Builders' Forum Archives
Posted by Jim E on Feb 17, 2007
As promised in an earlier thread about puzzle joints, I made an experimental decorative joint using an insert. The idea is to reinforce a butt joint without using a doubler or glass tape. I changed my mind about using a butterfly dovetail shape, since there's no need to mechanically lock the joint together. The diamond shape is easier to make, stronger, and still looks nice.
The top picture shows the layout. There's a single 1" x 4" diamond joining a 2" wide strip of 4mm okoume. There would be four of these across an 8" wide panel. I did all the cutting using the knife and metal ruler in the picture. To glue it, I put unthickened epoxy on the edges, let it soak in for a few minutes, then applied a fairly thick cabosil mix. It cured for four days under weights, then was scraped smooth.
Picture 2 shows the joint under load. Those water-filled 1/2 gallon milk containers are about 4lb each. There'a nice even curve with no hard points.
The last picture shows what it looked like after I deliberately broke it. The butt line was the weak point as expected. Interestingly the epoxy cracked apart there, leaving the wood untouched. Perhaps it would have been stronger if I had used microfibers rather than cabosil.
I conclude that, in order to be as strong as a scarf join, the diamonds would need to be right next to each other, with no butt line between. But even this "50%" join is probably adequate for a panel that's going to be glassed on both sides. The panels would need careful handling until the hull was wired up, though.