Abstract:
A computer-controlled band saw sharpener employing electrically powered, X and Y axis drivers, each having a reciprocating element. The X-axis driver actuates a feed finger mechanism, to index the saw in repetitive, stepped fashion, while each tooth is successively ground. In concert with the X-axis driver, the Y axis driver repetitively lowers and raises a grinding head, provided with a rotary grinding wheel. Both the X and Y axis drivers include servomotors to rotate the shaft of an elongated ball screw. A carrier box houses a non-rotatable ball screw nut. The nut travels back and forth along the ball screw shaft in accordance with its direction and extent of rotation. Drive arms for the feed finger mechanism and the grinding head mechanism are bolted at their lower ends to a threaded bushing in the nut carrier box. The computerized controller is programmable, to produce virtually any desired tooth shape in the saw. Standard-tooth profiles, variable-pitch tooth profiles, and variable depth tooth profiles may be selected by touching a button on the controller&#39;s panel. The apparatus may be used either in new sharpener construction, or as a retrofit to prior art sharpeners to replace mechanical cam-actuated drive mechanisms.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 09/539,267 filed on Mar. 30, 2000, now U.S. Pat. No. 6,374,703, issued Apr. 23, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to devices for sharpening band saws used in the lumber industry and for sharpening saws used in other industrial and commercial applications. More specifically, the invention pertains to a computer-controlled system, for driving the saw blade advancing and saw blade grinding components of a saw sharpener. 
     2. Description of the Prior Art 
     Automatic band saw sharpeners have been available for many years. These sharpeners are used to sharpen large, industrial ban saws, used primarily in the lumber industry. Typically, such sharpeners include a main sharpener frame and a plurality of outboard support posts. These posts maintain the blade in a horizontal, oval-like configuration during the sharpening operation. Prior art sharpeners employ a feed finger mechanism to index the band saw in sequential, stepped fashion through the sharpener, while the grinding wheel of a grinding head assembly is successively lowered into and raised from the blade to shape each tooth. 
     Cam-actuated mechanism have previously been used to drive both the feed finger and the grinding head in the proper timed relationship. For example, in the prior art band saw sharpener manufactured by the Armstrong Manufacturing Company of Portland, Oreg., an electric motor rotates a drive shaft upon which at least two separate cams are located. A treadle arm, connected to the grinding head assembly, includes a cam follower on its lower end for engagement with one of the cams. In similar fashion, a feed finger arm is connected to the feed finger mechanism, and includes a cam follower on its lower end for engagement with the other cam. Rotating the drive shaft effects X-axis movement of the feed finger and Y-axis movement of the grinding head assembly, so as to grind the desired tooth configuration into the saw. 
     If single lobe drive cams are used, such a sharpener will produce teeth having the same pitch and the same gullet depth. However, it has been determined that variable tooth patterns in the saw may be desirable, as certain operational advantages are provided. For example: the feed speeds for sawing the subject lumber may be increased; the “washboard” pattern that same pitch and same depth teeth impress in the lumber may be eliminated or reduced; and, the size of the kerf produced in the cut lumber may also be reduced, saving wood product. 
     To satisfy this need, prior art sharpeners were developed which use drive cams having multiple lobes to produce variable pitch and variable depth gullets. It has also been advocated to provide up to three pairs of such cams on a single drive shaft, so that different pitches and different gullet depths may appropriately be selected for the particular saw to be sharpened. However, it is time consuming to move and relocate the cams and then set up the sharpener so it is adjusted properly for the different grinding operation. Moreover, a considerable investment must be made to acquire and maintain the cams corresponding to each selected pitch and depth. 
     Another disadvantage of the cam-actuated system is the mechanical wear that results from long-term use of the sharpener. Mechanical wear in drive components, such as the drive cams and the cam followers, can cause improper and erratic grinding patterns, resulting in undesirable tooth size and shape. In addition, replacing the cams is expensive, and results in down time for the sharpener. 
     In recognition of these disadvantages, efforts have been made to redesign or replace the cam-actuated mechanisms in saw sharpeners. Specifically, a number of patents have issued for more modern saw blade sharpeners, some of which incorporate programmable computers or controllers. Other sharpeners disclosed in recently issued patents employ alternative drive mechanisms for their saw blade driving and grinding head components. 
     For example, U.S. Pat. No. 5,488,884, issued to Andrianoff et al., shows an apparatus for side-grinding saw blade teeth. This apparatus uses a commercially available programmable controller, and includes pneumatic cylinders to move the grinding head assembly. In U.S. Pat. No. 5,471,897, granted to Wright, a “conventional computer numeric controller” is used to provide sharpening functions. The Wright reference also shows the use of servo motors and ball screw mechanisms, to effect movement of the grinding wheel assembly, in the X and Y axes. And, in U.S. Pat. No. 5,826,465, issued to Iseli, a band saw manufacturing method is disclosed which uses hydraulic rams for advancing and retracting the “grinding wheel arrangement”. 
     Nevertheless, the need exists for an apparatus which may be installed as a retrofit to a prior art cam-actuated sharpener, giving it vastly improved flexibility and performance: and, the need further exists for economical drive and control systems which may be incorporated into new saw sharpeners, which systems provide reliability and accuracy not attainable by known prior art devices. 
     SUMMARY OF THE INVENTION 
     The present invention uses a programmable computerized controller, for actuating both an X-axis driver and a Y-axis driver. Each of these drivers employs a rotatable ball screw, a non-rotatable ball screw nut in threaded engagement with the ball screw, a reciprocating carrier box for the screw nut, a servomotor driving one end of the ball screw, an encoder to provide feedback information to the controller regarding the rotational position of the servomotor, and a micro switch positioned to actuate at a predetermined “home” location for the carrier box for the ball screw nut. 
     If the invention is implemented as a retrofit conversion to a prior art band saw sharpener, the drivers replace an existing motor driven cam mechanism. This mechanism typically includes an electric motor driving a shaft provided with two cams. The cams are engaged by respective cam followers, connected to respective drive arms. One drive arm moves a saw feed mechanism in a horizontal, or X-axis direction, and the other drive arm moves a grinding head mechanism in a vertical, or Y-axis direction. The coordinated horizontal movement of the saw blade and the vertical movement of the grinding head, effect grinding of the desired saw tooth shape in the blade. 
     To install the present invention as a retrofit conversion, the electric motor for driving the cam shaft, the shaft, the cams, the bearings supporting the shaft, and the cam followers, are all removed. A sub-frame, including parallel shafts strategically located to support each of the new X-axis and Y-axis drivers, is then installed. The sub-frame is conveniently installed to the main frame using the same holes used to secure the just-removed cam shaft bearings. The carrier box of each driver is bolt-connected directly to a foot or an end of the existing drive arms in the same place where the cam followers were previously attached. 
     The computerized controller is mounted to the sharpener frame, and cables extending therefrom provide electrical interconnections to each of the drivers. The controller appropriately actuates the driver servo motors, which in turn effects rotation of the ball screws, resulting in reciprocating X and Y axis movement of the drive arms. In this manner, the retrofit drivers control the saw blade feed and grinding head components just like the prior art cam-driven mechanisms did. 
     However, because the computerized controller is programmable, virtually any tooth shape can be produced by the sharpener without changing any of its mechanical components. For example, the controller can effect standard-tooth profiles, variable-pitch tooth profiles, and variable depth tooth profiles. The controller may be switched from one tooth profile to another in seconds, merely by touching a button. The controller has two grinding modes for each tooth profile, a standard grind and a retip grind. In the retip grinding mode, the grinding feed rate is slowed down while the top and the face of the new tip are shaped, and then the normal grinding speed is resumed while shaping the remainder of the tooth profile. 
     If the invention is implemented as part of a new band saw sharpener construction, substantially the same components are used, except the entire electric motor, shaft, cams, and cam follower components of the prior art device are never used or installed, resulting in substantial savings. 
     Therefore, it is an object of the present invention to provide a computer-controlled band saw sharpener, which may be implemented either as a retrofit modification to an old sharpener, or as part of a newly constructed sharpener, and which can grind standard-tooth profiles, variable-pitch profiles, and variable depth profiles, without changing any mechanical components. 
     It is a further object herein to disclose an electronically actuated X-axis driver and a Y-axis driver, each employing a servo motor, a rotatable ball screw, and a non-rotatable ball nut for improved accuracy, flexibility, and control over the operation of a feed finger mechanism and a grinding head assembly in a saw sharpener. 
     It is yet a further object herein to disclose a computer-controlled driver for use with a sharpener, in which the driver includes a rotatable ball screw driving a ball nut fixed within a movable carrier box, in which the carrier box is supported entirely by the ball nut and captive balls. 
     It is yet a further object herein to disclose a computerized controller for electronically actuated X and Y axis drivers in a saw sharpener, which controller is programmable to effect a wide variety of different pitch and gullet depths for saw teeth, by a button selection made by the user. 
     It is yet a further object herein to disclose a lever actuated lifting mechanism, for raising the grinding head assembly of a saw sharpener into a disabled position, above the saw. 
    
    
     These and other objects of the present invention will become apparent by reference to the accompanying text and drawings which follow. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a left front perspective view of a band saw sharpener incorporating the features of the present invention, and further including outboard saw support posts, a back feed unit, and a band saw in position for sharpening; 
     FIG. 2 is a left front perspective view of the sharpener shown in FIG. 1, taken to an enlarged scale; 
     FIG. 3 is right front perspective view of the sharpener shown in FIG. 1, taken to an enlarged scale; 
     FIG. 4 is a perspective view showing the X and Y axis drivers, respectively connected to the driver arms for the feed mechanism and the grinding head assembly, the general outline of the sharpener frame being shown in broken line; 
     FIG. 5 is a perspective view of the sharpener with a portion of the frame being broken away to show the Y-axis driver and its attachment to one of the support shafts of a sub-frame; 
     FIG. 6 is a side elevational view of the sharpener and the Y-axis driver, with the grinding head assembly in a lowered position; 
     FIG. 7 is a view as in FIG. 6, but showing the grinding head assembly in a raised position; 
     FIG. 8 is fragmentary, side elevational view of one end of the sharpener, showing the grinding head assembly lifter arm rotated downwardly; 
     FIG. 9 is a fragmentary view of the sharpener as in FIG. 6, but showing the lifter arm rotated downwardly, to raise the grinding head assembly into a disabled position; 
     FIG. 10 is a perspective view of the sharpener with a portion of the frame being broken away to show the X-axis driver and its attachment to the other support shaft of the sub-frame; 
     FIG. 11 is a fragmentary, side elevational view of the sharpener, showing the rearward motion of the X-axis driver and the saw feed finger mechanism; 
     FIG. 12 is a view as in FIG. 11, showing the forward motion of the saw feed finger mechanism; 
     FIG. 13 is a left front perspective of the Y axis driver, a portion of the two bellows being broken away to show the ball screw shaft; 
     FIG. 14 is a side elevational view of the Y axis driver, a cover plate for the carrier box being removed to show the ball screw nut; 
     FIG. 15 is a fragmentary, cross-sectional view of the ball screw shaft and ball screw nut; 
     FIG. 16 is a schematic representation showing the computerized controller, the X and Y axis drivers, the back feed unit, and a pneumatic pump; 
     FIG. 16A is an inset detail, taken on the line  16 A— 16 A, shown in FIG. 16; and, 
     FIGS. 17A-17L, inclusive, are successive side elevational views of a saw showing the feed finger of the saw feed mechanism and the grinding wheel of the grinding head assembly, the Figures collectively depicting a complete grinding cycle for a saw tooth using the sharpener of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to FIG. 1, a saw sharpener apparatus  11  includes a frame  12 , for housing and supporting the various components to be described herein. A portion of a band saw  13  is secured within a horizontal saw carriage  14 , mounted along a top portion of the frame  12 . The primary purpose of the carriage  14  is to help maintain the saw  13  in secure relation during the grinding operation, while allowing the saw to be indexed successively through the sharpener until the sharpening operation is completed. To that end, the carriage includes a horizontal inner recess and an outer vertical wall, that together with the adjacent portion of the frame, define a groove  16  through which the bottom edge of the saw slides. An elongated clamping bar  17 , pivotally mounted to the top frame  12 , pneumatically clamps the saw  13  against a frame-mounted alignment plate (not shown) during the first portion of the grinding cycle for each tooth. This grinding cycle, including the clamp down of the saw, will be explained in more detail below. The clamping bar includes a rectangular clamping plate  15 , translated by a “pancake” pneumatic cylinder  20 , to clamp the saw against the alignment plate at the appropriate time during the grinding cycle. The clamping bar  17  is pivotally mounted to the frame so that it can be swung out of the way when the saw is being installed into or removed from the sharpener. 
     The remaining portion of the saw is supported and maintained horizontally in a generally oval-shaped configuration by four vertical support posts  18  and a back feed unit  19 . A slotted guide  21  is included at the top of each post, to hole the saw  13  in slidable relation therein. 
     The Feed Finger Mechanism 
     The saw is successively indexed through the sharpener  11  by the cooperative actions of a feed finger mechanism  22  and the previously mentioned back feed unit  19 . The feed finger mechanism includes an elongated head  23  fitted at its front end with a transversely extending finger  24 . A clevis  26 , secured to the rear end of the head, is pivotally attached to a drive arm  27 . A fixed rod  28  is journalled through a median portion of the drive arm  27 . Lastly, an attachment foot  29 , extends from the lower end of arm  27  at an oblique angle. 
     In a retrofit application of the invention herein, all of the components of the feed finger mechanism  22  described to this point may be left unmodified from the prior art cam-driven arrangement. However, as explained above, for the retrofit modification to proceed, the cam follower of the prior art sharpener must be removed from the end of the attachment foot  29 . Also, the remainder of the X and Y axis drive components are removed as well, including the drive motor, the cams, the cam shaft, and the bearings supporting the cam shaft. 
     In lieu of the removed components, a sub-frame  31  is first installed. Sub-frame  31  includes a pair of parallel end plates  32 , spanned by a fixed, lower support shaft  33  and a fixed, upper support shaft  34 . Bolts  36  are used to mount the plates  32  to the inner wall of frame  12 , using the same holes which were previously used to mount the cam shaft bearings. This particular sub-frame construction makes the retrofit modification of the cam-actuated prior art sharpener a relatively straightforward process. However, it may also be desirable in a particular application not to use the sub-frame arrangement, in which case the support shafts could be individually mounted, directly to the frame. It is also apparent that the housing, or base plate, of the X and Y axis drivers discussed below could be extended to reach an opposing wall of the frame, and be pivotally attached thereto. 
     The X Axis And Y Axis Drivers 
     One end of an X axis driver  37  is connected to the upper support shaft  34 . The same end of a Y-axis driver  38  is connected to the lower support shaft  33  (see. FIG.  4 ). Because the X and Y axis drivers are substantially identical in construction, only the structural features of the Y-axis driver  38  will be explained in detail, it being understood that the X-axis driver is structurally identical, except where differences are specifically noted below. In addition, for simplicity and clarity, the same numerals will be used for identifying common components of both drivers, wherever appropriate. 
     Making specific reference now to FIGS. 13-15, the Y-axis driver  38  includes an elongated, driver frame or plate  39 , upon which the major components of the driver are mounted. A pillow block  41 , including a journal bearing  42 , is secured to one end of the plate  39 . In this case, lower shaft  33  is passed through the journal bearing  42  when the Y-axis driver  38  is installed. This mounting arrangement accommodates the slight rotation the drivers make around their support shafts during normal operation. 
     A servo motor  43  is mounted to a motor plate  44 . A first bearing plate  46  is bolted to plate  39  in parallel, spaced relation, from motor plate  44 . A second bearing plate  47  is bolted to the other end of plate  39 . A ball screw shaft  48  extends between plates  46  and  47 , and is mounted for rotation therein by means of respective ball bearings  49  and  51 . A coupler  52  interconnects an output shaft of motor  43  with an adjacent end of shaft  48 , so that rotary motion of the motor is imparted to the shaft. A cover plate  53  maintains the pillow block  41 , the motor plate  44 , and the first bearing plate  46  in secure relation. 
     The ball screw shaft  48  passes through opposing end walls  54  of a reciprocating element, or carrier box, generally identified by the numeral  56 . Secured within one of the end walls is a bushing  57 . A ball screw nut  58  is fixed within the bushing  57 . Balls  59 , held captive within nut  58 , effect a translation of carrier box  56 , in response to rotation of ball screw shaft  48 . A plate (removed for clarity, in FIG. 14) normally seals off one open side of carrier box  56 . A bearing  60  is mounted within a circular cutout in the other side  62  of the carrier box  56 . Bearing  60  is secured therein by means of a snap ring  65 . An internally threaded bushing  61 , is press-fitted within the bore of the bearing  60 . A first bellows  63  extends between an end wall  54  and the first bearing plate  46 , covering a first enclosed section of the threaded ball screw shaft. A second bellows  64  extends between the other end wall  54  and the second bearing plate  47 , covering a second enclosed section of the threaded ball screw shaft. The bellows  63  and  64  expand and contract in response to movement of the carrier box. The combination of the sealed carrier box and the two bellows is effective to protect the ball screw and the ball nut from airborne contaminants such as metal filings and dust. 
     A particular advantage of the foregoing arrangement is that all linear guides or rails for the ball nut or its carrier box have been eliminated. In other words, in a conventional ball screw drive assembly, two or more parallel guides are typically used to support the ball nut or its housing as they are driven by the ball screw. In the present construction, the reciprocating carrier box has no external support, so most of the weight of each driver is supported by side loading forces between the ball, the ball screw, and the ball screw nut. By eliminating the need for linear guides, the X and Y axis drivers of the present invention are considerably less expensive to manufacture. Moreover, the difficulty in isolating the linear guides from the dust and metal fragments generated during the sharpening operation is avoided. 
     A position encoder  66  is mounted on the rear end of servo motor  43 , and is mechanically coupled to the motor&#39;s drive shaft. The position encoder  66  produces pulses which correspond to the extent and the direction of rotation of the servo motor and the connected ball screw shaft. These pulses are carried over output wires  67  to a computerized controller  68  (see, FIG.  16 ). In this manner, the controller is able to determine the precise location, and the rate and direction of movement of the carrier box  56 . 
     Controller  68  has a user interface including an LCD screen display  69  and control buttons  71 ,  72 , and  73 . A rotary speed control knob  74  is also provided on the front panel of controller  68 . Internal components of the controller which are not shown include a microprocessor, programmable memory, servo motor amplifiers, and high current drivers. The drivers control various electric solenoids to be discussed herein. 
     A micro switch  77  is mounted to a median region of the base plate  39  of Y-axis driver  38 . Micro switch  77  includes a lever arm  78 , having a tip element  79  adjacent first bearing plate  46 . When carrier box  56  of the Y-axis driver is driven to a proximate location, adjacent plate  46 , box  56  impinges upon element  79 , and a signal confirming that event is sent to controller  68 , over micro switch wires  81 . The X-axis driver  37  includes a micro switch  82 , located at a distal end of its base plate  39 , adjacent second bearing plate  47 . Switch  82  also includes a lever arm  83  and a tip element  84 . The location of micro switch  82  is such that when carrier box  56  of the X-axis driver is driven to a distal location, adjacent plate  47 , box  56  contacts element  84 , and a signal confirming that event is sent to controller  68 , over micro switch wires  86 . Controller  68  uses this information regarding the location of the two carrier boxes, to place the feed arm mechanism and the grinding head assembly in a “home” position, during an initialization sequence for the sharpener  11  upon power up. 
     Electrical power to actuate the servo motor  43  of the X-axis driver is provided by controller  68  over motor wires  87 , and electrical power to actuate the servo motor  43  of the Y-axis driver is similarly provided over motor wires  88 . Controller  68  supplies current of the proper polarity to drive servo motors  43  either clockwise or counterclockwise, depending upon the phase of the grinding cycle for a tooth, to be described below, resulting in cyclic, reciprocating movement of the respective carrier boxes. 
     The X-axis driver  37  is interconnected to attachment foot  29  by means of a bolt  89  threaded into bearing  61 . As can be seen in FIGS. 10-12, the movement of carrier box  56  toward servomotor  43  causes drive arm  27  to pivot about rod  28 , resulting in a rearward movement of feed finger  24 . And, the movement of carrier box  56  away from the servomotor  43  causes the drive arm to pivot about the rod  28  in an opposite direction, effecting a forward movement of feed finger  24 . The axial orientation of head  23  is slightly offset from that of saw  13 , in a forwardly diverging direction. This offset is such that at the beginning of a grinding cycle, feed finger  24  engages a gullet  91  of a tooth  92  (see, FIG.  17 A). Later in the cycle, after the tooth and the saw have traveled a certain distance, the feed finger disengages from the gullet (see, FIG.  17 H). Then, the servo motor is reversed, and the feed finger is driven rearwardly for a new cycle. 
     While the above-described reciprocating action results in successive indexing of the saw through the sharpener  11 , the back feed unit  19  is also included to drive the other side of the saw in identical fashion. This is necessary so that “bunching” of the saw during the sharpening operation will not occur. Back feed unit includes a back feed arm  93  and a back feed finger  94  (see, FIGS.  1  and  16 ). Back feed arm  93  is pivotally mounted at one end, where it is driven in reciprocating fashion by a double action pneumatic cylinder (not shown). Pneumatic pressure is provided by an air compressor or pump  96 . The pressurized air is diverted to directional ports on the pneumatic cylinder by an electric solenoid (not shown). The electric solenoid is actuated by controller  68  through back feed solenoid wire  97 . When back feed finger  94  is driven forwardly in synchronism with forward action of feed finger  24 , it pushes against the gullet of the saw tooth and drives the saw in a forward direction. When back feed finger  94  is retracted rearwardly, the finger rides up the back of the next tooth, and then drops into its gullet. Then, the cycle is repeated, so that the entire extent of the saw is indexed identically, during the sharpening operation. 
     A portion of the output of pump  96  is also feed to an electric solenoid (not shown) within clamping bar  17 . A solenoid wire  100  from the controller  68  carries control signals for selective actuation of the solenoid during a portion of the grinding cycle. When actuated, the solenoid provides pressurized air to cylinder  20 , effectively clamping saw  13  in place. 
     The Grinding Head Assembly 
     Having explained the mechanisms which effect the indexing of the saw, we can now turn to a description of the saw grinding components. A grinding head assembly  98 , of conventional design, includes an arbor carriage  99 , an arbor  101 , and a grinding wheel  102  (see, FIG.  6 ). The arbor carriage  99  is pivotally mounted at a lower end to the frame, so that the upper end may be raised and lowered about the pivot point. It is during successive raising and lowering of the grinding head assembly  98 , that the grinding wheel  102  shapes each saw tooth. 
     To that end, the grinding head assembly  98  further includes a lifting rod  103  and a drive arm  104 . An upper threaded end of the lifting rod  103  is adjustable in length, and is adapted to support arbor carriage  99 . A lower end of the lifting rod includes a drive head  106 , having an elongated slot  107  therein. Drive arm  104  is pivotally mounted at one end, through a shaft  108 . A drive pin  109  extends outwardly from arm  104 , to engage slot  107  in slidable relation. When arm  104  is raised to pivot about shaft  108 , drive pin  109  also raises, pushing against the upper end of slot  107 . The lifting rod  103  and the arbor carriage  99  are thereby lifted upwardly as well (see, FIG.  7 ). When drive arm  104  is pivoted into a lowered position, gravity draws the carriage and the lifting rod downwardly, under the support and control of the drive pin  109  (see, FIG.  6 ). 
     In the cam-actuated prior art sharpener, the lower end of drive arm  104  was bolted to a cam follower, which in turn, engaged a cam. In making the retrofit modification of this prior art device, the cam follower and cam are removed, as discussed above. In lieu thereof, Y-axis driver  38  is installed onto lower shaft  33 . All that remains to complete the modification, is to attach the lower end of the drive arm  104  to the reciprocating carrier box  56 , by means of bolt  111 . In this manner, the head grinding assembly  98  is now driven upwardly and lowered downwardly by the Y-axis driver  38  of the present invention, so that the grinding wheel  102  can shape the saw teeth in the desired fashion. 
     The Grinding Head Lifting/Disabling Mechanism 
     During adjustments of the sharpener, and to observe the operation of certain of its components in isolation, the need arises to disable the drive of the grinding head assembly. A simple yet effective mechanism has been developed to provide this feature. Making particular reference now to FIGS. 7-9, inclusive, a lifting bar  112  will be noted. Lifting bar  112  is located outside frame  12 , for convenient access by the machine operator. One end of the bar  112  is attached to an outer end of a rotatable shaft  113 . The other end of bar  112  is provided with a grip  114 . A bracket  116 , extending from frame  12 , supports an inner end of shaft  113  for rotation. An intermediate portion of shaft  113  passes through the frame  12 . A lever arm  117  is attached at its proximate end to the inner end of shaft  113 . A lifting plate  118  is hingeably attached at one end to frame  12 . The other end of plate  118  includes a hole through which a lower end of a threaded connecting rod  119  passes. An upper end of connecting rod  119  has a hook, which passes through a hole at the distal end of lever arm  117 . A nut  121  is provided on the threaded portion of rod  119 , so that the relative position of plate  118  can be adjusted for proper operation of the mechanism for disabling the grinding head assembly. 
     During normal operation of the sharpener, lifting bar  112  is in a vertical position, as shown in FIG.  4 . This position of the bar  112  maintains a slight gap between the lower end of the drive head  106  and the upper surface of the lifting plate  118 , even when the grinding head assembly is fully lowered (see, FIG.  6 ). This gap is increased, of course, when the assembly  98  is fully raised (see, FIG.  7 ). When the operator desires to disable the grinding head assembly, so that grinding wheel  102  cannot come into contact with saw  13 , lifting bar  112  is rotated downwardly, as shown in FIG.  8 . This rotates shaft  113  and lever arm  117 , lifting both rod  119  and plate  118 . The upper surface of plate  118  impinges upon the lower end of drive head  106 , raising the grinding head assembly  98  into a disabled position (see, FIG.  9 ). In this disabled position, drive pin  109  continues to move within slot  107 , but the pin does not come into contact with the upper end of the slot. Consequently, all systems of the sharpener continue to operate, except the grinding head assembly does not raise and lower during each grinding cycle. Thus, the saw will still be indexed through the sharpener  11 , allowing observation of the sharpener&#39;s operation without concern for damaging the saw if adjustments are needed. This mechanism also provides a safety feature by raising the grinding wheel away from the grinding area during an emergency. 
     The Computerized Controller And Its Software 
     Before an actual grinding cycle is described, a detailed description of the operational modes and features of the computerized controller  68  will be presented. The main program of the software for the controller  68  is the motion control program. This program controls the movement of the head grinding assembly and the feed finger mechanism by powering and monitoring the positions of the servo motors in the X-axis and Y-axis drivers. The main program also operates electric solenoids which control air pressure directed to the saw clamping mechanism as well as the air pressure driving the back feed unit. The software also employs two prioritized multi-tasking sub-programs. The first priority program is the user input handler, which constantly monitors the push buttons  71 - 73 , for an input from the machine operator. The second priority program is the message handler. This program runs at all times in the background, and displays messages sent to it by either of the programs on display screen  69 . 
     On power up, execution of the main program beings. Initialization of program variables and flags is performed, followed by initiation of the two sub-programs and the display of a “welcome” message. Next, the display shows a HOMING message, the pushbuttons become operative, and the grinder motions commence. 
     Motion Control Algorithms 
     All motions performed by the X and Y axis drivers are closed loop. When a servo control signal is sent from the controller to the servo amplifier and its respective servo motor  43 , the position change due to this signal is reflected in a corresponding signal sent from the position encoder  66  to controller  68 . All motion is controlled using a Proportional, Integral Derivative (“PID”) filtering technique. The terms P. I. and D refer respectively to empirically determined constants used in the Proportional gain function (larger correction for larger error), the Integrator function (improves accuracy), and the Derivative function (damping) of the PID filter. The PID values are chosen so that the desired speed or position is reached as quickly as is physically possible, while concurrently meeting the level of stability and accuracy desired. 
     For any motion, the error between the desired position or speed and the actual position or speed is computed. This error is then digitally processed by the PID filter. The result is the new control signal for the servo motor. This cycle of checking the error and modifying the existing control signal is repeated thousands of time per second, with the ideal goal of having the actual motion match the desired motion. 
     The most basic motion undertaken by the X and Y axis drivers is the rotation of the ball screw shaft  48  at a predetermined rate. This rate, in turn, determines a corresponding known rate of linear motion of the ball screw nut  58 , because the pitch of the ball screw is known. The software of the controller  68  first determines the current speed of rotation by counting pulses from the servo motor encoder for a known period of time. The rotational speed, in revolutions per minute, is then calculated by division, since the number of pulses per revolution is known. The current speed is subtracted from the desired speed, with the resultant error, if any, being filtered by the PID filter. The output of the PID filter is the next value the controller sends to the servo motor amplifier. This reiterative process occurs thousands of times per second as the software attempts to make the error zero. 
     Another type of motion undertaken is the movement of the ball screw nut  58  to a specific position at a desired velocity, or movement to a specific position within a given length of time. This is accomplished essentially in the same manner as just described, for setting the rotational speed of the ball screw. Thus, a rapidly as possible, the error between the desired and the actual velocity or position is calculated, filtered, and fed back to the servo motor amplifier as the next control signal. However, the number of calculations that must be rapidly performed increases, as the number of different target variables, such as location, velocity, or elapsed time, increases. 
     The most computation intensive operation undertaken by the software is the coordinated vector movement. To accomplish this type of movement, additional calculations are performed to coordinate the motion of the two ball screw nuts in the X and Y axis drivers. Since the mechanical mechanism converts the ball screw nut motions into X axis motion of the saw blade and Y axis motion of the grinding wheel, any saw tooth shape desired can be generated. In addition, computations are performed to control the speed of grinding along the selected shape. The two vector movements used to describe all saw tooth shapes are the line and the arc. 
     The Homing Operation 
     The homing operation allows the computerized controller  68  to establish a known mechanical position on each axis from which to base all future moves of the ball screw nuts. Each axis driver has a micro switch mounted at a particular extreme limit of ball screw nut motion, for that driver. On power up, the controller first checks each micro switch for closure, checking for confirmation that the ball screw is already at the extreme position. If the ball screw is not at that position, the controller moves the ball screw nut toward the micro switch until switch closure is detected. 
     Then, the ball screw nut motion is reversed until the encoder  66  on the servo motor outputs an index pulse. The index pulse is a once per revolution pulse that the encoder produces in addition to a 4000 pulses per revolution output signal. Once the controller has received this index pulse, the controller will count all subsequent encoder pulses, using the index pulse as a reference for zero. This homing operation is performed for both axes, and when completed, the state of the sharpener changes to Ready, and this fact is shown on the display screen  69 . The main program halts, and waits for input from the machine operator. 
     The Indexing Operation 
     Before grinding of a variable-pitch or variable-depth saw can commence, the feed finger  24  must be positioned on an index tooth, having a distinctive physical characteristic. This ensures that the grinding operations will be proper for the different pitches or the different gullet depths found on the saw. When this operation is initiated, the controller automatically moves the feed finger  24  to a predetermined index position. Then, the operator examines the saw for the distinctive physical characteristic. One such characteristic might be the longest tooth, or the tooth having the deepest gullet. Having located that tooth, the operator manually moves the saw so that the feed finger is in the gullet of the index tooth. The saw and the controller are now in synchronization, and the grinding operation can begin. It should also be noted than an automatic index tooth detection system, for example, optical or mechanical sensors, may be used to identify the index tooth. In this case, the controller  68  could perform the indexing procedure automatically. 
     The Grinding Operation 
     After an input signal to commence grinding has been received, the software checks which tooth profile the machine operator has set to be current. Then, the software accesses the appropriate subroutine, and grinding of the first tooth begins. The grinding of each tooth is made in accordance with a subroutine, called in the order determined by the main program. Each tooth shape is defined and stored as a sequence of lines and arcs, stored as their start point, end point, and radius. The point 0.0 was previously defined during the homing operation. Consequently, all subsequent vector movements are defined relative to the 0,0 position, and are determined in units corresponding to encoder counts. Each tooth subroutine also contains commands to turn the pneumatic saw clamp cylinder  20  on and off at the appropriate time, and to actuate the remote back feed unit  19  in a manner appropriate for the pitch of the particular tooth. The controller follows the subroutine instructions using the PID filter method described above. 
     The speed of the grinding operation is set by the operator turning the adjusting knob  74  on the front panel of controller  68 . The resistance of a potentiometer, mechanically linked to knob  74 , is read by the user input subprogram. The overall speed of the sharpener is set by this subprogram, and passed to the main program. However, the overall speed is further modified, as necessary, for some portions of the X and Y axis motion sequence, to obtain the desired grinding speed at all points on a given tooth shape. 
     Upon completion of the subroutine for the first tooth, the main program calls for the next and subsequent subroutines to grind additional teeth, in accordance with the desired profiles. Thus, it is possible to grind each of the teeth identically, or to grind sets of teeth in particular and different ways in a sequence, which may be repeated for the entire extent of the saw. The controller also has a retip mode, which directs each tooth subroutine to use slower grinding speeds over the tip of each tooth. 
     Illustration Of A Grinding Cycle For One Tooth 
     Turning now to FIGS. 17A through 17L, inclusive, a complete grinding cycle for one tooth is shown. In FIG. 17A, the cycle is initiated with grinding wheel  102  beginning its angular descent from a fully raised position. At this point in the cycle, the feed finger  24  is stationary. The controller  68  sends a signal to an electric solenoid, providing pressurized air to pneumatic cylinder  20 . This effectively clamps saw  13  securely in place, in preparation for the next step. As grinding wheel  102  continues to descend, it enters the upper portion of the gullet  91  (see, FIG.  17 B). Because the saw is clamped, it will not be shifted by any forces created during this phase of the grinding operation. As shown in FIG. 17C, when the grinding wheel reaches the bottom of the gullet, the feed finger  24  begins applying X axis forces to the saw. Concurrently, the controller deactivates the solenoid, and the cylinder releases its clamping force on the saw. The grinding wheel also begins an upward movement, as the back side of the tooth  92  is formed. In FIGS. 17D-17G, the grinding operation continues in identical fashion, until the tip of the tooth  92  is reached. It can be seen that in FIG. 17H, the grinding wheel  102  has now cleared the tip of the tooth, as feed finger  24  reaches the greatest extent of its forward movement. In FIG. 17I, the feed finger  24  has begun its rearward movement, which continues in FIGS. 17J and 17K. Lastly, to complete the cycle, the tooth is urged slightly forwardly in FIG. 17L, so it is in contingent relation with the upper portion of the gullet, as the grinding wheel  102  begins is next descent. 
     As described above, the saw sharpener  11  of the present invention may be used advantageously both as a retrofit modification to prior art saw sharpeners, and in newly constructed saw sharpeners. It is evident that the specific construction of the X-axis and Y-axis drivers may need to be modified considerably for the particular application at hand. For example, the direct drive between the servo motor shaft and the ball screw shaft could be modified through the use of belts and gears so the axes of the two components could be offset in parallel relation, or perpendicular. Such an arrangement may be necessary or desirable to shorten the overall length of the drivers, or to provide different configurations for the drivers. In lieu of the preferred servo motor, an electro-hydraulic drive may be substituted. Such a drive would include a reversible hydraulic drive motor, and electrical systems for determining the direction and amount of hydraulic flow and the position of the carrier box. 
     It is also evident that the drivers may be relocated, in a newly constructed sharpener. Rather than being below, housed within the frame of the sharpener, one or more of the drivers may be located above or on the side of the frame. It is also contemplated that the teachings of the present invention may be applied with similar improved results to sharpeners for other types of saws, such as gang saw sharpeners or circular saw sharpeners. 
     It will be appreciated, then, that we have disclosed herein an computer-controlled band saw sharpener, which has applications both as a retrofit to improve the performance of prior art sharpeners, and as part of a newly constructed sharpener, and that the teachings may be applied for use with other sharpeners for other types of saws.