US3769867A

US3769867A - Veneer clipping apparatus

Info

Publication number: US3769867A
Application number: US00221531A
Authority: US
Inventors: W Schilling
Original assignee: MacMillan Bloedel Ltd
Current assignee: MacMillan Bloedel Ltd
Priority date: 1972-01-28
Filing date: 1972-01-28
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06
Also published as: CA966415A

Abstract

Veneer clipping apparatus including a rotatable drum having a plurality of axially-extending knives arranged around and projecting from its surface, and power means connected to the knives for selectively moving the knives radially outwardly and inwardly between extended and retracted positions. The apparatus can be operated automatically by scanning means or manually from a position ahead of the drum with reference to the direction of movement of veneer between the drum and an anvil.

Description

[ Nov. 6, 1973 United States Patent 11 1 Schilling VENEER CLIPPING APPARATUS 465,427 12/1891 83/337 1,753,967 4 1930 R b 1 as 296 75 Inventor: Walter w. Schilling, Delta, Br1t1sh 3,641,857 2,1972 lai i Columb1a, Canada Primary Examiner-J. M. Meister Attorney-Kellard A. Carter [57] ABSTRACT Veneer clipping apparatus including a rotatable drum having a plurality of axially-extending knives arranged around and projecting from its surface, and power [73] Assignee: MacMillan Bloedel Limited,

Vancouver, British Columbia, Canada [22] Filed: Jan. 28, 1972 [21] Appl. No.: 221,53l

means connected to the knives for selectively moving the knives radially outwardly and inwardly between exlnt. 823d 25/12 83/337, 338, 285-291,

Field of Search..............

tended and retracted positions. The apparatus can be operated automatically by scanning means or manually from a position ahead of the drum with reference to the direction of movement of veneer between the drum and an anvil.

[56] References Cited UNITED STATES PATENTS 1,965,523 7/1934 MaeFarren.......................

3,318,180 Bauman............................ 83/337 X 17 Claims, 8 Drawing Figures PAYENTEDMUY 6l975 v 3.769.867

SHEET 2 BF 4 PAIENTEUnnv 6 I975 SHUT 4 [if 4 VENEER CLIPPING APPARATUS This invention relates to clipping apparatus particularly for wood veneers used in the manufacture of plywood, but which can be used for selectively clipping other materials.

The main purpose of clippers in this field is to clip a ribbon of veneer travelling from a lathe which has cut the veneer off a log being rotated around the longitudinal axis thereof. The clippers cut the veneer into individual sheets of different lengths with reference to the direction of travel of veneers, and in the majority of cases, the. clippers are used as well to clip out defects, such as,

a areas of less than full width ribbon,

b knot holes over a certain size,

c pitch pockets, and

d shake or cracksover a certain size.

There are a number of veneer clippers in use, but they all have some disadvantages. With one known type of veneer the clipping is done by the action of a horizontal knife which is driven down in a vertical plane through the travelling veneer to meet a soft anvil under the veneer. Then the knife is reversed to its starting position, allowing the veneer to move before'clipping again. The action of the knife is very fast, the full cycle time'being estimated at about 150 milliseconds. This type of clipper can be used for selective clipping.

The accuracy of the reciprocating blade clipper is variable. When manually controlled to clip out defects or to produce random widths of veneer, the error of clip may average plus or minus inches from the proper clip position. When this apparatus is controlled by an automatic timer, the clip error is approximately plus or minus 2 inches. These errors represent a substantial loss of wood. However the main objection to these clippersis that they are limited to a speed of up to about 300 feet per minute. Higher speeds produce cracks and weak wood on the leading edges of the sheets. While clipping random widths from relatively small sheets of veneer, the'error of clip is greater and the edges may not be parallel due to the poor contro of the travel of these sheets.

Another clipper generally used is one consisting of a drum having one axially-extending blade which makes one cut per revolution of the drum. This drum can travel very fast, but it only cuts sheets of predetermined widths, and cannot be used for selective clipping.

Clipping apparatus according to the present invention comprises a drum having a peripheral surface, a plurality of axially-extending knives on the drum at the peripheral surface thereof and mounted for individual radial movement, said knives having cutting edges facing'outwardly of the drum and normally being in'retracted positions, and power. means connected to the knives for moving selective ones of said knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted positions thereof. I

The knives clip the veneer against a rotating anvil beneath the drum, said anvil preferably having a soft metal, wood or plastic surface. The peripheral speed of the clipper and anvil is the same as the veneer speed, and when a clip is necessary, a knife is activated to project radially outwardly to clip the sheet against the anvil. The knife is activated in advance of the point of the clipping, thus giving it time to move into cutting position. The knives can be activated by means of a scanner, and/or manually.

A preferred form of veneer cutting apparatus in ac cordance with this invention is illustrated in the accompanying drawings, in which FIG. 1 is a diagrammatic plan view of the clipping apparatus,

FIG. 2 is a diagrammatic side elevation of the apparatus,

FIG. 3 is a perspective view of one of the knives,

. FIG. 4 is a fragmentary longitudinal section through the drum showing one knife in elevation in its retracted position,

FIG. 5 is a view similar in its extended position,

FIG. 6 is a view similar to FIG. 4, but showing a knife which is operated from the opposite end of the drum,

FIG. 7 is an isometric view of one end of clipper drum, showing the operating means for the knives, and

FIG. 8 is an enlarged longitudinal section taken on the line 8-8 of FIG. 1.

Referring to FIGS. 1 and 2 of the drawings, 10 is veneer clipping apparatus in accordance with the present invention which includes a drum 12 mounted for rotation above a rotatable anvil 14. Veneer strips 16 are directed into the nip 17 between the drum and anvil by conveyors 18and 19, while the clipped veneers 22 are conveyed away from the drum and anvil by a conveyor 23. A suitable scanner 25 of conventional design is located between the adjacent ends of conveyors l8 and 19, and is located ahead of drum 1.2,with reference to the direction of movement of the veneer, as indicated by arrow 27, a distance less than the length of the circumference of said drum.

Drum 12 is rotatably mounted at its

ends

30 and 31 in

suitable bearings

32 and 33. As the drum is mounted in the same manner at both ends, and as the elements associated with the respective drum ends are the same, only one end, namely end '30, will now be described in detail.

A hub 38 is secured to and projects outwardly from end 30 of the drum and is rotatably supported in bearing32. A support plate 39 is secured to hub 38 and radiates therefrom. A collar 41 projects from plate 39 and extends around the longitudinal axis of the drum and hub, indicated at 42. Hub 38 has an extension 44 of reduced diameter projecting outwardly therefrom through collar 41 and through a stationary housing 45 having a removable cover 46.

Drum 12 is rotated in a suitable manner, such as by means of a sprocket 48 mounted on collar 41 and suitably connected to a source of power, such as an electric motor, not shown.

Anvil 14 is mounted at its opposite ends in bearings 49, and is driven in any suitable manner, such as by to FIG. 4 showing the knife means of a sprocket 50 connected to a suitable source of power, such as an electric motor, not shown. Drum 12 and anvil 14 are driven at the same peripheral speed, and it is preferable to use variable speed motors for driving these elements so that the drum and anvil can be rotated at the same speed as the veneer strip 16 supplied thereto. This strip is usually from the logpeeling lathe of a mill, and the drum and anvil are rotated at the same speed as the lathe. The surface of the .anvil is preferably formed of a comparatively soft material, such as soft metal, wood or plastic.

The drum and anvil may be of any desired length, but in usual mill practice, the veneer strips are cut off logs that are a little more than 8 feet long so that the veneer strip is a little over 8 feet wide in order to provide suitable lengths for 8 foot plywood panels.

A plurality of knives or blades 55 are mounted on and extend axially of drum 12 on the peripheral surface 56 thereof. These knives are relatively thin and have cutting edges 58 along their outer edges opposite to their inner edges 59 which extend over the drum surface. It is desirable to provide as many knives as possible around the circumference of the drum as the more knives there are the more accurate is the cutting or clipping of the veneers and the less waste of wood there will be. These knives are normally in retracted positions relative to the drum, and are selectively shifted longitudinally of the latter, and means is provided for moving the knives radially into cutting position during the longitudinal shifting thereof. In order to get a relatively high number of knives onto the surface, the mechanism for moving each second knife is located at one end of the drum while the mechanism for the remaining knives is located at the opposite end of the drum. 8

The following dimensions are provided for illustrative purposes only, and are not to be considered in a limiting sense. These are:

drum diameter -33] inches number of knives l28 spacing of knives O.828 inch thickness of knives inch width of knives 2-% 4- /2 inches length of knives 112 inches R.P.M. at 600 feet/min. e 100 FIGS. 3, 4 and illustrate one knife 55 and the mounting thereof.

The inner edge 59 of knife 55 is formed with a plurality of V-shaped projections 63 throughout the length thereof. These projections have inclined surfaces 65 on one side thereof which are parallel to each other, and inclined surfaces 66 on their opposite sides which also are parallel to each other. The V-shaped projections 63 normally fit into correspondingly-shaped annular grooves 68 formed in the peripheral surface of drum 12. An incline of about to the axis of the drum has been found to be satisfactory for surfaces 65.

Blade 55 is also formed with a plurality of fingers 70 which are inclined in the same direction as and are parallel to the projection surfaces 66. Each of these fingers extend into an annular inclined pocket 72 in the peripheral surface of the drum. With this arrangement,

blades 55 actually rest upon a plurality of inclined surfaces which are parallel to each other, these surfaces being surfaces 66 and the internalsurfaces of pockets 72. With this arrangement, when knife 55 is drawn axiin order to reciprocate it axiallyof the drum. FIG. 5

. anvil 14. FIG. 6 illustrates a knife 55a which is reciprocated from the opposite end of the drum by a piston rod 76a. The only difference between the knife 55a and knife 55 is that the fingers a of the former are inclined oppositely to fingers 70 of blade 55, and are parallel to the inclined surfaces 65 of projections 63 of blade 55a. As a result of this, blade 55a is moved to its extended position when it is shifted to the left, as indicated by arrow 78 in FIG. 6.

The operation of the apparatus described so far is relatively simple. When the beginning of a defect, such as a pitch pocket 82, see FIG. 1, reaches scanner 25, a signal is sent to cause one of the

blades

55 or 55a to be extended so that when the front edge of the pitch pocket moves over anvil 14, the selected blade is in a position to cut through the veneer. Similarly, when the trailing edge of the pitch pocket passes the scanner, another blade is extended so that the second cut is made to cut out the veneer strip containing the pitch pocket. The scanning mechanism for this purpose is well known in the industry, and does not require description herein. It is also possible to provide controls which will cause the veneer strip to be cut into desired lengths with respect to the direction of movement of the veneer strip. When the front end of a veneer strip reaches the scanner, a knife is activated on the drum at the right time to cut a veneer of a desired length. This type of control is also well known in the industry.

Alternatively or in addition to the scanner, suitable control switches may be provided for an operator who can then cut the veneer strip where desired through manual controls. In this case, he would be watching for defects at a shadow line cast onto veneer strip 16 at scanner 25.

FIGS. 7 and 8 illustrate an example of apparatus for reciprocating the knives longitudinally of drum 12. The air cylinders 80 of piston rods 76 are mounted on a support 81 radiating from hub 38 spaced outwardly a little from the adjacent drum end. These rods are preferably made extensible by means of turnbuckles 82.

Support plate 39 is in the form of a disc, and concentric air supply and

exhaust manifolds

85 and 86 are mounted on the rim thereof. An annular wiring duct 87 surrounds and is mounted on supply manifold 85.

Electrical current is supplied for operating each of the knives 55 by wires 94 which are connected to brushes 95 in contact with conductor rings 96 on a drum 97 which is mounted on and secured to the hub extension 44 within housing 45, see FIGS. 7 and 8.

As the controls and operation of the air cylinders 80 are the same, only one will now be described in detail, namely cylinder 80a shown in FIG. 8.

A solenoid controlled air valve 100 is mounted on support plate 39 for each operating cylinder. This valve is connected to supply manifold 85 by a tube 102, and to the exhaust manifold 86 by another tube 103. Tubes and 106 extend from valve 100 to

passages

108 and 109 extending longitudinally through hub 38. Another pair of tubes 111 and 112 extend from

passages

108 and 109 to opposite ends of cylinder80a.

In view of the large number of pressure cylinders 80 required and the limited available space, these cylinders are arranged in pairs, such as the

cylinders

80a and 80b shown in FIGS. 7 and 8. In this case, the tube 112 for cylinder 80b is connected to block ll5'located at one end of cylinder 80a, said block having a passage therethrough, not shown, which is connected to a tube 116 that extends to an end of cylinder 80b. The opposite end of this cylinder is connected by a pipe 118 to an annular support ring 120 which has a passage therethrough, not shown, which is connected to a tube121 for cylinder 80b. Tubes 111 are connected to some of the passages 108, while tubes 121 are connected to others of said passages.

A plurality of wires 125 extend from rings 96 through protective tubes 126 to the wiring duct 87. The wires 125 for the power cylinders 80a travel from duct 126 through a protective pipe 129 to the control units 130 of valves 100.

Air under pressure is directed by a pipe 135 from a suitable source, not shown, through a suitable rotary fitting 136 to a passage 137 centrally and extending longitudinally of the extension 44 of hub 38. Another pipe 140 extends from the inner end of passage 137 out to air supply manifold 85. Exhaust air is discharged from the apparatus in any suitable manner, such as through a pipe 142 extending from the exhaust manifold 86 into collar 41, see FIG. 8.

The veneer strip 16 flows over anvil 14 and beneath drum 12, and when it is desired to make a cut in the veneer, the appropriate knife 55 is activated or moved to its extended position. This action takes place before the selected knife reaches the cutting position over the anvil. The actual electrical system for energizing the right knife does not form part of the present invention. The system is such that each signal for a cut will energize whatever knife 55 is a predetermined distance around the circumference of drum 12 from the cutting posi tion. The electrical signal operates the control unit 130 of valve 100 of the power cylinder of the selected knife 55, for example, cylinder 80a. The control unit operates this control valve 100 to direct pressure air from manifold 85 through tube 105, passage 108 and tube 111 to the inner end of cylinder 80a. At the same time, air is exhausted from the outer end of cylinder 80a through tube 112, passage 109, tube 106 and tube 103 to the exhaust manifold 86. This movesthe piston rod 76 of that cylinder to shift knife 55 connected to this rod to the right, as seen in FIG. 8, thereby extending the knife relative to the periphery of drum 12. As the extended knife swings downwardly over anvil 14 it cuts into the veneer moving over said anvil. If the selected knife is operated by a cylinder 80b, the operating air travels through passage 108 and

tubes

121 and 118 to this cylinder, and the exhaust air travels through

tubes

116 and 112 into passage 109.

The instant the cutting action takes place, another electrical signal energizes control unit 130 which reverses valve l00'so that pressure air is directed into the outer end of cylinder 80a through pipe 1 12 and exhaust air is removed therefrom through pipe 111 to shift piston rod 76 and its blade 55 back to the left, thus retracting the knife.

As the knives 55 are positioned very close to each other on drum 12, the veneer can be clipped very accurately so as to remove defects with very little waste of wood. The drum and its selected blade are spaced from scanner 25 or the manual operators position so that the blade has sufficient time to reach its fully extended position before arriving at the cutting position. The blade is moving in the same direction as the veneer at the cutting time and this makes it possible to rotate the drum and anvil at any desired speeds, such as the speed of the cutting lathe with which this apparatus is associated.

This clipping apparatus can cut defects out of the veneers accurately while operating at any lathe speed,

such as, for example, 1,000 ft./min. At 500 ft./min. the clipper would rotate at 56.7 R.P.M. The time available to actuate a knife at 1000 ft./min. is 0.53 seconds, which is the time for l revolution of the drum. When the spacing of the knives is 0.828 inch, the maximum error of clip is 0.42 inch.

As stated above, half of the knives are operated by air cylinders and controls at the end 31 of the drum, as diagramatically illustrated at in FIG. 1.

Some of the advantages of this apparatus are as follows:

1. Selective clipping.

2. Operates at any lathe speed.

3. Can be set to clip at predetermined lengths, for example, 53 inches and 26 inches.

4. On random cutting has accuracy of plus or minus 0.42 inch.

5. No breaking of veneer.

6. Operates at low R.P.M.

7. Extreme knife life due to the high number of knives.

8. Knives seldom require sharpening.

9. Simple to adjust and replace knives.

10. Can be programmed by a scanner or operated manually.

11. No veneer damage due to speed of travel.

12. No accumulated error due to speed.

13. The minimum clip the same at any speed.

14. Can clip any size within 0.42 inch.

I claim:

1. A selective high speed veneer clipper comprising a drum having a peripheral surface, a plurality of closelyadjacent and axially-extending thin knives mounted side by side on the drum and radiating therefrom at the peripheral surface thereof and mounted for individual axial and radial movement, said knives substantially covering the peripheral surface of the drum and said peripheral surface supporting the knives throughout the lengths thereof, said knives having cutting edges facing outwardly of the drum and :normally being in retracted positions, power means beyond an end of each knife and mounted to rotate with the drum around the axis of the latter, connecting means extending between and interconnecting each power means and the end of the adjacent knife, operation of each power means moving the knife connected thereto in an axial direction, means between the drum and each knife for moving said each knife. 'radially during axial movement thereof, and means operatively connected to the power means of all of the knives for selectively moving during rotation of the drum selected knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted position.

2. A veneerclipper as claimed in claim 1 including means co-acting between the drum and each knife for retaining said each knife on the drum while permitting radial and axial movement thereof.

3. A veneer clipper as claimed in claim 2 in which said co-acting means for each knife is such that reciprocal endwise movement of the knife shifts the latter outwardly and inwardly relative to the drum.

4. A veneer clipper as claimed in claim 3 in which said power means includes a power unit connected to each knife to reciprocate the knife in an endwise direction.

5. A veneer clipper as claimed in claim 4 in which the power units of the knives are alternately arranged with the units-of some knives at one end of the drum and the units of the other knives at the opposite end of the drum.

6. A selective high speed veneer clipper comprising a drum having a peripheral surface, a plurality of axially-extending knives on the drum at the peripheral surface thereof and mounted for individual radial movement, said knives having cutting edges facing outwardly of the drum and normally being in retracted positions, means co-acting between the drum and each knife for retaining said each knife on the drum while permitting radial and axial movement thereof, and power means connected to the knives for moving selected ones of said knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted positions thereof, said co-acting means of each knife comprising a plurality of fingers inclined in the same direction on an inner edge of the knife, and a plurality of pockets in the peripheral surface of the drum and inclined in the same direction as the fingers, said fingers slidably fitting in the pockets, whereby reciprocal movement of the knife in an endwise direction causes the knife to move outwardly and inwardly of the drum.

7. A veneer clipper as claimed in claim 6 in which said power means includes a power unit connected to each knife to reciprocate the knife in an endwise direction.

8. A veneer clipper as claimed in claim 6 in which the power units of the knives are alternately arranged with the units of some knives at one end of the drum and the units of the other knives at the opposite end of the drum. i

9. A veneer clipper as claimed in claim 4 in which eachpower unit comprises a fluid cylinder mounted at an end of the drum and operatively connected through said connecting means to the adjacent end of one of the knives.

10. A veneer clipper as claimed in claim 4 in which each power unit comprises a fluid cylinder mounted at an end of the drum and having a piston therein, and said connecting means connecting said piston to the adjacent end of one of the knives.

11. A veneer clipper as claimed in claim 1 in which said connecting means is adjustable.

12. A veneer clipper as claimed in claim 6 in which each power unit comprises a fluid cylinder mounted at an end of the'drum and operatively connected to the adjacent end of one of the knives.

13. A veneer clipper as claimed in claim 6 in which each power unit comprises a fluid cylinder mounted at an end of the drum and having a piston therein, and connecting means connecting said piston to the adjacent end of one of the knives.

14. A veneer clipper as claimed in claim 13 in which said connecting means is adjustable.

15. A selected high speed veneer clipper comprising a drurri having a peripheral surface, a plurality of axiallyextending thin knives mounted side by side on the drum and radiating therefrom at the peripheral surface thereof and mounted for individual radial movement, said knives having cutting edges facing outwardly of the drum and normally being in retracted positions, power means mounted on the drum and connected to an end of each of the knives for moving selected ones of said knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted positions thereof, means co-acting between the drum and each knife for retaining said each knife on the drum while permitting radial and axial movement thereof, said co-acting means for each knife being such that reciprocal endwise movement of the knife shifts the latter outwardly and inwardly relative to the drum, a plurality of axially-spaced annular sloping surfaces formed on the peripheral surface of the drum,

\ said sloping surfaces being substantially parallel with each other, and said knives being formed with sloping surfaces on inner edges thereof corresponding to and resting upon said drum sloping surfaces, whereby when each blade is moved axially by the power means connected thereto said each blade is moved by said sloping surfaces radially outwardly relative to the drum.

16. A veneer clipper as claimed in claim 15 in which the coacting means of each knife comprises a plurality of fingers inclined in the same direction on the inner edge of the knife, the incline of said fingers being substantially parallel to said sloping surfaces of the drum, and a plurality of pockets in the peripheral surface and inclined in the same direction as the fingers, said fingers slidably fitting in the pockets.

17. A selective high speed veneer clipper comprising a drum having a peripheral surface, a plurality of axially-extending knives on the drum at the peripheral surface thereof and mounted for individual radial movement, said knives having cutting edges facing outwardly of the drum and normally being in retracted positions, a plurality of axially-spaced annular sloping surfaces formed on the peripheral surface of the drum, said sloping surfaces being substantially parallel to each other, a plurality of annular pockets in the drum peripheral surface inclined in the same direction as and parallel to said sloping surfaces, sloping surfaces formed on inner edges of the knives corresponding to and resting upon said drum sloping surfaces, a plurality of sloping fingers on the inner edges of the knives and slidably fitting in said inclined pockets, an operating air cylinder connected to an end of each knife and mounted on a first support rotatable with the drum, annular air supply and exhaust manifolds mounted on a second support rotatable with the drum, an air control valve mounted on the second support for controlling each cylinder, tubes connecting each valve to the supply manifold and the exhaust manifold and to the opposite ends of the cylinder controlled by said each valve, an electrical control unit for operating each control valve, and ring and brush means electrically connected to each control unit.

Claims

1. A selective high speed veneer clipper comprising a drum having a peripheral surface, a plurality of closely- adjacent and axially-extending thin knives mounted side by side on the drum and radiating therefrom at the peripheral surface thereof and mounted for individual axial and radial movement, said knives substantially covering the peripheral surface of the drum and said peripheral surface supporting the knives throughout the lengths thereof, said knives having cuttIng edges facing outwardly of the drum and normally being in retracted positions, power means beyond an end of each knife and mounted to rotate with the drum around the axis of the latter, connecting means extending between and interconnecting each power means and the end of the adjacent knife, operation of each power means moving the knife connected thereto in an axial direction, means between the drum and each knife for moving said each knife radially during axial movement thereof, and means operatively connected to the power means of all of the knives for selectively moving during rotation of the drum selected knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted position.

2. A veneer clipper as claimed in claim 1 including means co-acting between the drum and each knife for retaining said each knife on the drum while permitting radial and axial movement thereof.

5. A veneer clipper as claimed in claim 4 in which the power units of the knives are alternately arranged with the units of some knives at one end of the drum and the units of the other knives at the opposite end of the drum.

8. A veneer clipper as claimed in claim 6 in which the power units of the knives are alternately arranged with the units of some knives at one end of the drum and the units of the other knives at the opposite end of the drum.

9. A veneer clipper as claimed in claim 4 in which each power unit comprises a fluid cylinder mounted at an end of the drum and operatively connected through said connecting means to the adjacent end of one of the knives.

12. A veneer clipper as claimed in claim 6 in which each power unit comprises a fluid cylinder mounted at an end of the drum and operatively connected to the adjacent end of one of the knives.

15. A selected high speed veneer clipper comprising a drum having a peripheral surface, a plurality of axially- extending thin knives mounted side by side on the drum and radiating therefrom at the peripheral surface thereof and mounted for individual radial movement, said knives having cutting edges facing outwardly of the drum and normally being in retracted positions, power means mounted on the drum and connected to an end of each of the knives for moving selected ones of said knives outwardly in a radial direction relative to the peripheral surface and the adjacent knives and back to the retracted positions thereof, means co-acting between the drum and each knife for retaining said each knife on the drum while permitting radial and axial movement thereof, said co-acting means for each knife being such that reciprocal endwise movement of the knife shifts the latter outwardly and inwardly relative to the drum, a plurality of axially-spaced annular sloping surfaces formed on the peripheral surface of the drum, said sloping surfaces being substantially parallel with each other, and said knives being formed with sloping surfaces on inner edges thereof corresponding to and resting upon said drum sloping surfaces, whereby when each blade is moved axially by the power means connected thereto said each blade is moved by said sloping surfaces radially outwardly relative to the drum.