US3782039A

US3782039A - Self sharpener for rotary cutter

Info

Publication number: US3782039A
Application number: US00253735A
Authority: US
Inventors: D Warwick
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-12-09
Filing date: 1972-05-16
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01

Abstract

A rotary mower blade sharpener device including a resiliently deformable body portion provided with an abrasive upper surface, and an anchor sheet fixed to the lower surface of the body portion to extend outwardly thereof. Relative rotation of the mower blade with respect to the upper surface of the body portion effects sharpening of the blade; the body portion being constrained from rotation by the wheels of the mower pressing the anchor sheet into engagement with the ground or other expansive surface. The anchor sheet may be formed of a pliable material and of sufficient size to permit its being folded or wrapped upwardly to enclose the body portion in a decorative shipping and/or storage envelope.

Description

United States Patent [191 Warwick *Jan. 1, 1974 SELF SHARPENER FOR ROTARY CUTTER The portion of the term of this Notice: I v

patent subsequent to May 30, 1989, has been disclaimed.

[22] Filed: May 16, 1972 [21] Appl. No.: 253,735

Related U.S. Application Data [63] Continuation-impart of Ser. No. 96,565, Dec. 9,

1970, Pat. NO. 3,665,658.

[52] US. Cl 51/250, 5l/288, 56/250,

76/82.] [51] Int. Cl B24b 19/00 [58] Field of Search 51/250, 246, 149,

5l/l6l, 228, 288; 76/82.]; 56/250 3,665,658 5/l972 Warwick 5l/250 Primary Examiner-Othell M. Simpson Assistant Examiner-Nicholas P. Godici Attorney-John B. Bean et al.

57 ABSTRACT A rotary mower blade sharpener device including a resiliently deformable body portion provided with an abrasive upper surface, and an anchor sheet fixed to the lower surface of the body portion to extend outwardly thereof. Relative rotation of the mower blade with respect to the upper surface of the body portion effects sharpening of the blade; the body portion being constrained from rotation by the wheels of the mower pressing the anchor sheet into engagement with the ground or other expansive surface. The anchor sheet may be formed of a pliable material and of sufficient size to permit its being folded or wrapped upwardly to enclose the body portion in a'decorative shipping and- /or storage envelope.

13 Claims, 5 Drawing Figures SELF SHARPENER FOR ROTARY CUTTER REFERENCE TO RELATED APPLICATION BACKGROUND OF THE INVENTION In my co-pending application Ser. No. 96,565, Filed Dec. 9, 1970, now US. Pat. No. 3,665,658 there is disclosed a rotary mower blade sharpening device comprising a resiliently deformable body portion having its opposing surfaces sized to substantially correspond to the diameter of an envelope defined by rotation of the blade to be sharpened, whereby permitting the device to be freely received within the protective shroud normally surrounding the blade.

In a preferred embodiment of my prior invention, each of the opposing surfaces of the body portion is covered with abrasive material, such as is provided by commercially available flexible abrasive sheets including fine grit sandpaper, garnet paper or emery cloth; and a centered opening extends through the opposing surfaces to freely receive the stub end of the blade mounting drive shaftwhen the blade to be sharpened is positioned in surface engagement with one or the other of such surfaces.

The thickness of the sharpening device, when in noncompressed condition is chosen to be slightly in excess of one of the conventional blade height grass cutting settings normally provided for commercially available lawn mowers. Thus, when the sharpening'device is laid on a supporting surface such as the ground or concrete driveway with one of its abrasive surfaces facing up, the blade is arranged at the height adjustment which essentially corresponds to the thickness of the device and the lawn mower is set down over the device, the blade will be positioned in surface engagement with the upwardly facing abrasive surface of the device and the downwardly facing surface of the device will be maintained by the blade in frictional engagement with the supporting surface. Thereafter, momentary rotations of the blade, caused by either switching on or off an electric lawn mower or pulling the starter cord of a gasoline engine driven lawn mower with the spark plug disconnected, will effect sharpening of the blade cutting elements. The friction gripping engagement of the downwardly facing abrasive surface of the device with the supporting surface is relied upon to either prevent or retard rotations of the device with the blade.

Alternatively, my prior sharpening device may be provided with abrasive material on only one of its opposing surfaces. In this case, the other surface would have friction gripping properties imparted thereto by forming the device from a material having a high coefficient of friction, such as foam rubber, or by roughening or providing ribs projecting from a relatively slippery plastic foam material, such as polyurethane foam.

SUMMARY OF THE INVENTION The present invention relates to improvements in a lawn mower blade sharpening device of the type disclosed in my co-pending application Ser. No. 95,565, filed Dec. 9, I970, now US. Pat. No. 3,665,658.

In accordance with the present invention, only a relatively upper surface of the body portion is provided with abrasive properties, and an anchor device is fixed 2 adjacent the lower surface of the body portion and extends outwardly thereof. Rotation of the body portion with the blade to be sharpened is constrained by placing the wheels of the mower in pressing or clamping engagement with the anchor device.

The anchor device is preferably formed of pliable sheet material of sufficient size to permit it to be wrapped upwardly about the body portion, so as to define a decorative sales and/or storage envelope or container therefor. Instructions in the use of the device may be printed directly on the anchor sheet or, if desired, the anchor sheet may be formed from a transparent plastic material and infonnation printed on an insert sheet viewable through the wrapping.

THE DRAWINGS The nature and mode of the present invention will now be more fully described in the following detailed description taken with the accompanying drawings wherein:

FIG. 1 is a perspective view of a packaged" form of a blade sharpening device of the present invention;

FIG. 2 is a side elevational view of a conventional lawn mower positioned above the blade sharpening device;

FIG. 3 is a fragmentary perspective view showing the rotor blade to be sharpened in surface engagement with the device of the present invention;

FIG. 4 is a fragmentary view showing a single cutting element in contact with the sharpening device of the present invention; and

F IG. 5 is a sectional view illustrating a modified form of the sharpening device of the present invention.

DETAILED DESCRIPTION Now referring to the drawings in greater detail, 10 generally designates a blade sharpening device formed in accordance with a preferred embodiment of the present invention, and 12 generally designates a rotary type lawn mower of substantially standard construction.

For purposes of reference, mower 12 as shown in FIG. 2 as having a single mower blade 14 defining cutter elements 14a arranged adjacent opposite ends thereof. Blade 14 may be suitably fixed, as by lock nut 16, to mower drive shaft 18 for rotation about a generally vertically extending axis. A protective shroud, such as may be defined by mower housing peripheral wall 20 or cut grass directing inserts, not shown, is arranged outwardly of an envelope defined by rotation of blade 14. As is conventional, mower 12 is provided with suitable means, such as operating handles 22, for the purpose of adjusting lawn mower wheels 24 relative to the frame of the mower in order to vary the cutting height of blade 14, that is, the distance at which such blade is spaced from the ground, grass, driveway or other expansive supporting surface 26 on which wheels 24 are normally supported.

When lawn mower I2 is of the type employing a gasoline engine 28, momentary rotations of blade 14 may be effected by pulling starter cord 30 when the ignition wire to spark plug 34 is disconnected. When lawn mower 12 is of the type employing an electric motor, not shown, momentary rotation of blade 14 may be effected by merely. turning on and off the electric motor control switch, also not shown. As will be more fully hereinafter described, one or more momentary rotations of blade 14 relative to sharpening device are employed to effect self sharpening of cutting elements 14a.

Now referring particularly to FIGS. 2-4, it will be seen that blade sharpening device 10 includes a main body portion 40 having opposite and normally like sized upper and lower surfaces 42 and 44, respectively. Body portion 40 is formed of a resiliently deformable material, such as for instance a sheet of readily compressible rubber or plastic foam material, which may be either of the open or closed cell variety. The material forming the body portion should possess a relatively fast recovery rate and be of a thickness sufficient to accommodate for deflections of upper surface 42 and to place such surface at a height convenient for effecting sharpening of the mower blade in the manner to be described.

At least upper surface 42 is dimensioned to substantially encompass an envelope defined by rotation of blade 14, while the body portion as a whole is dimensioned to permit its insertion upwardly within shroud 20, as illustrated in FIG. 2. As by way of example, when device 10 is fabricated for use with mowers having a single cutting blade and thus a substantially cylindrical protective shroud, body portion 40 is preferably shaped as a disc with a diameter at least equal to the diameter of the envelope developed by rotation of the cutting elements or edges of the blade, but less than the minimum transverse dimension of shroud in order to permit at least the upper portion of the device to freely pass inwardly of the shroud for engagement with the blade. Devices may of course be fabricated in a series of sizes and shapes accommodating for the range of sizes and designs of commercially available mowers.

In the preferred embodiment of the present invention, which is illustrated in FIGS. 1-4, a flexible abrasive sheet 46 and an anchor device 48 are suitably attached, as by adhesive bonding, to surfaces 42 and 44, respectively, and an opening 50 is arranged to extend centrally through at least body portion 40 and sheet 46. In the construction shown, opening 50 is dimensioned to freely receive lock nut 16 and the lower end of drive shaft 18 in order to prevent frictional contact thereof with device 10. For mower constructions employing blades supported for rotation about parallel drive shafts, body portion 40 and sheet 46 may have an oval plan view configuration and two through openings in order to insure continuous sharpening contact of blade cutting elements with the device, as well as permitting the device to be freely received within the blade enclosing shroud. In either construction opening 50 performs the additional function of permitting the device to be hung up on a nail or other suitable support so as to permit convenient storage thereof.

Sheet 46 may be selected from commercially available abrasive sheet materials including, for example, fine grit sand papers, garnet papers, emory cloths and fast cutting industrial cloths, whereas the sheet bonding adhesive may be of any permanently flexible adhesive permitting flextures of the associated sheet and body portion without separation. This results in a construction, wherein the abrasive sheet becomes a floating table, which insures accurate following of the blade cutting elements, while preventing tearing of the foam support.

Altemately, it is anticipated that abrasive sheet 46 may be dispensed with and abrasive granules bonded directly to the upper surface of the body portion, particularly when the latter is mold formed with a thin pliable skin, which tends to act as a substitute for the flexible sheet or cloth.

Anchor device 48 is preferably formed of sheet material, which is permanently fixed to surface 44, as by adhesive bonding, and sized to extend outwardly thereof at least sufficiently to underly mower wheels 24 when mower 12 is positioned over device 10 in the manner illustrated in FIG. 2. As will be apparent, the weight of the mower acting through its wheels serves to clamp or press the anchor sheet into frictional engagement with the ground, whereby to constrain rotation of body portion 40 during the blade sharpening operation to be described. If desired, anchor sheet may be apertured to exposed portions of surface 42 or friction material may be applied to its lower surface with a view to assisting in the constraint of rotations of the body portion. Also, the anchor device could be imbedded within body portion 40, during its forming operation, in order to negate the need for a separate attaching operation.

Referring particularly to FIG. 1, it will be seen that when the anchor device is formed of a suitably sized pliable material, it may be wrapped upwardly around body portion 40, so as to provide a sales and/or storage envelope or container therefor. The marginal edge portions of the upwardly wrapped anchor sheet may be secured by a suitable tie or band 52, as indicated in FIG. 1, or merely stuffed downwardly into opening 50. The plan view configuration of the anchor sheet may be of circular, oval, rectangular, cross or other shape, as desired. Pliable sheet materials having utility in the practice of the present invention include by way of example woven fabrics such as cotton duck, molded or cast sheets of plastics including polyethelene and PVC, and open nettings.

Instructions in the use of the sharpening device may be printed directly on the anchor sheet when it is employed as a wrapping or, if desired, on a removable overwrap for the device. Alternatively, desired information may be printed on an insert sheet viewable through the anchor sheet wrapping if the latter is formed of a transparent plastic material or open netting.

Further, it is anticipated that the anchor device may be in the form of a rigid sheet of material, such as for instance plywood, aluminum or plastic, and not sold as an integral part of the sharpening device. In this case, a purchaser would separately purchase an anchor sheet and body portion surface 44 provided, if desired, with an adhesive layer or coating suitable for bonding the device to the anchor sheet. A conventional stripable cover sheet would be provided to protect the adhesive layer prior to use.

While not as versatile or inexpensive as a simple pliable sheet, which I prefer to employ, it would also be possible to form an anchor device with mower wheel receiving recesses or cutouts in order to more positively lock the body portion in accurate alignment with the blade to be sharpened.

While the overall thickness of device 10 may vary, depending upon the construction of mower 12 for which it is designed for us, it is preferable to employ sheets of a resiliently deformable foam material having a thickness greater than about 1 inch in order to effectively isolate surfaces 42 and 44. However, since commercially available lawn mowers conventionally provide for a pouplar 2 inch grass cutting height setting for blade 14, body portion 40 is preferably formed from commercially available 2 inch sheet stock material in order not to require the average user to readjust the setting of the mower wheels each time the blade is to be sharpened. The effective non-compressed thickness of device is slightly greater than the 2 inch blade setting or distance of the blade from the ground, due to the thickness of sheet 46, which will be normally less than about 1/16 inch.

Whenever it is desired to sharpen blade cutting elements 14a, it is merely necessary to lay device 10 on the ground or other generally flat supporting surface 26, as illustrated in FIG. 2. Care should be exercised to insure that the device is laid down upon a relatively clean and dry surface in order to prevent damage to and/or shippage of anchor sheet 48. Thereafter, mower 12 is mowed over device 10 and blade 14 lowered into engagement with the upwardly facing sheet 46, as for example by pivoting mower 12 between its phantom and full line positions shown in FIG. 2; wheels 24 being simultaneously placed in overlying engagement with anchor sheet 48. When blade 14 is thus positioned, it serves to very slightly deflect sheet 46 and thus compress body portion 40, such as is shown in magnified form in FIG. 4. The degree of deflection of sheet 46 need only be sufficient to insure proper sharpening contact of blade 14 therewith, since the weight of mower 12 acting through wheels 24 is primarily relied upon to retain anchor sheet 48 in frictional engagement with the ground.

The thus constrained anchor sheet serves in turn to prevent or constrain rotation of the body portion 40 relative to the supporting surface, whenever mower blade 14 is rotated while in engagement with upwardly facing abrasive sheet 46.

It will be understood that during rotations of blade 14, body portion 40 serves to provide a substantially uniform bias for maintaining sheet 46 in sharpening engagement with blade cutting elements 14a; the body portion permitting momentary flextures of those portions of sheet 46 engaged by the blade to an extent necessary to accommodate for variations in or vertical displacements of the rotational tracks of the cutting elements. Moreover, body portion 40 is believed to permit slight rotational displacements of

sheets

46 and 48, thus aiding in preventing the pulling of sheet 48 from beneath wheels 24 at the initiation of blade rotation.

For mower blades in reasonably good cutting condition and with abrasive sheet 46 in new or only partially worn condition, sharpening of cutting elements 14a may be effected by one or more momentary rotations of blade 14, as for instance produced by pulls on starter cord 30 when the ignition wire is disconnected from spark plug 34.

As by way of example, a cutting device demonstrating a long service life has beenformed by cutting a disc from a two inch thick sheet or resiliently deformable or elastic polyurethane (charcoal polyether) foam material having a density of about 1.25 lbs. per cubic foot, the foam material demonstrating a relatively fast recovery rate and about 25 percent compressibility under an applied load of about psi. This type of foam material is an open cell in structure and commercially available for use as a gasket and package cushioning material. A sheet of fast out drum paper manufactured and sold by the Carborendum Co. under the designation A4- 0E655F was cut to size and bonded to the upper surface of the disc by standard type of permanently flexible rubber cement. An anchor sheet of cotton duck having the weight of medium or sportsweight" denim cloth was bonded to the lower surface of the body portion using a rubber based cement. Cutting devices have also been formed by employing pliable polethelene and PVC plastic anchor sheets, which are in the range of about 25 mils thickness.

Of course, the efficiency cost and useable life expectancy of the sharpening device is expected to vary substantially depending upon the type of foam material and/or abrasive sheet employed.

FIG. 5 illustrates a modified form of the sharpening device of the present invention generally designated as 10. Device 10' differs from the construction described hereinabove in that a pair of abrasive sheets, such as 46', is affixed in juxtaposition to the upper surface of body portion 40; and opening 50' is only of a depth sufficient to freely accommodate any blade attaching device or grass guard depending below the mower blade.

The above construction may be further modified in accordance with the teachings of my co-pending application by forming the centered opening of a relatively large diameter in order to accommodate for mower blade constructions of the type wherein the radial dimension of the blade cutting elements is relatively short, such as for instance wherein two or more blades are mounted to project radially from a circular blade support which is in turn secured to the mower drive shaft.

The term wheel as used herein and the appended claims is meant to include any mower supporting device including for instance endless treads or stationary supporting structures, such as may be carried by an air cushion supported mower vehicle.

I claim:

ll. A method for sharpening the blade of a rotary mower having supporting wheels, wherein said blade is supported for rotation about the axis of a vertically extending drive shaft, including the steps of:

providing a sharpening device formed substantially of a resiliently deformable material wherein an upper portion of said device having abrasive properties is dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition and a lower portion of said device is fixed to anchor means sized to extend outwardly thereof;

laying said device on a substantially horizontal generally planer supporting surface with said upper portion facing upwardly and said anchor means in engagement with said supporting surface; lowering said mower towards said supporting surface so as to place said blade in surface engagement with said upper portion and to place said wheels in engagement with said anchor means; and

effecting at least one momentary rotation of said blade relative to said upper portion while maintaining the plane of said envelope substantially parallel to said upper portion, thereby to effect sharpening of said blade.

2. A method for sharpening the blade of a wheel supported rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and is positioned within a protective shroud arranged outwardly of an envelope defined by said blade upon rotation thereof, said mower supporting wheels being mounted on said mower to position said blade in at least one cutting distance from an expansive mower supporting surface, including the steps of:

providing a sharpening device having a body portion formed of a resiliently deformable material and an anchor sheet, said body portion having upper and lower surfaces, said upper surface carrying abrasive material and being dimensioned to encompass said envelope while being freely receivable inwardly of said protective shroud, said anchor sheet being affixed to said lower surface and being sized to extend outwardly thereof, and the thickness of said device as measured between said opposing surfaces being of a dimension slightly greater than said one distance;

laying said device on said supporting surface with said upper surface facing upwardly and said anchor sheet facing downwardly in engagement with said supporting surface;

lowering said mower to position said wheels in engagement with said anchor sheet with said body portion arranged within said shroud and said blade arranged in surface contact with said upper surface; and

effecting one or more momentary rotations of said blade sufficient to effect sharpening of said blade by said device.

3. A device for sharpening the blade of a rotary mower having wheel means for supporting said mower above a supporting surface, said blade being supported for rotation about the axis of a vertically extending drive shaft, said device comprising:

a body portion formed substantially of a resiliently deformable material, said body portion having an upper surface carrying abrasive material and dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition; and

anchor means, fixed to said body portion and sized to extend outwardly thereof for engagement by said mower wheels when said device is arranged on said supporting surface with said upper surface facing upwardly and said anchor means in engagement with said supporting surface and said blade is juxtaposed in engagement with said upper surface.

4. A device according to claim 3, wherein said anchor means comprises a sheet of material fixed adjacent to a lower surface of said body portion.

5. A device according to claim 4, wherein said sheet is pliable and sized to permit wrapping thereof upwardly to at least substantially enclose said body portion.

6. A device according to claim 4, wherein said body portion is formed of a plastic foam in the shape of a disc and said upper surface of said body portion has a sheet of abrasive material bonded thereto.

7. A device for sharpening the blade of a wheel supported rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and positioned within a protective shroud arranged outwardly of the envelope defined by said blade upon rotation thereof, and wherein said blade is arrangeable in at least one cutting distance from a supporting surface for said mower, said device comprising:

a body portion and an anchor means, said body portion being formed substantially of a resiliently deformable material and dimensioned to permit at least an upper portion thereof to be freely received inwardly of said protective shroud, said body portion having an upper surface carrying abrasive material and a lower surface fixed to said anchor means, said anchor means being dimensioned to extend outwardly of said body portion suficiently to underly said mower wheels when said body portion is received within said shroud, the distance between said body portion surfaces being such that when said anchor means is laid down on said supporting surface and said blade is lowered towards said upper surface and arranged at said cutting distance, said body portion is arranged within said shroud and said blade is positioned in contact with said upper surface, whereby said blade may be sharpened by one or more momentary rotations thereof relative to said body portion while in contact with said upper surface, rotation of said body portion with said blade during momentary rotations thereof being constrained by the weight of said mower acting through said mower wheels to press said anchor means into engagement with said supporting surface.

8. A blade sharpening device according to claim 7, wherein said upper surface having abrasive properties is characterized as including at least one sheet of flexible abrasive material adhesively bonded to said body portion.

9. A blade sharpening device according to claim 8, wherein at least two of said sheets of material are provided, said sheets of material being removably joined together in juxtaposition.

10. A blade sharpening device according to claim 7, wherein said anchor means is formed of pliable sheet material dimensioned to permit wrapping thereof upwardly to at least substantially enclose said body portion.

11. A blade sharpening device according to claim 10, wherein said sheet material is plastic.

12. A blade sharpening device according to claim 10, wherein said sheet material is a fabric.

13. A blade sharpening device according to claim 10, wherein said body portion is of disc shaped configuration and formed of a plastic foam, and said upper surface having abrasive properties is characterized as including at least one sheet of flexible abrasive material adhesively bonded to said body portion.

Claims

1. A method for sharpening the blade of a rotary mower having supporting wheels, wherein said blade is supported for rotation about the axis of a vertically extending drive shaft, including the steps of: providing a sharpening device formed substantially of a resiliently deformable material wherein an upper portion of said device having abrasive properties is dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition and a lower portion of said device is fixed to anchor means sized to extend outwardly thereof; laying said device on a substantially horizontal generally planer supporting surface with said upper portion facing upwardly and said anchor means in engagement with said supporting surface; lowering said mower towards said supporting surface so as to place said blade in surface engagement with said upper portion and to place said wheels in engagement with said anchor means; and effecting at least one momentary rotation of said blade relative to said upper portion while maintaining the plane of said envelope substantially parallel to said upper portion, thereby to effect sharpening of said blade.

2. A method for sharpening the blade of a wheel supported rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and is positioned within a protective shroud arranged outwardly of an envelope defined by said blade upon rotation thereof, said mower supporting wheels being mounted on said mower to position said blade in at least one cutting distance from an expansive mower supporting surface, including the steps of: providing a sharpening device having a body portion formed of a resiliently deformable material and an anchor sheet, said body portion having upper and lower surfaces, said upper surface carrying abrasive material and being dimensioned to encompass said envelope while being freely receivable inwardly of said protective shroud, said anchor sheet being affixed to said lower surface and being sized to extend outwardly thereof, and the thickness of said device as measured between said opposing surfaces being of a dimension slightly greater than said one distance; laying said device on said supporting surface with said upper surface facing upwardly and said anchor sheet facing downwardly in engagement with said supporting surface; lowering said mower to position said wheels in engagement with said anchor sheet with said body portion arranged within said shroud and said blade arranged in surface contact with said upper surface; and effecting one or more momentary rotations of said blade sufficient to effect sharpening of said blade by said device.

3. A device for sharpening the blade of a rotary mower having wheel means for supporting said mower above a supporting surface, said blade being supported for rotation about the axis of a vertically extending drive shaft, said device comprising: a body portion formed substantially of a resiliently deformable material, said body portion having an upper surface carrying abrasive material and dimensioned to encompass an envelope defined by rotation of said blade when in juxtaposition; and anchor means, fixed to said body portion and sized to extend outwardly thereof for engagement by said mower wheels when said device is arranged on said supporting surface with said upper surface facing upwardly and said anchor means in engagement with said supporting surface and said blade is juxtaposed in engagement with said upper surface.

7. A device for sharpening the blade of a wheel supported rotary mower of the type wherein said blade is supported for rotation about the axis of a vertically extending drive shaft and positioned within a protective shroud arranged outwardly of the envelope defined by said blade upon rotation thereof, and wherein said blade is arrangeable in at least one cutting distance from a supporting surface for said mower, said device comprising: a body portion and an anchor means, said body portion being formed substantially of a resiliently deformable material and dimensioned to permit at least an upper portion thereof to be freely received inwardly of said protective shroud, said body portion having an upper surface carrying abrasive material and a lower surface fixed to said anchor means, said anchor means being dimensioned to extend outwardly of said body portion suficiently to underly said mower wheels when said body portion is received within said shroud, the distance between said body portion surfaces being such that when said anchor means is laid down on said supporting surface and said blade is lowered towards said upper surface and arranged at said cutting distance, said body portion is arranged within said shroud and said blade is positioned in contact with said upper surface, whereby said blade may be sharpened by one or more momentary rotations thereof relative to said body portion while in contact with said upper surface, rotation of said body portion with said blade during momentary rotations thereof being constrained by the weight of said mower acting through said mower wheels to press said anchor means into engagement with said supporting surface.