Abstract:
A grinding apparatus for grinding the hard metal inserts or working tips of drill bits (percussive or rotary), tunnel boring machine cutters (TMB) and raised bore machine cutters (RBM), said grinding apparatus having means for holding one or more bits to be ground and a grinding machine carried by an arm or lever system that permits movement of the grinding machine relative to the longitudinal axis of the button of the bit to be ground wherein means are provided to assist with aligning the grinding machine with the longitudinal axis of the button to be ground.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to improvements in apparatus for grinding the hard metal inserts or working tips of drill bits (percussive or rotary), tunnel boring machine cutters (TBM) and raised bore machine cutters (RBM) and more specifically, but not exclusively, for grinding the tungsten carbide cutting teeth or buttons of a drill bit or cutter. 
   In drilling operations the cutting teeth (buttons) on the drill bits or cutters become flattened (worn) after continued use. Regular maintenance of the drill bit or cutter by regrinding (sharpening) the buttons to restore them to substantially their original profile enhances the bit/cutter life, speeds up drilling and reduces drilling costs. Regrinding should be undertaken when the wear of the buttons is optimally one third to a maximum of one-half the button diameter. 
   Manufacturers have developed a range of different manual and semi-automatic grinding machines including hand held grinders, single arm and double arm self centering machines for setting up two or more bits to be ground, mobile machines for grinding on the road or in a workshop and grinders designed specifically for mounting on drill rigs, service vehicles or set up in the shop. The present invention is particularly applicable to mobile grinding apparatus of the type described in U.S. Pat. No. 5,193,312 and semi-automatic grinding machines as described in U.S. Pat. No. 5,070,654. 
   These types of machines utilize a grinding cup having the desired profile rotated at high speed to grind the carbide button and the face of the bit/cutter surrounding the base of the button to restore the button to substantially its original profile for effective drilling. When grinding buttons, the centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button. On buttons where wear is uneven, typically gauge buttons, this may result in regrinding the button off center from its longitudinal axis. 
   The conventional grinder designs switch between grinding pressure and balance pressure to achieve the desired effect. This, for example, does not allow for a grinding pressure equal to zero. In conventional grinder designs, the minimum grinding pressure is equivalent to the weight of the arm or lever section and the components attached to it. This may, for example, result in grinding pressure that is too great in relation to what is optimum for the size, type and profile of the button being sharpened. 
   The gauge buttons are mounted in the bit at an angle relative to the face of the bit. In order to properly regrind a worn gauge button the bit must be tilted to correspond to the angle at which the gauge buttons are mounted in the bit. In order to regrind all the buttons on the bit, the operator must continually, tilt and rotate the bit as he proceeds from button to button and/or bit to bit. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide means to help align, and optionally lock, the grinding machine with the longitudinal axis of the button to be ground. 
   It is a further object of the present invention to provide a biased side load to the grinding machine to help align the grinding machine with the longitudinal axis of the button to be ground. 
   It is a further object of the present invention to provide a balance pressure to the arm section which controls the movement of the grinding machine along the longitudinal axis of the bit or button when not in use and grinding pressure when in use. 
   It is a further object of the present invention to provide a controlled combination of balance pressure and grinding pressure to the grinding machine, whenever necessary. 
   It is a further object of the present invention to minimize operator set up and movement of the bit during regrinding. 
   Accordingly the present invention provides a grinding apparatus for grinding the hard metal inserts or working tips of drill bits (percussive or rotary), tunnel boring machine cutters (TBM) and raised bore machine cutters (RBM). The grinding apparatus consists of a means for holding one or more bits to be ground. In the preferred embodiment the means to hold the bit(s) is a table mounted in or on a frame, box or stand and is provided with one or more apertures to hold one or more bits to be ground. A grinding machine is carried by an arm or lever system that permits movement of the grinding machine relative to the bit or button to be ground. This is normally horizontal and vertical movement. Means are provided to help align the grinding machine with the longitudinal axis of the button to be ground. In one embodiment the means to help align the grinding machine with the longitudinal axis of the button provides a biased side load, optionally adjustable, to the grinding machine. 
   Another aspect of the present invention relates to means to provide a balance pressure to the arm or lever section that controls the movement of the grinding machine in the direction of the longitudinal axis of the button or bit when not in use and grinding pressure when in use. 
   Another aspect of the present invention relates to means to provide a controlled combination of balance pressure and grinding pressure to the grinding machine. This allows for a greater range of grinding pressure, or reduction of minimum grinding pressure by countering the combined weight of the arm or lever section and all other components contributing to feed during grinding. 
   Another aspect of the present invention relates to a grinding apparatus for grinding the hard metal inserts or working tips of drill bits (percussive or rotary), tunnel boring machine cutters (TBM) and raised bore machine cutters (RBM). The grinding apparatus includes a means for holding one or more bits to be ground. The means for holding the bit(s) is tiltably or pivotably mounted in or on a frame, box or stand with means to control the tilting/pivoting action. The means for holding the bit(s) is preferably a table provided with one or more apertures to hold one or more bits to be ground. A grinding machine is carried by an arm or lever system that permits horizontal and vertical movement of the grinding machine relative to the bit or button to be ground. 
   A further aspect of the present invention relates, where the grinding machine uses a hex drive configuration as described in U.S. Pat. Nos. 5,639,273 and 5,727,994, to a means to easily align and attach the grinding cup and detach the grinding cup after use. 
   Further features of the invention will be described or will become apparent in the course of the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be more clearly understood, the preferred embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a perspective view of one embodiment of an improved grinding apparatus according to the present invention having a grinding machine carried for vertical and horizontal adjustment by an arm or lever system journaled on a stand and with a tiltable table, pivotally mounted in a box, for holding the bit(s) to be ground; 
       FIG. 2  is a top plan view of the box and tilting table of the grinding apparatus of  FIG. 1 ; 
       FIG. 3  is a top plan view, partially cut away, of the apertures for holding the bits in the table of  FIG. 2 ; 
       FIG. 4  is a plan view of the left side of the grinding apparatus of  FIG. 1  showing the means of tilting the table; 
       FIG. 5  is a plan view of the right side of the grinding apparatus of  FIG. 1 ; 
       FIG. 6  is a plan view of the left side of a first arm section of the arm system of  FIG. 1 ; 
       FIG. 7  is a top plan view, partly broken away, of the first arm section of  FIG. 6 ; 
       FIG. 8  is a close-up plan view of one end of the first arm section of  FIG. 6 ; 
       FIG. 9  is a side view in cross section of a first control box and second arm section for setting the balance pressure and feed pressure for the grinding machine of  FIG. 1 ; 
       FIG. 10  is a front plan view of the grinding machine and second control box of the arm system of  FIG. 1 ; 
       FIG. 11  is an enlarged side view, partially in cross section, of the second control box of  FIG. 10 ; 
       FIG. 12  is a front plan view of another embodiment of an improved grinding apparatus according to the present invention having a grinding machine carried for vertical and horizontal adjustment by an arm or lever system journaled on a stand or frame and with a tiltable table for holding the bit(s) to be ground; 
       FIG. 13  is a front plan view of the frame for the grinding apparatus of  FIG. 12 . 
       FIG. 14  is a left side plan view of the frame of  FIG. 12 ; 
       FIG. 15  is a right side view of the frame of  FIG. 13 ; 
       FIG. 16  is a front plan view of the arm or lever system of  FIG. 12 ; 
       FIG. 17  is a top plan view of the arm or lever system of  FIG. 16 ; 
       FIG. 18  is a top plan view of the tiltable table for holding the bits to be ground of the grinding apparatus of  FIG. 12 ; 
       FIG. 19  is a right side plan view of the table of  FIG. 18 ; and 
       FIG. 20  is a plan view of bottom of the frame of  FIG. 13 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   With reference to the  FIGS. 1 to 11  one embodiment of a grinding apparatus according to the present invention is generally indicated at  1 . The grinding apparatus  1  includes means for holding one or more bits to be ground generally indicated at  2 . In this embodiment the means for holding the bit(s) consists of an open box  3  having a table  4  mounted within the box. A grinding machine  5  is carried by an arm or lever system, generally indicated at  6 , journaled on a stand  7  attached to the rear  8  of box  3 . A compressed air feed is provided to operate various aspects of the grinding apparatus as discussed in detail below. 
   In order to minimize operator set up and movement of the bit during regrinding, table  4  is tiltably mounted within box  3  at pivot points  9 , 10  on each side  11 ,  12  of the box  3  (see  FIGS. 4 and 5 ). The table  4  is provided with one or more apertures  13  to hold one or more bits to be ground. When a bit(s) (not shown) is positioned in aperture  13  the bit is held in place by pressure plate  14  controlled by a locking cylinder  15 . A shield  16  is attached to and moves with the pressure plate  14  and fully covers the opening between the rear of pressure plate  14  and back  17  of aperture  13 . The shield  16  protects the piston rod of the cylinder  15  and prevents accidental pinching of fingers, etc. when the locking cylinder and pressure plate are retracted. The locking cylinder  15  can be depressurized and backed off slightly to rotate the bit (to the next button to be ground) within the aperture  13  without full retraction of the locking cylinder  15  and pressure plate  14  attached to it. 
   If the button to be ground is a gauge button, it is typically mounted in the bit at an angle relative to the face of the bit. The grinding machine  5 , in order to properly regrind a worn button, should be aligned with the longitudinal axis of the button. Accordingly to regrind the gauge buttons, in the embodiment shown, table  4  is tilted to correspond to the angle at which the gauge buttons are mounted in the bit. Alternatively, the grinding apparatus could have, for example, a tilting feature or positioning feature allowing the grinding machine to be aligned with, and provide balance and grinding pressure along, the longitudinal axis of the button, without tilting the bit or button. 
   The means of tilting table  4  is best illustrated with reference to  FIG. 4 . An arced slot  20  is provided in the side  11  of box  3 . A similar slot is provided in the other side  12  of box  3  so the means for tilting the table can be mounted on either side of the box. A scale  21  is preferably provided to indicate the angle at which the table  4  will be tilted. A stop  22  is positioned within the slot  20  at the desired angle and locked in place by lever  23 . Once set for a particular bit type, the angle is fixed and doesn&#39;t have to be reset for each bit or button to be reground. A cylinder  24  is provided on the side  11  of the box  3  and the end  26  of the cylinder rod  25  is connected to the side  27  of table  4  at point  28 . When air is fed to cylinder  24 , extension of the rod  25  will tilt table  4  until further extension is prevented by stop  22 . As shown in  FIG. 4 , controls, generally indicated at  29 , for tilting the table and locking the bit(s) in place are provided at the side  11  of the box  3 . One switch  30  controls the cylinder  24  for tilting the table  4  and a second switch  31  controls the locking cylinder  15  and pressure plate  14 . Switches  30 ,  31  to control the locking cylinder  15  and pressure plate  14  of the second aperture  13  and the tilting of the table may be provided on the other side  12  of box  3 . Flow controls are provided to regulate the speed of movement of the table and the pressure plate. The tilting means can be mounted on either side of the box so that two boxes may be mounted side by side, while leaving the tilting means easily accessible 
   Large down the hole bits to be reground typically have a relatively long shaft that fits through aperture(s)  13 . In order to regrind smaller bits a floor plate  32  that can be pivoted (slid) in and out of position under aperture  13  is provided. Knob  33  and slot  34  in table  4  control the location of the floor plate  32 . Adapters (not shown) for holding multiple small sized bits can be inserted into aperture  13 . An air-actuated adapter having a pair of adjustable outer plates to accommodate different sizes of bits and air actuated locking plates is preferred. Use of the adapters eliminates repetitive set up time for the operator. 
   A splash-guard  35  is provided at the front  37  of the box  3  that can be raised and lowered along a slot  36  on each side of the front  37  edge of box  3 . A counter balancing spring assists in the ease of operation of splashguard  35 . The splashguard  35  can be set and retained at different heights as desired. 
   The arm system  6  for carrying and positioning grinding machine  5  as noted previously is journaled onto stand  7 . With reference to  FIGS. 1 ,  2 ,  4 , and  6 – 9 , the arm system  6  consists a first arm section  39  having one end  40  journaled to stand  7 . The other end  41  of the first arm section  39  is journaled to the backside  42  of a first control box  43 . The first arm section  39  controls the horizontal location of the grinding machine relative to the bit to be reground. To the front side  44  of control box  43  is pivotally mounted a second arm section generally indicated at  45 . The second arm section  45  consists of a pair of parallel arms  46 ,  47  with one end  48 , 49  of each arm  46 , 47  pivotally mounted to the front side  44  of the first control box  43 . The other end  50 , 51  of each arm  46 ,  47  is pivotally connected to the backside  52  of a second control box  53 . The second arm section  45  controls the vertical movement of the grinding machine up and down. 
   Within the second control box  53  ( FIG. 11 ), is a rotation motor  54  and bearing arrangement  55  for providing an orbital rotation to grinding machine  5 . The grinding machine  5  is attached to control box  53  by means of plates  56 . The grinding machine  5  has an electric motor in the embodiment shown but can also utilize an air or hydraulic motor. Each of the plates  56  is provided with an acruate slot  57 . The angle of attachment of the grinding machine  5  relative to control box  53  can be adjusted by means of slots  57  and locking levers  58 . By having the grinding machine slightly off vertical, nipple formation on the button being reground is minimized and uneven wear on the grinding cup avoided. 
   A circuit board  60  is provided within the second control box  53 , said circuit board  60  containing the control system for the grinding apparatus including controls and microprocessor to control grinding time on each button and grinding pressure. In the embodiment shown pressure controls  61  are provided with digital read outs on the front  62  of the second control box  53  to permit the operator to set, or increase or decrease grinding time and grinding pressure. The microprocessor can be used to provide other functions either manual or automatic. For example, the microprocessor, in the case of an electric motor, can monitor the amperage being used and if it reaches a preset limit automatically decrease the grinding pressure to prevent motor burn out. The microprocessor can also control the flow of coolant to the face of the button during grinding. 
   Within the first control box  43  ( FIG. 9 ), is means to provide a balance pressure, generally indicated at  63 , to the arm or lever section that controls the movement of the grinding machine in the direction of the longitudinal axis of the button or bit when not in use and grinding pressure when in use. In the embodiment shown, the means to provide a balance pressure  63  to the arm or lever section that controls the vertical movement of the grinding machine is a cylinder  64  connected to an end  49  of the lower arm  47  of the second arm section  45 . The end  49  of lower arm  47  extends out from the pivot point  65  at which the lower arm  47  is connected to the first control box  43 . Cylinder  64  provides a balance pressure to the second arm section when the grinding machine is not in use and grinding pressure when in use. The grinding pressure can be adjusted. In addition, by applying a controlled balance pressure while in zero grinding pressure mode, a lesser grinding pressure can be achieved than that provided by gravity; effectively increasing the available range of grinding pressure. 
   When grinding buttons the self-centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button. On buttons where wear is uneven, typically gauge buttons, this may result in regrinding the button off center from its vertical axis. One aspect of the present invention provides means  66  to help align the grinding machine with the longitudinal axis of the button to be ground. In the embodiment shown the means  66  to help align the grinding machine with the longitudinal axis of the button consists of, a cylinder  67  ( FIG. 6 ) having one end  68  connected to stand  7  and the other end  69  connected to the bottom  70  of the first arm section  39 . The cylinder  67  provides a side load to grinding machine  5  to help align the grinding machine with the longitudinal axis of the button. In the embodiment shown, the cylinder  67  is automatically activated when the table  4  is tilted by the pilot feed from cylinder  24  through valve  71  ( FIG. 7 ). Valve  79  controls the water flow. The side load biases the grinding machine to grind more on either the outside or the inside of the gauge buttons as required thereby tending to shift the grinding machine over the true center of the button. The means to help align the grinding machine with the longitudinal axis of the button to be ground can alternatively include a locking system to lock the arm in place to prevent movement in a direction normal to the longitudinal axis of the button while permitting movement in the axial direction. Suitable side load can also be provided by means other than by cylinder  67  such as counterweights etc. 
   While typical grinding apparatus are aligned so that the longitudinal axis of the bit is generally vertical during grinding, in the case of very large bits, or in drilling equipment where bits or cutters are mounted in a clustered pattern, grinding may be done with the bit aligned horizontally or some other suitable angle. The present invention is equally applicable to this situation. In this situation the grinding machine may be carried on an arm or lever system and the grinding pressure applied in a horizontal or other suitable direction. 
   As best illustrated in  FIG. 10 , the grinding machine illustrated utilizes a hex drive system of the type described in U.S. Pat. Nos. 5,639,273 and 5,727,994. In order to make the operation of the apparatus operator friendly, means  72  are provided to easily align and attach the grinding cup and detach the grinding cup after use. A spring-loaded button  73  when depressed will fit into a slot in the rotor  74  and prevent it from rotating. This enables the operator to align the hex drive section of the grinding cup with the drive section  75  of the rotor  74  and then push the grinding cup on. To remove the grinding cup after use the operator presses lever  76  towards the grinding machine. The lever  76  pivots around point  77  and the extending arms  78  push the grinding cup away from the drive section of the rotor facilitating removal of the grinding cup from the grinding machine. 
   With reference to  FIGS. 12 to 20  a second embodiment of an improved grinding apparatus according to the present invention is generally indicated at  100 . While the present invention is applicable to all grinding apparatus having a grinding machine carried for vertical and horizontal adjustment by an arm or lever system journaled on a stand or frame and preferably with a tiltable table for holding the bit to be ground, the grinding apparatus shown is of the type intended to be mounted on drill rigs, service vehicles or set up in the shop, optionally installed inside a cabinet enclosure. 
   The grinding apparatus  100  includes means  101  for holding one or more bits to be ground. In this embodiment the means  101  for holding the bit(s) consists of a frame  102  supporting a table  103 . The frame  102 , of the illustrated embodiment, consists of a generally rectangular box  104  having a back panel  105 , left and right side walls  106 ,  107  normal to said back panel, two extending support arms  108 ,  109 , a top plate  110 , bottom plate  111  and hinged cover  112 . 
   A grinding machine  113  is carried by an arm or lever system, generally indicated at  114 , journaled on a cylinder rod  115  of cylinder  116  attached to the frame  102 . A compressed air connection is provided to operate various aspects of the grinding apparatus as discussed in detail below. 
   In order to minimize operator set up and movement of the bit during regrinding, table  103  is tiltably mounted to the frame  102  at pivot points  118 ,  119  on the appropriate places of the extending arms  108 ,  109  (see  FIG. 12  and  FIG. 18 ). The table  103  is provided with one or more apertures  121  to hold one or more bits to be ground. A bit (not shown) is positioned in aperture  121  with the button to be ground in the vertical position. The bit is held in place by pressure plate  122  controlled by a locking cylinder  123 . The locking cylinder can be backed off slightly to rotate the bit (to the next button to be ground) within the aperture  121  without full release of the pressure on the locking cylinder. 
   The grinding machine  113 , in order to properly regrind a worn button, should be aligned with the longitudinal axis of the button. Accordingly to regrind the gauge buttons, table  103  is tilted to correspond to the angle at which the buttons are mounted in the bit. The bit is then indexed in the table so that the longitudinal axis of the button to be ground is in the vertical. The means of tilting table  103  is best illustrated with reference to  FIGS. 18 and 19 . The front edge  124  of right side  107  is bent to form an inwardly extending flange  125 . A longitudinal slot  126  is provided in flange  125 . A scale  127  is preferably provided to indicate the angle at which the table  103  will be tilted. A stop  128  is positioned within the slot  126  at the desired angle and locked in place by lever  129 . Once set for a particular bit type, the angle is fixed and doesn&#39;t have to be reset for each bit or button to be reground. A cylinder  130  is connected to stop  128  and the end  131  of the cylinder rod  132  is connected to the side  133  of table  103  at point  134 . When air is fed to cylinder  130 , retraction of the rod  132  will tilt table  103  until the rod is fully retracted. As shown in  FIG. 12 , controls, generally indicated at  135 , for tilting the table and locking the bit(s) in place are provided on the instrumentation panel  136  on the hinged protective cover  112 . One switch  137  controls the cylinder  130  for tilting the table  103  and a second switch  138  controls the looking cylinder  123  and pressure plate  122 . Flow controls are provided to regulate the speed of movement of the table and the pressure plate. 
   Large down the hole bits to be reground typically have a relatively long shaft that fits through aperture  121 . In order to regrind smaller bits a floor plate  139  that can be slid in and out of position under aperture  121  is provided. Knob  140  and slot  141  in table  103  control the location of the floor plate  139 . Adapters for holding multiple small sized bits can be inserted into aperture  121 . Use of the adapters eliminates repetitive set up time for the operator. 
   A splashguard  142  (see  FIG. 20 ) connected to bottom plate  111  of frame  102  is provided at the front of the grinding machine  113  that can be raised and lowered. 
   The arm system  114  for carrying and positioning grinding machine  113  as noted previously is journaled onto cylinder rod  115 . With reference to  FIGS. 12 and 16 , the arm system  114  consists of a first arm section  143  having one end  144  journaled to cylinder rod  115 . The other end  145  of the first arm section  143  provides the mount for the second arm section  146 . One end  147  of the second arm section  146  is journaled to the first arm section  143 . The other end  148  of the second arm section  146  is journaled to the back mount  149  connected to control box  150 . Locking lever  151  allows control box  150 , with back mount  149  to be moved into position in relation to the axis of the second arm section and then to be locked. Cylinder  116  through cylinder rod  115  controls the vertical movement of the grinding machine up and down. Cylinder  116  through cylinder rod  115  provides a balance pressure to the arm system when the grinding machine  113  is not in use and grinding pressure/feed when in use. The grinding balance pressure and pressure/feed can be adjusted. 
   Within the second control box  150 , is a rotation motor  152  and bearing arrangement  153  for providing an orbital rotation to grinding machine  113 . The grinding machine  113  is attached to control box  150  by means of plates  154 . The grinding machine  113  has an electric motor in the embodiment shown but can also utilize other motor type such as air or hydraulic motor. Each of the plates  154  is provided with an arcuate slot  155 . The angle of attachment of the grinding machine  113  relative to control box  150  can be adjusted by means of slots  154  and locking lever  156 . By having the grinding machine oscillate slightly off vertical, nipple formation on the button being reground is minimized and uneven wear on the grinding cup avoided. 
   When grinding buttons, the self-centering aspects of the grinding machine tend to center the grinding machine over the highest point on the button. On buttons where wear is uneven, typically gauge buttons, this may result in regrinding the button off center from its vertical axis. To substantially align the grinding machine with the longitudinal axis of the button, the second arm section  146  is provided with a cylinder  157  having one end  158  connected to the cylinder  116  and the other end  159  connected to the mount  160  of the second arm section  146 . The cylinder  157  provides a biased load to grinding machine  113  to help align the grinding machine over the button. In the embodiment shown, the cylinder  157  is automatically activated when the table is tilted by the pilot feed from cylinder  130  through valve  161  in instrumentation panel  136 . The biased load causes the grinding machine to grind more on the inside or outside of the gauge buttons, as required, thereby tending to shift the grinding machine over the true center of the button. The suitable biased load can be provided by other means such as counter weights, etc. Another method of aligning the grinding machine with the longitudinal axis of the button, in cases where deemed necessary, is by using one or more cylinder(s)  157  connected to cylinder  116  and cylinder rod  115  or frame  102  to lock the position of grinding machine  113  in the plane normal to and aligned with the longitudinal axis of the button to be ground. Additionally or alternatively cylinders, mechanical locks etc. can be incorporated to achieve the desired effect. During grinding the grinding machine will then grind the button in alignment with the button axis. Other methods such as mechanical locks can also be used for locking the horizontal distance or position in the horizontal plane of grinding machine  113  from, for example, cylinder  116  and cylinder rod  115  or frame  102 . 
   Variations of the above described principle of biased side loads, counter balancing and position fixing, that can be used to allow for grinding at angles other than vertical, are within the scope of the present invention. Combinations of variations of the above described principle of biased side loads, counter balancing and position fixing can be used to substantially eliminate the need for tilting/pivoting the bit when switching between grinding of face buttons and gauge buttons. This principle would be ideal in cases where tilting or pivoting of the bit is difficult due to size, weight, etc. 
   Having illustrated and described a preferred embodiment of the invention and certain possible modifications thereto, it should be apparent to those of ordinary skill in the art that the invention permits of further modification in arrangement and detail. 
   It will be appreciated that the above description related to the preferred embodiment by way of example only. Many variations on the invention will be obvious to those knowledgeable in the field, and such obvious variations are within the scope of the invention as described and claimed, whether or not expressly described.