Patent Publication Number: US-2013239509-A1

Title: Invisible butt joint fastener

Description:
BACKGROUND 
     Butt joint is one of the simplest wood joints both in terms of construction and appearance. Many modern furniture used mechanical fasteners, such as the cam-dowel described in German Auslegeschrift 2625182 and U.S. Pat. No. 4,131,376, to recreate the simple look; however, each fastener has to expose certain tightening/releasing interface, and hiding the interface inside a furniture may not aways be feasible. These fasteners also typically require two interconnected holes, which are harder than single hole to drill. 
     The present invention is a butt joint fastener, designed with the objectives to eliminate or minimize its visibility, and simplify the making and assembling of furniture. The new fastener is primarily designed to be used in groups to connect structural pieces so that they can all be locked in one sliding action between the two pieces along the abutting surfaces, rather than being tightened individually like traditional fasteners, thus no exposure to the fastener is needed as the tightening and releasing forces are directly applied to the structural pieces instead of the fasteners. The fastener requires only one groove on the abutting surface of the structural piece, and can be routed out relatively easily. 
     SUMMARY 
     A fastener for connecting two structural pieces over abutting surfaces is presented. Each fastener comprises a male and a female member. A series of such fasteners are typically fixed into the abutting surfaces of the structural pieces to be connected, with the two members on opposite structural pieces and leaving only the tongues of the male members sticking out of the abutting surfaces. The male tongue can slide inside the female socket when the two structural pieces slide against each other along the abutting surface, and the socket surface is partially covered with raised ribs, that can catch the ribs on the male tongue when the tongue is at the locking position but not at the releasing position. All the ribs are slopped relative to the sliding direction to convert part of the sliding action into a pulling action, so as to create a preload between the structural pieces when connected. The lead angle between the ribs and the abutting surface is small, so friction can keep the fastener from sliding open even when a pulling force perpendicular to the abutting surface is applied, but an optional elastic retaining clip may further prevent the joint from accidental sliding open. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective partial section view of the preferred embodiment of the fastener. 
         FIG. 2  is a parallel side view of the preferred embodiment of the fastener. 
         FIG. 3  is a typical application of the fastener, where a series of the fastener are used to butt join two structural pieces. 
         FIG. 4  is a lockable embodiment of the fastener. 
         FIG. 5  is the typical application of the lockable embodiment. 
         FIG. 6  is a perspective view of a curved embodiment of the current invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The Preferred Embodiment—FIGS.  1  Through  3   
       FIG. 1  is a perspective view of the preferred embodiment of the current invention, with the female member drawn in section. The male member has a generally elongated body  10  with a head end  11 , a tail end  12 , a fore-face  13 , a tongue  14  extending from the fore-face  13 , and a series of raised ribs  15  on the surface of the tongue  14 . The female member has an elongated body  20  with a head end  21 , a tail end  22 , a fore-face  23 , a socket  24  indenting into the fore-face  23 , and a series of raised ribs  25  on part of the inner surface of the socket  24  near the head end  21 . 
       FIG. 2  shows the same preferred embodiment in parallel projection from a side. The male tongue  14  can be inserted into the rib-free region of the socket  24  near the tail end  22 , and slide towards the head end  21  to get locked inside. The unlocking process is just opposite the locking process, and both processes are shown with the arrow-ended line in the  FIG. 2 . The male ribs  15  runs with a small lead angle between the rail edge  150  and the fore-face  13 , inclining towards the fore-face  13  near the tail end  12 . The rail edge  250  of the female rib  25  also runs with a lead angle matching that of the rail edge  150 . The two rail edges  150  and  250  can glide against each other to pull the two members closer together in the locking action. The male member fore-face  13  has a small hump near the tail end  12  to form a retaining clip  130  that may clip to the inside wall of the female socket  24 . Due to the natural elasticity of the material, the retaining clip  130  does totally prevent the tongue  14  from sliding out of the socket  24 , but does make accidental releasing much harder. 
     The  FIG. 3  shows the typical application of the fastener in connecting two structural pieces  30 . The bodies  10  and  20  are pre-glued into grooves, leaving only the male tongues  14  extending out of the abutting surface. All fasteners are lined up in a row so all male tongues  14  can be inserted into female sockets  24  simultaneously, and be locked with one sliding action between the two structural pieces  30 . 
     Alternatives—FIGS.  4  Through  6   
     The lead angle between the ribs  15 / 25 , or more specifically the rails  150 / 250 , and their respective fore-faces  13 / 23  is small, so friction can keep the fastener from sliding open even when a pulling force perpendicular to the abutting surface is applied to the locked fastener; therefore, the retaining clip  130  can be eliminated in applications where no side-way forces are expected on a joint. 
     On the other hands, when side-way motion is frequent or severe, a more secure lock such as the latch  16  in  FIG. 4  may be needed. The latch  16  is designed to be higher than the retaining clip  130  above the fore-face  13 , so the latch  16  cannot be opened by side-way forces alone when locked inside the female socket  24 . The only way to unlock the latch is to push it directly down towards the body  10 . A gap  161  between the bottom of the latch  16  and the male body  10  allows the latch to bend down, and a narrow waist  160  makes the bending easier. The sides of the latch are beveled  162  to allow easier tool access to the top of the latch  160 . As shown in the  FIG. 5 , a screw driver  40  is inserted from the side of the latch, through a channel  31  in the structural piece  30 , to push the latch  16  down when unlocking is needed. Although the fastener is not totally invisible in this case, the exposure is still minimal comparing to traditional fasteners. 
     Butt joints are used not only for connecting straight edges of structural pieces.  FIG. 6  showed a curved embodiment of the current invention that can be used to connect curved edges, which are often seen in trade-show display construction. A moderate degree of flexibility can also be achieved with certain plastic materials without making the fastener overly stretchy. 
     The  FIG. 6  also showcased several minor variations to the preferred embodiment. The ribs  15  on both sides of the male tongue  14  are connected at the head end  11 . In fact, they can be connected at the tail end  12  too, or broken somewhere as long as gliding on female rib  25  is not interfered, or even leaving one entire side of the tongue flat. The same is true for female ribs  25  as well. Many of the detailed features of the ribs  15 / 25  shown in the  FIG. 1  through  FIG. 5 , such as the rounded endings and the chamfered non-rail edges, are more for a smooth looking than for any function, can be eliminated as shown in the  FIG. 6 . 
     The male body  10  and female body  20  may be glued into structural pieces  30  as shown in the  FIG. 3 ; however, other means of fastening are equally valid with straightforwards modifications to them. For example, the female body  20  may include holes at the bottom to be screwed into grooves, or the male body  10  may be made thinner with screw holes on it.