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Finger Joint Process Questions

Don Silsbe

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A friend is starting a Spindrift 12 kit build.  We are both wondering about the details on how to assemble a finger joint.  My quandary is how you get the bottom glass tape in place without messing up the joint.  Can one of y’all please explain, and/ or provide photos?  I’m sure she’ll be on this forum soon.  But in the meantime, I’d like to know how this is done.

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I am confused by the question as there is no glass involved in gluing the joint together. I prefer to call them finger scarfs rather than finger joints. Finger joints are typically parallel sided and equally spaced fingers. This means that roughly 50% of the joint is a butt joint which is not very strong. We used to use step scarfs which gave us a strong joint but the edges were vulnerable to handling and were often damaged by the time it was used. The other problem was quality control. We used 9 steps in 6mm ply which gave us steps of .013" . If the end of the sheet was not perfectly flat to the table the scarf was rejected which messes up a good sheet of ply. If a new shipment was 10 or 15 thousand's off in thickness our cut file did not work and we had to make another one until the next batch.


Most of our competitors went to a puzzle joint. We tested them and realized that up to 2/3 of the joint was a butt joint. I made up all kinds of joints and tested them. I discovered the worst part with them is that to assemble, you have one part on the table and the other part is positioned above it's partner and lowered and pressed place. A lot of the glue gets squeezed out of the joint. When I cut the joint open I felt that if I was lucky, 60% to 70 % was effectively glued. 


The finger scarf edge gets loaded with glue and the joint is brought together horizontally compressing the glue and squeezing out the excess glue with 100% glue contact. While a small part of our finger scarf could be called a butt, it is minimized  by keeping it to the bit diameter, typically 1/4".  We have tested it on lots of different applications and it is working well. We like to think of it as scarf turned on it's side. we do not usually glass the joint at all. Any joint in 6mm ply is fine if you are going to glass the joint.


As with all joints it is not perfect. It does not automatically align the joint. When the joint is brought together you need to look to see if there is any gap at the at the end of the fingers at one side of the joint. If there is, you need push the panels together with bias to the side that is open. On wide panels it is hard to get it wrong but if one is careless with narrow panels it could happen. Another possibility is that if the fingers are not in plane and level with each other when you bring the joint together, the wedge effect lock them together out of plane. Usually you can tap a piece of wood with a hammer over the fingers and massage them back to being level with each other. You do not need to use much force for the joint. The main thing is that 6mm ply is not perfectly flat and you are bringing two pieces together that need to be flat in the region of the joint. We have a bunch of 2 1/2# lead weights that we put down wherever they are needed. Clean off the excess glue and you should have a fine joint.

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I've got an album here that has some video of how I do it. I screw side 1 down to a scrap piece of flat 3/4" ply with plastic underneath. Then first coat with neat epoxy the edges of the fingers. Then, with fingers spaced about 1" apart,  fill it up with thickened epoxy using a putty knife. Then push together then screw the second side down. Add screws if needed in specific spots to push a non-cooperative finger down. Don't squeeze between boards as this offers no way to A: ensure the fingers are lined up or B: clean up the squeeze out. 


This wastes a fair amount of glue but it's important to get 100% fill of the joint. If you have multiple finger joint lined up to do at once then you can re-use the squeeze out for the next one in line. 



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As Alan mentioned, it’s the best way to hold everything in place. Even though the finger joints are tight, it can be a bit difficult to make sure they are lined up perfectly as there can be movement. I also drew thick pencil lines across the joints once I had then aligned and dry fitted perfectly. Once the epoxy went on, I could still see most of them.

I also used screws to align all the interior bulkheads - I screwed partially into the hull in front of and behind the bulkheads to secure them vertically as of allow for some tack welding.

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