Golf Club Heads with Variable Face Thickness

This application is a continuation of U.S. application Ser. No. 18/116,532, titled “GOLF CLUB HEADS WITH VARIABLE FACE THICKNESS” (Atty. Docket No. CLG-00800-CON2), filed on Mar. 2, 2023, which is a continuation of U.S. application Ser. No. 17/577,322, titled “GOLF CLUB HEADS WITH VARIABLE FACE THICKNESS” (Atty. Docket No. CLG-00800-CON), filed on Jan. 17, 2022, and issued as U.S. Pat. No. 11,612,791 on Mar. 28, 2023, which is a continuation of U.S. application Ser. No. 16/920,504, titled “GOLF CLUB HEADS WITH VARIABLE FACE THICKNESS” (Atty. Docket No. CLG-00800), filed on Jul. 3, 2020, and issued as U.S. Pat. No. 11,247,106 on Feb. 15, 2022. The entire contents of the foregoing applications and patents are hereby incorporated by reference.

Golf club heads have mass and performance properties that affect the quality and consistency of shots when hitting a golf ball. Such mass and performance properties are often related to the mass or the distribution of mass in the golf club head. Examples of such mass and performance properties can include the location of a Center of Gravity (CG) for the club head, Coefficients of Restitution (CORs) or Characteristic Times (CTs) at various locations on a striking face of the club head, and Moments of Inertia (MOIs) about different virtual axes passing through the CG.

As example of a mass property affecting performance, the location of the CG can affect, for example, how high a golf ball is hit, the amount of spin on the golf ball, or the forgiveness of a club head in terms of ball speed and straightness for shots where the impact occurs at off-center locations away from a “sweet spot” on the striking face. As conventionally defined, the sweet spot is the point on the striking face from which a normal projection passes through the club head's CG. For example, moving the CG lower toward the sole, and back from the striking face of an iron type club head can advantageously increase the height of shots for longer distance and result in more backspin on the golf ball for a more controlled shot. Locating the sweet spot closer to the center of the striking face may also better align the sweet spot to a player's expected sweet spot location. Due to the asymmetric shaping and mass distribution of traditional iron-type golf club heads, a laterally centered CG location typically requires, for example, including high density weights, which can be costly and negatively affect swing weight.

As another example of a mass property affecting performance, greater MOIs in a club head mean that the club head is more resistant to twisting when the golf ball is hit at off-center positions on the striking face that are farther from the sweet spot. Increasing the MOIs of the club head generally results in the club head being more stable or forgiving for off-center shots, allowing such off-center shots to be straighter and have a faster ball speed due to the greater MOIs.

As an example of a performance property, the COR is a measurement of energy loss or energy transfer between the striking face and the golf ball. Higher measured CORs on the striking face translate to less energy loss or better energy transfer when the striking face impacts the golf ball. More energy is transferred to the golf ball with a higher COR, which translates to a faster ball speed that typically results in a farther shot. The COR can be measured, for example, using conventional cannon testing in keeping with the United States Golf Association's (USGA's) prescribed method for determining the COR. In this regard, the USGA has migrated from using the COR to using a different performance property referred to as a Characteristic Time (CT) measurement to quantify the elasticity of the striking face. For all purposes herein, the CT refers to characteristic time as described in the USGA's “Procedure for Measuring the Flexibility of a Golf Clubhead” (Rev. 1.0.0, May 1, 2008).

The improvement of mass and performance properties of a club head are balanced against structural requirements for the intended use of the club head, such as stress properties. Mass and performance properties are also balanced against other limits, such as limits prescribed by regulatory bodies, such as the USGA, concerning the CT, dimensions, and club head mass. In addition, players generally have implicit expectations for club heads, such as an overall appearance with respect to size, or an overall expected weight of the club head for the type of golf club or the loft angle of the golf club.

The present inventors recognized a need for a variable face thickness pattern for golf club heads, particularly iron-type club heads, that improves mass and performance properties of club heads, while maintaining similar stress limits, appearance, and overall club head weight. As discussed in more detail below, the improved mass and performance properties can include, for example, Coefficients of Restitution (CORs), Characteristic Times (CTs), Moments of Inertia (MOIs), and/or a Center of Gravity (CG) location for the club head. In some example embodiments, a cavity-back or a hollow bodied, iron-type club head has an improved variable face thickness pattern that allows for discretionary weight to be moved from the striking face of the club head to other areas of the club head to improve mass and/or performance properties of the club head. Advantageously, such club heads may have improved mass and performance properties, such as higher CORs on the striking face, higher MOIs, and more laterally centered, deeper, and lower CG locations than comparable club heads, while maintaining similar stress limits. Additionally, such club heads do not sacrifice traditional appearances, dimensions (e.g., blade length, topline thickness), and overall club head weight (e.g., swing weight) that may be preferred by some players.

Reducing weight in the face while maintaining an overall club head weight can be important for players who may associate specific lofts of a golf club head with a certain mass, and have a preferred golf club swing weight. Generally, when presented in a set, iron-type club heads increase in mass with loft. For example, the mass of iron-type club heads may adhere the following equation:

where mh is a club head mass in grams, LA is the loft angle of the club head when orientated in a reference position, and a is between 190 g and 210 g. In one or more embodiments, a golf club head maintains such a head mass mh, while having an improved face thickness pattern. Such a club head may have an improved face thickness pattern with a vertical MOI extending through the CG, Izz, that satisfies:

In one or more aspects of the disclosure, a golf club head, when orientated in the reference position, includes a golf club head main body having a toe, a heel opposite the toe, a sole, and a top portion opposite the sole. The club head has a mass mh that satisfies Equation 1. In addition, the club head has a blade length less than 80 mm. The striking face of the club head defines a face plane and has a face center, and a virtual center plane extends vertically through the face center perpendicular to the face plane. As used herein, a face center of a striking face is determined according to the procedure described in the USGA's “Procedure for Measuring the Flexibility of a Golf Clubhead” (Rev. 2.0, Mar. 25, 2005). A CG of the club head is located not more than 2.0 mm from the virtual center plane, and an MOI about a vertical axis extending through the CG, Izz, satisfies Izz>mh*9.3 cm.

In some aspects, the striking face includes a central region including the face center, an intermediate region at least partially surrounding the central region, an upper region above the central region, an upper region above the central region, a lower region below the central region, and a toe region toe-ward of the central region. Each of the central region, the upper region, the lower region, and the toe region include a maximum width and an average thickness, and the intermediate region is disposed between the central region and each of the upper region, the lower region, and the toe region. The intermediate region has an average thickness greater than that of each of the central region, the upper region, the lower region, and the toe region. In one or more embodiments, the intermediate region fully surrounds the central region.

According to some aspects, at least one of the toe region, the upper region, and the lower region includes, on a rear surface thereof, an elongate groove or recess having a width no less than about 2.0 mm. Alternatively or additionally, the upper region, the lower region, and the toe region respectively include, on a rear surface thereof, an upper groove or recess extending generally in a heel to toe direction, a lower groove or recess extending generally in a heel to toe direction, and a toe groove or recess extending generally in a top to bottom direction.

In one or more aspects of the disclosure, a golf club head, when orientated in a reference position, includes a golf club head main body having a toe, a heel opposite the toe, a sole, and a top portion opposite the sole. A face insert of the club head has a mass mf fixedly attached to the golf club head main body and includes a striking face that defines a face plane. The club head has a mass mh that satisfies Equation 1. The club head has a blade length less than 80 mm, and an MOI, Izz, about a vertical axis extending through a CG of the club head that satisfies Izz>mh*9.3 cm. In addition, a ratio mf/mh is less than or equal to 0.22. In one or more embodiments, the ratio mf/mh of an iron-type golf club head is less than or equal to 0.20.

In some aspects, the striking face includes a sweet spot corresponding to a first COR, COR1, and an auxiliary location spaced at least 7.5 mm from the sweet spot corresponding to a second COR, COR2, where: COR2≥0.98*COR1. In some implementations, a variable thickness of the striking face may provide for a higher COR near the sweet spot, increase the COR in a region including the sweet spot, and/or provide a larger area of a higher COR near the sweet spot. In another aspect, the relocation of mass from the striking face can move the CG so that the sweet spot corresponds to an area with a higher COR and/or a more frequently hit area of the striking face by players. For example, the central region of the striking face may include a heel-side region that has a greater thickness than a toe-side region so as to improve the COR in areas of the striking face that are more commonly hit by players.

The recesses or grooves on the rear surface of striking faces of the present disclosure not only increase the COR of the striking face, but can also improve weight distribution of the club head by relocating mass from the striking face to other areas of the club head to increase MOIs and/or to better locate the CG of the club head for better performance. The recesses or grooves may also be determined with a stress limit on the striking face as a constraint so that the striking face is comparable to prior art club heads when tested for durability, despite the reduced mass of the striking face.

In one or more aspects of the disclosure, a golf club head oriented in a reference position, includes a golf club head main body having a toe, a heel opposite the toe, a sole, and a top portion opposite the sole. A striking face of the golf club head includes a central region including a face center, a first average thickness, and a variable thickness. An upper region of the striking face is above the central region and includes an upper groove having a width greater than 5.0 mm and a second average thickness that is less than the first average thickness. A toe region of the striking face is toc-ward of the central region and includes a toe side groove having a width greater than 2.0 mm and a third average thickness that is less than the first average thickness. An intermediate region of the striking face at least partially surrounds the central region and is disposed between the central region and each of the upper groove and the toe groove. The intermediate region has a fourth average thickness that is greater than the first average thickness.

In one or more aspects of the disclosure, a golf club head oriented in a reference position includes a golf club head main body having a toe, a heel opposite the toe, a sole, and a top portion opposite the sole. A face insert of the golf club head is fixedly attached to the golf club head main body and includes a striking face having a face center. A central region of the face insert includes the face center, a first average thickness, and a variable thickness. An upper region of the face insert is above the central region and includes an upper groove having a width greater than 5.0 mm and a second average thickness that is less than the first average thickness. A toe region of the face insert is toc-ward of the central region and includes a toe side groove having a width greater than 2.0 mm and a third average thickness that is less than the first average thickness. An intermediate region of the face insert at least partially surrounds the central region and is disposed between the central region and each of the upper groove and the toe groove.

In one or more aspects of the disclosure, a method of manufacturing a golf club head includes forming a golf club head main body having a striking face, a heel portion, a toe portion opposite the heel portion, a sole, a top portion opposite the sole, and a blade length no greater than 80 mm. A thickness pattern of the striking face is formed by defining on the striking face a central region including the face center, an intermediate region at least partially surrounding the central region, and at least one of an upper region above the central region, a lower region below the central region, and a toe region toe-ward of the central region. The intermediate region can be disposed between the central region and each of, or at least one of, the upper region, the lower region, and the toe region. The central region is recessed such that the central region has a thickness less than the intermediate region. At least one of the toe region, the upper region, and the lower region is recessed such that the recessed region has a thickness less than that of the central region. The variable face thickness pattern is formed such that the striking face includes a sweet spot corresponding to a first COR, COR1, and an auxiliary location spaced at least 7.5 mm from the sweet spot corresponding to a second COR, COR2, where COR2≥0.98*COR1.

In one or more aspects of the disclosure, a method of manufacturing a golf club head includes forming a golf club head main body having a striking face, a heel, a toe opposite the heel, a sole, and a top portion opposite the sole. A variable thickness pattern is determined with a computing device by defining on the striking face a plurality of parameterization zones, including a central zone having the face center. Each of the parameterization zones includes at least one of a variable first parameter and a variable second parameter. A target value is set for at least one of a respective first constraint, second constraint, and third constraint. Each of the at least one variable first parameter and second parameter is varied for each of the parameterization zones. Impact of the striking face with a golf ball is simulated, and resultant values are evaluated against the target value for the at least one of first constraint, second constraint, and third constraint. The determined variable thickness pattern is formed on the striking face based on the evaluation. In some implementations, the first constraint is a striking face mass, the second constraint is mechanical stress on the striking face, and the third constraint is a weighted COR representing an overall effective or expected COR for the striking face based on the CORs for different portions of the striking face that have been weighted by their expected golf ball impact probabilities. In addition, the variable first parameter and the variable second parameter, in some implementations, may include a variable maximum width and a variable thickness for the parameterization zone or region.

In one or more aspects of the disclosure, a method of manufacturing a golf club head includes forming a golf club head main body having a striking face, a heel, a toe opposite the heel, a sole, and a top portion opposite the sole. A variable thickness pattern is determined with a computing device by defining on the striking face a central region including a face center of the striking face, an intermediate region at least partially surrounding the central region, an upper region above the central region, a lower region below the central region, and a toe region toe-ward of the central region. Each of the central region, the upper region, the lower region, and the toe region includes a variable width parameter and a variable thickness parameter. The intermediate region is disposed between the central region and each of the upper region, the lower region, and the toe region. A target value is set for at least one of a respective first constraint, second constraint, and third constraint. Each of the variable first parameter and the variable second parameter is varied for each region of the striking face. Impact of the striking face with a golf ball is simulated, and resultant values are evaluated against the target value for the at least one first constraint, second constraint, and third constraint. The determined variable thickness pattern is formed on the striking face based on the evaluation.

The various exemplary aspects described above may be implemented individually or in various combinations. The foregoing features and advantages, as well as other features and advantages, of the golf club heads of the present disclosure will become apparent to those of ordinary skill in the art after consideration of the following description, the accompanying drawings, and the appended claims.

Representative examples of one or more novel and nonobvious aspects and features of the golf club heads and methods of manufacturing such club heads as disclosed below are not intended to be limiting in any manner. Furthermore, the various aspects and features of the present disclosure may be used alone or in a variety of novel and nonobvious combinations and sub-combinations with one another.

is a front view of exemplary golf club headaccording to one or more embodiments. As shown in, club headincludes toe portion, heel portion, topline portion, and sole portion. Club headalso includes hoselthat extends from heel portion. Hoselmay include an open end for receiving a golf club shaft (not shown) of a golf club. Hosel axisextends axially through the center of hosel, and lies in a virtual vertical hosel plane (e.g., virtual vertical hosel planeshown in). Club head, including striking face, may be formed, for example, of a steel material.

In, club headis oriented in a reference position with sole portionin contact with virtual ground plane, and with central hosel axisin the virtual vertical plane. As used herein, a club head is orientated in the “reference position” when the sole of the club head (e.g., sole portion) is in contact with a virtual ground plane (e.g., virtual ground plane), its central hosel axis (e.g., central hosel axis) is positioned in a vertical plane, and its score-lines (e.g., score-lines) are parallel to the ground plane. In the reference position, club headis positioned at a predetermined Loft Angle (LA) (i.e., LA in) and a predetermined lie angle (i.e., α in). Unless otherwise indicated, all parameters of the various embodiments in this disclosure are specified with the club heads orientated in the reference position.

In one or more embodiments, LA ranges from about 18 degrees to about 40 degrees. In other embodiments, the golf club head is a wedge-type golf club head and LA ranges from about 40 degrees to about 64 degrees.

As shown in, club headincludes striking faceconfigured to strike a conventional golf ball. In some implementations, striking facemay form part of a face insert that is fixedly attached to a main body of club head. In other implementations, striking facemay be integrally formed as part of the main body of club head. Striking faceis provided with one or more grooves or score-lines, which impart additional spin to the golf ball when struck. In, striking faceincludes face center, which is located on virtual center planethat extends vertically through face centerperpendicularly to a face plane defined by striking face(e.g., face planein). As used herein, a “face center” of a striking face is determined according to the procedure described in the United States Golf Association's (USGA's) “Procedure for Measuring the Flexibility of a Golf Clubhead” (Revision 2.0, Mar. 25, 2005). In the example of, face centerdenotes a point on striking facethat is midway between the heel-to-toe extents of score-lines, and midway between the sole-to-topline extents of striking face. In other embodiments, score-lines may extend to a toe-side edge of the striking face. In such embodiments, the lateral dimension of the face center is determined as midway between the heel-most extent of the score-lines and a club face apex, such as club face apexin.

In the example of, sweet spotis located on striking facea horizontal distance, CG, toward heel portionfrom virtual center plane. Sweet spotis located on virtual vertical CG planesuch that sweet spotis located on striking facewhere a virtual line projected normal to a face plane of striking face(e.g., face planein) passes through a CG of club head(e.g., CGin). As used herein, a club head's “sweet spot” is defined as a location on the club head's striking face from which a virtual line projected normal to a face plane of the striking face passes through the club head's CG location.

As discussed in more detail below, striking facehas been formed with a variable thickness in different regions or parameterization zones of striking faceto provide improved mass and/or performance properties of club head. Such properties can include, for example, greater Coefficients of Restitution (CORs) and/or greater Characteristic Times (CTs) on a larger area and/or more commonly hit area of striking face, greater Moments of Inertia (MOIs) about a virtual vertical CG axis (e.g., virtual vertical CG axisin) and/or about a virtual horizontal CG axis (e.g., virtual horizontal CG axisin), and/or an improved CG location for club head. The improvement of these mass and performance properties can be accomplished by the selective thinning or thickening of the different regions or parameterization zones and/or the relocation of discretionary mass from the striking face to other portions of the club head. As used herein, a striking face thickness is measured perpendicular to a face plane defined by the striking face (e.g., face planeinand face planein).

A total mass of the club head may serve as a target total mass comprised of structural mass and discretionary mass. Structural mass as used herein generally refers to mass necessary to establish a minimum structural integrity for the club head to be operable for its intended use. Discretionary mass, on the other hand, can refer to the remaining mass that, given a target mass, is not needed to establish the minimum structural integrity of the club head, and may therefore be located primarily to adjust mass and/or performance properties of the club head.

For example, the thickness of different regions or parameterization zones of striking facecan result in mass being moved from such regions or parameterization zones to other locations in club headto provide higher MOIs of club headand an improved location for the CG of club head(e.g., CGin), while increasing COR values in particular locations on the striking face. For example, mass removed from particular areas of the striking face can improve the COR of the striking face and the removed mass can be relocated in the club head so that the CG of club headcan be advantageously located closer to virtual center plane, closer to virtual ground plane, and farther behind striking face. As a result, sweet spotcan be advantageously located closer to face centerto better correspond to a player's expected sweet spot location and/or a more frequently hit area the striking face, and to provide a more forgiving club head to result in better off-center shots in terms of shot height, straightness, and distance. In this regard, sweet spotin some implementations can be located horizontally no greater than 2.0 mm from face centeras a result of the relocation of mass from striking facein accordance with the present disclosure. In other words, the CG of club head(e.g., CGin) in such implementations can be located not more than 2.0 mm from virtual center plane. In one or more embodiments, golf club heads having this lateral CG location do not include any high-density materials (e.g., tungsten alloys).

As noted above, the variable thickness pattern of the striking face discussed in more detail below can increase the COR at locations on striking facecorresponding to more commonly hit locations or a larger area of striking face to provide better energy transfer for off-center shots or for a statistically greater number of shots. Additionally or alternatively, the disclosed variable thickness patterns for a striking face can increase the area of the striking face that has a relatively high COR. For example, in some implementations, striking faceinmay include a maximum COR no less than 0.80 at a first location, and a COR of no less than 98% of the maximum COR at an auxiliary location on the striking face that is no less than 7.5 mm from the first location. In such implementations, the first location corresponding to the maximum COR may be at or near sweet spot, such as within 5 mm of sweet spot. Some implementations of variable thickness patterns discussed below for improving CORs on the striking face include, for example, a central region of the striking face having a heel-side thickness greater than a toe-side region.

is a rear view of an exemplary cavity-back club head according to one or more embodiments. In this regard, club headinincludes rear cavitybehind at least a portion of striking face, and rear musclenear sole portion. For the purposes of case of illustration,provides a rear view of club headfrom. However, those of ordinary skill in the art will appreciate with reference to the present disclosure that club headmay include a different construction in other implementations, such as the hollow body construction shown in, for example.

As shown in, CGis located on virtual horizontal CG axis. A horizontal MOI of golf club head, Ixx, is shown about virtual horizontal CG axis, which extends through CGand is parallel to striking face. As noted above, the reduction of mass achieved by varying the thickness of striking facecan allow for an increased Ixx, and thereby improve performance of golf club headfor off-center shots in a vertical direction along striking face(e.g., toward topline portionor toward sole portion).

Club headinhas a Blade Length (BL) measured between a toe-most extent of club headat virtual vertical toc planeand the intersection of hosel axisand ground plane, which also defines lie angle a. In some implementations, club headcan have a BL less than 80 mm. This blade length may, for example, correspond to an expected BL for an iron-type club head. In this regard, changes can be made to the thickness of striking facewithout sacrificing the conventional outer dimensions of club head, such as the BL or topline thickness of topline portion(e.g., TLin). In addition, the overall or target club head mass of club head(e.g., swing weight) in some implementations may correspond to expected masses for iron-type club heads.

As noted above, the mass for iron-type club heads typically vary based on the Loft Angle (LA). When presented in a set, iron-type club heads can increase in mass with loft. For example, the mass of iron-type club heads may adhere the following equation:

where mh is a club head mass, LA is the loft angle of the club head when orientated in a reference position, and a is between 190 g and 210 g. In some implementations, club headmaintains such a head mass, mh, while having an improved face thickness pattern.

is a heel side view of club headaccording to one or more embodiments. As shown in, the LA of club headis defined between face planeand virtual vertical hosel plane. As noted above, hosel axisextends axially through the center of hosel, and lies in virtual vertical hosel plane. Face planeis defined such that striking facelies in face plane. With reference to Equation 1 above, the club head mass of club headmay vary depending on the LA of club headsuch that higher numbered clubs with larger angles for LA have a greater club head mass.

As shown in, a distance between face planeand rear side planedefines Top Line Thickness (TL), which corresponds to a thickness of top line portionshown in. The TLof club headis no greater than 6.5 mm. This TLmay correspond to an expected TLfor an iron-type club head. In this regard, changes can be made to the thickness pattern of striking facewithout sacrificing the traditional outer dimensions of club head, such as the TLof club head, which may be preferred by some golfers.

is a cross-section view of club headtaken along cross section lineinaccording to one or more embodiments. As shown in, a rear surface of striking facefacing rear cavityand rear muscleincludes upper region groove, central region recess, and lower region groove. In implementations where striking faceincludes a face insert, a rear surface of the face insert can include upper region groove, central region recess, and lower region groove.

The rear surface of striking facealso includes intermediate regionat least partially surrounding the central region including central region recess. In this regard, intermediate regionincludes upper intermediate regionand lower intermediate region, above and below central region recess, respectively. Each of the central region, the upper region, and the lower region including central region recess, upper region groove, and lower region groove, respectively, has an average thickness that is less than the average thickness of intermediate region, which may have an approximately uniform thickness. Upper region grooveand lower region groovemay extend in generally a heel to toe direction, as in the examples of upper region groovesandand lower region groovesandin, respectively.

In some implementations, at least one of upper region grooveand lower region groovecan be an elongate groove having a width no less than approximately 2.0 mm. In addition, thickness of central region recessmay taper in some embodiments such that a heel-side region of the central recess may be thicker than a toe-side region of the central recess, as in the example of central region recessin. As another example, the central recess can include a heel-side region that has a greater thickness than a toe-side region, as in the example of heel-side regionand toc-side regionin. In some implementations, the thickness of the central region may decrease stepwise from a heel-side of the central region toward a toe-side of the central region.

In, Izz, is centered about virtual vertical CG axis. Discretionary mass removed or saved from striking faceto form upper region groove, central region recess, and lower region groovecan be relocated to heel portionand toe portionto increase Izz. In some implementations, Izz may satisfy:

where mh is the mass of club head. As noted above, increasing the MOI about virtual vertical axisextending through CGimproves the forgiveness of club headso as to cause less bending of club headabout virtual vertical axisduring off-center shots in a horizontal direction along striking face(e.g., shots that are more toe-ward or heel-ward of sweet spot).

In addition, the variable thickness pattern of striking facecan increase the COR at locations on striking facecorresponding to more commonly hit locations or a larger area of striking face to provide better energy transfer for off-center shots or for a statistically greater number of shots. The variable thickness pattern of striking facewith upper region groove, central region recess, and lower region groovecan increase the area of the striking face that has a relatively high COR.

For example, mass removed from particular areas of striking facecan improve the COR of striking face, and the removed mass can be relocated in club headso that CGcan be advantageously located closer to a lateral center of striking face, closer to virtual ground plane, and farther behind striking face. In such an example, mass removed or saved from striking faceto form upper region groove, lower region groove, and central region recess, such as by machining (e.g., grinding, milling) or by a known casting or forging process, can be relocated to rear muscleto lower the location of CGand move CGfarther behind striking face. As another example, mass removed from striking facecan be relocated from a heel-side of striking faceto a toe-side of striking faceto move CGaway from heel portiontoward toe portion.

Those of ordinary skill in the art will appreciate with reference to the present disclosure that other implementations may vary from the arrangement shown in. For example, other implementations of a cavity-back club head may include a different shape of rear cavityor rear muscle. As another example variation, the cross-section shapes of one or more of upper region groove, central region recess, and lower region groovemay differ from what is shown inin other implementations. As yet another example variation, some implementations may not include central region recess, and only include one or more grooves adjacent a periphery of the rear surface of striking face, such as upper region grooveand/or lower region groove.