Fig. 6. Brace pivoting pick-up transport arm. Fastening Beam Components A number of approaches to fastening the braces to the caps to construct the 1 m beam were explored, some simply on paper and others by development testing. Concepts which required prefinished holes and insertion of rivets, screws or other similar fastening devices were eliminated due to inherent alignment problems which could exist, depending upon the size of the parts being assembled. Concepts which would result in metal vaporization, such as electron beam or laser welding, were also eliminated. Concepts which involved punching or punch and bending were attempted and eliminated for one of two reasons: (1) they produced debris (selfpiercing rivets or screws) or (2) the fastening technique produced cracking in either the brace or cap material at the fastening points (punch and upset in a fashion similar to a grommet or tab and bend). Ultrasonic welding (Figure 9) and resistance spot welding (Figure 10) seemed to be the only readily available approaches which satisfied all design conditions. Ultrasonic welding yielded inconsistent results and was therefore abandoned. Development tests of series resistance spot welding (Figure 11) gave consistent, predictable results, which led to the clamp and weld mechanisms utilized in the machine (Figure 12) and shown conceptually in Figure 13. Each pair of electrodes is actuated individually by a separately driven cam within each weld block. To minimize peak power, each pair of welds is made sequentially after a set of braces has been clamped in place, first the verticals and then the diagonals, to complete each beam bay. Because the peak power required for resistance welding exceeds the peak power
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