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Modeling the B&O’s St. George float bridge


Once the pillow blocks were dry, and keeping the pieces correctly oriented, I turned the chords upside-down and marked the locations of the floor beams. Again, doing all of them at the same time ensured uniformity. After placing a pair of chords, one upper and one lower, in yet another jig, I temporarily installed a set of diago- nals in each of the end panels, then measured for the height of the vertical posts. The vertical and diagonal mem- bers were glued into place starting with the vertical posts at the ends. There are three “layers” of diagonal timbers. First, all the “far side” diago- nals were added, taking care to reverse the direction at the mid-point of the truss. I also added the third vertical post (along with the extra one). Making sure the pieces were down tight in the jig, I let everything dry for a bit. Going back to the end where I start-


ed, I began gluing the “middle layer” diagonals into place. They run in the opposite direction to the first ones and also reverse their direction at the mid- point of the truss. I now had a series of X-bracing the length of the truss. I fin- ished by adding the third, or “near side” diagonals, which run the same as the first layer. When the glue set I carefully removed the truss from the jig and laid it on something flat. I


placed a piece of ³₄″ thick plywood over it, then some heavy weights evenly dis- tributed over the length of the truss. This was left for several days for the glue to cure thoroughly. Each truss was made in this manner. The holes for the “iron” tie rods (brass rod installed later) were drilled next. When done, the two “outer” mem- bers of each set of X-bracing sloped up- wards, towards the center of the truss, with the single center diagonal sloped in the opposite direction. The next step was by far the most te- dious of the project, and it also made me quite popular with my local hobby shop. I ordered 1,500 or so Grandt Line nut-bolt-washer castings in several sizes for detailing the trusses. Foolish- ly, I assumed that this number would be more than enough. At the same time I also ordered a dozen each of the drill bits needed to install them. A few weeks later I was back at the hobby shop making a similarly-sized Grandt Line order, plus one for more drill bits. I estimate that there are between 1,000 and 1,100 bolts on each truss; countless castings ended up on the floor, to say nothing about the apron and finger piers. Since I used a pin vise to drill the holes the local drug store also made out well selling me the small-size Band Aid®


brand adhesive


bandages to cover blisters on my fin- gers. (I have since purchased a small modeler’s drill press.) I held off adding any of the n.b.w.’s on the top of the trusses until the floor was completed. Attaching all these n.b.w.’s is really quite easy (once the holes are drilled, of course). After drilling the holes, I re- moved a bunch of the bolts from their sprues, leaving as much as possible of the “bolt” attached to the casting. With a pair of fine-pointed tweezers, I picked up a bolt just under the washer and in- sert it into the hole. Then, with the point of the tweezer, I pushed it down until the casting was seated flush on the surface. After doing this a few times, I took a fine-pointed 000 or 0000 brush and, after dipping it in Tenax 7R or something similar, carefully touched it to the wood next to a casting. The wood absorbed some of the glue and the plas- tic shank adhered to the wood. I positioned the three trusses upside- down at the correct spacing, holding them in place using weighted blocks. Checking to make sure they were plumb and parallel to each other,


I


glued four floor beams into place on their marks, spacing them evenly along the length of the bridge. I let these set up, removed the blocks, and placed the whole structure right-side up on a flat surface. Checking again that the truss-


64


DECEMBER 2011


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