Abstract:
Method and apparatus for sharpening a cutting tool. In some embodiments, an abrasive endless belt is rotated in tension along a neutral plane between spaced apart first and second rollers. A guide assembly has spaced apart first and second guide surfaces which collectively converge to an intervening base surface to form a guide channel. Upon insertion of a blade of a cutting tool into the guide channel, a selected side of the blade contactingly slides against at least a selected one of the first or second guide surfaces and a first portion of a cutting edge of the blade contactingly engages the base surface to serve as a plunge depth limit stop for the blade. The endless belt is contactingly deflected by a second portion of the cutting edge away from the neutral plane to sharpen the second portion while the first portion remains in contact with the base surface.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation of copending U.S. patent application Ser. No. 14/213,264 filed Mar. 14, 2014, which is a continuation of copending U.S. patent application Ser. No. 12/809,522 filed Jun. 18, 2010 now issued as U.S. Pat. No. 8,696,407 and which is a 371 of International Patent Application No. PCT/US2008/068412 filed Jun. 26, 2008 and which in turn claims benefit to U.S. Provisional Patent Application No. 61/016,294 filed Dec. 21, 2007. 
     
    
     BACKGROUND 
       [0002]    Cutting tools are used in a variety of applications to cut or otherwise remove material from a workpiece. A variety of cutting tools are well known in the art, including but not limited to knives, scissors, shears, blades, chisels, machetes, saws, drill bits, etc. 
         [0003]    A cutting tool often has one or more laterally extending, straight or curvilinear cutting edges along which pressure is applied to make a cut. The cutting edge is often defined along the intersection of opposing surfaces (bevels) that intersect along a line that lies along the cutting edge. 
         [0004]    In some cutting tools, such as many types of conventional kitchen knives, the opposing surfaces are generally symmetric; other cutting tools, such as many types of scissors, have a first opposing surface that extends in a substantially normal direction, and a second opposing surface that is skewed with respect to the first surface. 
         [0005]    More complex geometries can also be used, such as multiple sets of bevels at different respective angles that taper to the cutting edge. Scallops or other discontinuous features can also be provided along the cutting edge, such as in the case of serrated knives. 
         [0006]    Cutting tools can become dull over time after extended use, and thus it can be desirable to subject a dulled cutting tool to a sharpening operation to restore the cutting edge to a greater level of sharpness. A variety of sharpening techniques are known in the art, including the use of grinding wheels, whet stones, abrasive cloths, etc. A limitation with these and other prior art sharpening techniques, however, is the inability to precisely define the opposing surfaces at the desired angles to provide a precisely defined cutting edge. 
       SUMMARY 
       [0007]    Various embodiments of the present invention are generally directed an apparatus and method for sharpening a cutting tool. 
         [0008]    In some embodiments, a sharpener includes a plurality of spaced apart rollers comprising at least first, second and third rollers rotatable about parallel first, second and third axes, respectively, wherein the first, second and third rollers are placed in a substantially triangular arrangement so that a distance between the first and second axes is substantially equal to a distance between the first and third axes and greater than a distance between the second and third axes. A flexible abrasive belt having a selected linear stiffness is routed along the plurality of spaced apart rollers to define a belt path having a first extent that extends along a first plane tangential to the first and second rollers and a second extent that extends along a second plane tangential to the first and third rollers, each of the first and second planes symmetric with respect to a centerline passing through a central axis about which the first roller rotates, the central axis bisecting an overall angle between the first and second planes. 
         [0009]    A tensioner assembly is connected to at least one of the plurality of spaced apart rollers and adapted to exert a bias force thereon to translate the corresponding axis of the at least one of the plurality of spaced apart rollers with respect to the axes of the remaining rollers to maintain a tension along the flexible abrasive belt and resist bending of the flexible abrasive belt out of at least one of the first or second planes. An electric motor is adapted to drive the flexible abrasive belt in at least a first selected rotational direction along the belt path. 
         [0010]    A sharpening guide assembly is secured adjacent the flexible abrasive belt. The sharpening guide assembly has a first guide slot adjacent the first extent of the flexible abrasive belt along the first plane and a second guide slot adjacent the second extent of the flexible abrasive belt along the second plane. The first and second guide slots have first and second cutting edge guide surfaces adapted to contactingly engage the cutting edge of the cutting tool during presentation of the respective first and second sides of the cutting tool against first and second cutting edge guide surfaces, the first and second cutting edge guide surfaces being symmetric about the centerline. 
         [0011]    The first guide slot is configured to contactingly support a cutting tool in a selected orientation during presentation of a first side of the cutting tool against the first extent of the flexible abrasive belt. The second guide slot is configured to contactingly support the cutting tool in said selected orientation during presentation of an opposing second side of the cutting tool against the second extent of the flexible abrasive belt. The first and second extents of the flexible abrasive belt respectively deform at a radius of curvature responsive to said presentation of the respective first and second sides of the cuffing tool thereagainst. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1A and 1B  provide respective isometric and side elevational views of a cutting tool sharpener system (sharpener) constructed in accordance with various embodiments of the present invention. 
           [0013]      FIG. 2  shows the sharpener of  FIGS. 1A-1B  with a guide housing removed to expose various features of interest including an abrasive belt and three rollers. 
           [0014]      FIG. 3  is a schematic depiction of  FIG. 2 . 
           [0015]      FIG. 4A  provides an end view of the arrangement of  FIG. 3  with the use of crowned rollers. 
           [0016]      FIG. 4B  provides an alternative end view of the arrangement of  FIG. 3  with the use of guide rollers. 
           [0017]      FIGS. 5A and 5B  show side and top plan views of portions of a first belt. 
           [0018]      FIGS. 6A and 6B  show side and top plan views of portions of a second belt. 
           [0019]      FIGS. 7A and 7B  provide schematic depictions of the sharpener to generally illustrate a twisting (localized torsion) of the unsupported abrasive belt during a sharpening operation upon a cutting tool. 
           [0020]      FIGS. 8A and 8B  generally illustrate different torsion effects that may be encountered by the abrasive belt during the sharpening of the cutting tool of  FIG. 7 . 
           [0021]      FIG. 9  shows a sharpening guide of the sharpener guide housing in greater detail. 
           [0022]      FIGS. 10A-10C  generally depict a progression of symmetrical sharpening operations that may be advantageously performed upon a cutting tool to provide the tool with a desired final geometry. 
           [0023]      FIG. 11  generally illustrates asymmetrical sharpening operations upon a cutting tool to provide a final desired geometry. 
           [0024]      FIGS. 12A and 12B  illustrate additional types of cutting tools with various cutting edge features that can be sharpened using the sharpener. 
           [0025]      FIG. 13  shows relevant portions of the sharpener in accordance with another embodiment configured to sharpen other types of cutting tools. 
           [0026]      FIG. 14  shows a side elevational view of  FIG. 13 . 
           [0027]      FIG. 15  provides a flow chart for a SHARPENING OPERATION routine generally illustrative of steps carried out in accordance with preferred embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIGS. 1A and 1B  generally depict an exemplary cutting tool sharpener system  100  (“sharpener”) constructed in accordance with various embodiments of the present invention. The sharpener  100  is configured to sharpen a number of different types of cutting tools in a fast and efficient manner. 
         [0029]    The sharpener  100  includes a main drive assembly  102  with a housing  104  which encloses a drive assembly (generally denoted at  105 ). The drive assembly  105  can take any suitable configuration depending on the requirements of a given application. Preferably, the drive assembly  105  includes an electric motor which rotates at a selected rotational rate. 
         [0030]    Suitable gearing or other torque transfer mechanisms can be used to provide a final desired rotational rate. In some embodiments, the rate and/or the direction of rotation can be adjusted, either automatically or manually by the user, for different sharpening operations. User control switches are generally depicted at  106 . 
         [0031]    The sharpener  100  further generally includes a sharpening assembly  108  coupled to the drive assembly. The sharpening assembly  108  preferably includes a substantially triangularly-shaped guide housing  110  with opposing sharpening guides  112  extending therein. The guides  112  enable a particular cutting tool, such as a kitchen knife  114 , to be alternately presented to the sharpener  100  from opposing sides. 
         [0032]      FIG. 2  provides another view of the sharpener  100  of  FIGS. 1A and 1B . In  FIG. 2 , the guide housing  110  has been removed to reveal a continuous, flexible abrasive belt  116  which is routed around rollers  118 ,  120  and  122 . The roller  118  is characterized as a drive roller which is powered by the aforementioned drive assembly. The roller  120  is a fixed idler roller, and the roller  122  is a spring biased idler roller with an associated tensioner assembly  124 . 
         [0033]    The tensioner assembly  124  preferably includes a coiled spring  126  or other biasing mechanism which applies an upwardly directed tension force upon the belt, as generally depicted in  FIG. 3 . The rollers  118 ,  120  and  122  are preferably crowned to maintain centered tracking of the belt  116 , as generally represented in  FIG. 4A , although guide rollers can additionally or alternatively be used, as generally represented in  FIG. 4B . While a substantially triangular path for the belt  116  is preferred, such is not necessarily required as any number of other arrangements can be used as desired. 
         [0034]    For example, in an alternative embodiment the belt  116  is routed around just two rollers rather than the three shown in  FIG. 3 . The rollers can be the same diameter to provide a substantially oval shaped path, or a larger roller can be used in lieu of the two lower rollers shown in  FIG. 3  to maintain a substantially triangular path. More than three rollers can also be used to provide other path configurations. It will be appreciated that in each of these embodiments, the system can be characterized as aligning the belt along a first selected plane between first and second supports (e.g., such as on the left hand side of  FIG. 3 ), and aligning the belt along a second selected plane between a third support and the first support (e.g., such as on the right hand side of  FIG. 3 ). 
         [0035]    The belt  116  nominally rotates at a speed and direction around the rollers  118 ,  120 ,  122  as determined by the operation of the drive assembly. It is contemplated that a population of belts will be supplied for use with the sharpener  100 , each belt having different physical characteristics and each being easily removable from and replaceable onto the sharpener  100  in turn. 
         [0036]    By way of illustration,  FIGS. 5A and 5B  provide respective side and top views of a first belt  116 A. The belt  116 A preferably includes a layer of abrasive material  128 A affixed to a backing (substrate) layer  130 A. The abrasive layer can take any number of fonns, such but not limited to diamond particles, sandpaper material, etc., and will have a selected abrasiveness level (roughness). The backing layer  130 A can similarly be selected from a wide variety of materials, such as cloth, plastic, paper, etc. 
         [0037]    In the present example, the first belt  116 A is contemplated as having an abrasiveness level on the order of about  400  grit. It is contemplated that the relative width, thickness and roughness of the first belt  116 A will make the belt suitable for initial grinding operations upon the cutting tool in which relatively large amounts of material are removed from the tool. 
         [0038]      FIGS. 6A and 6B  show a second exemplary belt  116 B. The second belt  116 B also has an abrasive layer  128 B and a backing layer  130 B. The abrasive layer  128 B is contemplated as comprising a finer grit than that of the first belt  116 A, such as order of about  1200  grit. The exemplary second belt  116 B is contemplated as being generally more flexible than the first belt  116 A. 
         [0039]    The second belt  116 B is shown to be narrower than the first belt  116 A, to demonstrate that the sharpener  100  can be readily configured to accommodate different widths of belts. However, in preferred embodiments, all of the belts utilized by the sharpener  100  will have nominally the same width and length dimensions. Further, for reasons that will be discussed below, it is preferred that belts of coarser grit (such as the first belt  116 A) will be configured to have successively higher levels of linear stiffness, whereas belts of finer grit (such as the second belt  116 B) will be configured to have successively lower levels of linear stiffness. 
         [0040]    As used herein, the term “linear stiffness” generally relates to the ability of the belt to bend (displace) along the longitudinal length of the belt (i.e., in a direction along the path of travel) in response to a given force. Generally, a belt with a higher linear stiffness will provide a larger radius of curvature as it is deflected by an object, since the belt has a relatively lower amount of flexibility along its length. Conversely, a belt with a lower linear stiffness, due to its relatively higher level of flexibility, will provide a smaller radius of curvature as it is deflected by the same object. 
         [0041]    Accordingly, the second belt  116 B is particularly suited for subsequent grinding or honing operations upon the cutting tool in which relatively smaller amounts of material are removed from the tool. It will be appreciated that the relative dimensions represented in  FIGS. 5-6  are merely exemplary in nature and are not limiting. For example, all of the belts may be of the same general thickness with different flexibilities established by other characteristics, such as the material used to form the belts, the composition of the backing layers, etc. Also, any number of additional belts can be provided with other dimensions and levels of abrasiveness, including belts with a grit of 40 or lower, belts with a grit of 2000 or higher, etc. 
         [0042]    It is contemplated that all of the belts will have generally the same circumferential length, but this is also not necessarily required as at least some differences in belt length can be accommodated via the tensioner  124 . Indeed, as will now be explained beginning with  FIGS. 7A-7B , a number of factors including the tensioner force and the belt length, width, thickness and stiffness are preferably selected to provide specifically controlled amounts of linear and torsional deflection of the belt during sharpening. 
         [0043]      FIGS. 7A and 7B  provide schematic representations of the sharpener  100  to illustrate preferred operation of a selected belt  116  during a sharpening operation upon a cutting tool  132 .  FIG. 7A  shows the cutting tool  132  prior to engagement with the belt  116 , and  FIG. 7B  shows the cutting tool  132  during engagement with the belt  116 . 
         [0044]    For reference, the cutting tool  132  is shown in a canted orientation, and for purposes of the present example the cutting tool is characterized as a conventional kitchen knife with handle  134 , blade  136  and curvilinearly extending cutting edge  138 . 
         [0045]    As shown in  FIG. 7B , the belt  116  preferably twists out of its normally aligned plane, as indicated by torsion arrow  140 , in the vicinity of the knife  132  as the cutting edge  138  is drawn across the belt  116 . More specifically, the user preferably grasps the handle  134  and pulls the knife  132  back in a substantially linear fashion, as indicated by arrow  141 . The moving belt  116  will undergo localized torsion (twisting) to maintain a constant angle of the abrasive layer  128  against the blade  136  irrespective of the specific shape of the cutting edge  136 . In this way, a constant and consistent grinding plane can be maintained with respect to the blade material. 
         [0046]    The amount of torsional displacement of the belt along a particular cutting edge can vary widely in relation to changes in the curvilinearity of the cutting edge. A typical amount of twisting may be on the order of 30 degrees or more out of plane. In extreme cases such as when the distal tip of a blade passes across the belt, twisting of up to around 90 degrees or more out of plane may be experienced. The torsion is generally a function of the length of the extent of the belt presented to the tool in comparison to the belt width, as well as a function of the tension applied to the belt applied by the tensioner assembly  124 . Thus, it is contemplated that, generally, each of the belts respectively installed onto the sharpener  100  will undergo substantially the same amount of torsion irrespective of the abrasiveness or linear stiffness of the belt. 
         [0047]    The direction of belt twist will be influenced by the relation of the cutting edge  138  to the belt  116 . In  FIG. 8A , a first portion  142  of the cutting edge  138  at the base of the blade  136  adjacent the handle  134  is generally concave with respect to the belt  116 . This will generally induce torsion in a counter-clockwise direction, as indicated by arrow  144 , as that portion of the blade passes adjacent the belt  116 . 
         [0048]    In  FIG. 8B , a second portion  146  of the cutting edge  138  near the point of the blade  136  is generally convex with respect to the belt  116 . Passage of the second portion  146  adjacent the belt will generally induce torsion in the opposite clockwise direction, as indicated by arrow  148 . 
         [0049]    In a preferred embodiment, the retraction of the knife  132  across the belt  116  is controlled by the aforementioned sharpening guides  112  in the guide housing  108  ( FIG. 1 ). One of the guides  112  is generally depicted in  FIG. 9 . A slot is formed by facing surfaces  150 ,  152  and a base surface  154 , although other configurations can be used, including angled surfaces that form a v-shape. During the sharpening steps of  FIGS. 8A and 8B , the knife  132  is inserted into the slot above the belt  116  and moved downwardly until the base of the cutting edge  138  (portion  142  in  FIG. 8A ) comes into contacting abutment against the base surface  154  (also referred to as a cutting edge guide surface). 
         [0050]    While maintaining a small amount of downward pressure upon the handle  134 , the user slowly draws the knife  132  back (i.e., direction  141  in  FIGS. 8A-8B ) so that the cutting edge  138  remains in contact with, and slides against, the base surface  154 . Preferably, the blade  136  is also lightly pressed against the vertical guide surface  152  so as to slidingly pass in contacting engagement with the surface  152  during the sharpening operation. 
         [0051]    Although not shown in  FIG. 9 , a suitable retention feature, such as a spring clip or a magnet, can be incorporated into the guide  112  to maintain the knife  132  in contacting engagement with the surfaces  152 ,  154 . The knife  132  is preferably passed across the belt several times in succession, such as 3-5 times, to sharpen a first side of the blade  136 . The knife  132  is then preferably moved to the other guide (see  FIG. 1 ) and these steps are repeated to sharpen the other side of the blade  136 . 
         [0052]    In some embodiments, the belt continues to rotate in a common rotational direction so that the belt moves “downwardly” with respect to the cutting tool on one side and “upwardly” with respect to the cutting tool on the other side. In other embodiments, the belt rotational direction is changed so as to pass downwardly on both sides, thereby drawing material down and past the cutting edge on both sides of the blade. Such change in belt rotational direction is not required in order to achieve effective levels of “razor” sharpness of the tool, but may be nevertheless be found to be beneficial in some applications. In such case, it is contemplated that the alternative directions of belt rotation can be manually set by the user, or automatically implemented by the sharpener  100  such as, for example, from the incorporation of a pressure switch or a proximity switch in each of the guides  112  to sense the presence of the cutting tool therein. 
         [0053]      FIGS. 10A-10C  generally illustrate a preferred sharpening sequence upon a blade  160 . As will be recognized by those skilled in the art, the ability to obtain a superior sharpness for a given cutting tool will depend on a number of factors, including the type of material from which the tool is made. It has been found that certain types of processed steel, such as high grade, high carbon stainless steel, are particularly suitable to obtaining sharp and strong cutting edges. It will be appreciated, however, that the sharpener  100  can be readily adapted to provide extremely sharp cutting edges for any number of materials, including relatively lower grades of steel, high quality Damascus steel, ceramic blades, tools made of other metallic alloys or non-metallic materials, etc. 
         [0054]    As set forth by  FIGS. 10A-10C , the sharpener  100  generates a novel, convex grind surface geometry.  FIG. 10A  shows the blade  160  in conjunction with a first belt  162  which, when alternately applied to opposing sides of the blade  160 , provides continuously extending, substantially convex surfaces  164 ,  166  which converge and intersect along a cutting edge  168 . The first belt  162  is characterized as having a relatively coarse abrasive level, and relatively high linear stiffness characteristics. 
         [0055]      FIG. 10B  shows a subsequent grinding operation upon the blade  160  using a second belt  172  that forms opposing surfaces  174 ,  176  and a cutting edge  178 .  FIG. 10C  is a side view depiction of the blade  160  at the conclusion of the operation of  FIG. 10B . It will be appreciated that due to the torsional operation of the respective belts  162 ,  172 , the cross-sectional geometries represented in  FIGS. 10A-10B  are nominally consistent along the entire longitudinal length of the blade (e.g., from substantially the tip of the blade to a position adjacent the handle). 
         [0056]    The sharpening operation of  FIG. 10A  with the first belt  162  constitutes a relatively coarse, first stage grinding operation upon the blade material, and provides a relatively large radius of curvature upon the opposing sides  164 ,  166  of the blade  160 . This radius of curvature (denoted as R 1  at  169 ) is primarily established as a result of the relatively higher linear stiffness of the belt  162 . Substantially this same radius of curvature is applied along the entire extent of the blade  160 . (It will be appreciated that the length of the radius RI is relatively large with respect to the scale of  FIG. 10A , and therefore the origin of the radius does not fit on the page). 
         [0057]    While the sharpening geometry of  FIG. 10A  can produce an extremely sharp cutting edge  168 , a limitation that may be experienced with this particular sharpening geometry is the fact that the blade  160  is relatively thin for a substantial extent of the width of the blade  160 . This can result in an undesirably weak blade that will deform, dull or break relatively easily if large forces are applied to the cutting edge  168 . 
         [0058]    Accordingly, it is contemplated that at the conclusion of this first stage of the sharpening operation, the first belt  162  is preferably removed from the sharpener  100  and the second belt  172  is installed, as depicted in  FIG. 10B . The blade  160  is once again presented to the sharpener  100  and the second belt  172  applies a relatively fine (honing) grind upon the blade  160 . This results in a correspondingly smaller radius of curvature (R 2  at  179 ) upon each of the surfaces  174 ,  176  due to the reduced linear stiffness of the second belt  172 . 
         [0059]    As before, the second belt  172  undergoes torsion as the blade  160  is drawn across the belt so that the smaller radius of curvature shown in  FIG. 10B  is consistently applied along the extent of the blade  160 . As noted above, the respective belts  162 ,  172  will preferably undergo substantially the same amounts of torsion during the respective grinding operations. 
         [0060]    The smaller radius of curvature established by the more flexible second belt  172  generally localizes the honing operation to the vicinity of the end of the blade  160 . The new cutting edge  178  (and the opposing surfaces  174 ,  176 ) result from the removal of material in  FIG. 10B  over what was present at the conclusion of the operation of  FIG. 10A . 
         [0061]    The effects of this localized honing operation in the vicinity of the cutting edge  178  are depicted in  FIG. 10C . Generally, score (scratch) marks  180  may be present on the blade as a result of the relatively more aggressive abrasive of the first belt  162 . The ends of these score marks  180 , however, may be honed out of the blade in the vicinity of the final cutting edge  178  as a result of the secondary sharpening operation. 
         [0062]    An advantage of the secondary sharpening process set forth by  FIG. 10B  is that the blade  160  now has the slicing advantages provided by the first surfaces  164 ,  166  of  FIG. 10A , as well as greater blade strength due to the greater thickness in the vicinity of the cutting edge  178  resulting from the greater curvature of the second surfaces  174 ,  176 . 
         [0063]    While two belts have been discussed above, it will be appreciated that such is merely illustrative and not limiting. For example, sharpening can be accomplished using any number of belts of various abrasiveness and stiffness that are successively installed onto the sharpener  100  and utilized in turn. Conversely, sharpening operations can be effectively carried out using just a single belt of selected abrasiveness and stiffness. 
         [0064]    For example, once the blade  160  has become dulled due to moderate use, all that may be required to restore the blade  160  to the sharpness of  FIGS. 10B and 10C  would be to re-present the blade  160  for sharpening against the second belt  172 , thereby realigning the material along the cutting edge  178 . Conversely, if greater wear or damage is incurred, the sharpness of the blade  160  can be restored by application of both belts  162 ,  172  to the blade. 
         [0065]    The two belt sharpening process of  FIGS. 10A-10C  is particularly suitable for relatively harder materials such as laminated and/or high carbon steels, or other materials with a relatively high Rockwell Hardness level (such as on the order of e.g., 60 or higher). Such materials are sufficiently strong and hard to be able to transition from the relatively coarse grinding provided by the first belt  162  to the relatively fine grinding provided by the second belt  172  without undergoing deformation or other effects that would cause deviation from the displayed geometries. 
         [0066]    Indeed, subjecting such relatively hard material to just the second belt  172  would ultimately result in the cutting edge  178 , although such may require an extended period of time since the finer abrasiveness of the second belt will generally take longer to remove the requisite material from the blade to arrive at this final configuration. The use of multiple belts of varying abrasiveness is thus preferred for purposes of efficiency, but is not necessarily required. Similarly, it may be desirable to apply just the coarse grind of  FIG. 10A  for certain applications. 
         [0067]    Softer materials such as lower grade steels with relatively lower Rockwell Hardness (such as on the order of, e.g., 45-50) may benefit from the use of higher numbers of sequential grinding stages. For example, a sequence of three different belts of 400 grit, 800 grit and 1200 grit may be respectively used in turn. This would tend to reduce the transitions between different belts, thereby reducing the risk of undesirably inducing folding or other deformations of the blade material in the vicinity of the cutting edge. Indeed, any number of belts, including 5-10 different belts or more, and belts of upwards of 2000 grit or more, can be progressively used as desired, depending on the requirements of a given application. 
         [0068]    While the geometries set forth by  FIGS. 10A-10B  are symmetric, similar geometries can readily be established for asymmetric blades, such as an exemplary blade  200  shown in  FIG. 11 . The asymmetric blade  200  is typical of certain types of cutting tools such as pocket or utility knives with scallops (serrations) along a portion thereof (not separately shown), as well as some types of shears, scissors, etc. 
         [0069]    The blade  200  has a first surface  201  that extends in a substantially vertical direction, and an opposing second surface  202  that curvilinearly extends to provide a convex grind surface similar to the surface  174  in  FIG. 10B . It will be appreciated that the asymmetric blade  200  can be readily sharpened simply by applying the aforementioned sharpening sequence to just the second surface  202 . 
         [0070]      FIGS. 12A-12B  provide further examples of tools that can be readily sharpened using the aforementioned sharpening sequence.  FIG. 12A  shows a first style of utility knife  204  with a blade  205  and handle  206 . The blade  205  includes opposing, curvilinearly extending cutting edges  207  and  208 . The cutting edge  207  further includes a concave recess  209  useful, for example, in cutting fibrous materials such as a rope. The knife  204  can be sharpened by the sharpener  100  simply by applying the sequence of  FIGS. 10A-10B  while the knife  204  is in the orientation of  FIG. 12A  (to sharpen edge  207 ), flipping the knife over, and repeating (to sharpen edge  208 ). The aforementioned torsional and bending characteristics of the respective belts are readily capable of providing so-called “razor” sharpness to the entire extents of the edges  207  and  208 . 
         [0071]      FIG. 12B  shows a second type of utility knife  210  with blade  211  and handle  212 . The blade  211  has a complex geometry with a lower curvilinear edge  213 , a straight cutting edge  214 , and scallops (localized serrations)  215 . The cutting edges  213  and  214  can be readily sharpened as set forth above. In many cases scallops such as  215  can also be sharpened, albeit in a manner similar to that shown in  FIG. 11 . It will be noted, however, that the torsional stiffness and width of the belts may need to be adjusted in relation to the relative size of the scallops  215  in order to maintain substantially the same initial geometries of the scallops at the conclusion of the sharpening operation. 
         [0072]    It will be noted at this point that complex geometries such as depicted in  FIGS. 10-12  with maximum levels of sharpness can generally be obtained only to the extent that the sharpening angle (i.e., the angle between the tool and the abrasive) is maintained within close tolerances during each sharpening pass. Too much variation in the sharpening angle from one pass to the next can actually result in a cutting edge becoming duller as a result of the sharpening operation, since the variations prevent formation of the desired intersection of the respective opposing surfaces. This constitutes a major drawback with most prior art sharpeners. 
         [0073]    Even state of the art sharpeners that employ multiple stages of guides and rotating grinding wheels to provide highly controlled sharpening operations are not immune to such variability. Such sharpeners will often require the user to rotate the tool as the tool is drawn back so that the tool takes a curvilinear path to match the curvilinear extent of the cutting surface. While such sharpeners may produce high levels of sharpness, it will be immediately apparent that variations will occur to the extent that the user does not (and cannot) draw the curved blade back at the exact same angle during each pass. 
         [0074]    It will thus be seen that the sharpener  100  advantageously provides highly repeatable and controllable sharpening angles for substantially any shape cutting edge, since the sharpening angle is established and maintained by the adaptive torsion of the belt as it reacts to the differences in curvilinearity of the cutting edge. It has been found that sharpeners constructed in accordance with the exemplary sharpener  100  disclosed herein readily achieve levels of sharpness that exceed what is sometimes generally referred to in the art as “scary sharpness” (razor sharp, scalpel sharp, etc.) even for cutting tools with less-than superior metallic constructions. 
         [0075]    While the various embodiments discussed above have been configured for the sharpening of bladed cutting tools, such as knives, which can be inserted into the guides  112 , it will be appreciated that any number of different types and styles of tools can be sharpened using the sharpener  100  by removal of the guide housing  110  ( FIG. 3 ) and presentation of the tool to the respective exposed extents of the belt  116 . Accordingly, any number of other styles and types of cutting tools, such as lawn mower blades, machetes, scissors, swords, spades, rakes, etc. can be effectively sharpened by the sharpener  100  in like manner to that discussed above. 
         [0076]    An alternative embodiment for the sharpener  100  is generally depicted in  FIG. 13 , which uses an alternative drive configuration and belt path for the belt  116 . Unlike the symmetric arrangement of  FIG. 3 , the alternative arrangement of  FIG. 13  provides an asymmetric triangular path for the belt. As before, the belt passes over rollers  118 ,  120 ,  122  and is tensioned by the tensioner  124 . 
         [0077]    The arrangement of  FIG. 13  provides only a single side of the belt for sharpening, such as for a cutting tool  216  characterized as a set of pruning shears. The shears  216  include spring biased handles  218 ,  220  which, when closed, bring a blade portion  222  with cutting edge  224  into proximity with a shear portion  226 . 
         [0078]    As further shown in  FIG. 14 , the configuration of the shears is such that the cutting edge  224  lies in close relation to the intersection with the shear portion  226 , making the shears difficult to sharpen in this vicinity using conventional processes such as a grinding wheel, due to the lack of clearance. However, generally the only limiting factor with the sharpener  100  is the thickness of the belt  116 , so that substantially the entire extent of the cutting edge  224  can be sharpened without the need to disassemble the tool  216 . That is, in both the embodiments of  FIGS. 3 and 13-14 , sufficient clearance is provided behind the belt  116  to provide a bypass clearance to enable a portion of the tool to be disposed behind the belt. 
         [0079]      FIG. 15  provides a flow chart for a SHARPENING OPERATION routine  300 , generally illustrative of steps carried out in accordance with various preferred embodiments of the present invention. It will be appreciated that  FIG. 15  generally summarizes the foregoing discussion. 
         [0080]    Initially, at step  302  a first abrasive flexible belt (such as  116 A in  FIGS. 5A-5B or 162  in  FIG. 10A ) is selected and installed onto the sharpener  100 . This first abrasive belt will have a selected abrasiveness level and a selected linear stiffness as discussed above. Once installed, the first belt is driven at step  304  via the drive assembly  105  ( FIG. 1A ) in a selected direction along a selected plane between a first support and a second support (such as between the rollers  122  and  118  in  FIG. 3 ). 
         [0081]    At step  306 , a cutting tool (such as  114 ,  132 ,  204 ,  210 ,  216 , etc.) is presented in contacting engagement against the abrasive surface of the belt. This induces torsion of the belt out of the selected plane to conform to the cutting edge of the cutting tool (as generally depicted in  FIGS. 7-8 ) and/or bending of the belt out of the selected plane at a radius of curvature determined in relation to said linear stiffness to shape a side surface of the cutting tool with said radius of curvature (as generally depicted in  FIGS. 10A-10C ). 
         [0082]    At this point it will be noted that while preferred embodiments configure the belt to both deflect in a torsional mode to follow changes in the contour of the cutting edge and to deflect in a bending mode to provide a desired radius of curvature to the formed cutting edge, both deflection modes are not necessarily required. That is, while both modes are preferably utilized together, each has separate utility and can be implemented without the other. For example and not by way of limitation, a given tool may be rotated as the tool is drawn back across the belt, thereby removing the advantageous torsional operation of the belt upon the cutting edge. Indeed, the sharpener could be readily configured to support the belt and prevent such torsion, as desired. Accordingly, the flow of  FIG. 15  shows that torsion and/or bend modes of deflection are induced during presentation of the tool. 
         [0083]    Preferably, the sharpening operation is applied to opposing sides of the tool, such as depicted in  FIGS. 10A-10C , so  FIG. 15  applies the foregoing step to the other side of the tool at step  308 . The operations at steps  306  and  308  can be carried out via the sharpening guides  112 , or can be carried out on the belt  116  with the guide housing removed, as depicted in  FIGS. 2 and 13-14 . 
         [0084]    A determination is made at decision step  310  as to whether additional sharpening operations are desired; if so, a new belt is installed onto the sharpener at step  312  and steps  304  through  310  are repeated using the new belt. Preferably, the new belt has a finer abrasiveness level (e.g., 1200 grit v. 400 grit, etc.) and less linear stiffness than then first belt. This sequence will generally result in the generation of a new cutting edge along the cutting tool, as depicted in  FIGS. 10B-10C . Once all of the desired sharpening stages have been completed, the routine ends as shown at step  314 . 
         [0085]    While step  312  sets forth the removal of an existing belt and the installation of a new replacement belt onto the sharpener  100 , it will be appreciated that such is not necessarily limiting to the scope of the claimed subject matter. Rather, the sharpener  100  can be readily adapted to concurrently operate multiple belts so that the tool is merely moved from one belt to another during the above sequence. 
         [0086]    Any number of sharpener configurations can be employed as desired. As noted previously, the respective bending and twisting modes are dependent on a number of factors relating to the configuration, speed and tension force upon a given abrasive belt. 
         [0087]    For purposes of reference, it has been found in preferred embodiments to utilize relatively narrow abrasive belts with lengths on the order of about 12 inches to 18 inches and widths of about 0.5 inches. The distance (journal length) between adjacent supports (e.g., such as the distance along the belt from rollers  118 ,  122  in  FIG. 3 ) can preferably vary from as low as around 2 inches to up to about 6 inches or more. The linear speed of the belt can also vary, with a preferred range being from about 1,500 feet/minute (ft/min) to about 5,000 ft/min. A preferred tension force supplied to the belt (such as via the tensioner spring  126 ) is on the order of around 4 pounds (lbs), with a preferred range of from about 0.5 lbs to upwards of about 10 lbs. It will be appreciated that the foregoing values and ranges merely serve to illustrate preferred embodiments and are not limiting. 
         [0088]    It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.