Need Help With Aluminum Lofting

Discussion in 'Boat Design' started by jdnelson97, May 24, 2011.

  1. jdnelson97
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    jdnelson97 Junior Member

    Thanks D. I love using Rhino ( used Solidworks and Autocad for years). The only issue I was having was the triangulation of compound surfaces vs. flat pattern. The parts were still usable, but I was unhappy with the result. Phil sent me a great breakdown on " Multiconic Development of Hull surfaces". But I am open to any suggestions or great design tips with Rhino.

    Thanks
     
  2. DCockey
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    DCockey Senior Member

    UnRollSrf should be more exact than Rabl's method "Multiconic" method if the surface is a true developable surface.
     
  3. Kiwifinn
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    Kiwifinn Junior Member

    I've been using Rhino exlusively for designing aluminium boats with developed surfaces with great success. Life became easier with Rhino 4 having the History function. I use the LOFT command set to DEVELOPABLE with the history enabled. This way even if my original chine and keel lines don't produce the desired result I can go back and tweak the lines, often a very small shift in one of the original curves produces good results. Sometimes I adjust the FIT TO setting to 0.1mm or so, quite accurate enough to be considered perfect.
    Always compare the surface areas of the panels in 3D and their developed shapes as well as lengths of all the edges. Done correctly these match perfectly.
    I work in millimetres with the absolute tolerance set to 0.01
    I've experimented with the DEVSURF plug-in but I prefer the LOFT command.
     
  4. Kiwifinn
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    Kiwifinn Junior Member

    Oh yes, UNROLLSRF is to be used. I like the report it gives comparing the area of the original to the developed surface.
    Good luck!
     
  5. jdnelson97
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    jdnelson97 Junior Member

    I currently use Rhino 4. I have attatched a sketch file and a surface file. The surface looks good to the eye, but when flatened they are off a few inches in the curves because the software does not compensate for the material flex/bow out.Can you show me how you come up with the correct flat pattern?
     
  6. jdnelson97
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    jdnelson97 Junior Member

    Test Sketch and Test Surface.
     

    Attached Files:

  7. DCockey
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    DCockey Senior Member

    I'm looking at you files. How did you create the surfaces?

    How are you determining the flattened surfaces are off? What are you comparing them to?

    What material flex/bow out are you refering to?

    David
     
  8. jdnelson97
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    jdnelson97 Junior Member

    I created these surfaces with Sweep 2 rail command. I pick the inside chine flat and the keel line for the 2 rails and I pick the transom line (deadrise angle)as the cross section. This creates half of the bottom plate surface. For the sides I use the same command but use the shear line and the outside chine flat for the rails. Then I use the keel line and the transom side for the cross section curves. This gives me the side plates. The transom is based off design and customer request. The reason I know these parts are off is because I have designed a few 32' boats already this year and all the parts were spot on except for entry of side sheets and the entry of hull plate. The builder referred to this as the " blow" (How far the material wanted to bow out on the curve sections. I use the unroll srf command and in some cases the squish command when creating the flat pattern. I am familiar with working with sheetmetal, but have not mastered this area yet. I have attatched a picture of a 32' boat that I had this issue with.
     
  9. jdnelson97
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    jdnelson97 Junior Member

    Pictures of 32' boat under construction
     

    Attached Files:

  10. daiquiri
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    daiquiri Engineering and Design

    The Rhino model you have attached in the post #21 shows lines which make surface discontinuities, visible with the EMap tool:

    Discontinuity.jpg

    I'd be interested to know if you have found these discontinuities on the real boat too?
     
  11. DCockey
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    DCockey Senior Member

    Sweep2 as you used it can create nice surfaces.
    - but -
    They won't always correspond to what a piece of sheet metal bent around the curves would do. Sometimes they will, or at least be close enough. But not always. Loft with the Develop option or DevSrf are better ways to create developable surfaces. Sometimes won't create a surface and that can indicated several things:
    1A) With Loft the curves need to be picked in different spots.
    1B) With DevSrf a different selection of the parameters for the surfaces is needed.
    2) The curves are "noisy" or have other problems even though the basic shape is okay.
    3) The basic shape of the curves won't work for a developable surfaces. Sometimes major changes are needed, sometimes minor.

    Use CurvatureGraph to check the curves. The curvature should be continuous without a lot of wiggles.

    The curves in the example you posted are polycurves with discontinuous curvature and many segments. It's better to work with a smooth curve. In spite of this the resulting surfaces seem to unroll okay. I used FitCrv with a tolerance of .01 inch to rebuild them as simply curves. Still a lot of noise though. Then I rebuilt them with RebuildCrvNonUniform and a tolerance of 0.2 in which helped.

    Next created surfaces using Loft. Had to experiment to find best locations to pick the curves. Then used UnrollSrf to unroll the surfaces and compared to what I got when I unrolled your surfaces.
    Sides - maximum deviation of around 0.17"
    Bottom - maximum deviation at the transom, around 0.55". Also deviation at the sides of around .017"

    Rhino file attached.
     

    Attached Files:

  12. daiquiri
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    daiquiri Engineering and Design

    DCockey,

    Yep, this one looks perfect. :)

    Discontinuity 2.jpg
     
  13. DCockey
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    DCockey Senior Member

    My guess is the discontinuities daiquiri found originate with the curves not being smooth.
     
  14. jdnelson97
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    jdnelson97 Junior Member


  15. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    ???
    Errrr... I'm really interested to hear the answer (post #25).
    I'd like to understand how does the discontinuity visible on the screen compare to what you've encounter on a real boat. If you don't mind sharing it with us, of course. :)
     
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