Golf club head

The present invention provides a golf club head comprising, a face, a plurality of score line grooves formed in the face, traces formed in the face by milling; and a pair of side surfaces of the score line groove including a first surface that is contiguous with the face and a second surface that is contiguous with the first surface in the depth direction of the score line groove. A first angle that is formed by each first surface of the pair of side surfaces is larger than a second angle that is formed by each second surface of the pair of surfaces. The face in which the traces are formed has the arithmetic mean deviation of the profile (Ra) of not less than 4.00 μm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a golf club head.

2. Description of the Related Art

The face of a golf club head include a plurality of grooves, known as marking lines, score lines, or face line grooves (hereinafter referred to as score line grooves), which affect the amount of spin on a ball. It is desirable to have the grooves on an iron club head, especially a wedge, in order to increase the amount of spin on the ball.

Japanese Patent Application Laid-Open No. 9-192274 discloses a golf club having grooves of V-shaped or trapezoidal cross section. Japanese Patent Application Laid-Open No. 9-70457 and No. 10-179824 disclose a golf club head with rounding on the edges of the score line groove, that is, where the score line grooves meets the face. The rounding has the effect of avoiding scratching or other damage to the ball. Japanese Patent Laid-Open No. 2003-93560 and No. 2005-287534 disclose a golf club head having score line grooves each of which has a side surface with two varying angles, such that the side surface is not constituted on a single, flat plane. Rules of golf regulate width and depth of score line grooves on a golf club head used in official competition play, and steps must be taken that satisfy the pertinent rules when considering applications in official play.

The amount of spin on the ball affects the surface roughness of the face as well. Japanese Patent Laid-Open No. 2005-169129 discloses a golf club head with a face having the surface roughness of not less than 40 Ra. Japanese Patent No. 3,000,921 discloses a golf club head with a face having a plurality of fine grooves that are distinct from the score line grooves. Rules of golf regulate roughness of a face on a golf club head used in official competition play, and steps must be taken that satisfy the pertinent rules when considering applications in official play.

Spin on a ball tends to decline when hitting in bad weather or in the rough, compared with hitting in fair weather or on the fairway. Increasing the volume of the score line grooves is one method of avoiding reduction in spin when hitting in bad weather or in the rough. Increasing the volume of the score line grooves makes it easier to get rid of grass and dirt that may be caught between the face and the ball into the score line grooves, and also improves drainage performance on the face.

Score line grooves with square cross-sectioning tends to have larger volume than score line grooves with other cross-sectioning, presuming an identical width, at the cost of increased damage to the ball, owing to a sharper angle on the edges of the score line groove.

Score line grooves with a V-shaped or trapezoidal cross-section may minimize damage to the ball, at the expense of reduced score line grooves volume, which tends to significantly reduce spin when hitting in bad weather or in the rough.

The golf club head disclosed in Japanese Patent Laid-Open No. 2003-93560 has score line groove edges with sharp angles that cause greater damage to the ball. The golf club head disclosed in Japanese Patent Laid-Open No. 2005-287534 may be unworkable, owing to the width of the interior of the score line groove being wider than the score line groove in the face. The score line groove edges also have sharp angles that cause greater damage to the ball. While Japanese Patent Laid-Open No. 2005-287534 discloses a golf club head with rounding on the edges of the score line groove, score line groove edges with significantly sharp angles, such as the score line grooves in Japanese Patent Laid-Open 2005-287534, may cause greater damage to the ball even if the edges are rounded. Even if the surface roughness of the face is modified, such as with the golf club heads disclosed in Japanese Patent Laid-Open No. 2005-169129 and Japanese Patent No. 3,000,921, poor drainage performance on the face will reduce spin.

SUMMARY OF THE INVENTION

The present invention has been made in order to overcome the deficits of prior art.

According to an aspect of the present invention, it is provided a golf club head comprising a face, a plurality of score line grooves formed in the face, traces formed in the face by milling, and a pair of side surfaces of the score line groove including a first surface that is contiguous with the face and a second surface that is contiguous with the first surface in the depth direction of the score line groove, wherein a first angle that is formed by each first surface of the pair of side surfaces is larger than a second angle that is formed by each second surface of the pair of surfaces, and wherein the face in which the traces are formed has the arithmetic mean deviation of the profile (Ra) of not less than 4.00 μm.

The golf club head is formed such that the first angle, which is formed by the first side, is larger than the second angle, which is formed by the second side surfaces. The first side surfaces of the score line grooves contribute to avoiding damage to the ball, and the second side surfaces contribute to ensuring volume in the score line grooves. Therefore, the golf club head can avoid significant declines in spin when hitting in bad weather or in the rough, as well as damage to the ball.

The arithmetic mean deviation of the profile (Ra) of not less than 4.00 μm in the face allows significantly greater spin through improved friction between the ball and the face.

DESCRIPTION OF THE EMBODIMENTS

FIG. 1is an external view of a golf club head A, according to an embodiment of the present invention. The embodiment depicted inFIG. 1applies the present invention to an iron club head. The present invention is optimized for club heads for which large amounts of spin are required, especially wedges such as sand wedges, pitching wedges, or approach wedges. The present invention may also be applied to golf club head for the wood type or the utility type.

The face10of the golf club head A comprises a plurality of the score line grooves20. The face10is the surface that strikes the golf ball. According to the embodiment, the respective score line grooves20are arrayed in straight lines in the toe-heel direction, all in parallel, with equal pitch between the respective score line grooves20. The face10comprises a plurality of striations30, which are traces formed by milling.

FIG. 2is a cross-sectional diagram in the vicinity of a score line grooves20, which cuts at right angles to the lengthwise, or toe-heel direction, of the score line grooves. In the embodiment, the cross-section of each score line groove20is constant in the lengthwise direction, except at the ends. The cross-sections are constant for each score line grooves20.

The score line groove20has a pair of side surfaces21and22and a bottom surface23. In the embodiment, the cross-section of the score line grooves20is symmetrical with respect to a center line CL. The pair of side surfaces21and22of the score line grooves20respectively comprises a first surface21aand22a, which are contiguous with the face, and a second surface21band22b, in the direction of the depth of the score line groove20, and which are contiguous with the first surface21aand22a. The bottom surface23is parallel to the face10, and also contiguous with the second surface21band22b.

The score line groove20has a bottom surface width Wb, a depth D, and a width W. The bottom surface width Wb refers to the distance between the ends of the bottom surface23. The depth D refers to the distance from the face10to the bottom surface23. The width W is the width of the score line grooves20intersecting at right angles to the lengthwise direction thereof, and refers to the distance between the edges of the score line grooves20, that is, the distance between the boundary between the first surface21aand the face10, and the boundary between the first surface22aand the face10. When rounding the edges of the score line grooves20by a radius r, as depicted inFIG. 3A, the width W will be measured from the point where the rounding begins, that is, the position of the dotted line inFIG. 3A.

The rounding of the edges has the effect of preventing damage to the ball, and rounding of a radius r of between 0.05 mm and 0.3 mm is preferable. In terms of spin on the ball, it is further preferable that the radius r be between 0.05 mm and 1 mm, inclusive.

The term “width of score line groove” used herein means the width W, as measured via the foregoing method, and is differentiated as the width measured via so-called the 30 degrees measurement rule in the R&A regulation which is a method for measuring groove width of a golf club head used for official games. As shown inFIG. 3C, the 30 degrees measurement rule refers to measuring the distance between points on a hypothetical line L, with a 30-degree inclination vis-à-vis the face10, and that connect the side surfaces21and22, as a width Wr. The width measured by the 30 degrees measurement rule will be referred to hereinafter as the rule-based width.

When rounding is applied to the edges of the score line grooves20, as depicted inFIG. 3A, the width W of the score line groove20W may differ from the rule-based width Wr. When rounding is not applied to the edges of the score line groove20, the width W will equal the rule-based width Wr. The rule-based width Wr is mandated as being not greater than 0.9 mm. The rules also mandate that the score line grooves depth D is not more than 0.5 mm.

Returning toFIG. 2, an angle θ1, which is formed by the first surface21aand22aof the score line groove20, is larger than an angle θ2, which is formed by the second surface21band22bof the score line groove20. The bigger the angle θ1gets, the bigger the angle of the edges of the score line groove20, i.e., the boundary between the first surface21aand22aand the face10, gets, and the more damage to the ball is avoided. In other words, the first surface21aand22aof the score line grooves20contribute to avoiding damage to the ball.

The fact that the angle θ2is smaller than the angle θ1contributes to enlarging the volume of the score line grooves20. In greater detail, composing the side surfaces21and22of the score line grooves20of the first surfaces21aand22aand the second surfaces21band22bwhich have different angles, allows making the bottom segment of the score line grooves20wider than when the score line grooves20is composed solely of the first surface21aand22a. It is thus possible to enlarge the volume of the score line grooves20. Therefore, portions of the score line groove20may share the function, i.e. the second surfaces21band22bof the score line grooves20contribute to ensuring the volume of the score line groove.

In such a manner, the golf club head1is capable of avoiding a significant decline in spin when hitting the ball in bad weather or in the rough, as well as minimizing damage to the ball.

The larger the cross section area of the score line groove20gets, the larger the volume of the score line groove20gets. The size of the cross section area of the score line groove20, or to put it another way, a cross section area ratio, is suggested as an indicator that evaluates the volume of the score line groove20hereinafter, according to the embodiment. Again, rules for golf club heads used in competition call for the depth D to be not greater than 0.5 mm. Accordingly, when the edges of the score line grooves20are not rounded, the maximum cross section area of the score line grooves20, when the rule-based width Wr applies, is Wr (mm)×0.5 mm=0.5×Wr (mm2), as depicted on the right-hand side ofFIG. 3B.

The cross section area ratio of the score line groove20for the cross section area S (mm2) as per the left-hand side ofFIG. 3B, vis-à-vis the maximum cross section area, is an indicator that evaluates the volume of the score line groove20. The cross section area ratio is displayed in equation (1). It is desirable for the cross section area ratio to be not less than 70%, as will be described hereinafter.
Cross Section Area Ratio (%)=S/(Wr×0.5)×100  Equation (1)

<Example of Score Line Grooves Cross Section Shape>

FIG. 4Adepicts a smaller angle θ1than the example shown inFIG. 2. As depicted in the example inFIG. 4A, the smaller the angle θ1gets, the bigger the cross section area of the score line grooves20gets, and thus, the bigger the volume of the score line grooves20gets. The smaller the angle θ1gets, however, the smaller the angle of the edge of the score line groove20gets, and thus, the more likely the ball is to be damaged. It is thus preferable to add rounding to the edges of the score line grooves20in this situation.

It is preferable to add rounding to the edges of the score line grooves20when the angle θ1is 50 degrees or less, and it is preferable to have the radius r of the rounding of the edges of the score line groove20be between 0.05 mm and 0.3 mm, inclusive, and to keep the radius r between 0.05 mm and 0.1 mm, inclusive. If the angle θ1is excessively small, however, the ball may be damaged even if the edges of the score line grooves20are rounded. Accordingly, it is preferable that the angle θ1be not smaller than 10 degrees.

Whereas the score line grooves20, as depicted inFIG. 2, has the bottom surface23, it is also possible to have the score line grooves20have no bottom surface23. Having the bottom surface will make it easier to enlarge the cross section area of the score line grooves, however.FIG. 4Bis a cross-sectional diagram of a score line groove120with no bottom surface. The score line groove120has a configuration similar to that of the score line grooves20, being formed from a pair of side surfaces121and122, and excepting the fact that the score line grooves120has no bottom surface. The score line grooves120cross-section is symmetric with regard to the central line CL, according to the embodiment. A pair of side surfaces121and122of the score line groove120are composed of a first surface121aand122a, which are contiguous with the face10, and a second surface121band122b, which are contiguous with the first surface121aand122ain the depth direction of the score line groove120. The angle θ1that is formed by the first surface121aand122aof the score line groove120is larger than the angle θ2that is formed by the second surface121band122b.

With reference toFIGS. 1 and 2, each striation30is of a significantly small form according to the embodiment, being smaller in cross section area than the score line groove20. In the embodiment, each striation30forms a circular arc, and is shaped so as not to overlap any other striation30. Also in the embodiment, each striation30is an arc of radius identical to every other striation30. Whereas a plurality of the striations30, formed by milling, were adopted as the traces in the face10in the embodiment, the shape of the trace is not limited thereto, and a variety of shapes may be so adopted.

An arrow d0inFIG. 1depicts an arrangement direction of the plurality of striations30. In the embodiment, each striation30is an arc of radius identical to every other striation30as described above. The arrangement direction d0is defined as the direction that passes through the center of the circle of arc of each striation30. An angle θ0, which is formed by the arrangement direction d0and the lengthwise direction of the score line groove20, is between 40 and 70 degrees, inclusive, as measured clockwise from the toe side end of the score line groove20. With regard to the striations30depicted inFIG. 1, the angle θ0is approximately 45 degrees.

The milling for forming the striations30may be performed using a milling machine, for example.FIG. 5is a schematic diagram illustrative of a forming method of striations30using a milling machine. The milling machine comprises a spindle2that rotates about a vertical axis Z, and a cutting tool (endmill)1is attached to the lower end of the spindle2. A golf club head A, that has not been formed with the striations30, fixed with the milling machine by way of a jig3so that the face10is horizontal. A cutting portion1aof the cutting tool is separated from the vertical axis Z by a distance rt, which is the radius of the circle of arc of each striation30.

FIG. 6. is a planar view diagram illustrative of a moving path of the cutting tool1when milling the striations30. The relative direction of movement, i.e., the horizontal direction, of the cutting tool1and the golf club head A, is identical with the arrangement direction d0of the striations30. As the cutting tool1is moved in the arrangement direction d0, relative to the golf club head A, the plurality of striations30is formed by milling the face10with the cutting tool1. The center of the circle arc of each striation30, or in other words, the position of the vertical axis Z, passes through the arrangement direction d0. Accordingly, the arrangement direction d0is the direction that passes through the center of the circle arc of each striation30. The depth, width, and pitch of each striation30is adjusted by the depth of the cut into the face10by the cutting tool1and the relative moving speed of the cutting tool1.

The face10face is formed so as to have the arithmetic mean deviation of the profile (Ra) of not less than 4.00 μm by such milling in the embodiment. By forming the face10with the arithmetic mean deviation of the profile (Ra) of not less than 4.00 μm, the surface roughness of the face10increases compared to giving the face10a mirrored finish. Increased surface roughness of the face10improves friction between the ball and the face10, which makes it easier to impart spin to the ball, nevertheless the ball is shot from the rough. The greater the surface roughness of the face10, the easier it is to impart spin to the ball, and the more likely the ball is to be damaged.

Accordingly, it is preferable for the surface roughness of the portion of the face10that forms the striations30to have the arithmetic mean deviation of the profile (Ra) of between 4.00 μm and 4.57 μm, inclusive. It is also preferable for the maximum height of the profile (Ry) to be not greater than 25 μm. Keeping the surface roughness of the face10within the specified range of values also meets the regulations pertaining to the surface roughness of the face of a golf club head to be used in official competition golf.

The larger the angle θ1of the score line groove20, the less likely the ball is to be damaged, and the less spin it is likely to receive. On the other hand, the arithmetic mean deviation of the profile (Ra) of the face10of not less than 4.00 μm improves the amount of spin on the ball, nevertheless the ball is shot from the rough. Accordingly, having the arithmetic mean deviation of the profile (Ra) of the face10of not less than 4.00 μm allows increasing the angle θ1of the score line groove20.

In other words, adjusting the angle θ1of the score line groove20and the surface roughness of the face10allows increasing the amount of spin on the ball, while avoiding damage thereto. According to the embodiment, the score line groove20also improves drainage performance of the face10, and makes it easier to get rid of grass and dirt that may be caught between the face10and the ball into the score line groove20. Accordingly, it is easier to impart spin to the ball without significantly decreasing the friction coefficient of face10, when hitting in bad weather or in the rough. Accordingly, it is possible to reduce the difference between the amount of spin imparted to the ball when hitting in good weather or on the fairway, and the amount of spin imparted to the ball when hitting in bad weather or in the rough.

Next, in the embodiment, since the angle θ0, which is formed from the arrangement direction d0of the plurality of striations30and the score line groove20, is between 40 degrees and 70 degrees, inclusive, it becomes easier to impart spin to the ball, allowing obtaining a greater amount of spin when using a golf club with the golf club head A when the face10is opened, as described inFIGS. 7A and 7B.

FIG. 7Adepicts a situation wherein the face10is facing directly in the target direction, andFIG. 7Bdepicts a situation wherein the face10is opened. The striations30have been omitted fromFIGS. 7A and 7B. The arrows inFIGS. 7A and 7Bdepict the direction of relative movement of the ball vis-à-vis the face10at time of impact.

In the embodiment, applying the plurality of striations30makes it easier to impart spin to the ball in both the situation shown inFIG. 7AandFIG. 7B. If the face10is opened, as depicted inFIG. 7B, results in the ball rubbing against the face10at time of impact in such a manner as to intersect the score line grooves20at an angle.

Presuming the angle θ0, which is formed by the arrangement direction d0of the plurality of the striations30and the score line grooves20, to be between 40 and 70 degrees, according to the embodiment, the number of striations30that rub against the ball is increased when the face10is opened, as depicted inFIG. 7B. To put it another way, the angle of the direction of relative movement of the ball and the striations30approaches a right angle. Accordingly, it becomes easier to impart spin to the ball, allowing obtaining a greater amount of spin.

While each striation30has been formed as a circular arc according to the embodiment, it is possible to form the striations30as a straight line as well.FIG. 8is an external view of an example of a golf club head A with striations in a different shape. The example given inFIG. 8is identical to the example shown inFIG. 1, except for the fact that a plurality of striations40shown inFIG. 8are formed of straight lines.

The plurality of striations40are mutually formed in parallel. When each striation40is straight lines, according to the embodiment, an arrangement direction d0′ is defined as a direction that is orthogonal to each striation40. An angle θ0′ formed from the arrangement direction d0′ and the lengthwise direction of the score line groove20is between 40 and 70 degrees, inclusive, as measured clockwise from the toe side end of the score line groove20.

Even if the striations40have a straight line shape, it is easier to impart spin to the ball, and it is particularly easier to impart spin to the ball when the face10is opened, making it easier to obtain a greater amount of spin on the ball in either case.

FIG. 10depicts the findings of an experiment that measured ball damage and spin for golf club heads No.1to No.5, and golf club heads No.11to No.19, each with different score line groove specifications. All of the golf club heads are sand wedges with a loft angle of 56 degrees, and milling has not been performed on their faces.

The experiment involved using golf clubs with each of the golf club heads No.1to No.5, and each of the golf club heads No.11to No.19attached, to hit a heretofore never used golf ball with a robot machine. The head speed of the sand wedge was set to 40 m/s. It was also decided to hit the ball 10 times each with a dry face and with a wet face, wherein the face was covered with a thin sheet of paper that had been soaked in water, in consideration of taking shots in good weather and the fairway and in bad weather and the rough.

The “score line groove specifications” section ofFIG. 10depicts the score line groove specifications for each of the golf club heads No.1to No.5, and each of the golf club heads No.11to No.19. The “Cross section shape” depicts the shape of the cross section for each of the golf club heads No.1to No.5, and each of the golf club heads No.11to No.19. The “single side surface (trapezoidal)” for the golf club heads No.1to No.3refers to the shape of the cross section of a score line groove220that is depicted inFIG. 9A. The shape of the cross section of the score line groove220is bilaterally symmetrical with respect to the center line. The angle θ1is formed from the side surfaces221and222, and the side surfaces221and222are unified, with angles that do not change along their surfaces. The depth D is the distance from the face10to a bottom surface223, and the width W of the score line groove220refers to the distance between the edges of the score line groove220.

The edges of the score line grooves of the golf club heads No.1to No.5, and of the golf club heads No.11to No.17are not rounded, that is, the rounding radius r=0, and, accordingly, the width W is identical in all instances to the rule-based width Wr, which is set to 0.9 mm, as depicted inFIG. 10. The edges of the score line grooves of the golf club heads No.18and No.19, on the other hand, are rounded, that is, the rounding radius r=0.2 mm, and the width W is not identical to the rule-based width Wr. The rule-based width Wr for both the golf club heads No.18and No.19is set to 0.9 mm or less, however.

The “single surface (V-shaped)” for the golf club heads No.4and No.5refers to the shape of the cross section of a score line groove320that is depicted inFIG. 9B, wherein the score line groove320is bilaterally symmetrical with respect to the center line. The angle θ1is formed from side surfaces321and322, and the side surfaces321and322are unified, with angles that do not change along their surfaces. The depth D is the distance from the face10to the intersection of the side surface321and the side surface322. The width W of the score line groove320refers to the distance between the edges of the score line groove320.

The “two-step side surface (with bottom surface)” of the golf club heads No.11to No.14, and of the golf club heads No.16to No.19refers to the shape of the cross section that is depicted inFIG. 2andFIG. 4A. The “two-step side surface (without bottom surface) of the golf club head No.15refers to the shape of the cross section that is depicted inFIG. 4B. In other words, the golf club heads No.11to No.19employ either the score line grooves20or the score line grooves120, according to the embodiment of the present invention.

The “angle θ1”, “angle θ2”, “width W”, and “depth D” inFIG. 10correspond to the symbols depicts inFIG. 2,FIG. 3,FIG. 4A, andFIG. 4B. The depth D is 0.5 mm, which is the maximum value allowed by the rules. The “cross section area S” is the cross section area for each score line groove. The “cross section area ratio” is calculated using the Equation 1. The “pitch” given inFIG. 10is the interval between the score line grooves, which is 3.60 mm for all golf club heads aside from the golf club head No.18.

Of the Findings, the “degree of scratches” is determined by visual and tactile examination of the level of scratches on the surface of the ball after each shot, when the face is dry, by three assessors, who rank the level of scratches on a 10-step scale. The experiment in question assigned a 10 to the ball whose surface had the most scratching, and a 1 to the ball whose surface had the least scratching. The “amount of spin” is calculated by marking the ball prior to the shot, and using a video camera to track the change in the location of the mark at time of impact. The scores shown are the average of 10 shots each for dry and wet conditions.

FIG. 11Agraphs the angle θ1versus the degree of scratches given in the Findings inFIG. 10. Low values of the Angle θ1signify a small angle of the edge of the score line groove, while high values of the Angle θ1signify a large angle of the edge of the score line groove. The golf club heads No.1to No.5, and the golf club heads No.11to No.19, show a similar trend, wherein the smaller the value of the Angle θ1, the greater the level of scratches on the surface of the ball, whereas the greater the value of the Angle θ1, the less scratches on the surface of the ball. The golf club heads No.18and No.19, however, which have rounding on the edges of the score line grooves, have less scratches than the golf club heads No.2and No.6, which have the same angle θ1, signifying that rounding on the edges of the score line grooves has the effect of avoiding damage to the ball.

A degree of scratches of eight or more signifies a level of scratches on the surface of the ball that would make it difficult, for all practical purposes, to use the ball for a number of holes in a row. Accordingly, an Angle θ1of not less than 50 degrees is preferable when the edges of the score line grooves are not rounded.

FIG. 11Bgraphs the relation between the Angle θ1and the Amount of spin, for both dry and wet circumstances, given in the Findings inFIG. 10. When the face is dry, The golf club heads No.1to No.5, and the golf club heads No.11to No.19, show a similar trend. The findings show no significant change in amount of spin versus the angle θ1, when the face is dry. While a change in amount of spin versus the Angle θ1is detectable when the face is wet, the overall trend is that the golf club heads No.11to No.19show a lesser decline in the amount of spin than do the golf club heads No.1to No.5.

The golf club head No.12, whose angle θ1is 60 degrees, and the golf club head No.14, whose angle θ1is 90 degrees, have lowered degradation in amount of spin than the golf club heads No.3and No.4, whose angles θ1are also 60 degrees and 90 degrees, respectively. It is deduced that the difference in the cross section area S also has an effect. In other words, the golf club heads No.11to No.19, with a two-step surface, have a larger cross section area, when the angle θ1is the same, and thus, it is conceivable that it increases the amount of water into the score line grooves, which may reduce the amount of spin that would be lost. The difference between the golf club heads No.1to No.5, and the golf club heads No.11to No.19, on the other hand, becomes insignificant as the angle θ1exceeds 100 degrees. Accordingly, it is desirable to have an angle θ1of not greater than 100 degrees when using a two-step surface, as with the golf club heads No.11to No.19.

The golf club heads No.2, No.11, No.18, and No.19, all with a common angle θ1of 30 degrees, experience a small decline in amount of spin under wet conditions. The golf club head No.18has the least decline in amount of spin among the four golf club heads, and it is conceivable that the fact that the golf club head No.18has a smaller pitch of the score line grooves than the golf club heads No.2, No.11, and No.19may have an effect. The golf club head No.19has the next lowest decline in amount of spin among the four golf club heads, and it is conceivable that the fact that it has a larger width W than the golf club heads No.2and No.11may have an effect.

FIG. 11Cgraphs the relation between the cross section area ratio and the amount of spin, for the wet circumstance, given in the Findings inFIG. 10. The golf club heads No.1to No.5, and the golf club heads No.11to No.19, show a similar trend, in that the amount of spin for the wet circumstance correlates with the cross section area ratio. One may see that the plot becomes increasingly steep, and the amount of spin for the wet circumstance starts to increase, around the point where the cross section area ratio exceeds 70%. The plot becomes steeper still around the point where the cross section area ratio exceeds 80%. Accordingly, it is desirable to have a cross section area ratio of not less than 70%, and particularly preferable that the cross section area ratio be not less than 80%, with two-step side such as with the golf club heads No.11to No.19.

Achieving a cross section area ratio of 80% or more with two-step side surfaces such as with the golf club heads No.11to No.19becomes increasingly difficult in score line groove design terms when the angle θ1exceeds 50 degrees. Accordingly, it is preferable that the angle θ1not exceed 50 degrees when the cross section area ratio is 80% or more. With regard to damage to the ball, it is desirable in such instance that the edges of the score line grooves be rounded, and furthermore, that the angle θ1be not less than 10 degrees.

Based on the experimental findings, with regard to the score line groove20that is depicted inFIG. 2, it is preferable that the angle θ1be between 50 and 100 degrees, inclusive, and that the cross section area ratio be not less than 70%, if the edges of the score line groove20are not rounded. Setting the angle θ2to a maximum of 30 degrees will make a cross section area ratio of not less than 70% easier to achieve from a design standpoint, and accordingly, it is preferable to set the angle θ2to a maximum of 30 degrees.

On the other hand, while a cross section area ratio of not less than 80% avoids further degradation in amount of spin in the wet circumstance, it is preferable that the angle θ1be between 10 and 50 degrees, inclusive, and that the edges of the score line grooves20be rounded. Setting the angle θ2to a maximum of 30 degrees will make a cross section area ratio of not less than 80% easier to achieve from a design standpoint, and accordingly, it is preferable to set the angle θ2to a maximum of 30 degrees, and furthermore, that the angle θ2be not more than 15 degrees.

Pursuant to the experimental findings, the score line groove specification was configured to make the rule-based width Wr a maximum of 0.9 mm. When using the golf club head that is the present invention in official competition golf, it is necessary that the rule-based width Wr be not larger than 0.9 mm. Making the rule-based width Wr excessively narrow, however, also narrows the score line grooves cross section area. Accordingly, it is preferable that the rule-based width Wr of the score line grooves of the golf club head that is the present invention be between 0.6 mm and 0.9 mm, inclusive.

FIG. 12depicts the findings of an experiment that measured amount of spin on the ball for golf club heads No.21, No.22, and No.31to No.37, each with different striation specifications. All of the golf club heads, No.21, No.22, and No.31to No.37, are sand wedges with a loft angle of 56 degrees, and the circular arc striations30depicted inFIG. 1have been formed in their faces by milling. All of the golf club head have common score line groove specifications and the cross-sectional shapes of the score line grooves are trapezoidal.

A cutting tool with radius (rt inFIG. 5) of 37.5 mm was used in milling the striations30for all of the golf club heads, No.21, No.22, and No.31to No.37.

The “θ0” inFIG. 12is the θ0depicted inFIG. 1, an angle formed by an arrangement direction of the striations30, i.e., the d0inFIG. 1, and the score line groove. The “Ra” is actual measured value of the arithmetic mean deviation of the profile on the face in which the striations are formed.

The “amount of spin” inFIG. 12depicts the amount of spin on the ball. The amount of Spin is calculated by marking the ball prior to the shot, and using a video camera to track the change in the location of the mark at time of impact.

The experiment involved using golf clubs with each of the golf club heads No.21, No.22, and No.31to No.37attached, and having three testers hit a golf ball out of the rough, aiming at a target 40 yards away. The three testers hit five balls with the face in direct line with the target direction, and five balls with the face opened. The angle at which the face was opened was left up to the testers' discretion.

The “normal”, under the amount of spin heading inFIG. 12, is the average value of the amount of spin when the face is in direct line with the target direction, and the “open” is the average value of the amount of spin when the face is opened.

FIG. 13Agraphs the relationship between the amount of spin and the Ra experimental findings depicted inFIG. 12. It is apparent that the rougher the face, the more spin on the ball, for both the normal and the open circumstance. The slope of the plot becomes steeper near the point where Ra reaches 4 μm, which suggests that the Ra of not less than 4 μm is preferable. Taking into account such factors as the fact that the rougher the face, the easier it is to damage the ball, as well as regulations pertaining to the surface roughness of the face on golf club heads that are used in official competition play, suggests that the arithmetic mean deviation of the profile Ra on the face of between 4.00 μm and 4.57 μm, inclusive, is preferable.

FIG. 13Bgraphs the relationship between the amount of spin and the θ0experimental findings depicted inFIG. 12for the golf club heads No.21, No.22, and No.35to No.37, all of which have identical surface roughness on the face, i.e., Ra: 4.4 μm.

It is apparent that the amount of spin increases as the θ0ranges from 0 to the vicinity of 55 degrees, and then declines as the θ0exceeds 55 degrees, for both the normal and the open circumstance. For the range of θ0between approximately 30 and 80 degrees, centering on the vicinity of 55 degrees, an amount of spin of 7000 rpm or more may be obtained in the open circumstance, which suggests that a sufficient amount of spin may be obtained in the open circumstance when the θ0is between 40 and 70 degrees, inclusive.

EXAMPLE

An experiment was performed to evaluate amount of spin on the ball in comparative examples, as well as the example of the present invention.FIG. 14Adepicts the specification of the example of the present invention and comparative examples 1 to 3, andFIG. 14Bdepicts the findings of the experiment performed on the specification of the example of the present invention and the comparative examples 1 to 3. The example of the present invention and the comparative examples 1 to 3 are all sand wedges with a loft angle of 56 degrees.

The meanings of the respective items listed under the “score line groove specifications” heading inFIG. 14Aare the same as the meanings for the respective items inFIG. 10. The cross-sectional shape of the score line groove in the example and the comparative example 3 are the cross-sectional shapes depicted inFIGS. 2 and 4A. The cross-sectional shapes of the score line grooves in the comparative example 1 and the comparative example 3 are the cross-sectional shapes depicted inFIG. 9A.

The “milling” inFIG. 14Arefers to the presence or absence of milling of a face. The example and the comparative example 2 have had their faces subjected to circular arc striation30milling as depicted inFIG. 1. An cutting tool with radius (rt inFIG. 5) of 37.5 mm was used in milling the striations30. The comparative example 1 and the comparative example 3 have not had their faces subjected to milling. The “Ra” inFIG. 14Aactual measured value of the arithmetic mean deviation of the profile on the face in which the striations are formed.

In summary, while the comparative example 1 and the comparative example 2 have common score line groove specifications, they differ with regard to the surface roughness on the face. While the comparative example 3 and the example have common score line groove specifications, they differ with regard to the surface roughness on the face. While the comparative example 2 and the example have the same surface roughness on the face, they differ with regard to the score line groove specifications.

The experiment involved using golf clubs with each of the golf club heads in the example and the comparative examples 1 to 3 attached, and having three testers hit a golf ball out of the rough, aiming at a target 40 yards away. The three testers hit five balls from the fairway, and five balls from the rough.

InFIG. 14B, the degree of scratches is determined by visual and tactile examination of the level of scratches on the surface of the ball after each shot from the fairway, by the three testers, who rank the level of scratches on a four-step scale. Damage to the ball is ranked from most to least in the order of X→Δ→◯→⊚.

InFIG. 14B, the “amount of spin” depicts the amount of spin on the ball. The amount of Spin is calculated by marking the ball prior to the shot, and using a video camera to track the change in the location of the mark at time of impact. Under the amount of spin heading inFIG. 14B, the subheading “fairway” lists the average amount of spin values for the balls hit from the fairway, and the subheading “rough” lists the average amount of spin values for the balls hit from the rough.

Turning to the degree of scratches, the comparative example 3 and the example had low levels of scratches on the surface of the ball, whereas the comparative example 1 and the comparative example 2 had high levels of scratches on the ball. It is conceivable that the resulting degree of scratches to the ball is a consequence of the score line groove specifications. As depicted inFIG. 14A, the angle θ1of the comparative example 1 and the comparative example 2 is smaller than the comparative example 3 and the example, and has edged score line grooves. In addition, the score line grooves of the comparative example 3 and the example have rounding on the edges, with a radius 0.2 mm, whereas the score line grooves of the comparative example 1 and the comparative example 2 do not have rounding on the edges.

In a comparison of the comparative example 1 and the comparative example 2, the comparative example 2 has the higher degree of scratches, whereas in a comparison of the embodiment and the comparative example 3, the embodiment has the higher degree of scratches. It is conceivable that the resulting degree of scratches to the ball is a consequence of the milling.

Turning the amount of spin,FIG. 15graphs the amount of spin from the findings of the experiment inFIG. 14B. There is no significant difference in the shots from the fairway with the example and the comparative examples 1 to 3. There is a difference in the shots from the rough, however.

Of the example and the comparative examples 1 to 3, it is apparent that the example shows the smallest difference between the shots from the fairway and the shots from the rough. While the comparative example 3 and the example have common score line groove specifications, a significant difference emerges in the amount of spin with the shots from the rough, suggesting, accordingly, that the presence or absence of the milling has an effect thereupon.

With an overall assessment of the degree of scratches and the amount of spin, the comparative example 1 and the comparative example 2 are inferior to the comparative example 3 and the example with regard to the degree of scratches. While the comparative example 3 fared best with regard to the degree of scratches, it also experienced significant degradation in amount of spin with the shots from the rough, suggesting, accordingly, that the example is the best of all.