Abstract:
The abrasion machining method according to the invention, for abrading a part that cannot be machined by conventional means, comprises the following steps:
       a pin is placed at a certain distance from the part to be machined, so that there is no contact between the part and the pin;   the pin is driven so as to rotate;   an abrasive liquid is injected between the rotating pin and the part in order to abrade the latter; and   the pin is moved translationally along the part.       
 
     Thanks to the method, the part may be machined precisely and reproducibly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the machining of parts and more particularly to the machining of parts made of ceramic matrix composites. 
         [0002]    Ceramic matrix composites are generally denoted by the acronym CMC. These composites are formed from a ceramic matrix, for example based on carbon (C) or silicon carbide (SiC) within which fibers of a material having good tensile strength properties extend so as to take up the forces to which the composite is subjected. The ceramic matrix provides the bonding between the fibers, ensures that they are spaced apart and transfers the forces to which the CMC part is subjected to said fibers. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    CMC materials are used especially for the manufacture of parts for applications in the aeronautical field, in particular for the manufacture of turbine blades, turbine seals, combustion chambers, nozzles, guide vanes, etc. CMC parts are abradable (i.e. they can be abraded). The known methods for machining CMC parts are milling, grinding with a grinding wheel, water jet cutting, electron beam cutting and ultrasonic machining. 
         [0004]    Problems arise when machining parts formed from a CMC material, irrespective of the method chosen. The methods most often used, which are grinding and water jet cutting, are either expensive or do not provide high-precision machining. This is particularly problematic when the parts are of complex shape, especially in the aeronautical field that imposes standards with low tolerances. Moreover, the use of a tool such as a grinding wheel or a milling cutter for machining a CMC part causes fouling of this tool, since particles of the CMC part become lodged between the grains of the material forming the tool. Furthermore, electron beam cutting and ultrasonic cutting are complex operations that are onerous to implement. 
         [0005]    The invention has been developed in order to solve specific problems associated with CMC parts to be machined and therefore applies particularly well to this type of part. However, the Applicant does not intend to limit the scope of its rights to just this application and this is why the invention applies more generally to composite parts (for example those with an organic matrix) and even more generally to parts intended to be machined. 
       SUMMARY OF THE INVENTION 
       [0006]    Thus, the invention relates to a method of machining a part by abrasion, in which:
       a pin is placed at a certain distance from the part to be machined, so that there is no contact between the part and the pin;   the pin is driven so as to rotate; and   an abrasive liquid is injected between the rotating pin and the part in order to abrade the latter.       
 
         [0010]    Thanks to the invention, the pin serves as support and as driving means for the abrasive liquid that exerts the abrasion function. The pin therefore participates indirectly in the machining of the part via the abrasive liquid. The method thus has all the advantages of abrasion machining, without having the drawbacks thereof In particular, since the pin is not used directly for abrading the part, it is less consumed or fouled by it and its shape remains more stable. This enables the method to be repeated with constant good quality, only the liquid being consumed. Moreover, it is easy to adapt the method to the part to be machined, by adapting the nature of the abrasive liquid and the various parameters that may be involved in the method, including the distance between the part and the pin, and therefore the pressure exerted by the pin on the liquid and the part, the rotation speed of the pin, the possible speed of displacement of the pin relative to the part, the amount of liquid injected between the part and the pin, etc. 
         [0011]    Finally, the method of the invention includes characteristics both of a milling method and a grinding wheel method: of a milling method since it employs a rotated pin and of a grinding wheel method since the material is removed from the part by abrasion. Thus, the use of a rotating pin gives the method of the invention great precision and controllability in its implementation, whereas the abrasion of the part may take place very progressively, since the action of the abrasive liquid, driven by the pin, is exerted tangentially to the abraded surface of the part. 
         [0012]    According to a preferred embodiment, the pressure applied to the part is adapted by adjusting the distance between the pin and the part. 
         [0013]    According to a preferred embodiment, liquid is injected continuously during the process, to ensure constancy of its action on the part. 
         [0014]    According to a preferred embodiment, the abrasive liquid comprises a liquid with a suspension of particles of an abrasive material. In particular, the effects of the method may be adapted by modifying the nature of the liquid, the particle size distribution of said particles, their concentration, their hardness, etc. Preferably, the abrasive particles used are made of boron carbide. 
         [0015]    Preferably, since the pin is formed from a material having a certain hardness and the part is formed from a material having a certain hardness, the abrasive material of the abrasive liquid has a hardness between the hardness of the pin and the hardness of the part. Thus, it is guaranteed that the liquid has an abrasive effect on the part but not on the pin, and that the pin is not consumed, thereby improving the stable reproducibility of the method over time. 
         [0016]    According to one embodiment, the pin is also driven so as to move translationally along the part, for example in a manner similar to that of a conventional lateral milling operation. This makes it possible to machine a larger area of the part with the pin, in a controllable and precise manner. 
         [0017]    The method is particularly suited to the machining of parts made of materials of the CMC type. 
         [0018]    The invention applies particularly well to the machining of parts used in the aeronautical field, for which the requirements and the standards are very stringent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be better understood with the aid of the following description of the preferred way of implementing the method and the preferred embodiment of the device of the invention, with reference to the appended plate of drawings, in which: 
           [0020]      FIG. 1  shows a schematic view of the preferred embodiment of the machining device of the invention; 
           [0021]      FIG. 2  shows a perspective partial view of a blade that can be machined using a device of the type shown in  FIG. 1 ; and 
           [0022]      FIG. 3  shows a pin that can be used in the device of  FIG. 1 , specifically for machining the root of the blade shown in  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring to  FIG. 1 , a device  1  for the abrasion machining of a part  2  comprises a pin  3 , of axis A, mounted on a shaft  4  that also extends along the axis A and is itself supported by a frame  5 . The pin  3  may also be denoted by the expression “rotary head”  3 . The shaft  4  can be rotated (as represented schematically by the arrow  6  in  FIG. 1 ) by rotary drive means, not shown, comprising here an electric motor, said means being mounted on the frame  5 . The shaft  4  then rotates the pin  3  which is integral therewith. The device  1  also includes means (not shown) for positioning and fixing the part  2  relative to the frame  5 , in a manner known to those skilled in the art. 
         [0024]    The device also includes all the elements of a center (or device) for conventional machining (motors, position sensors, processing unit, etc.). It is not necessary to describe them in detail because their structure and their function are well known to a person skilled in the art who, to implement the device and the method of the invention, may use a conventional machining center that he will adapt so that it includes the means specific to the device and method of the invention. 
         [0025]    The pin  3  is a part axisymmetric about an axis A, that is to say a part that has an external surface having a shape generated by the revolution of a straight or curved segment, called a generatrix, about the axis A. The pin  3  shown in  FIG. 1  has a cylindrical shape and its generatrix is therefore a straight line parallel to its axis of revolution A. 
         [0026]    The device  1  also includes means for injecting an abrasive liquid between the pin  3  and the part  2 , in this case including an injection system with a pump and a nozzle  7  for injecting this liquid. The liquid may thus be injected into the interface between the pin  3  and the part  2 , as shown schematically by the arrow  8  in  FIG. 1 . 
         [0027]    The abrasive liquid comprises in this case water (which is the “base liquid” or solvent of the abrasive liquid solution) into which boron carbide (which is the solute of the solution) is mixed, the boron carbide being the abrasive material or agent of the liquid. This solution, of boron carbide mixed with water, has an abrasive effect when it is frictionally applied to an “abradable” material, i.e. one that can be abraded. 
         [0028]    In this case, the external surface of the pin  3  is smooth. As such, the pin  3  has no abrasive effect and it is the abrasive liquid driven by the pin  3  that has an abrasive effect on the part. 
         [0029]    The hardness of the abrasive agent of the abrasive liquid is in this case between the hardness of the part  2  and the hardness of the pin  3 . Thus, the liquid has an abrasive effect on the part  2  but not on the pin  3 . 
         [0030]    The pin  3  in this case is made of diamond. This material has the advantage of being very hard, enabling it not to be affected, and therefore not consumed, by its contact and friction with the abrasive liquid. Preferably, it is a single-crystal diamond, this type of diamond having an even higher hardness than polycrystalline diamond (which nevertheless can also be used). 
         [0031]    The machining method will now be described with reference to its implementation with the device of  FIG. 1 . 
         [0032]    The pin  3  is positioned at a certain distance from the surface of the part  2  to be machined, without contact therewith. This distance is determined by a person skilled in the art and constitutes one of the parameters of the method since it determines, depending on the nature and the amount of injected abrasive liquid, the pressure of the liquid driven between the external surface of the pin  3  and that of the part  2 . 
         [0033]    The motor is switched on, thus rotating the shaft  4  and therefore the pin  3 . The injection system is also switched on, so as to inject abrasive liquid onto the surface of the part  2  to be machined. The liquid is thus driven between the rotating pin  3  and the surface of the part  2  to be machined. 
         [0034]    Thus, a film of abrasive liquid is formed on the part  2  and this liquid is driven between the rotating pin  3  and the part  2 . Consequently, an abrasive effect is applied on the surface of the part  2  facing the surface of the pin  3  driving the liquid, the abrasive effect being exerted by the abrasive liquid. In other words, since the pin  3  faces the part  2  at an interface between them, that separates the space into two half-spaces E 1  and E 2 , and since the pin  3  is rotating, on one side of the part  2 , from one half-space E 1  to the other E 2 , the abrasive liquid is injected into the half-space E 1  in which the external surface of the pin  3  approaches, through its movement, the part  2  in order to pass into the other half-space E 2 . Thus, the pin  3  does drive the abrasive liquid along the part, from one half-space E 1  to the other E 2 . From the standpoint of the part  2 , the liquid is driven tangentially to its surface and therefore exerts an abrasion force tangential to its surface. The pin  3  also exerts a certain pressure on the liquid and on the part  2 , perpendicular to the abraded surface of the latter. The abrasion force exerted by the liquid on the part  2  depends in particular on this pressure combined with the tangential driving force. 
         [0035]    The pin  3  may also be driven so as to move along the surface of the part  2  to be machined, as shown schematically by the arrow  9 . Since the pin  3  moves from the upstream end toward the downstream end, it is rotated in such a way that its rotating surface, facing the part  2 , moves from the downstream end toward the upstream end, whereas the abrasive liquid is injected at the interface between the pin  3  and the part  2  on the downstream side of the pin  3 . 
         [0036]    The abrasion machining method makes it possible to achieve high-precision machining since a pin  3  can be moved along a part  2  very precisely. Moreover, using a smooth pin  3  in combination with an abrasive liquid gives the abrasion a very fine character. The part  2  therefore has a very smooth surface in the zone that has been machined by abrasion, that is to say the part  2  has a very good surface finish. Since the abrasive effect is exerted by the liquid and not directly by the pin  3 , the latter is barely damaged or consumed by implementing the method, or even not at all. The method is even more regular and reproducible when the abrasive liquid is fed continuously into the zone to be abraded, so that the amount of liquid per unit area is constant over time. 
         [0037]    Various parameters may be modified in order to optimize the method and/or adapt it according to the nature of the part  2  to be machined and/or to the desired rendering. The main parameters are, among others, the following:
       the nature of the material constituting the pin  3 , and therefore its hardness;   the surface state (roughness) of the pin  3 ;   the nature of the base liquid used for manufacturing the abrasive liquid;   the amount of liquid injected between the pin  3  and the part  2 ;   the nature of the abrasive agent of the abrasive liquid;   the particle size of the abrasive agent of the abrasive liquid and its concentration in the liquid;   the distance between the pin  3  and the part  2 , and therefore the pressure applied on the part  2  by the pin  3 ;   the peripheral rotation speed of the pin  3 ;   the speed of advance of the rotating pin  3  along the part  2 ; and   the nature of the movements to which the pin  3  is subjected.       
 
         [0048]    The shape of the part  2  once machined depends, on the one hand, on the shape of the pin  3  and, on the other hand, on the path that it travels along the part  2 . Depending on the shape to be machined, the pin  3  may be stationary relative to the part  2 . 
         [0049]    According to one particular embodiment, when the part to be machined has a surface with a particular shape, when this surface is seen side-on, the pin  3  has a surface of revolution, the generatrix of which has the particular shape of the surface of the part  2  when seen side-on. 
         [0050]    An example of this embodiment is described with reference to  FIGS. 2 and 3 , for machining a blade  2   a , more precisely for machining a blade  2   a  having what is called a “fir tree” root  2   b , that is to say the lateral surface of which has, when seen side-on, a curved shape with at least two changes of curvature. This type of blade root is well known to those skilled in the art of aeronautical engine manufacture.  FIG. 3  shows a pin  3  having an external surface with a shape specially designed for machining the fir-tree root  2   b  of the blade  2   a . Thus, this pin  3  has an external surface generated by the revolution, about the axis A of the pin  3 , of a generatrix segment having the same shape as the profile of the root  2   b  of the blade  2   a , namely a curved shape with two changes of curvature, as may be seen in  FIG. 3 . 
         [0051]    With a pin  3  having the same shape as the profile of the root  2   b  of the blade  2   a , it is possible to machine the root  2   b  in a single pass of the pin  3 , the pin  3  simply being moved along a straight path parallel to the root and abrading the latter so as to give it, seen side-on, the shape of its generatrix.