Patent Publication Number: US-8522595-B2

Title: Guided keeper assembly and method for metal forming dies

Description:
CROSS REFERENCE TO RELATED APPLICATIONS AND CLAIM TO PRIORITY 
     The present application is a continuation of commonly assigned, U.S. patent application Ser. No. 13/114,208 filed May 24, 2011 now U.S. Pat. No. 8,074,486, which is a division of commonly assigned U.S. patent application Ser. No. 12/762,400 filed Apr. 19, 2010 (now U.S. Pat. No. 7,950,262), which is a continuation of commonly assigned U.S. patent application Ser. No. 11/515,477 filed Sep. 1, 2006 (now U.S. Pat. No. 7,730,757), which are incorporated herein by reference, and claims priority thereto under 35 U.S.C. §120. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to metal forming dies and the like, in particular, to a guided keeper assembly and associated method. 
     Metal forming dies, such as stamping dies and the like, are well known in the art. Progressive metal forming dies are unique, very sophisticated mechanisms which have multiple stations or progressions that are aligned longitudinally, and are designed to perform a specified operation at each station in a predetermined sequence to create a finished metal part. Progressive stamping dies are capable of forming complex metal parts at very high speeds, so as to minimize manufacturing costs. 
     Heretofore, the dies used in metal forming presses have typically been individually designed, one-of-a-kind assemblies for a particular part, with each of the various components being handcrafted and custom mounted or fitted in an associated die set, which is in turn positioned in a stamping press. Not only are the punches and the other forming tools in the die set individually designed and constructed, but the other parts of the die set, such as stock lifters, guides, end caps and keepers, cam returns, etc., are also custom designed, and installed in the die set. Current die making processes require carefully machined, precision holes and recesses in the die set for mounting the individual components, such that the same are quite labor intensive, and require substantial lead time to make, test and set up in a stamping press. Consequently, such metal forming dies are very expensive to design, manufacture and repair or modify. 
       FIGS. 4 and 5  illustrate a prior art metal forming die that includes a die shoe  1  and a die pad  2 , which are interconnected for mutual reciprocation by a plurality of spools  3 . A spring mechanism  4  is mounted between die shoe  1  and die pad  2 , and resiliently urges die pad  2  to a fully extended position. A metal forming die  5  is mounted on the outer surface of die pad  2 . Each of the spools  3  includes an enlarged head  6  which reciprocates in an associated counter bore  7  in the bottom of die shoe  1 . The heads  6  of spools  3  engage the top of the associated counter bores  7  to positively retain die pad  2  in its fully extended position. The other ends  8  of spools  3  are attached to the corners of die pad  2 . While such constructions have been generally successful, they do not precisely control reciprocation between die pad  2  and die shoe  1 , particularly in high speed, progressive die applications. 
       FIGS. 6 and 7  illustrate another prior art configuration, wherein pressed in pins  10 , with locator bushings  11 , have been added to the spools  3  shown in  FIG. 1  to more precisely control the reciprocation between die pad  2  and die shoe  1 . 
       FIGS. 8 and 9  illustrate yet another prior art configuration, which includes guide pins  10  and bushings  11 , but substitutes footed keepers  13  and  14  for the common spools  3  to positively limit the reciprocation between die pad  2  and die shoe  1 . More specifically, footed keepers  13  are mounted to die pad  2 , and engage mating footed keepers  14  which are mounted on die shoe  1 . 
     While such prior art constructions are generally effective, they are complicated and expensive. A modular guided keeper which both precisely aligns the die shoe and die pad, and positively limits reciprocal travel therebetween would be clearly advantageous in simplifying metal forming die constructions and reducing the cost in designing, manufacturing, and repairing the same. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention is a metal forming die of the type having a die shoe, a die pad mounted a spaced apart distance from the die shoe for reciprocation between converged and diverged positions, and a biasing member disposed between the die shoe and the die pad for biasing the same to the diverged position. The metal forming die includes at least one guided keeper assembly, comprising a base block having a generally flat mounting face abutting an adjacent face of the die shoe, at least one fastener aperture extending axially through a marginal portion of the base block for detachably mounting the base block to the die shoe, a central aperture extending axially through a central portion of the base block, and a bushing mounted in the central aperture of the base block. The guided keeper assembly also includes a guide pin having a cylindrically-shaped central portion closely received in the bushing in the base block for precisely guiding reciprocal motion between the die pad and the die shoe, a first end having an enlarged head shaped to abut the mounting surface of the base block to positively limit travel between the die shoe and the die pad, and a second end, positioned opposite the first end, and having a shoulder with a rigid center post protruding outwardly therefrom to precisely locate the second end of the guide pin in the die pad. The guided keeper assembly also includes a first fastener extending through the fastener aperture in the base block and securely, yet detachably, connecting the base block with the die shoe, as well as a second fastener securely, yet detachably, connecting the second end of the guide pin with the die pad. 
     Another aspect of the present invention is a guided keeper assembly for metal forming dies of the type having a die shoe, a die pad mounted a spaced apart distance from the die shoe for reciprocation between converged and diverged positions, and a biasing member disposed between the die shoe and the die pad for biasing the same to the diverged position. The guided keeper assembly includes a base block having a generally flat mounting face shaped to abut an adjacent face of the die shoe, at least one fastener aperture extending axially through a marginal portion of the base block for detachably mounting the base block to the die shoe, a central aperture extending axially through a central portion of the base block, and a bushing mounted in the central aperture of the base block. The guided keeper assembly also includes a guide pin having a cylindrically-shaped central portion closely received in the bushing in the base block for precisely guiding reciprocal motion between the die pad and the die shoe, a first end having an enlarged head shaped to abut the mounting face of the base block to positively limit travel between the die shoe and the die pad, and a second end, positioned opposite the first end, and having a shoulder with a rigid center post protruding outwardly therefrom to precisely locate the second end of the guide pin in the die pad. The guided keeper assembly also includes a first fastener extending through the fastener aperture in the base block and securely, yet detachably, connecting the base block with the die shoe, as well as a second fastener securely, yet detachably, connecting the second end of the guide pin with the die pad. 
     Yet another aspect of the present invention is a metal forming die of the type having a die shoe, a die pad mounted a spaced apart distance from the die shoe for reciprocation between converged and diverged positions, and a biasing member disposed between the die shoe and the die pad for biasing the same to the diverged position. The metal forming die also includes at least one guided keeper assembly, comprising a base block having a generally flat mounting face abutting an adjacent face of the die shoe, at least one fastener aperture extending axially through a marginal portion of the base block for detachably mounting the base block to the die shoe, and a central aperture extending axially through a central portion of the base block. The guided keeper assembly also includes a guide pin having a cylindrically-shaped central portion closely received in the central aperture of the base block for precisely guiding reciprocal motion between the die pad and the die shoe. The guide pin has a first end with an enlarged head shaped to abut the mounting face of the base block to positively limit travel between the die shoe and the die pad, and a second end, positioned opposite the first end, and having a shoulder with a center alignment aperture disposed concentrically in the shoulder, as well as an alignment pin having one end thereof mounted in the die pad, and an opposite end thereof closely received in the center alignment aperture on the guide pin shoulder to precisely locate the second end of the guide pin in the die pad. The guided keeper assembly also includes a first fastener extending through the fastener aperture in the base block and securely, yet detachably, connecting the base block with the die shoe, as well as a second fastener securely, yet detachably, connecting the second end of the guide pin with the guide pad. 
     Yet another aspect of the present invention is a method for making a metal forming die of the type having a die shoe, a die pad mounted a spaced apart distance from the die shoe for reciprocation between converged and diverged positions, and a biasing member disposed between the die shoe and the die pad for biasing the same to the diverged position. The method includes forming a base block with a generally flat mounting face shaped to abut an adjacent face of the die shoe, at least one fastener aperture extending axially through a marginal portion of the base block for detachably mounting the base block to the die shoe, and a central aperture extending axially through a central portion of the base block. The method further includes mounting a bushing in the central aperture of the base block. The method further includes forming a guide pin with a cylindrically-shaped central portion shaped for close reception in the bushing in the base block, a first end with an enlarged head shaped to abut the mounting face of the base block to positively limit travel between the die shoe and the die pad, and a second end with a shoulder and a rigid center post protruding outwardly therefrom. The method further includes forming a through hole in the die pad at a preselected location, and forming at least one fastener aperture in the die shoe at a preselected location. The method further includes inserting the central portion of the guide pin into the bushing in the base block for precisely guiding reciprocal motion between the die and the die shoe, and inserting a fastener through the fastener aperture in the base block and engaging the same in the fastener aperture of the die shoe to securely, yet detachably, mount the base block to the die shoe. The method further includes inserting the center post on the second end of the guide pin into the through hole in the die pad to precisely locate the second end of the guide pin in the die pad, and securely, yet detachably, connecting the second end of the guide pin with the die pad. 
     Yet another aspect of the present invention is to provide a metal forming die and associated guided keeper assembly that has a small, compact footprint, with a heavy-duty construction that is very durable. The guided keeper assembly has a modular configuration that facilitates economical manufacture, and also simplifies metal forming die constructions to reduce the effort and cost of designing, manufacturing, repairing and/or modifying the same. Machine downtime is also minimized to realize yet additional efficiency. The guided keeper assembly is efficient in use, economical to manufacture, capable of a long operating life, and particularly well adapted for the proposed use. 
     These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a die shoe and die pad interconnected by four guided keeper assemblies embodying the present invention, wherein portions of the die pad and die shoe have been broken away to reveal internal construction. 
         FIG. 2  is a side elevational view of one of the guided keeper assemblies embodying the present invention. 
         FIG. 3  is a bottom perspective view of the guided keeper assembly shown in  FIG. 2 , wherein a portion thereof has been broken away to reveal internal construction. 
         FIG. 4  is a partially schematic plan view of a prior art metal forming die. 
         FIG. 5  is a side cross-sectional view of the prior art metal forming die shown in  FIG. 4 . 
         FIG. 6  is a partially schematic plan view of an alternative prior art metal forming die. 
         FIG. 7  is a side cross-sectional view of the prior art metal forming die shown in  FIG. 6 . 
         FIG. 8  is a partially schematic plan view of yet another alternative prior art metal forming die. 
         FIG. 9  is a side cross-sectional view of the prior art metal forming die shown in  FIG. 8 . 
         FIG. 10  is an exploded perspective view of the guided keeper assembly shown in  FIGS. 1-3  with associated fragmentary portions of the die shoe and die pad. 
         FIG. 11  is a top plan view of a base block portion of the guided keeper assembly. 
         FIG. 12  is a vertical cross-sectional view of the base block taken along the line XII-XII,  FIG. 11 . 
         FIG. 13  is a bottom plan view of the base block. 
         FIG. 14  is a top plan view of a guide pin portion of the guided keeper assembly. 
         FIG. 15  is a side elevational view of the guide pin. 
         FIG. 16  is a bottom plan view of the guide pin. 
         FIG. 17  is a partially schematic plan view of a metal forming die having a plurality of stations each with die pads connected to the die shoe by the guided keeper assemblies. 
         FIG. 18  is a partially schematic side cross-sectional view of the metal forming die shown in  FIG. 17 . 
         FIG. 19  is a fragmentary, perspective view of another embodiment of the present invention. 
         FIG. 20  is a fragmentary, vertical cross-sectional view of the guided keeper assembly shown in  FIG. 19  shown attached to a die pad. 
         FIG. 21  is a fragmentary, top perspective view of a guide pin portion of the guided keeper assembly shown in  FIGS. 19 and 20 . 
         FIG. 22  is an exploded side elevational view of yet another embodiment of the present invention having an alignment pin connecting the guide pin with the die pad. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For purposes of description herein, the terms “upper”, “lower”, “right”, “left”, “rear”, “front”, “vertical”, “horizontal” and derivatives thereof shall relate to the invention as oriented in  FIGS. 1 and 2 . However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise. 
     The reference numeral  20  ( FIGS. 1-3 ) generally designates a guided keeper assembly embodying the present invention, which is particularly adapted for use in conjunction with metal forming dies, such as the die  21  illustrated in  FIG. 1 , having a die shoe  22  and a die pad  23  mounted a spaced apart distance from die shoe  22  for reciprocation between converged and diverged positions. A biasing member  24 , which is schematically illustrated in  FIGS. 17 and 18 , is disposed between die shoe  22  and die pad  23  for biasing the same to the diverged position. Guided keeper assembly  20  ( FIGS. 1-3 ) includes a base block  25  having a generally flat mounting face  26  abutting an adjacent face  27  of die shoe  22 . Base block  25  has at least one non-threaded fastener aperture  28  extending axially through a marginal portion of base block  25  for detachably mounting base block  25  to die shoe  22 . Base block  25  also includes a central aperture  29  extending axially through a central portion of base block  25 , and a bushing  30  mounted in the central aperture  29  of base block  25 . Guided keeper assembly  20  also includes a guide pin  32  having a cylindrically-shaped central portion  33  closely received in bushing  30  in base block  25  for precisely guiding reciprocal motion between die pad  23  and die shoe  22 . Guide pin  32  also includes a first end  34  having an enlarged head  35  shaped to abut the mounting face  26  of base block  25  to positively limit travel between die shoe  22  and die pad  23 . Guide pin  32  also includes a second end  36 , positioned opposite the first end  34 , and having a shoulder  37  with a rigid center post  38  protruding outwardly therefrom to precisely locate the second end  36  of guide pin  32  in die pad  23 . A first fastener  40  extends through the fastener aperture  28  in base block  25  and securely, yet detachably, connects base block  25  with die shoe  22 . A second fastener  42  securely, yet detachably, connects the second end  36  of guide pin  32  with die pad  23 . 
     In the example illustrated in  FIGS. 17 and 18 , die  21  is an upper die half, and includes four separate stations  45 - 48 , each having a separate die pad  23  attached to a common upper die shoe  22  by a plurality of guided keeper assemblies  20 . In the illustrated example, each of the die pads  23  is attached to the common die shoe  22  by four guided keeper assemblies  20  disposed adjacent corner portions of the die pads  23 . However, it is to be understood that the precise number of guided keeper assemblies and their particular location on the die pad  23  will vary in accordance with the particular application. Also, guided keeper assemblies  20  can be used on the lower die shoe, and other similar applications, as will be apparent to those skilled in the art. 
     As best illustrated in  FIG. 10 , at each position or location the guided keeper assembly  20  is to be installed, die shoe  22  is prepared in the following manner. A circular clearance or through hole  52  is formed through die shoe  22  in vertical axial alignment with the position at which the guided keeper assembly  20  is to be installed. Through hole  52  has a diameter slightly larger than the head  35  of guide pin  32  to permit free reciprocation of guide pin  32  therein. The formation of through hole  52  is relatively simple, since it can be formed in a single boring operation, and need not be precise, since there is substantial clearance between the head  35  of guide pin  32  and the interior of through hole  52 . 
     In the example illustrated in  FIG. 10 , four threaded fastener apertures  53  are formed in the surface  27  of die shoe  22 , and are arranged around through hole  52  in a quadrilateral pattern for purposes to be described in greater detail hereinafter. Also, in the embodiment illustrated in  FIG. 10 , two locator apertures  54  are formed in the surface  27  of die shoe  22  on opposite sides of through hole  52  to precisely locate base block  25  on die shoe  22  in the manner described in greater detail hereinafter. Preferably, locator apertures  54  are reamed to provide improved precision. 
     In the arrangement illustrated in  FIG. 10 , die pad  23  is prepared in the following manner. A precision circular locator aperture  60  is formed through die pad  23  at a position in vertical alignment with the location at which the guided keeper assembly  20  is to be installed. Locator aperture  60  is a through hole, and is formed with a precise diameter shaped through reaming or the like, to closely receive the center post  38  of guide pin  32  therein to accurately locate the second end  36  of guide pin  32  on die pad  23 . In the illustrated example, six non-threaded fastener apertures  61  are formed through die pad  23 , and are arranged in a circumferentially spaced apart pattern that is concentric with the locator aperture  60 . Fastener apertures  61  have enlarged outer ends to receive the heads of fasteners  42  therein, and serve to securely, yet detachably, mount the second end  36  of guide pin  32  to die pad  23  in a manner described in greater detail hereinafter. 
     The illustrated base block  25  ( FIGS. 10-13 ) is made from steel, and has a generally rectangular plan configuration defined by an upper surface  26 , a lower surface  66  and sidewalls  67 - 70  which intersect at radiused corners  71 . The illustrated base block  25  includes four non-threaded fastener apertures  28  positioned adjacent each of the corners  71  of base block  25 . Fastener apertures  28  are mutually parallel and are arranged in a rectangular pattern identical to that of the threaded fastener apertures  53  on die shoe  22 , such that fastener apertures  28  are in vertical alignment with threaded fastener apertures  53 . The lower or die pad ends of fastener apertures  28  have enlarged counter-bored portions  72  to receive therein the heads of fasteners  40 . The illustrated base block  25  also includes two locator apertures  73  which are formed through base block  25  and are arranged in a mutually parallel relationship for vertical alignment with the locator apertures  54  in die shoe  22 . The illustrated base block  25  has a relatively small, compact plan configuration to facilitate die manufacture, and also permits the same to be pocketed or recessed into the die shoe  22 , if necessary, for a specific application. 
     The illustrated bushing  30  ( FIG. 10 ) is a maintenance-free split bushing, constructed from a suitable antifriction material, such as bronze, steel alloys or the like. In the uninstalled condition, the outside diameter of bushing  30  is slightly larger than the interior diameter of central aperture  29 , such that bushing  30  is press fit into the central aperture  29  of base block  25  and is securely retained therein by a friction fit. The inside diameter of bushing  30  is slightly greater than the outside diameter of the central portion  33  of guide pin  32 , such as 0.0010-0.0020 inches, to accommodate for thermal expansion between the guide pin  32  and the bushing  30 , yet maintain precise reciprocal alignment between die shoe  22  and die pad  23 . 
     As will be appreciated by those skilled in the art, bushing  30  may be formed integrally into base block  25 , or omitted entirely by forming the bearing or guide surface for guide pin  32  in base block  25 . For example, base block  25  could be constructed from bronze, or other similar antifriction materials, such that central aperture  29  itself forms the guide surface. Alternatively, the central aperture  29  of base block  25  can be plated or otherwise coated with an antifriction material to eliminate the need for a separate bushing  30 . 
     The illustrated guide pin  32  (FIGS.  10  and  14 - 16 ) has a generally cylindrical shape, which in the orientation illustrated in  FIGS. 14-16 , has enlarged head  35  attached to the upper or first end  34  of guide pin  32  and center post  38  protruding downwardly from the lower or second end  36  of guide pin  32 . The illustrated shoulder  37  and center post  34  are formed integrally in the lower end  36  of guide pin  32 , and center post  37  is precisely located at the center of shoulder  37  in a concentric relationship. The lowermost end of the illustrated center post  38  is flat with a circular indentation at the center which facilitates precise location and formation of center post  38  on guide pin  32 . The illustrated center post  38  is accurately machined to a tolerance of 0.0-0.0005 inches. In the example illustrated in FIGS.  10  and  14 - 16 , six threaded fastener apertures  75  are formed in the flat, radially extending shoulder  37  of guide pin  32  in a circumferentially spaced apart pattern that is concentric with center post  38 . Threaded fastener apertures  75  are positioned to align vertically with the six non-threaded fastener apertures  61  and die pad  23 . In one working embodiment of the present invention, guide pin  32  is constructed from pre-hardened 4140 steel, or the like, is cut to length and formed, and then case hardened and polished. 
     With reference to  FIG. 10 , the illustrated guided keeper assembly  20  includes an annularly-shaped, resilient washer or ring  80  that is disposed on guide pin  32  between enlarged head  35  and the mounting face  26  of base block  25 . Resilient washer  80  serves to absorb impact between head  35  and base block  25  during operation, and can be constructed from urethane, or the like. 
     In operation, guided keeper assemblies  20  are used to quickly and easily interconnect die shoe  1  and die pad  2  for reciprocation between converged and diverged positions. At least two guided keeper assemblies  20  are typically used to mount die pad  2  to die shoe  1 . However, it is to be understood that the specific number of guided keeper assemblies  20  used depends upon the specific die application. In any event, the die shoe  1  is prepared in the manner described hereinabove by providing the clearance or through hole  52 , four threaded fastener apertures  53  and two locator apertures  54  at each location at which guided keeper assembly  20  is to be installed. Similarly, die pad  2  is prepared by forming one locator aperture  60  and six unthreaded fastener apertures  61  at each location guided keeper assembly  20  is to be installed. The base blocks  25  are then mounted to the surface  27  of die shoe  22  at each of the designated locations by installed threaded fasteners  40  which are then inserted through fastener apertures  28  and anchored in the threaded fastener apertures  53  in die shoe  22 . The illustrated fasteners  40  are cap screws with nylon pellets which resist inadvertent loosening in die shoe  22 . Alignment dowels or pins  85  may be mounted in die shoe  22  and received in locator apertures  54  and  72  to achieve additional precision in locating base blocks  25  on die shoe  22 . Guide pins  32 , with resilient washers  80  installed thereon, are then inserted through the bushings  30  in each of the base blocks  25 . The center post  38  at the lower end  36  of each guide pin  32  is received closely within the locator apertures  60  in die pad  23 . Threaded fasteners  42  are then inserted through the fastener apertures  61  in die pad  23  and anchored in the threaded fastener apertures  75  in the shoulder portion  37  of guide pin  32  to securely, yet detachably, connect the lower end of guide pin  32  with die pad  23 . 
     The reference numeral  20   a  ( FIGS. 20-21 ) generally designates another embodiment of the present invention, having a single fastener  42   a  at the shoulder end  36   a  of guide pin  32   a . Since guided keeper assembly  20   a  is similar to the previously described guided keeper assembly  20 , similar parts appearing in  FIGS. 20-21 ,  1 - 3  and  10 - 16 , respectively, are represented by the same, corresponding reference numerals, except for the suffix “a” in the numerals of the latter. In guided keeper assembly  20   a , the lower or shoulder end  36   a  of guide pin  32   a  includes a center post  38   a  having a non-circular plan configuration, which is designed to prevent rotation of guide pin  32   a  relative to the associated die pad  23   a . In the illustrated example, the center post  38   a  of guide pin  32   a  has a generally square plan configuration with radiused or rounded corners. Furthermore, a single threaded fastener aperture  75   a  is formed concentrically through shoulder  37   a  and into guide pin  32   a , and is adapted to receive therein a single threaded fastener  42   a  along with annularly-shaped cap or locking collar  88 . A set screw  89  extends radially through the side of guide pin  32   a  to facilitate removal of base block  25 , and positively retain fastener  42   a  in threaded fastener aperture  75   a . Die pad  23   a  is prepared with a non-circular locator aperture  60   a  to closely receive therein the center post  38   a  of guide pin  32   a  therein and prevent axial rotation therebetween. 
     The reference numeral  20   b  ( FIG. 22 ) generally designates yet another embodiment of the present invention having a removable locator pin  92  at the shoulder end  36   b  of guide pin  32   b . Since guided keeper assembly  20   b  is similar to the previously described guided keeper assembly  20 , similar parts appearing in  FIG. 22 ,  FIGS. 1-3  and  10 - 16 , respectively, are represented by the same, corresponding reference numerals, except for the suffix “b” in the numerals of the latter. In guided keeper assembly  20   b , a cylindrical recess  93  is formed in the end  37   b  of guide pin  32   b , instead of center post  38   b . In the illustrated example, recess  93  has a generally circular plan configuration, and is precisely formed in the center of the shoulder  37   b  of guide pin  32   b . A mating through aperture  60   b  is formed through die pad  23   b  in vertical alignment with recess  93 . A separate, cylindrical locator pin  92  has one end closely received in recess  93 , and the opposite end closely received in locator aperture  60   b , so as to precisely locate the shoulder end  36   b  of guide pin  32   b  in die pad  23   b.    
     Guided keeper assemblies  20 ,  20   a  and  20   b  each provide a very effective, versatile, uncomplicated and inexpensive mechanism that both precisely aligns a die shoe with an associated die pad, and positively limits reciprocal travel therebetween. 
     In the foregoing description, it will be readily appreciated by those skilled in the art that modifications may be made to the invention without departing from the concepts disclosed herein. Such modifications are to be considered as included in the following claims, unless these claims by their language expressly state otherwise. 
     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.