Sunday, March 28, 2010

#002 07 Clear Creek Timber Howe Truss Bridge - The Drawings Part 2


So here is where I am with the drawing.  Determining the dimensions from the photos has been rather difficult, so I'm staying close to Harry Brunk's drawing.  When I finally evaluated the pitch of the trusses (35 degrees) I was able to draw in the trusses and figure out the rough length.  Seems Harry's drawing is not that far off.  Will I ever be able to draw it precisely?  No, I'm afraid not.

Click on Drawing to Enlarge

B.  Set up the drawing.  At this point I decide what scale to draw the drawing in and set up the Cad drawing space.  We'll draw it in O-scale since I can actually print it out full size.  I'll reset the grid (the graph lines) to be one foot apart, with a darker four foot line for reference.  I'll make the whole drawing area 70 feet long which gives me extra room for notes and dimensions.  Then I'll set the layers of the drawing.  We'll have the main layer (turning of the track component option), dimensions layer and a text layer.  I always keep the text separate so that I can remove it easily for drawing.  The last stage is to put in my drawing text block that tells what the drawing it, the scale, who did it and the revision.  As I draw I save the file like this.  For starters I use 1.001.  The first one is the major revision number.  The next three numbers are the drawing numbers.  As a rule I save a new revision drawing every 15-20 minutes, or when I make a major change in the drawing.   The file is now saved as Clear Creek Howe Truss Bridge 1.004 since I've made a few mods to the drawing already trying to figure out the length.  For more information on Cad drawing I recommend you join one of the many Cad specific Yahoo groups.

C.  Draw the side truss frame timbers.  As this is probably the most complicated part of the drawing, so we'll tackle it first.  This will be the diagram for building both side truss frames and we'll build on the drawing so it must be very accurate.  One thing I like about making my own drawings is that it forces me to understand the construction and to build the bridge in my head.  Assembly goes much faster this way and I make fewer and less costly mistakes.  We won't need the rod trusses underneath or the

D.  Draw in the details.  I like to complete detail a model piece while I can keep it flat on the table.  It is easier to drill and glue on a horizontal surface.  So I'll go ahead and draw in all the nbw castings.  There will be TWO sides no, an OUTSIDE and an INSIDE.  That is the fun of a Cad system because it is easy to copy and paste.  First I'll have to draw an nbw or two, then some side views, then just past them in place.  One of the things you have to watch is "what am I connecting with these bolts?" While checking the picture vs the photos I've already found that along the top beam there should be two nbw's between each truss, not one.  So we'll correct it on our drawing here.

E.  Make a list of the materials needed.  Since I'm building this in component sections and am familiar with the parts right now, I'll go ahead and make a list of materials and put it on the drawing.  This makes it into my own kit.

Material for two truss side frames.
  • 9 x 18 - 400 scale feet (100 inches)
  • 9 x 9 - 900 scale feet (225 inches)
  • 3" nbw - 28
  • 2" nbw - 100 (some extra)
  • Brass (Piano) wire - 168 scale feet (42 inches)

That's all for today.  We'll start on the base of the bridge tomorrow.

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