may be referred to as the transverse section.

First, with reference to Figures 1 to 4, the rack bar 1 polished by the polishing method of this embodiment will be described while showing a steering device 100 that uses the rack bar 1.

As shown in FIG. 1, the steering device 100 is a rack-and-pinion type steering device 100. The steering device 100 includes a pinion 2 connected to a steering shaft of a vehicle or the like, a rack bar 1 on which a rack 11a that meshes with the pinion 2 is formed, and a bearing member 3 that supports the rack bar 1.

The rack bar 1 has a shaft material 10. The shaft material 10 has a rack portion 11 in which a rack 11a is formed, and a sliding surface 13 on the surface of the shaft material 10 that slides against the bearing member 3.

The rack bar 1 is manufactured, for example, from solid (cylindrical) steel material or hollow (cylindrical) steel material. Examples of such steel material include carbon steel material for machine structures as specified in JIS 4051:2016. Among these, S45C steel material is preferred.

As shown in Figures 1 and 2, the rack 11a is composed of multiple teeth 111 whose tooth tips 113 are formed to extend along the width direction Y (the direction perpendicular to the paper surface in Figures 1 and 2). The teeth 111 are arranged, for example, at equal intervals along the axial direction X. The teeth 111 also form multiple grooves 112 between them into which the teeth of the pinion 2 can be inserted and removed. With this configuration, the rack bar 1 in the steering device 100 moves along the axial direction X as the pinion 2 meshes with the rack 11a rotates. At this time, the rack bar 1 moves so as to slide against the bearing member 3, and the sliding surface 13 comes into sliding contact with the bearing member 3.

The rack bar 1 has, as sliding surfaces 13, a tooth back sliding surface 13a located on the opposite side facing the thickness direction Z of the rack portion 11, and both end sliding surfaces 13b located at both end portions where the rack 11a is not yet formed. As shown in Figures 3 and 4, in cross section, the shape of the surface 131a of the tooth back sliding surface 13a is, for example, an arc shape. Also, in cross section, the shape of the surface 131b of both end sliding surfaces 13b is, for example, a circle shape.

In the steering device 100, the rack bar 1 is supported by a bearing member 31 called a support yoke at the tooth back sliding surface 13a, and is supported by bearing members 32 called rack bushes at both end sliding surfaces 13b. The bearing members 3 are usually made of a resin material.

In the steering device 100, the support yoke 31 is formed in a shape that covers the surface 131a of the tooth back sliding surface 13a from below. The support yoke 31 has a support surface on which the tooth back sliding surface 13a is placed, and supports the rack bar 1 with the support surface and the surface 131a in contact with each other.

In addition, in the steering device 100, the rack bush 32 is formed in a shape that covers the surface 131b of the sliding surfaces 13b at both ends in the circumferential direction CY1 of the rack bar 1. The rack bush 32 has an inner circumferential surface that covers the sliding surfaces 13b at both ends in the circumferential direction CY1, and supports the rack bar 1 with the inner circumferential surface and the surface 131b in contact with each other.

3 and 4, in cross section, the diameter of the arc described by the surface 131a of the tooth back contact surface 13a and the diameter of the circle described by the surfaces 131b of the both end contact surfaces 13b are substantially the same. If these diameters are designated as R1, the diameter R1 is, for example, 22 mm to 37 mm. The diameter R1 can be calculated by measuring the circumference at any five points on the both end contact surfaces 13b to calculate the average circumference C1, and then using the following formula (1). The value of pi is 3.14.
R1=C1/Pi...(1)

The tooth back contact surface 13a has a central angle θ1 of, for example, 170° in the cross section (YZ plane) of the arc described by the surface 131a. The central angle θ1 can be calculated by measuring the circumference at any five points on the tooth back contact surface 13a to determine an average circumference C2, and then using the following formula (2).
θ1=C2/C1×360°...(2)

The steering device 100 is configured such that the rack bar 1 moves along the axial direction X while sliding against the bearing member 3, and as a result, the sliding surface 13 comes into sliding contact with the mounting surface and the inner peripheral surface of the bearing member 3.

Next, the method for polishing the rack bar 1 of this embodiment will be described with reference to Figures 5 to 8.

In the polishing method of this embodiment, an untreated rack bar 1a (a workpiece after heat treatment (hardening)) having an oxide film formed on its surface through a heat treatment such as high-frequency induction heating is polished with an abrasive 4 to obtain a treated rack bar 1b. At this time, as shown in Fig. 7, the sliding surface 13 is polished so that the polishing direction D is along the axial direction X (the polishing direction D is approximately parallel to the axial direction X).
By such a grinding direction D, grinding marks are formed on the surface of the sliding surface 13 so as to extend along the axial direction X. That is, the grinding marks are formed along the direction in which the sliding surface 13 and the bearing member 3 slide against each other. Therefore, the frictional resistance between the surface of the sliding surface 13 and the mounting surface or the inner peripheral surface of the bearing member 3 is suppressed. More specifically, for example, compared to a rack bar ground along the width direction Y, the frictional resistance between the rack bar 1 and the bearing member 3 in the axial direction X is suppressed. This also makes it possible to suppress wear of the bearing member 3. Note that the grinding direction D includes directions inclined by ±5° from the axial direction X to the width direction Y or the thickness direction Z.

The polishing method of this embodiment may include a first polishing process for polishing the surface 131a of the tooth back contact surface 13a, and a second polishing process for polishing the surface 131b of the both end contact surfaces 13b. The first polishing process and the second polishing process may be performed simultaneously or separately. When performed separately, the first polishing process and the second polishing process may be performed in any order, and the order may be changed as appropriate. Also, only one of the first polishing process and the second polishing process may be performed.

In the polishing method of this embodiment, it is preferable to polish the tooth back contact surface 13a located on the opposite side facing the thickness direction Z of the rack portion 11.
As described above, in the steering device 100, the rack bar 1 is supported by a bearing member 31 called a support yoke 31 at a sliding surface (tooth back sliding surface 13a) located on the opposite side of the rack portion 11 facing the thickness direction Z. The support yoke 31 is usually biased toward the rack bar 1 by a spring member or the like. The tooth back sliding surface 13a is a sliding surface located on the opposite side of the rack portion 11 on which the rack 11a meshing with the pinion 2 is formed, and is therefore also a location to which stress is likely to be directly applied when meshing with the pinion. For this reason, frictional resistance is relatively likely to occur between the support yoke 31 and the sliding surface 13a. In contrast, according to the rack bar 1 obtained by the above-mentioned polishing method, the polishing marks formed on the surface of the sliding surface 13a that slides against the support yoke 31 are formed along the moving direction of the rack bar 1, so that the frictional resistance can be reduced more effectively.

In the polishing method of this embodiment, only the surface 131a of the tooth back sliding surface 13a that slides against the bearing member 31 may be polished. Here, depending on the steering device 100, a part of the surface 131a of the tooth back sliding surface 13a (particularly both end faces 133a, 133a (see FIG. 3) that intersect with the width direction Y) may not be in contact with the support yoke 31. In other words, in some steering devices 100, sliding contact with the support yoke 31 may occur concentrated on a part of the surface 131a. For this reason, in order to polish the tooth back sliding surface 13a more efficiently, only a predetermined part of the surface 131a may be polished. In other words, a surface that needs to be polished (surface that needs to be polished) may be set on the surface 131a of the tooth back sliding surface 13a, and only the surface that needs to be polished may be polished.

The surface that needs to be ground may be set, for example, at a location (symbol 132a) as shown in FIG. 5. More specifically, in the cross section (Y-Z plane), an X-axis overlapping a line segment that defines the diameter R1 of the surface 131a (more specifically, a line segment that is parallel to the tip 113 of the tooth portion 111 of the rack 11a and defines the diameter R1 of the surface 131a), a Y-axis perpendicular to the X-axis so as to bisect the rack bar 1, and an origin O of the X-axis and Y-axis are defined, the rack portion 11 is disposed on the negative side of the Y-axis, and the surface that needs to be ground 132a is set at a portion located on the positive side of the Y-axis. Then, the surface that needs to be ground 132a is set at a portion located on the positive side of the Y-axis via a pair of straight lines L1 that pass through the origin O and are inclined at ±65° with respect to the Y-axis. The surface that needs to be ground 132a is also set at a portion between the pair of straight lines L1 and a pair of straight lines L2 that pass through the origin O and are inclined at ±25° with respect to the Y-axis.
That is, the contact surface between the surface 131a of the tooth back sliding surface 13a and the support yoke 31 is set, for example, to a portion of the surface 131a corresponding to 25 to 65° of the central angle θ1 and a portion of the surface 131a corresponding to 115 to 155° of the central angle θ1, with the positive direction of the X-axis being 0° (FIG. 5). Also, as the wear of the support yoke 31 progresses, the contact surface becomes, for example, a portion of the surface 131a corresponding to 25 to 155° of the central angle θ1, with the positive side of the X-axis being 0°. Therefore, by setting the surface 132a to be ground to a portion corresponding to 25 to 155° in consideration of the wear of the bearing member as described above, the rack bar 1 can be made capable of sufficiently reducing the wear resistance.

As shown in FIG. 7, in the polishing method of this embodiment, the surface 131a of the untreated rack bar 1a fixed by a fixing member (not shown) is polished by an abrasive material 4 configured to be movable along the axial direction X of the untreated rack bar 1a.

The untreated rack bar 1a is fixed, for example, so that the surface 131a faces upward in the vertical direction (thickness direction Z) and the tooth tips 113 of the teeth 111 of the rack 11a face downward. The abrasive 4 is configured to be movable along the axial direction X of the untreated rack bar 1a, for example, in a state of contact with the surface 131a. Furthermore, the abrasive 4 may be configured to be movable along the vertical direction (thickness direction Z) so that it can be in both a contact state and a non-contact state with the surface 131a. Alternatively, the abrasive 4 may be fixed and the untreated rack bar 1a may be moved along the axial direction X. In order to reduce the occupied area of the polishing device, it is preferable to move the abrasive 4 rather than moving the long untreated rack bar 1a.

As shown in Figures 6 to 8, the abrasive material 4 in this embodiment is a rotating grindstone 4. The rotating grindstone 4 is formed in a disk shape and is rotatable around a rotation axis CY2. The rotating grindstone 4 has abrasive grains on its rotating outer peripheral surface 41. The rotating grindstone 4 is arranged so that its rotation direction B (the movement direction of the outer peripheral surface 41) is aligned with the axial direction X of the untreated rack bar 1a.

The rotary grindstone 4 is composed of the abrasive grains and a resin serving as a binder for binding the abrasive grains.
From the viewpoint of cost, alumina-based abrasive grains having a relatively low hardness, such as white alumina (WA), are used as the abrasive grains. A ceramic-based binder, such as vitrified, is used as the binder. In addition, a resin-based binder may be used as the binder, taking into consideration the remaining parts during polishing of the curved surfaces 131a and 131b.

The grit size of the abrasive grain is #60 to #800, preferably #120 to #220. A grindstone with such a grit size can sufficiently remove the oxide film that occurs during hardening before the grinding process when grinding the untreated rack bar 1a, which requires a grinding allowance (for example, 10 to 30 μm), and thereby can provide a surface roughness sufficient to reduce the above-mentioned wear resistance. In addition, when the rack bar 1 is assembled into the steering device 100, a lubricant such as grease is usually applied to the surfaces 131a and 131b to improve lubrication with the bearing member 3. For this reason, a grindstone with the above grit size can provide a surface roughness that allows the lubricant to be appropriately retained on the surfaces 131a and 131b.

The outer diameter of the rotary grinding wheel 4 is, for example, 200 mm or more and 360 mm or less. The contact pressure of the rotating outer peripheral surface 41 against the sliding surface 13 during the polishing process of the rotary grinding wheel 4 is, for example, 0.1 MPa or more and 1 MPa or less. The peripheral speed of the rotary grinding wheel 4 during the polishing process is, for example, 25 m/s or more and 30 m/s or less.

As shown in FIG. 6, the rotating grindstone 4 has, for example, an outer peripheral surface 41 formed in a concave shape corresponding to the shape (arcuate shape) of the surface 131a of the tooth back contact surface 13a. More specifically, for example, in the grindstone cross section overlapping with the rotation axis CY2, the outer peripheral surface 41 has an arcuate portion 411 having a diameter the same length as the diameter R1 of the surface 131a.

In the polishing method of this embodiment, the rotary grindstone 4 is rotated so that the outer circumferential surface 41 of the rotary grindstone 4 as described above is aligned along the axial direction X of the untreated rack bar 1a, and the sliding surface 13 is polished by the outer circumferential surface 41 while moving the rotary grindstone 4 or the untreated rack bar 1a (preferably the rotary grindstone 4) along the axial direction X. That is, polishing is performed such that both the rotation direction of the rotary grindstone 4 and the movement direction of the outer circumferential surface 41 of the rotary grindstone 4 are aligned along the axial direction X of the untreated rack bar 1a.
This makes it easier to form polishing marks on the surface of the sliding surface 13 along the axial direction X of the untreated rack bar 1a, thereby further reducing the frictional resistance between the sliding surface 13 of the untreated rack bar 1a and the bearing member.

In the first grinding process, when a grindstone 4 having a concave outer peripheral surface 41 is used, it is preferable to perform a first one-sided grinding process for grinding a part of the surface 131a, and then a second one-sided grinding process for grinding another part of the surface 131a. More specifically, as shown in FIG. 8, when the positional relationship between the untreated rack bar 1a and the grindstone 4 is viewed from one side of the axial direction X of the untreated rack bar 1a, the rack bar 1 is rotated in one direction (for example, counterclockwise) around the axial direction X so that the inclination angle θ2 of the tooth tip 113 of the tooth portion 111 with respect to the rotation axis CY2 of the grindstone 4 is 45±5°, and fixed in that state (state of FIG. 8(b)), and the grindstone 4 is moved downward to contact the untreated rack bar 1a, and the grindstone 4 is rotated and moved along the axial direction X of the untreated rack bar 1a, thereby performing the first one-sided grinding process. After the first single-sided polishing process is completed, the grindstone 4 is moved upward so that the surface 131a and the grindstone 4 are not in contact with each other (e.g., the state shown in FIG. 8(a)). Next, the rack bar 1 is rotated in the other direction around the axis (e.g., clockwise) so that the inclination angle θ2 is 45±5°, and fixed in that state (e.g., the state shown in FIG. 8(c)). The grindstone 4 is moved downward to contact the untreated rack bar 1a, and the grindstone 4 is moved along the axial direction X of the untreated rack bar 1a to perform the second single-sided polishing process.

The areas polished by the first single-sided polishing process and the areas polished by the second single-sided polishing process may overlap in part. This allows the polishing of the surface 132a that requires polishing to be focused. In other words, the areas that are likely to come into sliding contact with the surface 131a and the support yoke 31 can be polished relatively focused and efficiently.

The moving speed V of the grindstone 4 is set to, for example, 30 to 50 mm/sec (including a 10% error). More specifically, first, the amount of grinding (removal amount) is determined for each portion of the surface 131a corresponding to each tooth portion 111 of the untreated rack bar 1a. Then, for example, the moving speed V is set to 50±5 mm/sec in the portion where the removal amount is 10 to 20 μm, and for example, the moving speed V is set to 30±3 mm/sec in the portion where the removal amount is 20 to 30 μm.

In the second polishing process, the entire surface 131b of the sliding contact surfaces 13b at both ends may be polished. On the other hand, in order to perform polishing more efficiently, it is preferable to focus on polishing the portion of the surface 131b where sliding contact with the inner peripheral surface of the rack bush 32 in the steering device may occur intensively. In other words, it is preferable to set a polishing-required surface 132b on the surface 131b that needs to be polished.

The surface that needs to be polished 132b is set at a position corresponding to the position of the surface that needs to be polished 132a of the tooth back sliding surface 13a in the circumferential direction CY1 of the rack bar 1. This is efficient because the surface that needs to be polished 132b can be polished following the polishing of the surface that needs to be polished 132a without changing the fixed state of the rack bar 1. Also, in the steering device 100, the load of the rack bar 1 tends to cause sliding contact with the sliding surfaces 13b at both ends to be concentrated at the bottom of the inner peripheral surface of the rack bush 32. Therefore, by setting the position of the surface that needs to be polished 132b as described above, it is possible to focus on polishing the parts that are likely to cause sliding contact with the rack bush 32.

When carrying out the polishing method of this embodiment, the untreated rack bar 1a has a predetermined polishing allowance. The polishing allowance is, for example, 10 to 30 μm.

In the polishing method of this embodiment, the movement direction D of the grinding wheel 4 relative to the rotation direction B of the grinding wheel 4 may be the same direction as the axial direction X, or may be opposite directions.

In the polishing method of this embodiment, a coolant liquid may be used to suppress heat generation during polishing. The coolant liquid may be a water-soluble coolant liquid or an oil-based coolant liquid, but a water-soluble coolant liquid may be used from the viewpoint of cost.

According to the polishing method of this embodiment, the arithmetic mean roughness Ra of the sliding surface 13 in the axial direction X after polishing is 0.1 μm (measurement method: needle contact type measurement, evaluation length 4 mm, standard JIS 2001), and the arithmetic mean roughness Ra in the circumferential direction CY1 is 0.4 μm (measurement method: needle contact type measurement, evaluation length 4 mm, standard JIS 2001). That is, the rack bar polished by the polishing method of this embodiment has different roughness Ra in the axial direction X and the circumferential direction CY1. Specifically, the roughness Ra in the axial direction X is small, and the roughness Ra in the circumferential direction CY1 is large.
Here, the bearing member 3 (particularly the rack bush 32) also has a function of preventing rotation of the rack bar 1 in the circumferential direction CY1 in the steering device 100. Therefore, according to the rack bar 1 having the sliding surface 13 with a relatively large arithmetic mean roughness Ra in the circumferential direction CY1, the above function of the bearing member 3 can be enhanced, and therefore, the rotation of the rack bar 1 in the circumferential direction CY1 in the steering device can be more effectively prevented.

As described above, one embodiment has been shown as an example, but the rack bar grinding method according to the present invention is not limited to the configuration of the above embodiment. Furthermore, the rack bar grinding method according to the present invention is not limited by the above-mentioned effects. The rack bar grinding method according to the present invention can be modified in various ways without departing from the gist of the present invention.

For example, in the above embodiment, a rotating grindstone 4 is used as the abrasive material, but this is not limited thereto, and the abrasive material 4 may be, for example, abrasive paper cloth or a buff abrasive material.

In addition, in the above embodiment, either the rack bar 1 or the abrasive material 4 is moved, but this is not limited thereto, and both the rack bar 1 and the abrasive material 4 may be moved.

1: Rack bar,
10: shaft material, 11: rack portion, 11a: rack, 111: tooth portion, 112: groove,
113: tooth tip, 13: sliding surface, 13a: tooth back sliding surface, 131a: surface,
132a: surface to be polished, 13b: sliding surfaces at both ends, 131b: surface, 132b: surface to be polished,
133a: end surface,
2: Pinion,
3: bearing member, 31: support yoke, 32: rack bush,
4: abrasive material, 41: outer peripheral surface, 411: arc-shaped portion,
X: axial direction, Y: width direction, Z: thickness direction, CY1: circumferential direction, CY2: rotation axis,
θ1: central angle, θ2: tilt angle,
B: rotation direction, D: polishing direction (direction of movement of abrasive material),
L1, L2: Straight line, R1: Diameter

Claims (2)

A method for grinding a rack bar, comprising the steps of:
the rack bar is slidable relative to a bearing member that supports the rack bar when the rack bar is incorporated into the steering device, and includes a shaft member, the shaft member having a rack portion on which a rack that meshes with a pinion is formed, and a sliding contact surface that is in sliding contact with the bearing member,
A method for polishing a rack bar, comprising the steps of: using a rotating grindstone that is disk-shaped, rotatable along a circumferential direction, and has abrasive grains on its rotating outer peripheral surface as it rotates; rotating the rotating grindstone so that the rotation direction of the outer peripheral surface is along the axial direction of the rack bar; and polishing the sliding surface with the outer peripheral surface while moving at least one of the rotating grindstone or the rack bar along the axial direction . The method for grinding a rack bar according to claim 1 , further comprising grinding the sliding surface located on an opposite side in the thickness direction to the rack portion.