Bending 1/4" andBend down 1/4" and 3/8" plywood 14 degreesees a 3"/in 12" pitch

Discussion in 'Boat Design' started by rasorinc, Jun 24, 2015.

  1. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    Have not had to really bend plywood in decades. Now using 4'x 8' marine doug fir ply I need to bend the 8' side downward. The 3' part is a flat section and the 1' x 8' part bends down 3" in 12". Any problems---Solutions? Thanks much. PS site will not let me correct wording in title.
     
  2. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I take it you mean that it bends in an even curve to end up at a 3" pitch angle, or 14.5degrees. So the radius of curvature would be that of a circle that would have a circumference of 360/14.5 x 12" /2pi = 47.4 inches. According to this you can do that radius for 1/4" and 3/8" if the bend is cross grain, which I think you are so you should be ok, maybe get it wet with hot water but I wouldn't steam it. They do talk about steaming it here though, as long as it is exterior or marine grade. Choose the side and edge with the least knots.

    Minimum radius for bending plywood:
    http://schmeling.com/reference/pdf_files/plywood_bending_radius.pdf

    or did you mean bending down to a total deflection of 3"? That would be a tighter radius, like 2 feet maybe. In that case I think you might need kerfs. Someone with experience is going to have to speak to that. I think I would kerf it with a table saw every so many inches, down from the inside of the curve, however far you need to go so it will take the curve. Then get the kerfs wet with epoxy and sawdust and talc or whatever you are using and bend it before the sealant hardens. Then epoxy and glass over that if that side is ever going to be in tension under load. But I am only guessing.

    Kerfing video:
    https://www.youtube.com/watch?v=g49H35ie2eQ

    Whatever you do it would make sense to test ideas with 1" strips, same radius, before doing full 48" width.
     
  3. rasorinc
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    rasorinc Senior Member

    Thanks Jamie, that is a good video on kerfing. I think that will be the way to go as it is the 8' edge I wish to bend down 3" over 12".I appreciate your help. Stan

    Any one else have a comment to make????
     
  4. BertKu
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    BertKu Senior Member

    Does those pictures helps you? That is the way I have done it.
    Bert 272.jpg 003.jpg
     
  5. BertKu
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    BertKu Senior Member

    Forgot to tell you, I used a kettle to create some steam.
    Bert
     
  6. rwatson
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    rwatson Senior Member

    I got carried away with Youtube videos on kerfing.

    For this discussion, the use of a router instead of a saw might be helpfull.

    I am thinking now of where such serious kerfing might speed up interior fitout
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Wow. That does give some groovy ideas for interior work, leaving the kerfs unfilled. You could still give it a splash of epoxy to stabilize and seal the wood. I also dig the idea of using a router to make vee cuts.
     

  8. PAR
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    PAR Yacht Designer/Builder

    Kerfed plywood has some disadvantages. First is the very strong likelyhood it will crack along one of the kerfs. The next is you need to be damn uniform about the kerf depths or inconsistent bends will result. I have a data sheet for minimum bending radius on plywood, across different species and grades. Drop me an email and I'll sent it to you Stan.

    In most situations, relatively thin plywood (3/8" or less) can be bent quite easily with a home floor steamer. I stole one from the other half and it was designed to clean dirt off grout lines on tile floors. A minute with this will bend most stuff.

    The combing shown (aft corners) is 5 ply, 1/4" meranti BS-1088, about 3" tall, bent to about a 10" radius and way tighter than one would think possible. It took about 30 seconds with the steamer wand, to "convince" this stuff to live in this position.
     

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