Method for machining the trailing edge of a turbine engine blade

Abstract

A method for manufacturing a turbine engine blade including an airfoil with a profile defined by a theoretical digital model, the method including: manufacturing a blank having a thickened portion along a trailing edge of the airfoil relative to the theoretical profile; removing the thickened portion by adaptive machining, including: positioning the blank in a reference frame; acquiring, by probing at a predetermined number of points on a first surface of the blank along the trailing edge, positions of the points in the reference frame; determining differences, in one direction, in position from corresponding points of the theoretical model; producing machining grids on the blank surface, peaks of the grids determined from the points; determining an amount of material to be removed from the surface of the grids, based on the position of the points relative to the peaks and the deviations in position; and machining the airfoil.

Claims

1. A method for manufacturing a turbine engine blade, the blade including a vane, an inner platform, and an outer platform, a profile of the vane being defined by a theoretical digital model, comprising: manufacturing of a blank of the blade, the blank including the vane, the inner platform, and the outer platform, and having a machining allowance along a trailing edge of the vane with respect to the theoretical profile, and removing the machining allowance by adaptive machining, comprising: a) positioning the blank in a machining tool such that a rectangular axis system defining a reference frame of the blank coincides with a machining axis system of the machining tool; b) acquiring, by probing at a given number of points on a first face of the blank, along the trailing edge, a position of the points in the reference frame; c) determining differences in position, in one direction, from corresponding points on the theoretical model; d) producing machining grids on the first face of the blank, apexes of the grids being determined from the points; e) determining a quantity of material to be removed on a surface of the grids, depending on a position of the points on the grid with respect to the apexes of the grids and the position differences; and f) machining the vane using the machining tool, wherein the positioning includes mounting the blank in a support, the support including a first fixing mechanism which fixes the inner platform and a second fixing mechanism which fixes the outer platform, wherein the movement of the machining tool is along a principal direction in the machining axis system which is parallel to a radial axis of the vane, and wherein the determining the quantity of material to be removed on the surface of the grids is measured along the principal direction in the machining axis system, and for each machining grid and at each contact zone of the machining tool with a surface of the blade inside the grids, the quantity of material to be removed is defined by a position of contact of the machining tool in the each machining grid and a value of the position differences measured on each apex of the each machining grid using weighting coefficients for each apex of the each machining grid, a value of a respective weighting coefficient corresponding to a distance between a center of mass of the each machining grid and the respective apex, a sum of the weighting coefficients for the each machining grid being equal to 1.

2. A method according to claim 1, wherein the machining allowance to be machined is formed on a suction face and on a pressure face of the vane.

3. A method according to claim 2, wherein the machining allowance is further formed on a chord of the vane.

4. A method according to claim 1, wherein the points probed are at least three in number.

5. A method according to claim 4, wherein the points probed are arranged parallel to the trailing edge.

6. A method according to claim 1, wherein the apexes of the machining grids comprise the probed points and points that are deduced by translation along a chord passing through the probed points.

7. A method according to claim 1, further comprising machining the face opposite to the first face, the quantity of material removed being determined by the position with respect to the apexes of the machining grids of the first face.

8. A method according to claim 1, wherein the support is removable and the support is mounted on the machining tool which is a numerically controlled machine tool.

9. A method according to claim 8, wherein the blade is a rotor turbine blade which is fixed to the removable support by being gripped between a removable jaw and a fixed jaw, the fixed jaw being shaped to the theoretical profile of a face of the vane, leaving free the trailing edge to be machined.

10. A method according to claim 8, wherein the blade forms part of a turbine nozzle sector, the removable support is arranged for isostatic mounting of the turbine nozzle sector in the removable support.

11. A method according to claim 10, wherein the removable support comprises support pads for damping vibrations during machining.

12. A method according to claim 10, wherein the machining of each of the blades of the nozzle sector comprises a prior best readjusting the blade with respect to the machining axis system on the machine tool.

13. A method according to claim 1, wherein the differences in position are determined along the direction in the machining axis system which is parallel to the radial axis of the vane.

14. A method according to claim 1, wherein the machining grids are polygons with four sides.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention will be better understood and other aims, details, features and advantages thereof will emerge more clearly in the detailed explanatory description that follows of an embodiment of the invention given by way of purely illustrative and non-limitative example, with reference to the accompanying schematic drawings, in which:

(2) FIG. 1 is a turbine blade;

(3) FIG. 2 is a turbine nozzle sector;

(4) FIG. 3 is nozzle sector mounted on a removable support, the points constituting the machining grids defining the adaptive correction zone on the trailing edge being identified;

(5) FIG. 4 illustrates how the calculation of the position of the machining tool is determined;

(6) FIG. 5 is the machining tool in action along the trailing edge of a vane;

(7) FIG. 6 is a support for an isolated rotor blade;

(8) FIG. 7 is the support of FIG. 6 with a blade between the jaws as well as the rod and bead of the probe at various probing positions;

(9) FIG. 8 is the support for a turbine nozzle sector.

DETAILED DESCRIPTION OF THE INVENTION

(10) FIG. 1 shows a low-pressure turbine blade 1 seen from the side of the suction face thereof. This is a rotor blade, and is individual and comprises a root 3 by means of which it is mounted on a turbine rotor, and a platform 4 between the root 3 and the vane 2. A second platform 5 is formed at the apex of the blade 1. The vane 2 extends, between the two platforms, also along its chord between the leading edge BA and the thinner trailing edge BF. The rotation axis of the turbine rotor on which the blade is mounted is parallel to the axis of the blade root.

(11) FIG. 2 is a low-pressure turbine nozzle sector 10, in a single piece, obtained by metal casting. This sector comprises a plurality of vanes 11 to 15 arranged radiating in a sector of a disc between an internal platform 17 and an external platform 18, both in an arc of a circle. Such a low-pressure nozzle sector 10, like the low-pressure rotor blades, is manufactured according to the lost wax casting technique known per se. Despite the precision obtained, this technique does not make it possible to achieve the low thicknesses required at the trailing edges. Machining is therefore necessary. The invention provides a method for achieving such a finish, taking account of the deformations of the vane.

(12) The method applied to the finishing of the vanes 11 to 16 of a six-vane single-piece nozzle sector 10 is described. Said sector is first of all mounted on a removable support 20 as can be seen in FIG. 3. This is an isostatic mounting along the three axes of a rectangular axis system, in particular the reference axis system of the sector, involving holding in translation along the three axes of the reference axis system, by means of the flange 21 arranged on the side of the external platform 18 at the centre thereof and which provides the fixing along the three directions of the axis system, and three supports for rotational locking by means of the flanges 22, 23 at the ends of the internal platform 17 and the support 24 at the centre (not visible in FIG. 3). The mounting further comprises two clevises 25 and 26 for damping vibrations, arranged on either side of the flange 21.

(13) The angle of the plane of the sector is chosen so as to take into account that of the plane of the zone of the trailing edge with respect to the same plane. It is preferable for the machined surfaces of the trailing edge to be parallel to the axis of the machining tool such as a broach. In the case of a five-axis numerically controlled machine for example, this makes it possible to avoid errors in positioning of the rotary axes (that is to say the 4.sup.th and 5.sup.th axes) of the machine.

(14) The support 20 is itself mounted on the platform 30 of the numerically controlled machine (not shown) by means of a pedestal 31.

(15) Once the support 20 is mounted on the platform of the machine tool, the axis machining system is best fixed with respect to the vane to be machined. For this purpose, the position of a certain number of points on the vane is acquired, by probing by means of a suitable apparatus, and suitable software, for example using the least squares method, computes the corrections to be made to the reference axis system of the mounting on the machining machine. This correction is made by means of three rotations about the X, Y and Z axes and a translation of the origin in the X, Y and Z directions (the linear axes of the machine).

(16) The machining axis system coincides with the principal axes of the vane, 11 to 16 respectively, to be machined. The definition of the axis system is linked to the configuration of the machine as well as to the possibility of probing along three axes (the rotation axes being deactivated). The positioning of the machined zone of the vane parallel to the axis of the broach means that the X axis is the radial axis of the vane with respect to the driving axis, the Z axis the rotation axis of the platform, perpendicular to the plane of the platform, the Y axis completing the direct axis system. The plane XY consists of the support face of the platform. It should be noted that the fixing of the machining axis system must be done for each of the vanes of the nozzle sector.

(17) Once the fixing is effected, the so-called adaptive machining of the trailing edge BF is carried out. This machining technique is described in the patent application FR 11 55 424 submitted by the present applicant.

(18) Said machining technique comprises the acquisition, by probing by means of a suitable probing apparatus with or without contact, of the position of a certain number of points along the trailing edge BF of the vane to be machined. These points are those defined on the 3D theoretical model of the vane in the CADM software. The reference frame is that of the machining axis system.

(19) Here, the position is measured of the points N2 to N6 (FIG. 3) distributed between the internal platform 17 and the external platform 18 at one mm (one mm corresponds to a functional dimensioning on the reference plane for measuring the thickness of the trailing edge) and along the trailing edge BF of the blank vane 14 to be machined. The number of points is preferably at least three.

(20) From these points, the position of which has been measured, a plurality of induced points are defined. All the tool/part contact points defining the machining of the pressure face and suction face must be contained in grids, and therefore the point N1 and the point N7 and then the points N8 to N14 are added. Said points N8 to N 14 are at the intersection of the chord passing through the probed points and the trailing edge. The points N1 and N7 that are too close to the platform to be able to be probed are obtained by translation of the adjacent points N2 and N6. The points N1 to N14 thus delimit the machining grid apexes, C1 to C6.

(21) For each of the points N1 to N14, the value of the differences delta (N) between the measured or induced position and the position of the theoretical model is computed. Because of the fixing of the reference axes, this difference is measured along the principal axis X. For each machining grid and at each contact zone of the tool with the surface of the blade inside the grids, the quantity of material to be removed is defined firstly by the position of the contact of the tool in the grid and the value of the differences delta (N) measured on each apex of the grid concerned.

(22) The method for determining a quantity of material to be removed by machining is described more precisely with reference to the example in FIG. 4. In this example, the grids are polygons with four sides; they could however have a different number of sides.

(23) Each point P is defined with respect to the apexes or nodes of the grid in which it is situated by means of four coefficients, referred to as weighting coefficients CPi (i being the reference number of the node in question). Each weighting coefficient corresponds to the weight that it is necessary to allocate to the corresponding node so that the point P is the centre of mass of these four nodes. In other words, the closer the point P is to a node, the higher the coefficient allocated to this node, and conversely the nodes furthest away are allocated a small coefficient. In order to make the definition of these weighting coefficients unique, they are reduced proportionally to one another so that the sum thereof is equal to one. By way of example, if the point P is at the centre of the grid, the four coefficients are equal to 0.25; if it is close to one of the nodes, as can be seen in FIG. 4, the coefficient CP1 will be equal to 0.5 whereas the other three will be equal respectively to 0.35 for CP2, 0.10 for CP7 and 0.05 for CP8.

(24) The file formed by the weightings of the points P to be scanned by the machining tool and by the corresponding orientations of the axis of the tool is then converted into a format that can be understood by the numerically controlled machine and loaded into the software thereof.

(25) The following step consists of defining a delta positioning difference for all the contact points between the part and the tool during the final machining. For this purpose the computing of this delta takes into account the weighting coefficients of the point P computed previously and the differences delta (Ni) from the nodes of the grid in which the point P is situated. The delta at point P, that is to say the correction to be made to the point P on the surface of the vane, is defined as being equal to the sum of the values obtained by multiplying each delta of a node by the weighting coefficient that is associated therewith.

(26) In the example of the point P situated in the grid formed by the four nodes N1, N2, N7 and N8, the value of delta (P) is equal to CP1*delta (N1)+CP2*delta (N2)+CP7*delta (N7)+CP8*delta (N8).

(27) More generally, it can be expressed by the formula delta (P)=?CPi*delta (Ni), where i corresponds to the index of the apexes of the machining grids.

(28) This difference delta (P), which extends along the X-axis of the machining axis system at point P, determines the component of the correction to be applied to the Cartesian coordinates given to the program that controls the positioning of the tool during the final machining. In the present case, the correction is made only along the X axis.

(29) The machining is preferably done by a five-axis numerically controlled machine tool for which the position of the tool is continually corrected by the value issuing from the deltas of the nodes of the grid in question and the weighting coefficients of the point where said tool is situated. Preferably the machining is done by slab milling; that is to say by the cutting peripheral wall of the milling cutter.

(30) Once the first face has been machined to bring its position to that of the surface of the theoretical model, the machining correction is determined on the face opposite to the first, that is to say here the pressure face. As on the suction face, the starting point is the initial points N2 to N6. Since the probe cannot access between the vanes, the correction is made in the same way as corrections carried out on the suction face. The same grids as on the suction face are used, and only the tool/part contact points are different.

(31) One example of the performance of the operations of milling the vanes of a nozzle sector is as follows: rough milling of the chord of each vane, one blade after another, this milling aiming to keep a machining allowance, rough milling of the pressure face of each vane, one vane after another, semi-final and final milling of the suction face of each vane, one vane after another, final milling of the pressure face of each vane, one vane after another, final milling of the chord of each vane, one vane after another.

(32) The thickness removed during the rough machining is around a few tenths of a millimeter, or even a few millimeters (depending on the machining allowance of the smelting).

(33) The thickness removed during the final machining is around a few hundredths of a millimeter.

(34) The amount of the correction is a few tenths of a millimeter.

(35) FIG. 5 shows the position of a milling cutter 40 machining the suction face of the vane 16, running along the trailing edge. The tool is mounted on a tool holder able to move along five axes for example.

(36) FIGS. 6 and 7 show the mounting of a rotor blade on a support 50. The support comprises a fixed jaw 51, the surface 51a of which contacting the rotor blade is formed so as to receive the suction face of the rotor vane. FIG. 6 shows the fixed jaw 51 alone. Said jaw is fixed to the platform 30 and has a specified inclination. This inclination is chosen so as to allow orientation of the machining tool in a direction such as the direction of the axis parallel to the axis of the broach. FIG. 7 shows the rotor blade held between the jaw 51 and a removable jaw 52 so as to leave free the region of the trailing edge of the vane.

(37) The various positions of the probe S where its sphere is in contact with the surface of the zone of the trailing edge are also shown. The mounting enables the probe to be positioned at each of the points, the position of which it is wished to measure.

(38) This arrangement enables the tool to machine by running on the two faces, pressure and suction, of the vane as well as the chord. The two jaws are gripped by the screws 53. The clamping movement of the movable jaw against the fixed jaw is ensured by guidance along guide pins 54. The screws allow sufficient clamping to reduce the deformation of the vane.

(39) The fixed jaw 51 comprises a stop 55 for the blade root platform making it possible to fix the blade in an axial position.

(40) The machining is carried out in the same way as described above for a nozzle sector vane.

(41) FIG. 8 shows the support 20 for a nozzle sector without the part. The isostatic fixing elements having the attachment flange 21 on one side for immobilising in the three directions can be seen. On the other side the three supports for rotational immobilisation about the three axes can be seen.

(42) The clevises 25 and 26 for damping the vibrations are arranged on either side of the flange 21. Said clevises are fixed by screws to the base and comprise support pads bearing against the faces of the platform, preventing the platform from vibrating.