US3751855A

US3751855A - Knife grinder

Info

Publication number: US3751855A
Application number: US00220104A
Authority: US
Inventors: G Pinat
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-03-03
Filing date: 1972-01-24
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: CA908999A

Abstract

A grinder for automatically grinding knives used mainly in the production of wood chip. These chipper knives consist of a steel plate having a lateral marginal portion inclined relative to the main portion. The plates have a bevelled knife edge along said inclined as well as said main portion. The grinder includes a power-operated grinding wheel mounted for reciprocating movement along a straight path. A swinging support is mounted underneath the grinding wheel and carries adjustable knife holders designed to hold in proper position a plurality of chipper knives. Hydraulic cylinders serve to move the grinding wheel and its electric motor along said path and to swing said support so that the knife edge will be ground and sharpened along its entire length in one continuous operation.

Description

lllite States atent 1 [111 3,751,855 Pinat Aug. 14, 1973 KNIFE GRINDER [76] Inventor: Gaston Pinat, 5384 Jules Fournier Primary [gammy-Donald Kelly St" MontreaLNonh, Quebec Attorney-Pierre Lesperance Canada 22 Filed: Jan. 24, 1972 [57] STRACT A grinder for automatically grinding knives used [21] Appl- N03 220,104 mainly in the production of wood chip. These chipper knives consist of a steel plate having a lateral marginal 30 Foreign Application priority Dam portion inclined relative to the main portion. The plates Mar 3 1971 Canada 106 830 have a bevelled knife edge along said inclined as well as said main portion. The grinder includes a power- [52] Us Cu 51/5 51/46 51/56 operated grinding wheel mounted for reciprocating movement along a straight path. A swinging support is [51] Km- CL B24) 3/36 B24J 3/38 mounted underneath the grinding wheel and carries ad- [58] Field g jg 'gg 51/5 1 56 225 justable knife holders designed to hold in proper posi- /234 tion a plurality of chipper knives. Hydraulic cylinders serve to move the grinding wheel and its electric motor [56] References Cited along said path and to swing said support so that the knife edge will be ground and sharpened along its en- UNITED STATES PATENTS tire length in one continuous operation. 2,738,625 3/1956 Strnad 51/56 3,600,859 3/1971 Edgecomb 51/56 x 9 Claims, 23 Drawing Figures PAIENIEU 3. 751 .855

sum

1 or 4 I I IIHH FIG.2

PAIENIEM I 3.751.855

sum 2 or 4 FIG.3

KNIFE GRINDER The present invention relates to a knife grinder and, more particularly, to a grinder for automatically sharpening chipper knives, that is knives used in the production of wood chips and the like.

Chipper knives consist of generally rectangular steel plates having a lateral portion inclined with respect to the main flat portion of the plate. The plate has a knife edge along the main portion and extended along the inclined portion. Sharpening of these chipper knives is a time-consuming operation, because, previous to the present invention, there was no automatic machine for accomplishing this grinding work.

It is the general object of the present invention to provide a machine for automatically grinding chipper knives of the character above mentioned and which is fast and efficient in operation, being capable of simultaneously grinding several knives.

Another object of the present invention resides in the provision of a grinder of the character described, in which the knife holders are mounted on a swinging support automatically pivotable in dependence of the movement of the grinding wheel to sequentially and continuously grind the two mutually inclined bevelled machine of the character described, which is compact and relatively inexpensive to manufacture and of long life with respect to its output.

The foregoing and other objects of the invention will become more apparent during the following disclosure and by referring to the drawings, in which:

FIG. I is a side elevation of the machine;

FIG. 2 is a top plan view;

FIG. 3 is a partial side elevation showing the side opposite to that shown in FIG. I and showing the grinding wheel assembly in retracted position;

FIGS. 3A, 3B, and 3C are schematic views of the grinding wheel and of a knife showing the grinding sequence during advancing movement of the grinding wheel;

FIG. 4 is a view similar to that of FIG. 3 showing the grinding wheel assembly in fully advanced position with the actuating lever for the swinging support in rotated position;

FIGS. 4A, 4B, and 4C are views similar to FIGS. 3A, 3B, and 3C and showing grinding the inclined portion of the knife and the reverse grinding sequence during retracting movement of the grinding wheel assembly;

FIG. 5 is a perspective view of a chipper knife of the type to be ground and sharpened by the machine of the invention;

FIG. 6 is a vertical cross-section, taken along line 6--6 of FIG. 3;

FIG. 7 is a top plan view of the swinging support for holding the knives to be sharpened;

FIG. 8 is a top plan view of the guide plates adapted to be fitted on the machined surfaces of the swinging support of FIG. 7;

FIG. 9 is an end elevation of the guide plate of FIG.

FIG. I0 is a partial cross-section, taken along line 10-40 of FlG.-7, but showing the guide plate and knife holder mounted thereon;

FIG. I I is a view of the assembly of FIG. 10 but taken at right angles thereto;

FIG. 12 is a top plan view of the knife holder;

FIG. 13 is a cross-section along line 13-13 of FIG. 12;

FIG. M is a partial longitudinal section of the machine taken along line 14-14 of FIG. 2;

FIG. 15 is a top plan view of the ratchet assemblu for the automatic lowering of the grinding wheel and taken along line I5-I5 of FIG. 14";

FIG. 16 is a vertical section taken along line 16-46 of FIG. 15; and

I FIG. 17 is a schematic diagram of the hydraulic circuit of the machine.

In the drawings, like'reference characters indicate like elements throughout.

The machine of the invention comprises an open top boxlike frame 1 supported on the floor by means such as I-beams 2. Frame I is of generally rectangular shape, when seen in top plan view, and comprises

longitudinal side walls

3,3, front end wall 4 and back end wall 5. It may have a bottom wall, if so desired.

A carriage 6, in the form of a plate-like member, extends across the top of frame I and is slidably supported on said frame for reciprocating movement in a horizontal plane in accordance with double arrow 7 longitudinally of frame 1. As seen in FIG. 6, one side of carriage 6 has an elongated flat shoe 8 in sliding contact with a flat rail 9 secured to the top longitudinal edge of side wall 3, while the other side of the carriage 6 carries a longitudinally extending V-grooved shoe 10 in slidable contact with a V-shaped rail II carried by the top edge of the side wall 3' of frame rails 9 and II are parallel. L-shaped retainer members 12 are removably secured to the sides of carriage 6 by bolts 13 and their short legs engage underneath the laterally protruding portions of rails 9 and Ill with a small clearance.

Carriage 6 carries oil cups 14 serving to lubricate the engaging surfaces of rails 9 and Ill and shoes 8 and I0 respectively. A horizontally and centrally disposed double-acting hydraulic cylinder and piston unit I5 (See FIGS. ll, 2, and I4) is secured by a bracket 16 to the back wall 5 of frame I and its piston rod 17 is adjustably attached to carriage 6 at I8. Cylinder unit 15 reciprocates carriage 6 between limit positions, shown in FIGS. 3 and 6.

A standard 119 is secured on top of carriage 6 and provides vertical guideways for a motor bracket 20 cartying in cantilevered fashion an electric motor 21 mounted with its shaft vertical along the center line of frame I and carrying at its lower and a grinding wheel 22.

Grinding wheel 22 is vertically movable through a circular opening 23, made in carriage 6. The assembly of the motor bracket 20, motor 21 and grinding wheel 22 can be moved up and down with respect to the standard 19; the grinding wheel moves between a top position protruding through carriage 6, as shown in FIGS.

I, 3, and 4, and a lowered position below carriage 6, as shown in FIG. 14. To move the assembly up and down, a screw 24 (See FIGS. 1 and 14) is in threaded engagement with motor bracket and is rotatably supported at its upper end by standard 19 and at its lower end by carriage 6, being provided with a thrust bearing (See FIG. 16).

A handwheel 25 is secured to the top end of screw 24. A ratchet wheel 26 is secured to the lower end portion of screw 24 and an operating lever 27 is rotatably mounted on screw 24 below ratchet wheel 26 and carries a pivotally mounted ratchet engaging pawl 28 urged into engagement with ratchet wheel 26 by a compression spring 29.

A tension spring 30 is attached to lever 27 and to carriage 6 and urges lever 27 in a clockwise direction, as seen in FIG. 15. An adjustable stop 31 is carried by the back end of frame 1 in the path of operating lever 27 to actuate the same in anticlockwise direction, as shown by arrow 32, upon carriage 6 reaching its back limit position. Thus, pawl 28 rotates ratchet wheel 26 and, consequently, screw 24 to lower the grinding wheel 22 a predetermined amount. Thus, the grinding wheel is lowered each time carriage 6 moves through its back limit position. The grinding wheel may be raised manually by handwheel 25 and manually lowered by also disengaging finger 28 from ratchet wheel 26.

A swinging support 33 is mounted in the front portion of frame 1 for pivotal movement about a horizontal axis transverse to the direction of movement of the grinding wheel. Support 33 is adapted to adjustably hold the chipper knives to be sharpened in proper position with respect to the grinding wheel and to rotate the same, so as to grind the entire bevelled surface of each knife.

Swinging support 33 is more particularly shown in FIGS. 2, 6, and 7. It has a U-shape when seen in front elevation, comprising a flat bed plate 34 and lateral upstanding legs 35, from the top ends of which laterally extend axially aligned

pivot shafts

36 and 37 respectively. Shaft 37 is rotatably mounted in a bearing 38 secured to side wall 3 on the inside thereof, while shaft 36 extends through a bearing 39, secured to side wall 3, and extends outwardly of said side wall 3' to be connected to an operating arm 40, the outer end of which is pivotally connected to the other end of piston rod 41 of a double-acting hydraulic cylinder and piston unit 42 (See FIGS. 3 and 4).

The outer end of cylinder 42 is pivoted at 43 to side wall 3' of frame 1. Stops 44, secured to wall 3' in the path of operating arm 40, serve to limit the pivotal movement of support 33. In the position shown in FIG. 3, the bed plate 34 of swinging support 33 is horizontal, that is parallel to the grinding face of grinding wheel 22, while in the position shown in FIG. 4, the bed plate 34 of the swinging support 33 is inclined approximately 55 to the horizontal.

A tube-like support 45 is rigidly secured to the back center portion of bed plate 34 of swinging support 33 and carries at its upper end a vertically adjustable diamond 46 for dressing the grinding wheel 22.

The top edge of each leg is provided in alignment with the

shafts

37,36 with a notch 47 defining a shoulder for receiving a straight gauge bar 48 serving to level the knives to be ground.

Gauge bar 48 is provided with steps 49 at each end and facing one face of the bar and also engageable with the notches 47.

Bed plate 34 provides two generally rectangular shaped machined surfaces 50 raised from the rest of the plate and with their longitudinal axes equally inclined on each side of the center line of frame 1 when the support is assembled therewith.

A guide plate 51 (see FIGS. 8 and 9) is adapted to be mounted flat on top of each machined surface 50 and be adjustably secured thereto for limited rotation by means of bolts 52 (See FIG. 11) loosely extending through holes 53 of the guide plate 51 and screwed in threaded holes 54 of bed plate 34.

One end of guide plate 51 is provided with a laterally extending bolt 55 rigidly secured thereto and adapted to pass through an apertured ear 56 secured to bed plate 34 laterally of one corner of machined surface 50. Thus, the horizontal inclination of guide plate 51 with respect to the bed plate 34 can be adjusted by bolt 55 and nuts 57. Opposite ear 56, is an additional ear 58 carrying an adjustable bolt 59 adapted to bear against guide plate 51 to serve as a reference.

Guide plate 51 has lateral longitudinally extending guide flanges 60 for longitudinally guiding the base 61 of a knife holder 62, the base 61 being slidable on the top face 63 of guide plate 61. Knife holder 62 has a threaded rod 64 adapted to extend through an apertured ear 65 secured to bed plate 34 and to be adjustably secured thereto by means of nuts 66, so as to adjust the longitudinal position of the holder 62 with respect to guide plate 61 and, consequently, with respect to bed plate 34.

Bolts 52, securing guide plate 51 to the bed plate 34, serve also to rigidly secure the holder 62 to the bed plate 34 once the proper adjustment has been effected. As shown in FIG. 11, the bolts 52 extend loosely through holes 67 of base 61.

Holder 62 is formed at its upper end with a vertically inclined support plate 68, the inclination of which is transverse to the direction of adjustment movement along guide plate 51. Support plate 68 is inclined upwardly towards the center line of the machine.

Support plate 68 has a central longitudinal slot 69 (See FIGS. 12 and 13) at the lower end portion thereof for receiving and guiding a block 70 which is rigidly secured to the underside of a stepped foot 71. An adjusting screw 72 is threaded within block 70 and is rotatable within an ear 73 of support plate 68, but is prevented from axial displacement with respect to the same.

Bolt 72 is terminated by a head 74 which serves to rotate the same to adjust the position of the foot 71 along inclined plate 68. Clamps 75 are pivotally mounted at 76 to ears 77 secured to each side of the holder 62, such that their ends will clamp chipper knives to be .ground on top of support plate 68 with the lower edge of the chipper plates engaging the steps of stepped foot 71. The clamps are maintained in clamped position by bolt 75' screwed within the lower end of the clamps and engageable with the side of holder 62.

Support plate 68 has a flat top portion and a lateral downwardly inclined portion 78 on one side making an angle of approximately 55 with the main plate portion. Similarly, the stepped foot 71 has a downwardly inclined lateral portion 79 fitting portion 78.

The chipper knife to be ground by the machine in accordance with the invention is illustrated in perspective in FIG. 5. This knife, generally indicated at A, consists of a steel plate defining a flat main portion 80 and a lateral inclined portion 81 of lesser width. One edge of the plate is bevelled throughout its length to form a cutting edge 82, the bevelled surface being continuous and indicated by main bevelled surface 83, lateral bevelled surface portion 84 and intermediate bevelled surface portion 85 which is longitudinally convex and merges smoothly with

surface portions

83, 84. The trailing end of the knife A is provided with rearwardly opening slots 86 for receiving retaining bolts for securing the knife in operative position in a chipper machine.

Lateral portion 81 meets the main portion 80 at about 55 in the usual chipper knife.

A plurality of similar chipper knives A are mounted on each support plate 68 in stacked position with the toe end of the chipper knife resting on one of the steps of the stepped foot 71. The inclined lateral portions 81 of the knives mate with the inclined lateral portion 78 of support plate 68 and with the inclined lateral portions 79 of the steps of the foot 71.

The vertical inclination of the support 68 is equal to the angle of bevelled surface 83 of knife A, such that surface 83 is substantially horizontal, that is parallel to bed plate 34, when the knives are mounted on the support plate 68. Also, the height of the steps of stepped foot 71 is equal to the thickness of the knives, so that the knives lie flat one over the other, and the depth of the steps of foot 71 is such that all the bevelled surfaces 83 of the stacked knives form a continuous flat surface. Lateral surface portions 84 and curved portions 85 form also a continuous flat surface. The two set of assembled knives face each other centrally of the swinging support 33. The knives are clamped on support plate 68 by screwing the tightening bolts 75 to force clamps 75 down onto the knives A. The resulting assembly appears as illustrated in FIGS. 2 and 6.

The hydraulic circuit for the two double-acting cylinder and piston units and 42 is shown in FIG. 17.

An electric motor 87 drives two hydraulic vane pumps 88, 89, one for each cylinder. Pump 88 sucks hydraulic fluid from reservoir 90 through filter 91, flex ible hose 92a and feeds the fluid to solenoid-operated reversing slide valve 92 connected to the double-acting cylinders 15. On the line from the pump 88 to valve 92 is a pressure gauge 93 with its needle valve 94 and a relief valve 95 set, for instance, at 600 p.s.i. for returning A return line 97 is connected between electro valve 92 and reservoir 99 through filter 98. Pump 89-feeds fluid to electro valve 92' of cylinder 42. The circuit is identical to the circuit associated with cylinder 15 and like elements are indicated by the same reference numeral with a prime addition.

The reservoir 90 is provided with a filler breather 99 and a drain gate valve 100.

Referring to FIG. ll, reversing valve 92 of carriage actuatingcylinder 15 is operated by a toggle switch 101, the pivoted actuating double am 102 of which is alternately moved by

fingers

103, 104 adjustably secured to a slotted bar 105 secured to carriage 6. The

fingers

103, 104 are positioned so as to reverse the direction of the carriage when the latter reaches at the desired limit positions.

the fluid to the reservoir 90 through safety return line' FIG. 1 also shows an electrical control box 106 and a master switch 107 for the machine electrical circuit. FIGS. 3 and 4 show the control for the operation of the electro valve 92' of swinging support actuating cylinder 42. This control comprises a toggle switch 108, the pivoted actuating double arm 109 of which is engageable by fingers 110 adjustably longitudinally mounted along slotted bar 111 secured to carriage 6.

Fingers 118 are adjusted so as to operate cylinder 42 from the position shown in FIG. 3 to the position shown in FIG. 4 when the carriage 6 has reached a position near the front end of frame ll, that is during forward movement of the grinding wheel 22, as indicated by the sequence of FIGS. 3A to 3C, and FIG. 4A. The grinding wheel thus first grinds the main bevelled surface 83 of knives A, then the curved bevelled surface and, finally, the lateral bevelled surface 84. The grinding wheel then reverses its direction, grinds again surface 84, then cylinder 42 is actuated to return the swinging support 33 to horizontal position and the retracting grinding wheel 22 grinds the curved surface 85, as shown in FIG. 4B, and then main surface 83, as shown in FIG. 4C, until the grinding wheel reaches its back limit position.

Upon reaching said back limit position, operating lever 27 abuts adjustable stop 31 (See FIG. 13) to lower grinding wheel 22 to a predetermined amount and the cycle is repeated.

Diamond 46 is raised to dress grinding wheel 22 and is lowered after dressing. During wheel dressing and while a carriage 6 is travelling, the grinding wheel is progressively manually lowered until refacing is completed.

Before grinding is started, the sets of knives A are vertically adjusted on swinging support 33 by means of adjusting bolt 72 with its head 74. Gauge bar (See FIG.6) is positioned, with its steps 49 uppermost, on the notches 47 of the swinging support and the knives are raised till their bevelled surfaces are at a minimum clearance from the gauge bar. The gauge bar is removed and grinding can be started. The grinding cycle is completed when gauge bar is repositioned on notches 47, but with steps 49 engaging said notches with a minimum clearance between the knives bevelled surfaces and the gauge bar 48. Distance a is normally 0.0l5 inch for a normal grinding cycle.

It will be noted that the vertical distance of the top of the knives above the horizontal axis of

shafts

36,37 of the swinging support 33 determines the radius of curvature of curved "bevelled surface 85 of knives A.

The knife holders can be adjusted by means of screw, 55 and nuts 57 to vary the horizontal inclination of the knife edges with respect to the center line of the machine; this inclination depends ;on the angle of the bevelled surfaces of the knives and also on the angle between the lateral portions with the main portion of the knives. The knife holders can also be adjusted with respect to the swinging support 33 substantially longitudinally thereof by means of bolt 64 and nut 66 to adjust the offset of the knife edge of the lateral portion of the knives with respect to the knife edge of the main portion of said knife. By offset is meant the distance of the lateral knife edge from the straight plane containing the knife edge of the main portion and perpendicular to said main portion. This offset can be varied from zero to any suitable value.

I claim:

1. An automatic grinding machine for grinding and sharpening knives having a continuous knife edge along a side thereof defined by two mutually inclined bevcllcd surfaces and an intermediate convex surface merging with said bevelled surfaces, the side of the knife opposite said knife edge forming a toe end, said machinecomprising a frame, a carriage mounted for reciprocating movement on said frame along a substantially straight horizontal path, power means to reciprocate said carriage, a motor-operated rotatable grinding wheel mounted on said carriage for raising and lowering movement of said wheel relative to said carriage, means to lower said wheel step by step upon each completion of a reciprocating movement of said carriage, a swinging support pivotally carried by said frame for swinging movement about a substantially horizontal pivot axis transverse to said path, said swinging support including a bed plate located below said grinding wheel and said pivot axis, power means to pivot said swinging support between first and second positions in which said bed plate is substantially horizontal and inclined relative to said path respectively, at least one knife holder secured on said bed plate and providing a vertically inclined support plate and a stepped foot at the lower end of said support plate to hold a stack of knives with the toe end of said knives resting on the steps of said foot and with the lowermost knife of the stack resting directly on said support plate, the configuration of said steps adapted to produce a smooth surface out of the respective bevelled surfaces of the stacked knives, said swinging support and said holder in the first position of said swinging support maintaining one set of bevelled surfaces in a plane parallel to said path and to the grinding underface of said grinding wheel and in the second position of said swinging support maintaining the other set of bevelled surfaces in a plane parallel to said path and to the grinding underface of said grinding whecl, whereby both sets of bevelled surfaces may be sequentially ground by said grinding wheel during reciprocating movement of said grinding wheel with reciprocating pivotal movement of said swinging support.

2. An automatic grinding machine as claimed in claim I, further including means to adjust the position of said stepped foot along said inclined support plate, so as to vary the distance of said sets of bevelled surfaces from said bed plate, said holder capable of maintaining said one set of bevelled surfaces above the level of said pivotal axis at a distance substantially equal to the radius of curvature of said intermediate set of convex bevelled surfaces of said knives, so as to grind said convex surfaces during swinging movement of said swinging support.

3. An automatic grinding machine as claimed in claim 1, further including power-operated control means for controlling operation of said power means to pivot said swinging support in response to the reciprocating movement of said carriage.

4. An automatic grinding machine as claimed in claim 3, wherein said frame consists of an open top box-like structure having a pair of parallel side walls, said swinging support located within said box-like structure, the top edges of said pair of side walls providing horizontal parallel rails, said carriage slidably mounted on and supported by said rails, said carriage having an opening through which said grinding wheel moves during its raising and lowering movement.

5. An automatic grinding machine as claimed in claim 1, wherein the longitudinal center line of said support plate makes a small horizontal angle with said transverse pivot axis.

6. An automatic grinding machine as claimed in claim 5, further including means to adjustably secure said support plate to said swinging support to vary the horizontal angle made between the longitudinal center line of said support plate and the transverse pivot axis.

7. An automatic grinding machine as claimed in claim 6, further including means to adjustably vary the horizontal position of said support plate in the general direction of said path.

8. An automatic grinding machine as claimed in claim 1, further including a diamond carrying support member secured to said swinging support to hold a diamond in dressing engagement with the underface of said grinding wheel.

9. An automatic grinding machine as claimed in claim 1, wherein said swinging support includes upstanding legs at both ends of said bed plate, axially aligned stud shafts protruding outwardly from the upper ends of said legs and serving as said pivot axis, one of said stud shafts extending through said frame, a crank arm secured to the outer end of said one stud shaft and said power means to pivot said swinging support comprising a double-acting hydraulic cylinder and piston unit pivotally attached to said crank arm and to said frame.

Claims

1. An automatic grinding machine for grinding and sharpening knives having a continuous knife edge along a side thereof defined by two mutually inclined bevelled surfaces and an intermediate convex surface merging with said bevelled surfaces, the side of the knife opposite said knife edge forming a toe end, said machine comprising a frame, a carriage mounted for reciprocating movement on said frame along a substantially straight horizontal path, power means to reciprocate said carriage, a motor-operated rotatable grinding wheel mounted on said carriage for raising and lowering movement of said wheel relative to said carriage, means to lower said wheel step by step upon each completion of a reciprocating movement of said carriage, a swinging support pivotally carried by said frame for swinging movement about a substantially horizontal pivot axis transverse to said path, said swinging support including a bed plate located below said grinding wheel and said pivot axis, power means to pivot said swinging support between first and second positions in which said bed plate is substantially horizontal and inclined relative to said path respectively, at least one knife holder secured on said bed plate and providing a vertically inclined support plate and a stepped foot at the lower end of said support plate to hold a stack of knives with the toe end of said knives resting on the steps of said foot and with the lowermost knife of the stack resting directly on said support plate, the configuration of said steps adapted to produce a smooth surface out of the respective bevelled surfaces of the stacked knives, said swinging support and said holder in the first position of said swinging support maintaining one set of bevelled surfaces in a plane parallel to said path and to the grinding underface of said grinding wheel and in the second position of said swinging support maintaining the other set of bevelled surfaces in a plane parallel to said path and to the grinding underface of said grinding wheel, whereby both sets of bevelled surfaces may be sequentially ground by said grinding wheel during reciprocating movement of said grinding wheel with reciprocating pivotal movement of said swinging support.

2. An automatic grinding machine as claimed in claim 1, further including means to adjust the position of said stepped foot along said inclined support plate, so as to vary the distance of said sets of bevelled surfaces from said bed plate, said holder capable of maintaining said one set of bevelled surfaces above the level of said pivotal axis at a distance substantially equal to the radius of curvature of said intermediate set of convex bevelled surfaces of said knives, so as to grind said convex surfaces during swinging movement of said swinging support.