SMOOTHING METHOD

Abstract

A smoothing method includes: a step of attaching a workpiece to a rotating shaft of a rotating mechanism; and a step of performing direct pressure blasting on the workpiece while rotating the workpiece about the rotating shaft as an axial center. In the blasting, injection media are injected in a direction orthogonal to the rotating shaft, the injection media include a core material made of an elastic body and abrasive grains provided on a surface of the core material, and hardness of the core material is lower than hardness of the abrasive grains.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2022-128820 filed with Japan Patent Office on Aug. 12, 2022 and claims the benefit of priority thereto. The entire contents of the Japanese patent application are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a smoothing method.

BACKGROUND

A technique for smoothing the surface of a component by blasting is known. For example, Japanese Patent Application Laid-Open Publication No. 2009-202307 discloses a polishing method in which polishing particles made of hard fine particles are injected onto the surface of a rolling element to form oil reservoirs over the entire surface, and polishing particles made of an elastic body containing abrasive grains are injected onto the surface of the rolling element at a predetermined angle to smooth the surface while maintaining the oil reservoirs.

SUMMARY

In the polishing method described in Japanese Patent Application Laid-Open Publication No. 2009-202307, in order to smooth the surface while maintaining the oil reservoirs, polishing particles (injection media) are injected at the predetermined angle. In this case, since the injection media collide with the surface at the above-described angle, the amount of grinding may vary depending on the position of the surface. Therefore, there is a possibility that the surface cannot be processed uniformly.

The present disclosure describes a smoothing method that can reduce non-uniformity of smoothing on a surface of a workpiece.

A smoothing method according to one aspect of the present disclosure includes: a step of attaching a workpiece to a rotating shaft of a rotating mechanism; and a step of performing direct pressure blasting on the workpiece while rotating the workpiece about the rotating shaft as an axial center. In the blasting, injection media are injected in a direction orthogonal to the rotating shaft. The injection media include a core material made of an elastic body and abrasive grains provided on a surface of the core material. Hardness of the core material is lower than hardness of the abrasive grains.

According to each aspect and each embodiment of the present disclosure, it is possible to reduce non-uniformity of smoothing on a surface of a workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a smoothing method according to an embodiment.

FIG. 2 is a diagram schematically showing a part of a processing device used in the smoothing method shown in FIG. 1.

FIG. 3 is a diagram schematically showing an example of an injection medium used in the blasting step shown in FIG. 1.

FIG. 4 is a diagram for explaining the blasting step shown in FIG. 1.

FIG. 5 is a diagram for explaining the smoothing mechanism.

FIG. 6A is a graph showing the relationship between the injection angle and the maximum height.

FIG. 6B is a graph showing the relationship between the injection angle and the depth of grinding.

FIG. 7A is a graph showing the relationship between the injection pressure and the maximum height.

FIG. 7B is a graph showing the relationship between the injection pressure and the depth of grinding.

FIG. 8A is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.01 MPa.

FIG. 8B is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.05 MPa.

FIG. 8C is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.10 MPa.

FIG. 8D is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.20 MPa.

FIG. 9A is a graph showing the relationship between the injection distance and the maximum height.

FIG. 9B is a graph showing the relationship between the injection distance and the depth of grinding.

FIG. 10A is a graph showing the relationship between the rotational frequency of the workpiece and the maximum height.

FIG. 10B is a graph showing the relationship between the rotational frequency of the workpiece and the depth of grinding.

FIG. 11A is a graph showing the relationship between the abrasive grain content rate in the injection media and the maximum height.

FIG. 11B is a graph showing the relationship between the abrasive grain content rate in the injection media and the depth of grinding.

FIG. 12A is a graph showing the relationship between the particle size distribution of the injection media and the maximum height.

FIG. 12B is a graph showing the relationship between the particle size distribution of the injection media and the depth of grinding.

FIG. 13 is a graph showing a residual stress value of a workpiece subjected to peening and a residual stress value of a workpiece subjected to smoothing after the peening.

DETAILED DESCRIPTION

Outline of Embodiments of the Present Disclosure

First, an outline of embodiments of the present disclosure will be described.

(Clause 1) A smoothing method according to one aspect of the present disclosure includes: a step of attaching a workpiece to a rotating shaft of a rotating mechanism; and a step of performing direct pressure blasting on the workpiece while rotating the workpiece about the rotating shaft as an axial center. In the blasting, injection media are injected in a direction orthogonal to the rotating shaft. The injection media include a core material made of an elastic body and abrasive grains provided on a surface of the core material. Hardness of the core material is lower than hardness of the abrasive grains.

In this smoothing method, the direct pressure blasting is performed on the workpiece. Therefore, since the injection media move toward the workpiece without spreading from the nozzle, the density of the injection media becomes substantially constant within a region where the injection media are injected on the surface of the workpiece. In the blasting, since the injection media are injected in a direction orthogonal to the rotating shaft of the rotating mechanism to which the workpiece is attached, the distance between the tip end of the nozzle from which the injection media are injected and the surface of the workpiece is substantially constant. This makes it possible to reduce non-uniformity in the amount of grinding the workpiece. As a result, it is possible to reduce non-uniformity of the smoothing on the surface of the workpiece.

(Clause 2) In the smoothing method according to Clause 1, an injection pressure of the injection media may be set to 0.01 MPa or more and 0.10 MPa or less in the step of performing the blasting. As the injection pressure of the injection media increases, the amount of grinding increases and the airflow in the vicinity of the surface of the workpiece is liable to be disturbed. Due to the turbulence of the airflow, the injection amount of the injection media per unit area can vary depending on the position of the surface of the workpiece. When the injection pressure of the injection media is too low, the surface of the workpiece cannot be sufficiently ground. In contrast, when the injection pressure of the injection media is within the above-described range, it is possible to suppress the turbulence of the airflow in the vicinity of the surface of the workpiece without excessively grinding the surface of the workpiece. Therefore, it is possible to further reduce the non-uniformity of the smoothing on the surface of the workpiece.

(Clause 3) In the smoothing method according to Clause 1 or 2, an injection distance of the injection media may be set to 50 mm or more and 100 mm or less in the step of performing the blasting. As the injection distance of the injection media becomes shorter, the amount of grinding tends to increase. When the injection distance of the injection media is too long, the surface of the workpiece cannot be sufficiently ground. In contrast, when the injection distance of the injection media is within the above-described range, the surface roughness of the workpiece can be reduced without excessively grinding the surface of the workpiece.

(Clause 4) In the smoothing method according to any one of Clauses 1 to 3, a rotational frequency of the workpiece may be set to 30 revolutions per minute or less in the step of performing the blasting. As the rotational frequency of the workpiece increases, the relative velocity between the injection media and the workpiece also increases. Therefore, a stronger frictional force is generated on the surface of the workpiece, and the amount of grinding tends to increase. In contrast, when the rotational frequency of the workpiece is within the above-described range, the surface roughness of the workpiece can be reduced without excessively grinding the surface of the workpiece.

(Clause 5) In the smoothing method according to any one of Clauses 1 to 4, a content rate of the abrasive grains in the injection media may be 15% by mass or more and 26% by mass or less. As the content rate of the abrasive grains in the injection media increases, the number of times that the abrasive grains come into contact with the surface of the workpiece increases, so that the amount of grinding tends to increase. When the content rate of the abrasive grains in the injection media is too low, the surface of the workpiece cannot be sufficiently ground. In contrast, when the content rate of the abrasive grains in the injection media is within the above-described range, the surface roughness of the workpiece can be reduced without excessively grinding the surface of the workpiece.

(Clause 6) In the smoothing method according to any one of Clauses 1 to 5, the injection media may have a particle size distribution of 125 μm or more and 600 μm or less. When the particle size of the injection media is large, there is a high possibility that the injection media do not reach a part of the surface of the workpiece having a complicated shape. On the other hand, when the particle size of the injection media is small, the injection media can contact the entire surface of the workpiece even if the workpiece has a complicated shape. Therefore, as the particle size of the injection media is smaller, the amount of grinding tends to increase. In contrast, when the particle size of the injection media is within the above-described range, it is possible to enhance the possibility that the entire surface of the workpiece is processed without excessively grinding the surface of the workpiece.

(Clause 7) In the smoothing method according to any one of Clauses 1 to 6, a first period during which the workpiece is rotated in a first rotation direction about the rotating shaft as an axial center and a second period during which the workpiece is rotated in a second rotation direction opposite to the first rotation direction may be alternately repeated in the step of performing the blasting. When the injection media slide on the surface of the workpiece only in one direction, there is a possibility that the surface of the workpiece cannot be processed uniformly. In contrast, since the injection media slide not only in one direction but also in the opposite direction on the surface of the workpiece, it is possible to further reduce the non-uniformity of the smoothing on the surface of the workpiece.

(Clause 8) The smoothing method according to any one of Clauses 1 to 7 may further include a step of performing peening on the workpiece before the step of performing the blasting. When peening is performed on a workpiece, a compressive residual stress is applied to the workpiece. This compressive residual stress is maximum at a predetermined depth from the surface of the workpiece. Therefore, by performing blasting after peening, the surface of the workpiece is ground and a portion to which a high compressive residual stress is applied is exposed on the surface of the workpiece. This makes it possible to enhance the strength of the processed workpiece.

Exemplary Embodiments of the Present Disclosure

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted. In each figure, an XYZ coordinate system may be shown. The Y-axis direction is a direction intersecting (here, orthogonal to) the X-axis direction and the Z-axis direction. The Z-axis direction is a direction intersecting (here, orthogonal to) the X-axis direction and the Y-axis direction. For example, the X-axis direction and the Y-axis direction are horizontal directions, and the Z-axis direction is a vertical direction. In the present specification, a numerical range indicated by using “to” indicates a range including numerical values described before and after “to” as a minimum value and a maximum value, respectively. The individually described an upper limit value and a lower limit value can be combined arbitrarily.

A smoothing method according to an embodiment will be described with reference to FIGS. 1 to 5. FIG. 1 is a flowchart of a smoothing method according to an embodiment. FIG. 2 is a diagram schematically showing a part of a processing device used in the smoothing method shown in FIG. 1. FIG. 3 is a diagram schematically showing an example of an injection medium used in the blasting step shown in FIG. 1. FIG. 4 is a diagram for explaining the blasting step shown in FIG. 1. FIG. 5 is a diagram for explaining the smoothing mechanism. A smoothing method M shown in FIG. 1 is a method for smoothing a surface of a workpiece W. The workpiece W is, for example, a component having a complicated shape. Examples of the workpiece W include sliding parts or driving parts. In the present embodiment, a gear is used as an example of the workpiece W.

As shown in FIG. 2, a processing device 10 is used for the smoothing method M. The processing device 10 is a direct pressure blasting device. The processing device 10 includes a rotating mechanism 11 and a nozzle 12. The rotating mechanism 11 is a mechanism (device) for rotating the workpiece W attached to the rotating shaft 11a around (the central axis AX of) the rotating shaft 11a as an axial center. The rotating shaft 11a extends in the X-axis direction. The rotating mechanism 11 is configured to move the rotating shaft 11a forward and backward in the X-axis direction. The nozzle 12 injects injection media 30 toward the surface of the workpiece W attached to the rotating shaft 11a. An injection port 12a is provided at the tip end of the nozzle 12. The injection port 12a is located above the rotating shaft 11a and faces the rotating shaft 11a (workpiece W). The diameter (nozzle diameter) of the injection port 12a is, for example, 10 mm.

As shown in FIG. 3, an injection medium 30 includes a core material 31 and abrasive grains 32. The core material 31 is made of an elastic body. The elastic body is made of, for example, a thermoplastic resin. From the viewpoint of ease of manufacturing the injection medium 30, an example of a thermoplastic resin is a hot melt resin. The hot melt resin is a solid (solid phase) at room temperature, and becomes a liquid (liquid phase) by melting at a temperature higher than the melting point. The hot melt resin is bonded to a material by changing the hot melt resin into a solid phase while the hot melt resin in the liquid phase is in contact with the material. Since the hot melt resin has a low melting point and elasticity, the hot melt resin is easy to manufacture the injection medium 30.

For example, a hot melt resin having a melting point of 60° C. or more and 100° C. or less may be used. When the melting point is less than 60° C., the hot melt resin may become a liquid phase during the blasting. When the melting point exceeds 100° C., the fixing process of the abrasive grains 32 may be costly, and the softening point tends to become high temperature, which may make it difficult to control the rubber elasticity. The softening point is a temperature at which the hot melt resin begins to soften. A hot melt resin whose rubber elasticity changes with temperature in a temperature range of 80° C. or less may be used. For example, a hot melt resin having a rubber hardness change of 1.3 (A) or more with respect to a temperature change of 1° C. in a temperature range of 20° C. to 50° C. may be used.

The hot melt resin satisfying the above-described conditions contains, for example, ethylene vinyl acetate, polyurethane, low density polyethylene, polyester, polyamide, polyolefin, ionomer, or polyvinyl alcohol as a main component. A hot melt resin containing ethylene vinyl acetate as a main component has a melting point in the range of 60° C. to 97° C. and a softening point of 69° C. or less (when the melting point is 60° C., the softening point is 40° C. or less). The melting point of a hot melt resin containing polyurethane as a main component is 90° C.

The core material 31 may have a spherical shape, a plate shape, a columnar shape, a cone shape, or a polyhedron. The particle diameter (particle size) of the core material 31 may be 125 μm to 600 μm or 150 μm to 500 μm. The core material 31 may contain a resin other than a thermoplastic resin and other components.

The abrasive grains 32 are particles obtained by powdering a material harder than the workpiece W to a particle diameter within a predetermined range. The hardness of the abrasive grains 32 is higher than that of the core material 31. The abrasive grains 32 may be made of alumina, silicon carbide, cerium oxide, tungsten carbide, zirconia, boron carbide, diamond, or the like. The average particle diameter (particle size) of the abrasive grains 32 may be in the range of 1 μm to 25 μm.

The abrasive grains 32 are provided on the surface of the core material 31. The injection medium 30 in which the abrasive grains 32 are bonded to the surface of the core material 31 can be obtained by solidifying a resin melted by heat, which is the origin of the core material 31, by cooling in a state where the abrasive grains 32 are in close contact with the resin. Each abrasive grain 32 may be fixed to the core material 31 so that a part of the abrasive grain 32 is buried in the core material 31 and the remainder of the abrasive grain 32 is exposed from the surface of the core material 31. The entire abrasive grain 32 may be buried in the core material 31.

The content rate of the abrasive grains 32 in the injection medium 30 is, for example, 15% by mass to 26% by mass. The particle size distribution of the injection media 30 is, for example, 125 μm to 600 μm.

As shown in FIG. 1, the smoothing method M includes a preparation step S1 and a blasting step S2. Each step will be described in detail below.

<Preparation Step S1>

First, the preparation step S1 is performed. The preparation step S1 is a step of preparing the workpiece W. In the preparation step S1, the workpiece W is attached to the rotating shaft 11a of the rotating mechanism 11. The workpiece W is attached to the rotating shaft 11a so that the center of the workpiece W (gear) coincides with the central axis AX of the rotating shaft 11a.

<Blasting Step S2>

Subsequently, the blasting step S2 is performed. The blasting step S2 is a step of performing direct pressure blasting on the workpiece W while rotating the workpiece W around (the central axis AX of) the rotating shaft 11a as an axial center. In the blasting step S2, the injection media 30 are injected in a direction (Z-axis direction) orthogonal to the central axis AX of the rotating shaft 11a. That is, the center of the injection port 12a of the nozzle 12 is located substantially directly above the central axis AX of the rotating shaft 11a, and the injection angle is substantially 90°. The injection angle is an angle formed by the central axis AX of the rotating shaft 11a and the central axis of the nozzle 12. The term “substantially directly above” means a position which can be regarded as directly above, and a deviation of about ±5 mm from the central axis AX of the rotating shaft 11a is acceptable, for example. The term “substantially 90°” means an angle that can be regarded as 90°, and for example, a deviation of about 90°±2° is acceptable.

The blasting is performed under a predetermined processing condition. Various processing parameters are set so as to obtain desired amount of grinding (depth of grinding) the workpiece W and to reduce the surface roughness of the workpiece W. Hereinafter, amount of grinding the workpiece W may be simply referred to as “amount of grinding”, and depth of grinding the workpiece W may be simply referred to as “depth of grinding”. Here, the amount of grinding (depth of grinding) D of the surface of the workpiece W follows Preston's law. That is, the amount of grinding D is expressed by Equation (1) using the injection pressure P, the velocity Va of the injection media 30, the moving velocity Vw (rotational frequency) of the workpiece W, the injection time T, and the Preston coefficient k. As shown in Equation (1), the amount of grinding D is proportional to the injection pressure P, the relative velocity, and the injection time T. The relative velocity is obtained by adding the velocity Va and the moving velocity Vw. The relative velocity indicates the frictional force between the workpiece W and the injection medium 30. As the rotational frequency of the workpiece W increases, the relative velocity also increases. The velocity Va is a velocity of the injection medium 30 when the injection medium 30 collides with the workpiece W. Therefore, as the injection distance becomes longer, the injection medium 30 are affected by the air resistance, so that the velocity Va decreases.

[Equation 1]

D=k×Px(Vα+Vw)×T  (1)

For example, the injection pressure of the injection media 30 is set within a range of 0.01 MPa to 0.10 MPa. The injection distance of the injection media 30 is set within a range of 50 mm to 100 mm. The injection distance of the injection media 30 is a distance between the injection port 12a of the nozzle 12 and the workpiece W (in the present embodiment, a distance between the injection port 12a of the nozzle 12 and the tooth tip end of the gear which is the workpiece W). The rotational frequency of the workpiece W (the rotational frequency of the rotating shaft 11a) is set to 30 revolutions per minute (30 rpm) or less. The injection time of the injection media 30 is appropriately adjusted depending on the degree of processing.

As shown in FIG. 4, in the blasting step S2, a period T1 (first period) and a period T2 (second period) are alternately repeated. The period T1 is a period during which the injection media 30 are injected into the workpiece W while the workpiece W is rotated in a rotation direction C1 (first rotation direction) and moved in a direction D1. The period T2 is a period during which the injection media 30 are injected into the workpiece W while the workpiece W is rotated in a rotation direction C2 (second rotation direction) and moved in a direction D2. The rotation direction C1 is clockwise about (the central axis AX of) the rotating shaft 11a as an axial center. The rotation direction C2 is opposite to the rotation direction C1 and is counterclockwise about (the central axis AX of) the rotating shaft 11a as an axial center. The direction D1 is a direction away from the driving device for driving the rotating shaft 11a in the direction in which the rotating shaft 11a extends (X-axis direction). The direction D2 is opposite to the direction D1.

In the blasting step S2, the injection media 30 are injected from the nozzle 12 toward the workpiece W at substantially 90° in a state in which the workpiece W is rotated about (the central axis AX of) the rotating shaft 11a as an axial center. As shown in FIG. 5, the injection media 30 are injected toward the workpiece W along with the airflow F and collide with the surface of the workpiece W. At this time, the core material 31 is elastically deformed along the surface of the workpiece W, and the peripheral edge of the abrasive grain 32 is caught on the protrusion of the surface of the workpiece W. In the vicinity of the surface of the workpiece W, the airflow F is blocked by the surface of the workpiece W to be dispersed, so that the airflow F flows along the surface of the workpiece W. Therefore, since the injection medium 30 slides on the surface of the workpiece W along the surface of the workpiece W along with the airflow F, the abrasive grains 32 scrape off the protrusions of the surface of the workpiece W. Thereafter, the core material 31 returns to its original shape by elastic force, so that the injection medium 30 is separated from the surface of the workpiece W. As described above, the surface of the workpiece W is smoothed.

In the smoothing method M described above, the direct pressure blasting is performed on the workpiece W. Therefore, since the injection media 30 move toward the workpiece W without spreading from the nozzle 12, the density of the injection media 30 becomes substantially constant within a region where the injection media 30 are injected on the surface of the workpiece W. In the blasting, since the injection media 30 are injected in a direction orthogonal to the rotating shaft 11a to which the workpiece W is attached, the distance between the tip end (injection port 12a) of the nozzle 12 from which the injection media 30 are injected and the surface of the workpiece W is substantially constant. This makes it possible to reduce non-uniformity in the amount of grinding the workpiece W. As a result, it is possible to reduce non-uniformity of the smoothing on the surface of the workpiece W.

As shown in Equation (1), as the injection pressure of the injection media 30 increases, the amount of grinding increases. Further, when the injection pressure of the injection media 30 is high, the airflow in the vicinity of the surface of the workpiece W is liable to be disturbed. Due to the turbulence of the airflow, the injection amount of the injection media 30 per unit area can vary depending on the position of the surface of the workpiece W. On the other hand, when the injection pressure of the injection media 30 is too low, the surface of the workpiece W cannot be sufficiently ground. For this problem, in the blasting step S2, the injection pressure of the injection media 30 is set to 0.01 MPa to 0.10 MPa. This makes it possible to suppress the turbulence of the airflow in the vicinity of the surface of the workpiece W without excessively grinding the surface of the workpiece W. Therefore, it is possible to further reduce non-uniformity of the smoothing on the surface of the workpiece W.

As the injection distance of the injection media 30 becomes shorter, the velocity Va in Equation (1) becomes higher, so that the amount of grinding tends to increase. On the other hand, when the injection distance of the injection media 30 is too long, the surface of the workpiece W cannot be sufficiently ground. For this problem, in the blasting step S2, the injection distance of the injection media 30 is set to 50 mm to 100 mm. This makes it possible to reduce the surface roughness of the workpiece W without excessively grinding the surface of the workpiece W.

As the rotational frequency of the workpiece W increases, the relative velocity between the injection media 30 and the workpiece W also increases. Therefore, a stronger frictional force is generated on the surface of the workpiece W, and the amount of grinding tends to increase. For this problem, in the blasting step S2, the rotational frequency of the workpiece W is set to 30 revolutions per minute or less. This makes it possible to reduce the surface roughness of the workpiece W without excessively grinding the surface of the workpiece W.

As the abrasive grain content rate in the injection media 30 increases, the number of times that the abrasive grains 32 come into contact with the surface of the workpiece W increases, so that the amount of grinding tends to increase. On the other hand, when the abrasive grain content rate in the injection media 30 is too low, the surface of the workpiece W cannot be sufficiently ground. For this problem, in the blasting step S2, the abrasive grain content rate in the injection media 30 is set to 15 mass % to 26 mass %. This makes it possible to reduce the surface roughness of the workpiece W without excessively grinding the surface of the workpiece W.

When the particle size of the injection media 30 is large, there is a high possibility that the injection media 30 do not reach a part of the surface of the workpiece W having a complicated shape. On the other hand, when the particle size of the injection media 30 is small, the injection media 30 can contact the entire surface of the workpiece W even if the workpiece W has a complicated shape. Therefore, as the particle size of the injection media 30 is smaller, the amount of grinding tends to increase. For this problem, in the blasting step S2, the injection media 30 having a particle size distribution of 125 μm or more and 600 μm or less are used. This makes it possible to enhance the possibility that the entire surface of the workpiece W is processed without excessively grinding the surface of the workpiece W.

When the injection media 30 slide on the surface of the workpiece W only in one direction, there is a possibility that the surface of the workpiece W cannot be processed uniformly. For this problem, in the blasting step S2, the period T1 during which the workpiece W is rotated in the rotation direction C1 and the period T2 during which the workpiece W is rotated in the rotation direction C2 are alternately repeated. Therefore, since the injection media 30 slide not only in one direction but also in the opposite direction on the surface of the workpiece W, it is possible to further reduce the non-uniformity of the smoothing on the surface of the workpiece W.

The smoothing method according to the present disclosure is not limited to the above-described embodiments.

For example, in the blasting step S2, only one of the period T1 and the period T2 may be performed. In the period T1, the workpiece W does not have to be moved in the direction D1. In the period T2, the workpiece W does not have to be moved in the direction D2. As described above, each processing parameter can be appropriately changed so as to obtain a desired amount of grinding (depth of grinding) and reduce the surface roughness of the workpiece W.

The smoothing method M may further include a step of performing peening on the workpiece W before the blasting step S2. When the peening is performed on the workpiece W, a compressive residual stress is applied to the workpiece W. This compressive residual stress is maximum at a predetermined depth from the surface of the workpiece W. Therefore, by performing the blasting after the peening, the surface of the workpiece W is ground, and a portion to which a high compressive residual stress is applied is exposed on the surface of the workpiece W. Therefore, the strength of the workpiece W after the processing can be enhanced.

Examples

Hereinafter, in order to explain the above effect, the present disclosure will be described in more detail with reference to examples. The present disclosure is not limited to these examples. In the following examples, the smoothing method M shown in FIG. 1 was performed under a predetermined processing condition, and the influence of each processing parameter included in the processing condition on the smoothing was evaluated. In the following evaluation, the same processing conditions were set except for a parameter to be evaluated. The processing conditions were adjusted appropriately so as to clarify the influence of the parameter to be evaluated.

<Evaluation of Injection Angle>

The influence of the injection angle on the smoothing was evaluated under the following processing conditions. A round bar of 25 mm in diameter made of SCM material (chrome molybdenum steel material) subjected to vacuum carburizing was used as a workpiece. Injection media having a particle size distribution of 125 μm to 600 μm and an abrasive grain content rate of 20% by mass were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 10 mm in diameter was used. The injection pressure was set to 0.20 MPa, the injection distance was set to 25 mm, and the rotational frequency of the workpiece was set to 2 revolutions per minute.

The maximum heights Rz and the depths of grinding were measured when the injection angle was set to 30°, 45°, 60° and 90°. The measurement was carried out at each time point (injection time) when 1 minute, 2 minutes, 4 minutes and 10 minutes had elapsed from the start of injection of the injection media. The measurement results are shown in FIG. 6A and FIG. 6B. FIG. 6A is a graph showing the relationship between the injection angle and the maximum height. FIG. 6B is a graph showing the relationship between the injection angle and the depth of grinding. In FIGS. 6A and 6B, the horizontal axis represents the amount of injection of the injection media per unit area of the workpiece (unit: g/cm²). Hereinafter, the amount of injection of the injection media per unit area of the workpiece is referred to as “injection amount per unit area of the workpiece” or simply “injection amount per unit area”. The vertical axis in FIG. 6A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 6B represents the depth of grinding (unit: μm). Since the injection amount per unit area is proportional to the injection time, it can be said that it represents the injection time.

As shown in FIG. 6A, when the injection angle was 45° or 60°, the maximum height Rz greatly decreased at a relatively early time point from the start of injection and then stabilized at a constant value. When the injection angle was 30°, the maximum height Rz greatly decreased at a relatively early time point from the start of injection, but thereafter increased as the injection time elapsed. This is considered to be caused by the fact that since the injection angle is small, the frictional force between the injection medium and the surface of the workpiece becomes too large and the surface of the workpiece becomes excessively blasted (wavy surface). When the injection angle was 90°, the reduction rate of the maximum height Rz was smaller than that of other injection angles, but the value at which the maximum height Rz was stabilized was equivalent to that of other injection angles. Note that the reduction rate means a reduction amount per unit time.

As shown in FIG. 6B, as the injection angle decreased, the depth of grinding increased. This is considered to be caused by the fact that the smaller the injection angle is, the larger the force of the injection media in the direction along the surface of the workpiece becomes, and the larger the frictional force between the injection medium and the surface of the workpiece becomes. It should be noted that at the injection angles, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other.

It has been confirmed that when the injection angle is 90°, the maximum height Rz (surface roughness) can be reduced without excessively grinding the surface of the workpiece.

<Evaluation of Injection Pressure>

The influence of injection pressure on the smoothing was evaluated under the following processing conditions. A spur gear having a diameter of 150 mm and a length of 30 mm in the direction of the tooth trace, made of SCM415 subjected to vacuum carburizing, was used as a workpiece. Injection media having a particle size distribution of 125 μm to 600 μm and an abrasive grain content rate of 20% by mass were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 10 mm in diameter was used. The injection angle was set to 90°, the injection distance was set to 50 mm, and the rotational frequency of the workpiece was set to 30 revolutions per minute.

The maximum heights Rz and the depths of grinding were measured when the injection pressure was set to 0.01 MPa, 0.05 MPa, 0.10 MPa and 0.20 MPa. The measurement was carried out at each time point (injection time) when 1 minute, 2 minutes, 4 minutes and 10 minutes had elapsed from the start of injection of the injection media. The measurement results are shown in FIGS. 7A and 7B and FIGS. 8A to 8D. FIG. 7A is a graph showing the relationship between the injection pressure and the maximum height. FIG. 7B is a graph showing the relationship between the injection pressure and the depth of grinding. FIG. 8A is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.01 MPa. FIG. 8B is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.05 MPa. FIG. 8C is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.10 MPa. FIG. 8D is a graph showing the amount of grinding in the direction of the tooth trace when the injection pressure is 0.20 MPa.

The horizontal axis in FIGS. 7A and 7B represents the injection amount (unit: g/cm²) per unit area of the workpiece. The vertical axis in FIG. 7A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 7B represents the depth of grinding (unit: μm). The horizontal axis in FIGS. 8A to 8D represents the measurement length (unit: mm) in the direction of tooth trace, and the vertical axis in FIGS. 8A to 8D represents the depth of grinding (unit: mm) Since the origin (0,0) is arbitrarily set in each measurement, the values on the horizontal axis and the vertical axis in FIGS. 8A to 8D have variations. Therefore, the relative values of the depth of grinding were compared.

As shown in FIG. 7A, the maximum height Rz was stabilized at 1.1 μm to 1.4 μm at each of the injection pressures described above. Therefore, it has been found that the injection pressure within the above range does not affect the maximum height Rz. It is considered that the maximum height Rz depends on the particle size of the abrasive grains. However, since abrasive grains of similar particle size were used in this evaluation, it can be said that the maximum heights Rz after processing were substantially the same as each other.

As shown in FIG. 7B, as the injection pressure increased, the amount of grinding tended to increase. It is considered that as the injection pressure increases, the injection energy increases, so that the amount of grinding increases. It should be noted that at the injection pressures, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other. Further, as shown in FIGS. 8A to 8C, when the injection pressure is 0.01 MPa, 0.05 MPa or 0.10 MPa, the depth of grinding is substantially uniform in the direction of the tooth trace of the workpiece. However, as shown in FIG. 8D, when the injection pressure is 0.20 MPa, a deviation occurs in the depth of grinding in the direction of the tooth trace of the workpiece.

The standard deviation a of value obtained by subtracting the minimum depth of grinding from the maximum depth of grinding at each injection pressure was 0.12 at an injection pressure of 0.01 MPa, 0.06 at an injection pressure of 0.05 MPa, 0.81 at an injection pressure of 0.10 MPa, and 1.57 at an injection pressure of 0.20 MPa. When the standard deviation σ is larger than 1.00, it can be said that a deviation occurs in the depth of grinding.

It is considered that as the injection pressure becomes higher, the airflow in the vicinity of the surface of the workpiece is liable to be disturbed, and the deviation occurs in the depth of grinding due to the disturbance of the airflow. On the other hand, although not included in the measurement results, it is considered that if the injection pressure is too low, the surface of the workpiece cannot be sufficiently ground and the maximum height Rz cannot be sufficiently reduced.

It has been confirmed that the maximum height Rz (surface roughness) can be substantially uniformly reduced without excessively grinding the surface of the workpiece in a range of injection pressure from 0.01 MPa to 0.10 MPa.

<Evaluation of Injection Distance>

The influence of the injection distance on the smoothing was evaluated under the following processing conditions. A spur gear having a diameter of 150 mm and a length of 30 mm in the direction of the tooth trace, made of SCM415 subjected to vacuum carburizing, was used as a workpiece. Injection media having a particle size distribution of 125 μm to 600 μm and an abrasive grain content rate of 20% by mass were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 10 mm in diameter was used. The injection angle was set to 90°, the injection pressure was set to 0.10 MPa, and the rotational frequency of the workpiece was set to 30 revolutions per minute.

The maximum heights Rz and the depths of grinding were measured when the injection distance was set to 25 mm, 50 mm, 75 mm and 100 mm. The measurement was carried out at each time point (injection time) when 1 minute, 2 minutes, 4 minutes and 10 minutes had elapsed from the start of injection of the injection media. The measurement results are shown in FIG. 9A and FIG. 9B. FIG. 9A is a graph showing the relationship between the injection distance and the maximum height. FIG. 9B is a graph showing the relationship between the injection distance and the depth of grinding. The horizontal axis in FIGS. 9A and 9B represents the injection amount (unit: g/cm²) per unit area of the workpiece. The vertical axis in FIG. 9A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 9B represents the depth of grinding (unit: μm).

As shown in FIG. 9A, the reduction rate of the maximum height Rz was substantially equal at all the injection distances described above. The value at which the maximum height Rz was stabilized was substantially equal at all the injection distances described above. Therefore, it has been found that the injection distance within the above range does not affect the reduction rate of the maximum height Rz and the maximum height Rz. Although not included in the measurement results, it is considered that if the injection distance is too long, the surface of the workpiece cannot be sufficiently ground and the maximum height Rz cannot be reduced.

As shown in FIG. 9B, the depth of grinding tended to decrease as the injection distance increased, and when the injection distance was 25 mm, the depth of grinding was excessively large. At the injection distance of more than 50 mm, the depths of grinding were substantially the same as each other. It should be noted that at the injection distances, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other. Since the specific gravity of the injection medium is small, it is easily affected by air resistance. Therefore, it is considered that at the injection distance of 50 mm or more, the kinetic energy of the injection media attenuates due to the air resistance, and the depths of grinding becomes substantially the same as each other.

It has been confirmed that the maximum height Rz (surface roughness) can be reduced without excessively grinding the surface of the workpiece in a range of the injection distance from 50 mm to 100 mm

<Evaluation of Rotational Frequency of Workpiece>

The influence of rotational frequency of the workpiece on the smoothing was evaluated under the following processing conditions. A spur gear having a diameter of 150 mm and a length of 30 mm in the direction of the tooth trace, made of SCM415 subjected to vacuum carburizing, was used as a workpiece. Injection media having a particle size distribution of 125 μm to 600 μm and an abrasive grain content rate of 20% by mass were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 10 mm in diameter was used. The injection angle was set to 90°, the injection pressure was set to 0.10 MPa, and the injection distance was set to 25 mm.

The maximum heights Rz were measured when the rotational frequency of the workpiece was set to 2 rpm, 16 rpm, and 30 rpm. The measurement was carried out at each time point (injection time) when 1 minute, 2 minutes, 4 minutes and 10 minutes had elapsed from the start of injection of the injection media. The measurement results are shown in FIG. 10A and FIG. 10B. FIG. 10A is a graph showing the relationship between the rotational frequency of the workpiece and the maximum height. FIG. 10B is a graph showing the relationship between the rotational frequency of the workpiece and the depth of grinding. The horizontal axis in FIGS. 10A and 10B represents the injection amount (unit: g/cm²) per unit area of the workpiece. The vertical axis in FIG. 10A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 10B represents the depth of grinding (unit: μm).

As shown in FIG. 10A, the reduction rate of the maximum height Rz increased slightly as the rotational frequency of the workpiece increased. This is considered to be caused by the fact that when the rotational frequency of the workpiece increases, the relative velocity between the injection media and the workpiece increases, so that a stronger frictional force is generated on the surface of the workpiece. The value at which the maximum height Rz is stabilized is similar at all the above-described rotational frequencies. As shown in FIG. 10B, as the rotational frequency of the workpiece increased, the amount of grinding tended to increase. This is also considered to be caused by the magnitude of frictional force due to the relative velocity. It should be noted that at the rotational frequencies, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other.

It has been confirmed that the maximum height Rz (surface roughness) can be reduced without excessively grinding the surface of the workpiece in a range where the rotational frequency of the workpiece W is 30 rpm or less.

<Evaluation of Abrasive Grain Content Rate in Injection Media>

The influence of abrasive grain content rate in the injection media on the smoothing was evaluated under the following processing conditions. A vacuum carburized SCM material having a diameter of 40 mm and a length of 10 mm was used as a workpiece. Injection media having a particle size distribution of 125 μm to 600 μm were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 6 mm in diameter was used. The injection angle was set to 45°, the injection pressure was set to 0.20 MPa, and the injection distance was set to 1 mm.

The maximum heights Rz and the depths of grinding were measured when injection media having an abrasive grain content rate of 15 mass %, 17 mass %, 24 mass %, 25 mass %, 26 mass %, 29 mass %, 37 mass % and 39 mass % were used. The measurement was carried out at each time point (injection time) when 3 minutes, 6 minutes, 12 minutes and 30 minutes had elapsed from the start of injection of the injection media. The measurement results are shown in FIG. 11A and FIG. 11B. FIG. 11A is a graph showing the relationship between the abrasive grain content rate in the injection media and the maximum height. FIG. 11B is a graph showing the relationship between the abrasive grain content rate in the injection media and the depth of grinding. The horizontal axis in FIGS. 11A and 11B represents the injection amount (unit: g/cm²) per unit area of the workpiece. The vertical axis in FIG. 11A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 11B represents the depth of grinding (unit: μm).

As shown in FIG. 11A, the reduction rate of the maximum height Rz was substantially equal in all the above-described abrasive grain content rates. The value at which the maximum height Rz was stabilized was substantially equal in all the above-described abrasive grain content rates. Therefore, it has been found that the abrasive grain content rate within the above-described range does not affect the reduction rate of the maximum height Rz and the maximum height Rz. It is considered that the maximum height Rz depends on the particle size of the abrasive grains. However, since abrasive grains of similar particle size were used in this evaluation, it can be said that the maximum heights Rz after processing was substantially the same as each other. On the other hand, although not included in the measurement results, if the abrasive grain content rate is too low, it is considered that the surface of the workpiece cannot be sufficiently ground and the maximum height Rz cannot be sufficiently reduced.

As shown in FIG. 11B, the depth of grinding tended to increase as the abrasive grain content rate increased. It is considered that as the abrasive grain content rate increases, the number of contacts between the surface of the workpiece and the abrasive grains increases, so that the depth of grinding also increases. It should be noted that in the abrasive grain content rates, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other. The maximum height Rz is stable at about 1.5 in any abrasive grain content rate. Since the injection densities at that time were 400 g/cm² to 500 g/cm², when the depths of grinding at these injection densities were compared with each other, the depths of grinding were almost the same when injection media having abrasive grain content rates of 15 mass %, 17 mass %, 24 mass %, 25 mass % and 26 mass % were used. In the case where injection media having abrasive grain content rates of 29 mass %, 37 mass %, or 39 mass % were used, the depth of grinding was excessively large.

It has been confirmed that the maximum height Rz (surface roughness) can be reduced without excessively grinding the surface of the workpiece in a range where the abrasive grain content rate is 15 mass % or more and 26 mass % or less.

<Evaluation of Particle Size Distribution of Injection Media>

The influence of the particle size distribution of the injection media on the smoothing was evaluated under the following processing conditions. A vacuum carburized SCM material having a diameter of 40 mm and a length of 10 mm was used as a workpiece. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 6 mm in diameter was used. The injection angle was set to 45°, the injection pressure was set to 0.20 MPa, and the injection distance was set to 1 mm.

The maximum heights Rz and the depths of grinding were measured when injection media having a particle size distribution of 125 μm to 600 μm, 212 μm to 500 μm and 75 μm to 300 μm were used. The measurement was carried out at each time point (injection time) when 3 minutes, 6 minutes, 12 minutes and 30 minutes had elapsed from the start of injection of the injection media. It should be noted that the abrasive grain content rate in the injection media having the particle size distribution of 125 μm to 600 μm was 25 mass %, the abrasive grain content rate in the injection media having the particle size distribution of 212 μm to 500 μm was 20 mass % and the abrasive grain content rate in the injection media having the particle size distribution of 75 μm to 300 μm was 36 mass %.

The measurement results are shown in FIG. 12A and FIG. 12B. FIG. 12A is a graph showing the relationship between the particle size distribution of the injection media and the maximum height. FIG. 12B is a graph showing the relationship between the particle size distribution of the injection media and the depth of grinding. The horizontal axis in FIGS. 12A and 12B represents the injection amount (unit: g/cm²) per unit area of the workpiece. The vertical axis in FIG. 12A represents the maximum height Rz (unit: μm). The vertical axis in FIG. 12B represents the depth of grinding (unit: μm).

As shown in FIG. 12A, the reduction rate of the maximum height Rz was substantially equal in all the particle size distributions described above. The value at which the maximum height Rz was stabilized was substantially equal in all the particle size distributions described above. Therefore, it has been found that the above-described particle size distributions of the injection media do not affect the reduction rate of the maximum height Rz and the maximum height Rz.

As shown in FIG. 12B, the depth of grinding varied depending on the particle size distribution of the injection media. It should be noted that in the particle size distributions, the depths of grinding at the injection amount (injection density) per unit area at the time when the maximum height Rz reached a constant value were compared with each other. When the injection media having the particle size distribution of 75 μm to 300 μm were used, the depth of grinding was excessively large. When the injection media having the particle size distribution of 212 μm to 500 μm were used, a part of the surface of the workpiece was not sufficiently processed, although the depth of grinding was suppressed. When the workpiece has a complicated shape, it is more likely that the injection media having a large particle size will not reach a part of the surface of the workpiece. On the other hand, when the injection media have a small particle size, the range (coverage) of the surface of the workpiece where the injection media collide (contact) increases even if the workpiece has a complicated shape. Therefore, the probability that the injection media having a smaller particle size come into contact with the surface of the workpiece is higher than the probability that the injection media having a larger particle size come into contact with the surface of the workpiece. Further, as the particle size is smaller, the surface area of the injection medium increases, so that the abrasive grain content rate also increases. From the above, it can be considered that as the particle size is smaller, the depth of grinding increases.

It has been confirmed that in the case where the injection media having the particle size distribution of 125 μm or more and 600 μm or less were used, the possibility of processing the entire surface of the workpiece can be increased without excessively grinding the surface of the workpiece.

<Residual Stress Value of Workpiece>

First, a workpiece subjected to peening under the following processing conditions was prepared. A vacuum carburized SCM material having a diameter of 40 mm and a length of 10 mm was used as a workpiece. SBM210C (cast steel injection media having a particle size of 125 μm to 250 μm) manufactured by Sintokogio, Ltd. was used as the injection media. The injection angle was set to 90°, the injection pressure was set to 0.30 MPa, and the injection distance was set to 200 mm. The injection angle is an angle formed by a central axis of a rotating shaft to which a workpiece is attached and a direction in which injection media are injected. The injection distance is a distance between an injection port of a nozzle from which injection media are injected and a workpiece. The injection amount was set to 9 kg/min and the peening time was set to 12 seconds. The coverage of the peening was 300%. The residual stress values at several depths from the surface of the workpiece were measured.

The workpiece subjected to the peening was further subjected to smoothing under the following processing conditions. Injection media having a particle size distribution of 125 μm to 600 μm and an abrasive grain content rate of 20 mass % to 24 mass % were used. WA #2000 manufactured by Sintokogio, Ltd. was used as the abrasive grain for the injection media. A nozzle having an injection port of 6 mm in diameter was used. The injection angle was set to 45°, the injection pressure was set to 0.20 MPa, and the injection distance was set to 1 mm. The injection time was set to 6 minutes.

The measurement results are shown in FIG. 13. FIG. 13 is a graph showing a residual stress value of a workpiece subjected to peening and a residual stress value of a workpiece subjected to smoothing after the peening. The horizontal axis in FIG. 13 represents the depth (unit: μm) from the surface of the workpiece. The vertical axis in FIG. 13 represents the residual stress value (unit: MPa). A positive residual stress value indicates a stress value of tensile residual stress, and a negative residual stress value indicates a stress value of compressive residual stress. It should be noted that the measurement results shown in FIG. 13 were not measured using the same workpiece. Therefore, the curve showing the residual stress values of the workpiece subjected to the peening and the curve showing the residual stress values of the workpiece subjected to the smoothing after the peening are slightly different in shape due to the difference in the workpiece and the influence of measurement error and the like.

As shown in FIG. 13, it has been confirmed that a compressive residual stress was applied to the workpiece which has been subjected to the peening and not subjected to the smoothing, and the compressive residual stress value became maximum at a depth of about 20 μm from the surface of the workpiece. The surface of the workpiece was ground by a depth of about 7 μm to 10 μm by further applying the smoothing to the workpiece. At this time, the surface of the workpiece was ground with little influence on the compressive residual stress value. As a result, it has been confirmed that a portion to which a high compressive residual stress was applied was exposed on the surface of the workpiece.

Claims

1. A smoothing method comprising:

a step of attaching a workpiece to a rotating shaft of a rotating mechanism; and

a step of performing direct pressure blasting on the workpiece while rotating the workpiece about the rotating shaft as an axial center,

wherein in the blasting, injection media are injected in a direction orthogonal to the rotating shaft,

wherein the injection media include a core material made of an elastic body and abrasive grains provided on a surface of the core material, and

wherein hardness of the core material is lower than hardness of the abrasive grains.

2. The smoothing method according to claim 1, wherein in the step of performing the blasting, an injection pressure of the injection media is set to 0.01 MPa or more and 0.10 MPa or less.

3. The smoothing method according to claim 1, wherein in the step of performing the blasting, an injection distance of the injection media is set to 50 mm or more and 100 mm or less.

4. The smoothing method according to claim 1, wherein in the step of performing the blasting, a rotational frequency of the workpiece is set to 30 revolutions per minute or less.

5. The smoothing method according to claim 1, wherein a content rate of the abrasive grains in the injection media is 15% by mass or more and 26% by mass or less.

6. The smoothing method according to claim 1, wherein the injection media have a particle size distribution of 125 μm or more and 600 μm or less.

7. The smoothing method according to claim 1, wherein in the step of performing the blasting, a first period during which the workpiece is rotated in a first rotation direction about the rotating shaft as an axial center and a second period during which the workpiece is rotated in a second rotation direction opposite to the first rotation direction are alternately repeated.

8. The smoothing method according to claim 1, further comprising a step of performing peening on the workpiece before the step of performing the blasting.