Patent Publication Number: US-2006009306-A1

Title: Golf club heads with inserts under compressive stress

Description:
This application claims priority to U.S. Provisional Application Ser. No. 60/575,966, filed Jun. 2, 2004, herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to golf club heads, such as putter heads, with inserts that are under compressive stress, as well as methods for making such golf club heads. In preferred embodiments, the inserts are composed of low density material, such as granite, in order to help maximize the moment of inertia (MOI) of the golf club head.  
     BACKGROUND OF THE INVENTION  
      Forgiving performance is a major objective of most modem putter designers, especially those targeting non-professional players. Even if a club head is delivered square to the target line at impact, a golf ball may lose both distance and accuracy (e.g., go off line) if the ball is not struck precisely on an axis in front of the putter planar center of mass. The degree of distance loss and mis-direction from a particular miss-hit is related to the putter planar moment of inertia (MOI). This is determined by the clubhead or putterhead planar moment of inertia and the position of the shaft in the putterhead which shaft weight and the shaft&#39;s own axial moment of inertia contributes to the putter MOI. The higher the MOI, the less the distance loss and angular misdirection for a given miss-hit. What is needed, therefore, are methods for making golf club heads, such as putter heads, that help maximize the MOI in order to make golf club more forgiving.  
     SUMMARY OF THE INVENTION  
      The present invention provides golf club heads, such as putter heads, with inserts that are under compressive stress, as well as methods for making such golf club heads. In preferred embodiments, the inserts are composed of low density material, such as granite, in order to help maximize the moment of inertia (MOI) of the golf club head.  
      In some embodiments, the present invention provides an article comprising a golf club head (or other sports club head), wherein the golf club head comprises: i) a body component comprising an insert recess, and ii) an insert component seated in the insert recess, wherein the insert component is under compressive stress. In other embodiments, the present invention provides an article comprising a golf club head, wherein the golf club head comprises: i) a body component comprising an insert recess, and ii) an insert component seated in the insert recess, wherein the body component is under tensile stress. In certain embodiments, the present invention provides an article comprising a golf club head, wherein the golf club head comprises an insert component, and wherein the insert component is under compressive stress. In certain embodiments, the insert components is not as wide as the insert recess, or is the same width as the insert recess, or is wider that the insert recess.  
      In particular embodiments, the present invention provides methods of making a golf club head (or other sports club head) comprising; a) providing; i) a body component comprising an insert recess; and ii) an insert component, and b) heating the body component; and c) generating a golf club head by seating the insert component in the insert recess of the body component such that the insert component is under compressive stress (and/or the body component is under tensile stress) upon cooling of the body component. In additional embodiments, the present invention provides methods of making a golf club head comprising; a) providing; i) a body component comprising an insert recess; and ii) an insert component, and b) cooling the insert component; and c) generating a golf club head by seating the insert component in the insert recess of the body component such that the insert component is under compressive stress (and/or the body component is under tensile stress) upon warming of the insert component (e.g. allowing the insert component to come back to room temperature).  
      In some embodiments, in step a) of the method, the insert component is wider that the insert recess (but is otherwise configured to be seated in the insert recess) when both the body component and insert component are at about room temperature. In certain embodiments, in step a) of the method, the insert component is between 0.004 and 0.05 inches wider (or between 0.00001 and 0.005 inches wider) than the insert recess when both the body component and insert component are at room temperature. In other embodiments, in step a) of the method, the insert component is between 0.005 and 0.03 inches wider than the insert recess when both the body component and insert component are at room temperature. In particular embodiments, the insert component is between 0.008 and 0.02 inches wider than the insert recess when both the body component and insert component are at room temperature. In further embodiments, the insert component is between 0.005 and 0.07 inches wider (or between 0.001 and 0.01, or between 0.00001 and 0.005 inches wider) than the insert recess when both the body component and insert component are at room temperature. In some embodiments, the insert component is less than 0.010 inches wider (or less than 0.009 inches wider) than the insert recess when the body component and insert component are at room temperature. In other embodiments, the insert component is wider than the insert recess when both the body component and insert component are at room temperature.  
      In some embodiments, the present invention provides systems comprising; a) a body component comprising an insert recess, and b) an insert component, wherein the insert component is between 0.004 and 0.02 inches wider than the insert recess (but is otherwise configured to be seated in the insert recess) when the body component and insert component are both at room temperature. In certain embodiments, the insert recess is configured to accept the insert component while at an elevated temperature (e.g. 250, 350, 450, 500, 750, 1000, 1250 degrees Fahrenheit) such that a golf club head could be generated.  
      In particular embodiments, the present invention provides systems comprising; a) an insert component, and b) a body component comprising an insert recess, wherein the insert recess, while at room temperature, is too small to accept the insert component, and wherein the insert recess, while at an elevated temperature (e.g. 250, 350, 450, 500, 750, 1000, or 1250 degrees Fahrenheit), is configured to accept the insert component such that a golf club head is generated.  
      In certain embodiments of the present invention, the insert component has a density of less than 5.5 grams/cm 3  or less than 4.5 grams/cm 3  or less than 4.0 grams/cm 3 . In some embodiments, the insert component has a density of less than 3.0 grams/cm 3 . In other embodiments, the insert component has a density between 5.0 and 1.5 grams/cm 3 , or between 2.0-3.0 grams/cm 3  (e.g. about 2.75 grams/cm 3 ).  
      In particular embodiments, the insert component has a hardness of at least 4.0 on Mohs hardness scale. In some embodiments, the insert component has a hardness of at least 5.0 on Mohs hardness scale. In particular embodiments, the insert component has a hardness of at least 6.0 on Mohs hardness scale.  
      In additional embodiments, the golf club head is configured for putting a golf ball. In some embodiments, the golf club head is attached to a golf shaft. In certain embodiments, the golf club head has a Moment of Inertia (MOI) about the shaft axis of at least 25.0 oz-in 2  or at least 40.0 oz-in 2 . In further embodiments, the golf club head has a Moment of Inertia (MOI) about the shaft axis of at least 45.0 oz-in 2 , 50.0 oz-in 2 , 55.0 oz-in 2 , 60.0 oz-in 2 , 62.0 oz-in 2 , 64.0 oz-in 2 , or 66.0 oz-in 2 . In some embodiments, the golf club head has a Moment of Inertia (MOI) about the shaft axis of between 40.0 oz-in 2  and 66.0 oz-in 2  or between 60.0 oz-in 2  and 66.0 oz-in 2 .  
      In certain embodiments, the golf club head is configured for chipping or pitching a golf ball (e.g. the golf club head form part of a gap wedge, sand wedge, lob wedge, pitching wedge, 9-iron, or similar types of clubs). In further embodiments, the golf club head is configured for driving a golf ball over 50 yards or over 100 yards (e.g. the golf club head is part of a 9-iron, 8-iron, 7-iron, 6-iron, 5-iron, 4-iron, 3-iron, 2-iron, 1-iron, 9 wood, 8 wood, 7 wood, 6 wood, 5 wood, 4 wood, 3 wood, 2 wood, or driver).  
      In some embodiments, the insert component comprises stone, glass, or ceramic. In certain embodiments, the insert component comprises plastic (e.g. plastic composite, thermoplasic, plastic with high hardness, etc.). In preferred embodiments, the stone comprises granite or marble. In certain embodiments, the body component further comprises a hosel. In certain embodiments, a golf shaft is inserted into the hosel. In other embodiments, the golf club head comprises a counterweight.  
      In additional embodiments, the compressive stress is exerted by at least a portion of the body component. In further embodiments, at least a portion of the body component is under tensile stress. In some embodiments, the tensile stress is exerted by the insert component. In other embodiments, there is no bonding material (e.g. epoxy) between the insert recess and the insert component. In preferred embodiments, the insert component is in direct contact over its length with the insert recess.  
      In some embodiments, the present invention provides an article comprising a golf club head (or other sports club head), wherein the golf club head comprises: i) a body component comprising an insert recess, and ii) an insert component seated in the insert recess, wherein the insert component is under compressive stress is composed of a low crystalline quartz granite (e.g. the low crystalline quart granite contain less than 10%, less than 5%, less than 3%, or less than 1% crystalline quartz). In certain embodiments, the low crystalline quartz granite contains between 0-10% crystalline quartz, between 0-5% crystalline quartz, or between 0-1% crystalline quartz). Preferably, the low crystalline quartz granite has a high modulus of elasticity, a uniform fine texture, and/or tight porosity (e.g. any combination of characteristics that make the low crystalline quartz granite insert component resistant to water absorption and warping from humidity). For example, in some embodiments, the low crystalline quartz granite is black granite obtained from Standridge Granite Corp. (Santa Fe Springs, Calif.). In certain embodiments, the low crystalline quartz granite insert component is under compressive stress. In other embodiments, the low crystalline quartz granite insert component is not under compressive stress (e.g. adhesive is used to secure the insert component into the insert recess).  
     DEFINITIONS  
      To facilitate an understanding of the present invention, a number of terms and phrases are defined below:  
      As used herein “compressive stress” refers to the stress applied to materials resulting in their compaction (decrease of volume). When a material is subjected to compressive stress then this material is under compression.  
      As used herein “tensile stress” refers to the stress state leading to expansion (volume and/or length of a material tends to increase).  
      As used herein, the term “hosel” refers to the socket or neck in the head of a golf club into which a shaft is inserted. Examples of hosels are shown as part  40  in  FIGS. 1-2 . 
    
    
     DESCRIPTION OF THE FIGURES  
       FIGS. 1A, 1B , and  1 C show various exemplary putter heads of the present invention.  
       FIGS. 2A, 2B , and  2 C show various exemplary putter heads of the present invention. 
    
    
     GENERAL DESCRIPTION OF THE INVENTION  
      The present invention provides golf club heads, such as putter heads, with inserts that are under compressive stress, as well as methods for making such golf club heads. In preferred embodiments, the inserts are composed of low density material, such as granite, in order to help maximize the moment of inertia (MOI) of the golf club head. The present invention enables the use of a brittle insert in the face of the putter head body by reducing the risk of fracture by using, for example, a thermal compression fit. Preferably, inserts that are slightly larger than the recess into which they are to be seated are seated into the recess by heating up the putter body and/or cooling the insert. The insert is then seated into the insert recess in the putter body and the components are allowed to return to room temperature causing compressive stress on the insert (and tensile stress on the putter body). This fit provides a residual compressive stress enabling the use of brittle materials such as ceramic, stone, glass, etc which would normally not be suitable due to their high susceptibility to fracture. Brittle materials typically have high compressive yield strengths and very low tensile yield strengths. The residual compressive stress effectively raises the tensile yield strength of the insert material to make the insert less susceptible to fracture.  
      The putter head assembly (e.g. body component with insert under compressive stress) is expected to see a certain range of temperatures for normal use and storage (e.g. use on a golf course and storage in a garage). This range may be referred to as T operating . Within T operating , the insert would be too large to fit into the pocket in the body and would have a nominal interference. Heating the body and/or cooling the insert utilizing the respective materials coefficient of thermal expansion can obtain the compression fit. This heating and/or cooling would take place such that the volume of the body increases and/or the volume of the insert decreases to a point where the insert could then be assembled into the pocket in the body. When the body and head both return to the same temperature within T operating  the nominal interference will require the body to stretch and the insert to compress from their initial size. This introduces the residual compressive stress in the insert and tensile stress in the body. The compression fit also holds the insert in place from friction between the mating materials and does not require the additional use of adhesives or fastening devices.  
      In certain embodiments, brittle material is used for the insert component. Using a brittle material (e.g. for the putter face) has several desirable attributes. Some brittle materials are extremely hard yet low density. A low-density face enables putter head weighting schemes to maximize/or minimize the moment of inertia (MOI) to the desired effect by allowing for greater flexibility in locating the center of gravity (CG) and the weight distribution. Many golfers wish to maximize the MOI of their putters since the putter is more resistant to twisting from off-center hits of the ball. Many golfers also desire a hard putting face to maximize accuracy and minimize damping of the ball impact. Some brittle materials can also achieve a very high dimensional flatness, which would result in a very accurate putting face. Also, many ceramics and stones are very dimensionally stable across a range of environmental conditions such as temperature and humidity. As such, the present invention provides a solution to a long felt need by allowing the use of insert components that help maximize MOI and feel since brittle, low-density materials may now be employed, and materials that might dampen the transmissibility of the putting sensation, such as epoxy, need not be used.  
     DETAILED DESCRIPTION OF THE INVENTION  
      Although not limited to any particular configuration,  FIGS. 1-2  show various preferred golf club heads of the present invention. These exemplary embodiments are described below to further illustrate the present invention and are not to be construed as limiting in any manner.  FIG. 1  shows various exemplary golf club heads of the present invention.  FIG. 1A  shows a body component  10  with an insert recess  20 . The insert recess shown in this figure is open (i.e. not enclosed) at the top. The body component may be constructed of any suitable material, such as, for example, aluminum or aluminum composite material. Preferably, when at room temperature, the insert recess  20  is slightly smaller than the insert component  30 . The insert component  30  is configured to seat within the insert recess  20  as shown in  FIG. 1A  when the body component  10  is expanded (e.g. by heat), or insert component  30  is reduced in size (e.g. by cooling), or by expanding the body component and reducing the insert component. In this regard, the insert component  30  may be seated in the insert recess under compressive force, such that that no additional attachment means (such as epoxy) are required to hold the insert component in place. However, it is understood, that epoxy may also be used in addition to the compressive force.  FIG. 1A  also shows the golf club head with a hosel  40 , into which a golf shaft may be secured.  FIG. 1B  shows a similar golf club head, except the insert recess fully encloses the insert when it is seated.  FIG. 1C  shows yet another embodiment. This embodiment includes a counterweight  50 . The counterweight is preferably composed of high density material (e.g. tungsten) such that the center of gravity of the golf club head is moved away from the insert component (e.g. further away from the ball striking surface).  
       FIG. 2  shows additional exemplary embodiments of the golf club head of the present invention.  FIGS. 2A and 2B  show particular embodiments with the insert component  30  inserted, where the insert component is composed of stone (e.g. granite or marble). Preferably, the body component of the golf club head in  FIG. 2A  is composed of aluminum, while the body component of the golf club head in  FIG. 2B  is composed of stainless steel.  FIG. 3C  shows another embodiment of the golf club head of the present invention, including an alignment marking  60  on the top of the inserted insert component. The alignment marking  60  may be used for lining up golf shots, such as a putt on a golf green.  
      In preferred embodiments, in order to generate golf club heads inserts under compressive stress, inserts that are larger than the insert recess (at room temperature) are fabricated. Such inserts can be seated into the recess insert by expanding the size of the insert recess and/or decreasing the size of the insert using a change in temperature. The size of the insert depends on the material chosen (e.g. granite) and temperature used to heat the body component, as well as on the shape of the insert recess (e.g. which may be rectangular, oval shaped, circular, etc.). Preferably, the insert component is between 0.005 and 0.03 inches wider than the insert recess when both the insert component and body component are at room temperature (e.g. the insert component is between 0.008 and 0.02 inches wider, or between 0.005 and 0.07 inches wider, than the insert recess when both the insert component are at room temperature. It is noted that the insert component may have the same width, but differ in some other parameter from the insert recess. For example, the insert component may have a height that is slightly taller or shorter than the insert recess. Regardless of the dimension that is different, all that is required is that the insert component be able to be seated in the insert recess (e.g. when the body component is heated up) such that the insert recess is seated under compressive stress.  
      The following calculations may be used to generally determine how much larger the insert should be compared to the insert recess. For example, one can use the following equation to determine the change in length of a given material when exposed to different temperatures: Equation (I): ΔL=LO*CTE*ΔT, where ΔL=Final Length minus Initial Length, LO=Initial Length, CTE=Coefficient of Thermal Expansion, and ΔT=Final Temperature minus Initial Temperature. The CTE is a material specific property. Therefore, if one had a given body component with an insert recess and a given insert to be fitted together, the gap between those two pieces when assembled is: Equation (II): GAP=(LO_body+ΔL_body)−(LO_insert+ΔL_insert), where LO_body is initial length of insert recess in body component, ΔL_body=Final body length minus Initial body length, LO_insert is initial length of insert, and ΔL_insert=Final insert length minus Initial insert length. Assuming that there is no heating or cooling of the insert, these equations can be rearranged to solve for the contact temperature. The contact temperature is the point at which after the insert is seated in the body and the body is cooling down, (shrinking as it cools) the body contacts the insert: Equation (III): T_contact=−GAP/(LO_body*CTE_body)+T_body_assembly, where T_body_assembly=temperature of body when assembling the insert. As the body continues to cool, in order for it to shrink it must compress the insert. This introduces residual compressive stress in the insert and tensile stress in the body. After contact, the forces on the insert and body must be equal and opposite. Both pieces may be viewed as essentially acting as very stiff springs. Therefore, using the spring force equation: Equation (IV): F_insert=k_insert*[u−CTE_insert*LO_insert*(T_final−T_contact)], where F_insert=compressive force on insert, where Spring Rate: k_insert=A_insert*E_insert/LO_insert, where A_insert=cross sectional area of insert, E_insert=Young&#39;s modulus of insert. Equation (V): F_body=k_body*[u−CTE_body*LO_body*(T_final−T_contact)], F_body=tensile force on body, where Spring Rate: k_body=A_body*E_body/LO_body, A_body=cross sectional area of body, and E_body=Young&#39;s modulus of body. Rearranging and remembering that the forces are equal and opposite: Equation (VI): u=(CTE_body*LO_body*(T_final−T_contact)/[1+(LO_body*A_insert*E_insert)/LO_insert*A_body*E_body)]. This equation can be solved for “u” and plugged into the force equations. Convert the forces into stress and verify that the resultant stresses do not exceed the yield strengths of the respective materials: Equation (VII): Stress_insert=F_insert/A_insert, and Equation (VIII): Stress_body=F_body/ A_body.  
      Using the above equations allows one to determine the temperatures or size of the various components that are needed for a given insert component and insert recess in a body component. For example, a golf putter body may be created from 6061 Aluminum that has a insert recess of 2.500″ in length and the insert component of 2.505″ in length at room temperature (70 Fahrenheit). If the putter body is heated to 450 degrees Farenheit, Equation (I) could be employed: ΔL_body=2.500 inches*1.35E−5/F*(450 F−70 F)=0.013 inches. Therefore the GAP per equation (II) is 2.500 inches+0.013 inches−2.505 inches=0.008 inches. This represent the amount of clearance when placing the insert component into the insert recess of the putter body. If it was desired to have more clearance, one could change the temperature and/or the length of the insert.  
      Next, one could determine the contact temperature as the body cools and shrinks to contact the insert component using equation (III) T_contact=−0.008 inches/(2.500″*1.35E−5/F)+450 F=213 F.  
      Equation (VI) could then be used to solve for displacement (u). Assuming a cross sectional area of the insert component of 0.375 square inches and a cross sectional area of the putter body of 0.700 square inches for this example. Young&#39;s modulus for the aluminum is 1.00E7 psi and for the granite is 5.80E6. Plugging this into the equation yields u=−0.0037 inches.  
      Taking this displacement, u, and plug into the Force equations (IV) and (V). CTE_insert (the granite) is 3.00E−6. T_final is room temperature=70 F. Solving these equations yields 3198 pounds force for F_body and −3198 pounds for F_insert. These are equal and opposite. Next take these forces and determine the stress on the body and insert per equations (VII) and (VII). Stress_body=3198 lbs/0.700 sq inches=4569 psi. Stress_insert=−3198/0.375 sq. inches=−8528 psi. Compare the magnitude of these stresses to the respective materials yield strength to ensure that the parts will not fail after assembly. For 6061 aluminum the yield strength is 40000 psi. Therefore there is a factor of safety of 8.8. For the granite the yield strength is 30000 psi so there is a factor of safety of 3.5. As long as the factor of safety is greater than 1 the part should not generally fail.  
      The present invention is not limited by the material or materials used to construct the insert component. Any suitable material may be used. Surprisingly, the present invention allows the use of brittle material in the insert component as the compressive stress on the insert component reduces the risk of fracture. The compressive fit of the present invention provides a residual compressive stress on the insert component enabling the use of brittle materials such as ceramic, stone, glass, etc., which would normally not be suitable due to their high susceptibility to fracture. The residual compressive stress effectively raises the tensile yield strength of the insert material to make the insert less susceptible to fracture. As such, the present invention allows the use of materials for making the insert component that exhibit a susceptibility to brittle fracture mechanisms.  
      In preferred embodiments, the insert components of the present invention are composed of low density material. In certain embodiments, the inserts are composed of material with a density of 4.0 g/cm 3  or less, 3.5 g/cm 3  or less, 3.0 g/cm 3  or less, between 1.0-4.0 g/cm 3 , or between 2.0-3.0 g/cm 3 . In preferred embodiments, the insert is composed of stone. The present invention is not limited by the type of stone. Preferably, the stone has a low density (most types of stone have a density of 2.5-3.0g/cm 3 ). Examples include, but are not limited to, basalt (about 3.2-3.5 g/cm 3 ); granite (about 2.4-2.7 g/cm 3 ); hornblende (about 3.0 g/cm 3 ); magnetite (about 3.2 g/cm 3 ); marble (about 2.6-2.9) g/cm 3 ; quartz (about 2.5-2.8 g/cm 3 ), and serpentine (about 2.7-2.8 g/cm 3 ). In other preferred embodiments, the inserts are composed of glass, which generally has a density of about 2.4-2.8 g/cm 3 . In certain embodiments, the inserts are composed of ceramic material.  
      In certain embodiments, the insert components are composed of relatively hard material (e.g. material hard enough to strike or putt a golf ball without denting). In particular embodiments, the material is at least 5.0 on Mohs hardness scale. Mohs hardness scale is as follows: 1.0 Talc; 2.0 Gypsum; 3.0 Calcite; 4.0 Fluorite; 5.0 Apatite; 6.0 Orthoclase; 7.0 Quartz; 8.0 Topaz; 9.0 Corundum (ruby and sapphire); and 10.0 Diamond. In preferred embodiments, the inserts are composed of material that is at least 6.0, 7.0, 8.0, or 9.0 on Mohs hardness scale. In particular embodiments, the insert components are composed of orthoclase, quartz, topaz, corundum, ruby, sapphire, diamond, or similar minerals.  
      The body component of the golf club head may be composed of any suitable material (e.g. any material that is able to house an insert component under compressive stress). In certain embodiments, the body component is composed of steel, aluminum or aluminum composite material. In other embodiments, the body component is composed of engineered composites or plastics. The present invention is also not limited by the shape of the body component. Preferably, the body component is in the shape of at least a portion of a golf club head. Various golf club heads are provided in the following patents and applications: U.S. Pat. Pub. 20040063516; U.S. Pat. Pub. 20050037857; U.S. Pat. Pub. 20030228925; U.S. Pat. Pub. 20030199332; U.S. Pat. No. 6,569,032; U.S. Pat. No. 5,433,441; U.S. Pat. No. 6,723,007; U.S. Pat. No. 4,569,524; and U.S. Pat. No. 6,796,911 all of which are herein incorporated by reference in their entireties.  
      Generating the golf club heads of the present invention with an insert under compressive stress is not limited to using change in temperatures. Any other suitable technique for seating the insert into the body component under compressive stress may be employed. In certain embodiments, insert molding techniques are employed. This method takes advantage of the shrinkage of the plastic or metal that occurs naturally after the part is ejected from the manufacturing process. In other embodiments, mechanical methods are used to exert and maintain the load on the insert component such that it is under compressive stress. For example, this could be achieved by mechanical methods such as attaching a mechanical fastener through the body component. These fasteners, when tightened, could apply and hold compressive pressure to the stone and enabling similar performance characteristics. In other embodiments, the body component could be designed to accept a clamp, which would supply the compressive force. The clamp could be placed around the end walls of the body component and the mechanical fastener could apply the needed pressure on the clamp and subsequently on the body component in order to create the compressive stress on the insert component. Another mechanical method to enable the compressive force would be to apply a mechanical load onto the end walls of the body component while the insert component is seated. This force could be applied by placing the insert component into the housing and subsequently placing the body component into a hydraulic press which could apply the needed pressures.  
      The golf club heads of the present invention, with inserts under compressive stress, may form part of a complete golf club for use while playing golf. Preferably, the complete golf club is a putter. One such exemplary complete golf club has the following components and performance characteristics. First, the insert that is under compressive stress is composed of granite. Granite is a preferred material for its low density and hard surface characteristics, which promote surface flatness and leads to a putt that minimizes ball skidding and initiates true roll. The granite insert component preferably is not secured with any adhesive bonding, but instead is held in place because of the compressive stress it is under. While not important to understand or practice the present invention, it is believed that eliminating the need for adhesives or other bonding material improves putter performance as these materials might dampen the transmissibility of the putting sensation. Additionally, the compressive force on the granite helps eliminate the potential of fracturing since granite is considered a very brittle material. Preferably, rear weighting (e.g. with tungsten) is provided in the putter head, in order to provide an optimum center of gravity (e.g. with a high MOI) directly behind the putter face achieving a smooth pendulum feel in the swing. Also, it is preferred that the face of the putter is balanced and has a loft angle of 3-4 degrees. The exemplary putter head may have a hosel (e.g. 2″-3″ inches in length) that is right or left handed, with a shaft mounted therein (e.g. a heel mounted shaft that is 33″-36″ in length). The height of the putter head may, for example, be about 1.5″-2″, with the sole being less than 5″ in preferred embodiments.  
     EXAMPLES  
      The following examples are provided in order to demonstrate and further illustrate certain preferred embodiments and aspects of the present invention and are not to be construed as limiting the scope thereof.  
     Example 1  
     Putter Head with an Insert Under Compressive Stress  
      This examples describes methods used generate a putter head with a granite insert under compressive stress. A block of 6061 aluminum alloy was machined to a size and shape of a golf putter body. An insert recess was precision-machined into the golf putter body with an opening width calculated to be 2.750+/−0.002 inches. A piece of granite was cut in order to generate an insert component. The insert component was cut such that it was slightly larger than the insert recess, having a width of 2.758+/−0.002 inches, which is 0.008 inches wider than the insert recess width. The width of the inert recess and the width of the insert component were calculated such that when the aluminum golf putter body was heated, the insert would be able to be seated into the golf putter head.  
      In this example, the golf putter body was heated up to 500 degrees Fahrenheit for about 1 hour in an oven. This heating allowed the insert recess to expand such that the granite insert could be seated therein. The granite insert component was fully seated into the expanded insert recess of the golf putter body by hand placing the insert while wearing oven mitts for protection. The assembled golf putter head was allowed to cool in ambient air for about an hour.  
      This manufacturing procedure results in a putter assembly where the granite insert is securely seated in the putter body without the use of any adhesives or other means of connection. Instead, the granite insert has a compressive fit that prevents it from coming loose. This compressive fit causes the insert to be under compressive stress and also causes at least part of the putter body to be under tensile stress.  
     Example 2  
     Two Additional Putter Heads with Inserts Under Compressive Stress  
      This Example describes the construction and testing of two additional putter heads with inserts under compressive stress. The granite inserts for both putter heads were cut to a width of 3.110+/−0.002. The insert recess in the first golf putter body (hereinafter P3104) was machined to a size of 3.104+/−0.002, while the insert recess in the second golf putter body (hereinafter P3100) was machined to a size of 3.100+/−0.002. Therefore, the granite insert for P3104 was nominally 0.006 inches larger than the insert recess, while the granite insert for P3100 was nominally 0.010 inches larger than the insert recess.  
      Both P3104 and P3100 were heated to 500 degrees Fahrenheit for about 1 hour, and attempts were made to seat the granite inserts into the insert recesses. For P3104, the granite insert was able to fully seat into the insert recess to make an assembled golf putter head. However, for P3100, the granite insert was only able to get about 85% into place before it was unable to fully seat. The aluminum golf putter body cooled too quickly causing the pocket to shrink as the granite was placed in the recess. Attempts to fully seat the granite insert using a C-clamp resulted in cosmetic damage to the insert (a small chunk broke off). The granite however was fully clamped by the putter body.  
      To test the properties of the components in the assembled golf putter head, a series of performance tests were conducted. The first test was used to asses the thermal extremes that the assembled putter head is able to withstand (e.g. extremes which a putter head may experience during a lifetime of use). In order to perform this test, the assembled putter heads were subjected to −40 degrees Celsius for one hour and subjected to 85 degrees Celsius for one hour. Upon completion of each temperature exposure, the assembled putter heads were inspected for visual signs of fracturing, chipping, or cracking in both the aluminum body component and the granite insert component. The granite was also inspected to determine if the fit in the putter body was secure. The results of this inspection were positive with no rejects or anomalies identified.  
      To test the strength of the insert component (e.g. as granite is known to be brittle) and to simulate potential impacts an assembled golf putter head might experience when striking a golf ball, destructive drop testing was performed with an assembled golf putter head made by the method described above. The initial drop test was conducted by dropping the assembled golf putter head on its corner from 5 feet. The results of this drop testing revealed that the granite was not affected, while the aluminum encountered minor surface denting on the corner. In the next series of drops, the assembled golf putter head was dropped with the granite insert component parallel to the concrete surface. The drop was conducted such that the granite face would be the first surface to impact the concrete ground. Drop heights were 3, 4, and 5 feet, with the 5 foot drop repeated three times. The results of this testing revealed that the granite insert was unaffected beyond minor cosmetic damage. To achieve a point of failure, a one pound hammer was dropped head first onto the face of the granite insert while the test sample rested on concrete from a distance of three feet which caused the granite insert to fracture with a portion chipping out. This same three foot drop with a hammer was repeated on a bare granite sample (not part of a putter head assembly) which caused the granite to shatter into multiple pieces.  
      Finally, as a control sample, a drop test was conducted with an assembled golf putter head where the granite insert was smaller than the aluminum body recess and simply glued in place at room temperature. This putter head assembly was dropped from 3 feet with the granite face parallel to the ground. The drop was conducted such that the granite face would be the first surface to impact the concrete ground. The granite insert shattered on the first drop.  
     Example 3  
     Moment of Inertia Calculations  
      This Example describes Moment of Inertia (MOI) calculations made for an exemplary putter head of the present invention and a comparison to the MacGregor V-FOIL putter. The MOI is a measure of resistance to twisting. High performance putters that are on the market, such as the MacGregor V-Foil, focus on maximizing the MOI of the club head by itself. Measuring only the MOI of the putter head itself ignores the reality that the putter head will twist about the shaft axis in a golfer&#39;s hands. Therefore, a more accurate measurement of a putter&#39;s performance can be obtained by examining the MOI of the putter head about the shaft axis. This allows one to gain an understanding of how resistant the club head is to twisting from off-center hits. This is critical to understanding the twist resistance of the system as a whole.  
      MOI for a putter depends not only on the moment of inertia of the body about the center of gravity (CG), but also the distance that CG is from the shaft axis. One CAD generated prototype of the present invention, shown in  FIG. 1C , was found to have a MOI (I_c or I Centroidal Axis ) for the club head itself of 25.6 oz-in 2 . This prototype used a granite insert with a density of about 2.6 g/cm 3 , and a tungsten counterweight (part  50 ,  FIG. 1C ). This low density insert face and tungsten rear weighting moved the center of gravity away from the clubface as well as the shaft axis. Since the center of gravity was calculated to be 1.816 inches from the clubface, the following parallel axis theorem was used to determine the MOI about the shaft axis:  
      I Parallel Axis =I Centroidal Axis +Md 2 , where M is the mass of the body, and d is the perpendicular distance between the centroidal axis and the parallel axis.  
      For the prototype club head, the following calculations were made: 
 
 I=I   —   c+M*d   2  
 
 I=[ 25.6 oz-in 2 ]+[12.256 oz]*[1.816 in] 2 =66.0 oz-in 2  
 
 The same MOI calculation was used for MacGregor&#39;s V-Foil M5K putter as it has a similar design and mass, and is advertised as one of the most stable putters ever made: 
 
 I=[ 4998 g-cm 2 ]*[0.353 oz/g]*[(1 inch) 2 ]/[(2.54 cm) 2 +[12.416 oz]*[1.570 in] 2 =57.93 oz-in 2 . 
 
      As can be seen by these calculations, the ability to use low density inserts allows one to design putter heads with increased MOI.  
      All publications and patents mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described articles, devices, methods and systems of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention which are obvious to those skilled in the relevant fields are intended to be within the scope of the following claims.