Tuesday, March 29, 2011

Cast Strap Floors






Due to the size and difficulty of extracting these full size patterns from the sand I decided to cast them in three pieces and weld them together. The patterns were made using 5/8" poly-carbonate sheet. The tapers were cut into the plastic while it was flat and then with just the right amount of heat - about 250 degrees, the material becomes pliable and can be laid right on the bent frame and held until it cools (I wore welding gloves for this process) When it is removed it holds both the curvature and the twist of the frame. (I used a heat gun but the person I learned this trick from used an oven.)
The patterns were rammed into the sand mold.
The mold is then separated and the patterns are removed. Removal was tough even with 3 degree bevels on the patterns because of the twist. At first I attempted to patch any sand displaced by the removal process but subsequently decided it was easier to grind away small amounts of excess bronze. Both sides were cast in the same mold and connected to a common sprue.
Once all the pieces were cast, the three pieces that make up a complete floor were placed in the boat, they were clamped in place and checked for fit.
The three pieces were held in their final shape by gluing sheets of plywood to each side with foaming urethane glue. The corners of the plywood were cut away to facilitate welding the joints while the plywood was still holding them in place.

The plywood was removed and the complete floor was ground and sanded smooth.
The original floors were fastened all the way through the planking. I did the same in the same holes except that I made my floors go one plank higher than the originals and added one more rivet. The rivets were made from1/4" copper rod cut to length. Using a torch, a ball peen hammer and a jig, I heated the rivets and then peened heads onto each one. Pushing the rivets through the hull and the floor, my wife held the backing iron on the hull side while I peened each rivet tight. Yes we are still married.
The 5/8" keel bolts that pocket into the outer edge of the lead were made in the same manner using theTIG torch, a ball peen hammer and a steel jig. The other ends were threaded by hand. Keel bolts larger than 5/8" were threaded on a lathe at a local machine shop.

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