Using Finger Joints to Scarf Plywood
Boatbuilders have had to adapt their boatbuilding methods to use materials that were available locally. It is rare to get long flawless planks and for this reason many methods of joining wood to make it longer were developed.
Traditional Methods of Scarfing
Scarfing is a common technique to join 2 pieces of wood into one longer plank.
The goal of scarfing a plank is to make a strong joint that bends in a similar way to the rest of the board. The ends of the planks to be joined need to be tapered and overlapped. The overlap is often 8 to 10 times the thickness of the board to insure good strength and if glue is used, good adhesion. Pitch or tar mixtures were often used in traditional boatbuilding, between layers to help make the joint watertight and help keep it together.
Epoxy is now the adhesive of choice.
Joining plywood using finger joints
I was curious to see if I could cut finger joints using my router. I was hoping to simplify the scarfing process. I was also just curious. I deserve my nickname of Curious George.
Many years ago I had bought a jig to make hidden dovetails for drawers. I never used it but when I moved it came to the surface.
I wondered If I could use it to make finger joints to join plywood.
The slots and fingers are exactly one half inch wide so in theory they should be perfect.
One of the uses of this jig is to make box joints.
My trim bit is exactly one half inch and it fits nicely between the fingers of the jig, unfortunately it is deeper than the thickness of the jig so I will have to lift the jig so that there is room for the little bearing and the screw that keeps the bearing on.
I traced the finger template on a scrap and cut it with the bandsaw. It is not accurate. It only has to lift the plate so the router bit fits.
Everything is in place. I needed to add another piece of wood so the surface would be even.
I lined everything up and screwed the pieces in place. I had no easy way of clamping and for a test I don't care if there is a hole.
The set up paid off and the cut was very easy. There was a little bit of fuzz on some of the end grain but otherwise the fingers are perfect. I did the second piece and sanded the slight irregularities. I don't suppose I really had to sand.
I glued with a sloppy mixture of epoxy and silica. The wood absorbed some of the epoxy and the consistency was just right.
The fit was quite tight and I had to use a mallet to push the fingers together. I did not think to be careful to start the fingers right at the end so I ended up with a small gap. I'm not sure that's bad because epoxy is happy with a loose fit anyway.
The joint is stiff and strong even without the glue. I clamped it anyway. I had added a bit of extra thickened epoxy in the small void I had at the end of the fingers where I had not aligned the 2 pieces perfectly into each other. It was too tight to safely remove them so I deemed it an acceptable gap and carried on.
The joint was solid from the very beginning. I don't think I really had to clamp at all but old habits die hard and I did. After a couple of hours the epoxy had set and I took the clamps off. The joint is rock solid even after a couple of hours. I'll test it tomorrow but I can see it is good. I think these finger joints are a great success and would do just fine in any boatbuilding project.
Testing my finger joints
I cut a 3 finger thick sample. The ends of the finger have small indents where I did not push them together when I was glueing. Otherwise the piece is solid and stiff.
I put my sample in the vice and tightened it gradually. The photos are taken looking down on the sample as I tighten the vice.
I tightened the set up until the finger joint failed. It was bent quite a lot before cracking.
The wood failed partly where the fingers ended. I think it would be important to either fill them completely with epoxy or better to actually position the pieces closer in the first place.
I deliberately tested the piece with the supporting blocks quite close so the stress would be on the joint.
Bending the finger joints
I also tested the bending of the sample. I moved the little pieces of wood so the sample was being supported further apart. The bend is quite even and the joint does not seem to have much effect on the bending of the plywood.
Bending the finger joints
I try to be accurate and check my information, but mistakes happen.email me if you find mistakes, I'll fix them and we'll all benefit: Christine