Sunday, July 05, 2009

New centerboard (part 6) - pivot plates.

The plans call for a pair of brass plates to reinforce the pivot hook on the centerboard. After popping them out of the old centerboard, I used a mill file to bevel the top edges, to smooth the transition to the wood.

The plans say to use screws to hold the plates in place, but the original didn't have them. Spent some time with the drill to machine some counter-sunk holes. Drilling brass it tricky, but I got the job done eventually. I have an ancient J. C. Penny drill press that didn't help much. I gave it away a few days ago, and will get a good one if I decide I really need one.

Used a mirror-sharp chisel to cut slots to bed the plates in the centerboard. If the plate had been placed on the surface of the plywood, all shear forces would be taken by the screws. The plates are 1/8" thick. The 3/4" plywood is a little thinner than advertised, and cutting it down 1/8" on each side would leave less than 1/2" of wood to take the strain, in an area where strength matters. So I chiseled through 3 plies, which makes them about 1/16" deep. Seems like a good compromise. The plates fit very snugly, which helps make sure they can transfer the shear forces to the wood.

I also decided to use some Gorilla Glue to hold the plates to the board. It is strong stuff, and grows by 4x as it cures, and sticks to just about everything.

I practiced screwing in to scrap plywood and found that it was easy to drive too hard and strip out the wood in the hole. I decided to use my bit and brace ($3 at a used building supplies store!) to do the last turns of the screws. I also experimented with different size pilot holes to see what works the best.

So, the installation procedure was:
  1. Wet both the wood and the metal plates
  2. Dab on a very thin layer of Gorilla Glue
  3. Wait 5 minutes for things to get tacky (gold lamé?)
  4. Install one plate
  5. Drive screws with the drill most of the way in
  6. Use the brace to finish the screws off
  7. Flip and install the other side.
It was at "flip" that we saw a problem: the screws were slightly too long. The screw tips poked through the wood on the other side, which would stop the other plate from fitting properly. Meanwhile, the glue is drying...

I quickly grabbed a Dremel grinder bit (I don't have a Dremel, just a grinder bit, left over from making a lock pick set 12 years ago), tossed it in the drill, and ground down the tips of the screws. They are soft brass, so it went fast. Then I put the other plate in place and clamped it down while the glue cured.

Meanwhile I cut the tips off the other screws so they wouldn't be too long. I used the grinder to shape them to a little point, to help the screw get started. Gorilla Glue reaches 80% strength in a an hour or two. After lunch I unclamped and drove in the remaining screws.

The result worked out OK, although I do see one corner bulging because of a screw that's still too long. Oh, well, it's too late to change and will be OK.
From Jay Bazuzi's personal blog
As you can see, the slots in the brass overhang the wood by about 1/16", so they take the strain first.

- 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

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