Long puzzle joint

I could really use some guidance. 

I'm trying to make a start on my first boat, a Nutshell pram (not a CLC boat). I'm building from plans, because I want to learn all the stages of building. I chickened out of learning to scarf on 4' wide sheets of ply and bought wood from CLC with puzzle joints pre-cut—so now I have to learn to glue 4' wide puzzle joints instead.

I haven't measured, but if the puzzle joint is approximating the same gluing area as a typical 8:1 scarf joint, then it is going to meander through something in the range of 30 linear feet (4' x 8 = 32'). I have epoxy & slow hardener (from Raka), but I'm wondering: For my 6 mm ply, how much epoxy do I need to mix up, can I safely do it in one batch without setting fire to my home, and can I get it applied to that long seam quickly enough to make the joint without it beginning to harden too much? By the way, I'm not planning to glass the boat (that's not a normal requirement in Nutshells), so the joints won't get that kind of reinforcement. I hope I didn't start this project with a bad decision.

I know the alternative is to dry-fit the panels first, cut the strakes and then glue the puzzle joints, but that raises another set of potential problems—principally how to adjust the length of the planks to allow for the thickness of the epoxy within the puzzle joint.

Thanks in advance. Any advice greatly appreciated.


10 replies:

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RE: Long puzzle joint

So you have 8 sheets of 6mm with puzzle joints on each 4' wide end? Why can't you glue two sheets together at a time, working your way thru the batch that way?  A Nutshell's only 9'-6" long after all.

I can't figure out why you'd need to wrangle 7 joints all at the same time to end up with a piece of 6mm that's ~ 64' long?

I'm sure I'm missing something in your post that'd clear this up but then it's 0 degrees here today & things just aren't working like they should....

 

 

RE: Long puzzle joint

   Raka's slow is quite slow, and an easy resin to work with. If you have pumps, the min amount is about 3 oz - one squirt hardener, two of resin. With a cheap gram scale, use 43 parts hardener to 100 parts resin.

For a full width scarf/puzzle joint, you don't actually need a lot of material - the joints are pretty tight. Start with a minimal mix, use a small acid brush to coat both edges, look in your cup to see what you have left and if at last half still remains, just thicken it  to about ketchup consistency with cotton flock, or wood floor, or both, and use the same acid brush to coat the edges again, being sure there's some thickness on both sides of the whole joint. The epoxy on the joint will cure much slower than what is in the cup, so move efficiently but don't feel rushed. At worst, you'll need to mix another min batch part way through, and continue dabbing it onto the edges of the joint.

Make sure you have a plastic coated (packing tape works nice also) bottom platten, and one for the top, and the means to apply uniform consistent pressure all the way across.

If extreme neatness is your thing, you can first mask the joint, cutting away excess tape with an exacto knife. Make the tape margin pretty wide since there can be squeeze-out up to an inch or two from the joint. 

RE: Long puzzle joint

Thanks for the instant replies!

nemochad: Thanks for the answers; I'm much obliged. The thickeners I have on hand are wood flour & cabosil (or similar). You suggest wood flour; I think CLC's help-sheet advises the silica. Do you have an opinion of one over the other in this application? And second question: I'm not familiar with the term "acid brush". What is it and where do I get it?

spclark: Sorry to have misled you. I only have two joints to make, in each case extending a 4' x 8' sheet to 10' long by means of a 2' extension & a puzzle joint. I think where I lost you was when I guesstimated the length of the joint. My reasoning was:

- A standard scarf joint is 8 times as broad as the thickness of the wood.

- This makes the surface area of the joint ~8 times larger than a butt joint in the same piece.

- I assume a puzzle joint will have about the same gluing area as a scarf, for equal strength.

- To get the equivalent 8-fold increase in joint area with a puzzle joint, which is only as thick as the wood it's made in, it would have to be 8 times as wide as the width of the pieces to be joined.

- Hence, in a 4' wide sheet of ply, the width of the joint, measured along the zigzagging surface of the puzzle teeth, must be around 32'

Hence my worries are about what's involved in making a 30' long epoxy joint. Hope that makes sense.

Thanks again.

 

RE: Long puzzle joint

   'Acid brush' is those small, metal handled, natural fiber brushes, originally used by plumbers to spread flux on copper joints, but usable by us for small jobs like this.

They can be cleaned by swirling in a half-inch of denatured alcohol, then left to sit overnight in the alcohol pool. I get about 3 uses this way.

Use the wood flour to thicken your 2nd coat. Silica is too brittle, and too hard to sand on a flat panel like this. Save the silica for fillets and filling over-drilled holes and anything that needs to be hard/durable/stiff. Scarfs should be thickened with stuff that won't decrease the epoxy's flexibility.

RE: Long puzzle joint

   Thanks; wood flour it is, then. I think I know the brushes you mean. I'll track some down.

RE: Long puzzle joint

When I built my 18-ft schooner Puzzled Mullet a few years ago I used the CLC puzzle-jointed plywood. First I test-fitted it dry, then I used a disposable foam brush to pre-coat the end-grain in the joints, then a gloved finger to brush on a  epoxy/woodflour mix. I used slow hardener to give myself time.

The only problem I had was a sore lower back from doing it on the shop floor. If I was doing it again, I'd have some kind of workbench.

Have fun,

Laszlo

 

 

RE: Long puzzle joint

Laszlo,

Thank you for your advice. I hadn't thought of something as simple as the gloved-finger applicator approach—that's why rubes need experts.

Nice looking schooner, and nice looking kayak behind. I envy you your experience, abilities & confidence to modify and create. Given my age and rate of progress (and the fact that I live in a desert) this Nutshell will likely be my one and only. I'm trying to make it be something I'll be proud to lay claim to.

RE: Long puzzle joint

 

   PJM I guess I shoulda re-read your initial query before posting my reply. I see now what you were getting at: puzzle joint mating surface area vs. 1:8 aspect traditional scarf joint across 48" sheet width.

Others have since added how-to's & user feedback that ought to help you. I can offer up using a brad nailer (held at an angle) to help keep plattens / cauls secured during glue-up. Just don't let the gun drive their heads below the surface, they can then be pulled out when time comes to open the assembly. This method allows using fewer weights that can be moved along the joint length while bradding is done. The small holes left from 18 gauge brads usually aren't obtrusive underneath a layer of glass.

Another trick is using plastic glueing syringes like what Gougeon sells for your application of thickened epoxy. These help keep quantity applied more uniform which helps later on by minimizing squeeze-out. If you have substantial mix left after completing one joint you can toss syringe into a glass of ice water to cool it until your next joint is primed with unaugmented epoxy.

RE: Long puzzle joint

Must be a pretty deep dingy if it is only 9'6" long and has sides 48" high. ;-)

I just can't see any reason to glue up a 48" seam on a little boat.  I'm thinking the deepest your panel would be is a foot to foot and a half.  The bottom might be two sheets, but still less that 24" each wide piece.  Why not dry fit the puzzle joint. Layout the pieces (number both sides of the joint). Place the work on a layout table............I used a ping pong table once.   Cut out the pieces and then glue the joints of the smaller, easier to handle parts together.  Once laid out and numbered at the puzzle joints you can take them apart for ease in handling.   

The bigger the part, the bigger the joint, the heavier the thing the more likely this old fat guy will hurt himself or screw up the work. I make things small and easy to work with if I can.

 

RE: Long puzzle joint

   CLC's puzzle joints on the Skerry kit are a little different shape than those "fingers", but I'm sure they are all very tight. I asked CLC and they verified they are supposed to be that tight. A lot of my epoxy squeezed out, but at least I know I got everything wetted.  I followed the instructions to use the Cell-o-Fill that came with it.

After gooping the joints and pressing them together on a decently flat surface (big piece of good ply on the garage floor), I scraped most of the excess off, covered the joints with plastic and used a rubber mallet and board to pound them flat before putting weights on everything.  I still had a couple of fingers which weren't quite flush with each other.  I just squeegee'd a little coat of epoxy and filler over them and then sanded.  I'm not as picky or as good as some of the wood artists around here.

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