Golf club head and method for manufacturing same

Abstract

A golf club head having a hollow therein comprises a face portion made of a metal material. The face portion has a striking face for striking a golf ball and a back face opposite to the striking face. At least a part of the back face has a smooth back surface portion having an arithmetic mean roughness of 1 micrometer or less and a maximum height roughness of 10 micrometers or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a golf club head and a method for manufacturing the same.BACKGROUND ART

[0002] In recent years, there have been proposed various golf club heads improved in rebound effect of the face portion in order to improve the flight distance of the struck golf ball. With such golf club heads, as the rebound effect is improved, high stress is generated on the face portion when hit by the golf ball. Due to such high stresses repeatedly applied to the face portion, there is a possibility that the face portion is cracked or broken.

[0003] Conventionally, in order to improve the durability of the face portion of a golf club head, in Patent Document 1 below for example, there has been proposed a technique to improve the durability by providing thick ribs or the like on the backside of the face portion.

[0004] Patent Document 1: Japanese Patent Application Publication No. 2012-110430SUMMARY OF THE INVENTIONProblems to be Solved by the Invention

[0005] A primary objective of the present disclosure is to provide a golf club head improved in the durability of the face portion by taking a different approach from that of Patent Document 1.Means for Solving the Problems

[0006] According to the present disclosure, a golf club head having a hollow therein, comprises a striking face portion formed of a metal material and having a front striking face for striking a golf ball and a back face opposite the front striking face, wherein

[0007] the back face at least partially has a smooth back surface portion havingan arithmetic mean roughness of 1 micrometer or less anda maximum height roughness of 10 micrometers or less.Effects of the Invention

[0008] In the golf club head according to the present disclosure, by adopting the above-described configuration, the durability of the face portion can be improved.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG. 1 is a front view of a golf club head as an embodiment of the present disclosure.

[0010] FIG. 2 is a side view thereof viewed from the heel side.

[0011] FIG. 3 is a cross-sectional view of the face portion of the golf club head taken along line III-III in FIG. 1.

[0012] FIGS. 4A and 4B are diagrams for explaining an edge bounding the striking face of a golf club head, wherein FIG. 4A is a front view thereof viewed in a normal direction to the striking face, and FIG. 4B is a cross-sectional view of the front striking face taken along line s1 of FIG. 4A.

[0013] FIG. 5 is a cross-sectional view of a modification of the face portion.

[0014] FIGS. 6A and 6B are a plan view and a side view, respectively, of a test specimen used for bending fatigue test, showing the shape and dimensions.

[0015] FIG. 7 is a diagram for explaining the bending fatigue testing machine.MODE FOR CARRYING OUT THE INVENTION

[0016] Embodiments of the present disclosure will now be described in detail in conjunction with accompanying drawings.The drawings may contain exaggerations and dimensional ratios different from the actual ratios in order to aid the understanding of the present disclosure.For the different embodiments, the same reference numerals are given to the same or common parts, and duplicate explanations are omitted.The specific configurations illustrated in the embodiments and the drawings are for understanding the present disclosure, and the present disclosure is not limited to such specific configurations.

[0017] FIG. 1 is a front view of a golf club head 1 as an embodiment of the present disclosure (hereinafter sometimes simply referred to as the head 1). FIG. 2 is a side view of the head 1 viewed from the heel side thereof. FIG. 3 is a cross-sectional view of a face portion 2 thereof taken along line III-III of FIG. 1.

[0018] In FIGS. 1 to 3, the head 1 is in its standard state.

[0019] The standard state of a golf club head is such that the golf club head is placed on a horizontal plane HP while satisfying a lie angle α and a loft angle β specified for the head by the manufacturer and the like.The lie angle α and the loft angle β are usually listed in the manufacturer's catalog and the like.In the standard state, the center line CL of the hosel of the head lies within a reference vertical plane VP orthogonal to the horizontal plane HP.In this application (including the description and claims), the golf club head is described based on that the head is under its standard state unless otherwise noted.

[0020] Directions related to a golf club head will be explained using a x-y-z Cartesian coordinate system. The above-described standard state is associated with the x-y-z Cartesian coordinate system as follows:

[0021] the horizontal plane HP corresponds to the x-y plane;

[0022] the reference vertical plane VP corresponds to the x-z plane;

[0023] the x-axis is parallel to both the reference vertical plane VP and the horizontal plane HP;

[0024] the y-axis is orthogonal to the reference vertical plane VP and parallel to the horizontal plane HP; and

[0025] the z-axis is orthogonal to both the x-axis and the y-axis.

[0026] With respect to the golf club head,

[0027] a “toe-heel direction” is defined as being parallel to the x-axis,

[0028] a “front-back direction” is defined as being parallel to the y-axis, and

[0029] an “up-down direction” is defined as being parallel to the z-axis.In the front-back direction, the side of the face portion 2 is the front side, and the opposite side is the rear side.

[0030] In the present embodiment, as shown in FIG. 3, the head 1 is provided with a hollow (i) therein. The hollow (i) may be left as a void space or may be filled with foamed resin or the like.

[0031] The head 1 shown in FIG. 1 is configured as a wood-type golf club head as one of embodiments of the present disclosure. The term “wood-type” is used to include a driver (#1) as well as various fairway woods, for example.Aside from the wood-type, embodiments of the present disclosure may be configured as a hybrid type, an iron type, a putter type and the like as long as the head has the hollow (i) therein.

[0032] In the present embodiment, the head 1 comprises a face portion 2, a crown 3, a sole 4, a toe 5, a heel 6 and a hosel 7.

[0033] The face portion 2 is formed from a metal material.The metal material that forms the face portion 2 is not particularly limited. For example, pure titanium, titanium alloy, stainless steel, maraging steel, aluminum alloy, magnesium alloy, tungsten-nickel alloy and the like can be used.

[0034] In the present embodiment, the entire head 1, including the face portion 2, is made of the metal material. But, in other embodiments, at least a portion of the head 1 other than the face portion 2 can be made of a non-metallic material such as CFRP.

[0035] The face portion 2 comprises a striking face 21 on the front side for striking a golf ball, and a back face 22 on the rear side opposite the striking face 21.

[0036] The striking face 21 may be provided with markings such as face lines, punch marks, fine grooves, etc. to increase the friction with the golf ball, Such markings are omitted from the figures showing the present embodiment.

[0037] The striking face 21 is a region surrounded by the face peripheral edge E.In this application, if the face peripheral edge E of the face portion 2a can be identified as a clear edge in appearance, then it is defined by the identified clear edge.If the face peripheral edge E can not be identified as a clear edge, then it is defined as follows.

[0038] First, as shown in FIG. 4A, a normal line N (straight line) drawn normally to the striking face 21 from the center of gravity CG of the head is obtained.Incidentally, the intersecting point at which the normal line N intersects with the striking face 21 is the sweet spot SS.Then, cross sections s1, s2, s3, - - - of the face portion 2 each including the normal line N are obtained while changing the angle around the normal line N little by little.Next, in each of the cross sections s1, s2, s3 - - - , as shown in FIG. 4B, points Pe at which the radius of curvature r of the contour line Lf of the outer surface of the head first becomes 200 mm or less in the course from the sweet spot SS toward one side and toward the other side thereof, are determined (if there are small indentations such as the above-mentioned face lines and the like, they are Ignored when determining the points Pe).Then, the face peripheral edge E is defined by a closed loop line passing through the points Pe drawn on the outer surface of the head while smoothly interpolating between the points.Then, a closed loop line drawn on the outer surface of the head passing through the points Pe while smoothly interpolating between the points, is defined as the face peripheral edge E.The radius of curvature of a curve line (contour line) at any point may be defined by the radius of a circle passing through three points: that point, a point on the curve line 1 mm toward one side of that point, and a point on the curve line 1 mm toward the other side of that point. The “1 mm” is the distance along the curve line.The value of “1 mm” is just an example; other values are acceptable.

[0039] The striking face 21 has a face center FC as shown in FIG. 1.In this application, the “face center” FC is determined as follows.Step 1: an arbitrary first point P which is considered as a point located near the center of the striking face 21 with respect to both the vertical direction and the toe-heel direction, is determined.

[0041] Step 2: a first plane parallel to the toe-heel direction which includes a straight line extending from the first point P perpendicularly to the striking face 21, is determined.

[0042] Step 3: a first midpoint Px of a line of intersection between the determined first plane and the striking face 21, is determined.

[0043] Step 4: a second plane parallel to the vertical direction which includes a straight line extending from the first midpoint Px perpendicularly to the striking face 21, is determined.

[0044] Step 5: a second midpoint Py of a line of intersection between the determined second plane and the striking face 21, is determined.

[0045] Step 6: a third plane parallel to the toe-heel direction which includes a straight line extending from the second midpoint Py perpendicularly to the striking face 21, is determined.

[0046] Step 7: a midpoint of a line of intersection between the third plane and the striking face 21, is determined as the renewed midpoint Px.Then, using the renewed midpoint Px, Steps 3-7 are repeated while measuring the distance between the renewed midpoint Py and the last midpoint Py, and when the distance first becomes 0.5 mm or less, the renewed midpoint Py is determined as the face center FC.

[0047] The crown 3 is a portion of the head 1 that extends toward the rear of the head from the face peripheral edge E on the upper side of the striking face 21 and forms the upper surface of the head 1.

[0048] The sole 4 is a portion of the head 1 that extends toward the rear of the head from the face peripheral edge E on the lower side of the striking face 21 and forms the under surface of the head 1.

[0049] The toe 5 is an end portion of the head 1 that is located on the opposite side of the heel 6 in FIG. 1.

[0050] The heel 6 is an end portion of the head 1 on the side where the hosel 7 is provided in FIG. 1.

[0051] The hosel 7 is provided with a bore 7a into which the tip end of a golf club shaft (not shown) is attached. The bore 7a is a substantially cylindrical space of which center line defines the center line CL of the hosel.

[0052] As shown in FIG. 3, the back face 22 of the face portion 2 faces the hollow (i). The peripheral edge (e) of the back face 22 is defined by a line passing through intersecting points of the inner surface of the head and normal lines 23 to the outer surface of the head at the face peripheral edge E.

[0053] In the present embodiment, at least a portion of the back face 22 of the face portion 2 is formed as a smooth back surface portion 10 defined as having an arithmetic mean roughness Ra of 1 micrometer or less and a maximum height roughness Rz of 10 micrometers or less.

[0054] As a result of various studies, it was found that the mechanism by which the face portion 2 is destroyed by being struck by a golf ball can be explained by so-called low cycle fatigue, in which first, when the golf ball is struck, the tensile stress acting on the back face 22 causes an initial crack to occur in the back face 22, then the initial crack gradually progresses toward the striking face 21 side of the face portion 2, and finally the face portion 2 is unable to withstand the impact with the golf ball and fractures.In order to suppress the initial crack occurring in the back face 22 as a countermeasure against this type of destruction mechanism of the face portion 2, the inventors have focused on the surface properties of the back face 22 of the face portion 2, and discovered that it is possible to significantly improve the durability of the face portion 2 by providing at least a part of the back face 22 with a smooth back surface portion 10 whose arithmetic mean roughness Ra and maximum height roughness Rz are limited within specific ranges.

[0055] Specifically, the smooth back surface portion 10 is configured to have

[0056] the arithmetic mean roughness Ra of 1 micrometer or less and

[0057] the maximum height roughness Rz of 10 micrometers or less.

[0058] Such smooth back surface portion 10 can prevent tensile stress from concentrating at a specific position when hit by the golf ball. Thereby, initial cracks of the smooth back surface portion 10 can be prevented (can be retarded).

[0059] In this application, the arithmetic mean roughness Ra and maximum height roughness Rz refer to values measured according to the Japanese Industrial Standards (JIS) B 0601-2013.

[0060] In order to prevent the tensile stress from concentrating at a specific position when hit by the golf ball, it is preferable that the arithmetic mean roughness Ra of the smooth back surface portion 10 is as small as possible, for example, not more than 0.9 micrometers, more preferably not more than 0.8 micrometers, still more preferably not more than 0.7 micrometers.The lower limit of the arithmetic mean roughness Ra of the smooth back surface portion 10 is not particularly limited. But, from the viewpoint of workability and the like, the lower limit may be 0.3 micrometers or more, for example.

[0061] From the same perspective as the arithmetic mean roughness Ra, it is preferable that the maximum height roughness Rz of the smooth back surface portion 10 is as small as possible, for example, not more than 9 micrometers, more preferably not more than 8 micrometers, still more preferably not more than 7 micrometers.The lower limit of the maximum height roughness Rz of the smooth back surface portion 10 is not particularly limited. But, from the viewpoint of workability and the like, the lower limit may be 3 micrometers or more, for example.

[0062] In the head 1 of the present embodiment, by taking a different approach than before, namely, by providing the smooth back surface portion 10 in the back face 22 of the face portion 2 as described above, the head 1 can be improved in the durability of the face portion 2.

[0063] The present disclosure is for improving the durability of the face portion 2 by improving the microscopic surface properties of the back face 22 of the face portion 2.According to the present disclosure, therefore, regarding the thickness distribution of the face portion 2, the head 1 can adopt various configurations without being subject to any particular restrictions.

[0064] There are no particular restrictions on the position in the back face 22 of the face portion 2 where the smooth back surface portion 10 is formed.On the other hand, empirically, cracks or fractures often occur in the face portion 2 near the face center (FC) where the golf ball frequently hits. Therefore, it is preferable that the smooth back surface portion 10 is provided at least at a position of the back face 22 corresponding to the face center FC. That is, the smooth back surface portion 10 is preferably provided so as to include an intersecting point at which a normal line 24 to the striking face 21 at the face center FC intersects with the back face 22 as shown in FIG. 3.

[0065] In the present embodiment, the smooth back surface portion 10 is formed over the entire area of the back face 22 of the face portion 2 as the most preferable example.

[0066] As another example, the smooth back surface portion 10 can be provided in only a part of the back face 22 of the face portion 2. Even in this case, the effect of improving the durability of the face portion 2 can be achieved in the portion where the smooth back surface portion 10 is formed.The ratio of the area of the smooth back surface portion 10 to the overall area of the back face 22 is not particularly limited, but preferably, the ratio is set to be not less than 18%, more preferably not less than 25%.

[0067] FIG. 5 is a cross-sectional view of another example of the face portion 2.As shown, the face portion 2 in this example comprises a first portion 201 having a first thickness t1, and a second portion 202 having a second thickness t2 less than the first thickness t1. The first portion 201 is located in a central part of the face portion 2 including the face center FC. The second portion 202 is formed on the peripheral edge side of the first portion 201.Incidentally, the face portion 2 in this example can be used to constitute a golf club head as another embodiment of the present disclosure.

[0068] The back face 22 of the face portion 2 in this example is provided with the smooth back surface portion 10 in at least the back face of the second portion 202.Empirically, cracks or fractures of the face portion 2 may occur at locations where the face portion 2 is thin.In this example, since the second portion 202 has a smaller thickness, the back face of the second portion 202 is formed as the smooth back surface portion 10.Thereby, the occurrence of cracks in the second portion 202 can be suppressed to improve the durability of the face portion 2.

[0069] The area where the smooth back surface portion 10 is formed may be limited to only the second portion 202. In this case, the area that needs to be processed so as to provide the smooth back surface portion 10 can be reduced, and thereby, the productivity of the head may be improved.

[0070] In order to further enhance the effect of the smooth back surface portion 10 to improve the durability of the face portion 2, at least a surface layer of the face portion on the rear side where the smooth back surface portion 10 is provided, may have compressive residual stress.

[0071] As described above, tensile stress acts on the back face 22 of the face portion 2 when hit by the golf ball. Therefore, by preliminarily imparting compressive residual stress to the face portion so that at least a surface layer of the face portion provided with the smooth back surface portion 10 has a compressive residual stress, it is possible to reduce the tensile stress acting on the back face 22 when hit by the golf ball.Thereby, the occurrence of cracks can be suppressed, and even if cracks do occur, their propagation can be suppressed or retarded. Thus, the face portion 2 is further improved in resistance to fatigue fracture.

[0072] For example, the compressive residual stress is preferably not less than 440 MPa, more preferably not less than 540 MPa.

[0073] In general, when manufacturing the face portion from a metal material by pressing or machining process, a certain amount of compressive residual stress is imparted during the processing, and the values of such compressive residual stress are smaller than 400 MPa.By ensuring that the region of the face portion compressed by pressing or machining process has a compressive residual stress greater than these values, the tensile stress acting on the back face 22 of the face portion 2 when striking a golf ball can be effectively mitigated.The upper limit of the compressive residual stress in the compressed region is not particularly limited, but from the viewpoint of processability, it may be 1100 MPa for example, preferably 1300 MPa, more preferably 1500 MPa.

[0074] The compressive residual stress of the smooth back surface portion 10 can be measured by the X-ray diffraction method, more specifically by the sin 2ψ method.

[0075] In the present embodiment, the compressed region is formed over the entire area of the smooth back surface portion 10. As a modified example, the compressed region may be formed only in a part of the smooth back surface portion 10.The compressed region having compressive residual stress is expected to improve the durability, but requires further processing, so in order to reduce the processing burden, the formation of the compressed region may be limited only in regions where durability is particularly required. Thereby, the productivity and durability of the head 1 can be improved in a well-balanced manner.<Head Manufacturing Method>

[0076] Next, a method for manufacturing the head 1 of the present embodiment will be explained.

[0077] The manufacturing method as an embodiment of the present disclosure comprises a smoothing step for forming the smooth back surface portion 10, which has the arithmetic mean roughness Ra of 1 micrometer or less and the maximum height roughness Rz of 10 micrometers or less, on at least a part of the back face 22 of the metallic face portion 2 which comprises the striking face 21 for striking a golf ball and the back face 22 opposite the striking face 21.As other steps of the manufacturing method for manufacturing the head 1, those conventionally carried out in the similar manufacturing methods may be appropriately employed.

[0078] The processing method used in the smoothing step is not particularly limited as long as the above-mentioned smooth back surface portion 10 can be obtained.However, from a practical standpoint, preferably employed as the processing method is, for example, a polishing processing. The polishing processing can be made by various methods. For example, the polishing processing is made by polishing the back face 22 of the face portion 2 by hand or machine using abrasive paper such as emery paper and sandpaper, or a polishing tool.The abrasive papers having mesh numbers of #240 to #2000 are preferably used. Incidentally, the polishing processing is carried by changing the mesh numbers of the abrasive papers so that the abrasive particles become gradually smaller.

[0079] Aside from the polishing processing, the processing method used in the smoothing step includes, for example, ultra-precision machining by which surface processing with nano-level precision is possible.

[0080] Further, the manufacturing method of the head 1 may include a step of imparting residual compressive stress to at least the surface layer of the back face 22 by shot peening before the smoothing step. The shot material, pressure, shot duration, etc. of the shot peening are appropriately set according to the target value of the compressive residual stress to be imparted.The shot peening causes the back face 22 of the face portion 2 to become uneven. But, the back face 22 is subsequently smoothed by the smoothing step while retaining the compressive residual stress.

[0081] While detailed description has been made of preferable embodiments of the present disclosure, the present disclosure can be embodied in various forms without being limited to the illustrated embodiments.

[0082] In order to make sure the effects of the present disclosure, confirmatory tests were carried out, wherein test specimens simulating various face portions were prepared and subjected to bending fatigue tests.<Confirmatory Test 1>

[0083] In this test, test specimens 1 to 4 were formed from a rolled material made of α+β titanium alloy and having a thickness of 3.9 mm.

[0084] Each of the test specimens 1 to 4 has a substantially dumbbell-shaped planar shape as shown in FIG. 6A, and a constant thickness of 3.0 mm as shown in FIG. 6B.

[0085] Each test specimen was provided with eight through holes used to fix it to a jig, and in the bending fatigue test, bending deformation was repeatedly applied to the middle necked portion of the dumbbell-shaped planar shape.

[0086] In the confirmatory test 1, each of the test specimens 1 to 4 was cut out from the rolled material so that the rolling direction of the rolled material coincided with the longitudinal direction of the test specimen.

[0087] The manufacturing process for each test specimen 1 to 4 is described in more detail below.Test Specimen 1:

[0088] The rolled material as described above was processed by face milling the surface corresponding to the front striking face of the face portion, and by ball end milling the surface corresponding to the back face of the face portion in two steps of rough processing and fine processing so as to have a final thickness of 3 mm.Then, the processed rolled material was press-molded and cut out into the test specimen 1 having the shape shown in FIG. 6A.No further special processing has been done, therefore, the test specimen 1 did not have the smooth back surface portion.Test Specimen 2:

[0089] The test specimen 2 was different from the test specimen 1 only in that the surface corresponding to the back face of the face portion was provided with the smooth back surface portion formed by polishing using abrasive papers.In the polishing processing, emery papers were used gradually changing from #240 to #2000.Test Specimen 3:

[0090] The test specimen 3 was different from the test specimen 1 only in that the surface corresponding to the back face of the face portion was subjected to shot peening under the following conditions to impart compressive residual stress.Therefore, the test specimen 3 did not have the smooth back surface portion, but did have a compressed region having compressive residual stress.Projection material: steel balls of 0.5 mm diameter

[0092] Pressure: approximately 0.5 MPa

[0093] Projection time: about 5 secondsTest Specimen 4:

[0094] Based on the test specimen 3 subjected to the shot peening, the test specimen 4 was formed by polishing the surface corresponding to the face portion of the test specimen 3. In the polishing processing, emery papers were used gradually changing from #240 to #2000.Therefore, the test specimen 4 had both the smooth back surface portion and the compressed region having compressive residual stress.

[0095] Then, each of the test specimens 1 to 4 was subjected to a bending fatigue test simulating repeated bending deformation of the face portion caused by repeatedly striking a golf ball.The bending fatigue test was carried out using a bending fatigue testing machine 100 as shown in FIG. 7. The bending fatigue testing machine 100 comprises a pair of sliders 101 that can reciprocate only in the horizontal direction, and a pair of jigs 103 respectively connected to the sliders 101 rotatably via a bearing 102. Each of the jigs 103 has a horizontal mounting portion 104 for mounting the test specimen.The test specimen is fixed to the horizontal mounting portions 104 with the surface corresponding to the back face of the face portion facing upward.Therefore, by moving the sliders 101 toward or away from each other, the jigs 103 are tilted via the bearings 102, and, as a result, the bending fatigue testing machine 100 causes an upward convex bending deformation of the test specimen.Such bending deformation allows tensile stress to act on the upper surface of the test specimen, similarly to that exerted when the face portion is hit by a golf ball.

[0096] In the bending fatigue test, the number of bending deformations until the test specimen was fractured was counted.

[0097] In addition, for each of the test specimens, the compressive residual stress on the surface corresponding to the back face of the face portion was measured using X-ray diffraction.The measurement was carried out by the sin 2ψ method using an X-ray diffractometer (Rigaku Corporation, fully automatic multipurpose X-ray diffractometer “SmartLab”). CuKa rays were used as the X-ray source, the operating conditions were 45 kV-200 mA, and Rigaku HyPix-3000 was used as the detector.The measurement target was the α-Ti phase.The measurement angles ψ (between the normal direction to diffraction plane (crystal plane) and the normal direction to the specimen plane) were seven angles between 0 and 45 degrees.

[0098] The test results are shown in Table 1.TABLE 1CompressiveArithmeticNumber ofPolishingShotresidualmeanMaximumbending cyclesprocessingpeeningstress MParoughness μmheight μmuntil fracturetest specimen 1nono3023.0231.36044test specimen 2yesno4430.657.613690test specimen 3noyes4823.6636.55870test specimen 4yesyes5400.46.8920499<Confirmatory Test 2>

[0099] In this test, test specimens 5 to 8 were used, which were formed using the same rolled material as in the confirmatory test 1.But, the rolling direction in the test specimens was different such that each test specimen was cut out from the rolled material so that the rolling direction of the rolled material coincided with the short side direction of the test specimen.

[0100] Aside from the rolling direction, the test specimens 5, 6, 7 and 8 were the same as the test specimens 1, 2, 3 and 4, respectively, with respect to the manufacturing methods, namely, in terms of the presence or absence of the smooth back surface portion and the presence or absence of the compressed region.

[0101] Then, each of the test specimens 5 to 8 was subjected to the bending fatigue test and measured for the compressive residual stress as explained above.

[0102] The test results are shown in Table 2.TABLE 2CompressiveArithmeticNumber ofPolishingShotresidualmeanMaximumbending cyclesprocessingpeeningstress MParoughness μmheight μmuntil fracturetest specimen 5nono4183.0731.48680test specimen 6yesno5180.686.7914874test specimen 7noyes6054.1835.56791test specimen 8yesyes6880.717.0119123

[0103] From the test results, it was confirmed

[0104] that the test specimens 2 and 6 (corresponding to the face portion of the first embodiment of the present disclosure described above) had a significantly higher number of bending cycles until fracture as compared to the test specimens 1 and 5, respectively, and

[0105] that, as compared to the test specimens 2 and 6, the number of bending cycles until fracture was increased in the test specimens 4 and 8 which respectively corresponded to the test specimens 2 and 6 to which compressive residual stress was applied.

[0106] Therefore, it can be seen that the golf club head according to the present disclosure can be improved in durability of the face portion when hit by the golf ball.Statement of the Present Disclosure

[0107] The present disclosure is as follows.Present Disclosure 1

[0108] A golf club head having a hollow therein, comprises a face portion formed of a metal material, wherein

[0109] the face portion comprises a front striking face for striking a golf ball and a back face opposite the front striking face,

[0110] the back face at least partially has a smooth back surface portion having an arithmetic mean roughness of 1 micrometer or less, and a maximum height roughness of 10 micrometers or less.Present Disclosure 2

[0111] The golf club head according to Present Disclosure 1, wherein

[0112] the arithmetic mean roughness of the smooth back surface portion is not more than 0.9 micrometers.Present Disclosure 3

[0113] The golf club head according to Present Disclosure 1 or 2, wherein

[0114] the maximum height roughness of the smooth back surface portion is not more than 9 micrometers.Present Disclosure 4

[0115] The golf club head according to Present Disclosure 1, 2 or 3, wherein the smooth back surface portion has a compressive residual stress of not less than 440 MPa.Present Disclosure 5

[0116] The golf club head according to any one of Present Disclosures 1 to 4, wherein the front striking face includes a face center, and

[0117] the smooth back surface portion is formed so as to include a position at which a normal line drawn perpendicularly to the front striking face passing through the face center, intersects with the back face.Present Disclosure 6

[0118] The golf club head according to any one of Present Disclosures 1 to 5, wherein

[0119] the smooth back surface portion has an area of not less than 18% of the overall area of the back face of the face portion.Present Disclosure 7

[0120] The golf club head according to any one of Present Disclosures 1 to 6, wherein the face portion includes a first portion having a first thickness and a second portion having a second thickness less than the first thickness, and

[0121] the smooth back surface portion is formed in the second portion.Present Disclosure 8

[0122] A method for manufacturing a golf club head comprising a metallic face portion having a front striking face for striking a golf ball and a back face opposite the front striking face, comprises:

[0123] a smoothing step for forming, on at least a part of the back face, a smooth back surface portion having an arithmetic mean roughness of 1 micrometer or less and a maximum height roughness of 10 micrometers or less.Present Disclosure 9

[0124] The method for manufacturing a golf club head according to Present Disclosure 8, wherein the smoothing step includes polishing processing.Present Disclosure 10

[0125] The method for manufacturing a golf club head according to Present Disclosure 8 or 9, which comprises, before the smoothing step, a step of imparting compressive residual stress to the back face by shot peening.DESCRIPTION OF THE REFERENCE SIGNS1 head

[0127] 2 striking face

[0128] 3 crown

[0129] 4 sole

[0130] 5 toe

[0131] 6 heel

[0132] 7 hosel

[0133] 10 smooth back surface portion

[0134] 21 striking face of face portion

[0135] 22 back face of face portion

[0136] 201 first portion

[0137] 202 second portion

[0138] FC face center

[0139] i hollow

Claims

1. A golf club head having a hollow therein, comprising:a face portion formed of a metal material, and comprising a front striking face for striking a golf ball and a back face opposite the front striking face,whereinthe back face at least partially has a smooth back surface portion havingan arithmetic mean roughness of 1 micrometer or less anda maximum height roughness of 10 micrometers or less.

2. The golf club head according to claim 1, whereinthe arithmetic mean roughness of the smooth back surface portion is not more than 0.9 micrometers.

3. The golf club head according to claim 1, whereinthe maximum height roughness of the smooth back surface portion is not more than 9 micrometers.

4. The golf club head according to claim 2, whereinthe maximum height roughness of the smooth back surface portion is not more than 9 micrometers.

5. The golf club head according to claim 1, whereinthe smooth back surface portion has a compressive residual stress of not less than 440 MPa.

6. The golf club head according to claim 2, whereinthe smooth back surface portion has a compressive residual stress of not less than 440 MPa.

7. The golf club head according to claim 3, whereinthe smooth back surface portion has a compressive residual stress of not less than 440 MPa.

8. The golf club head according to claim 4, whereinthe smooth back surface portion has a compressive residual stress of not less than 440 MPa.

9. The golf club head according to claim 1, whereinthe front striking face includes a face center, andthe smooth back surface portion is formed so as to include a position at which a normal line drawn perpendicularly to the front striking face passing through the face center, intersects with the back face.

10. The golf club head according to claim 1, whereinthe smooth back surface portion has an area of not less than 18% of the overall area of the back face of the face portion.

11. The golf club head according to claim 2, whereinthe smooth back surface portion has an area of not less than 18% of the overall area of the back face of the face portion.

12. The golf club head according to claim 3, whereinthe smooth back surface portion has an area of not less than 18% of the overall area of the back face of the face portion.

13. The golf club head according to claim 4, whereinthe smooth back surface portion has an area of not less than 18% of the overall area of the back face of the face portion.

14. The golf club head according to claim 1, whereinthe face portion includes a first portion having a first thickness and a second portion having a second thickness less than the first thickness, andthe smooth back surface portion is formed in the second portion.

15. The golf club head according to claim 2, whereinthe face portion includes a first portion having a first thickness and a second portion having a second thickness less than the first thickness, andthe smooth back surface portion is formed in the second portion.

16. The golf club head according to claim 3, whereinthe face portion includes a first portion having a first thickness and a second portion having a second thickness less than the first thickness, andthe smooth back surface portion is formed in the second portion.

17. A method for manufacturing a golf club head comprising a metallic face portion having a front striking face for striking a golf ball and a back face opposite the front striking face, comprising:a smoothing step for forming, on at least a part of the back face, a smooth back surface portion having an arithmetic mean roughness of 1 micrometer or less and a maximum height roughness of 10 micrometers or less.

18. The method for manufacturing a golf club head according to claim 17, whereinthe smoothing step includes polishing processing.

19. The method for manufacturing a golf club head according to claim 17, which comprises, before the smoothing step, a step of imparting compressive residual stress to the back face by shot peening.

20. The method for manufacturing a golf club head according to claim 18, which comprises, before the smoothing step, a step of imparting compressive residual stress to the back face by shot peening.

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