Abstract:
Tools and method for aligning a first polyhedral workpiece with a central axis and for joining the first workpiece with a second workpiece also aligned with the central axis. The aligning tool includes two sets of urging members offset from one another along the central axis of the first workpiece, each set including urging members biased to urge the first workpiece in a direction transverse to the central axis and opposite from the bias direction of the other set. The invention is useful for example in aligning and joining a rotatable prism to a rotor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to tools and methods for aligning and assembling workpieces, and more particularly relates to such tools and methods for aligning and assembling a poly-plane workpiece and a base member along their central axes.  
           [0002]    A single panel scrolling color projection display system is characterized by a single light modulator panel such as a liquid crystal display (LCD) panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while the illuminated rows of pixels are synchronously addressed with display information corresponding to the color of the then incident stripe. See, for example, U.S. Pat. No. 5,410,370, “Single panel color projection video display improved scanning” issued to P. Janssen on Mar. 25, 1994, and U.S. Pat. No. 5,416,514, “Single panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valve” issued to P. Janssen et al. on May 16, 1995.  
           [0003]    Light engines for such single-panel color projection display systems commonly utilize spinning prisms to generate the scrolling illumination bars or stripes. These prisms typically are polyhedrons having four plane sides substantially parallel to a central axis of rotation, and a square cross-section. Although the prisms must be centered with great precision to the axis of rotation of the scanner motor, this is difficult in practice due to the large dimensional tolerances associated with a low-cost manufacturing process.  
           [0004]    This centering is presently carried out by clamping the prism into the chuck of a lathe, measuring the (lateral) runout, adjusting the chuck and again measuring the runout. This process is repeated until the prism is centered within tolerance. Next, adhesive is applied to the end face of the prism adjacent to the rotor. The latter is held centered (by its shaft) in the lathe&#39;s stock. The stock is then moved forward until contact is made between the rotor and the prism is made. After the adhesive is cured, the assembly is released. This method tends to be cumbersome and time-consuming, and is thus unattractive for producing a large number of prism/rotor assemblies.  
         SUMMARY OF THE INVENTION  
         [0005]    In accordance with the invention, polyhedral workpieces having at least three plane sides substantially parallel to a central axis are aligned in an alignment tool having two sets of urging members biased in opposite directions to urge the workpiece into axial alignment with the central axis of the alignment tool.  
           [0006]    According to one embodiment of the invention, the alignment tool includes a mounting surface adapted for mounting a fixture, the fixture adapted for holding a second workpiece in contact with one end of the polyhedral workpiece for the purpose of assembly of the two workpieces.  
           [0007]    In another embodiment, an assembly tool is provided having first and second fixtures, the first fixture for holding the aligning tool with the first workpiece and the second fixture for holding the second workpiece, at least one of the first and second fixtures moveably mounted with respect to one another for moving the two workpieces along their aligned central axes into contact for assembly.  
           [0008]    In accordance with another aspect of the invention, a method of joining the workpieces is provided, including the steps of:  
           [0009]    a. aligning the central axis of the first workpiece with a reference central axis by urging the first workpiece into alignment by means of two sets of urging members positioned to exert balanced urging forces against opposing sides of the first workpiece, the two sets of urging members displaced from one another along the central axis of the workpiece and exerting the urging forces in opposite directions transverse to the central axis of the workpiece;  
           [0010]    b. aligning the central axis of the second workpiece with the reference central axis; and  
           [0011]    c. bringing the two workpieces into contact for assembly.  
           [0012]    In a prefered embodiment, the workpieces are joined with a radiation-curable adhesive layer, either or both of the workpieces are optically transparent, and light is directed through either or both of the workpieces to the adhesive layer to promote curing of the adhesive.  
           [0013]    The align-and-assemble method and tools of the invention enable centering and assembly of a polyhedral workpiece with another workpiece, regardless of geometrical imperfections, in much less time than that needed presently for the adjust-measure-repeat method. Use of a fast curing adhesive, such as a UV-initiated or UV and heat curable adhesive to join the workpieces can further reduce processing time. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0014]    [0014]FIG. 1A is a schematic view along the central axis of a cylindrical alignment tool of the invention, showing the relationship between two sets of oppositely-biased aligning spheres of the tool and a plane-sided workpiece (prism) of nominally square cross-section;  
         [0015]    [0015]FIG. 1B is a detailed perspective view of the biasing arrangement for a pair of oppositely biased spheres from different sets;  
         [0016]    [0016]FIG. 2 is a side view of an assembly tool of the invention, including the alignment tool of FIG. 1, and a mating fixture holding a second workpiece (rotor) in contact with the prism via a glue layer;  
         [0017]    [0017]FIG. 3 is a section view along the central axis of another embodiment of the alignment tool of FIG. 1, showing diametrically opposed aligning spheres maintained in position by a ribbed cage;  
         [0018]    [0018]FIG. 4 is a view similar to that of FIG. 1, showing the relationship between one set of aligning spheres and a prism of rectangular cross-section;  
         [0019]    [0019]FIG. 5 is a perspective view of a cage similar to that shown in FIG. 3, in which the aligning spheres have been replaced by cylindrically-shaped members; and  
         [0020]    [0020]FIG. 6 is a schematic elevation view of one embodiment of an assembly tool of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    Referring now to FIG. 1, a view along the central axis A of one embodiment of an alignment tool  10  of the invention, an alignment chamber  11  defined by a cylindrical wall  12  contains two sets  13  and  14  of spherical urging members  13   a  through  13   d  and  14   a  through  14   d , respectively. These urging members  13  and  14  are all of approximately equal size and all contact both the inner surface  12   a  of wall  12  and one side of a four-sided prism  15 . As will be appreciated, the more accurate the dimensions of inner surface  14   a  of chamber  14  and the urging members  13  and  14 , the more accurate will be the alignment of the central axis W of workpiece  15  to central axis A of alignment tool  10 .  
         [0022]    As best seen in the side view of FIG. 2, the two sets of urging members  13  and  14  are displaced from one another along the central axis W of workpiece  15 . Alignment of workpiece  15  is realized by biasing the two sets of urging members  13  and  14  in opposing directions transverse to the central axis W. Such biasing is achieved by means of springs, which push the spheres, through levers, against the sides of the prism  15 , forcing it to the center of the chamber  11 . Since the centering happens independently in two different locations, as shown in FIG. 2, the prism  15  will be centered along its entire length, even though it may have a rectangular cross-section, as shown in FIG. 4, or be tapered along its length.  
         [0023]    [0023]FIG. 1A shows one embodiment of a biasing arrangement in which a set of four springs  16   a  through  16   d  and eight levers represented by lines  17   a  through  17   h , bias both sets of spheres  13  and  14 . FIG. 1B is a schematic perspective view showing the biasing arrangement for two of the spheres  13   a  and  14   a . Each sphere is retained in a cage, each cage ( 21 ,  22 ) having a top ( 21   a ,  22   a ), bottom ( 21   b ,  22   b ) and side ( 21   c ,  22   c ). The bottom  21   b  of cage  21  and the top  22   a  of cage  22  extend through slots ( 23 ,  24 ) in wall  12 , and then bend downward and upward, repsectively, to form brackets ( 21   d ,  22   d ). Spring  16   a  is attached to brackets  21   d  and  22   d , resulting in a biasing force on sides  21   c  and  22   c , which act as levers corresponding to lines  17   a  and  17   b . This arrangement results in a bias of equal and opposite force on spheres  13   a  and  14   a  against sides  15   a  and  15   b  of prism  15 , respectively.  
         [0024]    [0024]FIG. 2 shows one embodiment of an assembly tool which is achieved by mounting a fixture  18  onto one end of wall  12 . Fixture  18  has an indented ledge  18   a  dimensioned to fit snugly against mounting rim  12   b  of wall  12 . Fixture  18  has a through hole  18   b  aligned with central axis A and dimensioned to accept spindle  19   a  of a second workpiece, rotor  19 . Since rotor  19  has a central axis R running through the center of spindle  19 , mounting of the fixture results in alignment of axes A, W and R.  
         [0025]    Mounting of the fixture  18  at one end of alignment chamber  11  brings mounting surface  19   b  of rotor  19  into close proximity to mounting surface  15   f  of prism  15 . Moving prism  15  slightly toward rotor  19  closes the gap and brings it into contact with glue layer  20 . Preferably, glue layer  20  is of a UV-curable adhesive, and curing is achieved by introducing a UV beam at surface  15   e  of prism  15 . The beam is then transmitted by total internal reflection from sides  15   a  through  15   d  and then through surface  15   f  to glue layer  20 .  
         [0026]    Preferably, a dual cure adhesive (ie, UV and heat curable) is used. This enables a short exposure in the alignment tool, followed by a full heat (and/or UV) cure in an oven. The latter cure can be a batch process, thus requiring minimal handling.  
         [0027]    [0027]FIG. 3 shows another embodiment of a biasing arrangement for an alignment tool  30  of the invention. In this arrangement, only two spheres ( 31   a ,  31   b ) are employed in each set. These spheres  31   a  and  31   b  are kept in diametrically opposed relationship by a rotatable cage  32  having ribs  32   a  and  32   b , attached to levers  33   a  and  33   b , which extend through apertures  34   a  and  34   b  of wall  34  and are attached to springs  34   a  and  34   b . Ribs  32   a  and  32   b  bear against the sides of the spheres through the biasing action of the springs.  
         [0028]    The arrangement of FIG. 3 of opposing spheres can be employed for any polyhedral workpiece having an even number of sides, so long as the opposing forces are balanced. For example, three spheres in each set may be used for a hexahedral workpiece and four spheres in each set for an octahedral workpiece.  
         [0029]    The urging members need not be spheres, so long as they have a circular cross-section transverse to the central axis of the tool. FIG. 5 shows a set of cylindrically-shaped urging members  50   a  and  50   b , urged against sides  15   a  and  15   c  of prism  15  by ribs  51   a  and  51   b  of cage  52 .  
         [0030]    [0030]FIG. 6 illustrates schematically one embodiment of an assembly tool  60  of the invention. Slidably mounted on base  61  is a pair of opposing fixtures  62  and  63 , each having a mounting chuck ( 62   a ,  63   a ) precisely aligned with a central axis A. When alignment tool  64  is mounted on fixture  62 , previously aligned workpiece  65  has its central axis W aligned with central axis A. At the opposite end of the base  61 , mounting of a second workpiece  66  on fixture  63  using chuck  63   a  results in its central axis R being aligned with central axis A. The two fixtures  62  and  63  are mounted on base  61  to slide longitudinally parallel to the central axis A, thus enabling the two workpieces to be brought together for assembly while maintaining their mutual alignment.  
         [0031]    The invention has necessarily been described in terms of a limited number of embodiments. From this description, other embodiments and variations of embodiments will become apparent to those skilled in the art, and are intended to be fully encompassed within the scope of the invention and the appended claims.