Tuesday, July 07, 2009

New centerboard (part 9) - Pouring in the real centerboard

With the test pouring done, we started making plans for the real thing.

In the test, as the lead cooled, it shrank and pulled away from the sides. To fix that, I wanted to use a smaller hole and slightly overpour, and then use a hammer to smush the middle, driving it towards the edges. Also, I read I should taper the edges of the hole, so the lead would flow around it, making the lead hold on to the shape of the wood. This would also smooth the transition when the lead shrank. In both cases, I needed a smaller hole, so I reduced from 6" square to 5" square for the first cut of the hole, with the plan to cut some more away for the taper.

The 1" spade drill bit provided the corners again. This time I used the jigsaw to cut the sides of the hole, since the circular saw wouldn't fit in the smaller space. I then turned the jigsaw to cut a 45 degree angle, and shaped the edges a bit. I followed with a rasp to take the edges closer to a 30 degree angle.

I didn't want to use screws on the inside edge again, since the lead hadn't flowed around them well. Nails seemed like a better choice. The plans show 3 nails per side, but getting them in without splitting the plywood seemed tricky, so I drove one in to each corner. Because the corners weren't tapered, they seemed stronger and needed help holding the lead in.

Because I wanted to overpour the lead for later pounding, I added a top mold, with a hole cut in the middle. The top mold also means that being perfectly level isn't required. So now the bottom layer is a 10" square; the middle layer is the centerboard with a 5.5" hole cut out, and the top layer is a 10" square with a 4" hole. A 1" pour hole would have been easier to cut (with my 1" spade bit), but harder to aim for. It would also have held in more heat, and I wanted the lead to cool faster so it would burn less wood.

I was told that lead was thin, so make sure everything is clamped tightly in place. I didn't have a clamp deep enough for the inside corner of the molds, so I used a lead-acid battery (in a case to protect from heat) to apply pressure in one corner. I've seen a section of railroad track used for this purpose. I'm not sure where to find one. Clamps did the other parts.

I decided to switch back to the cast iron pan instead of the coffee can. The coffee can didn't have a good pour spout, which was a problem during the test pour. I saw a lot of extra lead left behind in the can, so I used pliers to pull out some, and then melted out the rest. It took a long time to heat the can through the cast iron pan (pictured), so I removed the pan and heated the can directly. At this time I noticed the pan handle was so hot that I couldn't hold it long, even with the gloves. I timed it at ~10 seconds: not enough for a pour. I figured I could use the vise grips on the handle.

Once the extra lead was out of the coffee can, I put the cast iron pan back on, and then put the block of lead in. After 1/2 an hour of burning there wasn't any molten lead. It was breezy, which carried the heat away quickly. The cast iron pan had a lot of surface area and thermal mass, making it harder for the small campstove to do its job. Finally, the large block of lead was only touching the pan base at two small points, because the lead block was too big to fit in the pan. I needed to cut it like before, but now it was very hot.

I attacked it with a drill bit this time, drilling a series of holes in a line on one side, and then the other, and then poked at it until the lead split easily under my pliers. I gathered up all the shavings and put them in the pan, along with the two large blocks of lead. The shavings melted first, and helped transfer heat in to the block, which worked well.

To keep the heat from blowing away, I surrounded the setup with the panels from a plywood play house. When that didn't help enough, I added a piece of ceramic tile (from my "scary sharp" setup) on top of the pan, as a loose lid. Even better would have been heat reflectors, perhaps tile with foil over them.

After another round of waiting, the lead was melted. There was a little dross again, so I started skimming. After 5 minutes of "final" preparation, the lead had started to solidify at the edge of the pan. To work I had removed the plywood panel wind breaks and ceramic tile cover, and the wind was carrying away my heat.

Put them back in place, and waited again. Decided to do the pour quickly, so things would still be hot.

After another 15 minutes or so, the lead was all melted again. I clamped on the vise grips but found that it was too heavy to lift that way and stay under control. I used a second set of pliers to get both hands working together.

After the lead cooled, we removed the top mold, and saw that there was way, way too much lead. I shoulda stopped pouring when it was just ~1/16" up in to the top mold. Instead we had 1/4" or more. I experimented with removing it with a wood chisel and a block plane, but ended up buying a cold chisel to cut it away.

We spent a couple hours with hammer and cold chisel, to remove extra lead. Then we used the hammer to smooth the surface of the lead on the top, which caused it to bulge on the bottom. Hammered the bottom back to shape, then flipped and attacked the top again. After a while we decided it looked good enough. There was a gouge where we dug too deep at one point, and we were able to mostly repair it but not completely. Epoxy will have to fill in the hole.

I'm not certain if the centerboard will sink like it's supposed to. The new hole is considerably smaller than the old one (and the plans), and a substantial amount of lead is left over. However, the new centerboard is made of a denser, heavier wood, so it may be OK. If not, I can drill a few smallish holes and cast in the lead scraps.

Here's the good (bottom) side, in its final form.

- Clean up the cuts that went off the line
- Draw and cut the curve at the top
- Plane down the leading and trailing edges
- Cut a hole and pour in a lead sink weight (first time pouring lead!)
- Cut a gap for the pivot
- Prepare 1/8" sheet brass as pivot hole reinforcement (in progress)
- Drill 5 holes for fid and lanyard (ooh, the easy part)
- Epoxy and paint
- Install centerboard & lanyard
- Put boat back on trailer

No comments:

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.