Patent Publication Number: US-9902035-B2

Title: Compact grinding wheel

Description:
SUMMARY 
     A skate blade sharpening system is a specialized type of grinder specifically configured to sharpen ice skates. Conventional skate blade sharpening systems can have certain undesirable limitations. One dimension of limitation is related to size. Conventional systems may be relatively large and employ large components, such as grinding wheels and motors, with attendant high power consumption and cost. There are related performance issues, including sharpening results that are degraded by imbalance and other mechanical imperfections. Any irregularity in the trueness of the grinding wheel creates undesirable defects in the skate blade surface which will result in decreased skating efficiency. These effects are relatively large in conventional systems employing larger and heavier grinding wheels, for example, so it is difficult to attain a desired sharpness and smoothness of the skate blade. While systems are known that employ relatively smaller grinding wheels, the wheels have been high-precision solid metal wheels with associated high cost. 
     Apparatus is disclosed that can address the above and other limitations of existing sharpening systems, specifically limitations related to size and construction of the grinding wheel. 
     In particular, a grinding wheel for use in a skate blade sharpening system is disclosed that includes a metal grinding ring having an abrasive-coated outer surface for contacting a skate blade to be sharpened during sharpening, and a generally disk-shaped hub to which the grinding ring is fixedly mounted. The hub and grinding ring are configured for mating with an arbor on a rotating shaft of the skate blade sharpening system. Generally the hub is of a less expensive and lower-density material than the grinding ring. In one embodiment, the hub is of a lightweight material such as thermoplastic. 
     The use of separate grinding ring and hub enables the grinding wheel to have lighter weight than a conventional one-piece wheel of similar size, reducing undesirable effects of mechanical imperfections and attaining more precise sharpening results. The grinding wheel can also be manufactured at lower cost, and as a lower-cost item it can be replaced more often so as to maintain high sharpening performance without a severe cost penalty. 
     In one embodiment the grinding ring mates to an arbor on a rotating shaft of the sharpening system, specifically by contact between an inner surface of the grinding ring and a groove or shoulder of the arbor. Concentricity and thus balance are improved by this arrangement that utilizes the shoulder feature which can be created in the same manufacturing step as a central opening in the arbor defining its axis of rotation. 
     One challenge to the use of a grinding ring, especially when the hub is of a thermally resistive material such as thermoplastic, is its more limited heat sinking ability compared to conventional single-piece disk grinding wheels. The grinding ring has less ability to dissipate heat generated during the grinding process, and thus could be susceptible to undesirable operation temperatures. In one embodiment the grinding wheel mounts to the arbor in a way that the metal arbor contacts a metal edge of the grinding ring very close to the grinding surface, promoting efficient heat transfer to the arbor. Additionally, the arbor may be configured with vane features so as to generate air flow around the arbor and grinding ring during operation, providing greater convective cooling for heat dissipation and maintaining desired operating temperature of the grinding ring. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. 
         FIG. 1  is a perspective view of a skate sharpening system; 
         FIG. 2  is a schematic depiction of a grinding wheel contacting a skate blade during sharpening; 
         FIG. 3  is a perspective view of a metal frame and chassis of a sharpening system; 
         FIG. 4  is a perspective view of an interior of a sharpening system including a carriage assembly; 
         FIG. 5  is a perspective view of a skate blade clamp; 
         FIG. 6  is a block diagram of an electrical subsystem of a skate sharpening system; 
         FIGS. 7 and 8  are front elevation views of a sharpening system; 
         FIG. 9  is an exploded perspective view of a grinding wheel; 
         FIG. 10  is a perspective view of an interior of a sharpening system including a carriage assembly; 
         FIG. 11  is a rear view of a rear part of a radio frequency identification (RFID) antenna housing in a sharpening system; 
         FIG. 12  is a perspective view of an arbor; 
         FIG. 13  is a front elevation view of a carriage assembly; 
         FIG. 14  is a side elevation view of a carriage assembly; 
         FIG. 15  is a flow diagram of operation of a sharpening system. 
         FIG. 16  is a section view of the platform area of the chassis; 
         FIGS. 17 and 18  are plan views of clamping jaws; 
         FIGS. 19, 20 and 21  are section views of clamping jaws and guide blocks; 
         FIG. 22  is a bottom view of a slot cover; 
         FIG. 23  is a section view of one end of a carriage assembly; 
         FIG. 24  is a close-up view of a portion of  FIG. 23 ; 
         FIG. 25  is a schematic depiction of alignment between clamping jaws and a grinding wheel; 
         FIG. 26  is a side elevation view of an alignment tool; 
         FIG. 27  is a plan view of an alignment tool; and 
         FIG. 28  is a flow diagram of an alignment process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a perspective view of a skate sharpener  10  used to sharpen the blades of ice skates. It has a box-like housing with structural elements including a rigid frame  12  (bottom visible in  FIG. 1 ) and a rigid chassis  14 . Attached components include end caps  16  and a rear cover  18 . The chassis  14  includes a front platform portion  22 , also referred to as “platform”  22  herein. The platform  22  includes an elongated slot  24  for receiving the blade of an ice skate for sharpening, and the blade is retained by clamp jaws (not shown) on the underside of the platform  22  which are actuated by a mechanism including a clamp paddle  26 . Disposed on the platform  22  are slot covers or “scoops”  28  at respective ends of the slot  24 , each including a respective bumper  29  serving to sense contact with a skate blade holder. An outward-opening door  30  having a glass panel  31  and lower hinge portion  33  extends across a front opening. A user interface display panel  34  is disposed at top right on the chassis  14 . The skate sharpener  10  also includes a control module or controller, which is not visible in  FIG. 1  and may be located, for example, inside of the rear cover  18 . Further mechanical and electrical details are provided below. 
       FIG. 1  also shows a coordinate system  35  for references to spatial directions herein. The X direction is left-to-right, the Y direction front-to-back, and the Z direction bottom-to-top with respect to the skate sharpener  10  in the upright, front-facing orientation of  FIG. 1 . This coordinate system also defines an X-Y plane (horizontal), X-Z plane (vertical and left-to-right), and Y-Z plane (vertical and front-to-back). Using this coordinate system  35 , the slot  24  extends in the X direction and the skate blade is clamped in an X-Z plane during sharpening as described more below. 
       FIG. 2  depicts how a skate blade is sharpened. This is a schematic edge-on view of a lower portion of a skate blade  40  in contact with an outer edge of a grinding wheel  36 . With reference to the coordinate system  35 , this is a view in the X direction. As shown, the grinding wheel  36  has a convex rounded grinding edge  42 . In practice the grinding edge  42  may be generally hemispherical. The grinding wheel  36  rotates in the plane of the blade  40  (X-Z plane, into the paper in  FIG. 2 ), thereby imparting a corresponding concave rounded shape to a lower face  44  of the skate blade  40 . Two acute edges  46  are formed at the intersection of the curved lower face  44  and the respective sides  48  of the blade  40 . As material is removed, a clean and precise arcuate shape is restored to the lower face  44 , including sharper edges  46 . In practice, the radius of curvature of the lower face  44  is in the general range of ⅜″ to 1″, with one generally preferred radius being ½″. 
     It will be appreciated that the disclosed methods and apparatus may be used with other blade profiles, including flat and V-shaped, for example. 
     Returning to  FIG. 1 , basic operation with a complete skate is as follows. First a user may need to install a grinding wheel onto an internal carriage (not shown) accessible via the front opening. For this the user opens the door  30 , rotating it forward and downward about the horizontal hinge  33 , and then closes the door after successfully installing the grinding wheel. The nature of the installation will be apparent from the more detailed description below. The user then clamps the blade of the skate in the slot  24  and slides the scoops  28  inwardly until the bumpers  29  are engaged by the blade holder part of the skate. Each bumper  29  actuates a limit switch within the respective scoop  28 , so that the engagement is sensed by the controller to enable sharpening to proceed. The user then interacts with a user interface presented on the display panel  34  to initiate a sharpening operation. Subject to certain conditions as described more below, control circuitry of the control unit automatically operates both a grinding motor to spin a grinding wheel and a separate carriage motor (both described below) to move the rotating grinding wheel back and forth along the lower face of the skate blade a desired number of times. Each traversal of the grinding wheel across the length of the blade is referred to as a “pass”. In each cycle of two passes (one to the left and the other to the right), the grinding wheel is moved to a far-right position at one end of the skate blade to permit a communications exchange between circuitry on the wheel and the control unit. This communication and related control are described below. Upon completion of a desired number of passes, the control unit stops both the rotation and back-and-forth motion of the wheel  36 , and the user unclamps and removes the skate blade from the sharpener  10 . 
     It is noted that controls and locations could be reversed in alternative embodiments, so that the communications position would be a far-left position rather than a far-right position. 
     The above operation may also be used with bare removable skate blades of the type known in the art. In this case a blade holder or other mechanical aid of some type may be used to enable a user to position the bare blade in the slot  24  for clamping and to engage the bumpers  29  of the scoops  28  to permit operation. Alternatively, a bare blade could also be positioned without a blade holder. As described more below, a blade holder may engage limit switches on the slot covers  28  to enable sharpening operation, and enables a user to insert a loose skate blade in clamping jaws. 
       FIG. 3  is a view of the frame  12  and chassis  14 . In one embodiment, the frame  12  is made of a single piece of sheet metal, folded to form a bottom  50 , sides  52  and back  54 . The chassis  14  serves as a top for the sharpener  10  and provides support for key components including a skate clamp and a carriage assembly, both described below. The chassis  14  is a rigid component made of metal or other suitably strong material. In one embodiment, the chassis  14  is made of aluminum and formed by extrusion, which can provide very accurate dimensions and geometry in a highly repeatable manner. The chassis  14  may be made of other materials and by other methods, including machining for example, in alternative embodiments. 
     As shown, the chassis  14  has an S-like cross section defining the frontward platform  22  and a rearward shelf portion (“shelf”)  56  separated by a sloping wall  58 . The underside of the shelf  56  includes two rails  60  on which a carriage (not shown) moves, as well as a downward-projecting flange  62 . As described more below, a toothed “gear rack” that forms part of a rack-and-pinion mechanism for moving the carriage is attached to the flange  62 . On the platform  22  at each end of the slot  24  are rounded projections  64  on which the scoops  28  are slidably mounted. The projections  64 , also referred to as “arches”  64  below, have retention grooves  66  that engage with corresponding features in the scoops  28  to retain the scoops  28  on the projections  64  while permitting them to slide left and right. 
       FIG. 4  shows the sharpener  10  with several external components removed. The 4-sided sheet metal frame  12  is fully visible. A carriage assembly  70  includes a carriage  72  mounted on the two rails  60 , which are shown as separated from the rest of the chassis  14  in this view. The carriage assembly  70  includes a pivoting motor arm  78  to which a grinding wheel motor  80  is mounted. The grinding wheel  36  is mechanically coupled to the rotating shaft of the motor  80  by an elongated spindle  82 . The motor arm  78  has limited rotational travel about a horizontal pivot axis  83 , so that the grinding wheel  36  can move in a vertical direction to follow the profile of a skate blade when the sharpener  10  is in operation. In the illustrated embodiment, the motor arm  78  is biased toward an upper vertical limit by a spring  84  connected between the motor arm  78  and an upper portion of the carriage  72 . 
     One important feature of the presently disclosed skate sharpener  10  is use of a compact (small-diameter) grinding wheel  36 . Specifically, its diameter is less than the diameter of the grinding wheel motor  80  by which it is rotated. Use of a compact grinding wheel  36  can provide certain advantages including greater precision in operation and lower cost. 
     Also shown in schematic fashion in  FIG. 4  is a wire harness  86  providing electrical connections between the grinding wheel motor  80  and the above-mentioned controller as well as between the controller and a carriage motor mounted within the carriage  72  (not visible in  FIG. 4 ). In  FIG. 4  the wire harness  86  is shown separate from the rest of the unit for ease of illustration, but it is actually located inside the unit along the rear wall  54 . It preferably is self-supporting along its length in a manner that maintains its vertical position while permitting back-and-forth movement of the connectors attached to the carriage assembly  70 . An example of a suitable support element is a ribbon-like material of the type used in printers and other machines with translating components. This material can flex about a transverse axis while being stiff about a longitudinal axis, and thus can maintain horizontal straightness while also flexing in a desired curling manner about a vertical axis that follows movement of the carriage assembly  70 . 
     In operation, the grinding wheel  36  is rotated by the grinding wheel motor  80  via the spindle  82 , and the carriage assembly  70  is moved back and forth along the rails  60  by action of a rack-and-pinion mechanism that includes a motor-drive pinion gear  87  engaging a toothed rack on the underside of the chassis  14  (described more below). The pinion gear  87  is driven by a carriage motor mounted within the carriage  72 , not visible in  FIG. 4 . Each unidirectional pass of the grinding wheel  36  begins with the grinding wheel  36  located off one end of the skate blade and at the upper vertical limit position by action of the spring  84 . As the carriage assembly  70  is moved toward the opposite end of the sharpener  10 , the grinding wheel  36  encounters an end of the skate blade and is deflected downward to follow the profile of the skate blade across its length. At the end of the pass, the wheel  36  rides off the other end of the skate blade and returns to the vertical limit position by action of the spring  84 . 
       FIG. 5  shows the underside of the chassis  14 . It includes a skate blade clamping mechanism whose major components are a pair of clamp jaws  90 , specifically a front jaw  90 -F and a rear jaw  90 -R; a pull rod fork  92 ; a clamp cylinder  94 ; and a cam  96  at the underside of the clamp paddle  26  that rotates therewith. The clamp cylinder  94  is retained by a bracket  98 . Also shown is a jaw guard  100 . The clamp cylinder  94  has a pull rod  102  connected to the pull rod fork  92  and an internal spring-piston arrangement that actuates the pull rod  102  and thus the jaws  90  via the pull rod fork  92 . 
     As shown, the jaws  90  each include angled slots  104 , and in the slots  104  are arranged rectangular guide blocks  106  that retain the jaws  90  at the underside of the platform  22  with spacing to permit the jaws  90  to slide in the long direction of the slots  104 . The front jaw  90 -F is retained by one guide block  107  in a center slot  104 , while the rear jaw  90 -R is retained by respective guide blocks  106  in outer two slots  104 . This arrangement permits the front jaw  90 -F to rotate very slightly about a Z-direction axis extending through the single guide block  106 , while the rear jaw  90 -F is rotationally fixed. Additional details are provided below. 
     When the clamp paddle  26  is in the position shown in both  FIG. 5  and  FIG. 1 , i.e., extending horizontally away from the platform  22 , the cam  96  does not engage the internal piston of the clamp cylinder  94 , and the action of the internal spring is to retract the pull rod  102  (toward the left in  FIG. 5 ) so that the jaws  90  are brought toward each other by action of the angled slots  104  and guide blocks  106 ,  107 . This is a referred to as a “closed” position, in which the jaws  90  are either just touching each other or are only slightly spaced apart, less than the width of the thinnest skate blade to be sharpened. Because this position is created by the spring alone, it is referred to as a “biased closed” position. 
     When a skate blade is to be clamped for sharpening, a user rotates the clamp paddle  26  to open the jaws  90 . Referring to  FIG. 1 , the user pushes downward on the outer part of the clamp paddle  26 . In  FIG. 5 , the clamp handle  26  rotates out of the page, rotating the cam  96  accordingly and causing it to push against the piston within the clamp cylinder  94 . This force works against the spring bias to extend the pull rod  102  and push on the jaws  90 , causing them to move away from each other by action of the angled slots  104  and guide blocks  106 ,  107 . The space between the jaws in the open position is wider than the widest skate blade to be sharpened. The cam  96  and head of the piston may be co-configured to establish a detent with the jaws in the fully open position. The skate blade is then inserted through the slot  24  between the jaws  90 , and the user then rotates the clamp paddle  26  upwardly ( FIG. 1 ) to close the jaws  90  on the skate blade. It will be appreciated that the front jaw  90 -F automatically rotates as necessary to close snugly against the skate blade with balanced force across the length of the jaws  90 . In the absence of this rotating feature, any imperfection in alignment of the jaws  90  could create undesirable binding and/or rotational skewing of the skate blade, adversely affecting sharpening operation. 
     The jaw guard  100  protects against the possibility of contact between the grinding wheel  36  and the jaws  90 . If the sharpener  10  were to somehow be operated without a skate blade present, then without the jaw guard  100  the wheel  36  would move across the jaws  90  at its upper vertical limit position, potentially damaging the grinding wheel  36  and/or the jaws  90 . This is prevented by the jaw guard  100 , which would be encountered by the spindle  82  ( FIG. 4 ) and keep the grinding wheel  36  in a more downward position safely away from the jaws  90 . 
     Also shown in  FIG. 5  is the above-mentioned rack  120  that is part of the rack-and-pinion mechanism for moving the carriage  70 , as mentioned above. In the illustrated embodiment it is an elongated member, of a material such as metal or plastic, attached to the flange  62 . In an alternative embodiment, the rack  120  could be formed by machining or otherwise forming a toothed pattern in the flange  62  or similar feature of the chassis  14 . In yet other alternative embodiments, a different type of mechanism such as a belt drive might be used to move the carriage  70 . 
       FIG. 6  is an electrical block diagram of the skate sharpener  10 . A control unit  32  includes a processor  130  and one or more controllers  132 . The controllers  132  provide lower-level control of corresponding elements, such as the grinding wheel motor  80 , a carriage motor  134 , and a fan  136 . Also shown are the user interface (UI) display panel  34  and RFID interface circuitry  137  in radio communications with an identification tag  204  of the grinding wheel  36  (described more below). Both the controllers  132  and processor  130  are computerized devices including memory, I/O interface circuitry and instruction processing circuitry for executing computer program instructions stored in the memory. The controllers  132  may be specialized for low-level real-time control tasks such as achieving and maintaining a commanded rotational speed for a motor. The processor  130  may have a more generalized architecture and potentially richer set of programming resources to perform a variety of higher-level tasks, including interfacing to a user via the UI display panel  34 . The processor  130  executing instructions of a particular computer program may be viewed as circuitry for performing functions defined by the program. For example, the processor executing instructions of a sharpening operation controller may be referred to as sharpening control circuitry, and the processor executing instructions related to usage control may be referred to as usage control circuitry. As mentioned above with reference to  FIG. 1 , the controller  32  may be located within the rear cover  18 . 
       FIGS. 7 and 8  are front views illustrating the above operation. A skate  140  is present and its blade  142  is clamped into a sharpening position in which the lower portion of the blade  142  extends downward through the slot  24  ( FIG. 1 ) into the interior of the sharpener  10 . In  FIG. 7  the carriage assembly  70  is located at far left, and the grinding wheel  36  is at an upper vertical limit position just off the left (leading) edge of the skate blade  142 .  FIG. 8  shows the carriage assembly  70  and grinding wheel  36  in the middle of a pass. It can be seen that the grinding wheel  36  has moved downward as it has followed the profile of the blade  142 . As mentioned, this left-to-right pass ends with the grinding wheel  36  at the far right, off the right (trailing) edge of the blade  142 . Generally multiple passes are used in a sharpening operation for a given blade  142 , with the number of passes being determined by the amount of material removal that is necessary to achieve desired sharpness. The sharpener may use both left-to-right and right-to-left passes in sequence, i.e., the grinding wheel  36  travels back and forth in contact with the blade  142  in both directions. Assuming a single home position at one end, in practice each sharpening operation may have a number of two-pass cycles, each including a pass in one direction and a pass in the opposite direction. In alternative embodiments sharpening may occur in only one direction, i.e., the grinding wheel  36  is in contact with the skate blade  142  only for passes in one direction, which alternate with non-sharpening return passes in the other direction. 
       FIG. 9  shows details of the grinding wheel  36  in one embodiment. It is a multi-piece removable assembly that includes a metal grinding ring  200  disposed on a rigid hub  202 , such as by a press fit. The hub  202  has a shallow front-facing cavity  203  which receives an identification tag  204  and a tag capture disk  206 . The identification tag  204  (and an optional graphic label not shown in  FIG. 9 ) is covered by the capture disk  206 , which has a snap-fit to the hub  202 . The identification tag  204  may be adhered to the hub  202 . Once the capture disk  206  is snapped onto the hub  202 , disassembly is very difficult. In one embodiment the hub  202  and disk  206  are formed of thermoplastic or similar hard non-metallic material, and may be substantially transparent. The grinding wheel  36  is mounted to an axle  208  of the spindle  82  by a retention nut (not shown) that urges the grinding wheel  36  against a metal arbor  212  that forms part of the spindle  82 . 
     The grinding ring  200  has an abrasive outer surface for removing material from a steel skate blade during operation. In one embodiment the abrasive surface may include a diamond or cubic boron nitride (CBN) coating, deposited by electroplating for example. The grinding ring  200  is preferably of steel or similar rigid, strong metal, and it may be fabricated from steel tubing or bar stock. Although in general the grinding ring  200  may be of any size, it is preferably less than about 100 mm in diameter and even more preferably less than about 50 mm in diameter. Its thickness (radially) is substantially less than its radius, e.g., by a ratio of 1:4 or smaller. The ring shape, as opposed to a disk shape as used in more conventional grinding wheel designs, produces a much lighter grinding wheel  36  which can reduce the effects of wheel imbalance, eccentricity, and non-planarity. Reducing such effects can contribute to a smoother finish on a skate blade and a higher performance skate sharpening. 
     As shown, both the arbor  212  and hub  202  have shaped outer edges which mate with respective edges of the grinding ring  200 . The mating between the arbor  212  and ring  200  is a sliding contact mating that permits mounting and dismounting of the grinding wheel  36  while also providing for heat transfer between the grinding ring  200  and the arbor  212 . This relatively tight fit is also responsible for the centering of the grinding wheel. The heat transfer helps dissipate frictional heat generated in the grinding ring  200  as it rotates against a skate blade in operation. Specifically this mating is between a portion of an inner annular surface of the grinding ring  200  and an annular outer rim of the arbor  212 . Both the hub  202  and arbor  212  have notches or shoulders on which respective portions of the grinding ring  200  rest. Thus the shoulder portion of the hub  202  extends only partway into the grinding ring  200 , so that a remaining part of the grinding ring  200  extends beyond the arbor-facing end of the hub  202  and mates with the shoulder portion of the arbor  212 . 
     The arbor  212  may include vanes or other features to increase its surface area and/or enhance air flow for a desired cooling effect, further promoting heat dissipation and helping to maintain a desired operating temperature of the grinding ring  200  in operation. 
     One important feature of the grinding ring  200  is its relatively small size, as compared to conventional grinding wheels which may be several inches in diameter for example. Both the small size of the ring (outer diameter) as well as its ring geometry (in contrast to disk geometry of conventional grinding wheels) contribute to advantages as well as challenges. Advantages include low cost and ease of manufacture, so that it can be easily and inexpensively replaced to maintain high-quality sharpening operation. The size and geometry also reduce any contribution of the grinding ring  200  to imbalance and related mechanical imperfections of operation. Balance and related operational characteristics are more heavily influenced by the arbor  212 , which is preferably precision-formed and precision-mounted. One challenge of the geometry and size of the grinding ring  200  is heat removal, and this is addressed in part by the heat-conducting mating with the arbor  212  and heat-dissipating features of the arbor  212 . 
     The identification tag  204  has a unique identifier such as a manufacturer&#39;s serial number, and when packaged with a grinding wheel  36  into an assembly serves to uniquely identify that assembly including the constituent grinding wheel  36 . The identification tag  204  also includes memory capable of persistently storing data items, used for any of a variety of functions such as described further below. The identification tag preferably employs a security mechanism to protect itself against tampering and improper use, including improper manipulation of the contents of the memory. Memory protected in such a manner may be referred to as “secure memory”. The serial number should be a read-only value, while the memory is preferably both readable and writeable. As described below, a separate transceiver in the system  10  is capable of exchanging communication signals with the tag  204  for reading and writing data. In one embodiment, so-called “RFID” or radio frequency identification techniques may be employed. Using RFID, the identification tag  204  is read from and written to using radio-frequency electromagnetic waves by an RFID transceiver contained in the sharpening system  10  (described more below). Other types of implementations are possible, including optically interrogated tags and contact-based tags such as an iButton® device. 
     For security, the identification tag  204  may use an access code that is read by the control unit  32  and validated. The access code can be generated by a cryptographic hash function or other encryption algorithm that takes as input the serial number of the identification tag  204  and a confidential hash key. Using the serial number ensures that the access code created is uniquely paired with a specific identification tag  204 . This uniqueness can help prevent misuse that is attempted by copying an access code from one identification tag  204  to another. When the serial number of the other identification tag  204  is encrypted, the result will not match the copied access code, and appropriate action can be taken such as preventing use of the grinding wheel  36  that contains the apparently fraudulent identification tag  204 . 
       FIG. 10  shows the sharpener  10  having the carriage  70  located in a “home” position at the far right of the sharpener  10 . The right end wall  52  is cut away in this view in order to show pertinent detail. Attached to the right wall  52  is a housing  220  in which an electronic sensor module  222  is mounted. The sensor module  222  is connected by cabling (not shown) to the controller  32  ( FIG. 6 ). In this position the grinding wheel  36  is adjacent to an inner side of the housing  220  and vertically centered on the housing  220  by action of a shoulder member  224  of the housing  220 . Additional details of this arrangement are described below. 
     As mentioned above, the wheel  36  includes an identification tag  204  on which various data may be stored for a variety of purposes. In the illustrated embodiment this tag employs a wireless communication technique such as Radio Frequency Identification (RFID) communications. The sensor module  222  includes an RFID antenna (not shown) which becomes registered or aligned with the identification tag  204  when the grinding wheel  36  is in the illustrated home position, so that the tag  204  may be read from and written to using RFID communications. Generally the RFID antenna has one or more loops of conductive material such as wire or metal etch, with the loops having a circular or other shape (e.g., rectangular). The RFID communications may operate on any of a number of frequencies. Frequencies in common use include 133 kHz (Low Frequency or LF), 13.56 MHz (High Frequency or HF), and 900 MHz (Ultra High Frequency or UHF). 
     In the illustrated embodiment the identification tag  204  is within the circumference of the circular RFID antenna of the sensor module  222 , e.g., concentric with the antenna, during the reading and writing of data from/to the tag  204  as part of operation. By this arrangement the identification tag  204  can be read from and written to even when the grinding wheel  36  is rotating at full speed, which may be between 5000 and 25000 RPM. Reading and writing at full rotational speed has a distinct advantage of allowing the sharpener  10  to sharpen more quickly, because it is not necessary to slow/stop wheel rotation and then bring rotation back up to speed for each read/write operation. As described more below, in one embodiment reading and writing occurs once during each 2-pass cycle, so the time savings is proportional to the number of cycles in a sharpening operation. Additionally, reading and writing at full rotational speed can discourage any tampering with the grinding wheel  36 , because it is always moving during the sharpening process. In some embodiments it may be advantageous to maintain rotation but at a reduced rotational speed to improve the read/write communications with the tag  204 . 
       FIG. 11  is a view from inside the sharpener  10  toward the front, showing the inside-facing part of the housing  220  and other details. As shown, the shoulder member  224  has a sloped edge  226  and horizontal edge  228 . When the grinding wheel  36  is returning to the home position, moving right-to-left in  FIG. 11 , it initially is at its vertical limit position as indicated in phantom. The spindle  62  encounters the sloped edge  226  and follows it downward, then rides along the horizontal edge  228 . This motion of the spindle  62  brings the wheel  36  into a desired vertical position with respect to the antenna within the housing  220 , e.g., aligning the center of the wheel  36  with the center of the antenna. This alignment generally maximizes the RF coupling between the antenna and the tag  204 , resulting in robust and accurate transfer of RF signals therebetween. 
       FIG. 12  shows the rear face of the arbor  212 . It is a unitary component including a set of rearward-facing projections or “vanes”  230 , each extending generally radially with slight curvature as shown. With this configuration the arbor  212  creates airflow in the vicinity of the arbor  212  and grinding ring  200 , increasing convective heat dissipation from these components over an alternative lacking this feature. It will be appreciated that any of a variety of specific vane configurations may be employed, including non-curved vanes. 
       FIG. 13  shows the front of the carriage assembly  70 . The motor arm  78  is an oblong member mounted for rotation on a spindle axle  240  at the left side of the carriage  70 . A Y-adjustment knob  242  is mounted on a separate Y-adjustment axle below the spindle axle  240 . A height adjustment mechanism includes a rotating adjustment member  244  and a bracket  246  extending downward from the motor arm  78  and having a limit peg  248 . The adjustment member  244  includes a user handle  250  and a pointer feature  252  having a terminus at an array of numbers arranged on the carriage  70 . Its lower edge is scalloped by a series of faces having successively increasing distances from the center of rotation (proceeding clockwise along the edge). 
     As the adjustment member  244  is turned, it presents different faces of the scalloped lower edge at a rest position of the limit peg  248 . When the grinding wheel  36  is clear of the skate blade and the motor arm  78  rotates upward under the action of the spring  84 , the upward travel is limited by the limit peg  248  encountering a face of the lower edge of the adjustment member  244 . The different faces of the adjustment member  244  are at different radii from the center of rotation of the adjustment member  244 , thereby establishing different vertical locations for this rest position of the limit peg  248 . 
     In operation, a user rotates the adjustment member  244  to set a maximum vertical position of the grinding wheel  36 . The purpose of this adjustment is to set a vertical travel limit of the grinding wheel  36  when it comes off the edge of the skate blade. This feature helps tailor operation depending on the type of skate being sharpened. Regular ice hockey skates have rounded upturns at each end of the skate blade (e.g. toe or heel), and it is desired that the grinding wheel  36  move upward to follow the upturns. This can be accomplished by having a high maximum vertical position. The blades on so-called “goalie skates” are flatter and it is typically desired that the grinding wheel  36  not move as far upward as it leaves the end of the blade, but rather come off relatively straight. This can be accomplished by adjusting the height limit using the adjustment member  244  to set a lower maximum vertical position. 
     In  FIG. 13 , the grinding wheel  36  is shown in a downward position such as it occupies when riding along a skate blade, so the limit peg  248  is well away from the adjustment member  244 . It will be appreciated that upward rotation of the motor arm  78 , such as occurs when the grinding wheel  36  moves away from the skate blade, rotates the bracket  246  upward so that the limit peg  248  encounters the lower edge of the adjustment member  244 . 
       FIG. 14  is a view from the left side of the sharpener  10 , with the near end wall  52  partially cut away. This view illustrates several features related in some manner to the compactness of the grinding wheel  36 , i.e., its smaller diameter relative to that of the grinding wheel motor  80  ( FIG. 4 ). When conventional larger grinding wheels are used, there is inherently greater vertical space within which other mechanical components may be mounted, such as the grinding wheel motor, clamping jaws for the skate blade, etc. Using the compact grinding wheel  36  enables a corresponding compactness in the overall skate sharpener  10 , which is generally advantageous but also requires that more attention be paid to the design and organization of other mechanical features. 
     One feature visible in  FIG. 14  is the height difference between the rear shelf  56  and the lower front platform  22  of the chassis  14 . The relative height of the shelf  56  provides clearance for the carriage assembly  72  and the components it carries, including the grinding wheel motor  80  with its vertical movement on the motor arm  78  (see  FIG. 4 ). The lower platform  22  is closer to the grinding wheel  36 . The jaws  90  are located below the platform portion  22 , even closer to the grinding wheel  36  to permit the skate blade to be retained at a sufficiently low position that it can be contacted by the grinding wheel  36  in operation. The above-described protective function of the jaw guard  100  can also be appreciated in this view—the spacing between this component and the spindle  82  is smaller than the spacing between the grinding wheel  36  and the jaws  90 . 
     Another pertinent feature relates to a Y-adjustment mechanism permitting fine adjustment of the position of the grinding wheel  36  to align it with a retained skate blade in the X-Z plane (which is perpendicular to the page of  FIG. 14 ). The grinding wheel  36  is mechanically coupled to the carriage  70  by a series of components including the spindle  82 , motor  80 , and motor arm  78 , which is mounted to a spindle  250  having a spindle axle  240  mechanically fixed to the carriage  70 . The spindle  250  includes an interior mechanism causing fine translational movement (horizontally in  FIG. 14 ) in response to rotation of a spindle gear  252 . In some embodiments, the spindle  250  is located above a nominal position of the grinding wheel  36 , creating a desired arc of movement of the motor arm  78  and direction of force between the grinding wheel  36  and the skate blade. In order to actuate the Y-adjust mechanism of the spindle  250 , an adjustment axle  254  on which the adjustment knob  242  is mounted is located below the spindle  250  and has a gear  256  engaging the spindle gear  252 . This lower position enables a user to reach into the unit (from the front opening which is to the right in  FIG. 14 ) and rotate the adjustment knob  242  with their fingers, clearing the underside of the front platform portion  22  of the chassis  14 . 
       FIG. 14  also shows the above-mentioned carriage motor  260  that drives the pinion gear  87  in engagement with the rack  120 . 
     Use of Identification Tag  204   
     The grinding wheel  36  utilizes the identification tag  204  to carry important information and provide it to the control unit  32  of the sharpener  10 . The information carried by the tag  204  can be used to improve sharpening operation and reduce costs associated with the skate sharpener  10 . 
     Accurate and repeatable skate sharpening is obtained when the grinding wheel  36  is in good condition (e.g. running true, not excessively worn, not damaged). One of the limitations of existing sharpeners is that there is no indicator for the user that alerts them when the grinding wheel is not in good condition. Generally the user must make a judgment call on when to retire a grinding wheel. This may occur, for example, in response to a bad skating experience with skates that were sharpened with a grinding wheel that is no longer in good condition. 
     The disclosed sharpener  10  can use the data-carrying ability of the grinding wheel  36  to track usage, and employ the usage information in some way to promote delivery of consistent high quality sharpening. Generally this will involve comparing actual usage to a usage limit that has been predetermined as a dividing point between high quality sharpening and unacceptably low quality sharpening. When the usage limit is reached, some action is taken. For example, the control unit  32  may provide an indication to a user via the user interface display panel  34 . It may also prevent further use of the grinding wheel  36 , i.e., refrain from performing any passes with a wheel whose usage has reached the limit, even if such continued use has been requested by a user. 
     In one embodiment, the above usage tracking may be realized by initially loading the usage limit value onto the tag  204  and then subtracting or “debiting” the stored value as the grinding wheel  36  is used. The usage limit may be deemed to have been reached when the stored value reaches a predefined number such as zero. Generally the usage tracking and usage limit may be specified in any of a variety of ways, including a count of passes or cycles as has been mentioned, or alternatively by counting operating time (tracking the operating time for each sharpening and accumulating the time values over a period of successive sharpenings). If the usage limit value is specified as a maximum number of passes, then the value is decremented by two for each 2-pass cycle of the grinding wheel  36  over a skate blade during sharpening. In one embodiment, this decrementing can take place once each cycle, with the grinding wheel  36  passing through the home position ( FIG. 8 ) to enable the required RFID communications. In another embodiment, the updating may occur only once for a multi-pass sharpening operation. For example, once a number of passes has been specified (either by default or by actual user selection), the number of passes may be updated by the system immediately after the machine reads the tag  204  and just before the carriage motor  260  begins rotating. If the stored value were updated less frequently or at a different time, there may be more opportunity for a user to somehow “trick” the sharpener  10  into using a grinding wheel  36  longer than its useful life, which would jeopardize the quality of the skate sharpening. 
     A specific example is now provided for illustration. It is assumed that the useful lifetime of a grinding wheel  36  is on the order of 160 passes. This translates to approximately 10 sessions of sharpening a pair of skates if an average of 4 cycles (8 passes) is used per skate (8*2*10=160). 
     In a given embodiment, usage may be tracked in units of passes, cycles, blades sharpened (assuming some fixed or limited number of passes per blade), time, or some other scheme. The UI display  34  may be used to display remaining usable life for a grinding wheel  36  to the user. For example, it may be displayed as a fraction or percentage, or as more general ranges which could be indicated by colored indicators, for example—e.g., green for high remaining lifetime, white or other neutral color for intermediate, and red for low remaining lifetime. In one embodiment a linear array of indicators may be used, and indicators successively extinguished from one end as usage increases, and the end-of-life indicated by no indicators being lit. 
     Since there will be user-to-user variability in how many passes are done for a skate sharpening, the system may alert a user when the number of cycles needed to complete a sharpening exceed the number of cycles of remaining life of the grinding wheel  36 . The alert may be provided, for example, by dimming or flashing a set of indicators, and/or by stopping a sharpening that is in progress or preventing a new sharpening from beginning. Generally, it is desired that the display technique enable a user to accurately plan for use and avoid running out of usable grinding wheel lifetime in the middle of a sharpening 
     Beyond the usage tracking information, the tag  204  may also be used to carry system setup parameters that the control unit  32  can read and then apply to operation. This programming-type approach can enable a single sharpener  10  having a generalized design to be used in a wide variety of ways. For example, the tag  204  may contain parameters for the rotational speed of the grinding wheel motor  80 ; the speed of translation of the carriage assembly  70  across the skate blade; and the magnitude of a normal grinding force (i.e., the force applied by the grinding wheel  36  in a direction normal to the bottom face of the skate blade  40 ). Employing customizable settings in this manner can support variability in the materials, diameters, and grits used for different grinding wheels  36 . Larger wheel diameters for different skates, or different grits for different skate steels or surface finishes, will generally require different system settings (grinding wheel RPM and translation speed) for optimized use. In operation, the control unit  32  can read the parameters from the tag  204  and then apply the parameters prior to beginning a sharpening operation, such as by programming the appropriate controllers  132  ( FIG. 6 ). This programmability may also promote compatibility as designs of the grinding wheels  36  evolve over time. For example, if an innovation in grinding wheel abrasives happens in 5 years and this requires different system settings, the wheels produced in 5 years will store corresponding values of operating parameters to enable existing sharpener systems  10  to properly adjust themselves to produce an optimal sharpening. 
     The identification tag  204  may also store user-specific settings to be used for sharpening operations, such as a default number of passes for a skate sharpening. The control unit  32  can read such values and then use them unless they are overridden by a specific current selection by the user. One user may sharpen relatively frequently and typically use a small number of passes, such as two, while another user may sharpen less frequently and typically use a larger number of passes, such as eight. The user interface preferably would enable a user to modify or update any such persistently stored values. Saving user-specific values on the grinding wheel  36  also enhances “portability” of the customization. A user can carry their own grinding wheel  36  and mount it for use in different sharpener systems  10  at different locations while still obtaining the same user-specific operation. For example, an organization such as a hockey club or rink operator can provide access to a sharpener system  10  and allow users to swap grinding wheels  36 , so that each user receives a desired user-specific experience. 
     The sharpener system  10  may also have features for defeating counterfeiting or certain tampering with tags  204 . For example, it might record the unique tag identifiers (e.g., tag serial numbers) for every tag  204  that has been used over some interval on that sharpener, as well as recording the number of passes that were last seen on the tag  204 . If there is ever a time when a sharpener  10  sees a grinding wheel  36  that it has seen before but having remaining pass count greater than the number of remaining passes last seen on that wheel, the sharpener  10  could deem the grinding wheel  36  to be a counterfeit or tampered with and prevent its use. This might be done to insure that only grinding wheels  36  of sufficient quality are used, to obtain good sharpening results and avoid any unsafe conditions that could occur by using a defective or inferior grinding wheel  36 . The system  10  may store the most recent passes remaining count as individual numbers or as percentages similar to the way the system displays the grinding wheel remaining life to the user. 
     Yet another possibility is for the tag  204  to store system fault data, i.e., data describing fault conditions that have occurred during a sharpening operation. This can help users interact with technical service to diagnose problems they may be having with their machine. A manufacturer or service organization might request that the user send a grinding wheel  36  to that organization for review. The grinding wheel is smaller and thus far cheaper and convenient to send than is the entire system  10 . At the manufacturer or service organization, technicians can read fault data such as fault codes from the wheel  36 . In another embodiment, the identification tag  204  may be compatible with readers such as near-field communications (NFC) readers such as used on smart phones and similar small computing devices. When the user experiences a system fault, the user can remove the grinding wheel  36  and place it near the computing device. The device might immediately launch an application or navigate to a particular web site to provide information to the user about the particular fault that is identified by the fault data stored on the tag  204 . Another use for this type of interface is for repurchasing grinding wheels  36 . The application or website launched by the device may provide product ordering functionality, enabling a user to easily obtain replacement grinding wheels  36  as existing grinding wheels are used up. 
       FIG. 15  provides a high-level description of system operation with respect to the identification tag  204 . At  270 , the system  10  engages in communication with the identification tag  204  which is attached to a grinding wheel  36  mounted in the sharpening system  10 . As described above, the identification tag  204  has secure memory including a usage location for persistently and securely storing a usage tracking value. The communication both reads from and writes to the usage location. 
     At  272 , the system  10  tracks usage of the grinding wheel  36  for sharpening operations and writes updated usage tracking values to the usage location as the grinding wheel  36  is used for the sharpening operations. Usage may be tracked by counting passes, for example, in which case it may be convenient for the usage tracking value to be expressed as a pass count. The usage value may directly indicate an amount of usage that has occurred, e.g., as an increasing count of passes, or it may be directly indicate an amount of usage remaining, e.g., as a decreasing count of passes. 
     At  274 , the system  10  reads a current usage tracking value from the usage location and selectively enables and disables sharpening depending on whether a usage limit has been reached, as indicated by a relationship between the current usage tracking value and a predetermined usage limit value. When a decreasing or decremented usage value is used to indicate an amount of usage remaining, then the predetermined usage limit value can be used as the starting usage value, and the usage limit is reached when the usage value is decremented to zero. 
       FIG. 16  is a section view of the platform area  22  of chassis  14 . The clamp paddle  26  and left slot cover  28  ( FIG. 1 ) are shown, as well as various components of the blade clamping mechanism described above with reference to  FIG. 5 . 
     Referring first to the slot cover  28 , the button  27  is mounted for rocking on a horizontal axis and has a downward-extending rack  300  at the rear. The rack  300  engages a pawl  302  attached to the arch  64 . A spring (not shown) biases the button  27  so that its top is co-planar with the top of the slot cover  28  and the rack  300  engages the pawl  302 , locking the slot cover  28  in place. In use, a user depresses a front part of the button  27  (see  FIG. 1 ), lifting the rack  300  and enabling the slot cover  28  to slide left and right on the arch  64 . The left slot cover  28  travels between a far left position and a more rightward position in which it covers the left end of the slot  24 . A limit for the far left position is established by the rightmost wall of the slot cover  28  hitting a rightward wall or face of the arch  64  adjacent the platform  22 . A limit for the rightward position is established by the left wall of the slot cover  28  hitting the pawl  302 . There is a similar but mirrored arrangement for the right slot cover  28 . Additional details of the slot cover  28  are given below. 
     Referring next to the blade clamping mechanism, a vertex portion of the U-shaped pull rod fork  92  is shown, along with a pin  304  securing it to the pull rod  102 . The pull rod  102  extends through the clamp cylinder  94 , terminating at a piston head  306 . The pull rod  102  is disposed within bushings  307 ,  308 . A spring  310  is disposed between one end of the body of the clamp cylinder  94  and an external retaining ring  312  on the pull rod  102 . 
     When the clamp paddle  26  is in the position shown, the cam  96  presents a lower-radius face to the piston  306 , and the spring  310  urges the pull rod  102  to a maximum retracted position, to the left in  FIG. 16 . The pull rod fork  92  is under tension and pulls the clamp jaws  90  ( FIG. 5 ) in a closed position. If a skate blade is present then the clamp jaws  90  clamp the skate blade into place with a force geometrically related to the force created by the spring  310 . This arrangement is referred to as a biasing mechanism and the force as a bias force. 
     A user opens the clamp jaws  90  by pushing downward on an outer part of the paddle  26 , rotating it counterclockwise in the view of  FIG. 16 . The cam  96  has increasing radius in this direction and pushes the piston head  306  rightward against the force of the spring  310 . This action releases the clamping force between the jaws  90  and skate blade if present, and pushes the pull rod fork  92  rightward pushing the jaws  90  apart. The jaws are fully open when a maximum-radius part of the cam  96  is contacting the piston head  306 . This maximum-radius location can generally be anywhere in a range of about 10 degrees to 90 degrees from the closed position of  FIG. 16 . For smooth operation and good mechanical advantage it may preferably be somewhere in the smaller range of 40 degrees to 75 degrees. In one embodiment it is located at 60 degrees. As mentioned above, a configuration providing a detent action may be used. For example, the cam  96  may have a slightly flattened area at maximum-radius location for a slight detent action. 
       FIGS. 17 through 21  show details of the jaws  90  including connections to respective ends of the pull rod fork  92 .  FIGS. 17 and 18  show plan views of the bottoms of the rear and front jaws  90 -R,  90 -F respectively.  FIGS. 19 and 20  show sections through a guide slot  104  and guide block  106  of the rear jaw  90 -R, and  FIG. 21  shows a section through a guide slot  104  and guide block  107  of the front jaw  90 -F. 
       FIG. 17  shows the use of two guide blocks  106  at respective endmost slots  104  for the rear jaw  90 -R. The slots  104  are oriented at approximately 30 degrees with respect to the long axis of the jaws  90  (X direction). In response to force exerted by the pull rod fork  92 , the jaw  90 -R slides along the guide blocks  106 . When opening, the rear jaw  90 -R moves upward and to the left in the view of  FIG. 17 , and when closing it moves in the opposite direction. The rear jaw  90 -R maintains a fixed orientation substantially along the X axis. It establishes the orientation of the clamped skate blade, which should be highly co-planar with the X-Z plane of movement of the grinding wheel  36 . 
     As shown in  FIG. 18 , the front jaw  90 -F has a generally symmetrical configuration with respect to the rear jaw  90 -R, and it moves symmetrically as well, i.e., downward and to the left when opening in the view of  FIG. 18 . However, the front jaw  90 -F is secured with only one guide block  107 , located in the center guide slot  104 . As described more below, the guide block  107  is mounted in a manner permitting slight pivoting, while the guide blocks  106  for rear jaw  90 -R are not. Thus, the front jaw  90 -F also rotates slightly about the Z-direction axis of the single central guide block  107 . This enables the front jaw  90 -F to conform its orientation to that of the rear jaw  90 -R when a skate blade is clamped between them. It will be appreciated that this configuration avoids issues that could occur if the front jaw  90 -F had an orientation that was fixed but slightly different from that of the rear jaw  90 -R due to normal mechanical tolerances. These issues include mechanical binding, uneven force across faces of the jaws (higher at one end than at the other), as well as inaccuracy in the orientation of the skate blade, adversely affecting sharpening quality. The illustrated configuration avoids these issues by allowing the rear jaw  90 -R to serve as a mechanical reference, and the front jaw  90 -F to conform itself to that reference. 
       FIGS. 19 through 21  illustrate certain functionality provided by the configuration of a guide slot  104  (i.e., of its surrounding walls) and the guide blocks  106 ,  107 . As shown, the jaws  90  are spaced from the platform  22  by respective spacer blocks  343  which are rigidly secured to the underside of the platform  22 . The jaws  90  and guide blocks  106 ,  107  have a configuration that provides for spacing the jaws  90  slightly from the respective spacer blocks  343 , enabling the jaws  90  to slide easily between open and closed positions. The configuration also provides for closing this spacing when the jaws  90  are brought into the closed position, so that they rest flush against the spacer blocks  343 . This action make the jaw positioning precise and accurate. It also prevents the jaws  90  from tilting about their longitudinal axes, which would tend to occur if the space were not closed up as the jaws  90  are tightened against the skate blade  40 . Maintaining a predictable flat orientation of the jaws  90  provides for greater accuracy in the positioning of the clamped skate blade  40 . 
       FIGS. 19 and 20  show details for the rear jaw  90 -R. The guide blocks  106  for the rear jaw  90 -R are fastened to the spacer block  343  by bolts  338 . The jaw  90 -R and guide block  106  have respective sloped or angled surfaces  340 ,  342  contacting each other. The jaw surface  340  is one side wall of the guide slot  104  ( FIG. 17 ) in which the guide block  106  is located.  FIG. 19  is a section view showing these surfaces  340 ,  342  as lines at the intersection with the Y-Z plane of the paper. Referring back to  FIG. 17 , the surfaces  340 ,  342  are also angled in the direction of the guide slot  104 , which corresponds to a plane through the paper of  FIG. 19 , tilted about 30 degrees to the left of X-direction normal. In the view of  FIG. 19 , the front of the jaw  90 -R and skate blade  40  are at the left. The jaw  90 -R is pulled in the X direction out of the paper to be closed, and pushed in the opposite direction to be opened. The pulling and pushing cause corresponding leftward (closing) and rightward (opening) motion by action of the angled guide slots  104 . 
       FIG. 19  shows that the combination of the thickness of the rear jaw  90 -R, the width of the guide slot  104 , and the height and width of the guide block  106  is such that the top of the jaw  90 -R is slightly spaced from the bottom of the spacer block  343  in the illustrated position. This is a first condition in which the jaw  90 -R is slack, i.e., not exerting a clamping force. This could be either a fully or partially open position. The jaw  90 -R rests relatively loosely on the guide block  106  and is able to slide thereon without interfering contact with the spacer block  343 . There is a slight space  345  between the jaw  90 -R and guide block  106  as shown. 
       FIG. 20  is a similar view as  FIG. 19  but in a second condition in which the rear jaw  90 -R is pulled tightly by the pull rod fork  92  ( FIG. 5 ) and exerting a clamping force on the skate blade  40 . As the jaw  90 -R encounters the skate blade  40  it experiences a rightward force causing it to ride up the surface  342  of the guide block  106  until the top of the jaw  90 -R hits the bottom of the spacer block  343 . This movement closes the space  345  and opens a separate space  347  on the other side of the guide block  106 . Because the surfaces  340 ,  342  have precisely the same slope, the jaw  90 -R automatically assumes a position in which its upper surface is flush against the bottom surface of the spacer block  343 . As the motion ceases, the combined forces of the pull rod fork  92  and the skate blade  40  press and hold the jaw  90 -R at this upward position, tight against the guide block  106 . This action occurs consistently whenever the jaw  90 -R is closed, and thus the rear jaw  90 -R and skate blade  40  are consistently positioned. 
     The above motion reverses when the jaws  90  are opened. As the rear jaw  90 -R is pushed in the X direction, clamping tension is released and it slides downward in the Z direction, closing the space  347  and returning to the position of  FIG. 19  The configuration providing the space  347  in the closed position of  FIG. 20  also provides for the slight looseness of the jaw  90 -R that permits it to slide easily when slack. 
       FIG. 21  is an analogous view to that of  FIG. 20  but for the front jaw  90 -F, which is secured via only one guide block  107  as described above. The configuration and operation are essentially the same as for the rear jaw  90 -R—the front jaw  90 -F is pushed against the spacer block  343  and guide block  107  in the same manner, and has the same configuration providing for spaces  345  and  347 . However, the guide block  107  is secured to the spacer block using a shoulder screw  346  in a tightly toleranced counter-bored hole of the guide block  107 . The shoulder screw  346  and counter-bored hole of the guide block  107  are sized to create a slight gap  348 , so that the guide block  107  is not secured tightly to the spacer block  343 . Thus, the guide block  107  is free to rotate slightly about the Z-direction axis of the shoulder screw  346  to provide the above-described rotational compliance of the front jaw  90 -F. 
     In the illustrated embodiment as described above with reference to  FIGS. 19 through 21 , the jaw closing direction (left or right) is perpendicular to the direction of the actuating force (out of the paper), and the slots  104  are angled accordingly to translate the actuating force to the clamping force. Also, the actuating force is a pulling force, essentially pulling each jaw  90  up the surface  342  of the guide blocks  106 ,  107 . It will be appreciated that in alternative embodiments other configurations may be used, depending in part on the relative locations of the jaws and the force-generating actuator as well as the nature of the force as either compressing or tensioning the jaws. In particular, the slots  104  may be oriented at angles other than 30 degrees. Also, in the illustrated embodiment the jaw  90  is slightly thinner than the height of the guide block  106 , but this is not essential. 
     In the illustrated embodiment the jaws  90  are urged against a lower or bottom surface of the spacer blocks  343 , which are fixedly secured to the underside of the platform  22  of the chassis  14 . More generally the jaws  90  are urged against a surface that is in some manner referenced to the chassis  14 , i.e., having a fixed position with respect to the chassis  14 . In an alternative embodiment, the jaws  90  might be secured directly to a surface of the chassis  14  itself, such as the bottom surface. 
       FIG. 22  is a bottom view of a slot cover  28  and an arch  64  on which it is captured. The bottom of the button  27  is visible, including the rack  300  that moves in and out of the page in this view when the button  27  is operated as described above. The slot cover  28  is retained on the arch  64  by a latch-like rail mechanism including inner edges  318  of the slot cover  18  that fit within corresponding elongated grooves on the upper surface of the arch  64  where the central rounded portion  319  meets the lateral flat portions  321 . 
     In the illustrated embodiment, the bumper  29  is attached to the body of the slot cover  28  (at lower left corner in this view). The attachment is with a pin or similar fastener  320  that permits the bumper  29  to rotate. A face portion  322  contacts a skate blade holder in operation as described above ( FIG. 1  and related description). Another portion  324  extends to an actuation lever  326  of a limit switch  328 . The bumper  29  is biased (counterclockwise in this view) by a spring  330 . The limit switch  328  is wired to the controller  32  ( FIG. 6 ) to enable the controller  32  to sense its electrical state (open or closed). The wires are omitted in  FIG. 22  for ease of illustration. 
     In operation, the limit switch  328  is electrically open by default, due to the mechanical biasing action of the spring  330 . When the face portion  322  of the bumper  29  is depressed, the bumper  29  rotates (clockwise in this view) and the arm  324  depresses the limit switch lever  326 , electrically closing the limit switch  328 . The state of the limit switch  328  as open or closed is sensed by the controller  32 . In one embodiment, sharpening operation is permitted only when the limit switch  328  is sensed as closed, which normally occurs when a skate blade is clamped in position and the slot covers  28  have been moved inward to contact the skate blade holder. In these operating positions the slot covers  28  cover the outer ends of the slot  24  that would otherwise be open. This prevents the introduction of any objects through the outer ends of the slot  24 , where such objects might harmfully contact the rotating grinding wheel  36  as it moves along the slot  24  during a sharpening operation. If the limit switch  328  of either slot cover  28  is sensed as open, which normally occurs when either a skate or skate blade holder is not present or both slot covers  28  have not been moved inward to their operating positions, the controller  32  prevents sharpening operation, i.e., provides no electrical drive to the grinding wheel motor  80  and the carriage motor  260 . With these motors not rotating, it is safer to introduce objects (such as a skate blade during mounting, for example) into the slot  24 . 
     There are various alternatives to the configuration described above. An alternative to the bumper  29  may be a piston-like mechanism that moves linearly to actuate a switch, instead of rotating about a fixed pivot point as in the above. It is not necessary to use a limit switch with an actuation lever—in an alternative arrangement the bumper  29  (or analogous member) may directly push on the button of a limit switch. Also, in some embodiments a separate spring  330  may not be required. It may be possible to rely on the spring of a limit switch to provide a bias or return force. However, it may be desirable to use a separate spring to provide for adjustment of either/both the range of motion and actuation force of the bumper. 
     In yet another alternative, a contactless switch such as an optical emitter-detector pair could be used, with the skate or skate blade holder breaking the optical path to trigger the switch. 
     In the illustrated embodiment the slot covers  28  are affixed and always present, but in an alternative embodiment they could be separate components that are placed and locked onto the ends of the skate or skate blade holder by the user prior to sharpening. Also, while in the illustrated embodiment the slot covers  28  move by sliding, they could alternatively move by rotating on a hinge, telescoping, or rolling out (like a breadbox or garage door). 
       FIG. 23  is an end view of the carriage assembly  70 , similar to  FIG. 14  but showing a section view at the location of the pivot spindle  240 . Certain details are shown more clearly in the close-up view of  FIG. 24 . 
     The pivot spindle  240  is secured at each end to the carriage  72 . A pivot section  400  of the motor arm  78  is mounted on the pivot spindle  240  by a combination of bearings  402 ,  404  and bushings  406 ,  408 . Shown on the right in this view is a spring  410  disposed in compression between the front wall of the carriage  72  and an inner race  412  of the bearing  404 . Shown on the left is the spindle gear  252  which is disposed on a hub or nut  414  having screw threading engaging corresponding screw threading on the pivot spindle  240 . It will be appreciated that the gear and threading features may be integrated into a single component as an alternative. Arranged between the nut  414  and an inner race  416  of the bearing  402  is a washer  418  and a collar portion  420  of the bushing  406 , including a detent mechanism as described below. 
     The mounting of the motor arm  78  on the bearings  402 ,  404  permits the motor arm  78  to pivot about the pivot spindle  240  so that the grinding wheel  36  can follow the profile of the bottom face of the skate blade during sharpening (as described above with reference to  FIGS. 7 and 8 ). The bushings  406 ,  408  provide for low-friction transverse (Y-axis) movement of the motor arm  78  (left to right in  FIG. 23 ). The spring  410  provides a biasing force against a side face of the inner race  412  of the bearing  404 , urging the motor arm  78  rearward (leftward in  FIG. 23 ). The combination of the threaded nut  414 , washer  418  and collar portion  420  of bushing  406  act as a stop member against which the motor arm  78  is urged. Specifically the force from spring  410  is transmitted to the nut  414  via a set of mechanical components including the bearing  404 , pivot section  400 , bearing  402 , collar portion  420  of the bushing  406 , and washer  418  and detent mechanism described below. 
     The transverse or Y-direction (left to right in  FIG. 23 ) position of the motor arm  78  is varied by rotation of the nut  414 , which occurs by user rotation of the adjustment knob  242  ( FIGS. 13, 14 ) and resultant rotation of the adjustment axle  254  and gears  256 ,  252  as described above with reference to  FIG. 14 . As the nut  414  rotates, the screw action causes it to also move transversely in the Y direction along the pivot spindle  240 , and due to the pressing force from the spring  410  the motor arm  78  moves transversely along with it. The bushing  406  slides along an outer surface of the pivot spindle  240 , and the inner race  412  of bearing  404  is pressed onto bushing  408 , which slides along an outer surface of pivot spindle  240 . The bushing  408  may alternatively include a flange or collar portion similar to collar portion  420  of the bushing  406 . 
     The nut  414  and washer  418  are co-configured to form a detent mechanism providing several detent locations for a rotation of the nut  414 , helping prevent undesired transverse movement of the motor arm  78  after an alignment operation has been performed and a sharpening operation has begun. Specifically, the front face (rightward in  FIG. 23 ) of the nut  414  has a shallow depression in which is disposed a ball, and the washer  418  has an array of corresponding holes or depressions arranged in a circle. As the nut  414  is rotated the ball moves from one hole or depression of the washer  418  to the next, requiring a small force to push the ball sufficiently out of the first hole/depression to enable it to travel to the next. This force is easily generated by the user&#39;s rotation of the adjustment knob  242  but not by vibration or other mechanical forces occurring during sharpening operation. 
       FIG. 25  is a downward view encompassing the jaws  90  and the grinding wheel  36  and motor arm  78  underneath. The jaws  90  are shown in the closed position, slightly spaced apart as they are when retaining a skate blade (not shown). This view is of an aligned position in which a centerline  430  of the grinding wheel  36  is aligned with a centerline  432  of a sharpening position of the skate blade (midway between the clamping surfaces of the jaws  90 ). It will be appreciated that the grinding wheel  36  can be moved transversely (up and down in the view of  FIG. 25 ) by the above-described Y-adjustment mechanism, changing the position of the grinding wheel centerline  430  with respect to the centerline  432  of the skate blade. In general there is a small range of uncertainty in the position of the grinding wheel  36  relative to the centerline  432  based on mechanical tolerances as well as planned variability, such as varying sizes of grinding wheels  36  that the system supports, etc. The adjustment mechanism enables a user to obtain accurate alignment to achieve as closely as possible the idealized arrangement of  FIG. 2 , i.e., perfectly symmetrical curvature of the bottom surface  42  of the skate blade  40  about its centerline  432 , so that the edges  44  lie in the same plane perpendicular to the X-Z plane of the skate blade  40 . In the present context, the required accuracy of alignment is to within approximately +/−0.001″. It will be appreciated that this level of accuracy is generally not possible using simple naked-eye observation of the degree of alignment between the grinding wheel  36  and skate blade  40 . Thus features that aid alignment to this degree are disclosed. 
       FIG. 25  also shows certain features of the jaws  90  pertaining to alignment. First is a central open area  434  through which the grinding wheel  36  can be viewed and a separate alignment tool (described below) is received. Thus the jaws  90  are left with endward clamping portions  436 . Second are notches  438  formed in the front jaw  90 -F which receive corresponding protrusions from the alignment tool so that the alignment tool is properly oriented and located precisely in the left-to-right direction of  FIG. 25 . This precise locating in turn provides for close spacing of an alignment feature of the alignment tool with a corresponding feature of the grinding wheel  36 , as described more below. 
       FIG. 26  illustrates the alignment tool  440  as it is located during use. It has a lower blade-like portion  442  and an upper portion  444  holding a magnifying lens  446 . The blade-like portion  442  is clamped between the jaws  90  in the same sharpening position that the skate blade  40  occupies when being sharpened. In this view the front jaw  90 -F is omitted for ease of description. The blade-like portion  442  extends downward to support a flag  448  that functions as a first visual reference feature as explained below. In one embodiment the flag  448  is a thin member secured flat against a surface of the lower portion  442 . It is thus precisely spaced from the centerline  432  of the jaws  90  ( FIG. 25 ) when the alignment tool  440  is clamped in the illustrated position. In the illustrated embodiment this spacing is on the order of one-half the width of the grinding wheel  36 . Also shown in  FIG. 26  are machined shoulder portions  450  extending out of the page in this view. Bottom edges of the shoulder portions  450  sit on top of the endward clamping portions  436  of the jaws  90  ( FIG. 25 ), except for the slightly longer protrusions  452  that are received by the notches  438  ( FIG. 25 ). It will be noted that the flag  448  is opposite the grinding wheel  36  along a horizontal diameter. In other embodiments the flag  448  may be formed integrally with the lower portion  442 . 
     In use, a user opens the jaws  90  and inserts the alignment tool  440 , locating it so that the shoulder portions  450  sit on top of the endward clamping portions  436  of the jaws  90  and the protrusions  452  are received by the notches  438 . The user then closes the jaws  90  so that the alignment tool  440  is retained with the blade-like portion  442  in the same position as a skate blade  40  is retained during sharpening. The carriage  70  is then moved to bring the grinding wheel  36  to the position shown in  FIG. 26 , i.e., with its outer surface just slightly spaced from the flag  448 . This movement may be automatic or manual, and if automatic it may be user-initiated (such as via the user interface  34  of  FIG. 1 ) or in some manner auto-initiated by detection of the presence of the alignment tool  440 . 
     In one embodiment the movement of the grinding wheel  36  into the alignment position of  FIG. 26  may employ the same components used for moving the carriage  70  during sharpening, i.e., the carriage motor  260  and rack-and-pinion mechanism. The grinding wheel  36  may be moved until it encounters the alignment tool  440 , which can be sensed as an increase in the drive current through the carriage motor  260 . Upon sensing this encounter, the controller  32  provides one or more brief pulses of reverse drive current to move the grinding wheel  36  slightly away from the alignment tool  440  to allow for the Y-direction adjustment of the motor arm and grinding wheel  36  as described further below. The movement away from the encounter position could alternatively be guided by use of a position encoder on the motor, for example if greater positional accuracy is needed. 
       FIG. 27  is a view downward through the magnifying lens  446 . An area around the flag  448  is visible, with the grinding wheel  36  slightly spaced apart from it. The grinding wheel  36  has an annular notch  454  formed near its front face, which functions as a second visual reference feature as explained below. The notch  454  is precisely spaced from the centerline  430  of the grinding wheel  36  ( FIG. 25 ) by the same amount as the spacing between the flag  448  and the centerline  432  between the jaws  90 . Thus, when the flag  448  is aligned with the notch  454 , as is shown in  FIG. 27 , the centerline  430  of the grinding wheel  36  is precisely aligned with the centerline  432  between the jaws  90 , and hence with the centerline of the skate blade  40 . As indicated,  FIG. 27  shows the aligned position. It will be appreciated that when the centerline  430  of the grinding wheel  36  is not aligned with the centerline  432  between the jaws  90 , then the notch  454  is correspondingly offset from the flag  448  (in the up and down direction in  FIG. 27 ) as an indication of such misalignment. A user can look through the magnifying lens  446  to view the area of the flag  448  and simultaneously turn the adjustment knob  242  ( FIG. 14 ) to move the motor arm  78  and grinding wheel  36  in the transverse (Y) direction (up and down in  FIG. 27 ) to bring these centerlines into alignment, thereby accurately aligning the grinding wheel  36  with the bottom of the skate blade  40  for a sharpening operation. 
       FIG. 28  is a simplified flow diagram for a process of aligning a grinding wheel to a retained skate blade. The process includes at  460  visually observing an area in which first and second visual reference features of the skate blade sharpening system are located, where the first visual reference feature has a first predetermined location relative to a centerline of the retained skate blade, and the second visual reference feature is carried by a motor arm that also carries the grinding wheel and that has a second predetermined location relative to a centerline of the grinding wheel. In one embodiment the first visual reference feature may be a feature like flag  448  on a separate fixture or tool such as the alignment tool  440  that is clamped in the sharpening position, so that the first visual reference feature is temporarily placed in position for the alignment operation. In alternative embodiments the first visual reference feature may be built in to the sharpening system  10 , such as by incorporation into the jaws  90  for example. In one embodiment the second visual reference feature may be a notch or similar feature incorporated on the grinding wheel  36 , such as described above. 
     The process further includes at  462  operating an adjustment mechanism while visually observing the area where the visual reference features are located to bring them into alignment with each other. This brings the grinding wheel and the retained skate blade into an aligned position in which the centerline of the grinding wheel is aligned with the centerline of the retained skate blade. In one embodiment the adjustment mechanism may be configured and used such as described above, but the adjustment mechanism may be realized in different ways in alternative embodiments. 
     Referring again to  FIGS. 26 and 27 , the visual reference features in the form of the flag  448  and notch  454  provide for detection of parallax that could affect accuracy of the adjustment. As generally known, parallax is a phenomenon by which two objects that are actually misaligned in a particular direction nonetheless appear aligned when viewed from a different direction. In the present context, parallax could potentially occur if a user is not directly above the flag  448 . Because the flag  448  has a height much greater than its thickness, if a user were viewing from a slightly incorrect angle then the flag  448  would appear thicker than when viewed from directly above. A user can adjust his/her viewing angle until the thickness is minimized. Alternatively, if light is striking the sides of the flag  448  then the illuminated sides will be slightly visible when the flag  448  is viewed off-angle. The notch  454  also provides for parallax detection, because it will only be visible as a notch when viewed from directly above. When the area of the notch  454  is viewed off-angle, the notch is visually filled by its own inside surface. 
     Although the alignment process and apparatus as described herein contemplate a human user who looks through the magnifying lens  446  and rotates the adjustment knob  242 , it will be appreciated that in alternative embodiments a more automated process may be used. For example, some manner of machine vision or other apparatus may be used to monitor relative position between the grinding wheel  36  and skate blade  40 , and the adjustment mechanism may be driven by an adjustment motor provided with an electrical adjustment signal. A controller can then perform the process of  FIG. 28  based on position information from the position-monitoring apparatus and by generating the electrical adjustment signal to change the relative positions of these component accordingly until an aligned position is detected. 
     Is also noted that the placement of the notch  454  toward an edge of the grinding wheel  36  has significance. Proper grinding occurs at the center of the grinding wheel  36 , so if the alignment mark were placed at the center of the grinding wheel  36  then it would be affected by grinding and potentially lose its ability to function as an alignment mark. It might even be erased completely before the end of the usable lifetime of the grinding wheel  36 . When formed as a notch or similar feature, it might also compromise the quality of the sharpening. By placing the alignment mark in the form of the notch  454  nearer the edge or face of the grinding wheel  36  it is not affected by the normal wearing of the abrasive over a period of use, and it does not interfere with grinding. 
     While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.