Golf club head and method of manufacturing the same

A face portion 11 of a golf club head 10 is obtained press-forming a metal plate into a predetermined shape. On the front surface 11 a of the face portion 11 , score lines 12 are formed by means of die sinking of electrical discharge machining, and a mirror finished surface portion 13 is formed by means of finish processing of electrical discharge machining. On the rear surface 11 b of the face portion 11 , a rib structure 15 , and a high hardness layer 16 of an amorphous metal layer are formed. The electrical discharge machining is numerically controlled.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by way of preferred embodiments with reference to the attached drawings. Note that the term “processing” includes machining throughout the specification. &lsqb;First Embodiment&rsqb; &lsqb;Constitution of First Embodiment&rsqb; Referring to the FIGS. 1A to 1 E, a constitution of a golf club head according to a first embodiment of the present invention will be described. FIGS. 1A to 1 E show a golf club head of the first embodiment, which is a golf club head of a metal wood, and its members. The golf club head 10 of the first embodiment is fabricated in such a manner that a face portion 11 is joined to an opening 19 a of a body portion 19 made of stainless steel or titanium by welding or the like method. The face portion 11 is formed by press-forming a metal plate having a thickness of 2.8 mm into a predetermined shape, the metal plate being made of stainless steel alloy, titanium alloy or the like metal. As shown in FIG. 1 B, on a front surface 11 a , score lines 12 are formed by means of die sinking (i.e., processing of shaping and/or engraving a target material) of electrical discharge machining (hereinafter, die sinking of electrical discharge machining will be occasionally referred to as “electrical discharge die sinking”) using an electrode in the shape reverse to that of the score lines 12 . The electrical discharge machining is numerically controlled, that is, the steps of the electrical discharge machining are programmed in a computer and controlled by the computer, according to the first embodiment and the subsequent embodiments described below. As shown in FIG. 1 D, the score lines 12 , each of which has a width a of approximately 0.9 mm or less and a depth b of approximately 0.508 mm or less, are arranged with a spacing c therebetween of approximately 3×a (i.e., three multiplied by a) mm or more. During the process of the score lines 12 through the electrical discharge die sinking, a transmuted layer (i.e., layer whose property or quality is changed) serving as a high hardness layer 14 is formed on the front surface 11 a . A hardness of the front surface 11 a can be controlled by controlling the processing speed. Thereafter, the surface of the high hardness layer 14 is subjected to finish processing of electrical discharge machining (hereinafter, referred to as “electrical discharge finish processing”) so as to obtain a mirror finished surface portion 13 . A surface roughness of the mirror finished surface portion 13 can be made to a desired value by appropriately determining a processing speed and a clearance (i.e., gap) between the electrode and the face portion 11 . As shown in FIG. 1 C, on the rear surface 11 b of the face portion 11 , a rib structure 15 is formed by means of electrical discharge die sinking using a silicon electrode in the shape reverse to that of the rib structure 15 . The rib structure 15 is constituted by ribs each having a width d of approximately 2.0 mm and a height e of approximately 0.5 mm and arranged in the shape of mesh in which a distance f between neighboring ribs is approximately 3.0 mm. Note that the height a of the ribs obtained through electrical discharge die sinking can be made considerably high (that is, the rib structure obtained through electrical discharge die sinking has a considerably deeply engraved patterns). During the process of forming the rib structure 15 through electrical discharge die sinking, an amorphous metal layer serving a high hardness layer 16 is formed, as shown in FIG. 1E . Note that, in the case of the first embodiment, the amorphous metal layer serving as the high hardness layer 16 is formed only on the rear surface. This is because the front surface 11 a is polished and/or thereafter painted, and therefore even though the amorphous metal layer is formed on the front surface 11 a , the amorphous layer is removed or concealed, so that it is useless to form the amorphous metal layer on the front surface 11 a. The high hardness layer 16 has a thickness of approximately 4 &mgr;m. The layer 16 contains 16% of silicon, thereby exhibiting corrosion resistant and wear resistant properties. The high hardness layer 16 has a micro-Vickers hardness of approximately 1000 (in the case of stainless steel alloy), which is markedly high compared with that of the parent metal. Also, the layer 16 has a high toughness and a high strength (e.g., tensile strength), compared with the parent metal. Note that, while both the high hardness layer 16 of the amorphous metal layer and the high hardness layer 14 of the transmuted layer are common in exhibiting high hardnesses, the degree of hardness of the layer 16 is greater than that of the layer 14 because the amorphous metal layer exhibits a higher hardness than the transmuted layer. Further, in general, with respect to hardness, strength (e.g., tensile strength) and toughness, an amorphous metal layer exhibits a greater property than a transmuted layer if the other conditions are equal. With respect to the entire face portion 11 , the thickness t thereof is made to a small value of 2.2 mm. This is because the front surface 11 a has a high hardness layer 14 of the transmuted layer and because the rear surface 11 b has the high hardness layer 16 of the amorphous layer and the reinforcing rib structure 15 . &lsqb;Advantages of First Embodiment&rsqb; Owing to the above constitution, the golf club head 10 of the first embodiment exhibits the following advantages. Specifically, with respect to the front surface of the face portion 11 , owing to the constitution in which the high hardness layer 14 , and a mirror finished surface portion 13 , the surface roughness of which is controllable, are formed, the spin amount of the hit ball can be controlled. With respect to the rear surface of the face portion 11 , owing to the constitution in which the high hardness layer 16 of the amorphous metal layer is formed and accordingly the properties (i.e., hardness, strength and toughness) of the material itself is greatly enhanced, the thickness of the face portion 11 is made to a markedly small value compared with the face portion obtained through conventional machine processing. Also, owing to the constitution in which the rib structure having a deep engraved patters is formed on the rear surface for reinforcing the face portion 11 , the strength of the face portion 11 is greatly increased. With respect to the entire face portion 11 , the high hardness layer 14 of the transmuted layer is formed on the front surface 11 a , and the high hardness layer 16 of the amorphous metal layer and the reinforcing rib structure 15 are formed on the rear surface 11 b . Owing to this constitution, the thickness of the face portion 11 is made to be stronger and thinner, so that the rebounding performance of the face portion 11 is greatly enhanced. Further, since the steps of electrical discharge machining is numerically controlled, the machine for electrical discharge machining can be operated for full 24 hours. Accordingly, the quality of the respective processed portions can be standardized, and each member of the golf club head can be mass-produced, so that the labor cost (i.e., wage per worker) is lowered, thereby lowering the manufacturing cost. &lsqb;Second Embodiment&rsqb; &lsqb;Constitution of Second Embodiment&rsqb; Referring to FIGS. 2A to 2 E and FIG. 3, a constitution of a golf club head according to a second embodiment of the present invention will be described. FIGS. 2A to 2 E show a golf club head of the second embodiment, which is a golf club head of an iron, and FIG. 3 shows the golf club head seen from the rear side. As shown in FIG. 2 A, the golf club head 20 is fabricated in such a manner that the face portion 21 is attached to the attaching portion 29 a of the body 29 having a cavity back structure made of stainless steel or the like by welding or the like method. The face portion 21 is obtained by press-forming a metal plate having a thickness of 2.4 mm into a predetermined shape, the metal plate being made of stainless steel alloy, titanium alloy, or the like metal. As shown in FIGS. 2B and 2D , on the front surface 21 a , score lines 22 are formed by electrical discharge die sinking using a silicon electrode in the shape reverse to that of the score lines 22 . The score lines 22 , each of which has a width of approximately 0.9 mm or less and a depth of approximately 0.508 mm or less, are arranged with a spacing therebetween of approximately 2.0 mm or more (i.e., a value not less than three-fold of the width). During the process of the score lines 22 through electrical discharge die sinking, an amorphous metal layer serving as a corrosion resistant and wear resistant layer 24 is formed on the front surface 21 a . The layer 24 has a thickness of approximately of 4 &mgr;m, the layer 24 contains 16% of silicon, thereby exhibiting corrosion resistant and wear resistant properties. Accordingly, it becomes unnecessary to apply a posttreatment, such as plating and the like, to the front surface. The surface of the corrosion resistant and wear resistant layer 24 is subjected to electrical discharge finish processing so as to form a roughly finished surface portion 23 . A surface roughness of the roughly finished surface portion 23 can be controlled by controlling the clearance between the electrode and the face portion 21 . Accordingly, a desired surface roughness according to the item (number) of the iron can be obtained by determining the clearance to an appropriate value. As shown in FIGS. 2C and 2E , on the rear surface 21 b of the face portion 21 , a decoration layer 25 in the shape of mesh is formed using an electrode in the shape reverse to that of the mesh-shaped decoration layer 25 by means of electrical discharge die sinking. In the decoration layer 25 , grooves 25 a , each of which has a width g of approximately 6.0 mm and a depth h of approximately 0.3 mm, are arranged in the shape of mesh in which a spacing i between the grooves is approximately 2.2 mm. Also, recesses 25 b each having a diameter j of 2.2 mm and a depth k of approximately 0.3 m, and protrusions 25 c are formed. On the surface of the protrusions 25 c , numerous electrical discharge traces (i.e., traces formed through electrical discharge machining) are formed, thereby greatly enhancing the esthetic effect. Note that, while the main purpose of providing the decoration layer 25 is to enhance the esthetic effect of the rear surface 21 b , the layer 25 also exhibits a reinforcing performance as the rib structure 15 of the first embodiment because the decoration layer 25 has a rib-like structure. During the process of forming the decoration layer 25 through electrical discharge die sinking, a transmuted layer serving as a high hardness layer 26 is formed. The layer 26 has a micro-Vickers hardness of approximately 1000. With respect to the entire face portion 21 , the thickness t 2 thereof can be made to a small value of approximately 2.2 mm. This is because the rear surface 21 b has the high hardness layer 26 exhibiting a high hardness and the decoration layer 25 which has a rib-like structure exhibiting a reinforcing performance. &lsqb;Advantages of Second Embodiment&rsqb; As described above, in the second embodiment, the electrical discharge machining is employed as in the first embodiment. Accordingly, the second embodiment exhibits the advantages obtained through the electrical discharge machining in the first embodiment. Also, since the corrosion resistant and wear resistant layer 24 is formed on the front surface 21 a using a silicon electrode, the posttreatment to be applied to the front surface 21 a , such as plating, becomes unnecessary. Further, since the electrical discharge machining is employed, the esthetic effect can be greatly enhance. In this respect, the electrical discharge machining is the most effective processing method for manufacturing the golf club head 20 of the iron. More particularly, in recent years, various materials have been developed. Among such materials, maraging steel, titanium alloy or the like, each of which has a light specific gravity and a high tensile strength, are often used for a face portion of an iron, as well as for a face portion of a metal wood. When manufacturing the golf club head 20 of an iron, the body portion 22 made of stainless steel and the face portion 21 are manufactured separately, and thereafter they are joined to each other, as described above. Here, examples of the method of joining the face portion 21 (in most cases, made of a sheet material) and the body portion 22 include welding, press fitting and brazing. In terms of structure or decorative design of the golf club head of an iron, a hollow structure is rarely found, that is, most of golf club heads of an iron have a cavity back structure. This is because, in the cavity back structure, the weight of the golf club head is distributed in the circumferential portions of the golf club head so as to enlarge the sweet area, so that easiness in hitting and enlargement of the golf club head can be reconciled. In the cavity back structure, the most difficult problem puzzling the persons engaging in the development and designing of the golf club head is treatment of the cavity portion, that is, how the rear surface of the face portion 21 should be applied with complicated designs. This is because, in the case of the cavity back structure, it is so difficult to apply an elaborate decoration to the rear surface by means of conventional machine processing that the rear surface exhibits a poor appearance. Therefore, however beautifully the front surface may be processed, the appearance of the entire face portion 21 becomes poor because, in the case of the cavity back structure, the rear surface having such a poor appearance is seen from the rear side. Under the above circumstances, in conventional art, the decoration of the cavity portion was conducted in the following manner. Specifically, at the processing stage, at most, engraving or press-forming of a simple decoration such as a logomark can be conducted when engraving is applied or score lines are press-formed to the front surface. In the case of a finished golf club head, when a complicated decoration is intended to be applied to the cavity portion, plastic- or aluminum-made patches are attached on the cavity portion of the finished golf club head. In contrast, according to the second embodiment of the present invention, by taking advantage of the characteristic feature of the electrical discharge die sinking that processing can be conducted in a highly arbitrary manner so as to enable formation of complicated shapes, it becomes possible to form the decoration layer 25 which has a stable processed surface (that is, while the processed surface obtained through press-forming or engraving contains distortion formed during the press-forming or engraving, the processed surface obtained through electrical discharge machining is free from such distortion) and exhibits a deluxe appearance. As described above, according to the second embodiment, the performances of the iron can be exhibited as much as possible, and also excellent design or esthetic effect, such as a deluxe appearance, can be exhibited. &lsqb;Third Embodiment&rsqb; &lsqb;Constitution of Third Embodiment&rsqb; Referring to FIG. 4, a golf club head according to a third embodiment of the present invention will be described. FIG. 4 shows the golf club head 30 of the third embodiment, which is a golf club head of a short iron having a hollow structure. As described in conventional art, in the short iron (for example, Pitching Wedge &lsqb;PW&rsqb;, Approach Wedge &lsqb;AW&rsqb;, and Sand Wedge &lsqb;SW&rsqb;), the hitting sensation and the hitting direction are important. In the short iron, the approaching performance is also important. Here, the approaching performance is composed of two performances of allowing an actual hit distance to be as close as an estimated hit distance and of causing the hit ball to stop as close as possible to the point where the ball has fallen down. The approaching performance is important in the short iron because the short iron is used at a short play in which the pin (the pole with a flag standing in the hole) is aimed at deadly (i.e., directly). Note that degrees required for these performances differ according to the items of the short iron. In view of the above circumstances, in the second embodiment, a roughly finished surface portion 33 having a surface roughness RMS of 4.5 &mgr;m or less is formed on the front surface of the face portion 31 made of a metallic material by means of electrical discharge finish processing, the surface roughness being determined according to the item (number) of the short iron. In addition, materials which cause the hit ball to greatly spin are selected. As examples of metals having an excellent vibration absorbing performance, silicon carbide, soft iron, or the like can be mentioned. &lsqb;Advantage of Third Embodiment&rsqb; Owing to the above constitution, in the third embodiment, the friction between the face portion 31 and the ball can be controlled according to an item (number) of the iron, so that each of the above performances are realized at a degree suitable for the concerned item. &lsqb;Fourth Embodiment&rsqb; &lsqb;Constitution of Fourth Embodiment&rsqb; Referring to FIGS. 5A to 5 C, a constitution of a golf club head according to a fourth embodiment of the present invention will be described. FIGS. 5A to 5 C show a golf club head of the fourth embodiment, which is a golf club head of a putter, and its members. As shown in FIGS. 5A and 5B , the golf club head 40 of the fourth embodiment is fabricated in such a manner that the face portion 41 is attached to the attachment portion 49 a of the body 49 made of stainless steel by welding or the like method. As described in conventional art, in the putter, a hitting sensation, i.e., a performance of giving a soft hitting sensation at the time of hitting the ball, and an excellent hitting direction, i.e., a performance of causing the hit ball to reach the cup as a target are very important. In conventional putters, these performances are realized chiefly by selecting a suitable material rather than by adopting a suitable processing technique. In contrast, in the fourth embodiment, these performances are realized by, in addition to selecting a suitable material, employing the electrical discharge machining. Specifically, the face portion 41 is obtained by press-forming a metal plate having a thickness of approximately 1.0 mm into a predetermined shape, the metal plate being made of aluminum or other metal which is light and capable to be made thin. As shown in FIGS. 5B and 5C , on the front surface of the face portion 41 , a mirror finished surface portion 43 is formed by means of electrical discharge finish processing using a silicon electrode. During the process of the electrical discharge finish processing, an amorphous metal layer serving as a corrosion resistant and wear resistant layer 44 is formed. The layer 44 has a thickness of 4 &mgr;m and contains 16% of silicon, thereby exhibiting corrosion resistant and wear resistant properties. Accordingly, it becomes unnecessary to apply a treatment, such as plating, to the front surface of the face portion 41 . On the rear surface of the face portion 41 , electrical discharge machining is applied so that a transmuted layer serving as a high hardness layer 46 is formed. The high hardness layer 46 has a micro-Vickers hardness of approximately 1000. Owing to the layer 46 , the thickness t 4 of the face portion 41 can be made to a small value of approximately 0.8 mm. As described above, electrical discharge machining is employed as a processing method. Accordingly, a desired flatness of the face portion 41 is obtained, so that the hitting direction is enhanced. Also, a desired surface roughness of the front surface thereof is obtained, so that the contacting area between the face portion 41 and the ball is made to a small value. Owing to this constitution, the hit ball can roll smoothly, so that an accurate hitting direction and an excellent hitting sensation can be obtained. Further, according to the fourth embodiment, since the face portion 41 is made of a thin and light material, such as aluminum or the like, a greatly enhanced toe-heel balance is obtained. &lsqb;Modifications and Variations&rsqb; The present invention is not restricted to the above-mentioned embodiments, and various modifications and variations thereof which fall within the scope of the present invention is made possible as follows: In the second embodiment, the corrosion resistant and wear resistant layer 24 in the face portion 21 is constituted only by an amorphous metal layer containing silicon. However, the corrosion resistant and wear resistant layer 24 in the face portion 21 may be constituted only by a transmuted layer formed during electrical discharge machining. This is because a transmuted layer also exhibits corrosion resistant and wear resistant properties. Alternatively, the corrosion resistant and wear resistant layer may be configured as shown in FIG. 6 . Specifically, on the parent metal, the transmuted layer 24 A is formed during the electrical discharge die sinking. On the transmuted layer 24 A, finish processing is performed using a silicon electrode so as to form an amorphous layer 24 B which contains silicon and serves as another layer than the transmuted layer 24 A. The surface of the amorphous layer 24 B is subjected to finish processing so as to obtain a mirror finished surface portion 23 - 2 . In this way, the corrosion resistant and wear resistant layer 24 - 2 in the face portion 21 - 2 , which has another configuration than that of the face portion 21 , is formed. Thus, the corrosion resistant and wear resistant layer 24 - 2 is constituted by the combination of the transmuted layer 24 A and the amorphous metal layer 24 B. In short, in the second embodiment, the corrosion resistant and wear resistant layer may be constituted by: only a transmuted layer; only an amorphous layer; or a combination of a transmuted layer and an amorphous layer. Also in the other embodiments, the corrosion resistant and wear resistant layer may be constituted by: only a transmuted layer; only an amorphous layer; or a combination of a transmuted layer (including a transmute layer formed during electrical discharge die sinking) and an amorphous layer which is formed on the transmuted layer as another layer as a result of finish processing. Further, with respect to all the above-mentioned embodiments, the high hardness layer may be also constituted by: only a transmuted layer; only an amorphous layer; or a combination of a transmuted layer and an amorphous layer. Although in the fourth embodiment, the front surface of the putter is mirror finished, the front surface of the putter may be provided with dimple-shaped recesses obtained through electrical discharge die sinking. Further, although the rear surface of the putter according to the fourth embodiment has no rib structure, the rib structure may be formed. &lsqb;Advantages of the Invention&rsqb; As described above in detail, according to the present invention, the electrical discharge machining is employed in processing the front and/or rear (or inner) surfaces of the face portions of various types of golf club heads: the wood, iron, putter. As a result, the transmuted layer is formed on the front and/or rear surfaces. Owing to this constitution, the hardness, strength (e.g., tensile strength), and/or toughness of the material itself of the face portion are enhanced, so that the face portion having a greater strength and a smaller thickness and exhibiting an increased wear resistant property. In particular, when a hitting distance (an enhanced rebounding performance) is intended to be increased, electrical discharge machining using a silicon electrode is employed so as to change a part of the parent metal into an amorphous metal layer containing silicon. Owing to this constitution, the hardness, strength (e.g., tensile strength), and toughness and the like properties can be further enhanced, so that a face portion having a still greater strength and a still smaller thickness and exhibiting a further increased wear resistant property can be obtained. Further, both of the transmuted layer and the amorphous layer can increase, as well as being capable of increasing hardness and the like properties, the corrosion resistant and wear resistant properties, it becomes unnecessary to apply plating or the like treatment on the surfaces of the face portion. In terms of the processed surface, owing to the constitution in which the amorphous metal layer obtained through electrical discharge machining is formed, the corrosion resistant and wear resistant properties are exhibited, so that a posttreatment such as plating becomes unnecessary. Further, owing to the constitution in which the surface roughness of the mirror or roughly finished surface portion can be controlled, the functions of the surface portion, such as spin controlling function, can be controlled appropriately. Further, in terms of the outer appearance, owing to the constitution in which electrical discharge machining is employed, complicated shapes and deep unevenness, which have been unable to obtain through conventional processing methods, can be realized, so that the reinforcing and/or esthetic effects are greatly enhanced. Further, owing to the constitution in which the electrical discharge machining is numerically controlled and the processing can be operated in an accurate manner for full 24 hours, the quality of the manufactured pieces are stabilized, and the labor cost and accordingly the manufacturing costs can be greatly reduced. As described above, by employing the electrical discharge machining, satisfactory results can be obtained as to all the respects: with respect to performance and function; quality; design freedom and esthetic effect (deluxe appearance); capability of being mass-produced; and manufacturing cost, Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.