METHOD OF POSITIONING A HOLLOW WORKPIECE

Abstract

A method is for positioning a hollow workpiece obtained by casting and that enables the workpiece obtained in this way to be machined in accurate manner. The workpiece is obtained by a casting method involving a mold and a sacrificial core inserted inside the mold and serving to form at least one cavity in the workpiece. The workpiece includes surfaces of a first type defined during casting by the surfaces of the mold, and surfaces of a second type defined during casting by the surfaces of the core. A frame of reference for positioning the workpiece is constructed that includes at least three reference points (P1-P3) belonging to surfaces of the second type of the workpiece.

Claims

1. The positioning method for positioning a hollow workpiece obtained by casting; obtaining said workpiece by a casting method involving a mold and a sacrificial core inserted inside the mold and serving to form at least one cavity in said workpiece; wherein said workpiece comprises surfaces of a first type defined during casting by the surfaces of the mold, and surfaces of a second type defined during casting by the surfaces of the core; and wherein a positioning frame of reference for positioning the workpiece is constructed, which positioning frame of reference comprises at least three reference points belonging to surfaces of the second type of the workpiece; the method further comprising a step of putting the workpiece into place on tooling, said tooling having as many setting members as there are reference points in the positioning frame of reference, a respective setting member being positioned on each of the reference points of the positioning frame of reference of the workpiece.

2. A positioning method according to claim 1, wherein the positioning frame of reference comprises at least five reference points belonging to surfaces of the second type of the workpiece.

3. A positioning method according to claim 1, wherein the positioning frame of reference does not have any reference point belonging to surfaces of the first type of the workpiece.

4. A positioning method according to claim 1, wherein each reference point of the positioning frame of reference is situated on a surface that is locally plane.

5. A positioning method according to claim 1, wherein all of the reference points of the frame of reference are situated on final surfaces of the workpiece.

6. A positioning method according to claim 1, wherein the core defines a wall of the workpiece having at least two faces that are surfaces of the second type.

7. A positioning method according to claim 6, wherein the thickness of said wall is less than 1 mm.

8. A positioning method according to claim 1, wherein the workpiece is a turbine engine blade or vane.

9. A method of machining a hollow workpiece obtained by casting, the method comprising: a positioning step of performing the positioning method according to claim 1, wherein said tooling is drilling tooling; and a drilling step in which a hole is drilled in an inside wall of the workpiece having at least two faces that are surfaces of the second type.

10. A positioning method according to claim 1, wherein the positioning frame of reference comprises six reference points belonging to surfaces of the second type of the workpiece.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The accompanying drawings are diagrammatic and seek above all to illustrate the principles of the invention.

[0043] In these drawings, from one figure to another, elements (or portions of an element) that are identical are referenced by the same reference signs.

[0044] FIG. 1 is a perspective view of a hollow blade.

[0045] FIG. 2 is a cross-section view of such a hollow blade.

[0046] FIG. 3 is a diagrammatic longitudinal section view of such a hollow blade.

[0047] FIG. 4 is a perspective view of such a hollow blade provided with a positioning frame of reference.

DETAILED DESCRIPTION OF EMBODIMENTS

[0048] In order to make the invention more concrete, an example positioning method is described below in detail, with reference to the accompanying drawings. It should be recalled that the invention is not limited to this example.

[0049] In this example, the positioning method relates to a hollow blade 1 for a high-pressure (HP) turbine of an airplane turbojet. The hollow blade 1 is shown in FIG. 1. In this example, it is a rotor blade comprising in a single piece a blade root 2 of dovetail shape enabling the blade to be fastened on the rotor disk of the turbine, an airfoil portion 3, and a platform 4 extending transversely between the blade root 2 and the airfoil portion 3.

[0050] As can be seen more clearly in FIG. 2, which shows the airfoil portion 3 in cross-section, the blade 1 has a plurality of cavities 10 for passing a flow of a cooling fluid in the blade 1 in order to cool it while the turbojet is in operation. In conventional manner, cool air is injected into the blade 1 via a plurality of ducts passing through the blade root 2, and this cool air flows within the cavities 10 of the airfoil portion 3 and is discharged via small orifices formed in the outside walls of the airfoil portion 3, most particularly along its trailing edge.

[0051] Between them, these cavities 10 thus define internal walls 11 separating the various cavities 10.

[0052] Such a hollow blade can be fabricated using a conventional lost-wax casting technique by using a sacrificial core. It should be recalled that the main steps of such a method are as follows: making a first mold having the outside shape of the blade that is to be fabricated; installing a previously fabricated ceramic core of shape that corresponds to the negative of the shapes of the cavities in the blade to be fabricated; injecting wax into the mold and then cooling and obtaining a wax model of the blade that incorporates within it the ceramic core; coating the wax model with a slurry comprising a ceramic powder, while leaving feeder tubes; baking, discharging the molten wax, and obtaining a ceramic shell that forms a second mold, containing the ceramic core in position; casting a molten metal into the shell, followed by cooling and obtaining a metal blade containing the ceramic core; destroying the shell; and destroying the ceramic core, e.g. by means of a chemical bath, and obtaining the raw cast blade.

[0053] Thus, it is possible to define two different types of surface inside the raw cast blade. Surfaces 21 of a first type are surfaces that, while casting the molten metal, were defined, i.e. formed, by the surfaces of the shell. Conversely, surfaces 22 of a second type are surfaces that, while casting the molten metal, were defined, i.e. formed, by the surfaces of the core. Thus, in particular, the surfaces of the inside walls 11 separating the cavities 10 of the blade 1 are surfaces 22 of the second type.

[0054] A positioning frame of reference in accordance with the present disclosure is described below with reference to FIG. 4, which shows the airfoil portion 3 of the raw cast blade.

[0055] The airfoil portion 3 has a leading edge 31, a trailing edge 32, a suction-side wall 33, and a pressure-side wall 34. It extends between the platform 4 and a blade tip 35 having an indentation 36 sometimes referred to as a bath tub. The pressure-side wall 34 has a notched rear portion 34a in the form of a comb, formed by using the core during casting, and leaving uncovered the inside surface 33i of the suction-side wall 33 in the proximity of the trailing edge 32.

[0056] In this figure, the surfaces of the first type are shown in white, while the surfaces of the second type are shown with a stippled texture.

[0057] The positioning frame of reference comprises six reference points P1-P6 that are all situated on surfaces of the second type.

[0058] The first three reference points P1-P3 are situated on the inside surface 33i of the suction-side wall 33. The first reference point P1 is situated within the bath tub 36, substantially in the middle thereof, in the front to rear direction X, defined by the chord of the blade 1, and in the proximity of the top edge of the suction-side wall 33. A member for setting tooling can easily access this reference point and can become positioned therein by penetrating into the bath tub 36 from the top of the blade 1.

[0059] The second reference point P2 is situated in the top rear corner of the inside surface 33i of the suction-side wall 33, in the proximity of the blade tip 35 and of the trailing edge 32. It is accessible via the bath tub 36 from the top of the blade 1, or indeed from its pressure-side face through the notched portion 34a of the pressure-side wall 34.

[0060] The third reference point P3 is situated in the bottom rear corner of the inside surface 33i of the suction-side wall 33, in the proximity of the platform 4 and of the trailing edge 32. It is accessible from the pressure-side face through the notched portion 34a of the pressure-side wall 34.

[0061] The surfaces on which these reference points P1-P3 are positioned are locally plane and they are oriented in such a manner that their normal directions are substantially co-linear with the pressure side to suction side direction Y of the blade 1, orthogonal to the front to rear direction X. Also, given their positions on the inside face 33i of the suction-side wall 33, there is no need for these surfaces to be machined.

[0062] The fourth reference point P4 is situated at the bottom of a notch 34e formed in the top edge of the pressure-side wall 34, in the proximity of the leading edge 31. By means of this notch 34e, which is formed using a portion of the core during casting, the blade tip 35 can be machined without that affecting the position of the fourth reference point P4, since the bottom of the notch 34e remains untouched.

[0063] The first reference point P5 is likewise situated at the bottom of a notch 34f formed in the top edge of the pressure-side wall 34, at the top rear angle thereof. By means of this notch 34f, which is formed using a portion of the core during casting, the fifth reference point P5 is preserved, even in the event of machining the blade tip 35.

[0064] The surfaces on which the reference points P4 and P5 are positioned are likewise locally plane and they are oriented in such a manner that their normal directions are substantially co-linear with the bottom to top direction Z of the blade 1, and orthogonal both to the front to rear direction X and also to the pressure side to suction side direction Y. Under such circumstances, these two reference points P4 and P5 are easily accessible directly from the top of the blade 1.

[0065] The sixth reference point P6 is situated on the rear edge of the pressure-side wall 34, in the proximity of its top end, in a notch of the notched rear portion 34a. Given this position, no machining is going to affect this sixth reference point P6.

[0066] The surface on which this reference point P6 is positioned is also locally plane and it is oriented in such a manner that its normal direction is substantially co-linear with the front to rear direction X of the blade 1. Under such circumstances, this reference point P6 is accessible easily and directly from the rear of the blade 1.

[0067] By means of such a positioning frame of reference, it is possible to position the blade 1 on a machine tool, e.g. for drilling, by setting the blade 1 against six setting members of the machine tool having their ends arranged to correspond with respective ones of the reference points P1-P6 of the positioning frame of reference.

[0068] Once such setting has been performed, the position of the drill tool relative to the blade 1 is accurately known, thereby enabling very accurate holes to be drilled. For example, and as shown diagrammatically in FIG. 3, it is possible to make a hole 19 that passes longitudinally through an internal wall 11 of the blade 1 so as to connect together two distinct cavities 10c and 10d, while still ensuring some minimum amount of residual thickness between the hole 19 and the edge of the wall 11, preferably equal to 0.3 mm.

[0069] Although the present invention is described with reference to specific embodiments, it is clear that modifications and changes may be undertaken to those embodiments without going beyond the general ambit of the invention as defined by the claims. In particular, individual characteristics of the various embodiments shown and/or mentioned may be combined in additional embodiments. Consequently, the description and the drawings should be considered in a sense that is illustrative rather than restrictive.

[0070] It is also clear that all of the characteristics described with reference to a method can be transposed, singly or in combination, to a device, and vice versa, all of the characteristics described with reference to a device can be transposed, singly or in combination, to a method.