Rhino: Adding Thickness to Molded Offsets

Discussion in 'Software' started by bigjimslade, Jun 12, 2016.

  1. bigjimslade
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    bigjimslade New Member

    I have a hull form in Rhino 3d. It is based on molded dimensions.

    I want to add thickness to 3D print the model.

    No matter what I do OffsetSurface does not generate a solid. For example, where I have twin keels, the halfsiding at the stern does not extend to the offset sides of the keels. The defects are so large and numerous that manual patching would be impracticable.

    Does anyone have any alternatives to adding hull thickness to a Rhino model?
     
  2. wet feet
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    wet feet Senior Member

    I haven't had to use 3D printing and can't say for sure that it will work if you follow my recommendation:I would use the surface model as a basis for creating a solid (I think Rhino describes them as parasolid).You may have to add a deck surface and having done so you ought to be able to create a shell of the desired thickness,which you will probably have to export as an .stl file ready for printing.Good luck.
     
  3. DCockey
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    DCockey Senior Member

    Usually an outer surface generated using OffsetSrf from a surface generated from "molded" offsets needs to be extended using ExtendSrf and then trimmed to be joined into a watertight surface. Problems with ExtendSrf can/will arise if a surface has zero length sides, 180 degree corners and/or 0 degree corners.
     
    Last edited: Jun 12, 2016
  4. DCockey
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    DCockey Senior Member

    Rhino does not use parasolides. A solid in Rhino is a surface or a group of joined surfaces which are closed and "watertight".

    It appears that bigjimslade started with a set of "molded" offsets which are offsets to the inside, not outside, of the planking/plating/skin. He needs to add offset the surface by the thickness of the planking/plating/skin.
     
  5. SukiSolo
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    SukiSolo Senior Member

    A 'moulded' surface is of which type? An IGES file with lots of points? a spline traced and generated surface? or another type? Quite hard to give advice without more indication of this 'moulded' surface.

    I suspect there is some unfairness in the moulded surface and it may require rebuilding, at tight tolerance to obtain a better quality and simpler surface. Might also be that it needs breaking up into two or more individual surfaces and then offsetting and re trimming.

    As David says, be very careful of whether the draft angle gives a surface that then requires extending and a planar surface used to obtain a 'true' edge especially along the sheerline. This is the sort of place that extending a surface may pay.

    BTW Parasolid is a 3D modelling kernel AFAIK, one other one is Acis for example.
     
  6. DCockey
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    DCockey Senior Member

    I assume that bigjimslade created a NURBS surface in Rhino based on a table of offsets.
     
  7. SukiSolo
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    SukiSolo Senior Member

    Thanks David, I assumed it was a surface based on imported data, as he says a 'moulded' surface not modelled one. In this case the transom or equivalent feature could be part of the same surface as say for example a deep V hull, hence my advice about splitting the surface. Always a bit of fun dealing with imported points, single mesh, or less than fair surfaces....;)

    Perhaps the OP could give some idea of the actual geometry of the file?
     
  8. bigjimslade
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    bigjimslade New Member

    More Info

    Mr. Cockey is correct on all points.

    I have a Nurbs surface that is like a bathtub. It is moulded so that the surface is on the interior of the platting. I need to add thickness.

    Rhino's OffsetSrf does not generate a solid from the surface. I was wondering if you folks had some other technique. If would probably work if the hull were a canoe or a rowboat. The twin keels (skegs) cause problems.

    I was wondering if any of you folks had overcome this problem.
     
  9. wet feet
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    wet feet Senior Member

    My apologies for causing confusion,Rhino uses polysurfaces -not parasolids.
     

  10. SukiSolo
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    SukiSolo Senior Member

    Rhino is a Surface modeller not primarily a Solid modeller, but it will give you perfectly good solid objects. Thanks for the input bigjimslade and a description of the geometry.

    Personally I'd probably split off the bilge keels and work on the main surface first, if it is symmetrical maybe just half the surface?. This may or may not be healable ie use Untrim surface. You might want to put trim lines/surfaces at the points ie sheerline where you want to trim back to prior to using Untrim surface. Then offset by thickness desired. You might check whether all the 'Directions' are where you want and pointing so - use Direction command.

    Then work on the bilge keels, again one should be enough, you should be able to mirror/copy it. Possible this will require splitting further but do so as necessary until it offsets. Retrim and join all the surfaces and the only surface you will have to create is the 'thickness' at the sheerline. You need this to create a valid Polysurface/solid. There are tools to show where an incomplete solid is open, try Show naked Edge command.

    One other trick is the base tolerance in the model. This is dependent on the tolerance the model has been built at. If say, the base unit tolerance is 0.001mm it might pay to loosen this to 0.01mm, you're probably Imperial but you get the idea. Towards the end of model building it can help seal some gaps in a solid. Start tight and only with problems go a little looser.
     
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