Patent Publication Number: US-9429935-B2

Title: Methods of fabricating shims for joining parts

Description:
RELATED APPLICATIONS 
     This patent arises from a continuation of U.S. patent application Ser. No. 13/156,101, filed Jun. 8, 2011. The entirety of U.S. patent application Ser. No. 13/156,101 is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to techniques for fitting and assembling parts together within desired tolerances, and deals more particularly with a method of designing and fabricating shims used to fill gaps between part interfaces. 
     BACKGROUND 
     The parts of an assembly are sometimes required to be joined together with an accuracy that is within a preselected tolerance. For example, in the aerospace industry, some parts may be required to be assembled together with less than a 0.005 inch gap between them. When the gap exceeds the preselected tolerance, a shim or similar filler may be inserted into the gap in order to assure a within tolerance fit between the parts. The process of assembling and fitting parts together with the required accuracy may become more challenging when the assembly process must be carried out within confined spaces. 
     Several known methods have been used for measuring and filling part gaps during the assembly process. According to one method, a set of feeler gauges is used in a progressive trial-and-error process to measure the gap between two interfacing part surfaces. This approach is both time consuming and its accuracy may be dependent on the skill of the technician making the measurements. Using the manual gap measurements, a custom shim is constructed either manually or using automated machine tool processes. 
     A second method of measuring and fitting gaps between parts relies on manual probing of the gap using an electronic feeler gauge. Electronic feeler gauges may be difficult to use and the measurement results may also be dependent on the skill of the technician who carries out the measurements. 
     A third method of measuring and filling gaps between parts involves filling the gap with a plastic slurry material that cures in place to form a solid filler object. This solution to the problem may have several disadvantages in some applications. For example, the plastic slurry material must remain frozen until just before use and must be bonded to one of the parts but not to the opposite part. The parts to which the slurry material is to be bonded must be coated with a release agent in advance of application. In addition, the slurry material may exert a hydraulic pressure on the parts during the application process, which may deform or displace the parts slightly, reducing assembly accuracy. Another disadvantage of the slurry material is that the material may shrink in a non-uniform manner during curing. Also, the application of the material is time critical, and material may require an extended period in which to cure during which further work on the assembly may not be performed. 
     Still another method of filling the gaps between mating parts, sometimes referred to as predictive shimming, involves scanning the interfacing part surfaces in an attempt to predict the exact shape of the gap or void between these surfaces. The parts of the assembly are virtually fitted together and a shim is fabricated based on the virtually predicted relationship between the parts. The problem with this approach, however, is that the parts of the assembly, especially large assemblies, may experience significant relative movement of the parts between the time the parts are initially scanned and the time of assembly, resulting in changes of the shape and/or dimensions of the gap. Another disadvantage of this method lies in its dependence on relatively high global accuracy of measurement and assembly. 
     SUMMARY 
     Disclosed example methods of fabricating shims for joining parts, wherein the parts having mating surfaces separated by a gap, comprise: generating first digital data representing a first surface profile of a first part; generating second digital data representing a second surface profile of a second part to mate with the first part; determining distances between the first and second parts, as assembled, at multiple locations; generating a digital volume that closely matches a gap between the first and second parts based on the determined distances; generating a three dimensional digital representation of a shim to fill the gap using the first and second digital data; and automatically fabricating the shim matching the gap using a computer-controlled machine, the machine being controlled using the three dimensional digital representation of the shim. 
     Other disclosed example methods of fabricating shims for joining parts, wherein the parts have mating surfaces separated by a gap, comprise: generating first digital data representing a first surface profile of a first part; generating second digital data representing a second surface profile of a second part to mate with the first part; physically assembling the first and second parts into an assembly; placing a non-contact distance measurement device between the assembled first and second parts; measuring distances between the assembled first and second parts at multiple locations; generating a digital volume that substantially matches a gap between the first and second parts based on the measured distances; disassembling the first and second parts; generating a three dimensional digital representation of a shim to fill the gap by using the first and second digital data to map the first and second surface profiles onto the digital volume; automatically fabricating the shim matching the gap using a computer-controlled machine, the machine being controlled using the three dimensional digital representation of the shim; physically reassembling the first and second parts; and placing the fabricated shim between the parts to fill the gap. 
     Other features, benefits and advantages of the disclosed embodiments will become apparent from the following description of embodiments, when viewed in accordance with the attached drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is an illustration of a cross sectional view of an assembly of two parts having a gap therebetween. 
         FIG. 2  is an illustration of a cross sectional view of a shim for filling the gap between the parts shown in  FIG. 1 . 
         FIG. 3  is an illustration similar to  FIG. 1 , but showing the shim of  FIG. 2  having been installed between the parts. 
         FIG. 4  is an illustration of a flow diagram broadly showing the steps of a method of fabricating a shim according to the disclosed embodiments. 
         FIG. 5  is an illustration of a flow diagram showing additional detail of the method shown in  FIG. 4 . 
         FIG. 6  is an illustration of a cross sectional view of a part that joins itself. 
         FIG. 7  is an illustration of a cross sectional view of two parts that are joined to a third part using the disclosed method. 
         FIG. 8  is an illustration of a cross sectional view of two parts requiring the use of multiple shims. 
         FIG. 9  is an illustration of a cross sectional view of a shim used to join together multiple parts. 
         FIG. 10  is an illustration of a cross sectional view of two curved parts having substantially constant radii of curvature and having a curved gap therebetween. 
         FIG. 11  is an illustration of two parts joined together by substantially orthogonal shims. 
         FIG. 12  is an illustration of a cross sectional view showing multiple parts having various part joining interfaces joined together by individual shims. 
         FIG. 13  is an illustration showing details of several of the part interfaces shown in  FIG. 12 . 
         FIG. 14  is an illustration of a cross sectional view showing multiple part interfaces between trapped parts. 
         FIG. 15  is an illustration of a cross sectional view of a part whose thickness is being measured. 
         FIG. 16  is an illustration of a cross sectional view showing the part in  FIG. 15  having been assembled with a second part, wherein a gap between the two parts is being measured. 
         FIG. 17  is an illustration of gap measurements between a vertical tail stabilizer and the fuselage of an aircraft. 
         FIG. 18  is an illustration of a flow diagram of aircraft production and service methodology. 
         FIG. 19  is an illustration of a block diagram of an aircraft. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , first and second parts  30 ,  32  are to be joined together with a part fit-up that is within a desired tolerance. The parts  30 ,  32  have opposing, interface surfaces  36 ,  38  respectively, which are to be joined together at a joining interface  35 . Because the interface surfaces  36 ,  38  may not perfectly join each other along the interface  35 , a gap or void  34  may be present between the part interface surfaces  36 ,  38 . 
     In accordance with the disclosed embodiments, a method is provided of producing a shim  30  shown in  FIG. 2  that is designed and manufactured to substantially completely fill the gap  34  between the parts  30 ,  32  at the interface  35 . As used herein the term “shim” is intended to include, without limitation, joining parts, fillers and other elements that are used to fill one or more gaps or voids between two or more parts being assembled to achieve a fit of a prescribed tolerance. In some applications, the shim  40  may comprise one of the parts of an assembly of parts. In the present example, the shim  40  has an upper flat surface  42  substantially matching surface  36  of part  30 , while the lower surface  34  of the shim  40  is contoured to match the contoured surface  38  of part  32 . As will become apparent later however, the surfaces of the shim  40  that interface with the parts  30 ,  32  may have any shape or contour that substantially matches that of the parts  30 ,  32 . 
       FIG. 3  illustrates the shim  40  of  FIG. 2  having been installed in the gap  34  so as to substantially completely fill the gap  34  and thereby join the two parts  30 ,  32  together at the joining interface  35 . 
     It may be useful here to define several terms that used from time to time in the present description. The term “independent surface” is used herein is intended to mean a digital surface that stands alone in definition and is free from all constraints to other digital surfaces. “Semi-independent surface” refers to a digital surface that shares some constraints with other surfaces that have been determined to have sufficient accuracy to meet prescribed tolerances. “Joining interface” as used herein, is intended to refer to a collection of two or more surfaces on one or more parts that are to be joined into an assembly or part by means of custom formation of one or more of the surfaces or by means of a shim or joining part. “Unconstrained datasets” refer to a collection of independent surfaces, points or other digital data that will be used to create a digital shim or a digital joining part. “Partially constrained datasets” refers to a collection of semi-independent surfaces, independent surfaces, points and/or other digital data that may be used to create a digital shim or digital joining part. “Digital joining part” refers to a part on which digital surfaces are defined such that they may contact themselves or other parts in multiple places in a manner that each interface has a fit of a prescribed tolerance. “Shim” and “digital shim” refer to a digital defined shim or part that is composed of independent digital surfaces, points, or other digital data that has been related or constrained to one another to create the bounds of a volume to which a physical shim may be manufactured. 
     Attention is now directed to  FIG. 4  which broadly illustrates the steps of a method of producing a shim  40  ( FIG. 3 ) for fitting one or more parts  30 ,  32  together according to the disclosed embodiments. Beginning at  46 , the surface profile of the mating surfaces of two or more parts are determined. For example, the surface profiles of interface surfaces  36 ,  38  of parts  30 ,  32  shown in  FIG. 1  are determined. These surface profiles may be determined using any of various techniques, such as by calling up pre-existing 3-D CAD (computer aided design) design files that digitally define the surfaces  36 ,  38 , or by digitally scanning the surfaces  36 ,  38  either before or after the parts  30 ,  32  have been assembled, using a laser scanner, computer controlled coordinate measuring machine or other suitable equipment (all not shown). Next, at step  48  a three dimensional (3-D) digital volume is generated that substantially matches the gap  34  between parts  30 ,  32  (see  FIG. 1 ). The digital volume generated in step  48  establishes a basic inner volume definition of a shim object which would closely fill the gap  34 . At step  50 , a 3-D digital representation of the shim  40  is generated by mapping the part surface profiles obtained in step  46  onto the digital volume generated at step  48 . The results of step  50  is a digital solid definition of a shim  30  which would substantially fill the gap  34 . At step  52  the 3-D representation of the shim generated in step  50  is used to fabricate the shim  40  using any of various process, such as, without limitation, computer controlled machining. 
     Attention is now directed to  FIG. 5  which illustrates additional details of an embodiment for carrying out the method shown in  FIG. 4 . Beginning at step  54 , a first digital dataset is produced which defines each joining part surface such as surfaces  36 ,  38  of the parts  30 ,  32  shown in  FIG. 2 . The first digital dataset may be produced by digitally scanning the interface surfaces  36 ,  38  using any of various techniques, including, for example and without limitation, a computer controlled CMM (coordinate measuring machine), laser scanner, etc. Alternatively, digital files representing the interface surfaces  36 ,  38  may be imported from an existing source, such as one or more CAD design files that define the surfaces  36 ,  38  in 3-D. 
     At step  56 , the parts  30 ,  32  are assembled on a best-fit basis. The dataset produced in step  54  may be produced either before or after the parts  30 ,  32  are assembled in step  56 . At step  58  a second digital dataset is produced which defines the 3-D spatial relationship between the part interface surfaces  36 ,  38 . The second digital data set may be produced using any of various techniques which establish the relative positions of the interface surfaces  36 ,  38  in 3-D space. For example, following assembly of the parts  30 ,  32  in step  56 , a laser scanner (not shown) may be inserted into the gap  34  and used to scan the surfaces  36 ,  38 . This scanning process generates digital data representing the distance between the surfaces  36 ,  38  at a multitude of points representing the digital volume matching the gap previously discussed in connection with step  48  in  FIG. 4 . 
     Next, at  60 , the parts  30 ,  32  may be disassembled, as required, although in some applications the parts  30 ,  32  may remain in their assembled state until a shim  40  has been fabricated and inserted into the gap  34  between the parts  30 ,  32 . At step  62 , automated data processing implemented by a computer (not shown) may be used to produce a third data set that represents the shape and dimensions of the gap  34  to be filled. Step  62  is similar to step  50  shown in  FIG. 4  in which a 3-D representation of the shim  40  is generated corresponding to the shape and dimensions of the gap  34 . At step  64 , a shim or similar custom part is fabricated using the third dataset produced at step  62 . The shim  40  may be fabricated, for example and without limitation, using CNC machining. Finally, as shown in  66 , the shim  40  may be inserted into the gap  34  between the assembled parts  30 ,  32 , although if the parts  30 ,  32  have been previously disassembled at step  60 , then the parts  30 ,  32  are reassembled with the shim  40  inserted into the gap  34 . 
     The disclosed method may be employed to assemble and fit a wide variety of parts having differing shapes and interface surface contours. For example, the method may be used to assemble and fit parts having parallel joining interfaces, constant radius of curvature joining interfaces, orthogonal joining interfaces, and others (all not shown). Constraint relationships required to establish the relationship between joining surfaces may be established using any of a variety of techniques, including mechanically or electronically measuring the distance between the joining surfaces at multiple locations on the part surfaces  36 ,  38 . 
       FIG. 6  illustrates a single part  65  having opposing portions  70 ,  72  separated by a gap  68 . The disclosed method may be used to fabricate a shim  74  that substantially matches the shape of the gap  68  and results in fitting of the two portions  70 ,  72  together within a desired tolerance. 
     Referring to  FIG. 7 , the disclosed method may be employed to fabricate a joining part  80  which joins two other parts  76 ,  78  together, wherein the joining part  80  forms part of a part assembly  85 . In this example, parts  76  and  78  are joined along a joining interface  82  to which the joining part  80  conforms. 
       FIG. 8  illustrates an assembly of two parts  84 ,  86  representing a partially constrained dataset, and respectively having interface surfaces  90 ,  95 . Part  84  includes grooves  92  that define seven semi-independent interface surfaces  90 , while part  86  has a single independent surface  95 . In order to fit the interfacing and surfaces  90 ,  95  within desired tolerances, the disclosed method may be employed to fabricate seven shims (not shown in  FIG. 8 ) which are placed between surfaces  90 ,  95  to fill any gaps that may be present therebetween. 
       FIG. 9  illustrates a joining part  96  fabricated according to the disclosed method that may be employed to join multiple other parts  94  along differing interface surfaces  101 . A groove  98  through part  94   a  results in part  94   a  having two independent surfaces  100   a  fitted to the joining part  96 , while the other parts  94  have only one independent surface  101  fitted to the joining part  94 . 
       FIG. 10  illustrates an assembly of parts  102 ,  104  which respectively near-constant radius of curvature interfacing surfaces  106 ,  107  are separated by a gap  110 . According to the disclosed method, a shim  111  designed and fabricated by the disclosed method, has a suitable curvature and dimensions which closely fill the gap  110 . 
       FIG. 11  illustrates an assembly of parts  112 ,  114  that may be joined together by substantially orthogonal shims  116 ,  118  designed and fabricated in accordance with the disclosed method. 
       FIG. 12  illustrates an assembly  115  of sixteen generally square parts  120  fitted together by shims  122  produced in accordance with the disclosed method. In this example, each of the parts  120  may have regular or irregular interface surfaces  124  forming gaps  125  filled by segmented shims  122 . There may be no theoretical limit to the number of parts  120  or the size of the overall assembly  115  that may be fitted using the disclosed method using locally accurate shimming. Geometric changes within the assembly  115  may usually occur over a broad area. According to the disclosed method, segmented shims  122  may be used that are small enough that geometry changes in the assembly  115  due to warpage or the like do not shift the shims  122  out of tolerance. 
       FIG. 13  illustrates four generally square parts  130 , similar to those shown in  FIG. 12 , which have generally parallel interface surfaces  138 ,  140 ,  142 ,  144 ,  146 , some of which however, may include uneven surface contours, e.g.  140 ,  146 . The technique used to establish reference points at the interface surfaces  138 - 146  will vary depending upon the complexity of the contour of interface surface  138 - 146 . For example, the part interface shown at  132  comprises two complex contoured surfaces  138 ,  140 . In this case, both surfaces  138 ,  140  are referenced laterally with respect to each other, and surface reference points on both surfaces  138 ,  140  are therefore used for gap measurement. One technique for accomplishing this referencing is to tie the two adjacent parts  130  together using common features such as three common holes (not shown) in the parts  130 . Another technique would be to measure lateral differences between reference features and the assembled parts as well as the gap  135  between the surfaces  138 ,  140 . 
     At the surface interface shown at  134 , the opposing interface surfaces  142  are generally smooth and parallel, consequently, in order to establish the relationship between the opposing interface surfaces  142 , gap measurements need only be measured at three points on either of the surfaces  142 . Finally, as shown at part  136 , one of the interface surfaces  146  is relatively highly contoured, while the other opposing interface surface  144  is relatively smooth. In this case, the reference points for gap measurement need be placed only on the contoured surface  146 . 
       FIG. 14  illustrates trapped joining interfaces  156 ,  158 ,  160  between mating parts  152 ,  154  that are trapped within a common base part  148 . Thus, the two mating parts  152 ,  154  are constrained at both ends by the stack-up of the common part  148 . In this example, it is desirable to produce shims  150   a ,  150   c  and then perform the necessary measurements and fabrication steps to produce the third shim  150   b.    
     As previously discussed, a variety of techniques can be employed to establish the relationship between two parts  30 ,  32  ( FIG. 1 ) for the purposes of calculating a solid volume that will fill a gap  34  between the parts  30 ,  32 . For example,  FIGS. 15 and 16  illustrate a technique for measuring gaps between two parts  178 ,  186  ( FIG. 16 ) which have one or more holes  179 . In this example, part  178  may comprise an aircraft skin  178  having one or more holes  179  at known locations forming reference points on the skin  178 . As shown in  FIG. 15 , in order to first determine the thickness  180  of the skin  178 , the tip  184  of a depth probe  182  is inserted into the hole  179  and is brought into contact with a backing disk  186 , following which the thickness  180  of the skin  178  may be measured. Then, as shown in  FIG. 16 , a second part  186  is assembled onto the skin  180 , which may result in a gap  187  between the skin  178  and the part  186 . The tip  184  of the depth probe  182  is then reinserted into the hole  184  until it comes into contact with the part  186 , allowing a measurement of the distance “D” between the two parts  178 ,  186  at the location of the hole  179 . Using similar techniques, it may be possible to measure gap distances between more than two stacked parts. 
       FIG. 17  illustrates the assembly of a vertical tail stabilizer  212  on a fuselage  214  of an aircraft using the disclosed method. The vertical stabilizer  212  is brought into close proximity and held in position immediately above the fuselage  214 . Three gap measurements are then performed at both the front  216  and the rear  218  of the assembly thereby establishing the spatial relationship between the stabilizer  212  and the fuselage  214 . Based upon these gap measurements which establish the spatial relationship between the stabilizer  212  and the fuselage  214  and the surface profiles of these two parts, one or more suitable shims (not shown in  FIG. 16 ) may be fabricated to achieve a fit between the stabilizer  212  and the fuselage  214  within desired tolerances. 
     Embodiments of the disclosure may find use in a variety of potential applications, particularly in the transportation industry, including for example, aerospace, marine and automotive applications. Thus, referring now to  FIGS. 18 and 19 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  220  as shown in  FIG. 18  and an aircraft  224  as shown in  FIG. 19 . Aircraft applications of the disclosed embodiments may include, for example, without limitation, assembly and fitting fuselage skins, wings and wing skins, stiffeners, control surfaces, hatches, floor panels, door panels, access panels and empennages, to name a few. During pre-production, exemplary method  220  may include specification and design  226  of the aircraft  224  and material procurement  228 . During production, component and subassembly manufacturing  230  and system integration  232  of the aircraft  224  takes place. Thereafter, the aircraft  224  may go through certification and delivery  234  in order to be placed in service  236 . While in service by a customer, the aircraft  224  is scheduled for routine maintenance and service  238  (which may also include modification, reconfiguration, refurbishment, and so on). During any of stages  230 ,  232  and  238 , shims produced according to the disclosed method may be used to join parts, components or assemblies of the aircraft  224 . 
     Each of the processes of method  220  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 19 , the aircraft  224  produced by exemplary method  220  may include an airframe  240  with a plurality of systems  242  and an interior  244 . Examples of high-level systems  242  include one or more of a propulsion system  248 , an electrical system  248 , a hydraulic system  250 , and an environmental system  252 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the disclosure may be applied to other industries, such as the marine and automotive industries. The disclosed embodiments may be used to produce shims  241  that are employed to fit and join various parts, components and subassemblies of the airframe  240 . 
     Systems and methods embodied herein may be employed during any one or more of the stages of the production and service method  220 . For example, components or subassemblies corresponding to production process  230  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  224  is in service. Also, one or more apparatus embodiments, method embodiments, or a combination thereof may be utilized during the production stages  230  and  232 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  224 . Similarly, one or more of apparatus embodiments, method embodiments, or a combination thereof may be utilized while the aircraft  224  is in service, for example and without limitation, to maintenance and service  238 . 
     Although the embodiments of this disclosure have been described with respect to certain exemplary embodiments, it is to be understood that the specific embodiments are for purposes of illustration and not limitation, as other variations will occur to those of skill in the art.