8. 22 is no problem in this case since the procedure makes use of the total vacuum of space. The second problem is that there must be a very small space or gap between the two pieces being welded. In current EB welding this maximum gap ranges from .4 mm to .8 mm. The problem of machining parts to this tolerance, aligning them, and then keeping them aligned while welding is significant. As the welding progresses, the thermal strains generated tend to widen the existing gap. The problem of holding the work in place during welding is difficult on Earth when heavy hold down bars are used,but without these heavy and rigid clamps it is almost impossible (8.12). Thus EB welding can be used to fabricate the sections and bulkheads if this is done on a heavy bed with the use of automated hold down bars. VIII.5.3: Detail of Section and .Bulkhead Fabrication VIII.5.3.1: Lay-up: The heart of the lay-up and fabrication is the lay-up or hold down bed (see Figure 8.5). This is a large flat metal bed 25m x 25m in size to which are attached many automated clamps and hold down bars. The size is chosen to accommod~te the largest hull section (roughly 21m x 22m) and the 4 major stiffeners which attach to the edges of it. Working over the bed are several manipulator arms and two EB welding guns. The manipulator arms pick up the plates and minor stiffeners as they come through the air lock from the manufacturing shack. The manipulator arms then move the pieces of stock to their proper coordinates over the bed with proper angular orientation as directed by the automated control/central information processing system. The hold down clamps below the pieces then reach up and grab them,holding them ready to be welded. The EB welding head then comes overhead and welds the pieces together. The process then continues until the entire hull section or bulkhead is completed. Similarly, the pieces of the major stiffeners (strips 120 mm wide and Sm long) are brought in through the air lock, manipulated into position, and EB welded over the lay-up bed. These major stiffeners have T or X cross sections,depending on their position
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