US3604162A

US3604162A - Grinders for helical cutoff knives

Info

Publication number: US3604162A
Application number: US783983A
Authority: US
Inventors: Edward George Preston; Philip Arthur Maw
Original assignee: Molins Machine Co Ltd
Current assignee: Molins Machine Co Ltd
Priority date: 1967-12-15
Filing date: 1968-12-16
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14
Also published as: GB1238458A; FR1596869A; DE1814727A1

Abstract

A cutoff, suitable for a continuous rod cigarette-making machine, and having a helical knife. To grind one side of the knife, a stationary grinding wheel inclined at the appropriate angle to grind a bevel edge on the knife is used. To grind the other side of the knife, a grinder wheel is carried by a planetary mechanism to travel in a circular path and so that, as the knife is being ground, the grinder has a motion transverse to the path of the knife.

Description

United States Patent Inventors Edward George Preston;

Philip Arthur Maw, both of London, England Appl. No. 783,983

Filed Dec. 16, 1968 Patented Sept. 14, I971 Assignee Molins Machine Company Limited London, England Priority Dec. 15, 1967 Great Britain 57037/67 GRINDERS FOR HELICAL CUTOFF KNIVES 5 Claims, 10 Drawing Figs.

U.S. Cl 51/247 Int. Cl B24b 9/00 Field of Search 51/247, 246, 248

[56] References Cited UNITED STATES PATENTS 2,022,569 11/1935 Molins 51/247 Primary ExaminerLester M. Swingle AllorneyCraig, Antonelli, Stewart & Hill ABSTRACT: A cutoff, suitable for a continuous rod cigarettemaking machine, and having a helical knife. To grind one side of the knife, a stationary grinding wheel inclined at the appropriate angle to grind a bevel edge on the knife is used. To grind the other side of the knife, a grinder wheel is carried by a planetary mechanism to travel in a circular path and so that, as the knife is being ground, the grinder has a motion transverse to the path of the knife.

GRINDERS FOR I'IELICAL CUTOFF KNIVES This invention concerns improvements in or relating to grinders for a helical knife in a cutoff, such as a cutoff for severing the continuous cigarette rod into cigarette lengths (or multiples of such length) in a continuous rod cigarette-making machine.

In this specification the term helical.knife" is meant to in.- clude not onlya knife which is of true helical shape, but also a knife e.g. atlat knife, which, due to its limited angular extent, isa sufficient approximation to a helical knife, and is arranged at the helix angle.

The height of the knife should increase progressively from its leading to its trailing end, i.e. the distance of the cutting edge from the axis about which the knife rotates should increase from the end of the knife which first contacts the cigarette (or other) rod to. its other end, so that the rod is cut through progressively as the knife advances. The cutting edge of the knife needs to be continually reground, and accordingly one or more grinders may be provided which regrind the cutting edge with each kniferevolution. The grinders may be frustoconical, or cylindrical, and arranged at such an angle to the knife that a chamfered cutting edge is produced.

One side of the knife may be ground by a frustoconical or cylindrical grinder the axis of which is stationary, because the geometry is such that. the grinding surface can be inclined at such an angle to the knife that the conjoint effect of the increasing height of the cutting edge and the helix angle of the knife is to maintain the cutting edge in contact with the grinder at different positions along the grinder. With the other face of the knife, the requirement for a chamfered edge, the progressively increasing height of the knife as it passes the grinder, and the helix angle of the knife are conflicting factors.

According to the present invention there is provided a cutoff comprising a helical knife mounted for rotation about its axis and having a cutting edge at a progressively increasing distance from the axis of rotation, a grinder having a grinding surface to grind a chamfer along the knife edge, and means to move the grinder bodily in time relation with rotation of the knife to give the grinder a motion transverse to the knife as the knife passes the grinder, and at least substantially the same as the displacementin thidirection of the rotational axis of the knife of successive points along the cutting edge of the knife due to the helix angle of the knife.

The movement of the grinder may be in a circular path, the plane of which is parallel with'the rotational axis of the knife, the bodily movement of the grinder and the rotation of the knife being in such phased relation thatthe grinder travels along an are substantially at right angles to the travel of the knife during grinding.

The grinding surface may be a surface of revolution (e.g. a cylinder or frustrated cone) the grinder being mounted for rotation about its axis.

Rotation of the grinder may be caused by contact between the grinding surface and the knife edge or the grinder may be frictionally driven e.g. by contact with a stationary surface such asthe circular inwardly facing surface of an annulus, or the grinder may be driven by gearing, e.g. by the grinder carrying gear teeth which mesh with an annular gear ring.

One. embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, of which:

FIG. I is a side elevation of a cutoff for a continuous rod cigarette-making machine with a grinder according to the invention,

FIG. 2 is a view on the line 2-2 of FIG. 1 with parts broken away to show details,

FIG. 3 is a localsection on the line 33 of FIG. 2,

FIG. 4 is a local sectionon-the line 4-4 of FIG. 2.

FIG. 5 is a diagrammatic plan of the grinder and theknife illustrating the grinding action,

part of FIG.

FIG. 9 is a perspective view showing driving arrangements for the cutoff, and FIG. l0qis a partialsectional view showinga modification of'the arrangement of FIG. 7.

In FIGS. 1 and 2 the continuous cigarette rod is shown at CR moving in the direction of the arrow. At CRI is shown the end of the rod produced by the. previous cutting-off action. The cigarette which was cutoff by that action is not shown. but would be in advance of the rod CR.

The rod CR is continuously supported between two substantially disclike ledger elements I] and. l2ias disclosed in,British. Patent Application No. 57039/67 each of which consistsofa circular plate with a substantially cylindrical flange lIA and 12A respectively. The rim 11A. of the ledger 11 is shownin local section in FIG. 2 and has a peripheralv groove of a section corresponding to the section of the cigarette rod. The two ledger elements are situated adjacent to each other such that the cigarette rod is firmly supported in the passageway, formed between the two peripheral grooves without being crushed. The ledger elements 11 and 12: have driving shafts Band. 14. respectively which have mounted thereon identical gearwheels l5 and 16 respectively. The gearwheel 15' is driven by a further gearwheel 17 which in turn is driven from the. driving arrangements (not shown) of the cigarette-making machine so-that the peripheral speedof the ledger elements I1 and 12 is the same as the linear speed of the cigarette rod CR The

shafts

13 and 14 are mounted for rotation in a-fixedpillar. 18. This particular form of. ledger is not essential. to the present invention; any other suitable form of ledger could be employed. Thus, the ledger could be of the reciprocating type. operated by a crank mechanism.

The knife of the cutoff is shown at 19. The knife, I9 is of progressively increasing height and is shaped as a short are of a helix, the leading and trailing endsof-the. knife being shown at LE and TE respectively. The knife 19 is carried for rotation about an axis which is the axis of the helix by being mounted, on a rotatable drum 20 whose rotational axis coincides with the axis of the helix. The drum 20 is carried on a shaft 60, shown in FIG. 9, which is journaled for rotationin a gearbox 21. The gearbox 2I is mounted on structure 22 by means of a mounting plate 23.

Referring to FIG. 9, within the gearbox 21, the shaft 60 carries two bevel gears 61 and 62. The bevel gear. 62 isdriven by a bevel gear 63 mounted on a shaft 64 which extends into the gearbox 21 through the mounting plate 23. The shaft 64 carries a spur gearwheel 65, which in turn is driven by agearwheel 66 mounted on a shaft 67. The shaft 67isdriven from the cigarette-making machine through a gearwheel 68 which can be changed for one with a different number of teeth to alter the speed of rotation 6f the drum 20 when it is desired to alter the length of the cigarettes being cut off.

Extending upwards from the gearbox 21, as shown in FIGS. 1 and 2, is a cylindrical housing 24 which carriesat its upper end a further housing 25. A shaft 69, see FIG. 9, extends through the housing 24, its lower end carrying a bevel gear 70 which meshes with the bevel gear 61, and its upper end carrying a gearwheel 71 within the housing 25. The gearwheel 71 drives through an intermediate idler gear 72 in'the housing 25 a further gear wheel 26 (see FIGS. 2 and 9). The

housings

24 and 25 carry grinders and driving arrangements therefor which will be described later.

Also shown in FIG. 9 is a shaft 73 which is driven fromthe gearwheel 66 by means of a gearwheel 74 mountedthereon. At the other end of the shaft is an eccentric pin 75 which would be used to drive a reciprocating type of ledger, mentioned previously as an altemativeto the ledger arrangement shown.

The knife 19 is held between two clamping members 27 which are mounted within the drum 20. The mounting arrangement cannot be seen as the drum is closed by a plate 28. The clamping plates 27 protrude through a slot 29 in the drum. The knife 19 is thin and flexible and thus conforms to the shape of the clamping surfaces. The angular extent of the knife is small so that the knife may be flat without a significant departure from true helical shape. The clamping plates together with the knife can be adjusted bodily about an axis perpendicular to the rotational axis of the drum. Thus, the knife 19 can be made to conform with the desired helix for the cigarette length to be cut off.

The rotational axis of the drum 20 is inclined to the cigarette rod CR at the helix angle of the knife 19 so that, as the knife is passing through the cigarette rod, the part of the knife which is actually cutting is at right angles to the cigarette rod. When the helix angle of the knife is altered in order to cut off a different cigarette length the angle that the rotational axis of the drum makes with the cigarette rod is also altered to maintain the condition of the knife cutting the cigarette rod at right angles. To do this the mounting plate 23 and the gearbox 21 are together adjustable on the structure 22 so that the rotational axis of the drum pivots about the axis of the shaft 64. Thus, with the arrangement all the gears shown in FIG. 9 remain in mesh during such an adjustment. There is relative rotation between the

shafts

60 and 64 but this is negligible. Also the bodily movement of the drum 20 relative to the cigarette rod and the ledger is negligible.

To enable the knife 19 to pass through the cigarette rod CR the ledger element 11 has three substantially radial slots 11B spaced at 120 around the ledger. Similarly, the ledger 12 has three equispaced substantially radial slots 123. The slots are arranged so that as the knife 19 is about to enter the cigarette rod CR a slot 118 is aligned with a slot 128 for the knife to pass through. After a further revolution of the knife the next pair of slots 11B and 128 have become aligned with each other. Thus, the

ledger elements

11 and 12 make one complete revolution for each three revolutions of the knife 19. The

ledger elements

11 and 12 are disclosed more fully in the aforementioned application.

To regrind the knife after each cut the following arrangement is adopted. A frustoconical grinding wheel 30 is carried by its spindle being rotatably mounted in a housing 31 which in turn is carried by an adjustable bracket arrangement shown generally at 32, the other end of which is carried by the housing 25. The grinder 30 is adjustable up and down along its rotational axis by means of a knurled adjusting member 31A. The rotational axis of the grinder wheel 30 is adjustable in a plane parallel with the rotational axis of the drum 20 so that the rotational axis of the grinder 30 may be inclined to the plane of the drum 20 so that, due to this inclination and the frustoconical shape of the grinding surface, a chamfer will be ground along one side of the cutting edge of the knife 19. The spindle of the grinding wheel 30 carries a pulley 33, and a belt 34 passes around the pulley 33, two

idler pulleys

35 and 36 carried by a bracket 37 mounted on the housing 25, and a pulley 38 carried by the upper end of the shaft 69 which extends upwards through the housing 24. The driving arrangement for the grinder wheel 30 is such that, at the point of contact with the knife 19, the grinding surface is moving in the same direction as the knife 19, but at a greater speed.

The adjustable bracket arrangement 32 is shown in FIG. 4. A block 32A has two part-cylindrical surfaces 328 and 32C which have a common axis which passes through the point of contact between the grinder wheel 30 and the knife 19. The block 32A is secured to the housing 25 by two setscrews 320. Between the

surfaces

32B and 32C is a flat annular surface 3215. The housing 31 is secured to a block 32F which has two part-cylindrical surfaces 326 and 321-! (FIG. 2), of the same radius as the surfaces 328 and 32C respectively, and spaced apart by a similar flat annular surface 321. A slot 32] is provided in the member 32F and through this passes a setscrew 32K, with a knurled head 321., which is screwed into a tapped hole 32M in the block 32A. A clamp member 32N is provided between the head 32L of the screw 32M and the block 32F. By slackening the screw 32K the block 32F can be slid over the block 32A, and due to the shape of the cooperating cylindrical surfaces, the grinder 30 will be moved in a plane parallel with the rotational axis of the drum 20, and about the point of contact between the grinder 30 and the midpoint of the knife. It will be seen that the weight of the grinder 30 and housing 31 will be supported partly by the annular surface 325.

To grind the other side of the cutting edge of the knife 19 a frustoconical grinding wheel 39 is rotatably mounted towards one end of an arm 40. The arm 40 is secured to the lower end of a rotatable shaft 41, which is shown also in FIG. 9, which is journaled towards its lower end in a plain bearing 42, and has its upper end carried in a ball bearing 43. The bearing 42 is mounted on a plate 45 which is secured to an outwardly flanged insert 44, which in turn is received in a stepped aperture 46 in the housing 25. The outer race of the bearing 43 is carried in an externally threaded member 47, and is trapped between an internal flange 47A of the member 47 and a hollow cylinder 48A which is integral with a knurled adjusting member 48. At its upper end the threaded member 47 has an external flange 478 which is secured to the adjusting member 48. Adjustment of the member 48 thus moves the shaft 41 up or down and with it the arm 40 and the grinder wheel 39. The shaft 41 extends through hollow shaft 49 with which it is coaxial. The shaft 49 carries the gearwheel 26 and is rotatably supported in a ball bearing 50 mounted in the housing 25, and a ball bearing 51 which is trapped between an internal flange 44A on the member 44 and an annular rib 45A on the plate 45. The rotational speed of the arm 40 is the same as that of the drum 20. The

shafts

41 and 49 are drivingly connected by means of an internal key and keyway (not shown).

As may be seen in Figure 3, the grinder wheel 39 is rotatably mounted on a block 52 which is within and is slidable along the arm 40. The position of the block 52 along the arm 40 is adjustable by means of a knob 53 FIG. 2) which rotates a threaded rod 53A which is threaded through the block 52, the rod 53A being axially restrained in the arm 40. After adjustment of its position the block 52 is clamped in place by screws 54 which bear against a wedge member 55.

As illustrated, the grinder wheel 39 is arranged to be rotated about its own axis only by virtue of the grinding contact between its surface and the knife.

A weight 56 is secured to the end of the arm 40 to balance the rotatable assembly.

The cutoff and the grinders are adjusted and operate in the following manner.

The cigarette length which will be cutoff is determined by the linear speed of the cigarette rod CR and the rotational speed of the knife 19. The gear ratio of the driving arrangements for the cutoff by which it is driven from the cigarettemaking machine are adjusted to give the desired rotational speed of the cutoff in relation to the cigarette rod speed. The shape of the knife 19 is now adjusted by angular adjustment of the clamps 27, or by replacement of the clamps by ones of suitable shape, and by such angular adjustment.

The position and inclination of the grinder wheel 30 are altered by slackening the setscrew 32K and sliding the block 32F over the block 32A and then retightening the setscrew 32K, and by adjusting the knurled member 31A. These adjustments may be repeated until the knife correctly contacts the grinderwheel 30 as it passes it.

To adjust the path of the grinder wheel 39 the setscrews 54 are slackened and the knurled knob 53 is turned to move the block 52 along the arm 40, after which the setscrews 54 are tightened. This determines the radius of the path along which the grinder wheel 39 is carried bodily. The grinder wheel 39 is raised or lowered by turning the knurled knob 48. These adjustments may be repeated until the grinder 39 travels in the correct path so that it correctly contacts the knife 19 as it passes it. The manner in which the grinder wheel 39 grinds the edge of the knife 19 is diagrammatically illustrated in FIGS. 5 and 6.

positions of the rotational axis at the positions (A) (B) and (C) are shown at a b and c respectively. The first position is with the leading end LE of the knife 19 just beginning to contact the grinder wheel 39; the knife is shown at 19(A) and the grinder is shown at 39(A), both being shown in solid lines. The

1 second position is with the midpoint of the knife in contact with the grinder, as shown at MP, the knife and the grinder being shown in dotted lines at 19(8) and 39(8) respectively. The third position is with the trailing end TE of the knife in contact with the grinder, the knife and grinder being shown at 19(C) and 39(C) respectively in dot and dash line. Due to the progressively increasing height of the knife edge successive points along it come into contact with the grinder at progressively increasing diameters of the grinder. Thus, the circles 39(A) 39(8) and 39(C) which represent the grinder in FIG. 5 are of increasing sizes. Corresponding lines 39(A) 39(8) and 39(C) are shown on the grinder wheel 39 in FIG. 6 and represent the successive points around the grinder which contact the leading end, midpoint and trailing end respectively of the knife. It will be appreciated that a different diameter on the grinder wheel 39 corresponds to each point along the knife edge.

it will be appreciated that the bodily arcuate movement of the grinder 39 from position 39(A) to 39(B), which occurs while the arm 40 moves through an angle as shown in FIGURE 5, not only gives the grinder wheel 39 a motion transverse to the knife 19 substantially the same as the displacement in the direction of the axis of rotation of the point MP relative to the point LE on the knife, but also moves the grinder slightly to the right as viewed in these figures. Thus, in FIG. 6 the grinder wheel is shown in solid line at 39(A) and is shown in dotted line at 39(8). From position 39(8) to position 39(C) as the arm 40 rotates through a further angle 0 the grinder 39 moves back a corresponding amount to the left. The grinder 39 is not shown in dot and dash line in position 39(C) FIG. 6 because this position coincides with the position 39(A) shown in full line.

As the knife moves from position 19(A) to position 19(C) the grinder has to be given a transverse motion somewhat greater than that dictated purely by the helix angle of the knife due to the fact that successive points along the knife contact the grinder at progressively increasing radii thereon. The grinder therefore when moving through angle 0 has to be given an additional transverse motion equal to the difference between the radius of the grinder wheel 39 at the point of contact with the leading end LE of the knife, as shown at 39(A), and that at the point of contact with the midpoint LP of the knife, as shown at 39(B). This is achieved by choosing a suitable radius R (F IG. of the path of the grinder.

As has been mentioned, the rotational speed of the arm 40 is the same as that of the drum and the knife 19 so that as the knife 19 moves through 36026 from position 19(C) to 19(A) the grinder 39 will similarly move through 3600 from position 39(C) to 39(A), both the grinder 39 and the balance weight 56 passing over the drum 20 in so doing.

In the modification shown in FIGS. 7 and 8 the grinder wheel 39 is positively driven.

Referring first to F lG. 7, a drum 200 has a circular aperture to receive the plate 45, and the plate 45 and the drum 200 are secured to the insert 44 and the housing by countersunk screws 201. The cylindrical portion of the drum 200 extends downwardly to just below the arm 40 and is provided with a radial hole 200A to give access to the head 153 of a screw 153A. The arm 40 is slightly modified, a balance weight 156 replacing the balance weight 56.

The lower and open end of the drum 200 is partially closed by an annulus 202 which has an annular rib 202A which fits inside the drum 200. The annulus 202 is secured to the drum 200 by countersunk screws 203 (one only of which is shown). At its inner edge the annulus has a cylindrical flange 202B,

and the radially inner surface ofithe annulus and the flange present an inwardly facing cylindrical driving surface 202C which is coaxial with the rotational axis of the shaft '41.

The grinder wheel 39 is frictionally driven from the driving surface 202C of the annulus 202. Mounted on the spindle 139 of the grinder wheel 39 is a disc 139A which has an annular groove 139B. Surrounding the periphery of the disc 139A is a driving rim 139C of casehardened steel, which has an internal I annular groove 139D. The outer radius of the driving rim 139C is greater than the maximum radius of the grinder wheel 39. The drum 139C is mounted on the disc 139A by resilient means deformable radially of the grinder wheel 39. This resilient means is a soft rubber O-ring 139E which is trapped between the two annular grooves 1398 and 139D.

The position of the block 52 along the arm 40 is adjusted so that there is a small clearance as indicated at Y between the driving rim 139C and the cylindrical driving track 202C. When the cutoff mechanism is running at operating speed the speed of rotation of the arm 40 is such that the driving rim 139C is centrifugally displaced into contact with the driving surface 202C. This occurs due to the soft rubber ring 139E being compressed by the part of the driving rim 139C which is at any instant furthest away from the driving surface 202C. Thus, the effective rolling radius of the driving rim increases sufficiently to take up the clearance at Y.

This arrangement avoids excessive side thrust on the grinder wheel 39 and the arm 40, and on their bearings when the arm 40 is stationary. Such excessive side thrust could possibly occur at certain positions of the-arm 40 due to eccentricity or noncircularity of the driving surface 202C if the driving rim were to be rigidly secured to the grinder wheel spindle.

The clearance is chosen to besmall so that the driving rim will be positively rotated by the driving surface at operating speeds, but so that no contact occursbetween the driving rim 139C and any part of the driving surface when the cutoff is not operating, or is just starting to rotate. Thus the mass of the driving rim 139C and the softness of the rubber ring 139E are chosen for this to happen over the range of operating speeds of the cutoff, and over the operational range within which the block 52 may be adjusted along the arm 40.

As has been mentioned earlier, the shaft 41, and thus the arm 40 and the grinder wheel 39, are adjustable up and down. To cater for this the driving surface 202C is cylindrical. To cater for alteration of the radius of the circular path along which the grinder wheel 39 travels bodily, and produced by adjustment of the block 52 along the arm 40, the annulus 202 is replaced by another similar annulus with a driving surface 202C of suitable radius to provide the desired clearance at Y. Each of these annuli 202 will cater for a small range of adjustment of the block 52 and thus for a small range of cigarette lengths. The size of the range is determined by the amount that the static clearance at Y can be varied. Thus, a sufficiently large number of annuli 202, each with a driving surface of different radius, may be provided so that the total range of cigarette length that the cutoff can produce is catered for. Alternatively, it may be more convenient to use any particular annulus 202 only for one specific cigarette length.

Alternatively to the friction drive arrangement described above the spindle 139 could carry a small gear wheel 210 meshing with gear teeth 211 provided on the annulus 202, as seen in FIG. 10. The remaining elements correspond to the elements similarly designated in H6. 7 and have been described in connection with that figure.

What we claim as our invention and desire to secure by Letters Patent is:

l. A cutoff comprising a helical knife mounted for rotation about its axis and having a cutting edge at a progressively increasing distance from the axis of rotation, a grinder having a grinding surface to grind a chamfer along the knife edge, means to move the grinder bodily in a circular path, the plane of which is parallel with the rotational axis of the knife, in phased relation with rotation of the knife to give the grinder a motion along an are substantially at right angles to the travel of the knife as the knife passes the grinder, and at least substantially the same as the displacement in the direction of the rotational axis of the knife of successive points along the cutting edge of the knife due to the helix angle of the knife, the grinder being mounted for rotation about an axis, and its grinding surface being a surface of revolution about said axis, the grinder having a driving rim mounted thereon by resilient means deformable radially of the circular path of the grinder, there being provided an annulus coplanar with the driving rim and having an inwardly facing circular driving surface, a clearance being provided between the rim and the driving surface when the grinder is stationary, which clearance is taken up when the grinder is moving in its circular path by the rim being centrifugally displaced against the action of the resilient means towards the driving surface so that the grinder is rotated by the driving rim rolling on the driving surface as the grinder moves in its circular path.

2. A cutoff as claimed in claim 1 wherein the position of the grinder is adjustable to alter the radius of the circular path.

3. A cutoff as claimed in claim 1 wherein the grinder is a frustum of a cone.

4. A cutoff as claimed in claim 1 wherein the resilient means is a rubber ring which is trapped between an annular groove on the inside of the rim and an annular groove on the outside of the grinder.

5. A cutoff as claimed in claim 1 wherein the helical knife is mounted for rotation about its axis on a first shaft, the shaft carrying a first bevel gear, there being provided an input shaft carrying a second bevel gear meshing with the first bevel gear, the helical knife and the first shaft being angularly adjustable about the axis of the input shaft.

Claims

1. A cutoff comprising a helical knife mounted for rotation about its axis and having a cutting edge at a progressively increasing distance from the axis of rotation, a grinder having a grinding surface to grind a chamfer along the knife edge, means to move the grinder bodily in a circular path, the plane of which is parallel with the rotational axis of the knife, in phased relation with rotation of the knife to give the grinder a motion along an arc substantially at right angles to the travel of the knife as the knife passes the grinder, and at least substantially the same as the displacement in the direction of the rotational axis of the knife of successive points along the cutting edge of the knife due to the helix angle of the knife, the grinder being mounted for rotation about an axis, and its grinding surface being a surface of revolution about said axis, the grinder having a driving rim mounted thereon by resilient means deformable radially of the circular path of the grinder, there being provided an annulus coplanar with the driving rim and having an inwardly facing circular driving surface, a clearance being provided between the rim and the driving surface when the grinder is stationary, which clearance is taken up when the grinder is moving in its circular path by the rim being centrifugally displaced against the action of the resilient means towards the driving surface so that the grinder is rotated by the driving rim rolling on the driving surface as the grinder moves in its circular path.

3. A cutoff as claimed in claim 1 wherein the grinder is a frustum of a cone.