Knife sharpener, particularly for a microtome
United States Patent 1 1191 1111 3,874,120 Dalton, deceased et al. 1 Apr. 1, 1975 KNIFE SHARPENER, PARTICULARLY FOR 1,953.559 4/1934 Hughes 51/159 x A MICROTOME 2,287,317 6/1942 McD0nn.ell.... 51/153 2,391,260 12/1945 Mlller 51/59 R 1 Inventors: E a fi g as a of 2,431,624 11/1947 Smith 51/159 a on 1e .J.,by Ruth Dalton, executrixt Frederick Primary Evaminer-Harold D Whitehead gt f lll y i; John Attorney, Agent, or FirmSeidel, Gonda &amp;  
  ee, en mtown, a. Goldhammer [73] Assignee: Arthur H. Thomas, Company,  
  Philadelphia, Pa. [57] ABSTRACT [22] Filed: 6, 1973 A knife sharpener, particularly for a microtome knife, holds the kmfe agamst a reciprocating surface at a 1 1 pp Flo-13309127 first angular position relative to the direction of displacement of the surface, shifts the knife to another 152 US. Cl 51/59 R, 51/153, 51/159 angular P051901 relative/0 displacemen. [51] Int. Cl B24b 3/48 Surface after a predetermined amount of rehmve kmfe [58] Field of Search 5159 R 62 67, 153, 159 and surface displacement, lifts and turns the knife SW63 over to sharpen the opposite side of the edge 1n re sponse to a further measured amount of displacement [56] References Cited :ftfhe Sl]1rf 3C,tagt]tlln shifts the angular potsitiont offthe me re a we 0 e sur ace in response 0 ye a ur- UNITED STATES PATENTS ther relative displacement of the surface, then lifts and l 2 g/: 5 turns the knife over, and the entire cycle is repeated.  
  i1 erm l,878,791 9/1932 Magnano 51/159 18 Claims, 13 Drawing Figures /6 H I f 54 I O 1 86 I z0. /30 n n: &#34;j&#39;. .11. i g  
 / I. I 5 5 B5 25 7 I :r I T I PATENTED H975 3.874.120  
 SHEET 2 0F 5 FIG. 3  
  0.4. 246 RECTIFIER 204 2 06 246 W i I as? ATEHTED 1 !9175.  
 KNIFE SHARPENER, PARTICULARLY FOR A MICROTOME This invention relates to a knife sharpener. particularly for a microtome.  
  It is desirable to put an extremely fine edge on certain types of cutting devices, such as the microtome knife. Originally, knives were sharpened by hand. Hand sharpening involves holding the knife against the abrasive surface at a predetermined angle and appying sufficient pressure to hold the edge evenly against the surface. The amount of pressure must be gauged in accordance with the hardness of the knife and the fineness of the grinding surface. At best. hand sharpening of a microtome knife is an empirical process and the more experienced the operator. the better the edge.  
  Not unexpectedly. machines have been developed to assist in producing a quality edge on a knife such as a microtome. One such machine is the Model 7203 Thomas Fanz Microtome Knife Sharpener sold by Arthur H. Thomas Company, Philadelphia Pa. Such a machine is shown at page 769 of the Arthur H. Thomas Company catalog entitled Scientific Apparatus and Reagents, published in I968. The Thomas Fanz Microtome Knife Sharpener includes a reversible glass disc that is turned mechanically at a constant speed. A knife holder is mounted on a counterbalanced rod that is adjustable for inclination. A motor drive sweeps the knife across the diameter of the plate and an attachment automatically lifts and reverses the knife by means of a cam shaft, gears, lever and connecting cable.  
  Although hand sharpening has many drawbacks as explained above, the basic approach is still the best for providing a good edge on a microtome knife. In particular, the best technique is an angular. but straight line relative motion between the knife edge and the abrasive surface as shown in FIG. 3. The straight line mo tion between the knife edge and the abrasive surface produces straight microscopic scratches on the surface which define the edge ofthe knife. By holding the knife such that its edge is at an angle other than normal (90) to the direction of motion, the microscopic scratches are also at an angle other than normal. By shifting the knife so that the scratches are first at one angle and then at a substantially equal but opposite angle with respect to the normal. the microscopic scratches essentially form a cross hatching on the surface of the knife as illustrated hereinafter. The microscopic scratches are necessary to a good cutting action for a microtome and are normally generated by the hand action. The scratches on opposite sides of the surfaces that define the edge meet at the edge. As such they form desirable microscopic nicks in the edge; that is they form a saw or serrated edge which is required for good cutting.  
  The best edge is acquired if the knife is not permitted to shift, roll or change position during each stroke. Moreover, a sufficient and constant pressure should be applied. This is extremely difficult, if not impossible to accomplish by hand techniques. The present invention seeks to improve upon existing knife sharpening machines by fully duplicating the hand sharpening technique without incurring the difficulties normally attributed to such technique. In particular, the present machine seeks to fully duplicate the hand sharpening technique while eliminating the inherent deficiencies incurred when actually sharpening a knife by hand.  
  In particular, the present invention provides a machine wherein there is straight line reciprocating relative motion between the knife edge and the abrasive surface. Either the knife can move relative to a fixed abrasive surface, or the surface can move relative to a fixed knife. In operation, the machine makes a predetermined number of strokes on a first side of the knife with the knife being in a first angular position. Thereafter. the machine shifts the knife to a second angular position and an equal number of strokes on the same side of the knife are made. Next, the machine raises and turns the knife over. Thereafter, the same number of predetermined strokes in both the first and second angular position of the knife are made so that the knife is equally stroked on both sides. Finally. the knife is again turned over to the first side and the process is repeated.  
  For the purpose of illustrating the invention. there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.  
  FIG. 1 is a side elevational view of a knife sharpener in accordance with the present invention.  
  FIG. 2 is a sectional view of the knife sharpener taken along the line 2-2 of FIG. 1.  
  FIG. 3 is a sectional view ofthe knife sharpener taken along the line 3-3 in FIG. 1.  
 FIG. 4 is a bottom plan view of the knife sharpener.  
  FIG. 5 is an enlarged partial perspective view of the knife sharpener illustrating the knife turnover mechanism and knife angle shift mechanism.  
  FIG. 6 is an exploded view of a portion of the knife holder.  
  FIG. 7 is a partial perspective view ofa latch mechanism for the knife turnover mechanism.  
  FIG. 8 is a sectional view of the latch mechanism illustrated in FIG. 7 taken along the line 88.  
  FIG. 9 is an enlarged sectional view of the latch mechanism in FIG. 7 taken along the line 9-9.  
  FIG. 10 is a schematic circuit diagram of the drive. timing and solenoid control.  
 FIG. 11 is a perspective view of a microtome knife.  
  FIG. 1-2 is an enlarged plan view of the cutting edge of a microtome knive.  
  FIG. 13 is an enlarged partial sectional view of a microtome knife.  
  The machine described below provides a knife edge having a requisite sharpness on a repeatable basis. This is accomplished by duplicating the preferred knife stroking method using a machine that eliminates variations through human intervention. The preferred stroking method has been described above.  
  It has also been pointed out that the preferred knife stroking method generates microscopic parallel scratches in the facets or sides of the knife that define the cutting edge of the knife. Depending upon the quality of the cutting edge, these scratches can be reduced, but not eliminated. Indeed, it is not desirable to eliminate the scratches.  
  Referring to FIG. 11, there is shown a microtome knife 20 having facets 21 and 23 which define the cutting edge. Shown on the facets are a series of scratches 25 and 27 at equal but opposite angles with respect to a line drawn normal to the edge of the knife 20. These scratches would not ordinarily be observable by the naked eye. They are shown merely for the purpose of illustrating the desired result. The arrow shown at the side of the knife 20 shows its direction of motion when sectioning a sample. As best shown in FIGS. 12 and 13,  
 it is most desirable that the scratches intersect at the knife edge. Of course, such scratches are shown greatly exaggerated in size for purposes of illustration. Note that not only do opposte scratches meet or intersect to define serrations, but also the angled scratches on eact facet 21 or 23 also meet.  
  Referring now to the drawings in detail, wherein like numerals like elements, there is shown in FIG. 1 a knife sharpener, particularly for a microtome, designated generally as 10.  
  The knife sharpener is contained within a housing 12 that is divided into three compartments. The knife compartment 14 contains the sharpening plate 16 mounted on a reciprocable table 18. The compartment also contains knife 20 to be sharpened. The knife 20 illustrated in the drawings is a microtome blade. However, it should be understood that the microtome is illustrated herein as exemplary since other knives can equally well be sharpened using the knife sharpener 10. The compartment 14 also contains a portion of the knife support mechanism 22 and the knife angle shift mechaism 24, both of which are described in more detail hereinafter. The entire compartment 14 is enclosed by a hood 26, preferably made of a clear plastic so that the sharpening procedure can be readily observed. The hood 26 is hingedly fixed to the housing 12 by means of the hinge 28 as shown in FIG. 1. This permits access to the compartment 14.  
  Immediately behind compartment 14 is the motor compartment 30 (FIGS. 2 and 3) separated from the compartment 14 by the wall 32. Motor compartment 30 also contains portions of the knife support mechanism 22 and portions of the knife angle shift mechanism 24 which extend through the wall 32. Also located within motor compartment 30 are control microswitches and timing cams described hereinafter.  
  Immediately adjacent the motor compartment 30 is the control section which includes adjustment knob 36 for the timer 38 as well as the adjustment knob 40 for controlling the angle of attach for the blade 20 by adjusting the height of the knife support mechanism.  
  Bottom wall 42 (FIG. 2) separates the compartments 14 and 30 from the bottom compartment 44 which houses the driving mechanisms as hereinafter described. The bottom compartment 44 is closed by a lower bottom wall 46 to which appropriate resilient glides 48 are affixed.  
  The sharpening plate 16 may be a glass plate fixed on the reciprocable table 18 by conventional means, such as the abuting blocks 50, 52 and 54. The glass plate 16 is used for supporting a dry grinding paper. However, the plate can be modified to support a wet grinding compound if desired.  
  As shown in FIG. 4, slide blocks 56, 58 and 60 are affixed to the bottom of reciprocable table 18 and extend through slotted openings in bottom wall 42. Flanges 62 and 64 are riveted or otherwise affixed to bottom wall 42 so as to depend therefrom and provide support for retaining the slide rods 66 and 68. Slide rods 66 and 68 extend through holes in slide blocks 56, 58 and 60 and thereby provide means for supporting and guiding the table 18 as it is reciprocated. The reciprocal motion of the table 18 is in a straight line as defined by the guide rods 66 and 68.  
  The table 18 is driven in its reciprocable motion by crank 70 which is pivotally connected to the connecting rod 72. Rod 72 is also pivotally connected to the table 18 by the pin 74.Crank is directly fixed to the gear reduced output 76 of the drive motor 78 mounted within motor compartment 30.  
  As shown in FIG. 5, the knife 20 is wedge shaped. It. therefore can be retained within a similar wedge shaped slot 80 formed in the knife holder 82. The knife 20 is-wedged into position within the slot 80 by the thumbscrews 84 and 86 which extend through threaded holes in the holder 82 and bear against the back surface of the knife 20. Of course, other knife holders, as required, may be used.  
  The knife holder 82 is connected to the knife turnover rod 88 by the pin 90 extending through the aligned holes 96 and 92. Holes 96 are formed in the cam flanges and 102 which extend from the back of the knife holder 82. Hole 92 is formed in the flattened terminus portion 94 of the turnover rod 88. Terminus portion 94 fits snugly between the cam flanges 100 and 102. Pin 90 is held in position within the holes 92 and 96 by the screw 98.  
  The cam flanges 100 and 102 each define cam surfaces 104 and 106 at a predetermined angle with respect to the axis of turnover rod 88. Sleeve 108 is slidably mounted on turnover rod 88 and normally biased toward the cam surfaces 104 and 106 by spring 110. Spring 110 is held in position by collar 112 which is fixed to the turnover rod 88 by the set screw 114.  
  The force of sleeve 108 against either surfaces 104 or surfaces 106 of the cam flanges 100 and 102 holds the knife holder 82 and hence the knife 20 in position at an angle with respect to the axis of turnover rod 88. As used herein, the angle may be defined as the angle between an imaginary line normal to the edge of the knife 20 and the axis of the turnover rod 88. It should be understood, however, that the foregoing is for the purpose of defining the angle and therefore should not be considered as limiting since other methods of defining the position of the knife 20 may be appropriate. Since the turnover rod 88 is mounted to extend parallel to the direction of displacement of the table 18, it therefore follows that an imaginary line normal to the edge of the knife 20 is at the same angle with respect to the direction of the displacement of the table 18. In view of the foregoing, it follows that the cutting edge of the knife 20 is at a complementary angle with respect to the axis of the rod 88. In the embodiment illustrated herein, the angle is chosen to be 30. Such angle is defined by the angle of the cam surfaces 104 and 106 (FIG. 6). It therefore follows that the cutting edge of the knife 20 is at a complement of the angle, hence 60, with respect to the direction of displacement of table 18. It is in nowise intended that the angle be limited to what is specified herein. Such angles are exemplary only, and other angles may be chosen as desired.  
  The surfaces 106 and 104 are at equal angles with respect to the axis of turnover rod 88. Moreover, such surfaces combined with the force applied by the sliding sleeve 108 define an over center mechanism. The over center mechanism thus holds the knife holder 82 and hence the knife 20 in one of two angular positions with respect to the axis of turnover rod 88. Either position defines the same angle with respect to that axis, which angle may be 30? as defined above.  
  The turnover rod 88 is supported on a lever 116, FIG. 5, which in turn is pivotally connected to the wall 118 by means of a shaft and appropriate bushing 41 which extends through a slot 120. Shaft-bushing 41 permits lever 116 to freely rotate. ,The knob 40 and shaftbushing 41 provide a means for raising and lowering the level 116 with respect to the bottom wall 42 and lock the same in position. This changes the vertical angle of the axis of rod 88 with respect to the horizontal and hence provides a means to change the angle at which the knife rests against the plate 16. The particular angle selected depends upon the use to which the knife is to be put. By way of example, but not limitation, the length of slot 120 is such that the angle between the axis of rod 88 and the horizontal can be changed from 14 to 20. Of course, other ranges of angles can be used. This range of angles is sufficient for sharpening microtome knives. Other angular selections would be built into the machine for other types of knives.  
  The turnover rod 88 is supported on the lever 116 by appropriate bushings 122 (FIG. 5) extending through holes in the flanges 124 and 126 which depend from the lever 116. Turnover rod 88 terminates in a friction bearing 128 which prevents the rod from freely rotating except upon the application ofa turning torque as hereinafter described.  
  As previously indicated, the lever 116 pivots on the shaft-bushing 41. In doing so, it lifts the rod and hence knife 20 upward from the sharpening plate 16. This upward movement of the turnover rod 88 is used to cause the knife 20 to rotate 180. Such rotation turns over the knife 20 so that the opposite side can be brought to rest against the plate 16 when the rod 88 is returned to its lowered position.  
  Wheel 130 is connected to rod 88 through one-way clutch 132. As is conventional, one-way clutch 132 permits the wheel 130 to rotate freely in one direction with respect to rod 88 but blocks rotation in the opposite direction. In the embodiment shown herein, wheel 130 is free to move only in the counterclockwise direction.  
  The outer surface of wheel 130 is provided with a friction tire 134 preferably made of rubber or a resilient plastic. Friction tire 134 bears against pad 136 mounted on plate 138 which is affixed to wall 32. Pad 136 is made of rubber or a plastic material which has a sufficiently high friction to cause wheel 130 to rotate when friction tire 134 bears against it as turnover rod 88 is raised and lowered. Upon raising rod 88, the wheel 130 tends to rotate in a clockwise direction and hence drives the rod in the same direction through clutch 132. As the rod 88 is lowered, the wheel 130 turns freely and hence does not rotate the rod. The friction bearing 128 holds the rod in position as it lowers knife 20 toward sharpening plate 16.  
  The length of pad 136 is sufficient to rotate wheel 130 and hence the rod 88 180. Therefore, the knife 20 is turned over by the upward movement of the rod 88.  
  To be certain that the rod 88 rotates only 180, a stop 140 is affixed thereto. Stop 140 is provided with diametrically spaced shoulders 142 and 144. Shoulders 142 and 144 are positioned to engage the lip 146 fixed to wall 118 when the rod has turned a full 180. Once shoulder 142 or 144 engages lip 146, the wheel 130 merely slips along pad 136. This prevents any over rotation of the rod 88 and hence possible damage to the corners of the knife 20 when it is lowered to the plate. Thus, the rod 88 rotates exactly 180 each time it is lifted away from the plate 16 by turning the lever 116.  
  As best shown in FIG. 5, the mechanism for shifting the knife holder 82 from one angular position to another angular position as determined by the over center mechanism includes the pusher 148. Pusher 148 is slidably held adjacent wall 118 by a T-shaped block 150 that extends through slot 152. Block 150 is pivotally held to wall 118 by means of pivot pin 154. Thus, the pusher 148 is free to slide relative to block 150 and also is free to pivot upwardly and downwardly with block 150 on pin 154.  
  Pusher head 156 is fixed to the forward end of pusher 148 by resilient means such as a spring (not shown). The function of pusher 148 is to move forwardly to engage the knife holder 82 and cause it to shift from one angle to the other angle as determined by the over center mechanism. Pusher head 156 is resiliently mounted on pusher 148 so as to absorb any excess toward travel of the pusher 148. Such excess travel is designed in to accommodate the angular position of the mechanisms.  
  Pusher 148 is selectively caused to move forward to engage knife holder 82 by means of the bifurcated pusher block 158 fixed to reciprocable table 18. Pusher 148 engages the pusher block 1158 by means of slot 160. The pusher 148 is normally held out of engagement with pusher block 158 by the actuator 162 of solenoid 164. However, upon retraction of the actuator 162, the pusher 148 pivots downwardly by its own weight until it rests upon the bigth of the bifurcated block 158. When the table 18 reciprocates toward the wall 32, it will reach a position wherein the slot 160 overlies the bight of block 158 and the pusher 148 will fall into engagement therewith. As the table 18 reciprocates away from the wall 32, it will pull the pusher 148 with it. The length of pusher 148 is more than sufficient for the pusher head 156 to engage the knife holder 82 and cause it to rotate about pin 90 until the over center mechanism causes it to assume the other angle. When the table 18 reciprocates toward the wall 32, the actuator 162 is again extended. The block 158 carries the pusher 148 with it. As a result, the pusher 148 engages the actuator 162 along inclined cam 166 and thereby raises the pusher 148 sufficiently to disengage the front of slot 160 from the block 158. Thus, pusher 148 remains out of engagement with the block 158 until the actuator 162 is again withdrawin by the solenoid 164. Note the difference in length between the front wall and the back wall of slot 160. The back wall is longer. The length of the back wall is sufficient to be engaged by block 158 on all movements toward wall 32, if the push has first been moved forward.  
  As previously indicated, the knife 20 is turned over by pivoting lever 116. The lever 116 is intermittently pivoted by means of the apparatus described below.  
  As shown in FIG. 4, crank 168 is connected to the shaft 170 which is driven by gear 172. Connecting rod 174 is pivotally connected to the crank 168 at one end and to the slide block 176 at the other end. As best shown in FIGS. 7 and 8, slide block 176 is slidingly mounted on slide rod 178 and is maintained in position by means of guide 180 which moves in guide slot 182 formed in bottom wall 42. Guide 180 is fixed to slide block 176 by pin 184. It should be apparent from the foregoing that slide block 176 therefore reciprocates along guide rod 178 at a rate determined by the angular velocity of crank 168.  
  As shown in FIGS. 4 and 5, the lever 116 includes a tab which extends through an opening in bottom wall 44 and it is connected to channel bar 188 of the knife turnover latch mechanism 186. Channel bar 188 extends from the pivotal connection with lever 116 under the table 18 where it is slidingly connected to slide block 176. Slide block 176 includes a laterally protruding extension 190, as best shown in FIGS. 7 and 8. Extension 190 includes a portion that protrudes through an elongated slot 192 formed in the side ofchannel bar 188. Thus, reciprocating slide block 176 and its extension 190 provide support for channel bar 188 and can reciprocate relative to the channel bar.  
  From the foregoing, it should be apparent that by latching or otherwise connecting channel bar 188 to the reciprocating slide block 176, the channel bar 188 will be caused to move with the slide block and hence will pivot the lever 116 about shaft 41. As an end result, the blade 20 is turned over.  
  Channel bar 188 is latched to slide block 176 by means of primary pivot arm 194 and spring biased secondary pivot arm 196. As shown in FIG. 9, primary pivot arm 194 is pivotably connected to channel bar 188 by pivot arm 198. Secondary pivot arm 196 is pivotally connected to primary pivot arm 194 by means of hinge 200. Spring 202 normally biases secondary pivot arm 196 away from primary arm 194. Spring 203, normally biases the primary arm 194 outward from the channel bar 188. A limit stop is provided in hinge 200 so that secondary pivot arm 196 always maintains an acute angle with respect to primary pivot arm 194 even when fully biased outwardly.  
  Solenoid 204 is located to the side of primary pivot arm 194 and includes actuator 206. Solenoid 204 is mounted on plate 62. Upon energization, actuator 206 extends outwardly from solenoid 204 and pivots primary pivot arm 194 toward channel bar 188 against the bias of springs 202 and 203. This action latches channel bar 188 to slide block extension 190 in the following manner. The slide block extension 190 normally misses secondary pivot arm 196 as it reeiprocates within slot 192. However. upon movement toward channel bar 188 under the force applied by actuator 206, the extension 190 now engages the end of secondary pivot arm 196 and pushes it forward. The movement of pivot arm 196 carries with it the channel 188 and hence pivots lever 116. The weight of the knife 20 and the knife support mechanism 22 is sufficient to maintain secondary pivot arm 196 in contact with extension 190 throughout the cycle. Thus, the knife is not dropped back onto sharpening plate 16. Rather, it is lowered. Thereafter, actuator 206 is retracted and the turnover mechanism is not operated until it is again extended.  
  It should be obvious from the foregoing that there has been described a mechanism for sharpening the knife 20 at two different angles on one side, turning over the knife and then sharpening it on the other side at the same two angles. It should also be apparent that the sequence of the foregoing is determined by the actuation of solenoids 164 and 204. The timing sequence for the actuation of solenoids 164 and 204 is as follows.  
  All motive power for the knife sharpener 10 is derived from motor 78 (FIG. 3) mounted within motor compartment 30. The output of motor 78 drives shaft 76 (FIG. 4) to which is affixed gear 208. Gear 208 meshes with idler gear 210 which is in turn meshed with gear 172. Gear 212 is mounted in common with gear 172 on shaft 170. Gear 212 meshes with gear 214 mounted on shaft 216.  
  Gears 172 and 210 have the same diameter and the same number of teeth about their periphery. Hence, they rotate at the same but opposite angular velocities. Since gears 172 and 210 are larger in diameter than gear 208, they rotate at a slower angular velocity than gear 208. Accordingly, crank 168 turns at a slower angular velocity than crank 70. As a result, slide block 176 has a slower rate of reciprocation than table 18.  
  Gear 212 has a smaller diameter than gear 172. Gear 214 has a larger diameter than gear 212. Accordingly, gear 214, which is driven by gear 212, rotates at a slower angular velocity than gear 172.  
  Timing cam 218 is mounted on shaft 170 together with gears 172 and 212, as shown in FIG. 2. Hence, timing cam 218 rotates at the same angular velocity as shaft 170 and gears 172 and 212.  
  Timing cam 220 is mounted on shaft 216 in common with gear 214 and hence rotates at the same angular velocity as gear 214.  
  As best shown in FIG. 3, timing cam 218 controls the actuator 222 for knife shift microswitch 224. Timing cam 220 controls the actuator 226 for turnover microswitch 228 as well as the actuator 230 for power microswitch 232.  
  The function of the foregoing is best understood by reference to the schematic diagram shown in FIG. 10.  
  As shown in FIG. 10, power is drived from a conventional source of AC voltage and applied to the motor of timer 38 by closing timer switch 236. Timer 38 is conventional and therefore need not be described in detail. It is sufficient to state that upon rotation of knob 36, switch 236 is held closed until the timer 38 times out.  
  Power microswitch 232 functions as a supplemental power switch to bypass timer switch 236 through conductor 234. The function of this bypass is to maintain the operation of motor 78 until knife 20 has been lowered to sharpening plate 16. If timer 38 were allowed to merely time out, the possibility exists that this may occur while knife 20 is in a raised position. This is an extremely dangerous condition. To prevent this, power microswitch 232 is maintained in a closed (D) condition except at one position of the timing cam 220 when it moves to the open (C) condition by the engagement of actuator 230 (FIG. 3) with timing pin 254. Power microswitch 232 is illustrated in the open (C) condition in FIG. 10. Pin 254 is positioned on cam 220 so that microswitch 232 can be in the (C) condition only when knife 20 is on plate 16.  
  As illustrated in FIG. 10, motor 78 is connected to one side of the alternating current voltage source. The other terminal of motor 78 is connected to switch 232 and to recitifier 244. Rectifier 244 is connected across the source of AC voltage through switch 232. The negative side of rectifier 244 is connected to solenoid 204. The opposite terminal of solenoid 204 is connected through conductor 246 to terminal (A) of microswitch 228. Hence, microswitch 228 controls the actuation of solenoid 204, and hence the latch mechanism 186. As illustrated in FIG. 10, the circuit for energizing solenoid 204 is open.  
  The negative side of rectifier 244 is also connected to one terminal of solenoid 164. The opposite terminal of solenoid 164 is connected through conductor 248 to terminal (E) of microswitch 224. Microswitch 224, as shown, is connected through terminal (E) and conductor 250 to terminal (B) of microswitch 228. Thus, solenoid 164 is actuated upon movement of microswitch 228 from the position (A) to its opposite connective position (B) when microswitch 224 is in position (E).  
  As previously indicated. the timing sequence of the knife sharpener 10 is such that the knife is to be sharpened on one side at a first angle. sharpened on the same side at a second angle. turned over. and sharpened on the second side at the second angle. and then sharpened on the second side at the first angle. The knife is then turned over again and the process repeated. This process continues for a preset period oftime as determined by the timer 38. The sequence of the foregoing is determined by the timing cams 218 and 220. The movement of the timing cams is in turn determined by the angular velocity of the various gears within the gear train shown in FIG. 4.  
  By way of example. but not limitation. it may be desirable to reciprocate the table forward and back three times before shifting the angle of the knife 20. Accordingly. the gear ratio between gears 208 and 172 is selected at 3-to-l with idler gear 210 being at a ratio of l-l with gear 172. Thus. timing cam 218 is turning at an angular velocity which is one-third as fast as the an gular velocity of gear 208 and crank 70. Stated otherwise. timing cam 218 will make one revolution for each three revolutions of gear 208.  
  At the same time. the ratio between gears 212 and 214 is also 3-to- 1. Accordingly. gear 214 will make one revolution for each three revolutions of the gear 212 and one revolution for each nine revolutions of gear 208.  
  From the foregoing. it can be seen that the knife sharpener 10 operates follows:  
 Gear 208 makes three revolutions. thereby reciprocating plate 18 against knife 20 three times. (A complete cycle of reciprocation includes a forward and back stroke of plate 18 relative to wall 32).  
  Timing cam 218 turns one complete revolution. thereby bringing actuator 222 into notch 252. Switch 224 thereby shifts from the (F) position in which it has been held by cam 218 to the (E) position.  
  At the same time, timing cam 220 will have made one-third of a revolution. Actuator 226 follows the high portion of the cam 220 and holds switch 229 in the B position. Thus. a circuit is completed through switch 228 to the positive side of the rectifier 244, thereby energizing solenoid 164 and solenoid actuator 162. This causes pusher 148 to move forward in the manner described above and shift knife holder 82 to the angular position illustrated in FIG. 3.  
  Gear 208 continues to rotate and makes three additional revolutions which result in three additional reciprocations of table 18, this time with the knife 20 in its second angular position.  
  As the completion of the second three revolutions, switch 224 again moves from the (F) position to the (E) position, but it operates without effect since actuator 226 now follows the low or short radius portion of cam 220. Therefore, switch 228 is in the (A) position inhibiting switch 224.  
  Cam 220 having now made an additional one-third revolution and actuator 226 is now moved onto the low or short radius portion of the periphery of cam 220. The short radius portion extends over 120 of the periphery 0f cam 220. This moves switch 228 to the (A) position. This in turn completes a circuit through solenoid 204 and hence actuates the lift and turnover mechanism.  
  During the lift and turnover operation, gear 208 makes an additional three revolutions resulting in additional three reciprocations of&#39;the table 18. At the same time, cam 220 rotates an additional one-third revolution bringing it back to the position illustrated in FIG. 3; that is. with the actuator 226 on the high or large radius portion ofthe cam. During this sequence, actuator 222 falls into notch 252 on cam 218. But this does not operate the pusher 148 because switch 224 cannot complete the circuit through switch 228. This can only be done when switch 228 is in the (B) position.  
  As previously indicated, switch 232 prevents the knife sharpener 10 from timing out via timer 28 with knife 20 in the raised position. This is prevented by positioning switch 232 such that its actuator 230 is in position (D) except when lifted by timing pin 254 to position (C). This allows switch 232 to bypass timer switch 236, thereby maintaining power to motor 78 to complete the turnover operation. Upon completion of the turnover operation, actuator 230 will be moved to its (C) position by timing pin 254, thereby turning off the motor.  
  From the foregoing example. it can be seen that the knife 20 is sharpened by three complete reciprocating strokes on a first side at a first angle. by three additional complete reciprocating strokes at a second angle. and then the knife is turned over. The knife is turned over while the table 18 completes three additional reciprocating strokes. Thereafter. the knife is sharpened by three complete reciprocating strokes at the first angle. and then shifted back to the second angle and sharpened by three additional reciprocating strokes. Thereafter. the knife is again turned over to the first side and the process repeated.  
  It should be noted that when the knife is turned over from the position illustrated in FIG. 3, the side of the knife holder 82 on which thumbscrew 86 is located is now adjacent to the pusher 148. Thus. the pusher 148 need be only on one side of the turnover rod 88.  
  Although the operation of the knife sharpener 10 has been described in terms of a i3-to-l ratio between the various cycles, it should be understood that other ratios and timing cycles can be incorporated into the machine.  
  The present invention has been exemplified by describing the table 18 as moving with respect to the knife 20. It should be understood, however. that the invention is not limited to reciprocation of the table 18. It is within the scope of the present invention to reciprocate the knife 20 relative to a fixed table or to otherwise create the necessary relative motion. For example, the table could be moved in a crosswise fashion with respect to the knife.  
  The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and. accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.  
 1 claim:  
  1. in a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said knife and said surface relative to each other, the direction of relative displacement between said knife and surface being at an oblique angle with respect to the cutting edge of said knife, means for automatically changing said angle from a first predetermined angular position to a second predetermined angular position in response to a measured amount of relative displacement of said knife and surface, and means to turn the knife over to sharpen an opposite side of the edge in response to a further measured amount of relative displacement of said knife and said surface.  
  2. In a knife sharpener. means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface and said knife relative to each other, means for automatically shifting the angular position of the knife relative to the direction of displacement from a first predetermined angular position to another predetermined angular position in response to a measured amount of displacement of the surface, and means to turn the knife over to sharpen an opposite side of the edge in response to a further measured amount of displacement of said surface.  
  3. In a knife sharpener in accordance with claim 2 wherein said means for shifting the angular position of the knife relative to the direction of displacement of the surface comprises means to push the knife from one angular position to another angular position.  
  4. In a knife sharpener in accordance with claim 3 wherein said means for shifting the angular position of the knife includes means to bring said means to push the knife into operative engagement with the knife in response to a measured amount of displacement of the surface.  
  5. In a knife sharpener in accordance with claim 1 wherein said means to turn the knife over to sharpen an opposite side of the edge of a knife includes means for lifting and rotating the means for holding the knife.  
  6. In a knife sharpener. means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface and said knife in straight line linear motion relative to each othersmeans for automatically shifting the angular position of the knife relative to the direction oflinear displacement from a first predetermined angular position to a second predetermined angular position in response to a predetermined amount of displacement of the surface and knife relative to each other, and means to turn the knife over to sharpen the opposite side of the edge in response to a further predetermined amount of displacement of the knife and surface relative to each other.  
  7. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface in straight line linear reciprocable motion relative to said knife, means for automatically shifting the angular position of the knife relative to the direction of linear displacement of said surface from a first predetermined angular position to a second predetermined angular position in response to a predetermined number of reciprocations of said surface, and means to turn the knife over to sharpen an opposite side of the edge in response to a further predetermined number of reciprocations of said surface.  
  8. In a knife sharpener in accordance with claim 7 wherein said means for shifting the angular position of the knife relative to the direction of displacement of the surface comprises means to push the means for holding the knife from one angular position to a second angular position.  
  9. In a knife sharpener in accordance with claim 8 wherein said means for pushing the means for holding the knife includes a pusher and means for selectively engaging said pusher to the reciprocating abrasive surface to bring said pusher into engagement with said means for holding said knife.  
  10. in a knife sharpener in accordance with claim 7 wherein said means to turn the knife over includes means for lifting and rotating the means for holding the knife through 11. ln a knife sharpener in accordance with claim 10 wherein said means for lifting and rotating the knife holder includes means for selectively latching the means for lifting and rotating the knife holder to the reciprocable abrasive surface.  
  12. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, support means for the abrasive surface, drive means for displacing said support means in straight line linear reciprocable motion to thereby displace said abrasive surface in straight line linear motion relative to the knife, means for shifting the angular position of the knife relative to the direction of linear reciprocation of said abrasive surface from a first angular position to a second angular position, said means for shifting the angular position of the knife including a pusher for pushing the means for holding the knife from a first angular position to a second angular position and means for selectively engaging said pusher with said reciprocating support means, and meansto turn the knife over to sharpen its opposite side, said means to turn the knife over including means to lift and rotate the knife through 180, said means to lift and rotate the knife through 180 including means to selectively latch the means to lift and rotate the knife through 180 to the reciprocable support means.  
  13. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, support means for the abrasive surface, drive means for displacing said support means in reciprocable straight line linear motion relative to said knife edge, means to hold one side of said knife edge against said abrasive surface at a first predetermined angular position for at least one reciprocation of said surface, means to shift said knife edge to a second predetermined angular position, means to hold said knife edge at said second predetermined angular position for at least one reciprocation of said surface, means to lift and turn the knife over, means to hold the opposite side of the edge of said knife against said abrasive surface at a first predetermined angular position for at least one reciprocation of said surface, means to shift said knife edge to a second predetermined angular position and means to hold said knife edge at said second predetermined angular position for at least one reciprocation of said surface.  
  14. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said knife and said surface relative to each other, means for maintaining the cutting edge of said knife at an oblique angle with respect to the direction of relative displacement between said knife and said surface, means for changing said angle from a first angular position to a second angular position in response to a measured amount of relative displacement of said knife and surface, means to turn the knife over to sharpen an opposite side of the edge in response to a further measured amount of relative displacement of said knife and said surface, means for again changing said angle from a first angular position to a second angular position in response to a measured amount of relative displacement of said knife and surface, and means to turn the knife over to again sharpen the first side of the knife edge in response to a further measured amount of relative displacement of said knife and said surface.  
  15. In a knife sharpener in accordance with claim 14 wherein said means for changing the angle of relative displacement between said knife and surface comprises means to push the knife from one angular position to another angular position.  
  16. In a knife sharpener. means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface in straight line linear reciprocable motion relative to said knife, means for shifting the angular position of the knife relative to the direction of linear displacement of said surface from a first angular position to a second angular position in response to a predetermined number of reciprocations of said surface, said means for shifting ineluding a pusher for pushing the means for holding the knife and means for selectively engaging said pusher to the reciprocating abrasive surface to bring said pusher into engagement with said means for holding said knife, and means to turn the knife over to sharpen an opposite side ofthe edge in response to a further predetermined number of reciproeations of said surface.  
  17. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface in straight line linear reciprocable motion relative to said knife, means for shifting the angular position of the knife relative to the direction of linear displacement of said surface from a first angular position to a second angular position in response to a predetermined number of reciprocations of said surface, and means to turn the knife over to sharpen an opposite side of the edge in response to a further predetermined number of reciprocations of said surface, said means to turn the knife over including means for lifting the means for holding the knife and for rotating the same through and means for selectively latching the means for lifting and rotating the knife holder to the reciprocable abrasive surface.  
  18. In a knife sharpener, means for holding a knife against an abrasive surface for sharpening the knife edge, drive means for displacing said surface and said knife edge in straight line linear motion relative to each other. means to hold one side of said knife edge against said abrasive surface at a first and then a second angular position, means to move said knife in a first direction away from said surface, means to rotate said knife relative to said surface in a direction from said first direction to thereby turn the knife over, and means to hold the opposite side of the edge of said knife against said abrasive surface at a first and then a second angular position.