U.S. patent application number 12/974825 was filed with the patent office on 2011-06-30 for golf club head.
This patent application is currently assigned to BRIDGESTONE SPORTS CO., LTD.. Invention is credited to Hideo MATSUNAGA, Takaharu TAKECHI, Kozue WADA.
Application Number | 20110159987 12/974825 |
Document ID | / |
Family ID | 44188213 |
Filed Date | 2011-06-30 |
United States Patent
Application |
20110159987 |
Kind Code |
A1 |
TAKECHI; Takaharu ; et
al. |
June 30, 2011 |
GOLF CLUB HEAD
Abstract
A hollow golf club head has a head body having a face aperture
part and a face member held to the face aperture part by welding,
the face member formed by forging a rolled material of a titanium
alloy having an .alpha. phase, and the face member comprising a
thick-material part positioned in the center of the face member, an
outer peripheral part positioned at the periphery of the
thick-material part, and a thin-material part partially positioned
between the thick-material part and the outer peripheral part. The
thick-material part is formed in an area having a shape obtained by
depressing a substantially elliptical or substantially circular
shape in a substantially arcuate shape toward the center of the
ellipse at two opposite sides thereof. The thick-material part, the
thin-material part, and the peripheral part are formed by the
forging so as to destroy an orientation of the .alpha.-phase.
Inventors: |
TAKECHI; Takaharu;
(Shinagawa-ku, JP) ; WADA; Kozue; (Chichibu-shi,
JP) ; MATSUNAGA; Hideo; (Chichibu-shi, JP) |
Assignee: |
BRIDGESTONE SPORTS CO.,
LTD.
TOKYO
JP
|
Family ID: |
44188213 |
Appl. No.: |
12/974825 |
Filed: |
December 21, 2010 |
Current U.S.
Class: |
473/345 |
Current CPC
Class: |
A63B 53/0454 20200801;
A63B 2209/00 20130101; A63B 53/0466 20130101; A63B 53/0458
20200801; A63B 53/0408 20200801 |
Class at
Publication: |
473/345 |
International
Class: |
A63B 53/04 20060101
A63B053/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 2009 |
JP |
2009-298199 |
Claims
1. A golf club head having a hollow part, comprising: a head body
having a face aperture part; and a face member held to the face
aperture part by welding, the face member being formed by forging
of a rolled material of a titanium alloy having an .alpha. phase,
and the face member comprising a thick-material part positioned in
the center of the face member, an outer peripheral part positioned
at the periphery of the thick-material part, and a thin-material
part partially positioned between the thick-material part and the
outer peripheral part, wherein the thick-material part is formed in
an area having a shape obtained by depressing a substantially
elliptical or substantially circular shape in a substantially
arcuate shape toward the center of the ellipse at two opposite
sides thereof, wherein the thick-material part has a material
thickness that is thickest at the center part of the face part and
also gradually becomes thin from the center part toward the outer
peripheral part, wherein the thick-material part has a material
thickness that is thicker than the outer peripheral part and the
thin-material part has a material thickness that is thinner than
the outer peripheral part, and wherein the thick-material part, the
thin-material part, and the peripheral part are formed by the
forging so as to destroy an orientation of the .alpha.-phase.
2. The golf club head according to claim 1, wherein the forging is
hot forging.
3. The golf club head according to claim 1, wherein the
thin-material part is disposed so as to be inclined in the sole
direction at the toe side of the thick-material part, and to be
inclined in the crown direction at the heel side of the
thick-material part, and has a score line groove on the surface of
the face.
4. The golf club head according to claim 1, wherein the face member
further has a rib that passes through the center part of the
thick-material part and extends from the outer edge of both the
heel side and the crown side of the face toward the outer edge of
both the toe side and sole side, wherein the rib has a material
thickness that is thicker than the outer peripheral part.
5. The golf club head according to claim 1, wherein the rolling
direction of the rolled material is a direction at an angle within
.+-.5.degree. with respect to the score line.
6. The golf club head according to claim 1, wherein the face member
is either an .alpha. titanium alloy or an .alpha.-.beta. titanium
alloy.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority from Japanese Patent
Application No. 2009-298199 filed Dec. 28, 2009, which is
incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a golf club head, and more
particularly, relates to a golf club head having improved material
thickness on the back surface of the face part thereof, and also a
face part formed by forging of a rolled material.
[0003] The heads of a majority of recent wood clubs are formed with
at least the face part made of a metallic material. The material
thickness of the face part must be made thick in order to maintain
the strength to be able to withstand the impact with a ball.
Although heads are increasing in size, because the rules require
that they have a volume less than 460 cm.sup.3 plus a tolerance of
10 cm.sup.3, the majority of heads have become large 460-cm.sup.3
driver heads that are very close to the upper limit. When the head
size is increased, because the sweet area increases, the peripheral
weight distribution is emphasized, and the left-right and
top-bottom moments of inertia increase, it is possible to reduce
poor shots when an off-center hit occurs. However, when the head
size is increased and the head weight is also increased, the swing
balance becomes large and the head speed drops, and so the carry
might be reduced. Given this, a method is adopted of forming the
entire head from titanium, which has a low specific gravity and a
high strength, or an alloy thereof (these being referred to simply
as "titanium" unless otherwise noted) or of using a composite head
made of carbon and titanium.
[0004] Additionally, a large number of high-restitution heads,
which have not only an increased head size, but also an increased
coefficient of restitution, have been developed. Since 2008,
high-restitution heads having a coefficient of restitution of 0.830
or greater have been unusable in competition. At present, along
with an increase in head size, there is active use of
thick-material faces having an increased coefficient of
restitution. However, even with a high-restitution head, it is not
possible to achieve a spring effect for hits other than at the
sweet area, that is, off-center hits, and there has been a tendency
for a sharp decrease in carry in such cases. Also, it is known
that, from the standpoint of face strength, it is preferable to
make the material of a center part, which greatly deforms, thick,
and not to provide grooves such as score lines in the center part
of the ball-striking surface of the face.
[0005] Japanese Patent Application Publication No. 9-192273
discloses a golf club head having a thickness at the center
location that includes the sweet spot of the part that forms the
face is formed with a thickness to maintain strength sufficient to
withstand the impact with a ball, the material thickness of the
periphery of the center location being made thinner than the center
location, so as to impart springiness to the overall face.
[0006] Also, Japanese Patent Application Publication No.
2007-307143 discloses a golf club head having a face part formed
from a rolled titanium alloy material having an .alpha. phase, the
rolling direction of the face part being along the toe-heel
direction.
SUMMARY OF THE INVENTION
[0007] In the case in which the material at the center of the face
part, such as described in Japanese Patent Application Publication
No. 9-172273, is thick, there is the problem that, if the ball is
hit at a location that is removed from the sweet spot, the
restitution performance of the face is greatly reduced from the
case in which the ball is hit at the sweet spot, and if the
restitution performance at the sweet spot is suppressed so as to be
low, the restitution performance other than at the sweet spot is
considerably reduced. Also, generally there is a tendency that the
thinner the material of the face is made, the greater the
restitution performance of the face and the weaker the strength of
the face.
[0008] Additionally, in the case of using a titanium alloy having
an .alpha. phase for the face, the strength of the titanium alloy
that is obtained by rolling in one direction has a directionality
that is caused by the .alpha. phase crystal structure (close-packed
hexagonal lattice). For this reason, consideration must be given to
the rolling direction when using a rolled material, leading to the
problem of extreme difficulty of use, and there is a particular
tendency for cracks to form at score line grooves in the surface of
the face, the material of which has been made thin.
[0009] Given the above, in consideration of the above-noted
problems, the present invention has as an object to provide a golf
club head in which the restitution of the face part can be held to
within a range that conforms to the rules, while maintaining the
light weight and strength of the face part, the golf club head
being capable of preventing a large decrease of restitution
performance even in the case in which a ball is hit at a part away
from the sweet spot, and capable of preventing the occurrence of
cracks in the relatively low-strength thin-material part.
[0010] To achieve the above-noted object, the present invention is
a golf club head having a hollow part including a head body having
a face aperture part and a face member held to the face aperture
part by welding, wherein the face member is formed by forging of a
rolled material of a titanium alloy having an .alpha. phase; the
face member has a thick-material part positioned in the center of
the face member, an outer peripheral part positioned at the
periphery of the thick-material part, and a thin-material part
partially positioned between the thick-material part and the outer
peripheral part; the thick-material part, the thin-material part,
and the peripheral part are formed by the forging so as to destroy
the .alpha.-phase orientation. The thick-material part is formed in
an area having a shape obtained by depressing a substantially
elliptical or substantially circular shape in a substantially
arcuate shape toward the center of the ellipse at two opposite
sides thereof. The thick-material part has a material thickness
that is thickest at the center part of the face member and also
that gradually becomes thin from the center part toward the outer
peripheral part. The thick-material part has a material thickness
that is thicker than the peripheral part and the thin-material part
has a material thickness that is thinner than the outer peripheral
part.
[0011] In a golf club head according to the present invention, the
forging may be hot forging.
[0012] In a golf club head according to the present invention, the
thin-material part may be disposed so as to be inclined in the sole
direction at the toe side of the thick-material part, and to be
inclined in the crown direction at the heel side of the
thick-material part, and have a score line groove on the surface of
the face.
[0013] In a golf club head according to the present invention the
face member may further have a rib that passes through the center
part of the thick-material part and extends from the outer edge of
both the heel side and the crown side of the face toward the outer
edge of both the toe side and sole side, wherein the rib has a
material thickness that is thicker than the outer peripheral
part.
[0014] In a golf club head according to the present invention, the
rolling direction of the rolled material may be a direction at an
angle within .+-.5.degree. with respect to the score line.
[0015] In a golf club head according to the present invention, the
face member may be either an .alpha. titanium alloy or an
.alpha.-.beta. titanium alloy.
[0016] According to a golf club head according to the present
invention, because a thin-material part having a material thickness
that is thinner than the outer periphery part of the face member is
formed in a region that is caused to be recessed by forging of the
thick-material part of the face member, not only is it possible to
maintain the light weight of the face part and suppress the
restitution to within an amount that is compliant with the rules,
it is also possible to prevent a large decrease in the restitution
performance even in the case in which the ball is hit at a part
other than the sweet spot. Additionally, by forming the face member
by forging a rolled material, it is possible to weaken the
characteristic directionality of strength by destroying the
.alpha.-phase crystal structure of the titanium alloy, thereby
preventing the occurrence of cracks in the thin-material part that
has relatively low strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a front elevational view showing an embodiment of
a golf club head according to the present invention.
[0018] FIG. 2 is a rear elevational view showing a face member of
the golf club head shown in FIG. 1.
[0019] FIG. 3 is a front elevational view showing another
embodiment of a golf club head according to the present invention,
the embodiment having a rib.
[0020] FIG. 4 is a rear elevational view showing a face member of
the golf club head shown in FIG. 3.
[0021] FIG. 5 is a schematic cross-sectional view showing the face
member of FIG. 4 along the line A-A.
[0022] FIG. 6 is a schematic cross-sectional view showing the face
member of FIG. 4 along the line B-B.
[0023] FIG. 7 is a rear elevational view showing the angle of the
rib in the face member of FIG. 4.
[0024] FIG. 8(a) is a cross-sectional view showing an embodiment of
a forging process in the present invention before pressing.
[0025] FIG. 8(b) is a cross-sectional view showing one embodiment
of the forging process in the present invention after pressing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] Embodiments of a golf club head according to the present
invention will now be described with reference to the accompanying
drawings.
[0027] As shown in FIG. 1, a golf club head 1 is made up of a face
member 10 including a part of a face part, and a main body member
20 of the head. The body member 20 includes a crown part 4, a sole
part 5, a hosel part 6, and a side part 7, which are integrally
formed. The face member 10 and the body member 20 are joined by
welding, and thereby the interior of the head is made to be a
hollow structure. A construction appearing on the surface on the
hollow side of the face member 10, that is, the back surface of the
face member 10, is indicated by broken lines. On the back surface
of the face member 10, irregularities for altering the thickness of
the face member 10 are formed. The back surface of the face member
10 is explained.
[0028] As shown in FIG. 2, an elliptical line 12 on the back
surface of the face member 10 shows an elliptical region in which
the thickness of the face member is varied in contrast to the outer
peripheral part 11 having a flat surface. A thick-material part 15,
in which the thickness gradually increases toward the center of the
ellipse, is formed within a thick-material part outer periphery
line 19, which includes both end parts of the elliptical line 12
with respect to the minor axis of the ellipse and depression lines
14 that are depressed in an arcuate shape toward the center of the
ellipse in both end parts of the major axis of the ellipse.
[0029] As shown in FIG. 2, in the center part of the ellipse, there
is formed a circular central part 17, which has a flat surface and
has the greatest thickness in the face member 10. The
thick-material part 15 is configured so that its thickness
gradually decreases from the center part 17 to the elliptical line
12 or to the depression lines 14. An outer peripheral part 11 that
is formed outside the elliptical line 12 has a uniform thickness.
Thin-material parts 13 having a thickness smaller than that of the
outer peripheral part 11 are formed in parts of the ellipse that
are surrounded by the elliptical line 12 at both ends of the major
axis of the ellipse and the depression lines 14, respectively. As
shown in FIGS. 3 and 4, a singular rib 18 having a thickness
greater than that of the outer peripheral part 11 can be formed on
the back surface of the face member 10. FIGS. 5 and 6 show
schematic cross-sectional views of the face member of FIG. 4 along
the line A-A and the line B-B, respectively. In order to represent
the changes in thickness of the thick-material part 15 in FIGS. 1
and 2, contour lines 16 have been drawn.
[0030] FIGS. 1 to 6 are not drawn to scale. The various parts of
the face member 10 are described in further detail below.
[0031] The center part 17 includes a sweet spot of the golf club
head 1. The center part 17 includes the center point of the ellipse
indicated by the line 12. The center point of the ellipse and the
sweet spot may be identical or different. The radius of the center
part 17 is preferably at least approximately 3.0 mm and more
preferably at least approximately 3.5 mm. Also, the radius of the
center part 17 is preferably at most approximately 6 mm, and more
preferably at most approximately 5 mm. By making the radius of the
center part 17 in this range, the weight of the face part can be
kept low. The shape of the center part 17 is not limited to the
circular shape shown in FIG. 2, and may be may be elliptical or
polygonal such as tetragonal, e.g., rectangular and rhombic,
pentagonal and hexagonal. The thickness of the center part 17 is
preferably at least approximately 3.4 mm, and more preferably, is
at least approximately 3.6 mm. Also, the thickness of the center
part 17 is preferably at most approximately 4.0 mm, and more
preferably at most approximately 3.8 mm. By making the thickness of
the center part 17 in this range, the restitution coefficient of
the face part can be kept in the range specified by the rule.
[0032] The major axis of the ellipse indicated by the line 12 is
inclined so that the toe side 2 thereof shifts to the crown side
and the heel side 3 thereof shifts to the sole side. The reason for
this is as described below. In general, the variations in hitting
points of golfers are biased to the crown side on the toe side 2
and to the sole side on the heel side 3. By this inclination, more
hitting points at the time when a ball is hit by a face part
deviating from the sweet spot can be allowed to enter the region of
the thin-material parts 13. Specifically, as shown in FIG. 7, when
the golf club head is placed at a normal addressed position, an
inclination angle .theta.a of a major axis 42 of the ellipse with
respect to the horizontal line 40 is preferably at least
approximately 5.degree. and more preferably at least approximately
10.degree.. Also, the inclination angle .theta.a of the major axis
42 is preferably at most approximately 40.degree. and more
preferably at most approximately 30.degree.. However, the
thin-material parts 13 are not limited to the above-noted
disposition, as long as they are disposed to the outside of the
thick-material part 15. They may be disposed, for example, at the
crown side and the sole side.
[0033] The ratio of the length of the major axis of the ellipse
indicated by the line 12 to the length of the minor axis thereof is
preferably in the range of 100:50 to 50:50, more preferably in the
range of 95:50 to 70:50 (in the case in which the major axis and
the minor axis have an equal length, the shape is not elliptical,
but is circular). The thick-material part 15 is depressed by the
depressions 14 at two opposite sides of the ellipse or circle
thereof. It is preferable that the depressions 14 be formed to
match the disposition of the thick-material part 13, at either the
toe and heel sides or crown and sole sides. In the case of having
depressions 14 at the toe and heel sides, it is preferable that the
ratio between the length of the thick-material part 15 on the major
axis of the ellipse (that is, the length between the depressions
14) and the length of the thick-material part 15 on the minor axis
of the ellipse be in the range from about 5:4 to about 5:6. The
radius of curvature of the depression 14 on the heel side is
preferably at least approximately 12 mm, and more preferably at
least approximately 13 mm. The radius of curvature of the
depression 14 on the heel side is preferably at most approximately
25 mm, and more preferably at most approximately 20 mm.
[0034] As shown in FIG. 5, the thick-material part 15 has a curved
surface spreading toward the bottom such that the thickness thereof
continuously decreases from the center part 17 to the depression 14
or to the elliptical line 12, but it is not limited to this shape.
For example, the thick-material part 15 may have a stepwise surface
such that the thickness thereof decreases stepwise, or it may have
a surface of a truncated cone shape such that the thickness thereof
decreases continuously in a fixed ratio. As shown in FIG. 2, the
thick-material part 15 has four ridges, but it is not limited to
this. The presence or absence of and number and position of the
ridges can be changed as appropriate.
[0035] The outer peripheral part 11 has a uniform thickness and
occupies a region in which the thin-material part 13 and the
thick-material part 15 are not formed. The thickness of the outer
peripheral part 11 is preferably at least approximately 2.1 mm, and
more preferably, is at least approximately 2.2 mm. The thickness of
the outer peripheral part 11 is preferably at most approximately
2.5 mm, and more preferably at most approximately 2.4 mm. By making
the thickness of the outer peripheral part 11 be within this range,
it is possible to reduce the weight of the face part while
restraining the coefficient of restitution thereof.
[0036] The thin-material part 13 is formed to be thicker than the
outer peripheral part 11. The difference in thickness between the
thin-material part 13 and the outer peripheral part 11 is
preferably at least approximately 0.1 mm, and more preferably, is
at least approximately 0.2 mm. The thickness of the thin-material
part 13 is preferably at least approximately 1.8 mm, and more
preferably at least approximately 1.9 mm. The thickness of the
thin-material part 13 is preferably at most approximately 2.2 mm,
and more preferably at most approximately 2.1 mm. By making the
thickness of the thin-material part 13 be within this range, it is
possible to improve the restitution performance on the toe and heel
sides, on which the restitution performance is usually low.
[0037] As shown in FIGS. 3 and 4, the rib 18 has a thickness that
is at least greater than the outer peripheral part 11. As shown in
FIG. 5, the thickness of the rib 18 decreases continuously from the
thickness that is the same as the thickness of the center part 17
toward the outer peripheral part 11. By making the rib 18 thickness
continuously decrease from the center part 17 toward the outer
peripheral part 11 in this manner, it is possible to minimize the
decrease in restitution performance of the face surface, while
maintaining the strength of the face surface. The rib is not
limited to this configuration. The thickness of the rib 18 may be,
for example, made uniform. The thickness of the rib 18 is
preferably at least approximately 2.5 mm, and more preferably at
least approximately 2.7 mm. The thickness of the rib 18 is
preferably at most approximately 2.9 mm, and more preferably at
most approximately 3.6 mm.
[0038] The rib 18, as shown in FIG. 4, is formed so as to extend
substantially in a straight line, passing through the center part
17 of the thick-material part 15, from a crown side on the heel
side 3 of the outer peripheral part 11 to a sole side on the toe
side 2 of the outer peripheral part 11. The angle of inclination
.theta.b of the center line 44 of the rib 18 with respect to the
horizontal line 40 when the golf club head is place in the normal
addressed position, as shown in FIG. 7, is preferably at least
approximately 45.degree., and more preferably at least
approximately 50.degree.. The angle of inclination .theta.b is
preferably less than approximately 90.degree. and more preferably
at most approximately 80.degree.. By making the rib inclination
angle .theta.b be within this range, it is possible to maintain the
strength of the face surface without a large loss of restitution
performance in the case of an off-center hit because the variations
in the ball impact points are biased as discussed above. As shown
in FIG. 4, the rib 18 is formed to have a width that increases
along the direction from the center part 17 toward the outer
peripheral part 11, but it is not limited to this. The width of the
rib 18 can, for example, be uniform.
[0039] Over the entire surface area of the face part 10, the
proportionality of the surfaces that are occupied by the outer
peripheral part 11, the thin-material part 13, the thick-material
part 15 (including in this case the center part 17) and the rib 18
is preferably about 16-20:2-6:14-18:1-6, and more preferably about
17-19:3-5:15-17:1-5. By making the proportionalities be within
these ranges, it is possible to achieve a balance between weight
and strength over the entire face part. Also, in the case in which
the rib 18 is not provided, the proportionality of the surface that
is occupied by the outer peripheral part 11, the thin-material part
13, and the thick-material part 15 (including in this case the
center part 17) is preferably 8-10:1-3:7-9, and more preferably
17-19:3-5:15-17.
[0040] The above-noted constitution of the face member 10 is formed
by forging a rolled material made of a titanium alloy having an
.alpha. phase. An .alpha. alloy or an .alpha.-.beta. alloy may be
used as the titanium alloy having an .alpha. phase. In particular,
the .alpha.-.beta. alloy can be more preferably used than the
.alpha. alloy, because the .alpha.-.beta. alloy has a higher
strength than the .alpha. alloy, thereby improving the durability
of the face part of the club head, reducing the weight of the club
head by making the material of the face member thin, and increasing
the degree of freedom for designing the center of gravity by making
the material thin.
[0041] Exemplary .alpha. alloys include Ti-5Al-2.5Sn. Exemplary
.alpha.-.beta. alloys include Ti-4.5Al-3V-2Fe-2Mo,
Ti-4.5Al-2Mo-1.6V-0.5Fe-0.3Si-0.03C, Ti-8Al-1Mo,
Ti-1Fe-0.35O-0.01N, Ti-5.5Al-1Fe, Ti-6Al-4V, Ti-6Al-6V-2Sn,
Ti-6Al-2Sn-4Zr-6Mo, Ti-6Al-2Sn-4Zr-2Mo, and Ti-8Al-1Mo-1V. In
particular, Ti-6Al-4V and Ti-8Al-1Mo-1V are preferable.
[0042] The strength of the rolled material made from a titanium
alloy having an .alpha. phase exhibits directionality, because the
crystal texture (close-packed hexagonal lattice) of the .alpha.
phase titanium alloy has directionality. The close-packed hexagonal
lattice has two axes, the first axis being easily deformed, and the
second axis being substantially orthogonal thereto and being hard
to deform. When the alloy having the close-packed hexagonal lattice
structure is rolled in one direction, the easily deformed axis is
orientated in the rolling direction with the hard-to-deform axis
being orthogonal thereto, and thus, a prominent anisotropy with
regard to the strength of the alloy occurs.
[0043] Accordingly, the face member used in the golf club head of
the present invention is formed by forging the rolled material, at
which time the crystal directionality of the .alpha. phase is
destroyed.
[0044] The rolled material is manufactured by grabbing a titanium
alloy between a pair of rotating rollers by friction, and
subjecting it to rolling to reduce its thickness or cross-sectional
area. The rolling is performed repeatedly in the same one
direction. As a result, the easy-to-deform axis in the close-packed
hexagonal lattice is oriented substantially in parallel with the
rolling direction, and the hard-to-deform axis of the close-packed
hexagon lattice is oriented substantially perpendicularly to the
rolling direction, and thus, a prominent anisotropy in strength can
occur. The titanium alloy having an .alpha. phase rolled only in
one direction can exhibit a more prominent anisotropy than a .beta.
titanium alloy. It is preferable that the direction of rolling be
parallel to a score line in the face surface, or at an angle within
.+-.5.degree. thereof.
[0045] The rolled material manufactured in this manner is subjected
to plastic deformation by forging in order to form the
thick-material part and the thin-material part of the face member.
The rolled material is punched out using various methods such as
pressing or laser cutting. The punched out rolled material is
placed between upper and lower dies and is forged.
[0046] As shown in FIG. 8a, the upper die 50 of the dies used in
the forging has a pressing part 52 which is depressed in the center
so as to make the center part the deepest. The center part thereof
forms the center part 17 and the thick-material part 15 of the face
member. The upper die 50 also has a protruding pressing part 51 for
forming the thin-material part 13 of the face member, and a
depressed pressing part (not shown) for forming the rib 18 of the
face member. The lower die 53 has a shallow depressed part 54 with
a width that is slightly wider than a rolled material for face
member 55. The rolled material 55 is disposed on the shallow
depressed part 54 and is pressed between the upper and lower dies
50 and 53.
[0047] As shown in FIG. 8b, the upper die 50 presses the entire
rolled material 55 and causes it to plastically deform. The
thin-material part 13 of the face member is formed by the
protruding pressing part 51, and the thick-material part 15 is
formed by the center depressed pressing part 52. The directionality
of the .alpha. phase of the rolled material is changed by the
pressing so as to be in accordance with the plastic deformation of
the rolled material. Therefore, at least in the thick-material part
15 and the thin-material part 13, the pressing sufficiently
destroys the directionality of the .alpha.-phase crystal grains.
For example, in the thin-material part 13, the rolled material 55
is elongated in all directions, over 360.degree., and thus, the
.alpha.-phase orientation is destroyed in all directions, over
360.degree.. In the thick-material part 15, the rolled material 55
is pulled and elongated in the central direction and also is spread
outward in all directions, over 360.degree., thereby intricately
destroying the .alpha.-phase orientation. In the neighboring
periphery between the thin-material part 13 and the thick-material
part 15, the elongating and spreading of the rolled material 55 are
complexed together, thereby achieving a more intricate destruction
of the .alpha.-phase orientation. The .alpha.-phase crystal grains
(tissues) spread along the rolling direction are separated (finely
divided) by forging in this manner. In addition, the metal
structure is moved along the directions from the thin-material part
to the thick-material part, thereby, changing the orientation of
the .alpha.-phase metal tissue, i.e., the .alpha.-phase metal
tissue that had been uniformly oriented in the rolling direction is
no longer uniformly oriented. Therefore, the anisotropy in strength
is destroyed.
[0048] The forging process is preferably hot forging performed
after heating to at least the recrystallization temperature. FIGS.
8a and 8b show a single die for forging of the face material, but a
plurality of dies may be used to form the face member by a series
of forging steps.
[0049] The face member obtained by forging can be joined by welding
to the face aperture of the golf club head to manufacture the golf
club head. The volume of the golf club head 1 is preferably at
least approximately 100 cm.sup.3, and more preferably at least
approximately 350 cm.sup.3. The volume of the golf club head 1 is
also preferably at most approximately 500 cm.sup.3, and more
preferably at most approximately 480 cm.sup.3. The weight of the
golf club head 1 is preferably at least approximately 150 g, and
more preferably at least approximately 160 g. The weight of the
golf club head 1 is also preferably at most approximately 250 g,
and more preferably at most approximately 200 g.
[0050] As shown in FIGS. 1 and 3, the face surface of the golf club
may have score lines 8. The score lines 8 each have a groove shape
and are arranged parallel to each other. In the embodiment shown in
FIGS. 1 and 3, the golf club head has a design that does not have
score lines in the center of the face. Exemplary methods for
forming the score lines 8 include machining, forging, and casting
or other known conventional methods. This process may be performed
on the face member after forging or on the face surface after
welding to the golf club head. The rules allow score line grooves
to a depth of up to 0.5 mm. However, it is preferable that the face
surface of a hollow wood golf club be thin, and it is preferable
that the depth be within the range from approximately 0.1 mm to
approximately 0.3 mm, and more preferably in the range from
approximately 0.1 mm to approximately 0.2 mm. For example, this can
be made approximately 0.15 mm. In the case of making the material
of the face member thin, there is a problem with the occurrence of
cracking at the score lines. It is possible by forging to destroy
the anisotropy in strength caused by rolling, thereby preventing
cracking at the score line grooves.
[0051] FIGS. 5 and 6 show the ball-striking surface of the face
member as being flat to facilitate an understanding of the
constitution of the present invention, but a bulge having a radius
of curvature of about 250 mm to about 800 mm may be formed on the
ball-striking surface of the face member 10. In the same manner, it
is possible to form a roll having a radius of curvature of about
250 mm to about 800 mm on the ball-striking surface of the face
member.
[0052] FIGS. 1 and 3 show the embodiment with the face member 10 as
the center part of the face of the golf club head 1 and the
remaining part of the face as being formed integrally with the head
body, but the present invention is not limited to this embodiment.
For example, the entire face may be made the face member.
* * * * *