Patent Publication Number: US-9417624-B2

Title: Method of making a part and related system

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. Pat. No. 8,578,581 filed on Apr. 16, 2007, and issued on Nov. 12, 2013, the content of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to a method and system for compensating for differences between a nominal definition of a part and an actual part during a manufacturing process applied to the part. 
     BACKGROUND OF THE ART 
     When laser drilling holes or in other manufacturing processes, the machining tool must be positioned accurately with respect to the part being manufactured if the final part is to be made with any precision. In laser drilling, for example, the laser nozzle may be positioned in a nominal drilling location relative to a nominal surface of the part, the nominal drilling location being determined solely using a nominal computer model of the part, provided for example by a CAD (computer aided design) file. However, such an approach does not provide feedback as to whether the actual part is in the desired nominal location or has the desired nominal surface. Because of the differences between the actual surface and the nominal computer model of that surface, the holes may inadvertently be drilled at different locations and/or angles than those desired and defined in the nominal model, and as such may also have a diameter different from the nominal diameter. Thus, the final part may not be as desired. In the case of gas turbine parts in which cooling holes are drilled, the airflow rate and its direction through the cooling holes for the actual part may be different than for the nominal part, which can reduce the cooling effectiveness and as such the life of the actual part. 
       FIG. 1A  shows an example where a nominal straight profile as defined by the nominal computer model is indicated at N, and actual straight profiles located respectively inwardly and outwardly of the nominal profile N are indicated at I and O. A laser nozzle D is moved along its axis such as to keep a given distance SD with the actual surface of the part, and the nozzle D is aligned with the hole location of the nominal profile N. It can be seen that the hole locations in the actual profiles I, O, are different from one another and from the hole location set forth in the nominal profile N. As such, an undesirable variation d is introduced in the location of the hole depending on the actual profile of the part. 
       FIG. 1B  shows another example where actual, arcuate profiles I and O are located respectively inwardly and outwardly of an arcuate nominal profile N. In this case, as the nozzle D is aligned with the hole location of the nominal profile N, the variation between the nominal profile N and the actual profiles I and O not only introduces a variation in the location of the hole, but also a variation between the hole angle θ N  defined in the nominal profile and the hole angles θ I  and θ O  produced in the actual profiles I and O, with θ I &lt;θ N &lt;θ O . In addition, the drilling operation can also produce a back strike B when drilling the hole, thus damaging the interior surface of the part. 
     Similar variations between machined elements in an actual part and those same elements in a nominal part can also be observed in a variety of machining or other manufacturing operations. 
     Accordingly, there is a need to provide improved compensation for the differences between the nominal definition of a part and the actual part. 
     SUMMARY 
     It is therefore an aim to address the above-mentioned concerns. 
     In one aspect, there is provided a method of making a part, the method comprising providing a nominal tridimensional part definition including a nominal part surface, and a nominal location and orientation for at least one geometrical element on the nominal part surface, capturing an actual tridimensional surface of the part to obtain a corresponding digitized actual surface, performing a tridimensional comparison between the digitized actual surface and the nominal part surface, generating an actual location and orientation of each geometrical element on the digitized actual surface based on the tridimensional comparison, and providing each geometrical element on the actual surface based on the actual location and orientation of the element on the digitized actual surface. 
     In another aspect, there is provided a method of drilling a plurality of holes in a part, the method comprising providing a model of the part, the model including a nominal surface of the part, a nominal location on the nominal surface of each of the holes to be drilled in the part and a nominal orientation of each of the holes with respect to the nominal surface, obtaining a digitized actual surface representing an actual surface of the part, projecting the nominal location of each of the holes on the digitized actual surface based on a tridimensional comparison between the digitized actual surface and the nominal surface to determine an actual location on the digitized actual surface corresponding to each of the nominal locations on the nominal surface, determining an actual orientation of a drilling tool for each of the holes based on the nominal orientation of the hole with respect to the nominal surface and on the tridimensional comparison between the nominal surface and the digitized actual surface, and drilling each of the holes on the actual surface of the part at the corresponding actual location and following the corresponding actual orientation. 
     In a further aspect, there is provided a machining system for providing to a machine tool an actual location and orientation of a plurality of elements to be machined on a part, the system comprising a scanner for capturing an actual tridimensional surface of the part and defining a digitized actual surface representing the actual tridimensional surface, a comparator for performing a tridimensional comparison between the digitized actual surface and a nominal tridimensional surface of the part to determine a best fit between the digitized actual surface and the nominal surface, and a compensation calculator for generating the actual location and orientation of each of the elements on the actual digitized surface based on a nominal location and orientation for each of the elements on the nominal surface and on the best fit between the digitized actual surface and the nominal surface. 
     Further details of these and other aspects will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures, in which: 
         FIG. 1A  is a schematic view of a drilling process for a flat surface; 
         FIG. 1B  is a schematic view of the drilling process of  FIG. 1A  applied to a curved surface; 
         FIG. 2  is a block diagram of a machining system shown in interaction with other elements used in a machining operation; 
         FIG. 3  is a flow chart of a method of making a part, for example using the system of  FIG. 2 ; 
         FIG. 4  is a perspective view of a scanner of the system of  FIG. 2 , shown in combination with a part being scanned in a particular application of the system; 
         FIG. 5  is a schematic representation of a nominal profile and an actual profile of a part, illustrating a function of a comparator of the system of  FIG. 2 ; 
         FIG. 6  is a side view of a blade and encapsulation assembly, showing an alternate application of the system of  FIG. 2 ; and 
         FIG. 7  is a front view of the blade and encapsulation assembly of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  illustrates an example of a machining system  20 . The machining system  20  generally receives a nominal tridimensional definition of a part to be machined, for example in the form of a CAD file provided by a CAD program  22 . The nominal tridimensional definition corresponds to a model of the entire part or of a tridimensional surface of the part and includes at least a nominal surface  24  of the part, as well as a nominal location  26  and a nominal orientation  28  of each of a plurality of geometrical elements machined on that surface. 
     In the present specification and claims, the terms “geometrical element” and “element” are intended to encompass all the features that define a part, such as for example a surface, profile, ellipse, diameter, angle, plane, slot, hole, groove, etc., as well as one or many surfaces that can be used as datum. The term “nominal” as applied to a part, surface, geometrical element, etc. is intended to refer to the part, surface, geometrical element, etc. as defined in a theoretical model such as a CAD model, without tolerances, which may be used as a reference when machining one or a plurality of similar actual parts, surfaces, geometrical elements, etc. The term “actual” as applied to a part, surface, geometrical element, etc. is intended to refer to the real, physical part, surface, geometrical element, etc. at various stages of the manufacturing process, including any variation brought by that process. As such, a plurality of actual parts manufactured based on the same nominal part will usually be slightly different from one another and from that nominal part. 
     The machining system  20  transforms the nominal location  26  and the nominal orientation  28  of each element to be machined into an actual location  30  and an actual orientation  32  adapted to each real, actual part being machined, as will be described below, such that the actual location  30  and the actual orientation  32  of each element is fed to a computerized numerical control (CNC) machine tool  34  for machining that part. 
     In a particular embodiment, the various elements of the systems  20  communicate and interact without human intervention, and are for example actuated by a voltage signal generated by the CNC machine tool  34  or by an appropriate signal generated by an executable computer file. 
     In a particular embodiment, the machine tool  34  is a drilling machine such as described in U.S. patent application Ser. No. 11/218,785 filed Sep. 6, 2005, the entire specification of which is incorporated herein by reference. If required, the actual location  30  and the actual orientation  32  can be processed by a post-processor  36  before being sent to the machine tool  34 , such as to express them in an appropriate machine coordinate system usable by the machine tool  34 . 
     The machining system  20  will be described herein in relation to a drilling operation, which in a particular embodiment is a drill-on-the-fly (DOF) machining operation to define cooling holes in an annular part, such as for example a combustor liner, a combustor heat shield or a blade or vane ring in a gas turbine engine. However, it is understood that the machining system  20  and related method can be used with other types of parts, such as for example non annular parts such as an airfoil of a gas turbine engine, as well as for other types of drilling operations such as trepanning and laser percussion, and for various alternate machining operations such as for example electrical discharge machining (EDM), lathing, grinding and milling. One example of an alternate application is the machining and milling of integrally bladed rotors in a gas turbine engine, and particularly for correcting, if necessary, the leading edges of the blades. 
     Still referring to  FIG. 2 , the machining system  20  includes a scanner  38 , which captures an actual tridimensional surface of the part. In a particular embodiment, the scanner  38  is a line laser scanner, for example scanning 2000 points or traces at a time, allowing the capture of 6 million points per second. Alternate scanners  38  that can be used include a photogrammetry apparatus, a point laser scanner, a tomography apparatus, a coordinates measuring machine, etc. 
     The scanner  38  captures the actual tridimensional surface of each part to be machined and obtains a corresponding digitized actual surface  40  representing that actual surface, as illustrated by step  100  of  FIG. 3 . In a particular embodiment, the digitized actual surface  40  includes a point cloud formed of a plurality of points captured on the actual surface. Alternate digitized actual surfaces  40  that can be used include for example a meshed surface (such as for example a mesh surface generated from a point cloud), nurbs (non-uniform rational b-splines), etc. The digitized actual surface  40  thus approximates the actual surface, with the accuracy of this approximation depending on the resolution of the scanner  38  (for example the distance between captured points, nurbs, etc.). As such, in a particular embodiment, the digitized actual surface  40  includes a point cloud composed of millions of points (e.g. 5 million). The distance between adjacent points is preferably smaller than the size of the smallest geometrical element to be machined (e.g. the diameter of the smallest hole) such as to provide enough precision for adequate location of the machined elements. In a particular embodiment, the machined elements include a series of holes, the smallest of which has a diameter of 0.02 inch, and the distance between adjacent points of the cloud is between 0.0001 and 0.01 inch. 
     As shown in  FIG. 4 , the part  48  is received on a fixture  42  in a fixed manner, and the fixture  42  is rotationally received in a known position on a base  44  of the scanner  38 . The fixture  42  is rotated about its axis of rotation  43 . The scanner  38  includes a scanning head  46  which is positioned over the surface of the part  48  to be scanned, and remains stationary while scanning the actual surface of the part  48  as the part  48  rotates with the fixture  42 . In a particular embodiment, the scanning head  46  scans the part  48  as if it was a linear part and produces a point cloud representing a planar, “unfolded” version of the actual surface of the part. The point cloud is then “rolled” around the axis of rotation  43  of the fixture  42  such as to obtain the digitized actual surface  40  representing the tridimensional configuration of the actual surface of the part  48 . 
     Referring back to  FIG. 2 , the machining system  20  includes a comparator  50 , which receives the digitized actual surface  40  from the scanner  38 . The comparator  50  also receives the nominal surface  24  provided by the CAD program  22 , as illustrated by step  102  in  FIG. 3 . The comparator  50  performs a tridimensional comparison between the digitized actual surface  40  and the nominal surface  24  to find a correlation or best fit  52  therebetween, as illustrated by step  104  in  FIG. 3 . In the present specification and claims, “tridimensional comparison” is intended to include both the comparison of the digitized actual surface  40  and the nominal surface  24  with one another as well as the comparison of their relative position in a tridimensional environment, such as to determine corresponding points, meshed portions, nurbs, etc. between the digitized actual surface  40  and the nominal surface  24  and a relative displacement and rotation, where applicable, between these corresponding points, meshed portions, nurbs, etc. 
     In a particular embodiment, the comparator  50  compares the digitized actual surface  40  which is in the form of a point cloud with the nominal surface  24  using the Polyworks® software produced by InnovMetric Software Inc. Alternate calculating software that can be used by the comparator  50  depends on the form of the digitized actual surface  40  and include any adequate software that can be used to manipulate point clouds, meshed surfaces, nurbs, etc. such as for example Focus software provided by Metris, software provided by Geomagic, Prélude® provided by IBM&#39;s MdtVision, etc. 
     Referring back to  FIG. 2 , the machining system  20  further includes a compensation calculator  54 , which includes a calculator  56  receiving the correlation  52  between the point cloud  40  and the nominal surface  24  from the comparator  50 . The calculator  56  also receives the nominal location  26  and the nominal orientation  28  of each machined element provided by the CAD program  22 , as illustrated by step  106  ( FIG. 3 ). The calculator  54  then generates the actual location  30  and the actual orientation  32  for each element, as illustrated by step  108  ( FIG. 3 ). 
     The calculator  54  thus aligns the digitized actual surface  40  and the nominal surface  24  following the best fit  52  found by the comparator  50 , and projects the nominal location  26  of each machined element on the digitized actual surface  40  to obtain the corresponding actual location  30  for that element. In a particular embodiment, the calculator  56  receives the nominal location  26  of each machined element as a set of Cartesian coordinates (X, Y, Z), and projects each nominal location  26  perpendicularly to the nominal surface  24  and on the digitized actual surface  40  to obtain the actual location  30  as a new set of Cartesian coordinates (X′, Y′, Z′). Alternately, several other types of projection can be used, such as for example a radial or axial projection (particularly in the case of an annular part), or a projection parallel to an adjacent reference surface, for example a surface to be cooled by an airflow coming through holes being machined. 
     The calculator  56  determines the actual orientation  32  of each machined element based on the nominal orientation  28  and on a difference in orientation between the digitized actual surface  40  and the nominal surface  24  around each element. In a particular embodiment, and referring to  FIG. 5 , the calculator  56  receives the nominal orientation  28  of each machined element in the form of a nominal vector (I, J, K) defining the orientation of the machining tool for machining the element (e.g. the orientation of the axis of the hole). The calculator  56  then defines an actual plane  58  tangential to the digitized actual surface  40  around the actual location  30  of the element, a nominal plane  60  tangential to the nominal surface  24  around the nominal location  24  of the element, and a rotation necessary to bring the nominal plane  60  parallel to the actual plane  58 . The calculator  56  applies that rotation to the nominal vector (I, J, K), and projects the rotated nominal vector onto a plane extending through a central axis of the part and the actual location  30  of the element (represented by the plane of the sheet in  FIG. 5 ) to obtain a compensated vector (I′, J′, K′). As such, the angle θ′ defined between the compensated vector (I′, J′, K′) and the digitized actual surface is similar to the angle θ defined between the nominal vector (I, J, K) and the nominal surface, such as to obtain an actual orientation of the machine geometrical element (e.g. orientation of a drilled hole with respect to the surface of the part) as close as possible to the corresponding nominal orientation while ensuring that the machining process is performed with the machining tool oriented toward a center of the part. 
     In a case where the calculator  56  uses a projection parallel to an adjacent reference surface to determine the actual location (X′, Y′, Z′) of the elements, the orientation is also compensated with respect to that adjacent surface such that the actual orientation of each element maintains a nominal angle with respect to the actual reference surface. 
     In an alternate embodiment, such as for example when machining cooling holes in a heat shield or in a blade, it is not necessary for the machining process to be performed with the machining tool oriented toward a center of the part, and as such the compensated vector (I′, J′, K′) generated by the calculator  56  directly corresponds to the rotated nominal vector. 
     Referring back to  FIG. 2 , the compensation calculator  54  optionally includes a smoothing module  62 , which receives the compensated vectors (I′, J′, K′) from the calculator  56 . The smoothing module  62  examines each compensated vector (I′, J′, K′) in sequence, in the order the corresponding elements will be machined, and transforms each compensated vector (I′, J′, K′) into a smoothed vector (I″, J″, K″). In a particular embodiment, the smoothed vector (I″, J″, K″) of each element corresponds to the average of a group composed of compensated vectors (I′, J′, K′) of a given number of elements to be machined before and after that element. In a particular embodiment, the given number is selected between 3 and 10 inclusively. Alternate methods of determining the smoothed vectors (I″, J″, K″) include computing the smoothed vectors (I″, J″, K″) from a same row such that each extends at a same angle to the digitized actual surface than an average angle of the compensated vectors (I′, J′, K′) of that row, or computing the smoothed vectors (I″, J″, K″) such as to have a same rotation between each smoothed vector (I″, J″, K″) and the corresponding compensated vector (I′, J′, K′) as in a reference row of elements. The smoothed vectors (I″, J″, K″) are returned to the calculator  56 , and the calculator  56  outputs the actual orientation  32  of each element in the form of an actual vector which corresponds to the respective smoothed vector (I″, J″, K″). 
     In an alternate embodiment where the smoothing module  62  is omitted, each actual vector representing the actual orientation  32  of the respective element corresponds to the respective compensated vector (I′, J′, K′). 
     In an alternate embodiment, to minimize the computing time, the calculator  56  determines the compensated vector (I′, J′, K′) as described above for only selected ones of the machined elements, which are preferably regularly spaced apart with a number n of machined elements located in between. The compensated vectors (I′, J′, K′) of each group of n machined elements are set such as to produce a progressive transition between the respective angle θ′ defined by the compensated vectors (I′, J′, K′) of the selected machined elements delimiting the group. In this case the compensated vectors (I′, J′, K′) are thus naturally smoothed and the smoothing module  62  is omitted, the actual orientation  32  of each element corresponding to the respective compensated vector (I′, J′, K′). 
     The smoothing module  62  and the method described above where the compensated vectors of groups of n machined elements are selected such as to produce a progressive transition between selected machined elements delimiting each group are particularly useful in the case of fast machining operations such as drilling-on-the-fly, minimizing the changes of orientation of the machining tool between the machining of adjacent elements such as to minimize the risk of premature wear of the machining tool components. 
     When the actual location  30  and the actual orientation  32  of each element is determined, the elements are provided (e.g. machined) on the actual part, as illustrated by step  110  of  FIG. 3 . Alternately, each element can be machined as soon as its actual location and orientation are determined, e.g. as the actual location and orientation of other elements is being determined. 
     In a particular embodiment, the connected fixture  42  and part  48  are removed from the scanner  38  and installed in the machine tool  34 , with a position of the fixture  42  within the machine tool  34  being known, such as to machine the part  48 . Alternately, the scanner  38  and machine tool  34  can be provided in a same machine, such that the part  48  and fixture  42  are not moved between the scanning and machining operations. 
     In a particular embodiment, the scanner  38 , which can be for example the scanner  38  shown in  FIG. 4 , is optionally used to capture the real part after being machined as illustrated by step  112  of  FIG. 3 , through the determination of a final digitized actual surface  64 , for example a point cloud corresponding to a plurality of points on the surface of the machined part, representing the actual surface with the machined elements. The comparator  50  then compares the final digitized actual surface  64  with the nominal part (e.g. from the CAD program  22 ) to evaluate, through a best fit between the two, an accuracy of the machining operation and/or whether further machining is required, as illustrated by step  114  of  FIG. 3 . The final digitized actual surface  64  or digitized part can also be recorded as a digitized model for future analysis. The final digitized actual surface  64  preferably has a higher definition than the digitized actual surface  40  used for the determination of the actual location  30  and the actual orientation  32  of the elements to be machined, in order to be able to accurately capture the small machined elements in the machined part. For example, the scanner  38  can provide of a point cloud of more than a hundred million points, of which only a given number (e.g. 200) of the closest points around each hole are kept to define the final digitized actual surface  64  for analysis to reduce the necessary analysis time and complexity. 
     As the system modifies the nominal orientation and position of each individual geometrical element to correspond to the actual part, the nominal model is thus “compensated” for each geometrical element to be machined in each actual part. Prior art systems usually try to determine an overall position of the actual part such as to apply a same offset to all geometrical elements. By comparison, the present system thus allows increased accuracy in the position and orientation of each actual geometrical element machined into the actual part, such that the differences between the actual parts and between each actual part and the nominal part are minimized. 
       FIGS. 6-7  illustrate an alternate application of the system and method described above, namely the machining of rotor blades. An actual blade  70  is contained in an actual encapsulation  72 , formed for example of melting zinc alloy injected around the blade  70 , which is used as a holding module during the machining process. 
     In the prior art, the location of the blade with respect to its encapsulation is generally inspected, for example with a coordinate measuring machine (CCM), and the encapsulated blade is sent to machining only if the location of the blade in the encapsulation is within a given tolerance zone. If the location of the blade is not acceptable, i.e. outside the tolerance zone, then the encapsulation is removed, a new encapsulation is created, and the encapsulated blade is re-inspected. This method has the disadvantages of requiring substantial training of the operators performing the encapsulation, reducing the available tolerance for the machining process by allotting a portion of the manufacturing tolerance to a tolerance zone for the location of the blade in the encapsulation, as well as increasing time and cost of the manufacturing process each time an encapsulated blade is rejected and the encapsulation process needs to be redone. 
     By contrast, the present system, for example as shown in  FIG. 2 , allows machining of the blade regardless of its position within the encapsulation. The nominal surface  24  received by the comparator  50  define both the nominal encapsulation  76  and the nominal blade  74 , and the digitized actual surface  40  provided by the scanner  38  to the comparator  50  includes the digitized actual surface of both the actual encapsulation  72  and the actual blade  70  in which at least one geometrical element is to be machined (e.g. fir tree base, notch, etc.). The comparator  50  performs a tridimensional comparison between the digitized actual surface  40  and the nominal surface  24  to find a correlation or best fit between the digitized actual encapsulation and the nominal encapsulation, without regard to the location of the actual and nominal blades  70 ,  74 . 
     The calculator  56  determines reference elements on the digitized actual surface  40 , for example actual extreme sections  78  (uppermost and lowermost of several reference sections defined on the blade), an actual stacking line passing  80  through the X and Y zero location of each actual extreme section  78 , and a reference point A common on the casting and on the machined blade. This is preferably done through an iterative process such as to increase the precision of the position of the actual reference elements. The calculator  56  receives the nominal location  26  and nominal orientation  28  of similar reference elements on the nominal surface of the blade (e.g. nominal extreme sections  82 , nominal stacking line  84  and nominal point A′) and, aligning the digitized actual encapsulation with the nominal encapsulation following the best fit found by the comparator  50 , determines the deviation, including translation and rotation, between the reference elements on the digitized actual surface and the corresponding reference elements on the actual surface. In a particular embodiment, an average deviation is computed and expressed by a translation Δ(X, Y, Z) and a rotation vector Δ(I, J, K). The translation and rotation Δ(X, Y, Z, I, J, K) is then applied as an offset to the nominal location  26  and the nominal orientation  28  of each geometrical element to be machined to obtain the corresponding actual location  30  and actual orientation  32  of these elements. 
     In a particular embodiment, the calculator  56  is separated in a first module determining the deviation and encoding the calculated values for Δ(X, Y, Z, I, J, K) in a matrix which is embedded on the capsule, and a second module which may be integrated in the machine tool and which includes a code reader reading the matrix, the second module applying the deviation Δ(X, Y, Z, I, J, K) as an offset to the nominal location and orientation  26 ,  28  of each geometrical element to be machined to obtain the corresponding actual location and orientation  30 ,  32  of these elements. 
     As such, the system advantageously allows the use of the complete part tolerance for the machining of the part. The necessity to evaluate the accuracy of the blade position within the encapsulation and potentially having to remove and reform the encapsulation as a result of that evaluation is eliminated, thus simplifying the machining process. As such the complexity of training required for the operator responsible for the encapsulation is advantageously decreased because of reduced accuracy requirements of that encapsulation. Each actual blade is individually compensated in relation to its location within its encapsulation, and the consistency between actual parts being machined is thus increased. 
     The system and method described herein thus allow the nominal tridimensional definition of a part to be adapted to each one of a series of actual parts that are manufactured, such as to substantially reduce the variation in the location and orientation of the machined geometrical elements. The location and orientation of each of the machined elements, which can be for example a thousand holes pierced through a single part, is adapted to the actual, real surface of the part such as to obtain a part as close as possible to the part defined in the nominal model. The quality of the parts produced is advantageously increased, and the repeatability of the machining process is increased as well, thus reducing waste parts. 
     In the case where the part is a gas turbine engine combustor lining and the machined elements are a plurality of holes drilled through the lining, this adaptation of the hole location and orientation to the actual profile ensures that the hole angle with respect to the surface of the lining is similar to the nominal angle for each of the holes, and that the hole distribution in the real lining is similar to the nominal hole distribution, thus in use providing a more consistent airflow within the combustor lining through the machined holes, and a better performance uniformity between similar combustor linings. 
     The system also substantially reduces required accuracy in the position of the actual part to be machined within the fixture retaining the part during the machining operation. Increased accuracy in the actual part position is usually required in the prior art methods in order to ensure a minimum of consistency between successive machined parts. By contrast, the present system compensates for the position difference between the nominal and the actual part in addition to compensating for the differences in the nominal and actual surfaces, through the tridimensional comparison between the nominal part and each actual part. As such, regardless of the position of each actual part within the fixture retaining it during the machining (and scanning) operations, the position and orientation of the machined elements are compensated such as to be as close as possible to the position and orientation required by the nominal model. Accordingly, the consistency between successive machined actual parts is increased, while the necessity for precision in the design of retaining fixtures and the training for operators responsible for placing the parts to be machined in those fixtures is substantially reduced. 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without department from the scope of the invention disclosed. For example, in the present specification and claims, the terms “machining”, “machined”, etc. are used in a generic sense, and apply to manufacturing methods in which a part configuration or shape is changed by a tool: laser drilling, other drilling techniques as well as other material removing techniques whether by laser or with another adequate tool such as trepanning, cutting, boring, electrical discharge machining (EDM), lathing, grinding, milling, etc., operations such as bending or other forming operations, any suitable operation in which the shape or configuration of a part of workpiece is affected, or any other suitable operation. Modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.