November 14, 2009

Third Time is the Charm! Or was it the fourth time?



After several nights of work building a total of 16 linear feet of truss in tests of different dimensions I have coalesced on a final design in the third version (actually the forth if we count the first short section of truss using 50 percent reduction.) The drawing below shows the actual Potomac Creek bridge compared to the design of the selectively compressed version I am building. (click here for 1/4 O Scale enlarged version)

The actual bridge had 3 arched sections at 120 feet each, with a short length of truss at each end for an overall length of about 400 feet. The overall dimensions of my bridge are 192 feet long in three arched sections of 64 feet. The actual truss is 20 feet tall, the model is 13 feet. As such, the model truss could not be used as a through truss, as there is insufficient clearance.

Another key change in version three is that I used 3 inch treenails as pins, just as in the
prototype, instead of using the commercial dowels I bought. A 3 inch treenail scales out to 0.0625 inches. The model hobby industry does not sell dowels that small. The nominal 1/16th inch dowel actually measures about 0.085 to 0.090 inches. This meant the holes in the braces and chords to accommodate this size were too large, weakening the braces and looking clunky.

Ship modelers have long faced the problem in needing various sized dowels for masts. Actually they have an even bigger challenge as they frequently need to taper their dowels.

They use a draw plate, such as this one for sale from Model Expo, to reduce the diameter of the dowels. I made my own draw plate by drilling a series of ever decreasing sized holes in a used hack saw blade with 1/16th being the smallest. It worked very well. I laser cut strips at 0.070 wide in 1/16th inch thick basswood. These passed easily through my homemade draw plate to make the 0.062 inch dowels I needed for the treenails.

I have one truss of the third design done. It takes several hours to assemble this truss as there are hundreds of parts, though many of them are the same. I'll post photos later.


Just for kicks, we placed some of my HO scale ACW freight cars on the bridge section. They looked really cool. If I was modeling in HO scale, I could have modeled Potomac Creek full size with no compression! Such are the trade-offs in modeling in a larger scale.


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