Method and apparatus for tipping off refractory metal tubulation
BACKGROUND OF THE INVENTION 
     In the manufacture of the arc tube for a high pressure sodium discharge lamp it is necessary that the tubulation, which extends through at least one of the end caps to permit the evacuation and filling of the arc tube with the discharge sustaining medium, be effectively hermetically sealed off to complete the arc tube manufacturing process. 
     An effective and commercially feasible method for sealing off the refractory metal tubulation of a high pressure sodium discharge lamp arc tube is disclosed in U.S. Pat. No. 3,566,067 to Daniel A. Larson in which two sets of pinching jaws are employed, the first to flatten a section of the tubulation and the second to provide a narrowly pinched section which will serve as a point of separation when a predetermined electrical current is passed through the tubulation. Although this method of sealing off the refractory metal exhaust tubulation of a ceramic arc tube has proved reasonably successful, in some instances because of the very thin section of tubulation at the point of separation and the current levels necessary to cause the melting and separation, some of the metal in this thin area is flashed away resulting, in some instances, in unreliable seals. 
     An improvement on this method of tipping off the refractory metal tubulation of the high pressure sodium discharge lamp arc tube is disclosed in U.S. Pat. No. 4,117,299, issued to John Petro et al., which discloses a single four-directional crimp of the tubulation to form an X-shaped cross section. This latter method provided more metal in the area of the electrical resistance separation and provided an improved seal. 
     Although the foregoing methods reduced the tube diameter at the parting location significantly, the use of a single force to move the metal to a different shape and size provided some weakening and distortion of the metal structure, causing fine cracks both along the melted edge as well as in the remainder of the heavily squeezed area and in some instances has been the source of lamp failures. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention the metal exhaust tubulation is moved from its initial size to a pinched off condition slowly through a plurality of repetitive swaging operations to reduce the diameter of the tubulation to approximately 30% of its original size. In accordance with the present invention the refractory metal tubulation of the arc tube of a high pressure sodium discharge lamp is tipped off after the exhaust and fill operations of the arc tube manufacturing process by inserting the tubulation into a machine having a plurality of pairs of radially extending diametrically opposed tipping jaws extending normal to the tubulation. The plurality of radially extending pairs of tipping jaws are activated sequentially with a plurality of rollers having sequentially increasing diameters thereby causing the tipping jaws of said pairs to repetitively squeeze the tubulation into an increasingly smaller diameter. The tubulation is then removed from the machine and sufficient electric current is passed through the tubulation to cause the separation thereof in the area of smallest diameter. Each of the pairs of radially extending diametrically opposed tipping jaws squeeze the tubulation at least 10 times during this process. 
     The machine for reducing the diameter of the tubulation by the repetitive squeezing action comprises a cylindrical central stationary housing having a plurality of radially extending bores therein. Radially movable elongated tool members or tipping jaws are slidably disposed within each of the radially extending bores with the tool members each having a jaw surface on its interior end and a cam surface on its exterior end. A rotatable housing surrounds the stationary housing and is mounted on and is rotatable about the stationary housing. A plurality of rollers are mounted within the rotatable housing and are constructed and arranged to sequentially contact the cam surfaces on the tool members whereby upon rotation of the rotatable housing about the stationary housing the cam surfaces of the tool members will be contacted sequentially by the rollers causing the jaw surfaces of the tool members to be repetitively driven toward each other thereby applying a squeezing action to a tubular article disposed therebetween. The tool members are disposed in pairs and the axes of the rollers are disposed a uniform distance from the axis of the stationary housing with the rollers divided into sets of rollers of sequentially differing diameters with opposed rollers in each of the sets being the same diameter and move the tool members in each of said pairs an equivalent distance when the diametrically opposed rollers contact the curved cam surfaces of the tool members. The plurality of rollers are also divided into an upper tier of rollers and a lower tier of rollers and each pair of diametrically opposed tool members are contacted by only one of the two tiers of rollers. 
     The repetitive squeezing or swaging of the tubulation by the opposed tool members reduces the diameter of the tubulation to about 30% of its original diameter, essentially closing off the tubulation mechanically prior to the resistance tip-off accomplished by passing a predetermined electrical current through the tubulation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Many of the attendant advantages of the present invention will become more readily apparent and better understood as the following detailed description is considered in connection with the accompanying drawing in which: 
     FIG. 1 is a top plan view of the machine of this invention; 
     FIG. 2 is a sectional view taken along the line II--II of FIG. 1 modified slightly to illustrate contact of each of the sets of tool members by the actuating rollers associated therewith; 
     FIG. 3 illustrates a piece of tubulation prior to swaging; 
     FIG. 4 is a sectional view of the tubulation after swaging by the machine of FIGS. 1 and 2; and 
     FIG. 5 illustrates the tubulation after sufficient electrical current has been passed therethrough to cause separation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawing wherein like reference characters represent like parts throughout the several views, there is illustrated in FIGS. 1 and 2 a swaging mechanism generally designated 30 which is used to reduce the diameter of a piece of tubulation illustrated in FIG. 3 to the configuration illustrated at 34 in FIG. 4. Although the machine can be used to work many different kinds of metal tubulation the machine of this invention is particularly adapted for use in the tipping off of the refractory metal tubulation of the arc tube of a high pressure sodium discharge lamp. 
     In the process of manufacturing the arc tube 36 for a high pressure sodium discharge lamp the ends of a polycrystalline alumina arc tube body are generally closed off by an end cap 38 having tubulation 32 extending through at least one end thereof and carrying at its inner end a discharge sustaining electrode. The tubulation permits, during processing of the arc tube body the evacuation of the interior of the arc tube as well as the addition of the discharge sustaining ingredients as for example a rare earth starting gas and a sodium mercury amalgam. After the arc tube has been exhausted and filled with the discharge sustaining ingredients through the exhaust head 39 the tubulation must be sealed off hermetically. U.S. Pat. Nos. 3,566,967 and 4,117,299 are exemplary of methods by which the tubulation of the arc tube may be hermetically sealed. In each of these methods the arc tube is squeezed essentially in one or two steps to a constricted configuration and an electric current sufficient to separate the tubulation at its restricted area is passed therethrough. 
     In accordance with the present invention, rather than a single crushing press being applied to the tubulation which can affect the metallic structure significantly, a plurality of compressive squeezes are provided by the machine 30 to the tubulation 32 to slowly reduce the tubulation diameter to about 30% of its original diameter. The machine 30 operates on essentially a swaging principle whereby diametrically opposed tipping jaws repetitively squeeze the tubulation to its final configuration. 
     The machine of this invention includes a base or support 40 for the tipping jaw housing 42. The tipping jaw housing 42 is essentially T-shaped in cross section and carries at its upper end two pairs of diametrically opposed bores 44, 46 which communicate with a cylindrical central opening 48 extending through the tipping jaw housing 42. Each of the diametrically opposed pairs of bores 44, 46 carry slidably therein diametrically opposed tool members or tipping jaws A&#39; and B&#39; respectively. 
     Surrounding the tipping jaw housing 42 is the tool member or tipping jaw drive mechanism which includes a bearing housing 50, a roller housing ring 52 and a gear drive 54 which are rotatable in unison about the tipping jaw housing through bearing rings 56 spaced one from the other by an inner spacer 58 and an outer spacer 60. A bearing cover plate 62 secured to the bearing housing by bolts 64 and a locking collar 66 threaded to the tipping jaw housing 42 serve to retain the tipping jaw housing, bearings and bearing housing securely together. Journaled in the roller housing ring 52 are a plurality of roller pins 68 which are retained therein by a pin retaining plate 70. On each of the roller pins 68 there is mounted a pair of vertically aligned rollers designated as the upper tier of rollers A and a lower tier of rollers B which operate to actuate respectively tipping jaws A&#39; and B&#39;. 
     The two tiers of rollers A and B are disposed in four sets, two sets of six and two sets of five which are diametrically opposed and operate to actuate the tipping jaws progressively and sequentially to reduce the diameter of the tubulation 32. 
     Each of the bores 44, 46 has a slot 72 in the top thereof through which extends a vertically oriented pin 74 which is secured in the respective tool members or tipping jaws A&#39; and B&#39;. Each of the tipping jaws A&#39;, B&#39; have on their inwardly extending ends a tipping die 76 with a semi-circular shaped center portion 78 such that when the two tipping dies of the tipping jaws A&#39; meet the cylindrical opening defined by the two semicircular members form the final diameter of the pinch. At the other ends each of the tool members or tipping jaws A&#39;, B&#39; include a cam surface 80 which coacts with the various rollers to provide the squeezing operation. The vertically oriented restraining pins 74 are acted upon by the legs 82 of torsion spring 84 mounted on capped studs 86 which serve to thrust the tipping jaws A&#39;, B&#39; radially outwardly in the bores 44 and 46 against the rollers. 
     The operating rollers are designated 1 through 6, 8 through 12, 14 through 19 and 21 through 25 with the rollers 7, 13, 20 and 26 being merely guide rollers to present the cammed surface 80 to the first of the drive rollers of the next sequence. With the exception of the guide rollers 7, 13, 20 and 26 which are single vertically oriented rolls, each of the operating rollers 1 through 6, 7 through 12, 14 through 19 and 21 through 25 include an A tier and a B tier with, for example, rollers A1 and A14 being of the smallest diameter in the A tier while rollers B8 and B21 are of the smallest diameter in the B tier. The rollers having the largest diameter in each of the tiers are rollers A12 and A25 in the A tier and B6 and B19 in the B tier. 
     In operation, the tubulation is presented to the tipping jaws and the drive gear then drives the roller housing ring through a 180° rotation to accomplish the swaging or squeezing of, for example, a piece of tantalum or niobium tubulation from its original size of 0.125 inch OD with a 0.016 wall thickness to a pinched outside diameter of 0.040 inches. 
     The following chart is illustrative of a specific example of reducing the outside diameter of a 0.125 inch outside diameter tantalum or niobium tubulation to a 0.040 inch diameter which is also the diameter of the circumferential opening when the dies A&#39; or the dies B&#39; are together. The following chart is illustrative of a specific example giving both the diameters of the operating rollers as well as the space between the outer edges 88 of the respective tipping jaws A&#39; or B&#39; when the tipping jaws are closed to their fullest extent by the particular rollers. 
     
         __________________________________________________________________________
JAW A&#39;                   JAW B&#39;                                           
(Operates with top rollers)                                               
                         (Operates with bottom rollers)                   
Jaw    Operating                                                          
             Final                                                        
                  Diameter of                                             
                         Jaw    Operating                                 
                                      Final                               
                                           Diameter of                    
Position                                                                  
       Rollers                                                            
             Spacing                                                      
                  Oper. Rollers                                           
                         Position                                         
                                Rollers                                   
                                      Spacing                             
                                           Oper. Rollers                  
__________________________________________________________________________
Contact      .100 .316                                                    
 1st Squeeze                                                              
       A1 &amp; A14                                                           
             .070 .346    2nd Squeeze                                     
                                B8 &amp; B21                                  
                                      .070 .346                           
 3rd Squeeze                                                              
       A2 &amp; A15                                                           
             .068 .348    4th Squeeze                                     
                                B9 &amp; B22                                  
                                      .066 .350                           
 5th Squeeze                                                              
       A3 &amp; A16                                                           
             .064 .352    6th Squeeze                                     
                                B10 &amp; B23                                 
                                      .060 .356                           
 7th Squeeze                                                              
       A4 &amp; A17                                                           
             .058 .358    8th Squeeze                                     
                                B11 &amp; B24                                 
                                      .052 .364                           
 9th Squeeze                                                              
       A5 &amp; A18                                                           
             .050 .366   10th Squeeze                                     
                                B12 &amp; B25                                 
                                      .042 .374                           
11th Squeeze                                                              
       A6 &amp; A19                                                           
             .042 .374   12th Squeeze                                     
                                B1 &amp; B14                                  
                                      .034 .382                           
13th Squeeze                                                              
       A8 &amp; A21                                                           
             .034 .382   14th Squeeze                                     
                                B2 &amp; B15                                  
                                      .028 .388                           
15th Squeeze                                                              
       A9 &amp; A22                                                           
             .026 .390   16th Squeeze                                     
                                B3 &amp; B16                                  
                                      .022 .394                           
17th Squeeze                                                              
       A10 &amp; A23                                                          
             .018 .398   18th Squeeze                                     
                                B4 &amp; B17                                  
                                      .016 .400                           
19th Squeeze                                                              
       A11 &amp; A24                                                          
             .010 .406   20th Squeeze                                     
                                B5 &amp; B18                                  
                                      .008 .408                           
21st Squeeze                                                              
       A12 &amp; A25                                                          
             0    .416   22nd Squeeze                                     
                                B6 &amp; B19                                  
                                      0    .416                           
__________________________________________________________________________
 
    
     As will be seen from the foregoing, a 180 degree operation of the machine of this invention will cause the 44 operating rollers to squeeze the tubulation 22 times, 11 times by the A&#39; tool members or tipping jaws and 11 times by the B&#39; tool members or tipping jaws to provide for essential the complete closure of the hollow tubulation. The tubulation is then mounted in a conventional jig or indexed to the next position by an automatic machine and the tubulation contacted electrically on each side of the pinch in order that, in the conventional and well-known fashion, sufficient electrical current can be passed through the tubulation to cause heating at the high resistance area occasioned by the pinch to cause electrical separation at that point as illustrated in FIG. 5 at 90. 
     It will be apparent that in a completely automated system two arc tubes may be pinched off by the machine of this invention during one complete revolution of the roller housing ring 52. For example, when operating rollers B6 and B19 complete the final squeeze on a first tubulation and it is replaced by the next, guide rollers 26 and 13 will be in position to present operating rollers A1 and A14 to the tool member or tipping jaw A&#39; opposite from the one upon which they operated during the first 180° of rotation during the processing of the first tubulation, thus starting a complete new sequence of 22 squeezes of a second tabulation during the final 180° of a complete revolution.