Patent Publication Number: US-7901297-B2

Title: Golf club head

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a golf club head. 
     2. Description of the Related Art 
     Generally, on the face of a golf club head, a plurality of straight grooves are formed parallel to each other in the toe-and-heel direction. These grooves are called score lines, marking lines, face lines, or the like (to be referred to as score lines in this specification). These score lines have an effect of increasing the backspin amount of a shot or suppressing a significant decrease in the backspin amount of a shot in the case of a rainy day or a shot from the rough. 
     However, the amount of increase in the backspin amount of a shot obtained by changing the design of score lines is limited. To solve this problem, Japanese Patent Laid-Open Nos. 9-253250, 2008-23178, 2008-132168, 2008-132169, 2008-136619, and 2008-136833 each disclose an iron golf club head in which striations are formed on the face by milling in addition to score lines. Also, Japanese Patent Laid-Open No. 8-229169 discloses a patter head in which striations are formed on the face by milling. 
     Like the golf club heads disclosed in Japanese Patent Laid-Open Nos. 9-253250, 2008-23178, 2008-132168, 2008-132169, 2008-136619, and 2008-136833, forming striations on the face in addition to score lines can increase the backspin amount of a shot. However, the striations may clog with fine pieces of grass or the like. In this case, the backspin amount of a shot may become unsteady. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a golf club head which can provide a higher backspin amount more steadily. 
     According to the present invention, there is provided a golf club head including a plurality of score lines on a face, comprising a plurality of striations formed on said face and extending from a toe side to a heel side, wherein a depth Dp of each striation satisfies 10 μm≦D≦40 μm, a cross-sectional shape of each striation is a isosceles trapezoid having an upper base at the face side and a lower base at a back side of the golf club head, the upper base and lower base satisfy the upper base&gt;the lower base, and a width Pb of each protrusion formed between the striations and a width Wb of the lower base satisfies Pb&lt;Wb. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the outer appearance of a golf club head  1  according an embodiment of the present invention; 
         FIG. 2  shows a sectional view of a score line  20  and striations  30  in a direction perpendicular to the longitudinal direction (toe-and-heel direction), and an enlarged sectional view of the striations  30 ; 
         FIG. 3  is a view for explaining a width W of the score line  20  based on the 30 degrees measurement rule and a distance S between the score lines  20 ; 
         FIG. 4  is a view showing another example (score line  20 ′) of the score line  20 ; 
         FIG. 5A  is a view showing a case in which a face  10  is directed perpendicularly to the target direction, and  FIG. 5B  is a view showing a case in which the face  10  is open; 
         FIG. 6  is a view showing the outer appearance of a golf club head  2  in which striations  30  intersect score lines  20 ; 
         FIGS. 7A and 7B  are views for explaining the method of forming the striations  30  by an NC milling machine; and 
         FIG. 8  is a table showing the specifications of striations of golf club heads #1 to #4 and #11 to #24, the evaluation results of the backspin amounts and degrees of clogging of striations, and the rule conformance associated with the surface roughness of the face. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  is a view showing the outer appearance of a golf club head  1  according to an embodiment of the present invention.  FIG. 1  shows an example in which the present invention is applied to an iron golf club head. The present invention is suitable for iron golf club heads, and particularly for middle iron golf club heads, short iron golf club heads, and wedge golf club heads. More specifically, the present invention is suitable for golf club heads with loft angles of 30° to 70° (both inclusive) and head weights of 240 g to 320 g (both inclusive). However, the present invention is also applicable to wood or utility (hybrid) golf club heads. 
     The golf club head  1  has a plurality of score lines  20  formed on its face (hitting surface)  10 . The respective score lines  20  are straight grooves extending in the toe-and-heel direction and parallel to each other. The golf club head  1  also has a plurality of striations  30  formed on the face  10 . In this embodiment, the striations  30  are straight grooves parallel to the score lines  20  and extending in the toe-and-heel direction. However, they may be arcuate or elliptically arcuate grooves extending in the toe-and-heel direction.  FIG. 2  shows a sectional view of the score line  20  and striations  30  in a direction perpendicular to the longitudinal direction (toe-and-heel direction), and an enlarged sectional view of the striations  30 . In  FIG. 2 , a dashed line  10 ′ represents a virtual line on the same plane as the face  10 . The score lines  20  will be described first. 
     In this embodiment, the cross-sectional shapes of the score lines  20  are the same except in two end portions in the longitudinal direction. The score lines  20  have the same cross-sectional shape. In addition, in this embodiment, the cross-sectional shape of the score line  20  is symmetric with regard to a virtual center line CLa in the width direction. The virtual center line CLa is perpendicular to the face  10  and passes the middle point of a width W of the score line  20 . In this embodiment, the cross-sectional shape of the score line  20  is a trapezoid, but it may be another shape such as a V-shape. 
     The score line  20  has a pair of side surfaces  22  and a bottom surface  23 . An edge  21  of the score line  20  is the boundary portion of the side surface  22  and face  10 . The edge  21  may be rounded. An angle θ 0  is the angle between the side surface  22  and face  10 . The larger the angle θ 0 , the higher the backspin amount of a shot. The bottom surface  23  is parallel to the face  10 . A depth D 0  is the length from the face  10  to the bottom surface  23  as the deepest portion of the score line  20 . In the case of a golf club head for competitions, the depth D 0  must be 0.020 inches (0.508 mm) or less according to the rules. 
       FIG. 3  is a view for explaining the width W of the score line  20  based on the 30 degrees measurement rule and a distance S between the score lines  20 . The width W indicates the width measured based on the so-called 30-degree measurement method as a rule for a golf club for competitions. That is, the width W indicates the distance between the contact points of the respective virtual lines with an angle of 30° with respect to the face  10  and the respective edges  21  of the score line  20 . The distance S between the score lines  20  indicates the distance between the contact points of the respective virtual lines with an angle of 30° with respect to the face  10  and the respective edges  21  of the score lines  20  adjacent to one another. 
     For a golf club head for competitions, the cross-sectional area, width W, and distance S of the score line  20  need to satisfy a cross section area A (inch 2 )/(W (inch)+S (inch))≦0.003 (to be referred to as the area rule, hereinafter). The metric system expresses the cross section area A (mm 2 )/(W (mm)+S (mm))≦0.0762. In order to make the golf club head  1  as a golf club head for competitions, it is designed to satisfy the area rule. 
     As a rule about score lines of a golf club head for competitions, in addition to the area rule, each edge of a score line must be positioned within a virtual circle with a radius of 0.011 inches concentric with a virtual circle with a radius of 0.010 inches which internally touches the side surface of the score line and the face (to be referred to as a two-circle rule, hereinafter). 
     In order to satisfy the two-circle rule, however, it is necessary to decrease the angle (the angle θ 0  described above) between each side surface of the score lines and the face. In this case, not only a decrease in backspin amount but also a decrease in the volume of the score line occurs. Accordingly, a backspin amount may significantly decrease in the case of a shot from the rough or a shot on a rainy day. 
       FIG. 4  is a view showing another example (score line  20 ′) of the score line  20 . The same parts as in the score line  20  are denoted by the same reference numerals in the score line  20 ′, and the description thereof will not be repeated. Only the parts different from the score line  20  will be described. 
     In  FIG. 4 , a virtual circle C 1  is a circle with a radius of 0.010 inches which internally touches the side surface  22  and face  10 . A virtual circle C 2  is a circle with a radius of 0.011 inches which is concentric with the virtual circle C 1 . In order to conform to the two-circle rule described above, the edge of the score line needs to be positioned within the virtual circle C 2 . 
     In the score line  20 ′, a flat surface  21 ′ is formed in an edge portion to satisfy the two-circle rule. Note that the shape of the edge may be a rounding or notch other than the flat surface  21 ′. In order to make the golf club head  1  as a golf club head for competitions, it is designed to satisfy the two-circle rule as well. 
     The striations  30  will be described next with reference to  FIG. 2 . In this embodiment, a plurality of the striations  30  is arranged at an equal pitch in a direction (a direction perpendicular to the longitudinal direction of the score line  20 ) perpendicular to the longitudinal direction of the striation  30 . However, the striations  30  need not be arranged at an equal pitch. 
     The striation  30  has a pair of side surfaces  31  and a bottom surface  32 . The cross-sectional shape of the striation  30  is symmetric with regard to a virtual center line CLb in the width direction. The cross-sectional shape of the striation  30  is an isosceles trapezoid having an upper base (a width Wu) at the face  10  side and a lower base (a width Wb) at the back side of the golf club head  1 , in which the width Wu&gt;the width Wb. The cross-sectional shapes of the striations  20  are the same except in two end portions in the longitudinal direction. An angle θ is the angle formed by a pair of the side surfaces  31 . A depth Dp is the length from the face  10  (virtual line  10 ′) to the bottom surface  32 . 
     As a result of formation of the striations  30 , protrusions  40  are formed between the striations  30  on the face  10 . The protrusion  40  has a bottom portion with a width Pb and a top portion (top surface) with a width Pu, in which the width Pb&lt;the width Wb. The cross-sectional shape of the protrusion  40  changes depending on the specifications of the striations  30 , and is an isosceles trapezoid as shown in  FIG. 2  or a triangle (Pu≈0). A pitch P indicates the arrangement interval of the protrusions  40 , and has the same definition as the pitch of the striations  30 . 
     The protrusions  40  catch a ball surface at impact, thereby providing a backspin to the ball. In this manner, in this embodiment, it is possible to increase the backspin amount of a shot by the protrusions  40  (striations  30 ) in addition to the score lines  20 . 
     In this embodiment, the cross-sectional shape of the striation  30  is formed to be an isosceles trapezoid that satisfies the width Wu of the upper base&gt;the width Wb of the lower base. With this arrangement, grass or the like is not easily caught in the striations  30 , and grass or the like in the striations  30  readily comes out. Accordingly, it is possible to prevent the striations  30  from clogging with fine pieces of grass or the like. In addition, since the bottom surface  32  of the striation  30  is widely formed so as to satisfy the width Pb&lt;the width Wb, it is possible to further prevent the striations  30  from clogging with fine pieces of grass or the like. 
     When the depth Dp is too small, the effect of increasing the backspin amount decreases. For this reason, the depth Dp needs to be 10 μm or more. On the other hand, the larger the depth Dp, the more easily the protrusion  40  catches a ball surface, but a ball surface is easily damaged when the depth Dp is too large. For this reason, the depth Dp is set to be 40 μm or less. Accordingly, the depth Dp satisfies 10 μm≦Dp≦40 μm. Note that for a golf club head for competitions, the surface roughness of the face is determined to be 25 μm or less at a maximum height (Ry) by the rule. Therefore, in order to make the golf club head  1  as a golf club head for competitions, the depth Dp is set to satisfy 10 μm≦Dp≦25 μm. 
     When the width Pu of the top portion of the protrusion  40  is too large, the effect of increasing the backspin amount decreases. Accordingly, the depth Pu is preferably set to 10 μm or less. The smaller the width Pu, the more easily the protrusion  40  catches a ball surface, and the effect of increasing the backspin amount can increase. Accordingly, the width Pu preferably satisfies 0 μm&lt;Pu≦15 μm. 
     The striations  30  and protrusions  40  influence the arithmetic mean roughness (Ra) of the face  10 . The larger the surface roughness of the face  10 , the more the backspin amount increases, but a ball surface is easily damaged. Accordingly, the surface roughness of a portion of the face  10  with the striations  30  is preferably 1.0 μm to 5.0 μm (both inclusive) in arithmetic means roughness (Ra), and more preferably 1.5 μm or more. Note that for a golf club head for competitions, the surface roughness of the face is determined to be 4.57 μm or less in arithmetic mean roughness (Ra) by the rules. Therefore, in order to make the golf club head  1  as a golf club head for competitions, it is designed such that the arithmetic means roughness (Ra) of the face  10  is 4.57 μm or less. 
     When the angle θ is too small, the protrusion  40  becomes too thin and wears out quickly. Accordingly, the angle θ is preferably 30° or more. The larger the angle θ, the less easily the protrusion  40  catches a ball surface, and the effect of increasing the backspin amount decreases. Accordingly, the angle θ is preferably 140° or less. 
     The smaller the pitch P, the more easily the striations  30  clog with fine pieces of grass or the like, and the protrusions  40  do not easily catch a ball surface. Accordingly, the pitch P is preferably 100 μm or more. On the other hand, when the pitch P is too large, the number of protrusions  40  that catch a ball surface decreases, and therefore the effect of increasing the backspin amount decreases. Accordingly, the pitch P is preferably 600 μm or less. 
     When using a short iron or wedge, a player sometimes hits a golf ball with the face open so that backspin is easily provided to the ball.  FIG. 5A  shows a case in which the face  10  is directed perpendicularly to the target direction, and  FIG. 5B  shows a case in which the face  10  is open. Note that the striations  30  are not shown in  FIGS. 5A and 5B . In each of  FIGS. 5A and 5B , the arrow represents the relative displacement direction of a golf ball B with respect to the face  10  at impact. 
     When the face  10  is open as shown in  FIG. 5B , the golf ball B slides on the face  10  such that it diagonally crosses the score lines  20  and striations  30  at impact. When the direction (i.e., the arrangement direction) perpendicular to the longitudinal direction of the striations  30  is more parallel to the direction indicated by the arrow in  FIG. 5B , the number of the striations  40  that catch the surface of the golf ball B increases and backspin is easily provided to the golf ball B. For this reason, the striations  30  may be formed not to be parallel to the score lines  20  but to intersect them. 
       FIG. 6  is a view showing the outer appearance of a golf club head  2  in which striations  30  intersect score lines  20 . The golf club head  2  is different from the golf club head  1  only in an arrangement direction dr of the striations  30 . In the example shown in  FIG. 6 , an angle θr between the arrangement direction dr and the longitudinal direction of the score line  20  is about 45° in the clockwise direction from the toe side of the score line  20 . When the arrangement direction dr of the striations  30  is set as described above, the backspin amount of a shot with a face  10  open can be increased. 
     For example, the angle θr can be set within a range of 20° to 90° (both inclusive). For a short iron or wedge, since a player often hits a ball with the face  10  open, it is desirable to set the angle θr within a range of 40° to 70° (both inclusive). 
     The method of forming the striations  30  will be described next. The striations  30  can be formed as cut traces by milling of the face  10 . The striations  30  can be formed by milling by using, for example, an NC (numerically controlled) milling machine.  FIGS. 7A and 7B  are views for explaining the method of forming the striations  30  by an NC milling machine. 
     As shown in  FIG. 7A , a golf club head  1 ′ without the score lines  20  and striations  30  is fixed to the NC milling machine via a jig  3 . Note that in this embodiment, a case in which the face  10  is integrally formed with the golf club head is described. However, a face member which forms the face  10  and the head body may be prepared as separate members and joined together. In this case, the face member is fixed to the NC milling machine to form the score lines  20  and striations  30 . 
     The NC milling machine includes a spindle  4  which is rotatably driven around the axis Z. A cutting tool (end mill)  5  is attached to the lower end of the spindle  4 . The distal shape of the cutting tool  5  is selected depending on the cross-sectional shape of the striation  30 . As shown in  FIG. 7B , the cutting tool  5  having an isosceles trapezoidal shape can form one striation  30  without reciprocating the cutting tool  5 . 
     After setting the plane coordinates of the face  10  in the NC milling machine, the spindle  4  is rotatably driven. The face  10  (golf club head  1 ′) or cutting tool  5  is moved relatively in the formation direction of the striations  30  to cut the face  10 . When one striation  30  has been formed, the cutting tool  5  is separated from the face  10 . After that, the cutting tool  5  is moved relatively in the arrangement direction of the striations  30 , and the next striation  30  is formed. In this manner, the striations  30  are sequentially formed. When all the striations  30  are formed, the cutting tool  5  is changed to form the score lines  20 . 
     Note that as the method of forming the score lines  20  and striations  30 , forming methods other than milling such as electrical discharge machining, casting, or the like can be employed. 
     When the striations  30  are formed on the face  10 , since the surface hardness of the face  10  decreases, the face  10  may be easily worn out. For this reason, it is preferable to perform surface treatment for increasing the hardness of the face  10  after forming the striations  30 . As such surface treatment, cementing, nitriding, soft nitriding, PVD (Physical Vepor Deposition), ion plating, DLC (Diamond Like Carbon) treatment, plating, or the like is available. Particularly, surface treatment such as cementing or nitriding is preferable which reforms a surface without forming another metal layer on it. 
     EXAMPLES 
     Golf club heads #1 to #4 and #11 to #24 that were different in the specifications of striations were fabricated, and the backspin amounts and degrees of clogging of striations were evaluated using golf clubs respectively mounted with those golf club heads.  FIG. 8  is a table showing the specifications of striations of golf club heads #1 to #4 and #11 to #24, the evaluation results of the backspin amounts and degrees of clogging of striations, and the rule conformance associated with the surface roughness of the face. 
     All golf club heads #1 to #4 and #11 to #24 were sand wedges that were different only in the specifications of striations. A plurality of striations was formed parallel to score lines, like the golf club head  1  in  FIG. 1 . 
     In  FIG. 8 , “shape of striation” means the cross-sectional shape of a striation. Only in golf club head #1, the cross-sectional shape of the striation is rectangular, and in each of other golf club heads, the cross-sectional shape of the striation is an isosceles trapezoid as shown in  FIG. 2 . 
     “Wb/Pb” corresponds to the ratio of the width Wb of the lower base of the striation  30  and the width Pb of the bottom portion of a protrusion shown in  FIG. 2 . When Wb/Pb is smaller than 1, the width Pb is larger than the width Wb. When Wb/Pb is larger than 1, the width Wb is larger than the width Pb. In each of golf club heads #2 to #4, the width Pb is larger than the width Wb. “Pu” corresponds to the width Pu of the top portion of the protrusion  40  shown in  FIG. 2 . “θ” corresponds to the angle formed by a pair of the side surfaces  32  of the striation  30  shown in  FIG. 2 . “Dp” corresponds to the depth Dp of the striation  30  shown in  FIG. 2 . “P” corresponds to the pitch P of the protrusions  40  shown in  FIG. 2 . These are all set values. “Surface roughness (Ra)” indicates the arithmetic mean roughness of the face and shows actually measured values. 
     The backspin amounts and the degrees of clogging of striations were evaluated by hitting a plurality of golf balls with each of the golf clubs from the rough about 30 yards away from the green. “Spin amount” in  FIG. 8  was evaluated in four levels (A to D) by observing how readily the ball stopped on the green. Level A means that the ball was most readily stopped, that is, the backspin amount was the highest. “Degree of clogging” was evaluated by visually observing the degree of clogging of the striations after the test, and evaluated in three levels (A to D). Level A means that clogging was the least. In “rule conformance”, a golf club head with arithmetic mean roughness (Ra) of 4.57 μm or less and a maximum height (Ry) of 25 μm or less is indicated by (◯) meaning “conform”, and otherwise indicated by (X) meaning “not conform”. 
     From the comparison between golf club heads #1 and #11, it is obvious that when the cross-sectional shape of each of the striations is an isosceles trapezoid, clogging of striations is less. Although the degrees of clogging are C and D in golf club heads #2 to #4, the degrees of clogging are A to C in golf club heads #11 to #24. Accordingly, it is obvious that clogging of striations is less when the width Wb is larger than the width Pb. 
     From the comparison between golf club heads #11 to #14 having the same pitch P, it is obvious that the higher backspin amount can be obtained when the width Pu is smaller. Since a certain backspin amount was obtained in golf club head #14, the width Pu is preferably 15 μm or less. 
     From the comparison between golf club heads #15 to #18 and #20 having the same pitch P and the same depth Dp, it is obvious that clogging of striations is less when the angle θ is larger. Since a certain backspin amount was obtained in golf club head #15 but a backspin amount obtained in golf club head #18 was not very high, the angle θ is preferably between 30° to 140° (both inclusive). 
     From the comparison between golf club heads #19 to #21, it is obvious that the higher backspin amount can be obtained when the depth Dp is larger. Since a certain backspin amount was obtained in golf club head #20 but a lowest backspin amount was obtained in golf club head #19, the depth Dp needs to be 10 μm or more. On the other hand, in golf club head #21, the backspin amount was high but some flaws were identified on the ball surface from visual observation. Accordingly, from the viewpoint of how easily a ball is damaged, the depth Dp needs to be 40 μm or less. Note that as described above, for a golf club head for competitions, the surface roughness of the face is determined to be 25 μm or less at a maximum height (Ry) by the rule. Therefore, in order to make a golf club head for competitions, the depth Dp is set to satisfy 10 μm≦Dp≦25 μm. 
     From the comparison between golf club heads #22 to #24, it is obvious that clogging of striation is less when the pitch P is larger. Since a certain backspin amount was obtained in golf club head #25, the pitch P is preferably 600 μm or less. On the other hand, since there was a certain degree of clogging of striations in golf club head #22, the pitch P is preferably 100 μm or more. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2008-262060, filed Oct. 8, 2008, which is hereby incorporated by reference herein in its entirety.