8. 42 techniques (i.e., fasteners) suggest a subassembly scheme. By building subsections of the colony from stock on a lay-up bed permitting close alignment, one can take advantage of the EB welding technique. This subassembly procedure (described in Section VIII.5.3) reduces the number of parts to be assembled to 1020 20m x 20m hull sections and 1050 Sm x 20m bulkheads. These are then transported, positioned, and joined together during colony assembly, producing the structure described in Chapter VII: concentric inner and outer hulls, made from 20m x 20m sections bounded by major stiffeners, compartmented by inter-hull Sm x 20m bulkheads. These bulkheads tie into the hulls along the major stiffeners. The issue of joining technique for final assembly now arises. One possibility is the use of EB welding which may be economically feasible on 2070 pieces if not on 2 million. However, the problems are still severe. Section VIII.5.2.2 described the necessary gap for EB welding: .4 to .8 mm. Taking the maximum allowable gap to be .8 mm, let us see what this implies about the fabrication of the sections and bulkheads. Since there are always _at least two pieces that are welded together,this means that the contribution of each to the gap of .8 mm can be no more than .4 mm. For a section 20m by 20m, this requires its linear dimensions to be accurate to .4 mm,or one part in 50,000,and its 20m long edges to be straight to .4 mm or one part in 50,000. Further, the section must be perfectly square or skew to no more than .0023°. It is probably possible to fabricate the sections and bulkheads to these tolerances, assuming their edges are milled as a final step in fabrication. It is even possible to move the section to location and position it with these degrees of precision. However, maintaining the piece in place while welding presents some enormous difficulties. During welding,the area around the weld is heated and undergoes thermal expansion. This expansion tends to push the two pieces apart and widen the gap between them. In surfaces of compound curvature (as is the case with over 50 % of the hull area) the heating causes expansion in the plane of the work piece and deformation of the curvature of the edge being welded.
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