Abstract:
An apparatus and method for machining of thin panels, in particular, panels having a complex shape, specifically non-deformable panels, in which the panel to be machined is placed beforehand in an isostatic position, characterized by: one defines one or more areas of a predetermined extent for purposes of machining, named machining windows, in the area of the panel to be machined; and perpendicular to each machining window; one of surfaces of panel is held in position without introducing positioning hyperstatism; the actual shape of the aforementioned surface is measured; the desired machining operation is performed on the opposite surface by taking the aforementioned measured surface as a reference; and the aforementioned surface is released.

Description:
RELATED APPLICATION  
       [0001]     The present application claims priority to French Application No. 04 1276 filed Feb. 10, 2004.  
       TECHNICAL FIELD  
       [0002]     The present invention concerns machining through the removal of material from fine, flexible parts, in particular of a complex shape, such as panels of a non-deformable shape, e.g., aircraft fuselage panels. In particular the invention relates to a machining process and apparatus that is applicable to metallic panel types with or without stiffeners, composite panels, or “sandwich” type panels, on which machining operations affecting their thickness are to be performed, such as surfacing or the formation of recesses or cells, or routing or drilling operations.  
       BACKGROUND OF THE INVENTION  
       [0003]     The panels disclosed in this invention are panels having a double curvature, mainly located at the front of aircraft. These panels, generally made of a light alloy, have thicknesses ranging from 1 to 12 mm depending on the aircraft and the components of the panel (aluminum alloy, titanium alloy, metal composite or composite containing an organic resin).  
         [0004]     If the material permits, the production of these panels requires shaping by drawing on a necessarily convex mold, while panels of composite material are shaped by draping-joining-infusion and compaction methods.  
         [0005]     Because of its productivity and its flexibility, shaping by drawing is mainly employed.  
         [0006]     This type of shaping is performed by means of a combination of traction on the panel and “envelopment” of the aforementioned convex mold so that the geometrically known shape of the panel (that which was in contact with the drawing mold) is the internal (concave) surface. The drawing process generates a plastic deformation of the overall thickness of the panel and consequently, through constriction, leads to a thinning of the section. Due to the non-deformable nature of the shape, this “loss of thickness” is not uniform over all surfaces of the panel.  
         [0007]     The machining of such panels has alignment problems.  
         [0008]     According to the art of alignment, the positioning of a solid is known as isostatic positioning when the 6 degrees of freedom it has in space (3 rotations and 3 translations along the axes of a tetrahedron) are held fixed by means of contacts with 6 judiciously positioned physical points. The isostatic positioning of a non-deformable panel is accomplished as shown in  FIG. 1  of the drawings attached to the present description.  
       SUMMARY OF THE INVENTION  
       [0009]      FIG. 1  shows a thin metal panel  1  with a non-deformable shape that is obtained by drawing and into which two holes A and B, called pin registration holes, generally having a diameter of 12 and 14 mm are placed in the vicinity of two opposite edges. These holes are bored during the drawing operation when the rough piece is still located on the mold. They are used respectively for centering and orientation, and as illustrated by the figure, they stop translation along the axes X and Y as well as rotation about the axis Z (X being defined as the axis passing through the centers of the two holes A and B).  
         [0010]     The three other degrees of freedom are eliminated by three contacts points  2 ,  3  and  4 , which define plane in space.  
         [0011]     When it is oriented in space in this manner, panel  1  is not subject to any stress related to its positioning. On the other hand, if one wishes to increase the number of contact points that are fixed in space, the system becomes “hyperstatic,” and solid (panel  1 ) can only come in contact with the various points by deforming the solid and thus generating internal fastening stresses.  
         [0012]     In the case of panel  1 , which by definition has a low thickness compared with its volume, the “solid” to be aligned is deformable under the action of weak forces, such as the pressure applied in cutting it or even its own weight. Consequently, its positioning according to just isostatic principles is not sufficient to ensure a suitable alignment of the panel: It should be “rigidified” by supporting it with many more points than are necessary for its strict orientation in space. Based on this principle, there are two conventional methods for placing and holding complex parts that are flexible and of large size in position, namely aligning them on rigid tools and aligning them on a bed of suction cups.  
         [0013]     Alignment on rigid tools consists of producing rigid tools of the same shape as the part. It is held in position by fastening to indents. In the case of a non-deformable shape, this hyperstatic positioning does not make it possible to support the panel over its entire surface and it moreover strongly stresses the part. These fastening stresses introduce an evolving elastic strain into the thickness of the panel. The machining of a part that is thus stressed across its thickness involves an unbalance in the distribution of the stresses and consequently an evolution of the panel&#39;s shape.  
         [0014]     The alignment on a bed of suction cups is a solution, which, by limiting the number of support point, limits the degree of hyperstatism and consequently the internal fastening stresses. It is effective for all routing operations. On the other hand, the act of performing machining in the thickness direction requires a significant increase in the density of the supports. For example, if routing operations are satisfied with a space between supports of about 500 mm, machining operations for recesses require a space between supports of less than 150 mm. Consequently, this solution produces a similar result in terms of the quality of orientation compared with that obtained on rigid tools.  
         [0015]     Solutions that involve positioning over the entire surface of the panel do not make it possible to effectively identify its shape in space and generate significant fastening stresses, which will lead to changes in shape on being relaxed.  
         [0016]     The present invention seeks to mitigate the disadvantages of conventional alignment methods and proposes a machining process that allows a non-deformable panel, e.g., a molded panel to be held in position without stressing it, with alignment of its external convex surface based on its theoretical definition (internal surface) for purposes of the mechanical performance of precision machining operations such as those enumerated above.  
         [0017]     To this end, the object of this invention is a process for machining thin panels, in particular panels having a complex shape, particularly non-deformable panels, in which process the panel to be machined is first placed in an isostatic position, characterized by: 
        one or more machining zones of predetermined dimensions, called machining windows, are defined on the panel, in the sections that are to be machined, and     perpendicular to each machining window: 
            one of the surfaces of the panel is held in position without introducing positioning hyperstatism,     the actual shape of the aforementioned surface is measured,     the machining to be performed on the opposite surface is carried out by taking the aforementioned measured surface as a reference, and     the aforementioned surface is released.    
               
 
         [0024]     In such a process, one performs a new referencing that ensures an “automatic” correction for the possible variation in parallelism that can exist locally between the opposite surfaces of the panel with each windowing operation, i.e., with each machining accomplished perpendicular to each machining window.  
         [0025]     An object of this invention is also a device for implementing this process, including a means for isostatic positioning of the panel on the machine, characterized by including moreover: 
        a means for holding one of the surfaces of the panel in position over a predetermined extent without introducing hyperstatism,     a means for measuring the actual shape of the part of the aforementioned surface with regard to of aforementioned holding means,     a means for displacing the aforementioned holding means with regard to the aforementioned surface of the panel,     a multi-axis means for machining located over the other surface of the panel, and     a means for synchronously controlling: 
            the successive displacement and location of the aforementioned holding means with regard to one of the aforementioned machining windows,     the aforementioned machining means successively on the right of each machining window.    
               
 
         [0033]     According to an embodiment of the device disclosed in this invention, the aforementioned means for holding in position without hyperstatism consist of a support including: 
        at least two positioning stops concurrent with the aforementioned isostatic positioning means to ensure isostatic positioning,     multiple mobile, prehensile suction cups positioned toward the surface to be held,     and means for contacting and locking each suction cup in position for purposes of holding the aforementioned surface after isostatic positioning of the panel.        
 
         [0037]     According to another characteristic of the device disclosed in this invention, the aforementioned means for measuring the actual shape of the surface of the panel involving the aforementioned means of positioning consist, for each one of the aforementioned suction cups, of a transmitter of the position of the suction cup after it is locked in the position for holding the surface of the panel, with the aforementioned sensors being connected to a means for calculating the shape of the surface of the panel by interpolation of the positions of the various suction cups.  
         [0038]     The isostatic positioning of the panel is accomplished based on the principle mentioned above, utilizing two registration holes placed on the periphery of the panel to be machined, on two opposite edges, and three reference contact points on the panel.  
         [0039]     The process disclosed in this invention can be implemented according to two such positioning alternatives.  
         [0040]     According to a first embodiment, the aforementioned surface is placed in contact with three reference points located within the machining window for every machining window and before gripping and holding the surface of the panel.  
         [0041]     These three reference points can consist of three of the aforementioned positioning stops of the aforementioned means for holding in position.  
         [0042]     According to a second embodiment, always on each window and before gripping and holding the surface of the panel, the aforementioned surface is placed in contact with two reference points located in the window, the third reference point being one of the two aforementioned registration holes.  
         [0043]     The first two reference points can consist of two of the aforementioned positioning stops of the aforementioned means for holding in position.  
         [0044]     Furthermore, a device for applying the panel against the aforementioned positioning stops for purposes of isostatic positioning prior to fastening the surface of the panel perpendicular to the window is preferably associated with these means for placing the panel in an isostatic position.  
         [0045]     The process disclosed in this invention allows for significant improvements in comparison with existing processes, in particular in the production of recesses requiring a depth precision of less than 0.2 mm.  
         [0046]     This process allows in particular for: 
        isostatic positioning of the panel without stress (no deformation in the course of machining),     automatic machine programming (machining trajectories and positioning windowing) “within theoretical” limits,     referencing to the external surface of the panel (guaranteed base thicknesses of recesses),     instantaneous measurement of the actual shape nearest to the supports (no change of reference mark).        
 
         [0051]     Other characteristics and advantages will be elucidated by the subsequent description of modes of implementation of the process disclosed in this invention, which description is provided only as an example and refers to the attached drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0052]      FIG. 1  shows the placement of a part in the form of a panel with a double curvature in an isostatic position;  
         [0053]      FIG. 2  is a perspective view of a device for implementing the process for this invention;  
         [0054]      FIG. 3  is a perspective view of the device of  FIG. 2  from another angle;  
         [0055]      FIG. 4  is a partial view of the isostatic means of positioning of the device of  FIG. 2 ;  
         [0056]      FIG. 5  is a view of a complementary means for positioning the panel associated with the machining head;  
         [0057]      FIG. 6  is a perspective view of the means for gripping and holding one of the surfaces of a panel to be machined in a localized position;  
         [0058]      FIG. 7  is a more detailed view of the suction cup system of the aforementioned means for gripping and holding;  
         [0059]      FIG. 8  is an axial cross-section of the suction cup system of  FIG. 7 ;  
         [0060]      FIGS. 9   a,    9   b  and  9   c  illustrate the installation and locking in position of the suction cups;  
         [0061]      FIGS. 10   a  and  10   b  illustrate a first method for isostatic positioning;  
         [0062]      FIGS. 11   a  and  11   b  illustrate a second isostatic method of positioning;  
         [0063]      FIG. 12  is a diagram illustrating the positioning of the machining head during machining within a window;  
         [0064]      FIG. 13  is a diagram illustrating the method of displacement of the machining window in the case of isostatic positioning according to  FIGS. 11   a,    11   b;    
         [0065]      FIG. 14  is a flow chart describing the method for actuating the means for holding the suction cups in a mode in conformity with  FIGS. 10   a,    10   b;    
         [0066]      FIG. 15  is a flow chart describing the method of actuating the means for holding the suction cups in a mode in conformity with  FIGS. 11   a,    11   b;    
         [0067]      FIG. 16  is a synoptic diagram illustrating the principle of correction of the machining trajectory; and  
         [0068]      FIG. 17  is a view similar to that of  FIG. 6  showing the retracted position of some suction cups for machining, in particular for routing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0069]      FIG. 1  illustrates schematically the principle of isostatic positioning of a panel  1  having a double curvature.  
         [0070]     Panel  1  is a flexible, thin, non-deformable panel, e.g., a metal panel for covering an aircraft fuselage and has been shaped beforehand in a known manner by drawing on a convex mold.  
         [0071]     As described above, such a panel  1  displays variations in thickness over its entire surface. The geometrically known surface of panel  1  is the concave internal surface  1   a,  which was in contact with the drawing mold. The reference surface is the opposite surface  1   b  ( FIG. 2  and the following figures) and is convex, corresponding to the external wall of the fuselage that will be produced by employing the panel.  
         [0072]     Since panel  1  is machined on its geometrically known surface  1   a,  whereas the reference surface is surface  1   b  and the thickness of the panel has variations which are not known, this obviously presents a machining problem, particularly if one wants to create recesses whose depth is to conform to a very precise machining precision of, for example, less than 0.2 mm.  
         [0073]     The device for implementing the process disclosed in this invention shown schematically in  FIG. 2  includes a support  5  in the form of a gantry under which a doubled curved panel  1  that is to be machined is placed so that its concave surface  1   a  is turned towards the ground.  
         [0074]     A multi-axis machining head  6  is positioned below panel  1 .  
         [0075]     On the one hand, means  7  for supporting and positioning the panel carried by the gantry  5  are located on top of panel  1  and, on the other hand, means  8  for holding the upper convex surface  1   b  of panel  1  in a localized manner are also carried by the gantry.  
         [0076]     These means  7 ,  8  will now be described by also referring to  FIGS. 3 and 4 .  
         [0077]     Panel  1  is positioned by taking up two registration holes A and B in accordance with the drawing of  FIG. 1 , with one of these holes (A) being referenced in  FIG. 4  in the vicinity of an edge  9  of the panel and the other hole being located in the vicinity of the opposite edge  10 .  
         [0078]     Panel  1  is supported perpendicular to the holes A and B by an assembly  11  of three horizontal beams, which are in turn attached at their ends to two swing bars  12  that are positioned perpendicular to the beam and whose ends are in turn held by clamps  13 .  
         [0079]     The clamps  13  are in turn taken up by arms  14 , which are installed so that they can slide along the vertical axis Z on crosspieces  15  of the gantry  5 , by means of a sliding coupling symbolized by  16 .  
         [0080]     The swing bars  12  can oscillate on the ends of the clamps  13  around an axis  17  passing through one of the registration holes (A, B).  
         [0081]     At the lower end of the arms  14 , the clamps  13  can oscillate around a horizontal axis  18 , which is parallel to the axis X, which is itself parallel to the axis of the system of beams  11 , with the axis  18  passing through the registration hole (A or B) associated with the clamp.  
         [0082]     Lastly, the arms  14  can also slide along a vertical axis  19  passing through the associated registration hole (A or B).  
         [0083]     The means  8  for holding the convex surface  1   b  of panel  1  in a localized manner consists of a device with suction cups  20  installed on a multi-axis head  21  of a manipulator  22  carried by gantry  5 .  
         [0084]     The head  21  can move on the manipulator  22  along the axis Z, while the manipulator can move along the axis Y on a cross slide  23  of the gantry  5  as well as along the axis X via a displacement of the crosspiece  23  on tracks  24  tied to the gantry.  
         [0085]     As shown in  FIG. 3 , the crosspiece  15  supporting panel  1 , side  10 , is installed so that it slides on the same tracks  24  for the purpose of fitting the span between the centers A-B of different panels and also to provide the kinematics of a deformable parallelogram during the rotation of panel  1  about the axis  17 , while the crosspiece  15  located on the side  9  of the panel is held in a fixed position.  
         [0086]      FIG. 5  shows a magnified view of the machining head without the machining tool but equipped with a complementary device for positioning panel  1  consisting of a pair of impeller arms, one of which ( 25   a ) is in the active position while the other one ( 25   b ) is in a retracted, inactive position, with the operation of this device being clarified further on.  
         [0087]     The machining head has several degrees of freedom, e.g., five, and can in particular move along the axis X. It is, for this purpose, mounted so that it slides on guides  26  and rotates about an axis  27  ( FIG. 5 ), which can itself swivel in a horizontal plane ball and socket joint  28  between the support of the machining head  6  and the supporting feet  29 .  
         [0088]      FIG. 6  shows, in greater detail, the device with suction cups  20 , which is installed in a rotatable manner on a support arm  30 , which is in turn installed so that it can oscillate about a horizontal axis  31  on the head  21 , which is in turn mobile by translating along the axis Z and rotating about the axis of the latter.  
         [0089]     On the face facing panel  1 , the device  20  includes a rotatable support  32 , which is e.g., square, 500 mm on a side, from which, on the one hand, three stops  33 , arranged in a triangle and, on the other hand, regularly distributed suction cup devices  34  project, with all of the elements,  33 ,  34  covering the entire surface of the turntable  32 .  
         [0090]     The stops  33  are rectilinear locating pins with spherical ends placed perpendicular to the turntable  32  and are of the same length.  
         [0091]     The stops  33  are preferably as far away as possible from each other, and, as shown in  FIG. 6 , two are positioned at two angles of the turntable  32  and the third in the middle on the opposite side of the turntable.  
         [0092]     The suction cup devices  34  are preferably distributed uniformly and aligned over the entire remaining surface of the turntable, parallel to the stops  33 , with a constant spacing between the devices  34  of less than 150 mm.  
         [0093]     Each device  34  includes (FIGS.  6  to  8 ) a sleeve  35  that is installed so that it is axially mobile on the turntable  32  and in which a centering pin  36  slides, which is, at its end, fitted with a suction cup  37 .  
         [0094]     The sleeve  35  is moved by a drive-arrest mechanism, including a motor  38  actuating a ball screw  39  held in the turntable  32  by means of a bearing, a nut  40  engaging the screw  39  and a fastening device  41  which is integral with the nut  40  and whose restraints  42  are, when they are averted, laid against the internal wall of the sleeve  35  thus linking it to the nut  40 .  
         [0095]     The sleeve  35  carries the centering ( 36 ) and suctioning ( 37 ) devices at its end, including ( FIGS. 9   a,    9   b ) a sleeve  43  connected to the sleeve  35  via a ball and socket joint  44  and closed by a cover  45 , which is axially mobile with regard to the sleeve  43 , under the action of a pneumatic element called a bladder  46 , which is capable of contracting ( FIG. 9   b ) or elongating (relaxation,  FIG. 9   a ).  
         [0096]     Item  47 , in  FIG. 7 , shows a device for locking the sleeve  35  with regard to the turntable  32 . The device  47  is fixed on the turntable and encloses the sleeve  35  with two arms  48 .  
         [0097]     Item  49  represents a transmitter of the distance separating it from the surface of the panel in question, i.e., the distance of the suction cup  37  from the turntable  32 .  
         [0098]     The sensor  49  is fixed laterally to the sleeve  35  by a leg  50 .  
         [0099]     The operation of the device described above is as follows:  
         [0100]     Panel  1 , which is to be held in position without being deformed, is first brought into contact, e.g., on its convex surface  1   b,  with the three stops  33  of the device of  FIG. 6 , by means of the impeller arms  25   a,    25   b,  which are applied against the concave surface  1   a  in the direction of the support turntable  32  of the device.  
         [0101]     When contact is established, the suction cup devices  34  are initially positioned (step  51 ,  FIG. 9   c ) by actuating the motor  38  of the devices  34 .  
         [0102]     Setting the screw  39  in rotation, with the restraint  42  of the fastening device  41  being expanded and the arms  48  of the device  47  being loosened, involves a translational displacement of the sleeve  35 -centering pin  36  assembly in the direction of the face  1  of panel  1 .  
         [0103]     Once the sleeve  35  has made contact with surface  1   a,  with this position having been being detected by the sensor  49 , the motor  38  is stopped and the arms  48  are tightened by the immobilizing device  47  and thus the sleeve  35  with regard to the turntable  32 .  
         [0104]     The subsequent step (step  52 ,  FIG. 9   c ) is the lowering of the suction cups unit  37 . Panel  1 , which is thus “drawn in,” comes to be rest against the bottom of the suction cups. With these being shifted, e.g., by 1 mm, with regard to the theoretical position of the panel, the latter is locally stressed.  
         [0105]     The bladders  46  ( FIG. 9   a ) are then relaxed (step  53 ,  FIG. 9   c ), so that panel  1  regains its stress-free shape through elastic relaxation, while the cover  45  disengages from the sleeve  43 .  
         [0106]     The suction cup  37  is then attached to the panel and, with the unit  37 ,  45 , is held in position by the elasticity (in vacuum) of the pneumatic bladder  46 . Because of their inherently low weight and their being held in position by the elasticity of the bladder  46 , the aforementioned elements  37 ,  45  do not at all or hardly pull on the panel (step  54 ,  FIG. 9   c ).  
         [0107]     All of the centering pins  36  are thus simultaneously disengaged from their sleeve  35 .  
         [0108]     The following step consists of re-rigidifying the unit  32 - 34 , the panel having again assumed its natural shape, without stress.  
         [0109]     This step will be performed suction cup by suction cup so as not to deform the panel.  
         [0110]     Thus, for each device  34  (step  55 ,  FIG. 9   c ), the sleeve  35  is released externally by loosening the arms  48  and internally by retracting the restraint  42 . On re-contracting (step  56 ,  FIG. 9   c ) the bladder  46 , the unit  36 - 35  approaches panel  1  and the sleeve  43  re-engages the cover  45  ( FIG. 9   b ).  
         [0111]     The last step (step  57 ,  FIG. 9   c ) consists of re-immobilizing the sleeve  35  with regard to the turntable  32 , via the restraints  42  of the device  41  and the arms  48  of the device  47 .  
         [0112]     Thus, the operation is carried out successively with all of the devices  34 .  
         [0113]     Panel  1 , perpendicular to the turntable  32 , is perfectly immobilized, in a stable manner, without deformation nor stress on this part of the panel and, in addition, the actual shape of surface  1  with regard to turntable  32  is known from calculations based on the measurements provided by sensors  49 , which supply the precise position of surface  1   a  with regard to turntable  32  perpendicular to each suction cup device  34 . A calculation by interpolation based on these measured points makes it possible to know the actual shape of the aforementioned surface  1   a.    
         [0114]     The operation of the machine represented in FIGS.  2  to  5  is as follows.  
         [0115]     The first step for implementing the process disclosed in this invention is the isostatic positioning of panel  1  that is to be machined in the machine space, which is accomplished jointly by means  7  and means  8 .  
         [0116]     It will first be supposed that the positioning is accomplished according to the kinematics illustrated by  FIGS. 10   a,    10   b,  where  FIG. 10   b  is a bottom view of panel  1  of  FIG. 10   a  as suspended by the means of support  7 .  
         [0117]     The isostatic positioning is, in this case, provided by the registration holes A and B grabbed by the means  7  and by three complementary points of support  2 ,  3 ,  4  provided by the three stops  33  of means  8 , which thus define a reference plane.  
         [0118]     The positioning sequence proceeds in the following manner:  
         [0119]     The means  8  are initially placed in the theoretical position of surface  1   b  of panel  1 . The retractable arms  25   a,    25   b  of the machining head  6  are then made to emerge and are applied against the other surface  1  of the panel so as to push the latter against stops  33 . The means for controlling the motors for displacing elements  11 ,  13  and  14  along axes  17 ,  18  and  19  ( FIG. 4 ) lock the aforementioned elements in their positions.  
         [0120]     The suction cups  6  are then brought into the holding position of surface  1   b  of the panel, a vacuum is applied, and, finally, arms  25   a,    25   b  are retracted.  
         [0121]     The suction cup devices  6  adapt automatically to the shape of panel  1 . The transmitters  49  allow this shape to be accurately identified. The total time for adaptation to the practical shape of the panel is about a few hundred milliseconds, including the measurement of the profile of surface  1   b  of the panel.  
         [0122]     The means  8 , which are called the machining window, are positioned over an area of surface  1   b  where machining by the head  6  to be performed.  
         [0123]     Once the machining has been performed (drilling, routing or machining of a recess), the aforementioned window  8  is disconnected from the panel and is moved so as to be positioned at the height of another area of surface  1   b,  which may or may not be contiguous to the area that has just been machined.  
         [0124]     In the case of the positioning method of  FIGS. 10   a,    10   b,  the reference plane for machining is defined by window  8 , namely by the three stops  33  (support points  2 ,  3  and  4 ).  
         [0125]     The process for disconnecting window  8  and repositioning it in another place is illustrated by the flow chart of  FIG. 14 .  
         [0126]     At step E 1 , the suction cups  37  are disconnected and then, at step E 2 , window  8  is released.  
         [0127]     At step E 3 , panel  1  is repositioned by means  7 .  
         [0128]     At step E 4 , window  8  is repositioned with the assistance of arms  25   a,    25   b  and, at step E 5 , suction cups  37  are repositioned to capture surface  1   b  of the panel, including measurement of the profile of this region of the panel.  
         [0129]     This positioning kinematics and displacement of window  8  is preferable for the machining of recesses dimensions (extents) that are smaller than those of window  8 .  
         [0130]     The positioning of panel  1  can be accomplished according to a second method illustrated by  FIGS. 11   a  and  11   b,  with  FIG. 11   b  being a bottom view of the device of  FIG. 11   a.    
         [0131]     The isostatic positioning is, in this case, ensured by rigging the pinholes A and B engaged by the means  7  and by two complementary support points  3  and  4  ensured by two of the three stops  33  of window  8 , which points  3  and  4  define a reference plane along with one of the pinhole points (B).  
         [0132]     The positioning sequence proceeds as follows:  
         [0133]     The window  8  is initially placed in the theoretical position of surface  1   b  of panel  1  according to the same process as that described above concerning the positioning method according to  FIGS. 10   a,    10   b.    
         [0134]     The positioning method according to  FIGS. 11   a,    11   b  allows a progression of windowing operations, i.e., of machining operations from a given area of surface  1   a  to an adjacent area via a displacement of window  8  in the manner of a land-surveyor, as illustrated by  FIG. 13  and  FIG. 15 , which is a flow chart of the progression process of the window.  
         [0135]      FIG. 13  illustrates the passage of window  8  from a first position F 1  to a second immediately adjacent position F 2 , then to a third, also immediately adjacent position F 3 .  
         [0136]     The step E′ 1  In  FIG. 15  involves the detachment of suction cups  37  (for example, in case of the passage from a position F 1  to the subsequent position F 2 ).  
         [0137]     At step E′ 2 , retractable arms  25   a  and  25   b  are applied against the surface of the panel so as to press the latter against one of the stops  33  of window  8  (e.g., the point  4 ,  FIG. 13 ).  
         [0138]     At step E′ 4 , the second stop  33  being employed is released (point  3 ,  FIG. 13 ).  
         [0139]     At step E′ 5 , window  8  is made to swivel around the point  4  so as to carry it to the subsequent position F 2 .  
         [0140]     At step E′ 6 , suction cups  37  are repositioned to hold surface  1   b  of the panel, including measurement of the profile of this new area, which is adjacent to the preceding one.  
         [0141]     Thereafter, via an identical process, one passes from the windowing position F 2  to the position F 3  via a rotation of window  8  about the point  3  into its new position, the point  4  then being released, and the entire area of the surface of the panel to be machined is thus covered sequentially.  
         [0142]     This method for displacing window  8  is in particular applicable for machining recesses or for routing over areas of greater extent than that covered by window  8 .  
         [0143]     Regardless of the method of progression selected, the process of holding and fixing window  8  in position leads to a measurement of the position and the shape of the “external skin” ( 1   a ) of panel  1  and ensures its contact with the bottom of suction cups  37  (reference of the measurement). This information can be used to correct the trajectory corresponding to the desired machining operation.  
         [0144]      FIG. 16  is a synoptic of the principle of in situ correction of the trajectory.  
         [0145]     The curve represented by T is the actual profile of the trajectory. Based on measurements of surface M achieved when suction cups  37  are locked in position, the coordinates relating to these measurements are stored to a table known as of the variables V. The values in this table V are then read in order to substitute them for the variables of the trajectory. With the degree and the form of the interpolation function having been fixed in advance, the calculation of the control points based on the measured points (M) is a simple mathematical operation readily achievable by the machine&#39;s controlling computer. The time required for this process is a few milliseconds. The principle is presented here in the case of a 3-axis trajectory. It can be generalized to the programming of axes with the generation of two interpolated curves (one for the trajectory and the other for the orientation), both being related by derivation.  
         [0146]      FIG. 12  is a diagram illustrating the mutual positions of window  8  and the machining tool  6 , e.g., when a recess on surface  1   a  of panel  1  is produced.  
         [0147]     A routing operation is performed according to principles similar to the machining of a recess. A displacement kinematics of window  8  according to  FIG. 13  will preferably be used. Moreover, since the panel is traversed by the cutter, the suction cup devices  34  of window  8  that may be in the way of the cutter will be retracted as illustrated by  FIG. 17  where some of the devices ( 34 ′) are placed in a low, retracted position by actuating motors  38 , with arms  48  of the locking devices  47  being disengaged and restraints  42  of fixing devices  41  being retracted.  
         [0148]     Furthermore, compressed air is blown across the suction cups that are employed so as to avoid the depositing of chips. Routing is finally performed by leaving behind bridges every 500 mm, which corresponds approximately to the width of window  8  in the example being described, in order to preserve the cuttings attached to panel  1 .  
         [0149]     The process disclosed in the invention has the following advantages: 
        Isostatic positioning without stressing panel  1  (no deformation in the course of machining)     Automatic machine programming (machining trajectories and positioning windowing) “within theoretical” limits     Referencing to the external surface ( 1   b ) of the panel (guaranteed machining depth)     Instantaneous measurement of the actual shape nearest to the supports ( 33 ) (no change in points of reference)     Instantaneous trajectory correction     Two operating methods (associates with two positioning kinematics)     Possible change in operating method without a change in equipment     “Complex” controls (positioning, fitting of window  8 , trajectory correction, etc.) except for the machining phase (safety for part  1 ).        
 
         [0158]     It should be noted that the positions of tools  6  and holding means  8  can be reversed, with means  8  being applied to concave surface  1   a  of panel  1 .  
         [0159]     Of course, the invention applies to all kinds of panels, regardless of their constitutive materials, e.g., metal or composite, and regardless of the manufacturing process.