Abstract:
In a method for optimizing the joints between layers along portions of the layers which are flush with the surface of a part obtained by computer-aided modeling or prototyping involving layer decomposition, the connecting profile of two successive layers is mathematically and numerically defined using an algorithm in which the surface of the joint at the end zone adjacent to the flush portions is always substantially normal to the plane tangential to the surface of the part along the flush portions.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a method for optimizing the joints between layers along portions of the layers which are flush with the surface of a part obtained by computer-aided modeling or prototyping involving layer decomposition, as well as the elementary layers obtained by the method, and the parts resulting from their assembly. 
   Rapid prototyping methods are themselves known. For example, European Patent No. 0 585 502-B1 discloses a prototype part which is produced using software for decomposing the part to be produced into elementary layers. The layers are assembled together, and the final assembly is then externally reworked, in particular, to remove any roughnesses or imperfections resulting from assembly. 
     FIGS. 3 and 4  schematically show an assembly of layers which illustrate the problem encountered. In the illustrated assembly of two layers ( 1 ,  2 ), a portion of the joint ( 3 ) becomes externally flush, at ( 4 ). It will be understood that the zone ( 5 ) of the layer ( 1 ) comprises little material at this location, which, by machining and polishing, can result in the removal of material from the zone ( 5 ) of the joint ( 3 ), as is schematically shown in  FIG. 4 , at ( 6 ). This results in an imperfect part, with an irregular surface, which is unsatisfactory for certain applications. 
   Such conventional joints also have other disadvantages including poor strength, poor resistance to machining, poor resistance to mechanical stresses during use (in particular, compressive stresses, whether they be of mechanical or fluidic origin), poor resistance to all assembly operations (bonding, welding, cementing), and possible deformations during machining, during handling and during assembly. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, these disadvantages are remedied by providing a method for optimizing the joints between layers along portions of the layers which are flush with the surface of a part obtained by computer-aided modeling or prototyping involving layer decomposition. In this method, the connecting profile of two successive layers is mathematically and numerically defined using an algorithm in which the surface of the joint at the end zone adjacent to the flush portions is always substantially normal to the plane tangential to the surface along the flush portions. 
   A further understanding of the present invention is provided in the description of alternative embodiments given below, with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is a CAD representation of a part that is complicated to produce, with several possible alternative situations. 
       FIG. 1B  is a CAD representation of a layer obtained with the method of the present invention, illustrating the layer in three dimensions. 
       FIGS. 2A and 2B  represent a layer obtained in accordance with the present invention, seen from above in  FIG. 2A , and in section in  FIG. 2B . 
       FIGS. 3 and 4  illustrate the problems potentially encountered with the layers of the prior art. 
       FIGS. 5 and 6  schematically illustrate the operating principle of the method of the present invention. 
       FIGS. 7 ,  8 A,  8 B and  8 C illustrate simple alternative embodiments. 
       FIG. 9  illustrates an application to overhung and/or undercut profiles. 
       FIG. 10  illustrates a layer of the assembly shown in  FIG. 9  in detail. 
       FIG. 11  illustrates a layer of an undercut assembly shown in  FIG. 9 , showing two alternatives. 
       FIGS. 12A to 12F  illustrate application of the method of the present invention to a wall. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring first to  FIG. 5  of the drawings, the essential parameters of a joint obtained in accordance with the method of the present invention include an angle (α) between the tangent and the plane of a layer, a length (a) of a portion ( 7 ) of the joint ( 3 ′) (corresponding to portions of the joint ( 3 ) between the adjacent layers shown in  FIG. 3 ) which forms a profile connecting the two adjacent layers, an offset (b) of the plane of the layer, and a normal ({right arrow over (n)}) at the junction point ( 4 ′). In accordance with the present invention, the joint portion ( 7 ) is normal to the tangential plane (T) over a length (a). Note that if (a) is constant, (b) is f(α), and if α=π/2, b=0. 
   The above-discussed problems associated with a joint of this type are solved as a result; however, the layers obtained through such a calculation will be of variable thickness. In addition, the profile of the layer can vary all along the periphery, and the line of the joint (portions of which are formed at an incline in  FIG. 5 ) is not necessarily in one and the same plane. For angles (α) close to π/2, it will also be necessary to suitably position the layers relative to one another, as explained below. 
   Referring to  FIG. 6 , and with all other things being equal for the embodiment of  FIG. 5 , the following are carried out. The amount of material at the joint is controlled (the objective sought). The position of the layer (in X,Y) is controlled by a centering insert ( 8 ). The position of the layer (in Z) is controlled by a positioning profile ( 8 ′). This also results in control and reinforcement of the assembly and of its mechanical strength. 
   The positioning profile is calculated relative to the external contour of the layer, and the angle (α) is able to vary along this contour. The profile can be obtained by micromilling, by milling the profile, or with a form cutter. In the latter case, the form will be constant on the periphery. The interlock is “hyperstatic”, and it is possible to provide clearances for assisting with certain types of contact. 
   Various alternatives will now be briefly described. For example, in  FIG. 7 , the interlock cannot be taken apart due to the presence of an undercut (β) on the positioning profile ( 9 ). Assembly is possible due to the elasticity of the materials. 
   Referring to  FIGS. 8A ,  8 B and  8 C, it is also possible to produce an exterior joint as a function of the degree of sealing required. This can be done to make a joint resist, as shown in  FIG. 8A , or to add material as a supplement, which results in an external bead by deformation, as shown in  FIG. 8B  followed by  FIG. 8C . 
     FIGS. 9 ,  10  and  11  show examples of applications to overhangs and undercuts. The detailed views of the layers shown in  FIGS. 10 and 11  show that decompositions are possible for overhangs and undercuts, following the foregoing joint principle. It is also possible to choose the side of the interlock (upper or lower layer), and even their combination in space. 
   Finally,  FIGS. 12A to 12F  show various applications of the foregoing method to walls including applications without interlock ( FIG. 12A ); applications with external interlock only, and flat internal surfaces ( FIG. 12B ); applications with external and internal interlock, and flat internal surfaces ( FIG. 12C ); applications with external and internal interlock in the same plane ( FIG. 12D ); applications with simple normal decomposition ( FIG. 12E ); and applications with an offset double interlock ( FIG. 12F ). 
   From the foregoing, it will be noted that the digitization of the profile makes it possible to obtain a mathematically defined connection and nesting profile, which is functionally programmed. There is no limit to the profiles that can be obtained. The profile can be warped, and the joint surfaces can be complex and calculated. 
   It will be understood that the primary innovation herein lies in the principle of interlocking, the shapes being fully programmed and dependent on the cross-sectional area in which the nesting takes place. This can include flat surfaces, and can also include warped surfaces, as shown in  FIGS. 1B ,  2 A and  2 B. By using a geometric algorithm, the shape of the nesting joints is obtained by systematic computer calculation. Consequently, the shape of the joint depends on the layering plane and, therefore, cannot be known in advance. 
   At the interlocks, it is possible to provide functional portions of the induced functions in the final part. As a nonlimiting example, this can include regulating channels (cooling, heating, etc.) and/or channels for bringing assembly products and/or channels for the circulation of fluids. 
   The method of the present invention is applicable to all currently known fields involving layered parts designed by rapid prototyping and tooling, and all possible extensions that may later be developed by those skilled in the art for the decomposition of existing parts or for the design of new parts.