EP1465749B9 - Tool for surface treatment of an optical surface - Google Patents

Abstract

A tool for surface treatment of an optical surface, includes a rigid support having a transverse end surface, an elastically compressible interface which is applied against and overlaps the end surface, and a soft buffer designed to be applied against the optical surface and which is pressed against and overlaps at least partly the interface opposite and perpendicular to the end surface. The buffer includes a so-called central part which is located between the end surface and a so-called peripheral part which is located transversely beyond the end surface, elastic return elements connecting the peripheral part to the support.

Description

The invention relates to the surfacing of optical surfaces.

Surfacing means any operation to modify the surface condition of a previously shaped optical surface. These include polishing, grinding or etching operations to modify (decrease or increase) the roughness of the optical surface and / or to reduce the undulation.

US Pat. No. 1,665,292 already discloses a polishing wheel for sanding the surfaces of the bodywork of an automobile, comprising a thick felt pad and a support constituting a drive member itself driven for example by a flexible shaft. from an engine. The support has a flange to which are riveted flat elastic blades positioned radially. At their inner end, the resilient blades are sharpened so that they fit on the face of the flange located on the side of the felt pad, that is to say on the end face of the support. The outer ends of the elastic blades are received in a pocket made of a double layer of leather one layer rests against the pad and the other covers the curved outer ends of the blades. Between the flange and the central part of the buffer is disposed a helical spring welded to the support. Sanding by this grinding wheel is essentially done at its periphery.

The invention relates to a tool for surfacing an optical surface, which comprises a rigid support having a transverse end surface, an elastically compressible interface which is applied against and covers said end surface, and a flexible pad suitable for to be applied against the optical surface and which is applied against and at least partially covers the interface opposite and to the right of said end surface.

In order to reduce the roughness of the optical surface, the tool is brought into contact with the latter while maintaining sufficient tool pressure on it so that, by deformation of the interface, the pad conforms to the shape of the optical surface. .

While watering the optical surface with a fluid, it is rotated relative to the tool (or reciprocally) and is scanned by means of the latter.

Generally, the optical surface is rotated, its friction against the tool being sufficient to jointly drive it in rotation.

The surfacing operation requires an abrasive that can be contained in the buffer or in the fluid.

During surfacing, the elastically compressible interface makes it possible to compensate for the difference in curvature between the end surface of the tool support and the optical surface, so that the same tool is suitable for a range of surfaces. optical curvatures and different shapes.

When the transverse extent of the tool is comparable to the extent of the optical surface, which is generally the case for surfacing ophthalmic lenses, the range of optical surfaces that the same tool is able to plan is relatively limited .

Thus, this type of tool is particularly poorly suited to surfacing optical surfaces of complex shapes, called "freeform" in English, particularly aspherical, which by definition have a non-uniform curvature.

In addition, this type of tool is also poorly suited to optical surfaces having a convexity or concavity gap that is too pronounced relative to the tool: in the first case, the edges of the tool lose contact with the optical surface. ; in the second case, it is the central part of the tool that loses contact with the optical surface, resulting in incomplete surfacing.

To increase the range of optical surfaces that one tool is able to surface, two options are possible.

A first is to reduce the diameter of the tool, that is to say, its overall transverse dimension, so as to restrict and locate the portion of the optical surface in contact with the tool. On such a portion located in fact, the contact of the tool with the surface remains more homogeneous than considering this optical surface taken as a whole.

However, this restriction of the diameter of the tool is accompanied by a decrease in its "lift" or "sitting" and, consequently, its stability on the optical surface during surfacing.

It is then necessary to control, and thus to enslave, the orientation of the tool so that it is optimal at each moment, that is to say so that the axis of rotation of the tool is collinear or substantially collinear with the normal to the optical surface at the point of intersection of said axis with the optical surface.

But such servo requires the use of complex means such as a numerically controlled machine, the cost is generally high and may even be prohibitive for a surfacing operation.

A second option is, maintaining the diameter of the tool, to soften the interface, either by increasing its thickness, or by decreasing its elasticity.

But the latter then tends, under the effect of shearing forces, to twist or shift laterally, to the detriment of the efficiency and accuracy of the tool: In addition, the shear causes rapid wear, even a destruction, of the interface. Finally, the flexibility of the interface favors and accentuates the effects of scraping the pad against the edge of the lens, for ultimately risk leading to premature and / or untimely destruction of the tool.

In view of the foregoing, optical surface manufacturers, and in particular ophthalmic lens manufacturers, are resigned to using a large number of different tools, sizes and bends to cover the range of their surfaces. optics.

The invention aims in particular to solve the aforementioned drawbacks by proposing a surfacing tool which, while being adapted to a range of optical surfaces sufficiently large, in terms of curvatures (convexity, concavity) and shapes (spherical, toric, aspherical, progressive or any combination thereof, or more generally "freeform"), has good stability during surfacing, and allows surfacing at once safe, fast and of good quality while being of reduced cost.

For this purpose, the invention proposes a tool for surfacing an optical surface, which comprises a rigid support having a transverse end surface, an elastically compressible interface which is applied against and covers said end surface, as well as a flexible pad adapted to be applied against the optical surface and which is applied against and covers at least part of the interface opposite and to the right of said end surface, the buffer comprising a so-called central part which is at right of the end surface, and a peripheral portion which is transversely beyond said end surface, elastic return means connecting this peripheral portion to the support, the combination of said peripheral portion and return means forming means for stabilizing the tool during surfacing, said tool being adapted to perform a surfacing essentially at said central portion; the.

It is thus possible to polish an optical surface whose extent is much greater than the transverse dimension of the support without the problem of the stability of the tool.

It is then possible to use the same tool for a relatively wide range of optical surfaces to be surfaced.

In particular, the same tool is adapted to surface surfaces whose convexity - or concavity - has compared to that of the tool a gap relatively pronounced, as it is particularly suitable for surfacing surfaces of complex shape, including toro-progressive.

It is thus possible to cover the whole of a given range of lenses with a variety of tools (curvature, concavity, convexity) and, therefore, a fleet of tools restricted to the benefit of costs, including logistics.

Various embodiments correspond to the invention that has just been defined.

Thus, according to a preferred embodiment, the buffer is monoblock, its central and peripheral parts forming a single piece, to the benefit of simplicity of realization.

For example, the pad has a plurality of petals projecting transversely from its central portion, which corresponds to the usual form in which the surfacing pads are made.

Alternatively, the buffer comprises a ring surrounding the central portion, so that when the buffer is monoblock, it is, in the absence of stress, in the form of a disc.

Furthermore, the interface may also comprise a central portion which is at the right of the end surface of the support, and a peripheral portion, which is transversely beyond this end surface, and which is interposed between the peripheral portion of the buffer and the return means.

This results in greater flexibility of the whole.

The peripheral portion of the interface is, for example, in the absence of stress, in the form of a ring surrounding its central portion.

Furthermore, the tool may comprise a deformable ring transversely surrounding the support and interposed between the peripheral portion of the interface and the return means.

It has been found that this arrangement makes it possible to increase the regularity of the surfacing.

In order to further optimize this regularity of surfacing, the ring will preferably be chosen with a circular longitudinal section.

Furthermore, according to a particular embodiment, the interface is monobloc, its central and peripheral parts forming a single piece, to the benefit of simplicity of implementation.

Thus, the interface is for example, in the absence of constraint, in the form of a disk.

As regards the return means, they comprise for example an elastically flexible strip projecting transversely from the support, and connected to the support by a first end, and to the peripheral portion of the buffer by a second end.

Preferably, this strip is rigidly anchored in the support by its first end, in favor of good stability of the tool.

According to a particular embodiment, the return means comprise a starry part fixed to the support and provided with branches each forming an elastically flexible strip.

The use of such a part, which, moreover, is relatively simple, allows to obtain a good regularity in the reminder of the peripheral portion of the buffer during surfacing.

As regards the mounting of this starry part, the support comprises for example two jaws fixed to each other, the starry part having a central portion pinched between the two jaws and which project its branches.

Preferably, when the pad is in one piece and comprises, as a peripheral part, a plurality of petals, as previously mentioned in one of the exposed embodiments, each branch of the star piece is positioned at the right of a petal .

For example, the petals and branches are, respectively, seven in number, which is sufficient to ensure a surfacing both fast and good quality.

As for the end surface, it can be flat, concave or convex, which allows, with a small number of tools, to surface a large number of optical surfaces.

• la figure 1 est une vue en perspective éclatée d'un outil conforme à l'invention et d'une lentille ophtalmique présentant une surface optique à surfacer;
• la figure 2 est une vue en perspective de l'outil de la figure 1 représenté assemblé, au cours du surfaçage de la surface optique de la lentille de la figure 1 ; pour illustrer le mouvement de l'outil par rapport à la lentille au cours du surfaçage, on a représenté l'outil dans trois positions, dont deux sont matérialisées en traits mixtes ;
• la figure 3 est une vue partielle en coupe de l'outil et de la lentille de la figure 2, suivant le plan de coupe III-III ;
• la figure 4 est une vue d'élévation en coupe de l'outil de la figure 3 représenté seul, au repos ; la représentation en traits interrompus des moyens de rappel élastiques illustre la déformation de ces derniers au cours du surfaçage ;
• la figure 5 est une vue analogue à la figure 4, suivant une première variante de réalisation ;
• la figure 6 est une vue analogue aux figures 4 et 5, suivant une deuxième variante de réalisation ;
• la figure 7 est une vue schématique de dessus représentant une lentille ophtalmique en cours de surfaçage au moyen d'un outil conforme à l'invention, l'outil étant représenté au cours du balayage de la surface optique dans deux positions dont l'une est illustrée en traits interrompus.

Other features and advantages of the invention will emerge in the light of the following description of an embodiment given by way of nonlimiting example, description made with reference to the accompanying drawings in which:

• Figure 1 is an exploded perspective view of a tool according to the invention and an ophthalmic lens having an optical surface to be surfaced;
• Figure 2 is a perspective view of the tool of Figure 1 shown assembled during surfacing of the optical surface of the lens of Figure 1; to illustrate the movement of the tool relative to the lens during surfacing, the tool is shown in three positions, two of which are shown in phantom;
• Figure 3 is a partial sectional view of the tool and the lens of Figure 2, along the section plane III-III;
• Figure 4 is an elevational sectional view of the tool of Figure 3 shown alone, at rest; the dashed representation of the elastic return means illustrates the deformation of the latter during the surfacing;
• Figure 5 is a view similar to Figure 4, according to a first embodiment;
• Figure 6 is a view similar to Figures 4 and 5, according to a second embodiment;
• FIG. 7 is a schematic view from above showing an ophthalmic lens being surfaced by means of a tool according to the invention, the tool being represented during scanning of the optical surface in two positions, one of which is illustrated in broken lines.

FIG. 1 shows a tool 1 for surfacing an optical surface 2, in this case one of the faces of an ophthalmic lens 3. In FIG. 1, as in FIGS. 2 and 3, the optical surface 2 concerned is represented concave, but it could just as well be convex.

The tool 1 is formed of a stack of at least three parts, namely a rigid portion 4, an elastically compressible portion 5, and a flexible portion 6, which, in what follows, will be called respectively support, interface and buffer.

As it appears in particular in Figure 1, the support 4 comprises two jaws, namely a lower jaw 7 and an upper jaw 8 adapted to be superimposed being nested one into the other via a pin 9 protruding from one of the faces of the upper jaw 8, adapted to be housed in a complementary hole 11 formed, opposite, in a face 12 of the lower jaw 7.

As can be seen in FIG. 1, the support 4 is generally cylindrical with symmetry of revolution and has an axis of symmetry denoted X, which defines a so-called longitudinal direction.

N is the normal to the optical surface 2 at the point of intersection of the axis of symmetry X of the tool 1 with the latter.

Opposite its face 12 in which the hole 11 is made, the lower jaw 7 has an end surface 13 extended substantially transversely, against which is applied, covering the interface 5.

The buffer 6 is applied against the interface 5 on the other side of the latter with respect to the support 4.

More specifically, the pad 6 covers at least part of the interface 5 opposite and to the right of the end surface 13.

The friction of the pad 6 against the optical surface 2 will, by means of an abrasive contained in the watering fluid or incorporated in the pad 6 itself, to ensure a superficial removal of material on the optical surface 2 in order to to modify the surface state, as we will see later.

According to the invention, the buffer comprises, on the one hand, a central portion 6a which is at the right of the end surface 13, and a peripheral portion 14 which is, transversely, beyond the end surface 13.

This peripheral portion 14 is connected to the support 4 by means 15 of elastic return.

The peripheral portion 14 extends in the extension of the central portion 6a while being, at rest, substantially coplanar with it.

According to a preferred embodiment; illustrated in Figures 1 to 6, the buffer 6 is in one piece, the peripheral portion 14 being connected to the central portion 6a, so that they form in fact a single piece.

According to a preferred embodiment shown in bold lines in FIG. 1, the tampon 6 is in the form of a flower, and thus comprises a plurality of petals 14b which, projecting transversely from the central portion 6a, form the peripheral portion 14 of the buffer 6 and each extend transversely beyond the end surface 13.

According to a variant shown in phantom in Figure 1, the peripheral portion 14 is in the form of a ring 14a which surrounds the central portion 6a.

In this case, in the absence of stress, the buffer 6 is, when it is monobloc, in the form of a disc of material whose thickness is small in front of its diameter, as represented in FIG. the peripheral portion 14, 14a thus forming a flange with respect to the end surface 13.

The return means 15, which will be described later, can be interposed directly between the support 4 and the peripheral portion 14 of the buffer 6, that is to say, in practice, the flange 14a or the petals 14b.

However, according to a preferred embodiment illustrated in the figures, the interface 5 comprises not only a central portion 5a which is at the right of the end surface 13, but also a peripheral portion 16 which is transversely beyond of the end surface 13.

This peripheral portion 16 is in the extension of the central portion 5a, and is for example, in the absence of stress, in the form of a ring which surrounds the central portion 5a, and which is in fact interposed between the peripheral portion 14 of the buffer 6 and the return means 15.

As it appears in FIGS. 1 to 6, the interface 5 is in one piece, its central parts 5a and peripheral 16 being in fact connected together to form one and the same piece, the peripheral part 16 forming a collar with respect to the end surface 13.

Thus, in the absence of stress, the monoblock interface 5 is for example in the form of a disc of material whose thickness is small in front of its transverse dimension (that is to say, its diameter).

When the interface 5 and the buffer 6 are both monobloc, they have comparable transverse dimensions. In particular, when are each in the form of a disk of material, they will preferably be chosen, for constructive convenience, the same diameter. However, it will also be possible to use a pad of a diameter different from that of the interface, in particular of greater diameter in order to mitigate the edge effects of the tool on the worked surface.

In addition, for reasons which will appear later, it is provided, according to an embodiment illustrated in Figures 1 to 6, a deformable ring 17 interposed between the peripheral portion 16 of the interface 5 and the return means 15 .

In practice, this ring 17 is fixed on the peripheral portion 16 on the other side thereof relative to the pad 6, that is to say on the same side as the support 4, and so that it it is surrounded by ring 17.

Preferably, this ring 17 is of circular longitudinal section, but it could also be of more complex section, in particular oblong, polygonal, rectangular or square. Moreover, it is arranged on the peripheral part 16 concentrically with the support 4.

The return means 15 are now described.

These comprise at least one elastically flexible strip 18 which projects transversely from the support 4 and which is connected, on the one hand, rigidly to the support 4 by a first end 18a and, on the other hand, to the peripheral portion 14 of the buffer 6 by a second end 18b, said free end, opposite the first 18a.

In this way, under the effect of a force exerted longitudinally on the peripheral portion 14 to the right of the plate 18, the latter is deformed by exerting on the peripheral portion 14 a reaction opposite to said force.

In practice, the return means 15 comprise a plurality of such strips 18 distributed uniformly around the periphery of the support 4, to act on the entire peripheral portion 14 of the buffer 6.

According to an embodiment illustrated in particular in Figures 1 and 2, the return means 15 are in fact in the form of a starry part 19 rigidly fixed to the support 4.

This star piece 19 comprises a central portion 20 from which a plurality of branches 18 project each forming an elastically flexible strip extending radially in a transverse plane.

For fixing the star piece 19 to the support 4, its central portion 20 is, in practice, pinched between the jaws 7, 8 of the support 4, its centering being provided by means of a through hole 21 made in its center, crossed by the pin 9 of the upper jaw 8, the assembly being held by means of fastening means such as screws which, passing through the upper jaw 8 and the central portion 20 of the star piece 19, are caught in the jaw lower 7.

When, in accordance with an embodiment previously described, the one-piece stamp 6 comprises a plurality of petals 14b, there are provided on the star piece 19 as many branches 18 as petals 14b, the star piece 19 being oriented so that each branch 18 extends to the right of a petal 14b. Thus, when the pad 6 has seven petals 14b, the starry part 19 comprises, in turn, seven branches 18 each to ensure the elastic return of a petal 14b.

Although several embodiments are provided, as we have seen above, it has been found that the tool 1 corresponding to the embodiment illustrated in FIGS. 1 to 6 allowed a particularly satisfactory surfacing.

According to this embodiment, the buffer 6 and the interface 5 are both monoblock pieces, the interface 5 being in the form of a disk of material, the buffer 6 being in the form of a flower, while the return means 15 are in the form of a starry part 19 as previously described, and between the free ends 18b of its branches 18 and the interface 5 is interposed a deformable ring 17 of circular section.

The ring 17 is fixed to the interface 5 and the free ends 18b of the branches 18, this attachment can be provided by any means, the bonding being however preferred, especially for its simplicity.

In the embodiment shown, the diameters of the interface 5, the buffer 6, the star part 19, have a value at least twice that of the diameter of the support 4.

Moreover, when it comes to surfacing an ophthalmic lens, the diameters of the interface 5 and the buffer 6 are chosen substantially equal to the diameter of the lens 3, so that the diameter of the support 4 is much smaller than the diameter. of the lens 3.

The use of the tool 1 is illustrated in FIGS. 2 and 3.

This is in the case of surfacing or smoothing of a convex face 2 aspheric of an ophthalmic lens.

The lens 3 is mounted on a rotary support (not shown) by means of which it is driven in rotation about a fixed axis Y.

The tool 1 is applied against this face 2 with sufficient force so that the buffer 6 matches its shape. The tool 1 is here, meanwhile, free in rotation yet being off-center with respect to the optical surface 2. A drive forced rotation of the tool, by own means, however, can be provided.

The relative friction of the optical surface 2 and the buffer 6 is sufficient to rotate the tool 1 in the same direction as that of the lens 3, about an axis substantially coincident with the axis X of symmetry of the support 4.

The optical surface 2 is sprayed with a non-abrasive or abrasive watering fluid, depending on whether or not the pad performs this function.

In order to scan the entire optical surface 2, the tool 1 is moved during the surfacing along a radial trajectory, the point of intersection of the axis of rotation X of the tool 1 with the optical surface 2 effecting a plane. movement back and forth between two cusp points, namely an external cusp A and an internal cusp B both located at a distance from the axis of rotation Y of the lens 3.

The central portion 6a of the tampon 6 is deformed by conforming to the shape of the optical surface 2 thanks to the compressibility of the central portion 5a of the interface 5.

As for the peripheral portion 14 of the tampon 6, it deforms by conforming to the shape of the optical surface 2 thanks to the deformation of the flexible strips 18.

Given the rigidity of the support 4, the removal of material takes place in majority right of the end surface 13, that is to say that this removal of material is carried out essentially by the central portion 6a of the buffer 6.

As for the peripheral portions 14 of the buffer 6 and 16 of the interface 5, they essentially have a stabilizing role, on the one hand thanks to the increase in the lift or seating of the tool 1 compared to a conventional tool whose buffer and the interface would be limited to the central portions 5a, 6a and, secondly, thanks to the return means 15 which maintain a permanent contact between the peripheral portion 14 of the buffer 6 and the optical surface 2.

The deformable ring 17 allows a smoothing of the stress distribution exerted on the peripheral periphery of the interface 5 and hence on the pad 6 by the strips 18.

As a result, regardless of the location of the tool 1 on the optical surface 2 and irrespective of its rotational speed, its axis of rotation X is permanently collinear or substantially collinear with the normal n at the optical surface 2 , the orientation of the tool 1 is thus optimal at all times.

According to an embodiment illustrated in Figures 3 and 4, the end surface 13 of the support 4 is flat.

The tool 1 is thus adapted to surface a certain range of optical surfaces 2 of different curvatures.

In order to modify the adaptability of the tool 1, it is possible to prestress the biasing means 15 by twisting the flexible strips 18 so that they are already bent at rest, in one direction (FIG. 5) or in the other (Figure 6).

When, at rest, the strips 18 are straight (FIG. 4) or bent opposite the end surface 13 (FIG. 5), the tool 1 is intended for the concave optical surfaces 2, whereas when at rest the lamellae 18 are bent on the side of the end surface 13 (FIG. 6), the tool 1 is intended for the convex optical surfaces 2.

Moreover, according to a first variant illustrated in FIG. 5, the end surface 13 of the support 4 is convex, the tool 1 thus being intended for optical surfaces 2 having a more pronounced concavity.

According to a second variant embodiment illustrated in FIG. 6, the end surface 13 of the support 4 is on the contrary concave, the tool 1 thus being intended for optical surfaces 2 with a more pronounced convexity.

Of course, it is possible to combine the concave or convex embodiment of the end surface 13 with the preload of the return means 15, as described above.

In total, the use of three tools 1, as shown in Figures 4, 5 and 6, that is to say whose end surfaces 13 are respectively flat, convex and concave, is sufficient to cover a wide range of optical surfaces 2 to face, both convex and concave, and of various shapes: spherical, toric, progressive aspherical or any combination thereof, or more generally freeform type.

According to an alternative embodiment (not shown), the return means are in the form of a helical spring, anchored by a first end in the support and fixed by a second end to the peripheral portion of the buffer. This spring is for example frustoconical profile, and goes flaring from the support to said peripheral portion.

As we have seen, the use of a tool 1 as previously described corresponds to a conventional method well known to those skilled in the art, so that no particular adaptation of the machines usually used is necessary.

Claims (21)

1. Tool (1) for surfacing an optical surface (2), which tool comprises a rigid support (4) having a transverse end surface (13), an elastically compressible interface (5) that is pressed against and covers said end surface (13), and a flexible buffer (6) adapted to be pressed against the optical surface (2) and which is pressed against and covers at least part of the interface (5) on the side opposite to and in line with said end surface (13), characterised in that the buffer has a central portion (6a) that is in line with said end surface (13) and a peripheral portion (14) that is transversely beyond said end surface (13) and return spring means (15) join this peripheral portion (14) to the support (4), the combination of said peripheral portion (14) and return means forming stabilisation means for the tool during surface finishing, said tool being adapted to carry out surface finishing substantially at the level of said central portion (6a).
2. Tool according to claim 1, characterised in that the buffer (6) is of one-piece construction, the central portion (6a) and peripheral portion (14) forming a single component (6).
3. Tool according to claim 2, characterised in that the buffer (6) comprises a plurality of petals (14b) projecting transversely from the central portion (6a).
4. Tool according to claim 1 or claim 2, characterised in that the buffer (6) comprises a ring (14a) around the central portion (6a).
5. Tool according to claim 4, characterised in that the buffer (6) is of one-piece construction and when unstressed assumes the shape of a disc.
6. Tool according to any preceding claim, characterised in that the interface (5) has a central portion (5a) that is in line with said end surface (13) and a peripheral portion (16) that is transversely beyond said end surface (13) and is between the peripheral portion (14) of the buffer (6) and the return means (15).
7. Tool according to claim 6, characterised in that the peripheral portion (16) of the interface (5) when unstressed assumes the shape of a ring around the central portion (5a) of the interface (5).
8. Tool according to claim 7, characterised in that it further comprises a deformable ring (17) transversely around the support (4) and between the peripheral portion (16) of the interface (5) and the return means (15).
9. Tool according to claim 8, characterised in that the ring (17) has a circular longitudinal section.
10. Tool according to any of claims 6 to 9, characterised in that the interface (5) is of one-piece construction and its central portion (5a) and peripheral portion (16) form a single component (5).
11. Tool according to claim 10, characterised in that when unstressed the interface (5) assumes the shape of a disc.
12. Tool according to any preceding claim, characterised in that said return means (15) comprise a leaf spring (18) projecting transversely from the support (4).
13. Tool according to claim 12, characterised in that said leaf spring (18) is joined to the support (4) at a first end (18a) and to the peripheral portion (14) of the buffer (6) at a second end (18b).
14. Tool according to claim 13, characterised in that said leaf spring (18) is rigidly anchored in the support (4) at its first end (18a).
15. Tool according to any of claims 12 to 14, characterised in that the return means (15) comprise a star-shaped component (19) fixed to the support (4) and provided with branches (18) each forming a leaf spring (18).
16. Tool according to claim 15, characterised in that the support (4) comprises two jaws (7, 8) fixed together, the star-shaped part (19) having a central portion (20) that is clamped between the two jaws (7, 8) and from which its branches (18) project.
17. Tool according to claim 15 or claim 16, characterised in that the buffer (6) is of one-piece construction and comprises a plurality of petals (14b) projecting transversely from its central portion (6a) and each branch (18) is in line with a petal (14b).
18. Tool according to claim 17, characterised in that it comprises seven petals (17b) and seven branches (18).
19. Tool according to any preceding claim, characterised in that the end surface (13) of the support (4) is plane.
20. Tool according to any of claims 1 to 18, characterised in that the end surface (13) of the support (4) is convex.
21. Tool according to any of claims 1 to 18, characterised in that the end surface (13) of the support (4) is concave.

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