Monday, February 28, 2011

Where did you put the Tree Nails?

Tree Nails or Trunnels or pegs as they are referred to in timber framing and bridge building are used to hold the joined members together.

 Trunnels used in timber framed structures (3/4" to 1"diameter) hold joined members, usually mortise and tenon joints from being "pulled" apart.

Trunnels used in the Town Lattice Truss building are significantly larger in diameter (1.5" to 2") because of the way the rotational load forces act against them.

We will place 3 Trunnels where the web lattice members intersect the top and bottom chords and 2 trunnels where just the web members cross. Placement of these "Tree Nails" needs to be carefully though out so as to cut through the minimum "thru fiber" of the wood for maximum structural strength.

The picture (above right) shows a full scale mock-up using 1" foam insulation. Three 1.5" diameter green foam Trunnels through the intersection of a  2"x12" bottom chord and 2x10" web lattice.

Where do you get tree nails? A tree nail factory of course. That would be Northcott Wood Turning in Walpole, NH. This family owned business makes wooden pegs by the thousands from many types of wood in various lengths and diameters. Who knew?  For the 3/8 scale model we will be using 5/8" diameter Maple dowels from the local hardware. In the picture above you can see the pre-cut pegs and below a 60 degree chamfer is being turned using the metal lathe.

As a side note, years ago, when learning the "cobbler trade" I came across a container of 1/8" square 5/8" long wooden pegs in my shoe repair shop. They were hard Maple and sharpened on one end. I asked my Finder what they were for and of course he told me, "wooden shoes". After he had his fun with me he demonstrated how they were used on leather soled cowboy boots in the arch area to fasten the outer and inner soles around the shank. When driven into a hole punched by an awl I was amazed! Their holding power equaled that of the soft iron clinch nails (see picture above). For real. I am not making this up.

Puzzler - How many different length Trunnels do you think will be needed to build the bridge?

Tech Vocab - Web    As in "Oh what a tangled web we weave...?"

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