GB2031704A - A rotary cutter - Google Patents

Abstract

A rotary cutter (1) for a lawnmower has an undulate cutting edge (3) and the cutting edge region of the cutter is shaped by stamping to produce a series of formations (4). Each formation (4) presents on the top side of the cutter, shown, a quadrilateral-shaped planar surface (8) which is inclined downwards both in the direction from left to right and in the direction from top to bottom, in Fig. 1. Thus, the top surface of the cutter (1) in the cutting region is stepped whereas the corresponding bottom surface is ground to produce a continuous planar surface parallel to the plane of the cutter rotation. The cutter may be a cutter bar (1) as shown in Fig. 1 or it may be in the form of a disc. <IMAGE>

Description

SPECIFICATION A rotary cutter The invention relates to a rotating cutting knife, in particular for motor-driven lawn-mowers, and undulate cutting edges lying in a common grinding plane and permitting an oblique shearing or drawing cut.

The problem with which the invention is concerned may be illustrated by the example of the development of cutting knives in lawnmowers, though the invention is not restricted to this area of application.

In conventional lawn-mowers a cutter bar rotatably driven about a vertical axis is employed, which is provided on two diagonally opposite edge corners with a cutting edge running vertically to the direction of rotation, such as is shown for example in Fig. 5 of German Auslegeschrift 12 67 461. However, no oblique shearing or drawing cut can be achieved with such cutting edges.

It is known from German Utility Model 1 9 26 958 to give the cutting edge of such a cutter bar a curved configuration having a radius that is essentially smaller than the radius of the cutting edge, referred to the rotational axis of the knife. In this known knife the cutting edge is formed by tapering the wall of the cutter bar along a conical surface.

As a consequence, regrinding has to take place along this conical surface and thus along a curved path, which results in a not inconsiderable expense.

It is however also known in lawn-movers to employ a circular cutter disc rotating about a vertical axis, whose edge has a saw-tooth configuration. If crossed teeth are used, each individual tooth must be reground. If the cutting edges lie in one plane, as in the case of a rotatably driven bread knife, it is necessary to give the edge region of the cutter disc a frustoconical configuration and carry out regrinding along this frustoconical surface.

The object of the invention however is to develop a rotatably driven cutting knife so that, as regards the curved cutting edges that permit an oblique shearing or drawing cut, the regrinding of the said cutting edges can be carried out not along a conical surface but instead along a flat surface, while allowing all points of the curved cutting edges to be included in the regrinding.

Starting from the afore-mentioned basic shape of a rotating cutting knife, the essential feature of the invention is that the wall of the cutting knife in the region of the individual cutting edges is subjected to chipless forming in an angular position deviating from the circumferential direction and in a plane inclined to the grinding plane of the cutting edges, and the forming along the grinding plane of the cutting edges is worked in a chipforming manner, and is in particular trued or milled as well as ground off. In this connection, it is recommended to shape the base surface of the individual chipless forming in such a way that it approximately corresponds to an irregular, trapezoidal-like quadrilateral.

On account of the impression or stamping of the forming according to the invention and the subsequent removal of the forming along the grinding plane, a curved cutting edge which lies over its whole length exactly in the grinding plane is necessarily formed in each forming. A precondition for this is of course that the direction of the longitudinal axis of the individual forming deviates from the circumferential direction (in the case of cutter bars) or from the radial direction (in the case of circular cutter discs), and the base surface of this forming lies in an inclined position relative to the grinding plane and in a direction sloping towards the cutting edge.

All the formings considered necessary for the cutting knife can be carried out in a single work stage using a relatively simple stamping tool. Accordingly, the formed cutting knife needs to be worked in a chip-forming manner along the grinding plane, which is thus not curved, and the finished profiled cutting knife provided with curved cutting edges permitting an oblique shearing or drawing cut is obtained without any further treatment. The manufacture of such cutting knives as well as the regrinding is simple and inexpensive, and stationary knives can be reground extremely quickly using a plane-surface grinding disc.

The form of the curved cutting edges according to the invention can be used in cutter bars as well as in rotating cutting discs. Thus, the invention relates not only to the special form of cutting knives for lawn-mowers but also covers all other possibilities of application in widely differing fields in which an oblique shearing or drawing cut, produced by as many curved cutting edges as possible, is required.

Further features and structural details of the invention are given in the sub-claims and also illustrated in the drawing. The invention is illustrated diagrammatically and by way of example in the drawing, in which: Figure 1 is a plan view of part of a cutting knife in the form of a cutter bar, Figure 2 is a longitudinal section through the cutter bar according to Fig. 1, along the line ll-ll, Figure 3 is a cross-section through the cutter bar according to Fig. 1, along the line lil-lil, and Figure 4 is a partial plan view of a circular cutter disc as a variant of the cutter bar according to Fig. 1.

In the embodiment according to Figs. 1 to 3, a cutting knife such as is used in lawnmowers in a circular shape of a cutter bar, is denoted by the reference numeral 1. This cutting knife 1 rotates about a rotational axis 2 and has cutting edges 3 on diagonally opposite edges, referred to the rotational axis 2.

The cutting edges 3 are formed undulate and curved so that an oblique shearing or drawing cut is produced on rotation of the cutting knife about the axis 2. Furthermore, the cutting edges 3 lie along their whole length in the flat grinding plane 7, as shown by the chain-dotted line in Fig. 2.

The undulate cutting edges 3 can be formed in a very simple manner. A plurality of formings 4 is first of all produced in the region of each cutting edge 3 by chipless treatment. The base surface of the individual formings 4 approximately corresponds to an irregular quadrilateral, similar to a trapezoidal surface. The longitudinal axis of such a forming 4 is denoted by the reference numeral 6.

This longitudinal axis 6 is rotated by the angle a with respect to the circumferential direction 5 of the cutting knife 1, referred to a cutting edge point 1 6. The base surface of the forming 4 is also formed as an inclined plane 8, as shown in Fig. 2. This inclined plane 8 means that the formings 4 on the edges approximately parallel to the longitudinal axis 6 have unequal heights and depths. In Fig. 1, the deepest point of the forming is denoted by the reference numeral 9 and the highest point of the forming by the reference numeral 10.

When the cutting knife 1 is provided in the afore-described manner with formings 4, the cutting profile illustrated in Fig. 2 is then formed along the cutting axis ll-il of Fig. 1.

All that now remains to be done is to remove in a chip-forming manner along the grinding plane 7, for example by milling or truing, the regions of the formings 4 located on the underside of the cutting knife 1, and the curved undulate profile of the cutting edges 3 according to Fig. 1 is obtained simply by means of this procedure. It can easily be seen that a cutting knife manufactured in this way can be particularly easily reground along the grinding plane 7 with the aid of a planesurface grinding disc.

So that the grinding of the cutting edges 3 does not coincide with the base surface of the cutting knife 1, the whole region covering the cutting edges 3 and formings 4 is offset in a stepped manner 1 3 compared with the remaining part of the knife.

The profiles 4 can be produced in the same work stage with a suitably adapted stamping tool with the formation of ejectable blades 1 8 conventional in such cutting knives, as is illustrated diagrammatically in Figs. 1-3. The blades 1 8 serve for ejecting the grass or other material cut by the knives.

Cutting knives 14 having a circular base surface can however also be profiled in a similar manner according to Fig. 4. In this case too the direction of the longitudinal axis 6 of the formings 4 deviates from the radial direction 15, but the two dirctions pass through a common cutting edge point 1 6.

Likewise, the heights and depths of the formings 4 are formed along inclined planes, the deepest point 9 of the forming being located on the next internal curvature 11 and the highest point 10 of the forming being located on the next external curvature 1 2 of the cutting edge 3. In this case too it is recommended to provide an offset region 1 7 so that the cutting edges 3 can be ground along a flat surface without interfering with the remaining region of the cutting disc 14.

Without necessarily limiting the scope of the invention claimed, preferred embodiments of the invention may be summarised as fol lows: - 1. A rotating cutting knife, in particular for motor-driven lawn-mowers, with undulate cutting edges permitting an oblique shearing or drawing cut, characterised in that the wall of the cutting knife (1, 14) in the region of the individual cutting edges (3) is subjected to chipless forming in an angular position (a) deviating from the circumferential direction (5) and in a plane (8) inclined to the grinding plane (7) of the cutting edges (3), and the forming (4) along the grinding plane (7) of the cutting edges (3) is worked in a chip-forming manner, and is particular trued or milled as well as ground off.

2. A cutting knife according to Summary 1, characterised in that the base surface (8) of the individual chipless forming (4) corresponds to an irregular, trapezoidal-like quadrilateral.

3. A cutting knife according to Summary 1 or 2, characterised in that the central axis (6) of the forming (4) forms an acute angle (a) deviating from the direction of the rotational axis (2) of the cutting knife (1), the tangent being in the circumferential direction on the cutting edge point (16) located on the central axis (6).

4. A cutting knife according to any one of the preceding summaries, characterised in that the inclined plane (8) of the individual chipless forming (4) (impression) has a radial depression that increases outwardly referred to the rotational axis (2) of the cutting knife (1).

5. A cutting knife according to any one of the preceding summaries, characterised in that a cast profile is provided instead of the individual chipless forming.

6. A cutting knife according to any one of the preceding summaries, characterised in that the formings (4) or profiles are provided, on diagonally opposite edges of a cutter bar (1) whose cross-section and base surface is approximately rectangular.

7. A cutting knife according to any one of the preceding summaries, characterised in that the formings (4) or profiles are provided on diagonally opposite edges of a cutter bar (1) whose cross-section and base surface is approximately rectangular.

7. A cutting knife according to summary 6, characterised in that the region (13) enclosing the adjacent formings (4) or profiles is displaced in a stepped manner compared with the remaining region of the knife.

8. A cutting knife according to any one of the preceding summaries, characterised in that the base surface (8) of the forming has a radially outwardly sloping shape.

Although reference numerals have been used in the appended claims to improve the intelligibility of these claims, it is expressly stated that these reference numerals should not be construed as limiting the claims to the construction illustrated in the accompanying drawings.

Claims (11)

1. A rotary cutter (1 or 14) having at least one cutting edge (3) which is movable about a rotational axis and of undulate configuration as viewed in a direction parallel to the rotational axis cutter for obtaining an oblique shearing or drawing cut, characterised in that the cutter (1 or 14), in the region of the or each undulate cutting edge (3), has a plurality of formations (4) serially arranged along the cutting edge (3), each formation (4) presenting a cutting edge portion inclined at an angle to its direction of cutting movement (5) at any instant as viewed in a direction parallel to the rotational axis, each formation (4) presenting on one side of the cutter a planar surface (8) which is inclined with reference to planes extending perpendicular to the rotational axis, as viewed in direction perpendicular to the rotational axis, and the formations (4) together presenting on the other side of the cutter a continuous planar surface (7) extending perpendicular to the rotational axis.

2. A rotary cutter, according to Claim 1, characterised in that the planar surfaces (8) are quadrilateral-shaped.

3. A rotary cutter, according to Claim 1 or Claim 2, characterised in that the cutter is a rotary cutter bar (1) of a generally rectangular shape and of a generally rectangular crosssection, the cutter bar (1) having two diagonally-opposite cutting edges (3).

4. A rotary cutter, according to Claim 1 or Claim 2, characterised in that the cutter is a circular disc (14) having a continuous cutting edge (3) on its periphery.

5. A rotary cutter, according to any preceding claim, characterised in that each planar surface (8) is inclined in a direction from said one side of the cutter to said opposite side of the cutter as seen in a cross-section of the knife taken in a plane perpendicular to the cutting edge (3) whereby a depression formed in said one side of the cutter between each two adjacent planar surfaces (8) increases in depth in the direction towards the cutting edge (3).

6. A rotary cutting knife, according to any preceding claim, characterised in that the or each region (13 or 17) of the cutter including the formations (4) is offset from the remainder of the cutter in the direction of the axis of rotation of the cutter.

7. A rotary cutter, according to any preceding claim, characterised in that said formations (4) are produced by a method not involving the production of particles or chips.

8. A rotary cutter, according to Claim 9, characterised in that said method comprises stamping by a stamping tool.

9. A rotary cutter, according to Claim 9, characterised in that said method comprises casting.

10. A rotary cutter, according to any preceding claim, characterised in that said continuous planar surface (7) is formed by milling.

11. A rotary cutter, according to any one of Claims 1 to 9, characterised in that said continuous planar surface (7) is formed by grinding.

1 2. A rotary cutter, according to any preceding claim which is a rotary cutter for a lawn-mower.

1 3. A rotary cutter, substantially as here it before described, with reference to, and as illustrated in, the accompanying drawings.