The practice of the aerospace industry has been to adapt long established carpentry methods of joinery into aerospace commodities in an attempt to capture the advantages of reduced weight and assembly complexity. Substantial difficulties have sometimes been encountered however, because tolerances achievable with fine carpentry in wood-working are far superior than that which can be achieved using many aerospace materials, such as composite honeycomb sandwich panel type structures, and their related assembly methods. Thus, time proven joinery techniques that have been effectively implemented in wood-working applications, have proved ineffectual in the aerospace industry.
One such problem area for aerospace applications, which has been encountered in the prior art, has been how to maximize the volume of a defined envelope using extended tab and pocket cutout joinery methods, while also maintaining maximum joint strength. The rabbet joint has become the standard design for the majority of aerospace commodities that utilize extended tab and pocket cutout joinery. In a rabbet joint, the pocket cutouts are at the very edge of the panel, with the pocket sidewalls actually incorporated into the outer edge of the panel. This type of joint permits joint location to occur at the edge of a panel, thus providing the benefit of a non-interfering edge profile. The disadvantage of the rabbet joint, is that the joint must be adhesive bonded to secure the panel connection, and the primary load path is through the relatively weak adhesive bondline at the rabbet joint.
The standard alternate to the rabbet joint is commonly referred to as a mortise and tenon joint. Although, the term mortise and tenon has become somewhat generic in fine carpentry uses, aerospace usage has defined a mortise and tenon joint as a term of art, describing a joint utilizing sandwich panel construction with square cut tabs (tenons) and blind (joint not visible after joining has occurred) slotted pocket cutouts (mortises), without dovetailing.
The bonding process of a mortise and tenon joint also involves applying adhesive into the mortise pocket; however, since the pocket is fully enclosed in the mortise panel (not incorporated into the panel edge as in the rabbet joint), the primary load path is through the mortise panel itself and not the adhesive bondline. The disadvantage of the mortise and tenon joint is the existence of an edge margin of the mortise panel that extends from the mortise pocket to the actual edge of the panel. This interfering edge margin reduces the volume which can be achieved inside a defined envelope. It is desirable to have a joint that would provide the combined benefits of both a rabbet joint and a mortise and tenon joint.
Another unresolved problem in the prior art is that although tight tolerance control of the standard square cut mortise (panel hole) can be achieved through the use of NC panel and profile routers, large clearances are still usually required between the tenon tabs and mortise cutout sidewalls, in order to allow for variation in tenon panel thickness. The current state of the art in composite honeycomb sandwich panel production utilizes a multi-opening press (MOP) process, that does not presently afford a high degree of control of panel thickness variation.
Thus, relatively large clearances must be designed into mortise and tenon joint interfaces so that costly interference conditions do not occur, preventing the tenon tabs from fitting into the mortise pockets, and resulting in the scrapping of parts or expensive rework. These large clearances between the mortise pocket sidewalls and the tenon tab surfaces, increase the need for elaborate and expensive tooling to accurately locate and secure the panels. While the panels are held in place, an adhesive, which is used to bond the joint, is allowed the necessary time to cure. A joint structure with inherent self-tooling features that could eliminate the need for expensive additional tooling is highly desirable.
Still an additional unresolved problem in the prior art involves the efficient production of lightweight overhead stowage bins for aircraft. Currently, overhead stowage bins for aircraft are produced by joining together four (typically) composite sandwich panels using structural adhesives and either aluminum brackets fastened to potted inserts or modified box joints (typically rabbet joints). Both of these methods depend on the strength of the adhesives to carry the required structural loads.
This reliance on adhesives presents two major disadvantages. First, the loads that adhesives are typically capable of carrying are inferior to the loads that can be carried through the composite panels themselves. Second, adhesives present substantial manufacturing problems, in that parts must be jigged in the proper configuration while the adhesive cures; a time period generally of around eight hours. It is desirable to have a aircraft stowage bin that can be produced without structural adhesives.