Monday, July 06, 2009

New centerboard (part 7) - failed test pour

The centerboard is made of plywood, which is bouyant in water. We want it to stay down, so we have to add some weight. One way to do this is to melt and pour lead in to the board. The original was done that way, and I decided to do it again, reusing the lead.

Casting lead was all new to me, and it took a while to get things together. I decided to do a test pour first, and then repeat in the real centerboard.

To create the test setup, I started with two 10" x 10" pieces of the same plywood as the centerboard is made of. Neither was scrap, I'm afraid. The plans call for a 6" x 6" hole, to hold 10.9 lbs of lead. I drew a 6" square, then measured 1/2" in from each side. A small pilot hole, followed by a 1" spade drill bit in each corner got things started. To avoid tearing up the exit hole, I drilled 1/2-way through all 4 holes, then flipped and drilled the other side to match. Finally, used my small circular saw to cut the edges of the hole. That isn't very safe, as the saw wants to jump when the blade hits the surface. A jigsaw is safer, but straight cuts are harder and my circular saw is a better tool than my jigsaw.

Lead doesn't stick to wood, so I put nails and screws in the inside edge for it to mold around. I used both I wanted to see which would work better.

For a crucible (to hold the lead), I bought a 6" cast iron pan from the thrift store. Everyone recommends a coffee can, but I thought the built-in handle and pour spout would be useful.

I had a propane torch from a yard sale years ago, and decided to give it a go for melting the lead. It's a classic, probably from J. C. Penny. The instructions are amusing. In those days, you didn't need to worry about fumes from melting paint.

Applied the flame to the lead and all that happened was the lead and paint on the surface melted. I used the opportunity to scrape a lot away, but no lead melted. Not enough heat.

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