Technical field of the invention

​The present invention relates to a method for the electroerosion drilling of a hole in a part made of electrically conductive material, in particular for an aircraft turbomachine.

​Techn-plane back-to-plane

​The state of the technetium comprises, in particular, documents Wo-A1-2006/078096, US-A1-2008/17361 8, J P-A-S581 14821 and US-A -6140600.

​The EDM drilling or EDM drilling is a Machining method which consists in removing material from a workpiece by means of electrical discharges. Spark Machining is also described. This techn is characterized by its ability to machine all electrically conductive materials (electricity conductors) regardless of their hardness.

​The method of machining includes passing a current from an electrode to the workpiece through an electrical circuit to generate a vapor or vapor bubble that ionizes and resorts by imploding, causing destruction of the material of the workpiece. This destruction (micro-implosion) causes the spark. The high-intensity current drives a channel through the d-wire.​A disruptive discharge is then provided between the electrode and the workpiece, damaging it very locally (somewhat pm2). This method makes it possible to drill holes with very high precision and is suitable for very hard materials or even in cases where the complexity of the part requires, as is the case of parts of an aircraft turbomachine.

​The blades of a turbine engine of an aircraft turbomachine include, for example, holes drilled by an EDM method. The vane is hollow and the holes pass through the wall of the vane to provide ventilation air passages between the inner cavity of the blade and the vein of the turbine.

​The electroerosion Mach includes a head which is movable with respect to the workpiece to be drilled and which carries the EDM electrode. The electrode has an elongated shape and is to be translated along its axis of elongation. Los from an advancement of the electrode towards the workpiece, the spark is created and destruction of the material of the workpiece results in the formation of a hole with a predeceased diameter i. The electrode is then retracted and out of the hole in order to realise other drillings.

​The electrode is a consumable as it wears during drilling. The wear of the electrode is characterized by a reduction in its length. One of the problems of the EDM drilling is that it is difficult to accurately quantify this wear. When drilling a hole, wear (length reduction) of the electrode is generally greater than the depth of the drilled hole, but is however constant because it can vary from one hole to the other.

​This technical problem combines with two contradictory objectives which are to sufficiently pierce the part to realise a through-hole of a single shot (a single electrode advance), but not too advance the electrode so as not to impact and pierce the wall opposite the hole, as is the case in the aforementioned example of the drilling of a turbine blade wall.

​A solution to this problem could include controlling the unclogging of a hole by means of a pin that would be manually inserted by an operator into the hole. The pige would be at a prescaler diameter to check the diameter of the hole and would be pushed into the hole to ensure that the pie does not abut against a bottom of the hole in the case where it would not be

​through-opening. In the event that a hole would not be opened, the operator would have a step of retouching with the electroerosion Mach.

​Another solution is to use the EDM electrode as a pige. In such a case, the electrode used for drilling a hole would be used to verify that this hole is well open. After recoil and removal of the electrode from the hole (step b)), the electrode could again be advanced and inserted into the hole. In the event that the hole would be opened, it could be advanced by a length greater than the theoretical depth of the hole.​Otherwise, a bottom of the hole would be detected by sensing and a retouching step should be activated. However, this solution would not be reliable. Indeed, in practice, it would be difficult or impossible to detect by accurately sensing the bottom of the non-emerging hole because the feeler spark that should theoretically create between the iamber of the electrode and the bottom of the hole​would be in practice between the electrode and the peripheral edge or side wall of the hole upon entry of the electrode into the hole.

​Other solutions would consist in controlling only the unclogging of the hole (without having the value of its d-meter), by means of a thermal camera, by detecting the speculum through the hole, by injecting L into the internal cavity of the blade in the aforementioned example, etc.

​All these solutions are not entirely satisfactory because they are most often complex and long to implement.

​The present invention provides a simple, efficient and effective solution to this problem.

​SUMMARY OF THE INVENTION

​The present invention proposes a method for the electroerosion drilling of a hole in a part made of electrically conductive material, in particular for an aircraft turbomachine, the method using an electroerosion Mach comprising a head movable with respect to the part and carrying a consumable EDM electrode which has an elongated shape and which is displaced in translation along its axis of elongation, the method comprising the steps of:

​a) advancing the EDM electrode toward the workpiece to pierce a hole in the workpiece,

​b) reversing the EDM electrode and exiting the EDM electrode of the hole, characterized in that it further comprises a step of:

​c) lateral displacement of the revised head to a resistance representing less than 1 00% of the diameter of the hole to be drilled, d) advancing the EDM electrode towards the workpiece to palate the workpiece, and

​e) calculating the effective depth of the hole drilled in step a) from a plurality of ribs measured in a direction parallel to the axis, between a first position of the EDM electrode at the end of step a), and a second position of the EDM electrode in step d) when it palms the part.

​The method therefore proposes to determine the effective depth of a hole and, therefore, to remove it if the hole is open or not, via the electroerosion machine and its EDM electrode. The electrode is used in two different man-hours. It is first used to drill the hole in step a). It is used to palate the part away from the hole.​The above-mentioned drawback with the detection of the bottom of a hole does not exist here because the scanning spark can appear without difficulty between the iamber of the electrode and an outer surface of the part.

​In the present application, it is meant by sensing, sensing or electrical detection of an electrically conductive part by an electrode. The sensing or sensing of the workpiece has occurred when the electrode is sufficiently close to the workpiece to create a spark between the electrode and the workpiece. The effect of the spark is to be chosen so as not to damage the part since the purpose here is merely to detect the part for the purpose of breaking down the ribs.​The parameters of the electroerosion Mach are therefore d ifferful depending on whether the electrode is used for a drilling function or a probing function.

​Furthermore, the method according to the invention makes it possible to propose a lateral displacement of the head in step c) over a small distance (less than 100% of the diameter of the hole to be drilled). This has several advantages of low risk of drilling a hole or component adjacent to the hole to be drilled, a time gain on the displacement strokes of the electrode, an optimization of the drilling time of the hole and generally an optimization of the method as a whole.

​The method may include one or more of the features or steps below, considered alone or in combination with one another:

​-the method comprises additional steps of:

​f) comparing the calculated effective depth with a theoretical depth, and

​g) in the case where the effective depth would be less than the theoretical depth, advancing the EDM electrode towards the workpiece and into the hole drilled in step a) for further drilling;

​-step g) is regenerated in such a way that the hole is opened.

​-steps c), d) and e) are repeated after step g), and steps f) and g) are optionally repeated after step e);

​-a second side is relaxed and equal to the distance travelled by the EDM electrode in Lad between its first position and a third position of the EDM electrode at the end of step b), and a second dimension is determined and equal to the resistance travelled by the EDM electrode in said direction between this third position and the second position of the EDM electrode, the effective depth of the drilled hole being equal to the difference between the first and second sides;

​-the displacement in step c) is revised over a distance less than or equal to 5 mm, and preferably less than or equal to 1 mm;

​-the displacement in step c) is revised on a resistance between 40 and 70% of the d-meter of the hole to be drilled;

​-in step a), the EDM electrode is al immed with a voltage greater than or equal to 1 00V and a current greater than 1 ampere, and in step d), the EDM electrode is al immed with a voltage of less than 100V and a current of less than 1 ampere;

​-in step a), the EDM electrode is al immed by electrical pulses the ratio of the time of the pulses over time between the pulses is greater than 0.2, and preferably between 0.5 and 0.8, and in step d), the EDM electrode is al immed by electrical pulses of which the aforesaid ratio is less than 0.2;

​-in step d), the polarity of the EDM electrode is reversed with respect to the polarity of the EDM electrode in step a);

​-at the beginning of step a) and before drilling the hole, the EDM electrode is advanced towards the room up to a predetermined resistance capable of creating a spark gap;

​-said prescaler resistance is comprised between a free end of the EDM electrode and a surface of the workpiece, preferably said prescaler resistance is 10 mm;

​-in step d), the EDM electrode is moved to the room up to a predetermined distance capable of creating a feeler spark;

​-in step d), the resistance is comprised between an iamber of the EDM electrode and a surface of the part, preferably this predetermined resistance is 5 mm.

​BRIEF DESCRIPTION OF THE FIGURES

​Other features and advantages of the invention will become apparent during the reading of the detailed description of the invention for the understanding of which reference is made to the accompanying drawings in which:

​Fig. 1 is a very schematic view of an EDM electrode and a part to be drilled, and shows several steps of a drilling method according to the invention, and

​[FIG. 2] FIG. 2 is a view similar to that of FIG. 1 and showing other steps of the method.

​DETAILED Description OF THE INVENTION

​The present invention relates to a method for electroerosion drilling or EDM drilling which utilise an electroerosion Mach, only an EDM electrode 10 and a support head 1 2 of this electrode being represented in the drawings.

​The remainder of the Mach is not described and is part of the general knowledge of a special person skilled in the art in the EDM bore.

​The head 12 is movable in a plane H as well as in a direction perpendicular to this plane (Z-axis).

​The electrode 1 0 has an elongated shape along an axis of elongation parallel to the axis Z in the example shown, the electrode 10 passes through an orifice of the head and may be cut into this orifice.

​The electrode 10 is movable along the Z-axis and thus can be advanced or retracted, an advance allowing, for example, the drilling of a hole 1 8, and a recoil allowing it to come out of this hole.

​The part 16 to be drilled is positioned under the electrode 1 0 and the surface 16a on which the hole 1 is to be reshaped can be positioned perpendicular to the axis Z if a hole normal to the surface is to be made, or an inclined hole with respect to this axis X if a hole incised with respect to the surface is to be revised.

​The part 1 6 is, for example, made of metalic all based on n ickeel and cobalt. Alternatively, the part could be made of a composite electrically conductive material, for example of the CMC type.

​The electroerosion Mach is configured to utilise the 1 0 electrode in two different ways. This Mach is, for example, that of the Manufacturer Group Technologies, of the HSD6 type.

​The 1 0 electrode has a first electroerosion piercing function. The holes 1 8 to be drilled have, for example, a diameter of between 0.2 and 2 mm. They are preferably through. In the case where the holes 1 8 are oriented perpendicularly to the surface 1 6a of the part, the theoretical depth of the holes is equal to the thickness of the wall to be drilled of the part and is, for example, between 1 and 10 mm, and preferably between 2 and 5 mm.

​When used in the drilling mode, the electrode 1 0 is imparted with a high intensity current so that the spark produced between the electrode 1 0 and the part 1 6 is sufficient to destroy the material of the part and form the hole 1 8.

​The 1 0 electrode may be al immed with a voltage greater than or equal to 100V and a current greater than 1 ampere. The electrode 10 May be imparted by electrical pulses whose pulse time ratio over time between the pulses is greater than 0.2, and preferably between 0.5 and 0.8. Moreover, the electrode may be rel led to a positive or negative terminal, and therefore any polarity.

​The electrode 10 has another function of sensing or sensing the workpiece. The electrode 1 0 is then al immed with a lower current so that the spark prods between the electrode and the part does not deteriorate the part but makes it possible simply to detect its presence and position.

​The electrode may then be al immed with a voltage of less than 100V and a current of less than 1 ampere. The electrode may be al immed by electrical pulses having a ratio of less than 0.2. Finally, the polarity of the electrode is preferably inverted with respect to the polarity of the electrode in drilling.

​FIGS. 1 and 2 represent the steps of a method for re-drilling a method according to the invention for drilling a hole 18 in the part 1.

​FIG. 1 shows a plurality of different positions of the electrode 10 and the steps of the method. These positions are identified by the CH I to VI I

​The position I corresponds to the starting position of the electrode 1 0, this position corresponds to a side ZI on the axis Z in the example represented, and in general as a result of the description (unless otherwise mentioned), the side of the electrode is taken at the level of its upper end opposite the part to be drilled.

​In the position I I, the electrode 10 is advanced towards the workpiece until a first spark 20 is created, ie until the distance between the electrode surface 16a and the workpiece surface 16a is such that a spark 20 can be created between the electrode and the workpiece. This distance is typically 1 0 mm. The position I I corresponds to a side Zl on the axis Z

​The electrode is further advanced towards the part to pierce a hole 18. It then reaches a position i i i which corresponds to a side Z 111 on the axis Z The sum of the ribs Zl l and Z 111 corresponds to the descent stroke necessary for the drilling of the hole.

​This stroke is relaxed so that the hole opens but the wear U of the electrode that is difficult to accurately predict may result in unclogging of the hole. This stroke is for example between 5 and 20 mm, and is preferably between 10 and 15 mm. The electrode is in the i i i position while its L-end is at the ZIV side and the hole is not open.

​The respective positions I, I l and I I I correspond to a step a) of the method comprising advancing the electrode 10 towards the workpiece to pierce a hole.

​A second step b) of the method includes moving the electrode back and exiting the hole. The electrode is then moved to a position V located at a side ZV, located between the ribs Zl l and Z 111. the upper end of the electrode is at the side ZV '. Alternatively, the electrode could be moved to a position located at the shore Zl​However, wear min out of the electrode can be estimated and the Mach can be set so that the recoil of the electrode from the I I I position to the V position takes into account this wear. This recoil must be sufficient so that the electrode in the V-position is not likely to touch the workpiece.​Since a spark has been created in the position I I when the electrode was at the Z-side 11, it is understood that the electrode, after its recoil from the I I I position, could be at the I I position without the risk of touching the workpiece and also at the V-position without the risk of touching the workpiece taking into account that wear m in the electrode when drilling a hole.

​The optimization of the displacement strokes of the electrode is particularly important to optimize the drilling time of a hole and the method as a whole.

​The method further comprises a step c) of lateral displacement of the head and thus of the electrode to a position VI. In the present application, the term "head" is understood to mean a displacement of the head in a perpetual distance from the axis of the electrode. The electrode remains at the same side ZV. The amber of the electrode is at the side ZV '. The electrode is preferably located closest to the hole to be reshaped.

​For example, the displacement is carried out on a resistance of less than or equal to 5 mm, and preferably less than or equal to 1 mm. It may be revised over a distance of less than 100% of the diameter of the hole to be drilled, and preferably between 40 and 70% of that d imeter.

​The method then comprises a step d) of advancing the electrode 10 towards the part to pile the part. The electrode is moved until a sensing spark 22 could be created, ie, until the distance between the free end of the electrode and the surface 16a of the workpiece is such that a spark can be created between the electrode and the workpiece. This distance d2 is typically 5 mm.​It may be different from the resistor for the creation of the spark 20, insofar as the parameters of the machine are different. The position VI I corresponds to a rib ZVII on the axis Z the free end of the electrode is at the side ZVII'.

​The electrode may then be annealed and return to a position VIII at the side Zll.

​The method comprises a following step e) of calculating the effective depth of the hole drilled in step a) from the difference of ribs measured between the positions III and VII.

Plus exactement, les positions III et VII sont utilisées pour déterminer une première distance (par la formule ZIV-ZV’, en valeur absolue), qui correspond à la distance X1 parcourue par l’extrémité libre de l’électrode lorsque cette dernière est déplacée de la position III à la position V.Les positions VI et VII sont utilisées pour déterminer une seconde distance (par la formule ZVIII’ et ZV’, en valeur absolue), qui correspond à la distance X2 parcourue par l’extrémité libre de l’électrode lorsque cette dernière est déplacée de la position VI à la position VII.

La différence de côtes revient à mesurer la différence entre les distances X1 et X2 et représente donc la profondeur effective P du trou percé.

​The method may include additional steps f) and g) of comparing the calculated effective depth P with a theoretical depth, and in the case where the effective depth would be less than the theoretical depth, advancing the electrode towards the workpiece and into the hole drilled in step a) to drill further, and making it preferably open.

Ces étapes sont illustrées à la figure 2.

​The electrode is initially in position VIII and is moved laterally in position IX which is similar to position V, ie it is at a side Zll taking into account the wear of the electrode.

L’électrode 10 est avancée vers la pièce et dans le trou de façon à percer davantage le trou. Elle se retrouve dans une position X en entrée du trou, à une côte ZX, puis dans une position XI, à une côte ZXI en fin de perçage, dans laquelle son extrémité libre doit en principe être al ignée avec la surface interne 16b de la pièce ou être en dessous de cette surface. L’électrode a également subit une usure U’ lors de ce perçage complémentaire.

La course entre les côtes ZX et ZXI est déterm inée pour que le trou soit débouchant. Cette course peut être égale à la course entre les cotes Zl l et Z 111.

​The electrode is then annealed and removed from the hole to a position XI I located at the ZX side, located between the ribs Zl l and ZXI. As previously described, wear min out of the electrode can be estimated and the Mach can be set so that the recoil of the electrode depu is the position XI to the position XI I taking into account this wear. This recoil must be sufficient so that the electrode in position XI I is not likely to touch the workpiece.

​The electrode can then be brought into a position XI I located at the side Zl

​It is conceivable to check whether the hole retouched after steps f) and g) is well opened. It suffices to calculate the effective depth of the hole, as indicated above, and to compare it to the theoretical depth. It is therefore understood that steps c), d) and e) are repeated, and that steps f) and g) could optionally be repeated after step e) if it is found that the hole is not open even after a first retouching operation by steps f) and g).

WE CLAIMS

1 . ​Method for the electroerosion drilling of a hole (1 8) in a part (1 6) made of an electrically conductive material, in particular for a turbomach of an aircraft, the method using an electroerosion Mach comprising a head (12) movable with respect to the part and carrying a consumable EDM electrode (10) that has an elongated shape and that i is moved in translation along its axis of elongation, the method comprising the steps of:

​a) advancing the EDM electrode toward the workpiece to pierce a hole in the workpiece,

​b) reversing the EDM electrode and exiting the EDM electrode of the hole, characterized in that it further comprises a step of:

​c) lateral displacement of the head to a resistance representing less than 100% of the d-meter of the hole to be drilled,

​d) advancing the EDM electrode toward the workpiece to palate the workpiece, and

​e) calculating the effective depth of the hole drilled in step a) from a plurality of ribs measured in a direction parallel to the axis, between a first position of the EDM electrode at the end of step a), and a second position of the EDM electrode in step d) when it palms the part.

2. ​The method of claim 1, wherein it comprises additional steps of:

​f) comparing the calculated effective depth with a theoretical depth, and

​g) in the case where the effective depth would be less than the theoretical depth, advancing the EDM electrode towards the workpiece and into the hole drilled in step a) for further drilling.

3. ​The method of claim 2, wherein step g) is revised so that the hole is opened.

4. ​The method according to claim 2 or 3, wherein steps c), d) and e) are repeated after step g), and steps f) and g) are optionally repeated after step e).

5. ​The method according to one of the preceding claims, wherein a side-by-side electrode is determined and equal to the distance travelled by the EDM electrode in Lad between its first position and a third position of the EDM electrode at the end of step b), and a second side is determined and equal to the D-electrode traveled by the EDM electrode in said direction between this third position and the second position of the EDM electrode​, the effective depth of the drilled hole being equal to the difference between the first and second ribs

6. ​Method according to one of the preceding claims, wherein the displacement in step c) is revised over a distance between 40 and 70% of the diameter of the hole.

7. ​Method according to one of the preceding claims, wherein the displacement in step c) is revised over a distance less than or equal to 5 mm, and preferably less than or equal to 1 mm.

8. ​The method according to one of the preceding claims, wherein, in step a), the EDM electrode is al immed with a voltage greater than or equal to 1 00V and a current greater than 1 ampere, and in step d), the EDM electrode is al immed with a voltage of less than 100V and a current of less than 1 ampere.

9. ​The method according to one of the preceding claims, wherein, in step a), the EDM electrode is al immed by electrical pulses, the ratio of the time of the pulses over time between the pulses is greater than 0.2, and preferably between 0.5 and 0.8, and in step d), the EDM electrode is al immed by electrical pulses of which the aforesaid ratio is less than 0.2.

10. ​The method according to one of the preceding claims, wherein, in step d), the polarity of the EDM electrode is reversed with respect to the polarity of the EDM electrode in step a).

1 1 . ​The method of one of the preceding claims, wherein at the beginning of step a) and before drilling the hole,

​The EDM electrode is advanced towards the workpiece (1 6) to a prescaler distance capable of creating a spark gap (20).

12. ​The method according to the preceding claim, wherein said predetermined distance is between an L-axis of the EDM electrode (10) and a surface (16a) of the workpiece (1 6), preferably said predetermined distance is 10 mm.

13. ​The method according to one of the preceding claims, wherein in step d), the EDM electrode is moved towards the workpiece (16) to a predetermined distance (D2) capable of creating a sensing spark (22).

14. ​The method according to the preceding claim, wherein in step d), the distance (D2) is between an L-end of the EDM electrode (1 0) and a surface (1 6a) of the workpiece (16), preferably said predetermined resistance is 5 mm.