joining plywood with puzzle joints???

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

  1. Psuedomonas
    Joined: Aug 2005
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    Psuedomonas Psuedomonas

    I will soon be making 1/4" plywood into 16' lengths. Previously I used a common scarf joint, but with only 1/4" to work with, these are not too strong. Can anyone suggest a method or router jig to make interlocking joints across a 48" surface?
     
  2. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    here is a link on a method I'm going to use. I only need to buy the bar pattern and use lots of epoxy.http://www.duckworksmagazine.com/06/howto/puzzle/index.htm If you break the joint between frames you could put a but block out of ply on the inside or use regular wood. That wiil insure greater strength then the regular ply offers.

    PS Where did you buy your wet exhaust manifolds for the subi engines? Did you make them? Stan
     
  3. Ike
    Joined: Apr 2006
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    Ike Senior Member

    At the recent Wooden Boat festival I saw that Chesapeake Light Craft is using puzzle joints on their kit boats. It seems to be a good way to join them. I suppose the difficulty is cutting out the joint.
     
  4. wardd
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    wardd Senior Member

    you could take a 4' long piece of 1/4" thick steel as wide as 2 templates plus holding stock to a shop that has a cnc wire edm machine and they could cut it in half forming two interlocking templates perfectly matched
     
  5. CatBuilder

    CatBuilder Previous Member

    In case you still prefer scarfs:

    A quick note to say that scarfing even 3mm (3/32nd of an inch) plywood is a breeze if you do the following:

    Take every piece of plywood you want to scarf and stack them on a table, staggering them the appropriate (12:1) distance in a stair step pattern. (about 1.5" step for 3mm plywood)

    Carefully clamp down the plywood to the table so it doesn't move.

    Use a good low-rpm buffer/polisher with a sanding disc attached and sand that stair step into one smooth, uniform surface.

    Follow up with a palm sander to get things perfect.

    PRESTO! You can get perfect 12:1 scarf joints even on 3/32" plywood.

    I did it this way and was able to put a perfect 12:1 scarf into 18 sheets of 3mm ply - taking about an hour for all 18 sheets. I did 144 sheets total, scarfs on all 4 sides, in about a week's time.

    Was a hell of a good way to do scarf joints, just in case you still want to do them.
     
  6. tom28571
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    tom28571 Senior Member

    Why not just learn to make a scarf joint. Its not hard to do and is the best of all such methods. 1/4" scarf joints are routine. 1/8 is a bit harder but still not too difficult for any reasonable woodworker with opposable thumbs. Locking joints are easier in many instances for assembly of CNC kits but are more difficult to make with ordinary tools and generally weaker.
     
  7. michael pierzga
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    michael pierzga Senior Member

  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Bingo Tom, the usual choices of finger joint aren't especially easy to make with hand tools, aren't very attractive and are generally weaker then a scarf. A 6:1 or 8:1 scarf ratio in 1/4" ply, with epoxy is well past stronger then the plywood itself, so I'm not sure why there's an issue,
    If there's a strength concern, it's not the physical properties of a properly done scarf at these ratios.

    As has been mentioned, a scarf isn't particularly difficult to make. You can eyeball one fairly well with epoxy or use a jig, of which there are many types.

    Conversely, a Payson butt joint makes the scarf irrelevant. If the Payson joint is hollowed out and done under clamps, then it's about as simple as they get.
     

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  9. tom28571
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    tom28571 Senior Member

    Paul, I consider a simple scarf easier to make than a Payson butt joint if the joint is to be well faired out on the surface. I sometimes use the butt joint with tape on both sides without the hollowing and fairing if the joint is in a non stressed area and fairing is not an issue.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I too consider the scarf easier and considerably faster then the Payson butt joint. I do like the extra length you can get from the Payson joint and more importantly the nearly flawless faired result, with some setup on the hollowed version. Alignment issues are avoided with the Payson as well. This said I can hack out a good scarf in a few minutes and have it waiting for a cure in a few minutes more.
     
  11. lumberjack_jeff
    Joined: Oct 2010
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    lumberjack_jeff Sawdust sweeper

    A scarf joint is stronger, but one disadvantage is that a scarf creates a "hard spot" in the joined hull panel that will sometimes come out unfair.
     
  12. Psuedomonas
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    Psuedomonas Psuedomonas

    OK guys- you convinced me. I'll stick to the old scarf joint, but I'll try Catbuilder's method of stacking. Many thanks for the input
     
  13. michael pierzga
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    michael pierzga Senior Member

    To make a conventional slash scarf you need a good work surface to cut the scarf on and to glue up. Spend some time building a work surface. Hand planes, powerplanes, grinders, circular saws all will accomplish the task if you have a good work surface. By the way all the scarf techniques...Payson , conventional, fingerjoint have value according to the need. Those finger joints look great when naturally finished.
     
  14. tunnels

    tunnels Previous Member

    I like it its neat and because the fingers are short and long and are staggered it will be much stronger than the ordinary finger joining in a straight line that would break along the dotted line so to speak , but with staggered fingers makes it much better !!!!:D:p:p
     

  15. michael pierzga
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    michael pierzga Senior Member

    All the joint ..scarf techniques, have value. Its very difficult to pull off a scarph in plywood that will be bright finished. Very precise technique and skill is required if you expect to have a razor thin transition between sheets. That finger joint looks good. Id love to learn how to perform it.
     
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