Monday, March 7, 2011

How much difference could 1/4" make?

Glad you asked. Take a look at this. 

To the right you can see the handy-dandy little hole locating gage that I made for trunnel hole layout on the lattice web intersections.
See anything "out of place" in the picture above? Time for a little puzzler. #1? What is the actual size of a  1 x 4 board from the lumber yard? If you said 3/4" by 3 - 1/2" you are correct. Keep going. For the full size South Wayne bridge the lattice will be made from rough sawn 2" x 10" material. #2? In 3/8 scale what should the size of wood for lattice pieces be?  If you got out your slide rule and converted 3/8 to .375 and then multiplied that by 10" and said 3 - 3/4" you are absolutely correct again!  The difference between the width of the 1x4's I purchased from the lumber yard for the 3/8 scale model and what I really needed? You guessed it 1/4".  I dismissed this "slight difference" as no big deal.

Had you been working with me you could have pointed out the folly in this before, I learned it the hard way. You can see that the top hole is way too close (actually 3/8" in 3/8 scale) to the edge of the material. Arnold A. Graton advised me to keep 2" ( that's 3/4" in 3/8 scale) of material beyond the outside diameter of the trunnel and the edge of the piece.  The bottom hole is the way it should look.

Ah, there it is. Looks much better with a little more "meat" between the hole and the edge of the wood piece. Don't you agree? Much more strength.
 With the help of my trusty drill guide (used 1" PVC pipe and a couple of mounting blocks) I got down to the serious business of drilling over 120 - 5/8" diameter holes straight and true.
And here you have it. One Town Lattice truss, cambered, layed out, drilled and marked?

Puzzler - Since this truss must now be un-clamped and removed from the false work for transport and installation of the trunnels how would you mark the pieces??  Keep in mind they must be reassembled exactly as they were drilled or the trunnel holes will not line up properly. Flip, flop or flip and flop (there is a difference) one piece and BIG trouble.

Tech Vocab -  Flip -  Flop  -   Flip and Flop - Lead Screw - Back-up Block

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