Abstract:
An aircraft frame stringer is checked against designed specifications using a set of templates with stringer-engaging, upwardly-facing surfaces that match the designed contours of sections of the stringer at different locations along the length of the stringer. The templates are removably supported in mutually aligned and spaced relationship on a base assembly. The stringer is placed on the templates and measurements are made to determine if the stringer contours at the different locations match, within acceptable tolerance limits, the template contours at those locations. A computer model is used to determine the contours of the sections of the stringer at the desired locations thereon and to control the operation of a CNC machine for machining the templates.

Description:
FIELD OF THE INVENTION 
   This invention relates to a check fixture and method for checking the conformity of aircraft frame stringers to designed specifications. 
   BACKGROUND OF THE INVENTION 
   The frame of an aircraft includes longitudinally-extending stringers to which body panels are riveted at locations which match the inside mold line (IML) of the body of the aircraft. To ensure that the contour of a stringer matches, within tolerances, the IML of an aircraft body, aircraft manufacturers typically require that the stringers be checked for compliance with the designed IML at several locations or stations along the length of the stringer. Prior methods for checking if a stringer is within specifications have not been entirely satisfactory, being highly expensive and/or time-consuming. 
   SUMMARY OF THE INVENTION 
   A check fixture in accordance with this invention comprises a base assembly on which are mounted a row of mutually spaced, parallel, vertically-extending templates designed to mate with and support a stringer to be checked. The base assembly is designed to enable the templates to be positioned at equally-spaced stations to meet customer requirements for checking the stringers. Each template is manufactured to include a machined surface along an upper edge thereof that matches the shape of a corresponding transverse section or “slice” generated from a computer model of the stringer. Each slice taken from the computer file has substantially the same thickness as a corresponding template. 
   The base assembly may also be designed to enable templates to be located to support and check a stringer at other stations along the length of the base assembly. Airframe manufacturers in the United States of America typically require a stringer to be checked at 12-inch intervals along the entire length of the stringer. For such purpose, a base assembly in accordance with this invention is constructed to support an array of templates positioned at stations which are mutually spaced on 12-inch centers. In addition, to accommodate short stringers or the front ends of stringers, a base assembly constructed to support stringers every 12 inches along the length of the base assembly may also be constructed to support stringers midway between each of the 12-inch centers. Accordingly, for typical present-day requirements of U.S. airframe manufacturers, the base assembly of this invention may be constructed to be able to support templates at six inch intervals along its entire length. Of course, the base assembly could be provided with other locations for templates as may be needed to meet different stringer-checking requirements. 
   A stringer is checked for compliance with the customer&#39;s standard by simply placing the stringer onto the templates in proper registry and determining if the stringer matches margins of the notches in the templates within an acceptable tolerance. 
   Proper registry between the check fixture of this invention and the stringer to be checked is obtained by determining the distance from the aft end of the stringer to the designed location of the first, rearmost, template station and inserting that part of the stringer on the first template. To accomplish the proper registry of the templates and the stringer, the check fixture may be provided with a rear or bank end stop against which the rear or bank end of the stringer is abutted and the templates are stationed at predetermined spacings from the bank end stop. 
   The base assembly comprises an elongate base plate supported in a horizontal orientation on a table and a vertical back plate mounted on the rear edge of the base plate. Plural, mutually parallel, horizontally-extending template-support slots are located along the upper surface of the base plate, extending transversely to the longitudinal axis of the base plate. The template-support slots may be on equally spaced centers. The back plate has a like number of vertical slots which are open to corresponding base plate-support slots. Each template is inserted into one of the base plate slots and into the corresponding vertical slot so that the template is securely held in a vertical orientation. Means such as screws or bolts may be used to fixedly locate the templates on the base assembly. 
   A check fixture in accordance with this invention has several potential gauge stations, each gauge station comprising a template supported in one of the template-support slots. For ease of assembly and disassembly of the check fixture, each potential gauge station may be identified on the base assembly by a suitable numerical or alphabetical progressive set of indicia such as A, B, C, or 1, 2, 3, and so forth, with each template marked with the same type of indicia to identify the station at which the template is to be located. The check fixture can be converted from checking a set of stringers of one design to checking a set of stringers of a different design by simply removing the first set of templates used to check the first set of stringers and replacing them by the templates needed for checking the second set of stringers. Accordingly, the templates for each set of stringer can be retained for use at a later date. Reassembly into the base assembly is readily accomplished because of the obvious correlation between the indicia on the templates and the indicia identifying the slots on the base assembly. 
   Other features and advantages of the invention will become apparent from the following description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a front elevational view of a stringer check fixture made in accordance with this invention and a stringer in position to be checked in accordance with a method in accordance with this invention. 
       FIG. 2  is a partly exploded, fragmentary isometric view of the check fixture of  FIG. 1 , as viewed from above, the front, and the right end of the check fixture. 
       FIG. 3  is a fragmentary isometric view, partly in cross section, of the check fixture of  FIG. 1  and a stringer in position to be checked, as viewed from above, the front, and the right end of the check fixture. 
       FIG. 4  is a fragmentary end view of a template of  FIG. 1  and a transverse cross-sectional view of the stringer taken along lines  4 — 4  of  FIG. 3 . 
       FIG. 5  is a partly exploded, fragmentary isometric view, partly in cross section, of a portion of the check fixture of  FIG. 1 , as viewed from above, behind and the right end of the check fixture. 
       FIG. 6  is a transverse cross-sectional view of the parts of the check fixture shown in  FIG. 5  but assembled together, with a template shown in end elevation. 
       FIG. 7  is a fragmentary front elevational view of a second embodiment of a stringer check fixture made in accordance with this invention and a stringer in position to be checked in accordance with a method in accordance with this invention. 
       FIG. 8  is a partly exploded, fragmentary isometric view of the check fixture of  FIG. 7 , as viewed from above, the front, and the right end of the check fixture. 
   

   DETAILED DESCRIPTION 
   Referring first to  FIGS. 3 and 4 , a typical stringer  12  is illustrated in the form of a hat section having side walls  14  and a skin-engaging surface  16 , referred to herein as “base wall,” which is intended to be riveted to the skin of an aircraft body. Those familiar with the art are aware that stringers are often formed to follow paths that change along the length of the stringer in all three (x, y, and z) axes. The method and the apparatus of this invention are used to check the conformance of the base wall  16  along the length of the stringer  12  against design specifications. Before this invention is put to use to check a stringer  12 , the overall height and width dimensions of the stringer  12  have been determined to be within design tolerances by precedent processing steps. 
   A first embodiment of a check fixture, designated  20 , in accordance with this invention is illustrated in  FIGS. 1 through 6 . The check fixture  20  comprises a base assembly  22  and plural templates  24 . Individual templates  24  are further identified by indicia comprising the letters “A”, “C”, “E,” and so forth, beginning with the right or aft end of the check fixture  20 . Each of these letters correspond to a possible stringer station. 
   The base assembly  22  rests on a flat, horizontal support plate  26  which in turn rests on the top surface of a horizontally oriented, calibrated I-beam  28 . 
   With reference to  FIGS. 2 ,  5  and  6 , the base assembly  22  comprises a flat, horizontal base plate  30 , and a flat, vertical back plate  32  supported on top of the back edge of the base plate  30  and affixed thereto by plural bolts  34 . 
   Plural, mutually parallel, horizontally and transversely-extending template-support slots or grooves  36  are recessed in the top surface  38  of the base plate  30 . Similarly, plural, mutually parallel, vertically and transversely-extending, template-retaining, slots or grooves  40  are recessed in the front surface  42  of the back plate  32 . Each vertical template-retaining slot  40  is equidistant from the right or aft end of the back plate  32  to a corresponding template-support slot  36 . The aft ends of both plates  30  and  32  are coplanar so that the assembled base assembly  22  has a series of aligned horizontal template support slots  36  and vertical template-retaining slots  40 . As further described below, each pair of aligned slots  36  and  40  are available to support a template  24  in a vertical orientation. 
   A pair of through bores  44  may extend vertically through the base plate  30  adjacent the opposite ends thereof which are moved into alignment with respective bores  46  in the support plate  26 . Key pins  48  can be extended into the mutually aligned bores  44  and  46  to retain the base plate  30  and thereby the entire base assembly  22  in a fixed position on the support plate  26 , although that is not necessary in the embodiment of  FIGS. 1 through 6 . 
   With reference to  FIGS. 2 through 6 , each template  24  in accordance with this invention comprises a plate-like body having an upper surface shaped to receive a section of a stringer  12  including a recessed, stringer-receiving pocket  50  having an upwardly-facing bottom wall  52  shaped to conform to the designed shape of the stringer base wall  16  at a particular location along the base assembly  22 . Pocket  50  is located centrally of a larger pocket  54  that accommodates the upper portion of the stringer  12  as shown in  FIGS. 3 and 4 . 
   Each of the templates  24  may have drilled and tapped holes  56  in its bottom and rear side edges to permit the templates  24  to be secured to both base assembly plates  30  and  32  by bolts  58  which extend through countersunk holes  60  in the plates  30  and  32 . 
   To provide for accurately positioning a stringer  12  along the length of the check fixture  20 , an aft end locator or stop member  62  is supported by a mounting bracket  64  at the aft end of the base plate  30 , such that it will be aligned with the stringer-receiving pocket  50  in the first, aftmost, template  24 A. 
   A manner in which the shapes and orientations of the pockets  50 , and particularly their bottom walls  52 , are developed will now be described. The digital data for a stringer is obtained from the airframe designer or manufacturer in a 3-D model format. By computer, the stringer digital data is used to simulate a stringer in the “catch water” orientation, which is the same as the orientation shown in  FIGS. 3 and 4 . It may so orient the stringer (digitally) that its ends are equidistant from a common horizontal plane in space. Then a simulated “slice” having a width equal to the width of a template  24  is taken through the stringer at each template position or “stringer station line” and the digital data representing the outer perimeter of that slice is machined into a template  24 , taking into account allowable tolerances, by a CNC machine (not shown) to form the pocket  50  of the particular template  24  to be used at that stringer station line. All of the templates  24  for a particular stringer design are made in the same way. Each template may be inspected by a CMM to ensure that its cross-sectional profile matches the digital data for the stringer station line at which the template is to be located. 
   To check conformance of stringers of a given design, the templates  24  for that design are mounted in the appropriate base assembly slots  36  and  40 , the aft end of a stringer is abutted against the aft end stop member  62 , and the rest of the stringer lowered onto the template pockets  50 . The conformity or lack of conformity of the stringer being checked is then determined by use of a calibrated pin gauge (not shown) for checking the size of the gap, if any, between the base wall of the stringer and the mating surface of the template. Airframe manufacturers may permit use of a weight, such as a calibrated sand or bean bag, to bias a section of a stringer into conformity with a designed profile. Such bias can be obtained using the check fixture  20  of this invention by the simple expedient of placing the calibrated sand or bean bag on top of the section of a stringer in a template pocket  50 . 
   Although different requirements may be required, a gap of 0.030 inches between the base wall  16  of the stringer and the bottom wall of the template pocket  50  is usually considered acceptable. If the detected gap exceeds 0.030 inch (or any other tolerance permitted by the airframe manufacturer), the stringer profile would require correction. 
   It will be recognized that the templates  24  could readily be removed and replaced by other templates for stringers having different configurations. A set of templates for checking a stringers of one design can be marked with an appropriate part number and, after removal from the base assembly  22 , stored for future use should the need arise for checking the same stringers at a later date. 
   For ease of assembly, each of the templates is marked with indicia representative of stringer stations at which cooperating template support and retaining slots  36  and  40  are located, and the base assembly  20  has corresponding indicia so that the station at which a particular template is to be located is readily and accurately identifiable. For example, the station closest to the aft end of the base assembly may be marked as station “A,” and succeeding stations marked “B”, “C” and so forth, as shown in  FIGS. 2 and 3 . If, for example, templates are to be mounted at every other station, they would correspondingly be marked “A”, “C”, etc., as also shown in  FIGS. 2 and 3 . 
   A specific example of a check template  20  in accordance with this invention includes a 12 foot long base assembly  22 , thus having a base plate  30  which is 12 feet long, and a back plate  32  which is also 12 feet long. The several possible stations formed by the cooperating pairs of slots  36  and  40  are mutually spaced on six inch centers, the first pair of slots at station “A” are three inches from the aft end of the base assembly, and 1.25″ from the forward face of the aft end stop member  62 . Each of the templates is 0.25″ thick and the template-supporting and retaining slots  36  and  40  essentially the same width but with a tolerance to permit the bottom and rear edges of the templates  24  to be received therein. The slots  36  and  40  at the forward most station are located three inches from the forward end of the base assembly  22 . Of course, dimensions other than those mentioned above may apply. 
   The specific check fixture  20  described above is used to meet a typical requirement for checking stringer profiles every 12 inches. The stations midway between 12 the alternate stations separated by 12 inches are useful for those occasions in which the length of a stringer is such that an undesirably long unsupported length of the stringer would extend past one forward template  24  without reaching the next forward template  24 . Such a situation is avoided in the setup shown in  FIG. 1  in which there are two templates  24 W and  24 X that are only six inches apart to provide support for the forward end of the stringer  12 . Without the template  24 X in place, the forward end of the stringer  12  would extend past the stringer  24 X by over six inches. 
     FIGS. 7 and 8  show another embodiment of a check fixture, designate  120 , in accordance with this invention. The fixture  120  is actually two fixtures, having base assemblies  122   a  and  122   b , placed end-to-end on a support plate  126  that rests on a calibrated I-beam  128 , and maintained in their end-to-end relationship by key pins  148 . Base assembly  122   a  may be essentially identical to the base assembly  22  of the single fixture  20  shown in  FIG. 1 , and has an aft end stop  162 . Base assembly  122   b  can be nearly identical to the base assembly  22 , but is not provided with an aft end stop. Obviously, the fixture  120  of  FIGS. 7 and 8  can be used for longer stringers, such as the fixture  116  shown in  FIG. 1 . In  FIG. 7 , the several templates  124  mounted on the base assembly  122   a  are identified by letters A, B, C, and so forth, while those mounted on the base assembly  122   b  are identified by letters AA, AB, AC, and so forth. The same indicia may be marked on the templates  124  and on the base assemblies  122   a  and  122   b  for reasons previously discussed. 
   As is deemed evident, within the purview of this invention, other embodiments of a check fixture could have additional base assemblies aligned with the two base assemblies  122   a  and  122   b  of  FIGS. 7 and 8 , with additional templates  22  used therewith. 
   The base assembly plates, such as plates  30  and  32 , and the templates  24  forming a check fixture in accordance with this invention would typically be made from metal, such as aluminum, but other construction materials may be used.