Hollow granite box beam and method of forming
BACKGROUND OF THE INVENTION 
     This invention relates to beams of igneous rock. 
     It has long been known that igneous rock materials are extremely stable. Granite, for example, has a very low coefficient of thermal expansion, and very high tensile strength. As such, granite and like igneous rock materials have been widely accepted in the building and industrial arts where a material of high stability is required. 
     In particular, beams of granite have been used to fabricate precision guide surfaces for various precision instruments, such as coordinate measuring machines and circuit board drilling machines. Granite heretofore, however, has not proved to be a completely satisfactory material for such uses, largely because of its weight. Granite is very dense, and therefore a solid granite beam of a given size significantly outweighs an analogous hollow beam of steel, aluminum, or cast ceramics. Manufacturers of precision instruments have increasingly been using these materials in lieu of granite on account of lower transport costs, stress tolerances, and other weight-related factors. These materials, however, have also proved to be unsatisfactory. Cast ceramics, for example, has similar tensile strength and thermal expansion properties as igneous rock, but are generally much more expensive. On the other hand, steel and aluminum have higher coefficients of thermal expansion. 
     SUMMARY OF THE INVENTION 
     A hollow box beam of igneous rock comprises two component blocks, at least one of which has had igneous rock material removed to form an interior cavity. After the cavity is formed, the two component blocks are bonded together such that the interior cavity becomes an enclosed cavity in said box beam, thus forming a hollow box beam. 
     The method for constructing this box beam comprises forming at least one flat surface on each of the two component blocks, with each of these flat surfaces having identical dimensions for later mating and bonding. Igneous rock material is then removed from the flat surface of one or both these component blocks so as to form at least one open cavity in the component block. The thickness of the cavity wall is determined by the amount of material removed from the component block. Thus the thickness of the cavity wall is predetermined by the remover at the time of construction, and is not necessarily uniform though it may be so. The two component blocks are then mated and bonded together along the remaining unremoved portions of the flat surfaces, thus forming the box beam. 
     In another aspect of the invention, the two component blocks are formed by dividing a single elongated parent block in a direction parallel to the longitudinal axis of the parent block. The required flat surfaces are thus created by the division of the parent block into the two component blocks. 
     In yet another aspect of the invention, the flat surfaces of at least one of the two component blocks is ground such that there exists unremoved portions, constituting a gusset, positioned in the cavity so as to intersect with the other component block upon bonding. This gusset enhances the overall strength of the box beam after bonding. 
     In yet another aspect of the invention, a plurality of box beams are constructed from the two component blocks. This is accomplished by removing the igneous rock material from at least one of the component blocks so that a plurality of interior cavities are provided in each of the component blocks, each of these interior cavities being separated by unremoved divider portions having a width greater than that of the predetermined thickness of the cavity walls. After the igneous rock material has been removed the component blocks are separated into the respective box beams by dividing the component block longitudinally at approximately the center of the unremoved divider portions to form first sections of each of the plurality of box beams. The other component block is then longitudinally divided so as to form a second section of the plurality of box beams. The dimensions of each interfacial surface of the second sections are to be substantially identical with those of the corresponding first section. After the component blocks have been divided, the respective first and second sections of the plurality of box beams are bonded together along their respective interfacial surfaces. A gusset may be provided in this embodiment of the invention as well. The igneous rock box beam manufacturing system disclosed herein produces relatively low cost beams with intrinsic stability and reduced weight comparable to ceramic or metal beams. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective diagram illustrating the components of a granite box beam constructed according to the invention. 
     FIG. 2 is a schematic perspective diagram illustrating a variation on the box beam of FIG. 1 including a central gusset. 
     FIG. 3 is a schematic diagram illustrating one way of forming body and top blanks for the component blocks of FIG. 2. 
     FIG. 4 is a schematic diagram illustrating the grinding operation for forming the gussetted body block of FIG. 2. 
     FIG. 5 is a schematic perspective diagram illustrating the finished hollow granite box beam of the embodiment of FIG. 2. 
     FIG. 6 is a schematic perspective drawing illustrating a system for making a plurality of box beams like that of FIG. 2 from unitary blocks. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The manufacturing techniques described below are designed specifically for use in making hollow granite box beams for horizontal arms in coordinate measuring machines, for example, as a substitute for the solid granite cross beam in the Starrett RMS 1620-12 coordinate measuring machine Starrett Catalog No. 28, p. 443-444 2d ed. incorporated by reference herein. The system is applicable, however, to the manufacture of box beams for other applications. 
     As shown in FIG. 1, two elongated blocks 1 and 2 of granite or other igneous rock are shown. One component block 1 has had granite removed from the center by means of grinding, for example, whereby a central cavity is formed 3, surrounded by boundary walls 4, 5, 6 each having a predetermined, though not necessarily uniform thickness. The unremoved portions of the flat surface of the first component block 7, 8 are precision ground as is the mating underside of the second component 2. The precision ground areas are then gently sandblasted to frost the surfaces. Both surfaces are then bonded using a high tensile strength epoxy, such as Plast No. 67-A-83, available from Fibre Glast Development Corporation. 
     The component blocks 1, 2 may be from either separate pieces of granite, or from a single parent block. In the latter case, the parent block would be divided, preferably by a single saw cut, to provide the flat surfaces on both component blocks. 
     The preferred embodiment of FIG. 2 differs from that described above in that an unground portion 20 is left in the central region of the interior cavity of a first component block 17. This unground portion constitutes a strengthening gusset which is precision ground and sandblasted-- along the exterior unground portions 21, 22, and is to be co-planar with the exterior unground portions 21, 22, i.e., in that the plane, so that when the second component block 18 is bonded to the first 17, the gusset 20 is bonded by epoxy with the second component block 18, as are the exterior unground portions 21, 22. 
     As shown in FIG. 3, a single unitary parent block 16 is divided into a flat slab 18 and a body block 17 by cutting with a diamond saw blade 30 (rotary or band saw) on a work table 34. The cavities in body block 17 are then formed by a rotary diamond saw or a grinding wheel 36 as shown in FIG. 4. The plane of rotation of the wheel is parallel to the length of the block and to the boundary and gusset walls being formed. The block 17 is channelled all the way from one end to the other by running the grinding wheel 36 over the length of the block and then stepping the block laterally to a position where the next cut of the grinding wheel will overlap the preceding cut. Finally, the mating surfaces of the block 17 and slab 18 along their original interface are precision flat ground and sandblasted. The block 17 and slab 18 are then bonded together along their original interface to form the hollow granite gussetted box beam 37 as shown in FIG. 6, mounting holes 37a are preferably drilled before the steps of FIGS. 3 and 4 for attaching other equipment for a coordinate measuring machine, for example. However, the holes can be drilled afterwards, if desired. 
     After the beam has cured, the entire beam is precision surface ground and precision lapped. 
     The dimensions of a typical hollow granite block formed according to the invention for use in the previously identified coordinate measuring machine are as follows: 
     
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length l:           371/2   inches                                        
width w:            11&#34;     inches                                        
height h:           3 5/32  inches                                        
wall thickness      0.75    inch                                          
slab 18 thickness   0.75    inch                                          
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     The ground walls and gusset in block 17 are preferably the same thickness as the slab 18. 
     In another embodiment, as illustrated in FIG. 3, a plurality of box beams, in this case three, 41, 42, 43, are prepared from a single component block 38. The granite is removed from the central cavities of the component block, as in the above described embodiments, except that the unremoved divider portions 44 and 45 have an unground portion with a width greater than that of the other unground portions (the gussets). For example, if the gussets are 0.75 inch, the divider portions would be 1.50 inch plus the saw kerf. Like the above described embodiments, all interfacial surfaces 46-52 are precision ground to be coplanar and are then sandblasted. On the second component block 39 the underside (not shown) is likewise precision flat ground and sandblasted for bonding. The various box beams are then separated by sawing at approximately the midplane of the unremoved divider portions. See dashed lines on component block 38. Likewise the second component block 39 is divided into second sections of the final box beams along dashed lines on component block 39 with the dimensions of the second sections corresponding identically with the respective first sections of the plurality of box beams. 
     Other embodiments are within the following claims.