Processing method and processing apparatus for a workpiece.

Abstract

To provide a processing method of a machining object capable of imparting compressive residual stress to an inner wall surface of a hole regardless of a shape of the hole.SOLUTION: A processing method of a machining object according to one aspect incudes steps of: preparing a metal machining object having a hole that opens to a surface thereof; filling the hole with processing media; and applying external force to the processing media to impart compressive residual stress to an inner surface of the hole.SELECTED DRAWING: Figure 5

Description

Technical Field

[0001] The present disclosure relates to a method and an apparatus for processing an object to be processed.

Background Art

[0002] Holes such as water cooling holes are formed in metal parts such as molds or turbines. Since the inner wall surface of the water cooling hole is constantly exposed to the cooling water flowing through the cooling hole, corrosion is likely to occur on the inner wall surface of the water cooling hole. Further, due to the temperature difference between the outer surface of the metal part and the inner wall surface of the cooling hole cooled by the cooling water, thermal stress is repeatedly generated on the inner wall surface of the metal part. Therefore, damage such as stress corrosion cracking is likely to occur on the inner wall surface of the water cooling hole.

[0003] In order to prevent damage to the metal member, a technique of applying compressive residual stress to the inner wall surface of the water cooling hole to increase the fatigue strength of the inner wall surface is known. For example, Patent Document 1 describes inserting a nozzle into the water cooling hole of a mold and performing shot peening on the inner wall surface of the water cooling hole by injecting a projectile from the tip of the nozzle. In this method, when the projectile collides with the inner wall surface of the water cooling hole, compressive residual stress is applied to the inner wall surface.

[0004] Further, Patent Document 2 describes a vanishing tool that performs vanishing processing on the inner surface of a processing hole. This vanishing tool includes a cylindrical retainer, a plurality of rollers that are rotatable around an axis and can project radially from the retainer, and a mandrel that rotates around the axis and causes the plurality of rollers to protrude from and retract into the retainer. This vanishing tool rotates the mandrel inside the processing hole to tap and roll-pressure the inner surface of the processing hole with the rollers. As a result, the inner surface of the processing hole is plastically deformed, and compressive residual stress is applied to the inner surface of the processing hole.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

[0006] In the method described in Patent Document 1, if the water cooling holes have a complex shape, such as being curved in the depth direction, a special nozzle capable of being inserted into the interior of the complexly shaped water cooling holes is required. Furthermore, the burnishing tool described in Patent Document 2 extends linearly in the axial direction, making it difficult to insert into machined holes with complex shapes. Therefore, it is difficult to uniformly machine the inner wall surface of machined holes with complex shapes using this burnishing tool.

[0007] Therefore, the present disclosure aims to provide a processing method and apparatus for a workpiece that can impart compressive residual stress to the inner wall surface of a hole, regardless of the shape of the hole. [Means for solving the problem]

[0008] A method for processing a workpiece according to one embodiment includes the steps of: preparing a metal workpiece having a hole opening on its surface; filling the hole with a processing medium; and applying an external force to the processing medium to impart compressive residual stress to the inner wall surface of the hole.

[0009] In the processing method according to this embodiment, an external force is applied to the processing medium filled in the hole, and the external force is transmitted to the inner wall surface of the hole via the processing medium. At this time, the external force acts substantially uniformly on the inner wall surface of the hole, and compressive residual stress is imparted to the inner wall surface of the hole. Therefore, in this processing method, compressive residual stress can be applied to the inner wall surface of the hole regardless of the shape of the hole by applying an external force to the inner wall surface of the hole via the processing medium.

[0010] A processing method according to one embodiment may further include the steps of inserting a pressing member into a hole and applying a load to the pressing member inserted into the hole in order to apply an external force to the processing medium. In this embodiment, the load applied to the pressing member is transmitted to the inner wall surface of the hole via the processing medium. This external force can impart compressive residual stress to the inner wall surface of the hole.

[0011] A processing method according to one embodiment may further include a step of dropping a hammer onto a pressing member in order to apply a load to the pressing member. According to this embodiment, a load is applied to the pressing member from the hammer, and an external force acts on the inner wall surface of the hole via the processing medium, so that compressive residual stress can be applied to the inner wall surface of the hole.

[0012] A processing method according to one embodiment may further include a step of stirring the processing medium inside the hole. By stirring the processing medium inside the hole, the compressive residual stress applied to the inner wall surface of the hole can be increased.

[0013] A processing method according to one embodiment may further include a step of filling the hole with powder containing a metal material to be attached to the inner wall surface, prior to the step of applying an external force to the inner wall surface of the hole. By applying an external force to the processing medium after filling with powder containing a metal material, the powder is pressed against the inner wall surface of the hole, and a coating containing the metal material can be formed on the inner wall surface of the hole.

[0014] In one embodiment, a processing apparatus is provided for processing the inner wall surface of a hole formed in a metal workpiece. This processing apparatus comprises a pressing member and an external force application device. The pressing member is inserted into the hole filled with a processing medium. The external force application device applies an external force to the processing medium via the pressing member.

[0015] In the processing apparatus according to this embodiment, an external force is applied to the pressing member, thereby applying an external force from the processing medium to the inner wall surface of the hole. At this time, the external force acts substantially uniformly on the inner wall surface of the hole, thereby applying compressive residual stress to the inner wall surface of the hole. Therefore, with this processing apparatus, compressive residual stress can be applied to the inner wall surface of the hole regardless of the shape of the hole.

[0016] In one embodiment, the external force application device includes a hammer positioned above the pressing member and a lifting unit that moves the hammer vertically. When the hammer descends, it may collide with the pressing member, applying a load to the pressing member in the direction of the hole's depth. In this embodiment, since the load is applied from the hammer to the pressing member, an external force is applied to the inner wall surface of the hole via the processing medium. Therefore, compressive residual stress can be applied to the inner wall surface of the hole.

[0017] In one embodiment, the pressing member includes a cylindrical body and a stirring pin extending from the body parallel to the central axis of the body, and the stirring pin may be connected to the body at a position offset from the central axis. Since the stirring pin is connected to the body at a position offset from the central axis of the body, when the body rotates around the central axis, the stirring pin orbits inside the hole along the circumference centered on the central axis. As a result, the processing medium filled in the hole is stirred and agitated by the stirring pin. By agitating the processing medium, the compressive residual stress applied to the inner wall surface of the hole can be increased. [Effects of the Invention]

[0018] According to this disclosure, compressive residual stress can be applied to the inner wall surface of a hole regardless of its shape. [Brief explanation of the drawing]

[0019] [Figure 1] This is a flowchart showing a method for processing an object according to one embodiment. [Figure 2] This is a cross-sectional view showing an exemplary workpiece. [Figure 3] This is a schematic perspective view showing a processing apparatus according to one embodiment. [Figure 4] This is a diagram illustrating an example of a pressing member. [Figure 5] This diagram schematically represents the forces acting on the inner wall surface of the water cooling vent. [Figure 6]It is a flowchart showing a method for processing a workpiece according to another embodiment. [Figure 7] It is a diagram showing a modified example of a pressing member. [Figure 8] It is a diagram schematically showing the force acting on the inner wall surface of the water cooling hole. [Figure 9] It is a graph showing the relationship between the number of hammering and the residual stress. [Figure 10] It is a graph showing the relationship between the number of stirring and the residual stress.

Mode for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the description based on the drawings, the same elements or elements having the same function are denoted by the same reference numerals, and redundant descriptions are omitted. The drawings may be drawn with some parts simplified or exaggerated for ease of understanding, and dimensional ratios, angles, etc. are not limited to those shown in the drawings.

[0021] The method for processing a workpiece according to the present disclosure applies compressive residual stress to the inner wall surface of a hole formed in the workpiece. Examples of the workpiece include metal parts that require high fatigue strength and wear resistance, such as molds or turbines. Although not limited, the metal parts are composed of an iron-based alloy mainly containing iron, or an aluminum alloy, etc. The iron-based alloy is, for example, a steel material. Typically, martensitic steel after quenching and tempering is used for steel-made metal members, but the raw material before heat treatment may also be used.

[0022] Note that the workpiece may be a block-shaped metal part, or may be a multi-layer molded product three-dimensionally molded by laminating a plurality of metal layers. A hole such as a cooling hole is formed in the workpiece. Note that the hole formed in the workpiece is not limited to a cooling hole. In the following description, an example in which a mold having a cooling hole is adopted as the workpiece will be described.

[0023] Figure 1 is a flowchart showing a machining method MT1 of a workpiece according to one embodiment. In the machining method MT1 shown in Figure 1, first the workpiece, a mold 10, is prepared (step ST1).

[0024] Figure 2 is a cross-sectional view showing an example of a mold 10 prepared in step ST1. The mold 10 is, for example, a steel mold for die casting. Die casting is a type of mold casting method that produces a large number of high-precision castings by injecting molten metal under pressure. The surface of the mold 10 may be subjected to nitriding treatment. Nitriding treatment is a method of forming a nitrided layer on the surface of the mold 10 by heating the mold 10 in a nitriding gas such as ammonia gas. By nitriding the mold 10, thermal distortion of the mold 10 is suppressed.

[0025] As shown in Figure 2, the mold 10 is a block-shaped steel material having a first surface 11 and a second surface (surface) 12. A cavity, which is a space corresponding to the shape of the product, is formed on the first surface 11 of the mold 10. The mold 10 has a plurality of water cooling holes 13 that open to the second surface 12. The plurality of water cooling holes 13 are bottomed holes, and the mold 10 is cooled when cooling water flows through the water cooling holes 13 during casting using the mold 10. Each of the plurality of water cooling holes 13 has a side surface 14 and a bottom surface 15. The side surface 14 and bottom surface 15 constitute the inner wall surface 16 that defines the water cooling hole 13. Thermal stress is repeatedly generated on the inner wall surface 16 of the water cooling hole 13 due to the temperature difference between the first surface 11 of the mold 10 and the inner wall surface 16 of the water cooling hole 13, so the inner wall surface 16 of the water cooling hole 13 is particularly susceptible to stress corrosion cracking.

[0026] As shown in Figure 2, the multiple water cooling holes 13 extend from the second surface 12 of the mold 10 to just before the first surface 11. The water cooling holes 13 have a circular cross-sectional shape and have a substantially constant diameter throughout the depth direction. The water cooling holes 13 may have any cross-sectional shape, such as an elliptical or polygonal shape, and the cross-sectional area of ​​the water cooling holes 13 may change continuously or stepwise in the depth direction. In the embodiment shown in Figure 2, the water cooling holes 13 extend linearly in the thickness direction of the mold 10, but the water cooling holes 13 may be curved inside the mold 10.

[0027] Next, with reference to Figures 3 and 4, the processing apparatus used in the processing method MT1 for the workpiece will be described. Figure 3 is a schematic perspective view showing the processing apparatus 1 according to one embodiment. Figure 4 is a diagram showing a pressing member 30, which is part of the processing apparatus 1. The processing apparatus 1 comprises a base plate 21, an external force application device 28, and a pressing member 30.

[0028] The base plate 21 provides a support surface on which the mold 10 is placed. The mold 10 is placed and fixed on the base plate 21 with the openings of the multiple water cooling holes 13 facing upward. The external force application device 28 is a device that applies external force to the processing medium 25 via a pressing member 30 and includes a lifting unit 22 and a hammer 23. The lifting unit 22 is a linear motion mechanism that moves the hammer 23 in the vertical direction. The lifting unit 22 has a spline shaft 24 and a ball spline 26. The spline shaft 24 is erected on the base plate 21 and extends in the vertical direction. The ball spline 26 has an annular shape surrounding the spline shaft 24. The ball spline 26 has a plurality of rotatable rolling elements, and these rolling elements can move along the spline shaft 24 by rolling. The lifting unit 22 may further include a stopper that fixes the position of the ball spline 26 in the vertical direction. The lifting unit 22 initially holds the hammer 23 above the mold 10.

[0029] The hammer 23 is, for example, a metal plate and functions as a weight that applies a load to the pressing member 30 in the depth direction of the water cooling holes 13. Part of the hammer 23 is located above the multiple water cooling holes 13 of the mold 10. The hammer 23 is connected to a ball spline 26 so that it can move vertically above the mold 10. When the ball spline 26 is released, the hammer 23 falls (descends) toward the mold 10 together with the ball spline 26.

[0030] Figure 4(a) is a side view of an exemplary pressing member 30, and Figure 4(b) is a bottom view of the pressing member 30. The pressing member 30 is a metal member having a substantially cylindrical shape and extends along the central axis AX between one end 31 and the other end 32. The length of the pressing member 30 is formed to be longer than the depth of the water cooling hole 13 (the distance between the second surface 12 of the mold 10 and the bottom surface 15 of the water cooling hole 13 in the thickness direction of the mold 10). The diameter of the pressing member 30 is formed to be smaller than the diameter of the water cooling hole 13. In order to efficiently apply force to the processing medium 25, the diameter of the pressing member 30 may be 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more of the diameter of the water cooling hole 13. Therefore, the pressing member 30 is insertable into the water cooling hole 13.

[0031] Refer to Figure 1 again. The mold 10 prepared in step ST1 is fixed onto the base plate 21 with the openings of the multiple water cooling holes 13 facing upward. Next, the multiple water cooling holes 13 of the mold 10 are filled with a processing medium 25 (step ST2). The processing medium 25 is abrasive grains such as abrasive material that imparts residual stress to the mold 10. The material of the processing medium 25 is selected from various materials such as metal (e.g., iron, zinc, stainless steel), ceramic (e.g., alumina, silicon carbide, zircon), glass, and resin (e.g., nylon resin, melamine resin, urea resin) depending on the workpiece. The shape of the abrasive material is selected from various shapes such as spherical, polygonal, and cylindrical. Typically, steel balls with a diameter of about 0.2 mm to 1.2 mm are used as the processing medium 25.

[0032] In one embodiment, the multiple water cooling holes 13 of the mold 10 may be filled with a powder containing a metal material to be attached to the inner wall surface of the water cooling holes 13 together with the processing medium 25 (step ST3). For example, when forming a zinc coating on the inner wall surface 16 of the multiple water cooling holes 13, the multiple water cooling holes 13 of the mold 10 are filled with powdered zinc.

[0033] Next, the pressing members 30 are inserted into each water-cooling hole 13 filled with the processing medium 25 (step ST4). At this time, the pressing members 30 are inserted into the water-cooling holes 13 with their other end 32 facing the bottom surface 15 of the water-cooling hole 13. Therefore, one end 31 of the pressing member 30 is positioned above the second surface 12 with the mold 10 exposed, and the other end 32 of the pressing member 30 is positioned inside the water-cooling hole 13 with the processing medium 25 in contact.

[0034] Next, the ball spline 26 is released, and the hammer 23 positioned above the mold 10 is dropped toward the pressing member 30 (step ST5). As shown in Figure 5, the dropped hammer 23 collides with one end 31 of the pressing member 30, applying a load F toward the bottom surface 15 of the water cooling hole 13 to the pressing member 30. The load F applied to the pressing member 30 is transmitted to the processing medium 25, and the processing medium 25 is pressurized inside the water cooling hole 13. As a result, the inner wall surface 16 of the water cooling hole 13 is pressed by the processing medium 25. Due to the external force acting from the processing medium 25, the inner wall surface 16 of the water cooling hole 13 undergoes plastic deformation, and compressive residual stress is applied to the inner wall surface 16. At this time, since the force transmitted from the processing medium 25 acts substantially uniformly on the inner wall surface 16 of the water cooling hole 13, compressive residual stress is applied to the inner wall surface 16 with high uniformity.

[0035] Furthermore, when zinc-containing powder is filled into multiple water-cooling holes 13 of the mold 10 along with the processing medium 25, when force is applied from the processing medium 25 to the inner wall surface 16 of the water-cooling hole 13, the powder is pressed against the inner wall surface 16, and powdered zinc adheres to the inner wall surface 16. As a result, a zinc coating is formed on the inner wall surface 16. By forming a zinc coating on the inner wall surface 16 of the water-cooling hole 13, corrosion of the inner wall surface 16 by the cooling water circulating in the water-cooling hole 13 can be suppressed. As a result, the disappearance of the compressive residual stress layer from the inner wall surface 16 due to erosion of the inner wall surface 16 can be suppressed.

[0036] Next, the hammer 23 is raised by the lifting unit 22 (step ST6). The position of the hammer 23 after raising is determined according to the compressive residual stress to be applied to the inner wall surface 16 of the water cooling hole 13. Next, it is determined whether the dropping of the hammer 23 has been repeated a specified number of times (step ST7). If the number of times the hammer 23 has been dropped is less than the specified number, steps ST5 and ST6 are repeated until the specified number is reached, and a load F is applied to the pressing member 30. When the number of times the hammer 23 has been dropped reaches the specified number, the processing method MT1 of the workpiece according to one embodiment is completed.

[0037] As explained above, in the MT1 machining method for the workpiece, an external force is applied to the machining medium 25 filled in the water cooling holes 13 of the mold 10 via the pressing member 30. The external force applied to the machining medium 25 acts substantially uniformly on the inner wall surface 16 of the water cooling holes 13, thereby imparting compressive residual stress to the inner wall surface 16 of the water cooling holes 13 with high uniformity. Therefore, according to this MT1 machining method, compressive residual stress can be applied to the inner wall surface 16 of the water cooling holes 13 regardless of the shape of the water cooling holes 13.

[0038] Furthermore, in general shot peening, when the projectile strikes the workpiece, damage such as dents called tool marks may occur. If tool marks are present on the inner wall surface 16 of the water cooling hole 13, stress will concentrate in the area where the tool marks are formed, which may become the starting point for cracks in the mold 10. In processing method MT1, a large surface pressure can be applied to the inner wall surface 16 of the water cooling hole 13 by repeatedly applying a load F to the pressing member 30. Therefore, the irregularities on the inner wall surface 16 of the water cooling hole 13 can be smoothed out, and tool marks that cause damage to the mold 10 can be removed.

[0039] Next, a method for processing an object according to another embodiment will be described. In the following, the differences from the processing method MT1 described above will be mainly explained, and redundant explanations will be omitted. Figure 6 is a flowchart of the processing method MT2 for processing an object according to another embodiment. As shown in Figure 6, this processing method MT2 differs from the processing method MT1 shown in Figure 2 in that it further includes a step of stirring the processing medium 25.

[0040] In processing method MT2, an external force is applied to the processing medium 25 using a pressing member 40. Figure 7(a) is a side view of the pressing member 40, and Figure 7(b) is a bottom view of the pressing member 40. As shown in Figures 7(a) and (b), the pressing member 40 has a main body 41 and a stirring pin 42. The main body 41 is a metallic member having a substantially cylindrical shape and extends along the central axis AX between one end 43 and the other end 44. The length of the pressing member 40 in the direction of the central axis AX is formed to be longer than the depth of the water cooling hole 13. The diameter of the pressing member 40 is formed to be smaller than the diameter of the water cooling hole 13. Therefore, the pressing member 40 can be inserted into the water cooling hole 13.

[0041] The stirring pin 42 has a substantially cylindrical shape and a diameter smaller than the diameter of the main body 41. The stirring pin 42 extends from the other end 44 of the main body 41 in a direction parallel to the central axis AX and is connected to the main body 41 at a position offset from the central axis AX in the radial direction. Therefore, when the main body 41 rotates around the central axis AX, the stirring pin 42 revolves along the circumference of a circle centered on the central axis AX.

[0042] As shown in Figure 6, the processing method MT2 includes a step of stirring the processing medium 25 filled in a plurality of water cooling holes 13 (step ST8). The stirring of the processing medium 25 is performed, for example, by raising the hammer 23 and then rotating the main body 41 of the pressing member 40 around the central axis AX. In step ST8, the operator may rotate the pressing member 40 manually, or a power source such as a motor may be used to rotate the pressing member 40. When the main body 41 rotates around the central axis AX, the stirring pin 42 revolves along the circumference centered on the central axis AX. As a result, the processing medium 25 is stirred and agitated in the water cooling holes 13.

[0043] In one embodiment, as shown in Figure 8, the pressing member 40 may be rotated around the central axis AX while a load F is applied to it by a hammer 23 or the like. By rotating the pressing member 40 and stirring the processing medium 25 in the water cooling hole 13, the processing medium 25 can be made to come into even contact with the inner wall surface 16 of the water cooling hole 13, thereby improving the uniformity of the compressive residual stress applied to the inner wall surface 16. Furthermore, by applying an external force to the inner wall surface 16 while stirring the processing medium 25, the irregularities on the inner wall surface 16 of the water cooling hole 13 can be smoothed out, and tool marks can be removed from the inner wall surface 16.

[0044] In processing method MT2, after stirring the processing medium 25, it is determined whether the hammer 23 has been dropped a specified number of times (step ST7). If the number of times the hammer 23 has been dropped is less than the specified number, steps ST5, ST6, and ST8 are repeated until the specified number is reached. When the number of times the hammer 23 has been dropped reaches the specified number, processing method MT2 of the workpiece is completed.

[0045] As described above, according to the processing method MT2 for the workpiece, compressive residual stress can be applied to the inner wall surface 16 of the water cooling hole 13 regardless of the shape of the water cooling hole 13. The method of stirring the processing medium 25 is not limited to the example described above; for example, the processing medium 25 may be stirred in the water cooling hole 13 by applying vibration. Even when the processing medium 25 is stirred by vibration, it is possible to improve the uniformity of the compressive residual stress applied to the inner wall surface 16.

[0046] Next, the effects of the processing method for the workpiece described above will be explained based on the examples, but the processing method of this disclosure is not limited to the following examples.

[0047] (Example 1) First, in Example 1, a test piece made of alloy steel as specified in JIS (Japanese Industrial Standards) SKD61 was prepared, and multiple circular holes with a diameter of 6 mm and a depth of 25 mm were formed in this test piece. Then, the test piece was hardened and tempered.

[0048] In Example 1, steel balls made of JIS SUJ2 were filled into multiple holes in the test specimen as a processing medium. The diameter of the steel balls was 1.0 mm, and the Vickers hardness was 62. Then, the pressing members 30 shown in Figures 4(a) and 4(b) were inserted into the multiple holes filled with steel balls, and a hammer 23 was dropped onto the pressing members 30 using the processing apparatus 1 shown in Figure 3. The drop height of the hammer 23 was 100 mm. The total mass of the hammer 23 and the ball spline 26 was 1.34 kg.

[0049] In Example 1, the mold 10 was processed by varying the number of times the hammer 23 was dropped (i.e., the number of hammering strokes), and the residual stress applied to the inner wall surface of multiple holes was measured by X-ray diffraction. The residual stress was measured at a position on the side surface 12.5 mm away from the bottom surface of the hole.

[0050] The measurement conditions for residual stress using X-ray diffraction were as follows: ·Residual stress analysis method: cosα method ·Characteristic X-ray: Cr-Kα • Diffraction surface: Fe, 211 • Diffraction angle: 156.396 [deg] Voltage: 30kV ·Current: 1[mA] • X-ray elastic constant: 224 [GPa] Poisson's ratio: 0.28 ·X-ray incident angle: 35[deg] • X-ray irradiation diameter: 3 [mm] ·X-ray irradiation time: 30[s]

[0051] Figure 9 is a graph showing the relationship between the number of hammering cycles and the residual stress applied to the inner wall surface of the hole in the test specimen. In Figure 9, tensile residual stress is represented as a positive value, and compressive residual stress is represented as a negative value. As shown in Figure 9, in Example 1, it was confirmed that compressive residual stress was applied to the inner wall surface of the hole regardless of the number of hammering cycles. Furthermore, it was confirmed that the compressive residual stress applied to the inner wall surface of the hole increased as the number of hammering cycles increased.

[0052] (Example 2) In Example 2, the pressing members 40 shown in Figures 7(a) and 7(b) were inserted into multiple holes filled with the same steel balls as in Example 1, and a hammer 23 was struck against the pressing members 40 a specified number of times using the processing apparatus 1 shown in Figure 3. In Example 2, the pressing members 40 were rotated once or multiple times while the hammer 23 struck them the specified number of times to agitate the steel balls inside the holes. In all other respects, the test specimens were processed under the same conditions as in Example 1, and the residual stress applied to the inner wall surfaces of the multiple holes was measured by X-ray diffraction.

[0053] Figure 10 is a graph showing the relationship between the number of times the steel balls are stirred and the residual stress applied to the inner wall surface of the hole. As shown in Figure 10, it was confirmed that by striking the hammer 23 against the pressing member 40 while stirring the steel balls, a greater compressive residual stress can be applied compared to when the steel balls are not stirred.

[0054] Although we have described various methods for processing workpieces according to different embodiments, various modified forms can be constructed without being limited to the embodiments described above, as long as the gist of the invention is not altered.

[0055] For example, in the embodiment described above, compressive residual stress is applied to the inner wall surface 16 of the water cooling hole 13 formed in the mold 10, but compressive residual stress may be applied to the inner wall surface of holes other than the water cooling hole. Also, although the water cooling hole 13 described above is a bottomed hole, the water cooling hole 13 may be a through hole. For example, a processing medium 25 may be filled inside a through hole that is placed on a base plate 21 and has a closed bottom opening, and an external force may be applied to the processing medium 25.

[0056] Furthermore, in the processing methods MT1 and MT2 described above, powder is filled into multiple water-cooling holes 13, but it is not necessary to fill them with powder. At the very least, if a load is applied to the pressing members 30 and 40 while the processing medium 25 is filled, compressive residual stress can be applied to the inner wall surface 16 of the water-cooling holes 13. Moreover, if an external force can be applied to the processing medium 25, the shape of the pressing members 30 and 40 is not limited to the shapes shown in Figures 4(a) and (b) or Figures 7(a) and (b), but can have any shape. For example, the pressing member may have a conical shape that decreases in diameter as it approaches the bottom surface 15.

[0057] Furthermore, in the above-described embodiment, the external force application device 28 applies a load to the pressing member 30 by dropping a hammer 23, but the external force application device 28 may apply an external force to the processing medium 25 without using a hammer 23. For example, the external force application device 28 may use a hydraulic actuator such as a hydraulic cylinder to press the pressing member 30 toward the bottom surface 15 of the water cooling hole 13. Moreover, in processing methods MT1 and MT2, an external force is applied to the processing medium 25 via the pressing members 30 and 40, but it is not necessary to use the pressing members 30 and 40. For example, an external force may be applied to the inner wall surface 16 of the water cooling hole 13 by applying vibration or the like to the processing medium 25 filled in the water cooling hole 13, causing the processing medium 25 to flow inside the water cooling hole 13. Even in this case, compressive residual stress can be applied to the inner wall surface 16 of the water cooling hole 13. [Explanation of Symbols]

[0058] 1... Processing device, 12... Second surface (front surface), 13... Water cooling hole (hole), 16... Inner wall surface, 22... Lifting unit, 23... Hammer, 25... Processing medium, 28... External force application device, 30, 40... Pressing members, 41... Main body, 42... Stirring pin, AX... Central axis, F... Load.

Claims

1. A step of preparing a metal workpiece having a hole opening on its surface, A step of filling the aforementioned hole with a processing medium, A step of applying an external force to the processing medium to impart compressive residual stress to the inner wall surface of the hole, Includes, A method for machining an object, wherein the machining medium is abrasive grains for imparting residual stress to the object to be machined.

2. The steps include inserting a pressing member into the aforementioned hole, A step of applying a load to the pressing member inserted into the hole in order to apply an external force to the processing medium, The processing method according to claim 1, further comprising:

3. The processing method according to claim 2, further comprising the step of dropping a hammer onto the pressing member in order to apply the load to the pressing member.

4. A processing method according to any one of claims 1 to 3, comprising the step of stirring the processing medium inside the hole.

5. A processing apparatus for processing the inner wall surface of a hole formed in a metal workpiece, A pressing member is inserted into the hole which is filled with abrasive grains, which are the processing medium, An external force application device that applies an external force to the processing medium via the pressing member and applies the external force substantially uniformly to the inner wall surface of the hole via the processing medium, Includes, The external force application device is, A hammer positioned above the pressing member, A lifting unit for moving the hammer in the vertical direction, Includes, When the hammer falls, it collides with the pressing member and applies a load to the pressing member in the direction of the depth of the hole, thereby imparting compressive residual stress to the inner wall surface of the hole. The processing apparatus is such that the hammer is movable in the vertical direction so that it can be positioned at a starting position for dropping corresponding to the compressive residual stress to be applied to the inner wall surface of the hole.

6. A processing apparatus for processing the inner wall surface of a hole formed in a metal workpiece, A pressing member is inserted into the hole filled with the processing medium, An external force application device that applies an external force to the processing medium via the pressing member to impart compressive residual stress to the inner wall surface of the hole, Includes, The pressing member comprises a cylindrical body and a stirring pin extending from the body parallel to the central axis of the body, the stirring pin being coupled to the body at a position offset from the central axis, in a processing apparatus.

7. The processing method according to claim 3, further comprising the step of determining whether the hammer has been dropped onto the pressing member a specified number of times.

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