Patent Application
20120124834
The present invention relates to the field of the manufacture of components such as turbomachine blades using the forging technique, notably the precision forging technique. It relates more particularly to the manufacture of large fan blades in titanium alloy, such as turbojet engine fan blades and to the finishing of these blades to make the semifinished forged component geometrically compliant by using an adaptive polishing operation.
Turbojet engine fan blades are generally produced using precision forging. Precision forging involves striking successive blows to a rough form held in suitable dies until a semifinished component is obtained that has a shape and dimensional characteristics similar to that of the finished component. In the case of a forged semifinished fan blade, the airfoil does not comply, in terms of geometrical characteristics, with the final specifications, within accepted tolerance bands. These characteristics include for example the twist, which is a rotation of the sections of the airfoil along the stacking axis, the buckling which is a bending of the airfoil with respect to the stacking axis and the reference points, and ripple and shape defects.
The airfoil therefore has to be made compliant. Overall, this involves correcting the profile of the suction face side and of the pressure face side by removing material from those points of the airfoil where the thickness is greater than the theoretical profile. In the context of precision forging, the correction involves removing an excess thickness of up to a few tenths of a millimeter, generally of between 0.4 and 0.6 mm.
A number of points corresponding to the theoretical profile are determined, these points being distributed in a network along the axis of the airfoil and between the leading edge and the trailing edge. The geometric characteristics of the semifinished blade are measured at these points using three-dimensional sensor means. Patent EP 1596156 in the name of the applicant company describes such a means. The difference between the theoretical profile and the actual profile is thus determined.
According to the prior art, the next step is the thickness sorting operation, which involves analyzing and then protecting the thinnest zones of the component by applying a coating. This operation is performed chiefly by hand. The excess material is then removed, from between these protected zones, using chemical machining which involves keeping the component for a set length of time in a bath of acid capable of eating away at the metal. The out-of-tolerance zones which exhibit appearance defects and traces of the chemical machining are then manually reworked by local and repeated polishing. This operation is what is known as a first appearance polishing operation. By hand if necessary the component is tweaked until its shape falls within the prescribed tolerance band.
Finally, an automated polishing operation known as the final appearance polishing operation ensures the continuity of the aerodynamic profile and the surface finish necessary for the air to flow correctly. The automated polishing operation is generally performed using an abrasive belt. Use is made for example of a belt in which the abrasive material is silicon carbide. The belt is mounted on a wheel rotated tangentially with respect to the surface of the component. The movement of the wheel relative to the surface is controlled by a program that takes account of the geometry of the surface that is to be polished. Parameters such as the rate at which the abrasive belt travels across the surface, the rate at which the wheel travels with respect to the component and the pressure applied to the surface and the grit of the abrasive material are determined in such a way as to remove the required thickness of material and achieve the desired surface finish. A description of an abrasive belt polishing machine can be found in U.S. Pat. No. 5,193,314.
The manual operations, in particular when heavy components such as turbojet engine fan blades have to be worked on, are awkward for the operator and can potentially generate musculo-skeletal problems. Further, these operations have to be checked. There is a desire to replace manual operations with operations that free the operator and which allow several operations to be grouped into one. The applicant company has already developed a method for the automated polishing of titanium alloy using an abrasive belt made up of super abrasive grit made of industrial quality diamond or boron nitride; that method is described in EP 1525949.
The applicant company has set itself the objective of achieving geometric compliance and performing final polishing of the airfoil in one and the same step, and preferably automatically.
This objective is achieved using a method of manufacturing a component by forging, involving producing a semifinished component by precision forging and polishing the component using an abrasive belt, the nominal or compliant geometric characteristics of the component to be obtained being determined in a theoretical model, characterized in that it comprises the following steps:
Because the machine is numerically controlled, a specific program for the component that is to be polished is generated.
In the prior art technique for achieving compliance, the automated polishing machines are used for the final appearance polishing, using an abrasive belt suited to the desired surface finish. In the prior art, a uniform thickness of material is removed so as not to destroy the profile that has been made compliant by hand in the previous operation; the manual step of achieving compliance is now eliminated and incorporated into the final polishing operation.
The advantages of achieving compliance in the way described by the invention, which can thus be made automatic, are that the manual operations of sorting, of masking those zones that do not need to be treated and of reworking the components are eliminated.
Time in the component manufacturing cycle is also saved.
A reduction in the geometric spread which is associated with the manual rework is also noted.
Finally, the risks of repetitive strain injury are also eliminated.
According to another feature, a plurality of measurement points is defined beforehand at the surface of the component, the geometric characteristics of the semifinished component are measured at least some of said measurement points, the removal of material by said abrasive belt is controlled at said measurement points on the basis of the discrepancy between the geometric characteristics of the semifinished component and the nominal geometric characteristics.
According to another feature, a map of the removals of material is defined from the measurements of the geometric characteristics of the semifinished component, and said map is converted into a map of the control parameters for controlling the abrasive belt.
For preference, the control parameters for the abrasive belt are calibrated beforehand for each of the measurement points. The calibration operation is performed just once for a given type of component.
According to a preferred embodiment of the method, the belt is controlled by varying the relative feed rate of the component with respect to the abrasive belt, with the other abrasive belt control parameters kept constant. The other parameters are the rotational speed of the abrasive belt and the contact pressure of the wheel against the surface that is to be treated.
Thanks to this feature of the method it is possible in an advantageous way to overcome the difficulty of polishing the surface of the component while at the same time making it geometrically compliant.
In the calibration phase, a relationship such as a law or a look-up table is established between the controlled parameters and the amount of material removed. For example it is possible to use a calibration which is performed on the basis of the measurement, at each point, of the amount of material removed associated with at least two different feed rates.
In order to ensure polishing that is uniform at all points, a minimum amount of material corresponding to uniform polishing is removed at each measurement point.
As has been set out hereinabove, the method applies in particular to a turbomachine blade, more particularly to a turbojet engine fan blade.
The invention will be better understood and other objects, details, features and advantages thereof will become more clearly apparent during the course of the detailed explanatory description which follows, of a non-limiting embodiment given with reference to the attached drawings, in which:
The semifinished component that forms the subject of the method of the invention is, for example, a turbojet engine fan blade as depicted in
The contour of the airfoil is defined by a plurality of sections or cross sections extending between the platform and the tip, along an axis known as the stacking axis with respect to a reference system. The reference system is itself defined by elements or planes of the blade root. Thus, the blade is wholly geometrically characterized by knowledge of parameters associated with predefined points on each of the sections. This set of points constitutes the nominal geometric characteristics of the blade and forms the theoretical model. The nominal geometric characteristics can be defined in terms of dimensions, shapes, one or more coordinate(s) in space or else orientations or a combination of a number of these.
In the example depicted, that part of the airfoil that is situated between the platform and the pressure face aileron is defined by seven sections S1 to S7. In each of the sections, points on the surface of the airfoil between BA and BF have been identified. For example, the section referenced S4 comprises the points identified S41 to S49 between the trailing edge BF and the leading edge BA.
As in the method of the prior art, the starting point is to use a three-dimensional measurement robot to measure the geometric characteristics of the as-forged semifinished component.
The applicant company has described an example of a method and apparatus for simultaneously measuring the geometric characteristics of a plurality of points distributed over the surface of a blade in patent EP 1 596 156. The three-dimensional measurement of the coordinates of a set of predetermined points on the surface of a mechanical component with respect to a predetermined frame of reference involves:
Using this calculation, which is performed at each of the measurement points on the predefined sections, it is possible to note the non-compliant regions, i.e. the regions which, for the points considered, have excess thickness. For each of the non-compliant zones, a value of how much material needs to be removed in order to make them compliant is obtained.
In the prior art, the following successive operations would then be carried out:
The method of the invention involves, having determined the thicknesses that need to be removed, creating a map of the removals of material and polishing in a machine, preferably an automated machine, with direct removal of material at each of those points that correspond to the established map, without passing through the step of manually achieving compliance. In fact, the surface of the blade is polished while at the same time making it geometrically compliant.
One example of a polishing machine which can be suitable for the invention is described hereinbelow with reference to
The machine 1 illustrated in
To polish the component the belt is pressed locally and tangentially to the surface thereof, applying a determined pressure. The belt is set in movement and rotates with the wheel 111.
The amount of material removed and the surface finish are dependent on a number of parameters:
According to another feature of the invention, the difficulty of polishing the surface of the blade while at the same time achieving geometric compliance is solved by controlling the relative feed rate of the component with respect to the polishing belt, preferably keeping the contact pressure and the belt rotational speed constant.
The machine is controlled on the basis of the map of material to be removed. It is converted, for the benefit of the machine, into a map of relative feed rate of the component with respect to the polishing belt. This map is devised on the basis of a pre-established relationship between the feed rate and the amount of material removed. Such a relationship is established by learning at each point on the component.
The learning phase is carried out just the once for a given type of component. According to one particular embodiment of the method of the invention it involves measuring, at each point on the component, the amounts of material removed associated with a plurality of uniform different feed rates. This yields the amount removed, by interpolation; one particular example involves determining the removal for two different feed rates.
To sum up, the various steps in achieving compliance of a component the nominal geometric characteristics of which are known, involve measuring its geometric characteristics and identifying which zones are non-compliant. On the basis of these measurements a map of the material to be removed for the points corresponding to these zones is established. These data are entered into the control box of the polishing machine 1. The component that is to be processed is placed between the jaws of the machine and the machine is set in action. The abrasive belt is driven in rotation by the wheel and brought into position against the component. According to one feature of the invention, the rotational speed of the wheel is kept constant throughout the polishing operation, as is the pressure of the wheel against the component. The feed rate of the belt along the component is controlled by the control box into which the above data has been input.
The feed rate thus varies according to the amount of material that is to be removed. The method thus allows the component to be made compliant and undergo final polishing all in a single pass. For preference, minimum removal of material is planned for the entire surface in order to achieve a uniform final polishing.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0954398 | Jun 2009 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/058946 | 6/23/2010 | WO | 00 | 1/31/2012 |
