joining plywood with puzzle joints???

Discussion in 'Wooden Boat Building and Restoration' started by Psuedomonas, Nov 3, 2010.

  1. PAR
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    PAR Yacht Designer/Builder

    Finger joints long terrible under varnish and the only good ones are machine made. So, everyone pull out their personal 2 axis milling machine, plot out a finger joint and hope the stock doesn't move during the cut. Of course after this I'll have my robotic welder make up a new chair for me to sit in while this is going on . . .
     
  2. michael pierzga
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    michael pierzga Senior Member

    Well, I looking straight at a finger joint in a varnished teak plywood interior trim piece. Fabricated by a cabinet maker in the Netherlands 17 years ago and it still looks perfect. Obviously not suitable for general marine carpentry or structural work, but it is beautiful !!! so finely fit that only the grain of the wood gives it away.
     
  3. LP
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    LP Flying Boatman

    And yet another joint.

    Here is another scarf to throw into the mix. Obviously, If you don't have the hobbyists CNC machine, it's a moot point. Interesting concept none the less.

    http://clintchaseboatbuilder.blogspot.com/2010/03/cnc-boat-kits-and-prep-for-maine.html

    Paul, do you hollow(taper) the edges of the Payson joint individually or do you butt your pieces together and grind out the hollow as a unit? Then glue it, cure it and flip it do the other side?
     
  4. PAR
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    PAR Yacht Designer/Builder

    Yep, I've seen Clint's joints, but they do present a fair bit of localized stress under load. Again, these CNC made joints don't do much for me, because for them to work, you have to have a 3 axis machine and the precision that comes with it. It's not practical for use in the field, nor in the average guy's work shop, so I don't pay much attention to these joints, other then the novel shapes employed.

    The step or notched scarf can solve the feather edge issue with bright work, but does introduce some edge set stress along the joint, depending on how deep the steps or notches are.

    You can taper the Payson joint edges separately if you want and I'll do this if I have a bunch to do. I'll stack them up and stagger them back. Then I just plow out with a power plane or big grinder. It doesn't have to be pretty, just deep enough to have a place for the fabric and goo to live. Shoot for a good, smooth taper, which eliminates the stress risers.

    If doing one joint at a time, I'll butt the two pieces together and hit the seam with a 10" disk and some 24 grit. Once I'm through and fairly feathered on a couple veneers, it's time for goo. With practice, you can glue both sides at once. This helpful Greg?
     
  5. LP
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    LP Flying Boatman

    Yes, very helpful, Paul. Thanks. I really like the fact that you don't loose any length to the scarf. I will plan to use it the next I get a chance to build something.

    On thin material, have you tried a 1/2 Payson? (Single Payson?) Tapering only one side to bury the tape for the fair surface and then only taping the flat(nonfair) side.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Yea, you can do it both ways, but the buried way can be made fair with a coated board, clamped over the joint during the cure. I generally bury both sides, unless I know the area will be out of sight, though recently I did a panel, which was hidden inside a locker, but I still buried both sides, just in case someone did a repair and cussed about my poor workmanship after opening up the locker.
     
  7. LP
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    LP Flying Boatman

    One more question. What do you use on your boards as a parting agent? Does it leave behind a residue that needs to be removed?

    I've used polyethylene bags taped over filled areas to promote a more fair surface to start working. A poor man's vacuum bag. This though has limits, but the bags peel away very nicely.
     
  8. PAR
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    PAR Yacht Designer/Builder

    I use several different things, depending on a few factors. If it has to bend, but in a simple curve, Mylar and plastic packaging tape are favorites, especially Mylar, which is heavy enough to prevent wrinkles if placed under pressure. Packaging tape is cheap, fast and conforms to compound surfaces, but you get a tape line seam (which easily sands out). Polyethylene sheeting works, but is prone to wrinkle unless quite heavy. It can also be affected by heat and does tend to squirm under clamping pressure, which makes wrinkles.

    If I'm molding a part, I'll use plain old automotive wax and usually give the mold 3 coats, before I apply goo.

    A good poor man's vacuum bag are those closet organizers that use a vacuum to suck the air out and compress the blankets into a little ball. The bags are exactly cheap, but I've been known to cut out the valve and reuse it on other stuff. Hook up a shop vac, suck out the air and it'll usually hold until the goo kicks off.
     
  9. michael pierzga
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    michael pierzga Senior Member

    Wide Mylar tape does an excellent job. get some and practice on a few scraps. Very handy stuff to have around your shop.
     
  10. DGreenwood
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    DGreenwood Senior Member

    With a few hours of practice, even a ten thumbs hack can master making a nice scarf. The hard part is clamping. Learn to do that well and you will make beautiful, nearly undetectable scarfs every time. There are a couple of ways to make an ordinary shop compressor develop enough vacuum to do a great job of this too.
    Don't waste valuable time on complex and less reliable joints...spend it on the things that make a difference...like a fillet that doesn't look like it was done with a broom from five feet away.
     
  11. DGreenwood
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    DGreenwood Senior Member

    Every time I cut a cross section of some poor ******** work from many years ago and find myself cursing his sloppy technique, I try to imagine his discomfort in being forced to produce second rate work by whatever pressures he was suffering under at the time. Isn't it fun to be able to build something that will make that jerk that cuts through your work 50 years from now raise his eyebrows and say "wow, I never would have been so careful"!
     
  12. sabahcat
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    sabahcat Senior Member

    I have to ask
    I always do scarfs on thick ply but ion thinner stuff what about just but and glass?

    I have done several tests and they always break outside the glass joint and look at how the duflex z joint is done

    [​IMG]

    Only the thickness of the glass gets the 12:1 , not the whole material
     
  13. PAR
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    PAR Yacht Designer/Builder

    DGreenwood, as I've ripped open may boats over the years, I've seen both impressive and considerably less so workmanship. I've often cussed a former repair person, sight unseen, just to ask the same question in regard to the economic, personality or owner "restraints" placed on him, to force this type of repair or building decision. I too have had to hang upside down, through a hatch or freshly cut access, while applying a repair "as best as I can" given the situation or conditions. As I've gotten older, I've found I care more about what another might think 20 years later when he has to open up the area for what ever. I want them to be as impressed as I have been, when I've done the same to some another's previous work and not to just cuss my unknown name. I guess this is vanity.
     
  14. dbierman
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    dbierman New Member


  15. PAR
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    PAR Yacht Designer/Builder

    It's pretty easy to write an article and submit it with glowing remarks about the "latest joint", especially to an online magazine that doesn't read or vet their submissions. You're not going to be too impressed with the dog bone scarf either, especially after you spend all that time with setup and jigs. It's just another piece of wood added to the joint (more complication) and added to the joint cutting process. Adding more wood to the process is just introducing more potential for "slop" in the joint, not to mention several extra screw holes from the jig too. In the end, bend this joint across something, like a bulkhead and watch what happens. You'll hear the issues before they "present" themselves.
     
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