FR2518921A1 - PROCESS FOR FORMING A CAVITY IN A METAL PART AND APPARATUS FOR CARRYING OUT SAID METHOD - Google Patents

Abstract

THE INVENTION CONSISTS OF A METHOD OF SHAPING A PART WITH A VIEW TO GENERATING A CAVITY SHOWING A SPECIFIC THREE-DIMENSIONAL SHAPE, DURING TWO MACHINING PHASES. DURING THE FIRST PHASE A, HOLES ARE MECHANICALLY PUNCHED IN THE ROOM, IN A LARGE NUMBER, AND DISTRIBUTED UNIFORMLY IN THE THREE-DIMENSIONAL ZONE WHICH MUST CONSTITUTE THE CAVITY. THIS AREA IS LIMITED BY A PROGRAM PROFILE CORRESPONDING TO THE SEARCHED THREE-DIMENSIONAL CONTOUR, UNDERSTANDING THAT ONLY A MINIMUM OF MATERIAL, NOT REMOVED BY MECHANICAL MACHINING, SUBSIDIZED IN THE REGION TO BE DISCUSSED. DURING THE SECOND PHASE B, THE MINIMUM OF MATERIAL REMAINING BETWEEN THE HOLES AFTER MECHANICAL MACHINING IS REMOVED BY ELECTROEROSION USING AT LEAST ONE ELECTRODE WHOSE ACTIVE SURFACE IS COMPLEMENTARY TO THE CONTOUR TO GENERATE IN THE PART. THE SURFACE OF THE ELECTRODE IS POSITIONED SO AS TO BE PARALLEL TO THE PROGRAM PROFILE. THAT SAID, CONDITION B1 CONSISTS OF PASSING A MACHINING ELECTRIC CURRENT THROUGH THE REMAINING SPARKLING INTERVAL BETWEEN THE MACHINING SURFACE AND THE WORKPIECE OF A MULTIPLICITY OF HOLES, IN THE PRESENCE OF A MACHINING FLUID ELECTRIC, WHILE CONDITION B2 CONSISTS OF STARTING THE ELECTRODE-TOOL WITH A FEED MOVEMENT IN THE DIRECTION OF THE PART, WHILE MAINTAINING THE PARALLELISM AS QUESTIONED ABOVE AND KEEPING THE SPARKLING INTERVAL SUBSTANTIALLY CONSTANT . THIS OPERATION CONTINUES UNTIL THE WORKING SURFACE REACHES A POSITION SUCH THAT IT IS NO LONGER SEPARATED FROM THE PROGRAMMED PROFILE BY THE SPINNING INTERVAL DEFINED ABOVE.

Description

Method for forming a cavity in a metal part

  and apparatus for implementing this method.

  The present invention relates to the machining of three-dimensional contours and, more particularly, to a new and improved method and apparatus intended to form, in a workpiece, a cavity with a defined three-dimensional contour, in order to produce a mold or a matrix. , by

example.

  Generally, certain molds, dies and other products require to be machined in order to acquire very complex three-dimensional contours. These products

  have so far been made by milling, rectification

  fication, using a machine designed to work by electric discharge (EDM) or by electrochemical machining

  (ECM) or by a combination of grinding milling

  electrical or chemical cation and erosion However, as far as the applicant is aware, no attempt has been made

  made in this area to apply the techniques of -

  drilling in the production of a die, a mold or all

another object with complex contours.

  It is recognized that the machining processes of the electrical or electrochemical type are capable of practically reproducing the most complex three-dimensional contours with an extremely high degree of precision. On the other hand, their rate of material removal is relatively low. have also been developed with a particular orientation towards the machining of dies and molds In fact, these

251892 1

  techniques have been used for roughing out the contours of dies or molds These profiles, roughed out by milling or rectification, are generally finished manually by a highly specialized worker, which is both slow and expensive We have also proposed to use electrical machining techniques to finish rough contours

by milling or rectification.

  There remains a constant desire to reduce the total amount of time and labor invested in the production of a die, a mold or any other

  object with a complex three-dimensional outline.

  The present invention arises as an important object to offer a new and improved method for achieving a three-dimensional contour. This method is much more

  efficient than conventional methods; it reduces radica-

  the total time required to shape the cavity

  wanted in a block of raw material.

  Another object of the invention is to provide new equipment, of relatively small size,

  able to apply the method described.

  In accordance with the present invention, a first aspect presents a method of shaping, in a part, a cavity having a defined three-dimensional contour. This method comprises a first step: a) consisting in mechanically drilling a large number of holes in the part, know in the entire three-dimensional area intended to form the cavity This drilling operation is limited by a programmed profile corresponding to said three-dimensional area so that there remains on the workpiece only the minimum amount of material not yet removed During the second step b), this minimum quantity of material remaining between the holes and being in the programmed area of the impression is removed using at least one machining electrode whose surface has a shape complementary to the contour from the cavity The electrode, positioned to be parallel to the programmed profile, removes the residual material by (bl) the passage of an EDM current in the interval separating the machining electrode and the surface to be generated, in the presence of an electrical machining fluid which fills said holes and said interval Ltelectrode is animated by a movement of advance (b 2) by virtue of which it enters the workpiece while maintaining the parallelism mentioned above and keeping the machining interval relatively constant until the surface of the electrode is no more distant from the programmed profile than the value of said interval . Specifically, in the operation (ai, the multiple holes are drilled respectively at individually predetermined depths so that the profile traced by the location of the bottoms of the holes is relatively consistent and equidistant from the programmed profile Multiple holes, at least in their great majority, for ease of execution, are preferably arranged to be as much

as possible parallel to each other.

  Preferably, these multiple holes may also include a number of them drilled obliquely so as to cut the parallel bores. In addition, operation (b) includes an irrigation phase by virtue of which the machining fluid circulates through the oblique holes to create a dynamic flow-of-flỉde machining to

across the erosion interval.

  Preferably, operation (a) is carried out with drilling tools having different dimensions in order to produce series of holes of different diameters. These differentiated series of holes are preferably drilled

  successively in the region considered The paral-

  they are preferably oriented in a direction substantially corresponding to that of the advance of

  the tool electrode during the operation (b 2) Preferably

  rence, the holes, at least those having the largest diameter, are drilled in the region of the imprint so that their axes are equidistant from each other The holes of at least one of the other series are drilled in the intervals remaining between the holes of the largest diameter Preferably, another series of holes having the smallest diameter are drilled in the intervals remaining between the preceding series of

  holes, near the programmed contour of the cavity.

  Preferably, the single electrode referred to above will be replaced by several erosion tools comprising a roughing electrode and finishing electrodes. This means that, in phase (b), a large quantity of material, remaining after mechanical machining consisting of drilling multiple holes, will be removed by the roughing electrode and at least part of the material remaining between the end of travel of the drilling forests and the programmed curve will be removed by erosion using

the finishing electrode.

  The invention, in a second aspect, also includes the apparatus for carrying out the first aspect of the operation, namely a tool carriage capable of receiving at least one drilling tool and at least one electrode-tool for machining by erosion The invention also includes control elements, execution members capable of responding to signals from the control system with a view to advancing the tool carriage so that, on the one hand, it selectively brings the loaded drilling tool of the execution of phase (a) in the respective drilling position relative to the electrode and that, on the other hand, it allows the electrode to carry out phase (b) consecutively to phase (a ) and allows it to take, relative to the workpiece, a position suitable for the execution of the erosion operation The invention finally comprises the means for rotating the drilling tool during the phase {(a )

  as well as the means of supplying the holes and the inter-

  machining valle in electrical machining fluid during phase (b) There will also be means that can be used selectively in phase (b) to pass the electrical machining current between the tool electrode and the workpiece as well as means for advancing the drilling tool in phase (a) and the electrode in phase (b), on the one hand for drilling each of the multiple holes in phase (a) and, on the other hand, for electrically machining the balance of matter in

phase (b).

  These features of the present invention as well as other aspects and advantages will become more clearly apparent from

  reading the following description and examining the

  attached drawing, relating to certain intrinsic elements of the invention.

  Figure 1 (A) is a side schematic representation

  tick illustrating how multiple holes are drilled in the part by mechanical means, using a

  drilling tool, within the limits of a three-dimensional cavity

  The shape of which is produced by the method according to the present invention FIG. 1 (B) is a plan view schematically showing the

  room in which the holes were drilled.

  Figure 2 is a schematic side section showing the method of electrical machining of the part shown in the

Figure 1 (B).

  Figure 3 is a schematic representation of a recommended way to drill the holes in the room,

according to the invention.

  Figures 4 (A) and 4 (B) are sections schematically showing respectively the drilling tool and the workpiece and showing how to adjust the depth of each hole, in accordance

to the principle of the invention.

  Figure 5 is a side view, partially in

  section and partially in elevation, schematically showing a mechanism

  nism allowing to apply the method of

this invention.

  Figure 6 is a front view illustrating a possible form of the tool magazine forming part of the mechanism

Figure 5.

  Finally, Figure 7 is a side section showing a detail of the mechanism of Figure 5, designed to perform

  electrical machining of the previously drilled part.

  Figures 1 (A) Yet 1 (B) represent the part 1 constituted by a rectangular block in which it is proposed to machine the cavity 2-comprising the edge 2 a and the three-dimensional profile 2 b in view, for example, of constitute a matrix In application of the method constituting the present invention, one begins by drilling a certain number of holes 3, using the tool 4, in the part 1 and, in particular, in the zone intended to constitute the cavity 2 delimited by edge 2 a and profile 2 b After that, the pierced part 1 is subjected to electrical machining, as shown in FIG. 3 Machining, carried out either by electrical discharges (EDM), or by a process electrochemical (ECM), removes the residual matrix from the part and

  finish the three-dimensional contour 2 b using the electrode-

tool 5, of complementary profile.

  In the drilling phase, a single tool 4, of a given diameter, can be used to drill a certain

  number of holes 3 with a corresponding uniform diameter.

  However, it is generally preferable to use drilling tools of different diameters 41, 4 m, 4 n, as shown in FIG. 1 (A), in order to have several series of holes of different diameters 31, 3 m, 3 n, as shown in Figure 1 (B) Thus, tool 41, of large diameter, can be used to start drilling a series of equidistant, relatively large holes, each of which reaches a depth of up to a defined distance from the final contour 2 b That done, the 4 m tool, of intermediate diameter, is used to drill a number of holes of average size 3 m, also equidistant from each other but lying in the intervals between the large holes 31 Tool 4 n, of small diameter, can then be used to drill a certain number of holes 3 n, in a similar way, in the intervals between the large holes 31 and the medium holes 3 m In Figure 1 (B) , the small holes 3 N are located in the areas close to the edge 2 a of the cav ity, so that the holes 31, 3 m and 3 n, which punctuate the profile, are distributed

  so as to closely follow edge 2 a.

  It will be seen later, in detail, that it is preferable to mount the tools of different diameters on an automatic changer. They are then successively brought to drilling positions, programmed in advance in correlation with the drilling positions and the diameters of the other tools, in such a way that an optimal distribution is made between the series of holes 31, 3 m and 3 n, of different diameters, which occupy, as much as possible, the volume intended to constitute the cavity 2 In addition, as As shown in Figure 1 (A), it is recommended to drill a good number of oblique holes using one or more 4 S tools in order to constitute irrigation channels through which the erosion fluid can circulate during the later stage of machining to improve the efficiency of

  the electrical finishing operation.

  In the preliminary drilling operation which ensures a significant intrinsic removal of material (approximately 1000 grams / s in the steel), it is desirable to remove a maximum of material from the part 1 in the area thereof.

  located inside the imaginary contour 2 b In the ope-

  subsequent ration of electrical machining, only a minimum quantity of material remains to be removed, which favors

  obtaining a highly precise contour 2 b which characterizes

  The electrical machining phase should preferably be done in pure EDM and the

  description which we will give thereafter will be based

on this finishing process.

  For preliminary machining operations, any

  which method of drilling the series of holes 3, inside

  laughing of the profiles 2 a and 2 b of the part-1, can be adopted.

  FIG. 3 shows an example of a preferred method. We see that the series of large holes 31, of diameter D 1, is obtained using the drill 41 whose axis is successively located at the points whose coordinates are (X 1 , Y 2), (Xl, Y 3) ,, (X 2, Y 1), (X 2, Y 2), (X 3, Y 1), (X 3, Y 2) ,, o X 2- Xl = X 3-X 2 = = Y 2-Yl = Y 3-Y 2 = Y 4-Y 3 = = D 1 After having drilled the large holes 31, we pass to the medium holes 3 m in diameter D 2 each of which is located between the holes 31 and tangentially to them This second series of drilling is carried out using the tool 4 m Finally, it is the drilling tool 4 N which is used to drill the small holes 3 n, in the zone delimited by edge 2 a of the imaginary cavity 2 and the groups of holes 31 and 3 m the most

close to edge 2 a.

  In this case, the coordinates of successive points defining the edge 2 a can be recorded in a memory medium such as a perforated strip The centers of the holes of large, medium and small diameter 31 ', 3 m' and 3 n 'closest to edge 2 a, are positioned in such a way that the holes are relatively equidistant from edge 2 a, i.e. they are spaced from this edge by a distance wl compatible with the expression woewewl o wo

and wl are defined values.

  As shown in Figure 4, each hole 31, 3 m and 3 n is drilled at a depth located at a constant distance Z from the contour 2 b of the cavity Thus, the coordinates of the successive points defining the contour of principle 2 b are programmed and stored in a memory element such as a perforated tape The drilling tool 41 (4 m, 4 n) is advanced until its end reaches a position in which the minimum distance from the points (Xi, Yi, Zi) relative to the surface of the contour 2 b, in the direction of the axis Z, reaches a predetermined value of a Z. FIGS. 5 to 7 show a device designed to

  to practice the method of the present invention.

  As shown in Figure 5, the device comprises a frame whose horizontal surface carries the cross tables 11

  and 12 The upper table 12 is equipped with a tank.

  work 13, integral with the table in which the part 1 is fixed and positioned The table 11 is controlled by a motor lla which displaces the tank 13 and, consequently, the part 1, in the direction of the Y axis The table 12 is controlled by a motor 12 a which moves the tank and, consequently, the part 1, in the direction of the axis X. The device also comprises a vertical upright fixed to the frame 10 and containing the lead screw 14 which is oriented in the direction of the Z axis The lead screw controls the carriage 15 fitted with the tool-holder turret 16 It is controlled by

  the motor 17 and imparts a vertical movement to the holder

  tool 16 The assembly of the carriage 15 and the tool holder 16 is balanced by a mechanism constituted by the counterweight 18,

  the metal cable 19 and the pulley 20.

  The tool holder 16 is a turret capable of turning on the carriage 15, under the impulse of the motor 21 shown in dotted lines. As shown in FIG. 2, the turret 16 carries a number of drills of different diameters,

  141, 4 m, 4 n, 4 p, as well as the electrodes of electro-

  erosion 51, 5 m, 5 n Each turret drilling mandrel is associated with a bevel gear 22 When a tool arrives

  in the working position, under the impulse of the motor 21, the.

  corresponding bevel gear meshes with its counterpart 23 mounted on the shaft 24 This shaft 24 is controlled by the motor 25, by means of the bevel gears 26 and 27 Its rotation sets in motion the pinion 22 and, by

  Therefore, the corresponding drill.

  FIG. 7 shows that each erosion electrode 51, m, 5 n, is designed to be mounted on one of the stations of the turret 16, by means of a conductive cone 29,

  electrically isolated from the earth of the machine by the iso-

  lator 28 The c 8 ne 29 is connected by a cable 30 to a disc 31, itself mounted on a piston rod 32 the piston of which moves in the cylinder 33 All these elements are electrically connected by the cable 34 a to the one of the terminals of the erosion current generator 35 (Fig 5) The other terminal of the generator 35 is electrically connected to the part 1, via the cable 34 b A pressurized fluid, delivered by the hydraulic unit 36 , via the distributor 37, enters the jack 33 to bring the piston rod into electrical contact with the conductive disc 31 and thus send the erosion current to the electrode 51 (5 m, 5 n), - the part 1 being, moreover, connected to the generator 35. The erosion electrode 51 (5 m, 5 n) has an axial bore 5 a which opens onto the working surface 5 b and which is supplied with fluid-by a conical mouthpiece 38 This mouthpiece is applied to the male cone of a conduit 39 which is an integral part of the movable drawer 40 inside the cylinder 41 The latter is, in fact, a bore in the upright. A supply of fluid is therefore carried out through the slide 40 and the tubing 42 The pressurized fluid coming from group 36 is sent into the cylinder 41 to through the valve 42 It pushes the male cone 39 of the drawer 40 into

  the conical mouth 38 and thus connects the tubu-

  lure 42 with the axial bore of the electrode 51 (5 m, 5 n) A stream of machining fluid is then supplied by an irrigation group, it passes through the tubing 42 and arrives in

  the bore 5 a of the machining electrode.

  FIG. 5 shows the irrigation group for the EDM fluid It includes a reservoir 45 to which the fluid coming from the erosion tank 13 returns. This return takes place via the manifold 46, the valve 47 and the pipe 48 Le fluid contained in the reservoir 45 is taken up by the pump 49, sent to the filter 50 then to

  the pipe 51, the valve 52 and the erosion tank 13.

  The part 1 is shown securely clamped on the conductive table 53, electrically linked to the erosion current generator 35 by the cable 34 b The motors lia, 12 a, 20, 21

  and 25, are controlled by the digital control system 54.

  During the operation of the machine shown in FIGS. 5 to 7, a predetermined set of drills 41, 4 m, 4 n, 4 p and a predetermined set of electrodes-51, 5 m, are mounted on the turret 16 and a part 1 is firmly clamped on the table 53 A set of perforated tapes or other storage media, prepared by a CAD or CAM system, is used to control the digital system 54 and,

  through it, the motors lla, 12 a, 20, 21 and 25.

  For example, the motor 21 of the tool selector inter-

  comes first to put into service a drill 41 of large diameter This drill arrives at the work station where it is positioned in the vertical direction to work from top to bottom and it is brought in line with part 1, mounted on the

  table 53 Motors lia and 12 a, axes X and Y, inter-

  then come to position the part with respect to the drill and the latter can descend to pierce one of the large diameter holes To do this, the motor 14 rotates in the direction it takes to rotate the drill 41 and the motor 17 turns to move the carriage 15 and the turret 16, vertically from top to bottom, in order to impart to the drill 41 a drilling advance relative to the workpiece 1 The carriage 15 descends to the programmed depth after which the movement of the motor 17 is inverted and the drill 41 is brought upwards, in the initial position The digital system then sends to the motors lia and llb a programmed order to move the part 1 in order to bring the axis of the drill directly in line with the drilling position of a new hole The motor 17 is restarted to advance the drill 41 and continue this advance until the programmed drilling depth is reached In this way, a programmed series of holes is drilled in the piece, Inside the z one of machining defined by limits 2 a and 2 b After completion of this roughing by large diameter drill 41, the motor 21 receives a new order from the digital system 54 to change the turret 16 This rotates to bring into position working tool 4 m intermediate diameter This drill bit is brought into line with workpiece 1 and a new drilling program for holes of medium diameter takes place The drilling cycles for holes of different diameters follow each other and machine the workpiece with a high metal removal rate As a result, there will be only a reduced amount of material to be removed by electroerosion in the region of the part 1 intended for

form the cavity 2.

  After drilling the series of holes, the digital system 54 intervenes so that the motor 21 and the motors 11 a and 12 a bring the working electrode 5 whose shape is complementary to that to be given to the cavity 2 of the part 1 This cavity was then outlined by the

  different drilling cycles that we have just described.

  The electrode-tool 5 is positioned in such a way that its working surface is parallel to the imaginary contour 2 b The motor 17 then intervenes to immerse the electrode-tool 5 in the erosion tank 13 and to bring the electrode 5 in a position such, relative to the part 1, that the EDM process can take place The valves 37 and 42 are open to admit the pressurized fluid into the jacks 33 and 41 which, on the one hand, brings the piston 32 in electrical contact with the conductive disc 31 and, on the other hand, operates the junction between the bore 5 a of the electrode 5 and the pipe 43

  erosive fluid supply pump 49 starts

  step to send the erosion liquid into the bore 5 a of the electrode 5 and, from there, into the tank 13, and to make it circulate between this electrode and the erosion tank, via the line 48, the -filter 50 and channel 51 The erosion current generator 35 is put into service and establishes a machining voltage between the tool electrode 5 and

  part 1, that is to say between the two surfaces which delimit

  erode interval erode-electrode 5 continues to advance, allowing a succession of electric shocks, spaced over time, to burst through the erosion interval and remove material such portions of the part which are directly opposite the active face 5b of the tool electrode 5 and are therefore subject to electric discharges.

  portions of workpiece material that remain after use

  initial mechanical stroke of part 1, and separating between them the holes drilled by this machining, are gradually removed by electrical machining The turret 16 continues its

  advances towards the workpiece until the electrode-

  tool 5 has reached a programmed depth and completes the

  desired shape of the cavity 2 in the part 1.

  During the EDM phase, preferably, several similar tool electrodes will be used. Thus, a roughing electrode 51 is used first to generate a rough approximation of the cavity 2 with a relative material removal rate " -nt high (for example grams / s in steel), following the procedure described above The roughing electrode is then replaced by a 5 m finishing electrode using which the previously roughed cavity will be brought to its final form with a relatively low material removal rate (for example 0.5 gram / s in steel) Thus, the contours 2 a and 2 b of the cavity will be reproduced not only with a very high degree precision but still with an improved surface finish A semi-finishing operation can also take place using a 5 N electrode and will then be between the roughing phase and the finishing phase.

Claims (8)

Claims.   1 Process for forming a cavity of a three-dimensional contour   desired workpiece in a workpiece, including the steps of mechanically drilling a multiplicity of holes in said workpiece substantially everywhere but within a three-dimensional region which is intended to constitute the   cavity and is limited by a corresponding programmed periphery   laying out said desired three-dimensional contour, so that only a minimum of material is left unmachined in said part inside said region; and then b / EDM said minimal material   left unmachined among the holes and in said area -   programmed with at least one electrode,   plate having a machining surface which is complementary in configuration with said desired contour and is placed for this hole and in parallel with said periphery programmed by: bl / passing an electric machining current through the machining interval between said machining surface and said zone in the presence of an electric machining liquid in said holes and said gap, and b 2 / advancing said tool electrode in a relative movement in said workpiece, while maintaining   said parallelism and maintaining said interspace   substantially constant machining valley until said machining surface reaches a position spaced apart by   said spacing relative to said programmed periphery.   2 Method according to claim 1, characterized in that in the measurement a said holes are drilled respectively to individually predetermined depths such that a periphery defined by the floor regions of said holes conforms to the programmed periphery and in this   that they are substantially equally spaced from it   this. 3 Method according to claim 1, characterized in that at least most of the holes of the multiplicity are formed so as to extend substantially parallel each other.   4 Method according to claim 3, characterized in that said multiple holes comprise a plurality of holes obliquely intersecting said parallel holes, measure b further comprising the measure of flooding with said machining liquid the oblique holes to create a   dynamic flow of said machining liquid through said in- machining range.   Method according to one of claims 1, 2 or 3, charac-   terized in that measurement a is performed with a multi-   plicity of drilling tools with different tool dimensions to produce-the corresponding series of different holes in diameter.   6 Method according to claim 5, characterized in that the measure b consists in drilling said plurality of sets   holes successively in said region.   7 A method according to claim 5, characterized in that said parallel holes are oriented in a direction substantially the same as the direction in which at least one of the tool electrodes is advanced in measure b 2.   8 A method according to claim 7, -characterized in that said holes, at least in a set of the largest diameter of holes are drilled in said region with their centers equidistant from each other and said holes at least in another set are drilled in the interstices among   said holes with the largest hole diameter.   9 Method according to claim 8, characterized in that another set of holes having the smallest hole diameter are included in said multiplicity of holes and are drilled at least in an area of said region which   is closer to said programmed periphery.   A method according to claim 2, characterized in that at least one tool electrode comprises a plurality of electrodes comprising roughing and finishing electrodes in that in measure b at least a substantial portion of said material left unworked remaining   among said multiple holes is removed by electro-   erosion by said roughing electrode and at least a portion of said non-machined material remaining between said periphery defined by the floor regions of said   holes and said programmed periphery is removed by elect   tro-erosion by said finishing electrode.   11 Apparatus for implementing the method according to claim 1, characterized in that it comprises: a tool carriage for carrying at least one drilling tool and at least one electrode-tool for electrical machining; control means;   means operable in response to com signals   command from said control means to entrain said carriage tool to selectively bring said drilling tool, to execute measurement a, in a drilling relationship with said tool electrode, to execute measurement b following measurement a, in an electrical machining relationship with said part, respectively; means for rotating said drilling tool in measure a;   means for supplying said electric machining liquid   that in said holes and said machining interval in measure b; means selectively operable in measure b for supplying said electrical machining current between said tool electrode and said workpiece; and means for advancing said drilling tool in measure a and said tool electrode in measure b in said part for drilling each of said multiple holes in measure a and for EDM machining   said material left unmachined to the extent b   pectively 12 Piece obtained by implementing the method according to one of claims 1 to 10.