Butterfly joints for decorative and structural wood repair have been in use since ancient times. Probably introduced to the early Hawaiians from the ships that sailed to Hawaii in the whaling days, the Hawaiians were quick to adapt several styles of wood reinforcing joints and called the butterfly joints Pewas. In modern times it has become a value added item in woodworking and can often make the difference in a sale of a turned item with a crack that has been addressed with one or more butterflies. Here are two sites that will help you get started.
I know only what Kelly showed. He turned and hollowed the bowl. Then he glued a pewa routing template to the bowl with hot-melt glue and routed the pewa socket using a small router. He repeated this process for other pewas, if any. He then glued in the pewas and after the glue hardened, he carved and/or turned off the protruding bits of pewa.
I do a lot of them. That's exactly how I do it except I sand the high points, not turn. I don't (more like can't) do interior cracks. It doesn't matter because all you are attempting to do is stabilize the crack. Only install after the piece is completely dry and stable. Customers just want to see that you have acknowledged the crack. They seem to like it. They probably are not putting soup in the bowl.
I sand down the pewa that is proud of the surface because turning or chiseling it down may cause the wood to fracture along its grain and end up below the surface, which is a finishing problem. Install the patches after turning and sanding, but not after applying any oil, because the hot wax will not adhere to the oily wood and the template will shift around or just come off. Repositioning a shifted template is problematic. On a platter shape, if the crack goes through the wood, you can make the pewa on top and bottom by extending the 1/8" straight bit through the wood thickness. Then you can use two butterflies, one on the top and one on the bottom in the same hole cut from the bottom only. The bit will not extend much more than 7/16", if that, and the laser cut pewas are about 1/4" thick. The templates will usually work on the outside of a bowl only, except perhaps on a flat platter.
Kelly made his own template and cuts his own pewas. You can get kits from Big Island: http://www.bigislandengraving.com/ He recommends them. Their parts are quite a bit smaller than the Rockler. They are sized for turners. I have one of the templates and I bought a pile of precut pewas. Way easy to use.
Kelly applies the pewa from the outside after turning. He hot-melt glues the template onto the surface, then uses a palm router to cut the mortise. He does not cut through to the inside of the vessel. It is a shallow mortise so that there is glue surface on the bottom as well as the sides.
No affiliations. Just a happy customer and I too saw Kelly demo a couple of years ago. Joe
Sounds difficult to do unless the bowl is pretty big. Also, I can't grasp the geometry. If the template is flat it won't give a butterfly hollow on the curved surface of the bowl. If the template curves along the surface of the bowl then it won't fit a flat cut butterfly (will it?). Maybe I just better quit trying to understand how to do this and toss cracked turnings.
Quite easy - even on a small vessel. The template is flat and, therefore cuts a flat recess into the curved surface. The template is hot melt glued to the vessel, so it is stable on the curved surface. Once the pewa is glued in, it stands proud of the surface. It is then turned, carved and/or sanded to match the curve of the vessel.
I have successfully installed pewas on small turned boxes and pepper mills. These are about three inches in diameter. The thickness of the hot melt wax allows the flat template to adhere to the convex shape.
i believe that butterfly joints date back much earlier than the early Hawaiians. They were actually used by the ancient Egyptians dating back thousands of years. They were also used by the dutch to repair cracks in shipboards by their early sailors - hence the term "dutchman". Not sure if that predates the Hawaiians though. Ancient Japanese woodworkers also used them in furniture but I don't know what they called them.
