Robotic Lawnmower Cutting Arrangement, Robotic Lawnmower, Cutting Blade, and Methods

Abstract

A robotic lawnmower comprises a blade carrier (200) and a cutting blade (400) pivotally connected thereto, the blade comprising a substantially flat cutting portion (401) extending along a cutting portion plane (410), a blade carrier interface (404), and an offset portion (406) interconnecting the cutting portion (401) and the blade carrier interface (404). When operating the robotic lawnmower, the blade carrier interface (404) of the cutting blade (400) follows a circular path in a blade carrier interface rotation plane (415), and the cutting portion (401) follows a circular path in a cutting plane (402), offset from the blade carrier interface rotation plane N (415) by a cutting plane offset distance (408).

Claims

1. A robotic lawnmower cutting arrangement comprising a blade carrier configured to be rotated by a cutting motor about a blade carrier rotation axis, the blade carrier extending radially away from the blade carrier rotation axis and comprising a blade attachment interface radially offset from the blade carrier rotation axis the blade attachment interface being configured to pivotally hold a cutting blade; and a cutting blade comprising a substantially flat cutting portion provided with a cutting edge, and a blade carrier interface pivotally connected to the blade attachment interface of the blade carrier, the pivotal connection thereby allowing the cutting blade to pivot relative to the blade carrier about a blade pivot axis offset from the blade carrier rotation axis such that, when the blade carrier is rotated about the blade carrier rotation axis, the blade carrier interface of the cutting blade follows a circular path in a blade carrier interface rotation plane perpendicular to the blade carrier rotation axis, and the cutting portion follows a circular path in a cutting plane, wherein the cutting edge extends within the cutting plane, and the cutting plane is axially, with respect to the carrier rotation axis, offset from the blade carrier interface rotation plane by a cutting plane offset distance, or wherein the cutting blade further comprises an offset portion interconnecting the cutting portion and the blade carrier interface and extending in a direction transversal to the cutting portion plate, such that the cutting portion plane is axially, with respect to the carrier rotation axis, offset from the blade carrier interface.

2. The robotic lawnmower cutting arrangement according to claim 1, wherein the cutting plane offset distance exceeds 5 mm.

3. (canceled)

4. The robotic lawnmower cutting arrangement according to claim 1, wherein the pivotal connection between the blade carrier interface of the cutting blade and the blade attachment interface of the blade carrier is configured to allow free pivoting of the cutting blade about the blade pivot axis.

5. The robotic lawnmower cutting arrangement according to claim 1, wherein the blade carrier interface is integrally formed with the cutting portion, or wherein the cutting blade is integrally formed of a sheet material.

6. (canceled)

7. The robotic lawnmower cutting arrangement according to claim 6, wherein a maximum thickness of the sheet material of the cutting portion is less than 2 mm.

8. The robotic lawnmower cutting arrangement according to claim 1, wherein the blade carrier interface is made of a sheet material extending in a blade carrier interface plane, wherein the blade carrier interface plane is substantially parallel to at least one of a cutting plane and a cutting portion plane.

9. The robotic lawnmower cutting arrangement according to claim 1, wherein the cutting blade is detachably connected to the blade carrier.

10. The robotic lawnmower cutting arrangement according to claim 1, wherein the engagement between the blade carrier interface and the blade attachment interface has a radial play, with respect to the blade pivot axis, of at least 5 mm.

11. The robotic lawnmower cutting arrangement according to claim 1, wherein the engagement between the blade carrier interface and the blade attachment interface allows tilting the blade about a blade tilt axis extending through the blade attachment interface in the direction of travel of the blade attachment interface during rotation of the blade carrier.

12. (canceled)

13. The robotic lawnmower cutting arrangement according to claim 1, wherein the cutting blade is elongate and has a cutting blade length along its direction of elongation, wherein the cutting portion has a cutting portion length along said direction of elongation of at least 20% of the cutting blade length.

14. The robotic lawnmower cutting arrangement according to claim 1, wherein the blade carrier interface comprises a through-hole having an elongate shape along a plane perpendicular to the blade pivot axis.

15. The robotic lawnmower cutting arrangement according to claim 1, wherein the cutting blade extends between a proximal end, which is provided with said blade carrier interface, and a distal end, which is provided with said cutting portion.

16. The robotic lawnmower cutting arrangement according to claim 1, wherein the blade carrier is made of plastic, or wherein the blade carrier has a circular disc shape, or wherein the cutting edge is substantially straight.

17. (canceled)

18. A robotic lawnmower cutting blade comprising: a substantially flat cutting portion extending along a cutting portion plane, the cutting portion being provided with a cutting edge; a blade carrier interface; and an offset portion interconnecting the cutting portion and the blade carrier interface and extending in a direction transversal to the cutting portion plane, thereby holding the blade carrier interface at a position offset from the cutting portion plane.

19. A robotic lawnmower comprising the robotic lawnmower cutting blade of claim 18.

20. The robotic lawnmower according to claim 19, wherein the blade carrier rotation axis vertical when the robotic lawnmower is positioned in normal operating position on a horizontal surface.

21. The robotic lawnmower according to claim 19, wherein at least one of a cutting plane and a cutting portion plane is substantially horizontal.

22. The robotic lawnmower according to claim 19, further comprising a controller and a cutting motor, wherein the controller is configured to operate the cutting motor below a limit RPM adapted to give the cutting portion a maximum impact energy of less than 5 joules.

23. The robotic lawnmower according to claim 22, wherein the cutting motor is an electric motor configured to be operated at an output power of less than 200 watt.

24-27. (canceled)

28. A method of producing a robotic lawnmower cutting blade, comprising: bending a sheet metal blank to form a substantially flat cutting portion, a substantially flat blade carrier interface portion, and an offset portion separating the cutting portion from the blade carrier interface portion in a direction transversal to the plane of the cutting portion; and either attaching a cutting edge to the cutting portion, or grinding the cutting edge (403); and after grinding the cutting edge, locally treating the cutting edge.

29. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

[0036] FIG. 1 is a view in perspective of a robotic lawnmower system;

[0037] FIG. 2 is a block diagram illustrating functional blocks of a robotic lawnmower;

[0038] FIG. 3 is a side view in section of a robotic lawnmower cutting arrangement, and illustrates three different configurations of a pivotal attachment of a cutting blade to a blade carrier;

[0039] FIG. 4A illustrates a section of a cutting blade and blade fastener assembly of the robotic lawnmower cutting arrangement of FIG. 3;

[0040] FIG. 4B is a perspective view of the cutting blade of FIG. 4A;

[0041] FIG. 4C is a top view of the cutting blade of FIG. 4A;

[0042] FIG. 5 is a top view of the robotic lawnmower cutting arrangement of FIG. 3 when in operation;

[0043] FIG. 6 is a side view in section of the robotic lawnmower cutting arrangement of FIG. 3 when mowing a lawn;

[0044] FIG. 7 is a side view in section of the robotic lawnmower cutting arrangement of FIG. 3, with and without axial and radial play of the cutting blade relative to the blade pivot axis;

[0045] FIG. 8 is a side view in section of an alternative embodiment of a robotic lawnmower cutting arrangement;

[0046] FIG. 9 is a side view in section of yet an alternative embodiment of a robotic lawnmower cutting arrangement;

[0047] FIG. 10 is a side view in section of still another alternative embodiment of a robotic lawnmower cutting arrangement;

[0048] FIG. 11 is a perspective view of an alternative embodiment of a cutting blade;

[0049] FIG. 12 is a perspective view of yet an alternative embodiment of a cutting blade;

[0050] FIG. 13 is a perspective view of yet an alternative embodiment of a cutting blade; and

[0051] FIG. 14 is a flow chart illustrating two alternative methods of manufacturing a robotic lawnmower cutting blade.

[0052] All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the embodiments, wherein other parts may be omitted.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0053] FIG. 1 schematically illustrates an overview of a robotic lawnmower system 1 configured to mow a lawn 2 within a predefined work area 12 delimited by a boundary wire 11 emitting a magnetic field in the manner known in the art. The robotic lawnmower system 1 comprises a self-propelled robotic lawnmower 10 and a charging station 16. The robotic lawnmower is provided with wheels, such as a pair of front wheels 18 and a pair of rear wheels 20, for moving within the work area 12. Typically, at least one of the wheels 18, 20 is connected to a motor, such an electric motor, either directly or via a transmission (not illustrated), for propelling the robotic lawnmower 10 across the lawn 2.

[0054] FIG. 2 illustrates functional blocks of the robotic lawnmower 10. In the example of FIG. 2, each of the rear wheels 20 is connected to a respective electric propulsion motor 24. This allows for driving the rear wheels 20 independently of one another, enabling e.g. sharp turning of the robotic lawnmower 10. The robotic lawnmower 10 further comprises a controller 26. The controller 26 may be connected to sensors, actuators, and communication interfaces of various kinds, and may be implemented using a central processing unit executing instructions stored on a memory 28. Needless to say, different combinations of general and application-specific integrated circuits may be used as well as different memory technologies. In general, the controller 26 is configured to read instructions from the memory 28 and execute these instructions possibly in view of different sensor signals to control the operation of the robotic lawnmower 10. Typically, the controller 26 is configured to, based on the instructions, control the robotic lawnmower in an autonomous or semi-autonomous manner, i.e. with no, or only occasional, instructions from a human operator. The controller 26 also controls the operation of a cutter motor 300, which is configured to drive a cutting arrangement comprising a blade carrier holding a set of cutting blades in a manner which will be elucidated further below.

[0055] A wireless data transceiver 32 is connected to the controller 26, and allows the controller 26 to communicate with the charging station 16 or any other device, such as a remote control or a smart phone (not shown).

[0056] The robotic lawnmower 10 further comprises a navigation system 34. In the illustrated example, the navigation system 34 comprises an inertial navigation device 36, such as an accelerometer or a gyroscope, and a magnetic field sensor 38 configured to detect a magnetic field emitted by the boundary wire 11 (FIG. 1) on/in the ground. A boundary wire may be used for defining the boundaries of the area 12 to be treated, or to otherwise provide a reference to assist the robotic lawnmower 10 to navigate. The inertial navigation device 36 allows the robotic lawnmower 10 to keep track of its movement within the area 12 to be treated. The inertial navigation device 36 may be supplemented by a compass (not shown), to provide basic orientation information that may compensate for any drift of the inertial navigation device 36.

[0057] The controller 26 also controls the propulsion motors 24, thereby controlling the propulsion of the robotic lawnmower 10 within the area 12 to be treated. The propulsion motors 24 may be stepper motors, allowing the controller 26 to keep track of the respective numbers of turns of the motors 24, and thereby also the distance travelled by the robotic lawnmower 10, as well as any turning angle of the robotic lawnmower 10 when the motors 24 are operated at different speeds or in reverse directions. In this respect, the propulsion motors 24 may operate as odometers. Alternatively, the wheels 20 may be provided with odometer indexers configured to provide feedback to the controller 26 as regards the number of turns of each motor 24. The navigation system 34 further comprises a GNSS (Global Navigation Satellite System) receiver 42 Navigation information from the navigation system 34 and the motors 24 is fused in the controller 26 to provide an accurate position indication, in order to enable e.g. a systematic movement pattern of the robotic lawnmower 10, wherein the robotic lawnmower 10 traverses the lawn 2 along parallel, adjacent mowing tracks.

[0058] The controller 26, navigation system 34, transceiver 32, and electric motors 24, 30 are powered by a battery 40. The robotic lawnmower 10 is configured to navigate to the charging station 16 on a regular basis, and/or whenever the battery charge is running low, in order to dock with the charging station 16 for recharging the battery 40. The charging station 16 may be connected to receive power from the electric power grid.

[0059] Battery powered robotic mowers are limited by their battery capacity and energy efficiency for cutting grass. If energy consumption can be reduced, cutting coverage, cutting quality and cutting speed can be gained, while reducing wear and tear of the robotic mower machinery.

[0060] FIG. 3 shows an example of a robotic lawnmower cutting arrangement 100 of the robotic lawnmower 10 of FIGS. 1-2. The robotic lawnmower cutting arrangement 100 comprises a blade carrier 200 configured to be rotated by the cutting motor 300, via a cutting motor shaft 301, about a vertical blade carrier rotation axis 201. The blade carrier 300 may be configured as an injection-molded plastic component. The blade carrier 200 may be rotation symmetric and, in the illustrated example, has a circular shape as seen along the blade carrier rotation axis 201, which circular shape is concentric with the blade carrier rotation axis 201. The blade carrier 200 comprises a set of blade attachment interfaces 202 radially offset from the blade carrier rotation axis 201, each blade attachment interface 202 being configured to pivotally hold a respective cutting blade. In the illustration of FIG. 3, for clarity of illustration, only one of the blade attachment interfaces 202 is connected to a respective cutting blade 400.

[0061] The cutting blade 400 comprises, at a distal 424 end thereof, a substantially flat cutting portion 401 extending along a cutting portion plane 410 which is horizontal, i.e. parallel to the surface of the lawn to be cut. The cutting portion 401 is provided with a cutting edge 403 facing in a tangential direction with respect to the blade carrier rotation axis 201. A blade carrier interface 404 is arranged at a proximal end 422 of the cutting blade 400. The blade carrier interface 404 pivotally connects the cutting blade 400 to the blade attachment interface 202 of the blade carrier 200 via a cutting blade attachment screw 418, which operates as a pivot pin and thereby defines the blade pivot axis 405. The blade pivot axis 405 may be vertical, as is illustrated by the blade 400 attached to the blade carrier 200. According to other embodiments, however, the blade pivot axis 405 may be inclined radially inwards by an inwards inclination angle 407, or radially outwards by an outwards inclination angle 409, as is illustrated by the alternative positions 413′, 413″ of the cutting blade 400 and cutting blade attachment screw 418 relative to respective blade pivot axes 405′, 405″. The blade pivot axis 405 is radially separated from the blade carrier rotation axis 201 by a pivot axis radial offset 102, defined as the radial distance from the blade carrier rotation axis 201 to the point where the pivot axis 405 passes through a plane 415, which plane 415 passes through the blade carrier interface 404, and which plane 415 is perpendicular to the blade carrier rotation axis 201. A typical pivot axis radial offset 102 may be between 50 and 500 mm; and more typically, between 100 and 300 mm.

[0062] Between the cutting portion 401 and the blade carrier interface 404, the cutting blade 400 has an offset portion 406 which extends in a direction transversal to the cutting portion plane 410, and thereby defines an axial, with respect to the blade carrier rotation axis 201 as well as the blade pivot axis 405, cutting plane offset 408 between the cutting portion 401 and the blade carrier interface 404. The cutting plane offset distance 408 may be, by way of example, about 15 mm.

[0063] When operating the robotic lawnmower cutting arrangement 100, the blade carrier 200 is rotated about the blade carrier rotation axis 201 such that the blade 400 orbits the blade carrier rotation axis 201. Thereby, the blade carrier interface 404 of the cutting blade 400 follows a circular path in a blade carrier interface rotation plane 415 perpendicular to the blade carrier rotation axis 201, and the cutting portion 401 follows a circular path in a cutting plane 402, which planes 402, 415 are vertically offset from each other by the cutting plane offset distance 408.

[0064] FIGS. 4A, 4B and 4C illustrate the cutting blade 400 in greater detail, wherein the section of FIG. 4A also provides an exploded view of a fastener assembly for attaching the cutting blade 400 to the blade carrier 200. Starting with the section view of FIG. 4A, the cutting blade 400 is integrally formed of steel sheet. The thickness 419 of the steel sheet may be, for example, somewhat less than 1 mm. The proximal end 422 with the blade carrier interface 404 is integrally formed with the rest of the blade 400, and the blade carrier interface 404 extends in a blade carrier interface plane 412 coinciding with the blade carrier interface rotation plane 415. Thereby, the blade carrier interface plane 412 is parallel to the cutting plane 402 as well as the cutting portion plane 410. The fastener assembly comprises a cutting blade attachment screw 418 configured to penetrate the blade carrier attachment interface 404 and be secured in the blade attachment interface 202 of the blade carrier 200 (FIG. 3), and a washer 411 configured to be positioned between the cutting blade 400 and the blade carrier 200 (FIG. 3). The washer 411 is operative to reduce the friction in the pivotal engagement between the cutting blade 400 and the blade carrier 200. The blade attachment screw 418 is configured to, when secured to the blade carrier 200, offer the blade carrier interface 404 a slight axial play, such that the cutting blade 400 can pivot in, and slide along, the blade carrier interface rotation plane 415.

[0065] As is illustrated in FIG. 4B, the blade carrier interface 404 is configured as an elongate through-hole. The elongate shape of the through-hole 404 offers the engagement between the blade carrier interface 404 and the blade attachment interface 202 (FIG. 3) a radial play, with respect to the blade pivot axis 405, corresponding to the length of the elongate through-hole along the carrier interface rotation plane 415, minus the diameter of the blade attachment screw 418. A typical radial play may be, for example, of the order 10-50 mm.

[0066] Turning now to FIG. 4C, the cutting blade 400 comprises a pair of straight cutting edges 403, which extend along opposite side edges of the cutting blade 400. Each of the edges 403 faces in a respective tangential direction, relative to the blade carrier rotation axis 201. The cutting blade 400 is elongate, with its direction of elongation extending from the proximal end 422 to the distal end 424, and has a cutting blade length L1 along its direction of elongation. The cutting portion 401 has a cutting portion length L2 along said direction of elongation of about half the cutting blade length L1. The blade carrier interface portion 422, extending in the blade carrier interface plane 412 (FIG. 4A), has a blade carrier interface portion length L3 along said direction of elongation, which is also about half the cutting blade length L1. The proximal end 420 of the cutting blade has a pair of chamfers 430, which facilitate the cutting blade's 400 pivoting motion about the blade pivot axis 405 (FIG. 4B.

[0067] FIG. 5 illustrates the robotic lawnmower cutting arrangement 100 in operation, and as seen from below, illustrating four cutting blades 400 pivotally attached to the blade carrier 200. The freedom of each blade 400 to pivot about a respective pivot axis 405 (FIG. 4B), and the radial play of the cutting blade 400 in relation to its respective blade pivot axis 405 (FIG. 4B), allow the cutting blade 400 to freely rotate 504 about the pivot axis 405 (FIG. 3), move radially outwards 503 from the blade carrier rotation axis 201 in response to centrifugal forces, move radially inwards 501 towards the blade carrier rotation axis 201 in response to a collision with an uncuttable object 500, and also to pivot 502 about the pivot axis 405 in response to a collision with an uncuttable object 500. Referring back to FIG. 2, the controller 26 may be configured to operate the cutting motor 300 below a limit RPM adapted to give each respective cutting portion 401 a maximum impact energy of less than 2 joules, thereby resulting in a robotic lawnmower cutting arrangement which is relatively safe to people. For such a situation, it may be sufficient that the cutting motor 300 be operated at an output power of less than 20 watt.

[0068] FIG. 6 illustrates the robotic lawnmower cutting arrangement 100 in operation, and as seen from the side, illustrating two of the four cutting blades 400 of FIG. 5. While moving in a forward direction 600 across the lawn, the robotic lawnmower cutting arrangement 100 is rotated about the blade carrier rotation axis 201 by the cutting motor 300. Grass blades 501 entering the front end 610 of the robotic lawnmower cutting arrangement 100 will be cut from the side, in a tangential direction relative to the blade carrier rotation axis 201. When passing the trailing end 620 of the robotic lawnmower cutting arrangement 100, any remaining, un-cut grass blades will be given a second chance to be cut at the same cutting height. When passing the trailing end 620, the grass blades 501 will again be cut from the side, in an opposite tangential direction, thus leaving an even quality cut lawn after just one passage of the robotic lawnmower cutting arrangement 100. The non-cutting parts 404, 406 of the cutting blade 400 will rotate at a distance above the cut grass 503, minimizing friction against the grass.

[0069] As is apparent from FIG. 6, the robotic lawnmower 10 (FIG. 1) is operated by rotating each blade carrier interface 404 about the blade carrier rotation axis 201 in the blade carrier interface rotation plane 415; and rotating the cutting portions 401, with the cutting edges 403 facing in the tangential direction of travel of the cutting blade 400, wherein the cutting edges 403 are rotated in the cutting plane 402 below the blade carrier interface rotation plane 415.

[0070] As is illustrated in FIG. 7 an axial play in the engagement between the blade carrier interface 404 and the blade attachment interface 202, defined by an axial distance between the head of the blade attachment screw 418 and the blade carrier 200 exceeding the thickness of the washer 411 and the sheet material of the cutting blade 400, allows tilting the blade 400 somewhat about a blade tilt axis extending through the blade carrier interface 404 in the direction of travel of the blade carrier interface 404 during rotation of the blade carrier 200, i.e. in a tangential direction relative to the blade carrier rotation axis 201. The tilt play 450 thus obtained may, for example, allow a tilt angle of between −5 degrees and +5 degrees relative to a horizontal plane. Upon rotation about the blade carrier rotation axis 201, the cutting blade 400 will, in response to centrifugal forces acting thereupon, automatically assume a tilt position suitable for cutting.

[0071] FIG. 8 schematically illustrates an alternative embodiment of a robotic lawnmower cutting arrangement 100a. The robotic lawnmower cutting arrangement 100a comprises a flat blade carrier disc 200, on which a cutting blade 400a is suspended in a non-illustrated manner. The cutting blade 400a is similar in most aspects to the cutting blade 400 described in detail hereinabove, but differs from the cutting blade 400 in that the blade carrier interface plane 412a is non-parallel to the blade carrier interface rotation plane 415 (FIG. 3) and the cutting portion plane 410, and the pivot axis 405 of the carrier interface is non-vertical. Still, the cutting plane 402 and the cutting portion plane 410 coincide, and are horizontal. Thanks to the blade pivot axis 405 being inclined radially outwards, when hitting an uncuttable object, the cutting blade 400a may pivot about the pivot axis 405 to a raised position, higher above the ground. The inclination is oriented such that the blade pivot axis 405 lies in the same plane as the blade carrier rotation axis 201. When the blade carrier 200 is rotated about the blade carrier rotation axis 201, the path of the blade pivot axis 405 defines a cone having its apex pointing downwards.

[0072] FIG. 9 schematically illustrates yet an alternative embodiment of a robotic lawnmower cutting arrangement 100b. The robotic lawnmower cutting arrangement 100b is similar in most aspects to the robotic lawnmower cutting arrangement 100a described with reference to FIG. 8, but differs from the robotic lawnmower cutting arrangement 100a of FIG. 8 in that the carrier interface plane 412b is parallel to the cutting portion plane 410b, and both planes are inclined relative to a horizontal plane. Still, the carrier disc rotation axis 201 is vertical such that the cutting plane 402, i.e. the plane along which the grass tips are being cut, defined by the circle followed by the lowermost ends of the cutting edges of the cutting blades 400, is horizontal. The cutting portion plane 410b is inclined relative to the cutting plane 402, as seen in the illustrated vertical section comprising the blade carrier rotation axis 201 and the blade carrier interface.

[0073] FIG. 10 illustrates still another alternative embodiment of a robotic lawnmower cutting arrangement 100c. The robotic lawnmower cutting arrangement 100c is similar in most aspects to the robotic lawnmower cutting arrangement 100a described with reference to FIG. 8, but differs from the robotic lawnmower cutting arrangement 100a of FIG. 8 in that the carrier interface 404 extends along a carrier interface plane 412c which coincides with the offset portion 406.

[0074] FIG. 11 illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400d. The cutting blade 400d is provided with two blade carrier interfaces 404 and two cutting portions 201, and is functionally symmetric about its offset portion 406, such that it can be attached to the blade carrier 200 (FIG. 3) at either end.

[0075] FIG. 12 illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400e. The cutting blade 400e is integrally formed with a threaded rod 1200, which doubles as both offset portion 406 and blade carrier interface 404, as well as defines the blade pivot axis 405.

[0076] FIG. 13 illustrates still another alternative embodiment of a robotic lawnmower cutting blade 400f, which has its cutting portion plane 410 inclined relative to the cutting plane 402, as seen in a radial direction from the blade pivot axis 405.

[0077] Thanks to the ability of reaching very close to the ground with high precision and low friction, the methods and devices herein are suitable for cutting very short grass, in applications such as golf court fairways and golf court greens.

[0078] The cutting blade described above may be produced following a production method illustrated in the flow chart of FIG. 14, the method comprising the steps

[0079] 1101: providing a sheet metal blank;

[0080] 1102: bending the sheet metal blank to form a substantially flat cutting portion 401 (FIG. 4B), a substantially flat blade carrier interface portion 420, and an offset portion 406 separating the cutting portion 401 from the blade carrier interface portion 420 in a direction transversal to the plane of the cutting portion;

[0081] 1103: grinding a cutting edge 403; and

[0082] 1104: after grinding the cutting edge, locally treating the cutting edge to change the material properties thereof, for example to change its hardness.

[0083] In an alternative embodiment, steps 1103 and 1104 may be replaced by the step 1105: attaching a cutting edge 403 to the cutting portion 401. Also, the order of steps 1102 and 1105 may be reversed.

[0084] The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims. For example, a vertical a blade carrier rotation axis has been illustrated. However, also a robotic lawnmower cutting arrangement having a blade carrier rotation axis which is tilted relative to a vertical axis may benefit from the teachings disclosed herein. Hence, even though less preferred for cutting very short grass, such an arrangement is also intended to be within the scope of the claims. Similarly, a blade carrier without any free-rotating lower protection plate, such as that of EP0808096 B1, has been illustrated. However, also a robotic lawnmower cutting arrangement having a free-rotating lower protection plate may benefit from the teachings disclosed herein. Hence, even though less preferred for cutting very short grass, such an arrangement is also intended to be within the scope of the claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.