Safety razors

ES 2 223 943 T3

DESCRIPTION

Safety razors.

This invention relates to safety razors of the type in which an assembly of a blade unit is supported by a handle, and which includes at least one blade with a sharp cutting edge that travels across the surface of the blade. the skin being shaved by the means of the handle. A blade unit may be removably mounted on the razor handle, to allow the blade unit to be replaced by a new blade unit, when the sharp part of the blade has been reduced to a level. unsatisfactory, or it may be fixedly mounted to the handle with the intention that the entire razor be discarded when the blade (s) are blunt or dull. Disposable or replaceable blade units are commonly referred to as replacement shaving cartridges. The present invention relates in particular to the blades of safety razor blade units, and is specifically concerned with elongated blades, which have cutting edges extending along the blades.

Safety razor blade units include, in addition to at least one blade, a shield that defines a surface for contacting the skin in advance of the blades as the blade unit travels through the skin. in the execution of the shaving pass, and a so-called cap that defines a surface for contacting the skin behind the blade or blades. It is well known that in order to achieve the best possible efficiency, each blade has to be oriented with respect to the surfaces of the cap and guard so that it is inclined in a plane that is tangential to said surfaces (the "tangent plane"). By blade tilt is meant that the median plane of the blade material, the plane of which is bisector of the angle defined between the edge faces that form the cutting edge of the blade tip, sometimes referred to as the "tangent angle of the blade" with respect to the tangent plane. A blade may be fixed in position so that the tangent angle of the blade does not vary to any significant extent when using the razor, although it is also known that blades can be movably mounted within the frame so that their Tangent angles of the blade may change as a consequence of the movements of the blade in the execution of a shaving pass. In any event, it has been prior practice to support each blade in units of the safety razor blades, such that a substantially flat blade element including the cutting edge and the section of the blade extending from behind the blade. edge with faces at least several millimeters, is located at an appropriate angle to the tangent plane. In a well-known construction the flat blade elements are mounted on bent blade holders, which have a forward and upwardly inclined part, on which blade elements are supported, and vertical rear parts, which are guided in form slidable in the frame, in which the blades are mounted. The need to fix the tangent angles of the blades for correct positioning of the blades during mounting of the blade unit is a problem, and has a tendency to complicate the construction of the blade units, as well as the procedures of the blade itself. mounting. Additionally, in modern razors it is common to have two or three blades with parallel edges, and having a plurality of said blades located in respective inclined planes with respect to the tangent plane, which can inhibit the flexibility of the unit which would be desirable for had.

There have been proposals for safety razors with blade elements having circular cutting edges, which extend around holes in the elements. Examples of such a safety razor are described in US-A-5201253 and US-A5490329. Unlike the straight edge blade, blade elements with circular cutting edges cannot be inclined towards the sharp edge at a desired angle and position relative to the skin during shaving. Consequently, these blade elements do not require having to ensure the necessary shape and inclination of the edges, such as by means of special sharpening techniques, such as those disclosed in US-A-5490329, and / or the necessary shape can be imparted to the circular edges by deformation of the edges after sharpening, as described in US-A-5201253. These proposals have been specifically focused on solving problems inherent in blade elements with circular cutting edges, obviously not having application to blades with straight edges that do not present the same difficulties.

In US-A-5010646 a safety razor blade unit is disclosed which incorporates blades that are bent such that a cutting portion along which the sharp edge extends is at an angle of approximately 112 ° with respect to the base or part of the guide, both parts being corrugated to relieve tension, The blades bent in this way have a substantially reduced flexibility, and they do not offer any solution to the aforementioned drawbacks of the hitherto known flat blades.

A composite blade assembly for a safety razor is described in US-A3938250, which composite blade assembly includes a flat blade and a supplementary blade member attached to the underside of the flat blade, wherein the supplemental blade member is bent to increase the space between the cutting edges respectively provided on the flat blade and the supplemental blade member. Also, the tangent angle of the blade of the supplemental blade member is reduced by this blade member which is bent. The bent blade member is specifically adapted for the production of a compound tandem blade, and does not offer any obvious solution to the drawbacks of known configurations that limit flexibility, requiring that blades configured with certain inclinations provide the tangent angles of

ES 2 223 943 T3 the blades as desired.

With such a blade, a relatively small tip section can be bent, to achieve the tangent angle of the blade as desired, with the main part of the blade being substantially flat, and lying parallel to the tangent plane. , thus allowing improved flexibility, especially in blade units equipped with multiple blades.

The present invention aims to eliminate, or at least substantially reduce the limitations of prior art knife arrangements. The invention is particularly directed to a blade for a unit of a safety razor, comprising an elongated blade element with upper and lower surfaces, a forward edge section having a sharp cutting edge extending along and defined by portions of the surface tapering toward the tip of the cutting edge, and a substantially flat blade section extending rearwardly from the edge section, and through which the upper and lower surfaces are parallel, wherein a transition zone between the flat blade section and the blade section leading edge, the lower surface has a convex curvature and the upper surface is non-convex, preferably being concave.

In accordance with one aspect of the invention, a blade of the form described above is provided, wherein along at least most of the length of the blade, the cross-section of the blade has a second minimum moment of area not larger than 1.0 x 10^-4 mm⁴.

The second moment of the cross-sectional area depends on the dimension and shape of the cross-section, and is a measure of the flexibility of the blade. Although flexibility depends on the Young's modulus of the blade material, razor blade materials commonly used have a close Young's modulus, so the second moment of area is an excellent indicator of flexibility. of the blade. The second moment of the area is measured with respect to an axis, and by the second minimum moment is meant the moment of the determined area with respect to said axis, which gives rise to the lowest value for said cross section of the blade. It is preferable that the second minimum moment of the area does not exceed the value of 0.5 x 10^-4 mm⁴, and that the knives that have a second minimum moment have a value less than 0.35 x 10^-4 mm⁴, and even less than 0.20 x 10 ⁴ mm⁴, for example being possible about 0.14 x 10^-4 mm⁴ or lower.

According to another aspect, the invention provides a blade of the shape defined above, in which a plane dividing the tip of the blade edge in two, which is inclined towards the median plane of the flat blade section, with An angle in the range of 10 ° to 35 °. With the leading edge section set for such inclination, a tangent angle of the blade can be ensured, in which the flat blade section is located substantially parallel to the tangent plane. The angle between the bisector plane of the tip and the median plane of the flat blade section is preferably in the range of 15 ° to 30 °, and more specifically 20 ° to 25 °.

The blade according to a further aspect of the invention has the leading edge section extending rearwardly from the tip of the cutting edge, by a distance of not more than 1 mm, and preferably not more than 0.5 mm.

According to a preferred embodiment, the leading edge section extends rearwardly from the tip over a distance substantially not beyond the portions of the tapering surface, and in particular over a distance several times less than the distance a through which the flat section extends rearward from the front section.

In accordance with the different aspects of the invention mentioned above, the blade element conveniently comprises a metal band having a bend at the junction between the leading edge section and the flat blade section.

The invention also resides, according to a further aspect, in a blade unit of a safety razor that includes a guard surface, a cap surface, and at least one blade that comprises an elongated blade element with surfaces upper and lower, a leading edge section having a sharp cutting edge, extending along, and defined by the surface portions which taper toward the tip of the cutting edge, and a substantially flat blade section extending rearwardly from the leading edge section and through which the upper and lower surfaces are parallel, wherein a transition zone between the flat blade section and the leading edge section, the bottom surface has a convex curvature and the top surface is concave, Therefore, a plane that divides the tip of the edge of the blade in two is located at an angle with respect to a tangent plane, which is tangential to the surfaces of the protection and the cap, with a value greater than the angle of the median plane of the section of the flat blade with respect to the tangent plane.

The median plane of each blade is preferably substantially parallel to the tangent plane, which is tangential to the surfaces of the guard and the cap. With the surface of the upper blade being concave in the area of ​​the transition, the tip of the cutting edge lies above the plane of the upper surface in the section of the flat blade. A preferred blade element has the edge tip at a height of up to 0.25mm, particularly in the range of 0.05mm to 0.15mm, above the plane of the upper surface.

With a blade such as that provided by the invention, the tangent angle of the blade can actually be set during the production of the blades themselves. Additionally, since the main part of each blade can be flat, two or more blades can be arranged in an essentially flat array, allowing the production of a set of blades with substantially greater flexibility than is hitherto obtainable. The advantages of the invention are highlighted when a part of the upwardly inclined blade in the leading edge section is confined to a small area close to the cutting edge.

A plurality of blades, each in accordance with the invention as described above, may conveniently be assembled in a blade assembly, which form a sub-unit for mounting with other components in the manufacture of a blade unit. razor

ES 2 223 943 T3, the blades being interconnected such that their cutting edges are substantially parallel, and the flat sections of the blades are substantially coplanar.

The blades may be conveniently interconnected by transverse bands attached to the lower sides of the flat sections of the blades. These interconnecting bands can also serve to fix the blade assembly to a supporting structure of the blade unit.

In a blade unit of a safety razor incorporating the blades of the present invention, it is advantageous if the flat sections of the blades are coplanar or are located in a plane parallel to the tangent plane that is tangential to the surfaces. of the cap and of the protection, defined by the unit of blades. Such an arrangement makes it possible to take maximum advantage of the flexibility of the blade available by means of the invention.

The blade of the invention may comprise a blade element comprising a metal, for example steel, a band having a bend at the junction between the leading edge section and the flat section. The material of the blade may be conventional, and the cutting edge may comprise coatings such as those known in the art, including hard coatings, such as boron nitride, amorphous diamond-like carbon, which coatings are preferably deposited earlier. of the blade element bending near the tip, although it would be possible to apply one or more coatings subsequent to the bending of the blade element.

According to yet another aspect of the invention, there is provided a method of manufacturing a blade according to the invention as described above, the method comprising the steps of:

- providing a template of a substantially flat elongated blade element, having a sharp cutting edge extending along, and defined by surface portions tapering towards the tip of the edge; Y

- bending the template of the blade element in an area no greater than 1 mm back from the tip of the cutting edge, so that the tip of the cutting edge moves from the initial plane of the template.

During the bending operation, the tip of the cutting edge is preferably offset to a position above an upper surface plane of the template, for example, up to a height of 0.25 mm, such as 0.05 to 0, 15 mm above said plane.

Additionally, the invention resides in the stamping for making the blades according to the invention, the stamping comprising a first member having a flat pressing surface extending forward by an inclined ramp surface, and a second member having a flat pressing surface that faces the flat pressing surface of the first member, the members being guided for movement of the flat surfaces away from each other to press a jig of the blade element in between.

To inhibit unwanted movements of the blade element template during pressing, a backstop may be provided to contact the trailing edge of the blade element template and / or the ramp surface may be limited. by a front stop arranged for engagement by the front edge of the blade element template during pressing thereof.

The blades of this invention are particularly suction cups when used in a blade unit of a safety razor, having a blade support structure that complies with flexibility regulations, such as a blade unit as described and claimed in this applicant's UK patent application number 0025336.9 co-pending and an international patent application claim priority thereof, and whose contents are incorporated herein by reference.

A clear understanding of the invention will be obtained from the following detailed description, where reference is made to the accompanying drawings in which:

Figure 1 shows a safety razor, equipped with a blade unit having blades according to the invention, according to a front perspective view;

Figure 2 is a front elevation of the blade unit of the safety razor of Figure 1;

Figure 3 is an end elevation of the blade unit;

Figure 4 is a cross-sectional view taken along line IV-IV in Figure 2;

Figure 5 is an isometric view of the blade unit;

Fig. 6 is a view showing the blade unit viewed in the direction of arrow VI in Fig. 3;

Figure 7 is an exploded isometric view of the blade unit;

Figures 8 and 9 show the blade unit from the front and rear, respectively, in a deformed condition that can arise during shaving;

Figures 10 and 11 show the blade unit from the front and rear, respectively, in another deformed condition that may arise during shaving;

Figure 12 is a cross section through a blade of the blade unit, the blade being in accordance with the present invention;

Figures 13A, 14A and 15A are schematic illustrations showing successive stages in shaping the blade of Figure 2 by a pressing operation;

Figures 13B, 14B and 15B are enlarged views of the circled areas of Figures 13A, 14A and 15A, respectively; and Figures 16A and 16B are cross sections through two blades, indicating the axis with respect to which the second moment of the area is at a minimum value.

The razor shown in the figures

ES 2 223 943 T3 to 11, and which is described below, is also described and claimed in the aforementioned British application number 0025336.9 and in the claim priority of the international application thereof. The razor has a handle 1 on which a blade unit 2 is mounted. As shown, the handle 1 has a fixed support platform 3, to which the blade unit 2 is firmly attached, but the blade unit 2 could be equally well detachably connected to the handle 1, to allow replacement of the blade unit

two. The blade unit 2 comprises a support structure 4 on which the set of blades 5 is supported. The support structure 4 comprises a unitary rubber molding, or a material that has flexibility properties similar to those of rubber, and that includes an upper frame 6 on the upper face of which the set of blades 5 is positioned, a sub -frame 7 having the shape of a substantially flat sheet, and a base 8 that can also have the shape of a substantially flat sheet. The upper frame 6 and the sub-frame 7 are connected by a movable hinge 8 at their leading edges and are relatively positioned normally to diverge rearwardly from the hinge 9. The sub-frame 7 and the base 8 are connected similarly by a movable hinge 10 at their rear edges and are normally arranged to diverge from each other in the forward direction from hinge 10. With this configuration, the upper frame 6, the sub-frame 7 and the base 8, as seen in an end elevation (Figure 3), or in cross section (Figure 4), define a Z-shape, but with the angle α subtended between the upper frame 6 and the sub-frame 7 greater than the angle β subtended between the sub-frame 7 and the base 8, so that the upper frame 6 is normally fixed at an appropriate angle with respect to the shank of the handle 1, and to ensure the desired deformation characteristics of the support structure 4, as discussed below. Extending between and integrally connected with the upper frame 6 and the sub-frame 7, are the flexible trusses 12 spaced apart from each other, evenly distributed along the support structure 4. As shown there are six trusses 12, although they can use a higher or lower number. The louvers 12 are normally located in respective parallel planes perpendicular to the planes of the upper frame 6 and the sub-frame 7. The flexible lattices 12 constitute respective spring elements and each one is deformed by buckling, to allow the part of the upper frame 6 in the area of ​​said lattice to be displaced towards the subframe 6, where the deformed lattice 12 exerts a restoring force. substantially constant and independent of the degree of buckling, and therefore with the displacement of the upper frame 6. Since the louvers 12 act independently of each other, the different parts of the upper frame 6 along their length will easily move by different magnitudes towards the sub-frame 7.

Subframe 7 is similarly supported relative to base 8 by a series of six integral lattices 14 spaced evenly from each other along the blade unit. Conveniently, louvers 14 are aligned and coplanar with louvers 12. The lattices 14 that can also be deformed by buckling, serve as independent spring elements that act between the sub-frame 7 and the base 8, and allowing the local displacement of the sub-frame 7 towards the base 8 and therefore the handle 1 , meanwhile exerting a substantially constant restoring force that copes with said displacement. The flexible nature of the support structure 4 in the lattices 12,14 is such that the localized parts of the upper frame 6 and the set of blades 5 supported thereon can be deflected towards the handle 1 of the razor, with the in order to adapt the contours of the skin without necessarily influencing the dispositions of the other parts of the skin, and wherein the upper frame 6 and blade assembly 5 can be contorted accordingly to meet the undulations of the skin through which they move. Thus, the blade unit complies flexibly to ensure intimate contact with the skin across the total area encompassed by the blades.

Thus, Figures 8 and 9 show the knife unit 2 with the upper frame 6 and the knife assembly 5 deformed into a concave shape, in which the middle parts M are displaced towards the handle 1 by a greater amount than their end portions B with the louvers 12, 14 towards the center M of the blade unit buckling to a greater extent than the louvers 12, 14 located closer to the ends E of the knife unit 2. Figures 10 and 11 on the contrary show the knife unit 2 deformed in a convex configuration, in which the knife assembly 5 and the upper frame 6 are displaced downwards towards the handle 1 to a greater extent at the ends E of the blade unit than in the central part M of the blade unit, and in this case the lattices 12, 14 towards the ends E of the blade unit 2 are warped to a greater extent than the lattices 12, 14 closer to the center M of the blade unit 2. Although both sets of spring lattices 12,14 are shown bent in Figures 8-11, this is not unavoidable or essential. It is possible, for example, as a result of the downward shaving force applied towards the rear R of the blade unit of the louvers 12 to bend, such that the upper frame 6 and the blade assembly 5 move towards low adjacent to the rear edge R, without the louvers 14 bending and without any displacement of the upper frame 6 and of the blade assembly 5 at the front edge F. Likewise, a force applied near the front edge can cause a downward displacement of the upper frame 6 and of the blade assembly at its front edge F, due to the bending of the louvers 14 without the louvers 12 actually bending. As a consequence, the upper frame 6 and the blade assembly 5 comply both in the longitudinal direction of the blade unit and in the perpendicular direction, in order to closely conform to the contours of the area of ​​the skin being shaved. . Due to the subtended angle α between the upper frame 6 and the subframe 7 is greater than the subtended angle β between the subframe 7 and the base 8, the lattices 12 are somewhat longer and correspondingly weaker than the lattices 14, whereby the lattices 14 exert a greater resistance to the downward movement of the upper frame 6 and the set of blades 5, in

ES 2 223 943 T3 its front edge F, that the resistance to the downward movement exerted by the lattices 12 at the rear edge R of the upper frame 6 and the set of blades 5, whose characteristic is considered desirable as the greater shaving forces are they generally impart to the blade unit in the area of ​​the guard than with respect to the force exerted in the area of ​​the cap.

The blade assembly 5 comprises a guard member 15, and a plurality of elongated blades 16, guard member 15, and the blades 16, formed of flexible metal bands, and wherein the blades 16 have sharp edges 17 facing forward at parallel way. The guard member 15 and the blades 16 are interconnected by transverse bands 18, which are attached to the lower sides of the blades and the guard member 15. The maximum flexibility of the blade unit is guaranteed by the blades 16, and wherein the transverse connecting strips 18 are coplanar in the normal condition and without deformation of the blade assembly 5 and the blade unit 2. The guard member 15 is substantially coplanar also with the blades 16 and the connecting strips 18, although as shown in Figures 6 and 7, this guard member 15 has an upwardly sloping rear, and the slits 19 are spaced along the length of this part of the protection member 15, to enhance the flexibility of this member. The inclusion of the guard member 15 in the blade assembly may be advantageous in reliably defining the geometry of the shaving of the blades, and of the first blade in particular. The bands 18 have inverted T-shaped ends that engage with the grooves 20, 21 molded in the front and rear edges of the upper frame 6, in order to fix the blade assembly 5 in the support structure. The upper frame 6 includes the longitudinal members of the frame 22 in the front and rear members and in the transverse frame 23 in which the upper edges of the lattices 12 are fixed. In the assembled unit 2 the bands 18 of the blade assembly 5 extend above the respective frame members 23. The cap 24 of the blade unit 2 includes a flexible lubrication band 25, which is seated in a groove that extends along the rear longitudinal member 22 of the upper frame 6 and is held in position by the transverse bands 18 of the assembly. of blades. The support structure 4, at the front of the upper frame 6 in the hinged connection area to the sub-frame 7, forms a protection 26, which has the longitudinal ribs 27 molded thereon, although protrusions could be provided with other settings. Also, if preferred, a separate flexible shield element could be mounted on the support structure to provide a desired shield configuration.

In accordance with the present invention, each of the blades 16 comprises an elongated blade element with a section of the front edge 50 (Figure 12) forming the sharp cutting edge 17, the tip of this edge being confined between the parts of the blades. opposing surfaces 51, 52, hereinafter referred to as faces with a taper towards the tip, and a substantially flat blade section 54, extending rearwardly from the leading edge section 50. The leading edge section 50 is inclined upward toward the tip of the cutting edge 17 so that the plane P₁ bisecting the angle between the faces 51, 52 at the tip of the cutting edge, is inclined with an angle α of about 20 ° to 25 ° with the plane P_m of the flat section 54. The tip of the cutting edge 17 can be positioned above the plane P_or from the top surface in the flat section to a height of up to 0.25mm, preferably with a height in the range of 0.05 to 0.15mm. The inclination of the inclined front section 50 is obtained by bending a flat blade member, template 60 as shown in Figures 13A, 14A and 15A, by pressing on the template 60 of the blade member in a set. sharpening, comprising a first lower member 61 and a second upper member 62. The lower sharpening member 61 defines a flat pressing surface 64, which extends forward via a ramping surface 65. The upper sharpening member 62 has a pressing surface 66, which confronts the flat pressing surface. 64 of the lower member 61, and the upper member 62 is movable relative to the lower member 61 by pressing a jig of the blade element 60 between the flat pressing surfaces. The lower member 61 has a rear stop 67 for contacting the trailing edge of a blade element template 60, and a front stop 68 located on the ramping surface 65 for contacting the tip of the edge of the blade. knife. The stops 67, 68 can be adjustable to fit the width of the templates of the elements of the blade 60 to be pressed, and to be able to adjust the position of the fold to be produced with respect to the tip of the edge of the blade 17, the rear stop being 67 vertically adjustable and having a sloped front face for this purpose.

An elongated flat blade jig 60 with a finished cutting edge 16 along its leading edge is inserted between the upper and lower sharpening members 61, 62, being brought to rest against the flat surface 64 of the lower member with its trailing edge in contact with trailing edge 67, and wherein the tip of cutting edge 17 rests on ramping surface 65, and against front bumper 68 (Figures 13A and B). The template is initially inclined at a small angle to the flat surface 64. Upper member 62 is then driven downward, bringing its pressing surface 66 into contact with template 60, and causing the portion of template 60 between surfaces 61, 66 to be pressed flat against surface 64, and thus that the leading edge section 50 of the blade is pulled down against the ramping surface (Figures 14A and B). In one example, the blade edge faces 51, 52 extend rearward from the edge tip by 0.3mm, the blade thickness being 0.08mm, and wherein the leading edge of the blade Upper tool is aligned to contact the blade at a distance of 0.4mm from the tip. When the upper member 62 is raised again, the bend produced in the blade template 60 by the pressing operation recovers somewhat, but a permanent configuration remains in the blade template 60, such that the section of the

ES 2 223 943 T3 leading edge 50 is inclined at the desired inclination with respect to the flat blade section 54, extending rearwardly therefrom, and in a transition zone between the leading section and the flat section, having the lower surface a convex curvature, while the upper surface is not convex, and more particularly concave. After the completion of the blade shaping operation as described, a part of the template including the inclined leading edge section and a flat section several times wider than the inclined leading edge section, in the direction from the part Front to back, it can be cut or split from the template to provide a flexible blade band 16 of the desired width. The blades thus produced can then be used to produce blade assemblies as incorporated into the safety razor of Figures 1 to 11.

The blades 16 of the invention, manufactured as described above, are characterized in that they have a relatively high flexibility, regardless of being bent to raise the tip and tilt the cutting edge 17, to obtain a tangent angle of the blade to the section. plane 54 of the blade positioned parallel to the tangent plane. Flexibility can be conveniently expressed by the second moment of the cross-sectional area of ​​the blade, since the material of the razor blade uses Young's moduli which differ little from each other. The second moment of the area I of a cross-sectional area with respect to an axis X located in the plane of the cross-section, is the sum of the products obtained by multiplying each element of the area dA by the square of its distance and from the X axis. So:

T = fy²gives

It will be appreciated that the second moment of the area is dependent on the shape and size of the cross section.

The minimum area of ​​the second moment is the second moment of the area measured with respect to said X axis, which gives rise to the smallest value for the second moment of the area I_min, shown in Figures 16A and 16B, which correspond to the cross sections, taken in planes perpendicular to the length of the cutting edges, of the two blades 16, according to the invention. The blade 16 of Figure 16A is 0.08 mm thick and 0.80 mm wide, where the flat section 54 is 0.40 mm wide, and its second minimum moment of the area Im with respect to to the X axis is 0.322 x 10^-4 mm⁴. The blade of figure 16B has a thickness of 0.08 mm and a width of 0.80 mm, the flat section 54 being 0.10 mm wide, and in this case the second minimum moment of the area I_min 0.145 x 10 ⁴ mm⁴. As described above, the blades 16 of the invention are preferably positioned in a blade unit 2 with the median planes of their flat sections 54 substantially parallel to the tangent plane, tangential to the surfaces of guard 24 and cap 24 of the unit of blades, and the second moment of area I being measured around the axis parallel to these median planes, and with the intersection of the area centers for the blades shown in Figures 16A and 16B having values ​​of 0.443 x 10^-4 mm⁴ and 0.263 x 10 ⁴ mm⁴, respectively. To achieve a desired level of blade flexibility in an assembled blade unit 2, it is also found that the second moment of the area with respect to an axis passing through the center of the area, and parallel to the median plane of the flat section 54 shall not be greater than 1.0 x 10^-4 mm⁴, and more especially not more than 0.5 x 10 ⁴ mm⁴. If the blade 16 has a cross section that is constant along its length, the values ​​of I given above will be applicable to each position along the length. However, there are short sections of the blade where the values ​​of I are different, because the cross section is different, and the desired flexibility can be ensured by the blade having a shape such that the specified values ​​of I are applicable to along at least the length of the blade.

Although the invention has been previously described in relation to shaping a blade having faces at the edges, prior to bending of the blade, which are symmetrical with respect to the center plane of the blade, this is not essential and the edge is The blade could have an initial asymmetrical shape, including a so-called bevel-like edge, in which the tip is located in the plane of the upper surface of the blade.

Modifications to the specifically described embodiments are of course possible without departing from the principles of the invention, and may be incorporated by those skilled in the art. It will be understood, therefore, that the particular embodiments described above are provided by way of non-limiting examples, and it is intended that the scope of the invention should be limited only by the claims that follow.