Abstract:
A joinder method and article constructed according to the method in which two separate parts are fixedly joined into a unitary one-piece structure by an interference fit between precision mounting surfaces formed on the first and second parts along a mating joint. The dimensions of the first mounting surface are formed oversized with respect to the complementary dimensions of the second mounting surface. The first part is significantly cooled to cause sufficient contraction allowing the first mounting surface to be inserted within the second mounting surface. When warmed, the first part expands to its normal dimensional shape driving the first mounting surface into an interference fit with the second mounting surface. The first and second parts can be formed of dissimilar materials which, in the specific case of a golf club head, uses a heavier and harder material for the second part and a lightweight material for the top part. Optional surface irregularities on the second mounting surface forcibly engage the first mounting surface when the first part expands to its normal dimensional shape. Additional joint strength obtained by preventing oxidation of the mounting surfaces which enables diffusion bonding of the two parts along the mating joint. Temporary mounting tabs are formed about the periphery of each of the first and second parts during formation of the first and second parts for locating the parts in a fixture during formation of the precision mounting surfaces on the parts.

Description:
CROSS-REFERENCE TO CO-PENDING APPLICATION 
     This invention describes subject matter which is related to the subject matter disclosed in U.S. patent application Ser. No. 09/316,375, filed May 21, 1999, concurrently herewith for a “Weighted Golf Club Head and Method of Making the Same”, by Donald R. Cook. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present relates, in general, to metal fastening techniques and, more specifically, to methods for joining two metal components into a unitary structure. 
     2. Description of the Art 
     Various joining processes are available for unitarily joining two metal components or parts together into a unitary structure. Welding, adhesive bonding, and mechanical fasteners, such as screws, are widely employed to fixedly join two metal components or parts together. 
     Another metal joining technique utilizes a press fit or interference fit wherein one component or portion of one component has an outer dimension slightly larger or oversized with respect to a mating portion, such as a bore or recess, in another component. The two components are urged together to force the oversized portion of one component into the smaller sized portion of the second component to fixedly join the two components together. While effective in certain applications, a press fit or interference fit requires assembly equipment capable of generating and withstanding the high forces or pressures involved in creating the press fit between two components. 
     In certain applications, the aesthetic appearance of the assembled part requires that screw heads be covered thereby requiring additional assembly steps to finish the part. Similarly, welding two metal parts together along a joint between the two parts, while forming a secure joint, requires subsequent machining steps, such as grinding, to finish the joint to a smooth surface shape. 
     One example of a two-part metal component structure is a metal golf club head. Metal drivers have replaced older “wood” club head constructions. While it is possible to cast a hollow, one-piece golf club head, such fabrication techniques have presented difficulties in achieving high quality parts at a reasonable cost. Thus, a hollow metal golf club head is typically formed of two cast parts, such as a main body and a separate face plate or sole plate which are then welded together to form the complete golf club head. This is an expensive, time consuming process and requires additional finishing steps to smooth the weld bead to a smooth exterior surface shape on the golf club head. 
     Similar joinder techniques are employed in metal golf club irons wherein perimeter weights are mounted in recesses formed generally along the bottom edge of the iron to improve club head balance, to lower the center gravity of the club head and to enlarge the sweet spot on the club face. Such weights are mounted in recesses formed in the iron club body and then welded, brazed or soldered into place. Again, time consuming and the additional finishing steps are required to smooth the weld bead between the weight insert and the club body. 
     What is needed is a joinder technique or methodology for joining two metal component or parts together into a one-piece unitary structure which minimizes assembly steps of the one-piece structure, provides a secure joint between the two metal parts, and minimizes or preferably eliminates after-assembly finishing steps to lower the manufacturing cost of the one-piece structure or component. It would also be desirable to provide such a joinder methodology to the construction of golf club heads provided with a hollow driver configuration or perimeter weighted iron or putter configurations. 
     SUMMARY OF THE INVENTION 
     The present invention is a unique method for joining two metal parts into a unitary one-piece structure or member without welding, brazing, soldering, or the use of mechanical fasteners. The present invention also contemplates an article constructed to the inventive method. 
     According to one aspect of the present invention, a method of joining first and second parts with mating joint surfaces together into a unitary member comprises the steps of: 
     forming first and second parts with complementary mating surfaces; 
     forming the dimension of opposed portions of the mating surface of the first part oversized with respect to the corresponding dimension of the second part; 
     cooling the first part to sufficiently cause contraction in the dimension of the mating surface of the first part; 
     coupling the cooled first part with the second part at the mating surfaces; and 
     allowing the cooled first part to warm resulting in expansion of the first part to its original size and forming an interference fit between the complementary mating surfaces of the first and second parts. 
     The method also includes forming the first and second parts of the same material or dissimilar materials having different thermal expansion characteristics. 
     In one aspect, the method also forms the cooled first part of a material having a lower hardness than the hardness of the material forming the second part. 
     In another aspect, the inventive method forms surface irregularities on the mounting surface of the second part which forcibly engage the mating surface of the first part when the cooled first part expands its normal dimensions. 
     According to another aspect of the present invention, an article, such as a two-part golf club head, by example only, is constructed by the inventive method described herein. The golf club head comprises: 
     first and second separate portions having mating first and second mounting surfaces, respectively, to be joined together into a unitary one-piece member; 
     opposing portions of the first mounting surface of the first part having a normal dimension larger than the corresponding dimension of the mounting surface of the second part; and 
     an interference fit joining the first part to the second part at the mating first and second mounting surfaces by expansion of the first part to its normal dimensional shape after contraction caused by cooling. 
     In one aspect, the first and second parts are formed of dissimilar materials. 
     In another aspect, the material forming the second part is heavier and/or denser than the material forming the first part. 
     In another aspect, the second part is harder than the first part. 
     In yet another aspect, the second mounting surface of the second part has surface irregularities projecting outward therefrom which embed in the mounting surface of the first part as the first part expands. 
     The unique method and article formed by said method of the present invention provides joinder of two metal components into a unitary one-piece structure without the need for welding, brazing, or soldering operations which typically require subsequent surface finishing steps and a resultant increase in the manufacturing cost of the part as well as eliminating the need for mechanical fasteners to join the two parts together. By eliminating welding, brazing or soldering joinder operations, any possibility of heat tempering of either part is eliminated thereby minimizing any potential distortion in the shape or mating mounting surfaces of the two parts which could decrease the mechanical strength of the joint between the two joined parts. Thus, the method and article constructed according to the method of the present invention minimizes joinder failures due to inadequate joints. 
     When the article is a two-part golf club head constructed according to the inventive method, further advantages are obtained. When the bottom portion of the golf club head is formed of a heavier and harder material than a lighter weight top portion, the resulting golf club head, besides having a reduced manufacturing cost due to the elimination of surface finishing steps required by the prior art use of welding, etc., to join a two-part golf club head together, also has better weight distribution due to a low center of gravity since the bottom portion of the club is heavier than the top portion. This results in a better balance and higher performance of an inventive golf club head according to the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features, advantages and other uses of the present invention will become more apparent by referring to the following detailed description and drawing in which: 
     FIG. 1 is an exploded, perspective view of a golf club head constructed to the method of the present invention; 
     FIG. 2 is an inverted, exploded, perspective view of the golf club head shown in FIG. 1; 
     FIG. 3 is a top perspective view of the bottom portion of the golf club head shown in FIGS. 1 and 2; 
     FIG. 4 is a bottom, perspective view of the top portion of the golf club head shown in FIGS. 1 and 2; 
     FIG. 5 is an exploded, cross-sectional view, generally taken along line  5 — 5  in FIG. 1 of the assembled top and bottom portions of the golf club head; 
     FIGS. 6 and 7 are partial cross-sectional views, similar to FIG. 5, but showing alternate joint constructions according to the present invention; and 
     FIG. 8 is a partial cross-sectional view of an another aspect of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is a unique method of fixedly joining two metal components or parts together into a unitary one-piece structure as well as articles made by said method. It will be understood that the present method is adaptable to many different articles. 
     It will also be understood that the following description of the present inventive method and an article constructed using said method in which the article comprising a two-part golf club head, is by way of example only and not intended to limit the particular joint configuration or article made by the present method. 
     Referring to FIGS. 1-5, there is depicted an article which, by way of example only, is a two-part golf club head  10  formed of a bottom portion or part  12  and a top portion or part  14 . The bottom portion  12  and the top portion  14  of the golf club head  10  are joined together by the present method, described hereafter, into a unitary one-piece structure without the need for mechanical fasteners, welding, adhesives, etc., typically employed in joining two metal components together, particularly when forming a golf club head of two parts. 
     Again, by way of example only, the bottom portion  12  of the golf club head  10  has a generally concave shape formed of a bottom wall or sole  16 , a face or striking surface  18  and an upturned sidewall  20  terminating in a peripheral edge  22 . A hostel receiver  24  is formed at one corner of the bottom portion  12  for receiving a hostel to interconnect the club head  10  to a shaft. In addition, a pair of strengthening ribs  26 , by example only, are integrally formed on the sole  16  and project upwardly therefrom. 
     According to one aspect of the present invention, a mounting surface denoted by reference number  30  is formed about the peripheral edge of at least a portion and, preferably, substantially all of the peripheral edge  22  of the bottom portion  12 . By example only, the mounting surface  30  extends across the face surface  18  and the entire sidewall  20  of the bottom portion  12  except for a short distance around the hostel receiver  24 . 
     As shown more clearly in FIG. 5, the mounting surface  30 , according to one aspect of the present invention, has a step configuration formed of a top wall  32  extending radially inward from the peripheral edge  22 , an intermediate wall  34  depending from an inner edge of the top wall, and an inner wall  36 , generally parallel to the top wall  32  which extends from one end of the intermediate wall  34  to an edge of an inner surface of the sidewall  20  of the bottom portion  12 . 
     The intermediate wall  34  depends substantially perpendicularly from the top wall  32  and is also substantially perpendicular to the inner wall  36 . It will be understood that other joint configurations may also be possible as long as the intermediate wall  34  which forms the main resistive surface to expansion of the corresponding mating mounting surface of the top portion  14 , as described hereafter, is at an angle of no more than 90° with respect to the mating mounting surface of the top portion  14 . Thus, it is possible with the present invention to form the mounting surface  30  of the bottom portion  12  in which the intermediate wall  34  depends at an acute angle with respect to the lower inner wall  36  to form a dovetail configuration. The bottom edge of the mating mounting surface on the top portion  12  must contract sufficiently to clear the edge between the top wall  32  and the intermediate wall  34 . 
     As with the bottom portion  12 , the intermediate wall  48  of the top portion  14  is depicted as extending substantially perpendicularly from the inward extending wall  46  and the outer end wall  50 . It will be understood that the intermediate wall  48  may also project at an acute angle with respect the inner wall  50  for mating engagement with a similarly formed mounting surface on the bottom portion  12 . 
     The top portion  14  of the golf club head  10 , as shown in FIGS. 1,  2 ,  4  and  5 , has a top surface  40  having a slightly contoured shape and an outer peripheral wall  42  depending from the top surface  40 . A mounting surface  44  is formed inboard of the outer peripheral wall  42  and, as shown in greater detail in FIG. 5, is in the form of a depending leg having a radially inward extending wall  46 , an intermediate wall  48  depending from the inner wall  46  and an outer end wall  50  generally parallel to the inner wall  46  and projecting radially inward from the intermediate wall  48 . 
     The mounting surface  44  on the top portion  14  has a step configuration identical to the step configuration of the mounting surface  30  on the bottom portion  12  of the golf club head  10  for secure mating engagement therebetween. 
     As shown in FIG. 4, the mounting surface  44  on the top portion  14  extends about the entire outer peripheral edge  42  of the top portion  14  except for a small recess  52  which is disposed adjacent to the hostel receiver  24  in the bottom portion  12  when the bottom portion  12  and top portion  14  are joined together. 
     The mounting surfaces  30  and  44  are separately precision cut to an identical, complementary shape. This assures that when the top and bottom portions  14  and  12  are joined together, the mounting surfaces  44  and  30 , respectively, securely engage each other about their entire peripheral length. 
     As shown in FIG. 5, dimension  54  depicts the spacing or distance between the opposed points on the intermediate walls of the mounting surface  30  in the bottom portion  12 . Dimension  55  depicts the spacing or distance between two opposed points on the exterior surfaces of the opposing intermediate walls  48  of the top portion  14 . The dimension  54  will be understood to define a dimension between two diametrically opposed points or portions on the mounting surface  30  of the bottom portion  12 . Similarly, the dimension  55  represents the dimension between any two diametrically opposed points on the mounting surface  44  of the upper portion  14 , which points on the upper mounting surface  44  will be disposed in registry with the corresponding points on the mounting surface  30  depicted by the dimension  54  in FIG.  5 . Similar dimensions  54  and  55  will be established for each pair of diametrically opposed points on the bottom and top portions  12  and  14  between the sidewalls  20  or between the front and rear walls  16  and  18  of the bottom portion  12  and the corresponding portions on the top portion  14 . 
     Before describing the interaction of the mounting surfaces  30  and  44  according to the present inventive method, a brief discussion of the materials used to form the bottom portion  12  and the top portion  14  of the golf club head will be provided. Generally, any suitable material may be employed for the bottom portion  12  and the top portion  14 . Such materials include steel, stainless steel, aluminum, titanium and various alloys thereof. 
     Further, it is possible to form the bottom portion  12  and the top portion  14  of the golf club head  10  of the same material, such as aluminum, titanium, stainless steel, etc. However, further advantages may be obtained if a heavier weight or density material, such as titanium, is employed for the bottom portion  12  and a lighter weight material, such as aluminum, is used to form the top portion  14 . Such a construction provides better weight balance in a golf club by lowering the center of gravity of the golf club head due to the heavier bottom portion  12 . Thus, by example only, the bottom portion  12  is formed of titanium; while the top portion  14  of the golf club head  10  is formed of a lighter weight aluminum. 
     Thus, the dimension  55  defines an outer peripheral edge on the mounting surface  44  of the top portion  14  and dimension  54  defines an inner peripheral edge on the mounting surface  30  of the bottom portion  14  which are to be brought into engagement to prevent separation or disengagement of the top portion  14  from the bottom portion  12 . Even though the outer peripheral edge of the intermediate wall  48  on the top portion  14  is to be brought into secure engagement with the inner surface of the intermediate wall  34  of the bottom portion  12 , according to the present method, the dimension  55  representing the distance or spacing between two points or opposing portions of the intermediate wall  48  on the top portion  14  is intentionally made slightly oversize with respect to the dimension or spacing between the two corresponding points on the inner peripheral surfaces of opposing portions of the intermediate wall  34  of the bottom portion  12 . By way of example only, the dimension  55  on the top portion  14  is formed oversize by about 0.005 inches to about 0.010 inches with respect to the similar dimension  54  between spaced portions of the intermediate wall  34  of the bottom portion  12 , with an oversize amount of about 0.007 to about 0.008 inches being preferred. 
     The formation of this oversize dimension is the first step in the present inventive method. Next, the top portion  14  is subjected to substantially below 0° F., such as to about −280° F., cooling, by placing the top portion  14  in a liquid nitrogen tank or bath, or by spraying liquid nitrogen over the upper portion  14  to cause significant shrinkage or contraction of the top portion  14  in all directions or axes, particularly a reduction in the dimension  54  between two opposing outer surfaces of the intermediate wall  48  on the top portion  14 . 
     This shrinkage enables the mounting surface  44  on the top portion  14  to be inserted within the mating mounting surface  30  of the bottom portion  12  while the top portion  14  is still substantially cooled significantly below 0° C. Different cooling temperatures may be employed with different oversize dimensions  54  on the top portion  14  or different materials to achieve the same end result. 
     Next, when the top portion  14  has been sufficiently cooled to impart the desired amount of contraction or reduction in overall size, the top portion  14  is mated to the bottom portion  12  by insertion of the mounting surfaces  44  into the mounting surface  30  on the bottom portion  12 , respectively. The top portion  14  then immediately warms to ambient temperature or to the temperature of the bottom portion  12  causing an expansion of the top portion  14 , particularly between the depending portions of the mounting surface  44  of the top portion  14 . This forces the opposing portions of the depending leg or intermediate wall  48  of the mounting surface  44  of the top portion  14  into tight engagement with the corresponding portion on the intermediate wall  34  of the mounting surface  30  of the bottom portion  12 . 
     When the top portion  14  reaches ambient temperature, it will have expanded back to its original oversize dimension wherein the dimension  55  between two opposing portions of the outer surfaces of the intermediate wall  48  of the top portion  14  is larger than the mating portions of the bottom mounting surface  30 . This oversize dimension forces the depending leg of the mounting surface  44  of the top portion  14  into an interference fit with the mating portions of the mounting surface  30  of the bottom portion  12  thereby securely mounting the top portion  14  to the bottom portion  12  without the need for mechanical fasteners, welding or the significant forces and equipment for generating forces used to press fit or interference fit two parts together. 
     An alternate joint is shown in FIG. 6 in which the top portion  14 ′ has a mounting surface formed simply by forming the outer peripheral edge  42  as a flat wall  60  which seats against the inner surface of the intermediate wall  34  of the bottom portion  12 . An outer bottom edge of the top portion  14 ′ seats on the inner wall  36  of the mounting surface  30  of the bottom portion  12 . The critical dimensions  54  and  55  defining the mating joint between the top portion  14 ′ and bottom portion  12 , is between the opposing portions of the flat wall  60 . Otherwise, the two component construction shown in FIG. 6 is identical to that described above and shown in FIG.  5 . 
     Another alternate joint configuration according to the present invention is shown in FIG.  7 . In this joint configuration, the mounting surface  80  on the bottom portion  12  has a top wall  82  extending radially outward from an inner edge of the inner surface of the sidewall  20  of the bottom portion  12 . An intermediate wall  84  depends from the outer edge of the top wall  82  and terminates in a further radially outward extending outer wall  86 . 
     The top portion  14 , in this embodiment, has a mounting surface formed of an inner wall  88  extending radially outward from an edge of the inner surface of the top portion  14 . The inner wall  88  terminates in a depending intermediate wall  90  which extends substantially perpendicularly from the inner wall  88 . An outer wall  92  projects from the opposite end of the intermediate wall  90 . 
     In this aspect of the present invention, the bottom portion  12  is formed oversize, at least with respect to the dimension between diametrically opposing portions of the intermediate wall  84  by the above described amount between about 0.005 inches to about 0.010 inches. The bottom portion  12  is then cooled to approximately −280° F. to cause contraction of the entire bottom portion  12  in all directions. The mounting surfaces between the bottom and top portions  12  and  14  are then mated and the bottom portion  12  allowed to warm to ambient temperature. This causes an expansion of the bottom portion  12  to its normal dimensions thereby driving the intermediate walls  84  of the bottom portion  12  into secure, fixed engagement with the corresponding intermediate wall  90  of the top portion  14 . 
     Although the joint shown in FIG. 7 may not be as strong as the joint shown in FIG. 6 since the top portion would typically be formed of a lighter weight material in a golf club head which is a potentially weaker material then that used to form the bottom portion  7 , in certain applications, including golf clubs, as well as non-golf club articles, such a joint configuration may have certain advantages. 
     FIG. 8 depicts an alternate aspect of the present invention in which the mounting surface  30  is formed of a harder material, such as titanium, has an intermediate wall  34 ′ formed with surface irregularities  66  to enhance the mechanical connection between the top portion  14  and the bottom portion  12 . The surface irregularities  66  can take any one of a number of different forms, such as score lines similar to that used in rifle bores, serrated edges, radially inward facing projections, etc. In operation, during expansion of the cooled top portion  14 , the surface irregularities  66  embed themselves into the intermediate wall  48  of the top portion  14  for a more secure mechanical fit between the top portion and the bottom portion  12 . 
     Although the interference fit joint between two mating parts according to the present invention can take a variety of configurations, what is crucial is that the mating mounting surfaces of the two parts be formed such that the mounting surface of one part is disposed outermost of a corresponding mating mounting surface of the opposite part and has a wall portion, defined herein as the intermediate wall  34  of the embodiment shown in FIG. 5 or the intermediate wall  82  shown in the embodiment depicted in FIG. 7, which lies in the plane of expansion of the mating portion of the opposite part so as to engage and resist complete expansion of the opposite part and to thereby create the strong, interference fit joint between the two parts. 
     In a preferred configuration, the mounting surfaces of one of the parts defines a peripheral edge surrounding an opening into which the mounting portion of the opposite part is inserted. The interfering wall or surface of the one part which receives and resists expansion of the other part lies in a plane which is perpendicular to the insertion axis of one part into the other. 
     According to another aspect of the present invention, the strength of the mechanical joint between the top portion  14  and the bottom portion  12 , according to the method described above, may be further enhanced by providing diffusion bonding of the metals forming the top portion  14  and the bottom portion  12  at the mating joint. Typically, metal mating surfaces, such as aluminum or titanium, by example, oxidize to form a coating that prevents further surface reaction. By casting the bottom portion  12  and the top portion  14  and then joining the bottom portion  12  and the top portion  14  together, according to the method described above, immediately after casting and without allowing the mounting surfaces  30  and  44  of the bottom portion  12  and the top portion  14  to oxidize, the metals at the joint mounting surfaces  30  and  44  will diffuse bond together during expansion of the cooled top portion  14  into engagement with the bottom portion  12  to further increase the strength of the joint between the bottom portion  12  and the top portion  14 . 
     Referring to FIGS. 3 and 4, there is depicted another aspect of the present invention in which at least one and preferably a plurality of spaced mounting tabs, each denoted by reference number  70  for the bottom portion  12  and by reference number  72  for the top portion  14 , are integrally cast during the initial formation or casting of the bottom portion  12  and the top portion  14 . The tabs  70  and  72  can take any of a number of different shapes, but generally comprise a planar member projecting radially outward from the sidewall  20  or outer peripheral edge  22  of the bottom portion  12  as shown in FIG. 3 or from the outer peripheral edge  42  of the top portion  14  as shown in FIG.  4 . 
     The tabs  70  and  72  act as locators to accurately position the bottom portion  12  and the top portion  14  in a holding fixture during precision machining of the mounting surfaces or edges  30  and  44 . The tabs  70  and  72  fit into mating recesses or cavities in a fixture having a complementary shape to the shape of the tabs  70  and  72 . In one aspect, each tab  70  and  72  has a through bore drilled or otherwise formed therein which seats over a locating pin in the holding fixture to locate the bottom portion  12  or the top portion  14  in the proper position relative to a zero reference location on a machining table or machine. 
     The tabs  72  can also be used to support the top portion  14  in the liquid nitrogen tank or bath during cooling of the top portion  14 . 
     Otherwise, after the individual bottom portion  12  or top portion  14  is formed and the mounting surfaces  30  and  44 , respectively, precision formed thereon, the tabs  70  and  72  are cut off and the surface of the bottom and top portions  12  and  14  from which the tabs  70  and  72  projected ground or otherwise finished to a smooth exterior finish. 
     In summary, there has been disclosed a unique method of joining two metal parts together into a one-piece unitary structure as well as inventive articles constructed in accordance with said method. By substantially cooling one part, which is initially formed dimensionally oversize at the joint surface, to a sufficiently low temperature to cause a predetermined amount of contraction of the one part, the cooled part may be inserted into the mating part along adjoining mounting surfaces such that expansion of the cooled part back to its normal, oversize dimensional shape will cause an interference fit between the mounting surfaces of the two parts without the need for mechanical fasteners, welding, solder, brazing, adhesive, etc., which typically require additional machining or surface finishing steps to create a smooth joint between the two parts. The inventive method also eliminates heating of one or both of the parts, such as required by welding, brazing or soldering, which could affect the hardness of the materials forming the parts as well as potentially distorting their shape, particularly at the mating mounting surfaces. The interference fit mounting surfaces of the present method expands the possibilities for new appearances and designs of a large number of articles since the specific design requirements for forming a joint by welds, mechanical fasteners, etc., are eliminated.