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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to improvements in suspended ceiling grid construction and, in particular, to improvements in connector clips for ceiling grid members. 
       PRIOR ART 
       [0002]    Suspended ceiling grid members or runners typically comprise relatively long main runners and shorter cross runners. Both types of grid runners have connectors to join their ends to the ends of like members to construct a ceiling expanse of greater size than the length of individual main or cross runners. These end connectors, as the industry has advanced, are typically separate clip elements permanently attached to the grid runners themselves. The end connectors or clips are metal stampings, ordinarily of steel, formed with features that enable them to couple with identical units when one connector is pushed endwise into a lap joint with an opposing end connector. Depending on the clip design, the clips may directly abut or may have runner web areas disposed between them. In general, the features stamped or otherwise formed into a connector that establish a connection are a lateral projection and the edge of a hole. The projection of one connector is received in the hole of the opposing connector and, vise versa, the projection of the opposing connector is received in the hole of the one connector. The result is a joint with two locks. In practice, a connection may fail under tension at a force level substantially below the design or normally expected capacity of the joint. This can occur when the joined connectors slip sideways and disconnect one lock engagement resulting in a severe reduction in the load capacity of the joint. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention provides an end connector clip for suspended ceiling grid runners with improved clip-to-clip locking action. The improved locking function, in accordance with the invention, is achieved by orienting the locking surfaces with an angle relative to the plane of the clip body proper greater than what results from conventional practice. A preferred manner of forming the inventive locking surfaces is by increasing the clearance between the punch and die elements that create the locking surfaces. This technique, it has been found, develops an orientation of a locking surface that, in use, counteracts forces that tend to laterally separate mating locking surfaces of a pair of coupled clips which otherwise could result in a major loss of retention force. Ideally, the inventive technique is applied to both a locking projection and a projection receiving area of the clip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]      FIG. 1  illustrates portions of grid members, in the conventional form of inverted tees, for suspending ceiling panels; 
           [0005]      FIG. 1A  is an enlarged fragmentary perspective view of the locking surfaces of a grid member end connector or clip; 
           [0006]      FIG. 2  is a side elevational view of the clip and end portion of a grid tee; 
           [0007]      FIG. 3A  is a diagrammatic presentation of the locking areas of the clip; 
           [0008]      FIG. 3B  is a diagrammatic presentation of tooling, in vertical alignment with  FIG. 3A , used according to the invention to make the locking surfaces of the clip; and 
           [0009]      FIG. 3C  shows a cross-section of portions of joined clips in a longitudinal plane transverse to the planes of the main body of the clips. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0010]    Referring now to the drawings and in particular to  FIG. 1 , portions of generally conventional suspended ceiling grid runners, in the form of tees  10 ,  11 , are depicted. A main tee  10  has a vertically oriented slot  12 , one of many at regularly spaced intervals along its length in a central web  13 . End portions of opposed cross tees  11  are positioned in line with the main tee slot. The tees  10 ,  11  are preferably roll-formed from light gauge sheet metal stock as is customary. The main tee  10  can have a typical length of 10 or 12 feet or metric equivalent and the cross tees  11  can have lengths of 4 feet, 2 feet, and 1 foot, or metric equivalent. The cross tees  11  have identical end connectors or clips  14  fixed to their ends such as by staking portions of the tee sheet metal stock through holes provided in the connectors. 
         [0011]    The illustrated connectors  14  are of the general type disclosed in U.S. Pat. Nos. 5,517,796 and 5,761,868, the disclosures of which are incorporated herein by reference. Typically, the connectors  14  are stamped from steel sheet stock that is stronger and harder than that of the tees  10 ,  11 . The numeral  16  indicates the forward end of a connector  14 . 
         [0012]    When two connectors  14  are positioned from opposite sides of the main tee  10  into a common slot  12 , they form a joint of their respective cross tees  11  by establishing a double connector-to-connector lock. The relationship between a pair of joined connectors  14  is analogous to a handshake. More specifically, when clips  14  are joined they lap one another, preferably in direct abutment. The clips  14  are locked together when a lock area  17  of one clip  14  snaps or is otherwise received behind a forward one of two opposing projections  18 ,  19  stamped into the body of the other clip  14 . This same action occurs where the corresponding lock area  17  of the other clip is received behind the forward projection  18  of the one clip  14 . With both sets of lock areas  17  and projections  18  engaged, a double lock clip connection is established. The lock area  17  and projection  18  interengagement serves to resist tensile loads on the associated cross tees  11  tending to separate them and under proper conditions can sustain relatively high forces. 
         [0013]    Experience reveals that a joint between a pair of clips  14  will separate under relatively low forces if one of a set of locking area  17  and projection  18  slips laterally, i.e. perpendicular to the planes of the clips  14 . This can leave only one lock set between a lock area  17  and projection  18 . 
         [0014]    Such sidewise slipping may result, inter alia, from variations in the clip material, the clip manufacturing process, deviation from an ideal clip shape, installation technique, and eccentric forces imposed on the joined clips or combinations of these factors. 
         [0015]    The failure of a lock set by lateral movement between lock area  17  and projection  18  is related to the orientation of their respective contacting edges, designated  21 ,  22 . The closer these edge surfaces  21 ,  22  are to lying in planes that are perpendicular or are obtuse to the planes of the clip bodies, the greater the risk that they will separate laterally. Locking surfaces with such orientations have little or no resistance to forces tending to laterally separate the clips  14  and when the angle is measurably obtuse a reaction force is developed by the locking surfaces in response to a tensile force between the tees that may actually cause the clips to spring laterally apart and out of contact. A locking edge surface of a projection corresponding to the projection  18  when produced with conventional practice is prone to assume an obtuse angle relative to the plane of the clip. When this edge surface is originally formed by stamping a hole in the plane of the original sheet stock forming the clip it can be slightly acute, i.e. less than 90 degrees. However, when the projection is thereafter formed out of the plane of the main part of the clip body, the edge surface can be drawn into an obtuse orientation. 
         [0016]      FIGS. 3A and 3B  are diagrams projected vertically relative to one another illustrating aspects of the invention. In  FIG. 3A , the lock area  17  is rearwardly bound by a locking area edge surface  21 . Also in  FIG. 3A , the eventual projection  18  and an associated lock edge  22  are indicated. 
         [0017]    As shown in  FIG. 3B , punch elements  26 ,  27  cooperating with die sections  28 ,  29  form holes  31 ,  32 , respectively. A forward edge or boundary of the hole  31  is the lock area edge  21  and a forward edge of a bow tie shaped hole  32  forms the projection lock edge  22 . 
         [0018]    In  FIG. 3C , the forward ends of a pair of mating clips  14  are diagrammatically illustrated. The images of  FIGS. 3A and 3B  correspond to the clip  14  on the left in  FIG. 3C . 
         [0019]      FIG. 3B  shows, on an exaggerated scale, a high degree of clearance between the punches  26 ,  27  and die openings  33 ,  34  at locations corresponding to the lock edges  21 ,  22 . As a general rule in the metal stamping industry, a punch is slightly smaller than the hole or spaced from the die or cutting edge it operates with. Typically, the clearance between the punch and die at a side of a hole is about 8% to 10% of the thickness of the material being pierced. A hole punched in a metal sheet by a punch and die generally has a diameter or hole size at the punch side equal to the punch and at the die hole side equal to the diameter or size of the die hole. This means that the punched hole, if round, is actually slightly tapered, i.e. conical, across the thickness of the sheet material or if the hole has a different configuration its walls are tapered from the size of the punch to the size of the die hole or die edge. 
         [0020]    It has been discovered that by significantly departing from traditional practice and increasing the clearance between the punch elements  26 ,  27 , and die openings  33  and  34 , the angularity of the lock edges  21 ,  22  can be advantageously increased. For example, the clearance between the punch elements  26  and  27  and their respective die openings  33 ,  34  corresponding to the lock edges  21 ,  22  can be about 25% of the thickness of the sheet metal used to form the connector or clip  14 . The illustrated clip  14  can be formed of 0.015/0.017 inch high tensile steel (160 KSI), stress relieved or type  301 / 302  stainless steel, half hard.  FIG. 3B  shows that the punched or sheared locking edges  21 ,  22  are in planes forming acute angles with respect to the side of the clip engaged by the clip  14 R on the right. These edge surface angles are retained in the finish form of a clip  14 . In the case of the projection locking surface  22  which is stamped up out of the main plane of the clip body, the angularity, i.e. deviation from perpendicularity to the clip body, may be somewhat diminished but still prominent. 
         [0021]      FIG. 3C  shows that the angles of the locking edge  21  of the lock or receiving area  17  of one clip and the projection lock edge  22  of the other clip are complementary. Moreover, the angles of these surfaces  21 ,  22  create a force component biasing the clips  14  together when a tension force exists in the pair of tees  11  connected to the clips. Consequently, clips  14  with the acutely angled locking edge surfaces  21 ,  22  significantly increase the reliability of a connection. The clips  14  are less susceptible to separating at one lock area and then failing at a reduced tension level. 
         [0022]    Those skilled in the art will recognize the applicability of the invention to main tee clips such as shown, for example, in U.S. patent application Ser. No. 11/135,058 and U.S. Pat. No. 6,523,313. In the clips shown in U.S. Pat. No. 6,523,313, the material of the tee web is interposed in the area of the locks; nevertheless, the invention has application in such constructions where the connectors, while separated by grid runner stock, are lapped with one another and the locking edges serve the same function as described herein. 
         [0023]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

Summary:
An end clip for joining runners of suspended ceilings by coupling with an identical clip, the clip being stamped from sheet metal stock with lead and trailing ends, a lateral projection and a projection receiving area behind the lead end, the clip being arranged such that when an identical clip oriented in the opposite direction of the clip and caused to laterally overlap the clip the projection of the clip is locked in the receiving area of the identical clip and, vice versa, at least one of the projection and projection receiving area having a rearward facing sheared edge forming an acute angle with the clip plane resulting from being sheared with tooling having a clearance between tooling substantially greater than 10% of the thickness of the sheet from which the clip is stamped.