WO1988010167A1

WO1988010167A1 - Process for manufacturing an electric-discharge cutting tool, electrode, and corresponding tool

Info

Publication number: WO1988010167A1
Application number: PCT/FR1988/000316
Authority: WO
Inventors: Georges Voilmy
Original assignee: Georges Voilmy
Priority date: 1987-06-16
Filing date: 1988-06-16
Publication date: 1988-12-29
Also published as: FR2616695A1; FR2616695B1

Abstract

In a process for manufacturing a cutting or sharpening tool, the female electrode (1) is made from an electrode form having a surface roughness P1 less than the radius of curvature P2 of the cutting edges of the tool to be obtained. A tool form (2) is machined until its surface roughness P3 is less than the radius of curvature of the cutting edges of the tool to be obtained. The cutting tool is formed by subjecting the tool form (2) to an electric-discharge machining process in the presence of the female electrode (1). Application to the manufacture of tools for cutting or sharpening spot-welding electrodes.

Description

PROCESS OF FABR I CAT I ON OF A CUT OUT I L BY ELΞC TRO- ≡ROS I O, ELEC TRODE AND OUT I L C ORRESP OND

The invention relates to a method for manufacturing a cutting or sharpening tool by EDM as well as the processing electrodes and the tool thus obtained.

Currently, cutting or sharpening tools are normally made by milling a piece of high speed steel for example. In the case of making a tool for sharpening a spot welding electrode for example, the tool, due to the very varied shapes of the ends or active tips of this type of electrode, must have a complex profile. .

The realization of this type of tool by milling does not involve any rhédibitoire difficulty but, in many cases, in order to reduce the time and the corresponding costs of manufacture of these tools, it is necessary to realize these • • latter by making in multiple cutting and even in the most delicate cases, by adding inserts, previously machined by milling, in particular meticulously machined inserts brought into the central area of the tool.

Of course, all the operations necessary for the manufacture of this type of tool remain 'expensive because of the number of operations consuming working time and the tools thus obtained can hardly be subjected to a sharpening of their edges. cutting, due to wear, because such an operation requires a very important working time expenditure close to 60% of the time of implementation of a new tool. In addition, in most cases, the added parts of the tool, because of their complex shape and their location which is often difficult to access, can hardly be resharpened. The present invention aims to remedy the aforementioned drawbacks by implementing a method of manufacturing a cutting or sharpening tool in which the machining operations by milling the tool are eliminated.

Another object of the present invention is the implementation of a method in which the operations consisting in attaching one or more parts to the tool are eliminated.

Another object of the present invention is also the use of one-piece cutting or sharpening tools. Another object of the present invention is finally the implementation of cutting or sharpening tools capable of being resharpened with a very reduced working time expenditure, the resharpening of the same tool can, without disadvantage, intervene several times, the cutting profile of the tool being restored, after resharpening, with the same degree of precision as when it was first made.

The method of manufacturing a cutting or sharpening tool, object of the invention, is remarkable in that it consists in producing an electrode of female profile from an electrode shape having a surface state whose roughness corresponds to a radius of curvature less than the radius of curvature of the wire of the cutting edges of the tool to be obtained, to form a shape of starting tool having a surface condition whose roughness corresponds to a radius of curvature less than the radius of curvature of the wire of the cutting edges of the tool to be obtained, subjecting said form of tool in the presence of said female profile electrode to an EDM process to form said cutting tool.

The cutting tool obtained by the process which is the subject of the invention is remarkable in that it is in one piece.

The method and the cutting or sharpening tool which is the subject of the invention can advantageously be used in the field of automobile and mechanical engineering, for the sharpening of spot welding electrodes, and in any field. where such an assembly or connection technique is used.

The invention will be better understood on reading the description and on observing the drawings below in which:

FIG. 1a schematically represents the different stages of implementation of the method which is the subject of the invention,

FIGS. 1b and represent it, in a longitudinal section, two different different stages of implementation of the method which is the subject of the invention,

FIG. 2 represents a perspective view of an electrode making it possible to implement the method which is the subject of the invention for producing a tool for sharpening a spot welding electrode, FIGS. 3a and 3b in 1 and 2 represent a front view of the electrode shown in FIG. 2, according to advantageous alternative embodiments,

- the figure shows a detail of the edge of the electrode and the corresponding edge of the tool,

- Figure 5 shows a perspective view of a tool obtained according to the method of the invention, - Figure 5a shows in 1 and 2 a front view of the tool shown in Figure 5 according to two advantageous embodiments ,

FIG. 5b represents in 1 and 2 a front view of the tool represented in FIG. 5 according to two other advantageous alternative embodiments,

- Figure 6 shows a preferred embodiment of the tool according to the present invention, obtained by implementing the method of the invention.

FIG. 7a represents a view in longitudinal section of a tool according to the invention,

- Figure 7b shows a front view detail of Figure 7a and Figure 7c a full front view of Figure 7a.

The method of manufacturing a cutting or sharpening object of the invention will first be described in connection with Figure la.

As shown schematically in the aforementioned figure, the manufacturing process which is the subject of the invention consists first of all, as shown in this figure in point a), of producing a female profile electrode from a form of starting electrode denoted 1. By female profile electrode, one naturally understands an electrode having recesses or grooves, conferring on it the qualifier of female electrode, these grooves being intended, as will be described below. after in the description, in accordance with the process which is the subject of the invention, to cause the formation of ribs or cutting edges corresponding male on the cutting tool. The starting electrode shape 1 has a surface condition denoted SI in point a), the roughness of which corresponds to a radius of curvature denoted r 1 less than the radius of curvature denoted Pi of the wire of the cutting edges of the tool to get. In point a) of FIG. 1a, the wire of a cutting edge of the tool to be obtained is shown, the radius of curvature of which is noted P 2.

It will of course be understood, as has also been shown in FIG. 1a in point a) that the female electrode may in fact have projecting parts in a nonlimiting manner, this projecting part being represented in the center of the electrode substantially "

The method of manufacturing a sharpening cutting object of the invention further consists, as shown in Figure la in point b), to shape a form of starting tool, noted 2 The form of tool the start noted 2 has a surface condition 52, the roughness of which corresponds to a radius of curvature noted 3 less than the radius of curvature 2 of the cutting edge wire of the tool to be obtained.

The process for manufacturing a cutting or sharpening tool which is the subject of the invention furthermore consists in subjecting the form of tool 2 in the presence of the electrode 1 of female profile to an electro-erosion process shown diagrammatically in figure it in point c), to form the considered cutting tool.

Conventionally, the EDM process is carried out by placing the form of tool 2 and the electrode 1 of female profile in a bath of dielectric liquid, such as for example petroleum or a specific hydrocarbon or deionized water.

A more detailed description of the electro-erosion process will be given in conjunction with Figures 1b and. As described above, the EDM process is carried out, as shown schematically in Figure la in point c) by subjecting the electrode and the tool form 2 in the presence of the latter, to a series of electrical pulses , the assembly being immersed in the bath of dielectric liquid. During the conduct of the process, a very large number of electrical pulses are thus applied to the electrode and to the tool form, in order to generate destructive discharges allowing to carry out the removal of material on the tool form, in a manner known per se. During the conduct of the process, an average gap denoted g is permanently formed between the form of tool 2 and the electrode 1 of female profile.

As shown in particular in FIG. 1b, the female electrode 1, the shape of the starting tool 2 and the tool 1 being of revolution with respect to a longitudinal axis Oz, and the tool obtained by the mi implements the process which must have at the z coordinate in polar coordinate a determined profile r, the female profile electrode 1 has a corresponding female profile r, with respect to the same dimension taken with respect to the mean spacing g measured on the axis of symmetry of revolution Oz. Cri will of course understand as shown in Figure lb, in

^• - - section along a plane containing the axis of symmetry of revolution Oz, than the male profile r of the tool

• Obtained, of course, corresponds a female profile corresponding r of the electrode to a step or rib or cutting edge of the tool 2 corresponding to a groove of the same shape formed on the female electrode 1. By substantially revolution form of the shape of the starting tool and of the tool, it is understood, of course, the support form of the cutting edges, the latter however being able to exhibit any asymmetry allowing an adequate pattern to be obtained. In FIG. 1b, the distance or interstice between the electrode l and the shape of tool or tool 2 has been noted g, this interstice being measured on the axis of revolution Oz. It is quite obvious that for the implementation of the EDM process, the determining parameter is the distance d or dielectric thickness of the dielectric fluid considered. Of course, as the electro-erosion process is carried out, the female electrode 1 is lowered or brought closer to the form of tool 2, the gap g being, the mean gap maintained. substantially constant. The mean gap g can thus be maintained during the electro-erosion process, at a value of between 0.08 and 0.2 mm. According to an advantageous characteristic of the method which is the subject of the invention, the radius of curvature 1, 3 representative of the roughness of the surface state SI, respectively 52 of the shape of electrode 1 and of the shape of tool 2 is advantageously taken less than 5 micrometers. Thus, it is possible to form cutting edges having a wire at the level of the cutting tool 2, of the order of 5 micrometers. For this purpose, the parameters for adjusting the conduct of the EDM process can be chosen so that the degree of finish corresponds to a roughness whose radius of curvature is less than 5 micrometers, the radius of curvature of the wire J_Q of the cutting edge or cutting edges of the tool thus obtained, thus exhibiting such a degree of roughness or sharpening. Of course, for this purpose. the value of the gap g is set Moven to determine the finish of the surface finish tool, as well as forms of ^r wave constituting the pulses applied to the electrode 1 and the form tool 2 .

In a conventional manner, the EDM process is carried out by means of pulses of successive electrical voltages applied between the electrode 1 of female profile and the form of tool 2, the electrode being subjected to one or more descents during the process.

As shown in addition in FIG. 1 a, and in order to improve the degree of finish of the surface condition of the tool, the axes of longitudinal symmetry of the female electrode and of the shape d 'Tool 2 are offset by a distance of the order of a few micrometers and the shape of the tool is subjected to a planetary movement denoted P, relative to the longitudinal axis' of symmetry of the female electrode 1. 5 En furthermore, in order to improve the conditions for carrying out the removal of material by EDM, the female electrode 1 can be subjected relative to the mean gap g between the electrode 1 and the tool shape 2 a a periodic back and forth movement of small amplitude to remove the shavings of material removed by the phenomenon of EDM. This periodic back and forth movement thus allows, by the substantially vortex movements created in the dielectric fluid, to cause the displacement and the removal of the chips generated by the electro-erosion phenomenon. This ensures a substantially constant quality of the dielectric fluid, and in particular to avoid the creation of pure short circuits.

A more detailed description of a female electrode for implementing the process which is the subject of the invention will be given in connection with FIG. 2. In accordance with the above-mentioned figure, the female electrode 1 has a surface condition whose roughness corresponds to a radius of curvature i less than 5 micrometers.

The female electrode 1 can advantageously be made of a material such as graphite, copper, tungsten carbide,

10 tungsten or zinc, and more generally, an electrically conductive material. However, for reasons of service life and machining precision, it will be preferable to constitute the electrodes allowing the ^' implementation of the process which is the subject of the invention in a material such as graphite.

, r As shown in Figure 2, for the production of a point welding electrode sharpening tool, this type of spot welding electrode is most often made of copper or pocotungsten, a relatively expensive material, the female electrode object of the invention may include a central zone consisting of two elements

20 denoted 1 1, 12 in the form of a projecting circular sector. These elements 1 1, 1 2 are slightly off-center with respect to the longitudinal axis of symmetry Oz of the electrode 1.

In addition, the female electrode 1 comprises a surrounding area 1 3 platform of the two elements 1 1, 12 f circular sector elm, zone _j r device 13 consisting of a surface substantially of revolution, on which are formed the radial grooves denoted 130 intended to form on the tool for manufacturing corresponding cutting edges.

Of course, the machining of the female electrode 1 can be carried out by milling. However, it will be understood that the machining considered by milling, which may require a relatively long processing time, is not carried out

30 only once, that is to say for an electrode making it possible to manufacture a large number of tools. As shown in FIG. 3a, in point 1 in particular, the radial grooves 130 can advantageously be inclined to the left relative to the trace on the resolution surface 13, of a meridian plane noted

Q-

In FIG. 3a at point 2, on the contrary, the same electrode has been represented seen from the front in the case where the radial grooves 130 are, on the contrary, inclined to the right, with respect to the trace of the meridian plane Q. or to the right of course makes it possible to obtain a corresponding inclination of the cutting edges of the tool obtained by implementing the method which is the subject of the invention.

As has also been shown in FIG. 3b, at points l and 2, the radial grooves 130 may be in the form of a helix, the helices being inclined to the right or to the left relative to the trace, on the surface of revolution of the above-mentioned meridian plane Q.

In addition, the surface of revolution 13 of each female electrode 1 can be constituted by a frustoconical, parabolic, hyperboloidic or hemispherical surface or of any suitable shape. Of course, this shape will determine, in addition to the shape of the tool, the shape finally assigned to the active tip of the welding electrode rectified by means of the tool obtained by implementing the process which is the subject of the invention, when using a female electrode 1 previously described.

As also shown in the figure, according to a partial section, the female electrode 1, object of the invention, has at its extreme periphery a groove denoted 100, the groove of which has a bias profile 101. The groove and the groove of it allow to define the leading edge 201 of the tool 2, according to a profile having a corresponding draft angle. A description of a sharpening cutting tool obtained by implementing the process which is the subject of the invention will now be given in conjunction with FIGS. 5, 5a, 5b and 6.

In accordance with the subject of the invention, the cutting or sharpening tool obtained has a surface condition whose roughness corresponds to a radius of curvature less than 5 micrometers, in particular on the wire of the cutting edges of that -this. According to another advantageous characteristic of the cutting or sharpening device or tool which is the subject of the invention, it is made of a material such as high-speed steel, or tungsten carbide.

Compared to the cutting tools of the prior art, the cutting or sharpening tool which is the subject of the invention has the advantage of being made up of a monobioc element.

As shown in FIG. 5, the tool must be a central zone consisting, for example, without limitation, of two elements in the shape of a circular hollow sector denoted 21 and 22, slightly off-center with respect to the longitudinal axis of symmetry of the tool

Oz. It further comprises a peripheral zone consisting of a surface substantially of revolution denoted 23, on which are formed projecting edges 230 radial, forming cutting edges of the tool. Edges

^" radial projections denoted 230 are for example, as a function of the female electrode used inclined to the left or to the right with respect to the trace on the surface of revolution of a meridian plane denoted Q.

As has also been shown in Figure 5b, the projecting edges

230 radials are in the form of a helix and inclined to the left or to the right with respect to the trace on the surface of revolution of the abovementioned meridian plane Q. The surface of revolution can be a frustoconical, parabolic, hyperbolic surface, or in the form of a spherical half-cap.

In addition, as already shown in the figure, the tool has at its extreme periphery a projecting edge forming the leading edge of the tool, the leading edge having a bevel profile forming the corresponding draft angle.

In Figure 6, there is shown an advantageous embodiment of a tool obtained according to the method of the invention. The tool is substantially cylindrical in shape, a central area and a peripheral area being formed in each end section of the cylinder. In fact, it will be understood that the tool considered has two working surfaces, which correspond to the working surfaces of FIG. 5a or 5b for example. The projecting edges 230 of each peripheral zone 23 are respectively inclined to the right and to the left, to the right and to the right and to the left and to the left for example. It is thus possible to use the same cutting or sharpening tool or the sharpening machine in question proceeds to rotate the tool in one direction or another.

Of course, and advantageously, the tools can be made up of a set of tools comprising a plurality of tools of different dimensions and of different shape, as described above.

The tools obtained in accordance with the process which is the subject of the invention may be made of different material as described above, the differences in the material used depending on whether these tools are used manually or mechanically or equipped with robots, such as the Q automobile robots for example.

The aforementioned tools can be produced in single or multi-cut. Apart from the multiple geometric shapes encountered in resharpening or the manufacture of spot welding electrodes, the difficulty is in fact to achieve a scraping surface for the removal of very thin material on the active faces of the welding electrode, in fact just to refresh this active surface. It is thus possible to provide one of the end sections of the cylinder shown in FIG. 6, so that the latter comprises a scraping tool, while the other section comprises a machining tool. As regards the simple cutting tools, these are generally intended for resharpening of the welding electrodes, the resharpening being carried out manually. The tools obtained in accordance with the process which is the subject of the invention make it possible to define on them an absolutely arbitrary number of teeth, and this in any geometric form. ₅ Of course, the pitch of the cutting edges 230 can be chosen so as to stabilize the tool on the machine, and prevent any vibration of the tool when it is in use. The toothing or not of the cutting edges is then said to be tight and the entire welding electrode is then substantially covered, which makes it possible, for manual work, to obtain a surface condition of the welding electrode and a face. active practically identical to that 0 which would be obtained when using a fixed station. With regard to the use of the aforementioned tools, these can be used at low speed at 800 rpm, which makes it possible to increase the safety conditions for the users, although parts produced have made it possible to achieve of rotational speeds greater than 2000 rpm.

In the case of double-cut tools, the ease of determining the geometric shapes and the simplicity of implementation of the method which is the subject of the invention makes it possible to solve the problems in which the welding electrodes require to have an offset longitudinal axis. This problem appears with the need for offset due to the firmness of the part on a robotic or manual machine with automatic welding. The offset of the longitudinal axis of symmetry of the tool is defined in relation to the maximum and minimum dimensions of the electrode to be manufactured or resharpened. The new center of the tool can be produced with a very precise tolerance.

The best yields obtained by means of these tools are achieved on .- machines with rotating movement. The number of turns and the power of the step may vary depending on the time available to the user for resharpening or manufacturing the welding electrodes. Finally, the torques applied to the tool as shown in FIGS. 5, 5a, 5b or 6 can vary according to the speed of rotation of the tool, when the latter is mounted on pneumatic or electric machines.

The speed of rotation of the tool can be between 5 and 3000 revolutions per minute, depending on the conditions of resharpening or fabrication of the electrodes by the user. In order to allow the use of the tools according to the invention in such wide speed ranges, in the absence of a valid influence of the latter, the tools according to the invention as shown in FIG. 7a to 7c may have the following characteristics.

The tool is advantageously designed with a heel acting as a cut limiter and guiding the electrodes of all diameters, this heel being noted 210 in FIGS. 7a, 7b, 7c. The removal of material then depends on the duration of the rotation of the tool, its speed of rotation and the pressure exerted on it. The geometry of the cuts and specific cutouts of the tool allow the clamping of the clamp for fixing the closed tool. The zones 21 1 and 2 12 designate recesses for the evacuation of the chips. The diameter to obtain at the end of the electrode corresponds to the diameter of the heel limiter 210.

The tightening torque of the pliers has no influence on the rotation of the tool.

During tests, it was noted that the tool can be subjected to a tightening of the tool fixing clamps from 2 daN to 800 daN, the rotation of the tool being practically the same because the electrodes to be rectified or machining does not undergo any deformation.

Thus, the method which is the subject of the invention makes it possible to make up for the differences in geometry and alignment of the clamps for fixing the tool.

We have thus described a method of manufacturing sharpening tools or cutting αe particularly efficient insofar as the method allows the realization of any concave or convex geometric shape. ^' In particular, it is particularly easy to determine the parameter.

15 called preponderance which is in fact the length of the active zone of the active face of the spot welding electrode, this length being measured along the longitudinal axis of symmetry of revolution of the electrode. Of course, this length of preponderance corresponds to the free length of the central zone of the tool, that is to say the length of unhooking-

20 of the central elements 21 and 22 with respect to the termination in the central zone of the surface of revolution 23.

In addition, by its very design, the process which is the subject of the invention allows resharpening of the welding electrodes, and it is possible to redo the initial shape for a price of approximately 50% of the price of the initial value 5 of l 'new tool, and this at least twice depending on the shape of the tool, the material guard in the center and the state of the tool before resharpening, while keeping the initial geometry.

Since the point closest to the center of the active face of the welding electrodes must be produced in a particularly critical manner for the 0 spot welding electrodes, the method which is the subject of the invention makes it possible to solve this problem under advantageous conditions. On cuts or muiticuts the active face in relation to the welding electrodes can be reduced so as to obtain a center with a diameter of 0.7 to 0.8 mm, on simple cuts, this center can be brought from 0 , 3 to 0, mm on double cuts. Finally, it is possible to design the tools as a function of the number of sharpening repeats to be carried out, for each welding electrode per point, the repeating being defined as an interval in number of welding points at the end of which it is necessary to resharp the welding electrodes to maintain a substantially constant quality of spot welding.

Claims

CLAIMS j_. Method of manufacturing a cutting or sharpening tool, characterized in that it consists of:

- to achieve an electrode (1) of the female profile from a form of electrode (1) having a starting surface condition whose roughened _ty corresponds to a radius of curvature smaller than the radius of curvature of the thread ridges cutting the tool to be obtained,

- shaping a form of starting tool (2) having a surface condition whose roughness corresponds to a radius of curvature less than the radius of curvature of the wire of the cutting edges of the tool to be obtained,

- subjecting said form of tool (2) in the presence of said electrode. (1) of female profile to an electro-erosion process to form said cutting tool.

2. Method according to claim 1, characterized in that said electro-erosion process is carried out by placing said form of tool (2) and said electrode (i) of female profile in a bath of dielectric liquid, a mean gap (g) being permanently provided between said tool shape and said female profile electrode during said process.

3. Method according to one of claims 1 or 2, characterized in that the female electrode, the shape of the starting tool and the tool being of revolution relative to a longitudinal axis (Oz), the tool ( 2) having to present at dimension (z), in polar coordinates, a profile (r, âf _f said female profile electrode (1) has a corresponding female profile (1.0) with respect to the same dimension taken with respect to the mean spacing (g) measured on the axis of symmetry of revolution (Oz) Method according to one of claims 2 or 3, characterized in that the mean gap (g) is maintained during the process of 'EDM to a value between 0.8 and 2 mm.

5. Method according to one of claims 1 to 4, characterized in that the radius of curvature (p., P,) representative of the roughness of the surface condition respectively of the shape of the electrode (i) and of the tool shape (2) is less than 5 ^ ιm so as to form cutting edges having a wire at the cutting tool, of the order of 5 jsm. 6. Method according to one of claims 3 to 5, characterized in that the value of the mean gap (g) is adjusted to determine the finish of the surface condition of the tool.

7. Method according to one of claims 1 to 6, characterized in that the EDM process is carried out by means of pulses of successive electrical voltages applied between the electrode (1) of female profile and the form d 'tool (2), the electrode being subjected to a uυ several descents during the process.

8. Method according to one of the preceding claims, characterized ^'in that, in order to improve the finish of the surface finish tool, the longitudinal symmetry axis of the female electrode and form tool being offset by a distance j), said tool shape is subjected to a planetary movement (P) relative to the longitudinal axis of symmetry of the female electrode. 9. Method according to claim 8, characterized in that the electrode

(1) is subjected, with respect to the mean gap (g) between the electrode (1) and the tool shape (2), to a periodic movement back and forth of small amplitude to remove the shavings of material removed by the phenomenon of EDM. d_0. Electrode for implementing the method according to one of the preceding claims 1 to 9, characterized in that it has a surface condition the roughness of which corresponds to a radius of curvature P. less than 5 ιm.

1 1. An electrode according to claim 10, characterized in that it is made of a material such as graphite, copper, tungsten carb ure, tungsten, zinc.

12. Electrode according to one of claims 10 or 1 1, characterized in that for the production of a tool for sharpening a spot welding electrode, said electrode comprises: - a central zone consisting of two elements (1 1, 12) in the form of projecting circular sectors, slightly off-center with respect to the longitudinal axis of symmetry (Oz) of said electrode, - A peripheral zone (13) consisting of a surface substantially of revolution on which are formed radial grooves (130) intended to form on the tool corresponding cutting edges.

13. An electrode according to claim 12, characterized in that said radial grooves are inclined to the left or to the right with respect to the trace on the surface of revolution of a meridian plane (Q).

14. An electrode according to claim 12, characterized in that said radial grooves are in the form of a helix, inclined to the left or to the right with respect to the trace, on the surface of revolution, of a meridian plane (Q). J. Electrode according to one of claims 12 to 14, characterized in that the surface of revolution (13) is a frustoconical, parabolic, hyperboloidic, hemispherical surface.

16. Electrode according to one of claims 10 to 15, characterized in that it comprises at its extreme periphery a groove (100) -whose groove has a bias profile (101), said groove and the groove thereof- ci allowing to define the leading edge (201) of the tool according to a profile having a corresponding draft angle.

17. Cutting or sharpening tool, characterized in that it has a surface condition whose roughness corresponds to a radius of curvature less than

18. Tool according to claim 17, characterized in that it is made of a material such as high-speed steel, tungsten carbide.

19. Tool according to one of claims 17 or 18, characterized in that it is monobioc. 20. Tool according to one of claims 17 to 19, characterized in that it comprises:

- a central zone consisting of two elements in the form of a hollow circular sector (21, 22), slightly off-center with respect to the longitudinal axis of symmetry of the tool, - a peripheral zone consisting of a surface substantially of revolution (23 ) on which are provided radial projecting edges (230) forming cutting edges of the tool.

21. Tool according to claim 20, characterized in that said radial projecting edges (230) are inclined to the left or to the right with respect to the trace on the surface of revolution of a meridian plane. 22. Tool according to claim 21, characterized in that said radial projecting edges are in the form of helices inclined to the left or to the right with respect to the trace, on the surface of revolution of a meridian plane.

23. Tool according to one of claims 20 to 22, characterized in that said surface of revolution is a frustoconical, parabolic, hyperboloidic or hemispherical surface.

24. Tool according to one of claims 18 to 23, characterized in that it has at its extreme periphery a projecting edge forming the leading edge of the tool, the leading edge having a bevel profile forming the 'corresponding draft angle.

25. Tool according to one of claims 20 to 24, characterized in that it is substantially cylinder-shaped, a central area and a ^peripheral is formed in each end section of the cylinder.

26. Tool according to claim 25, characterized in that said projecting edges of each peripheral zone are respectively inclined to the right and to the left, to the right and to the right, to the left and to the left.

27. Tool set characterized in that it comprises a plurality of tools according to one of claims 20 to 26.