rhino unroll prob's!?

Discussion in 'Software' started by tugboat, Sep 20, 2013.

  1. DCockey
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    DCockey Senior Member

    Curvature is a Rhino command which when used on a surface shows half-circles corresponding to the maximum and minimum curvature at the selected location on the surface. If one of the half circles is a straight line then the surface (or a portion of the surface) is developable, and the straight line shown coincides with the ruling line going through the selected location.

    Curvature is 1/radius. Straight lines have 0 curvature. The half circles are the maximum and minimum curvature taking into account the sign of the curvature, not just the magnitude. When the signs of the curvatures differ then the curvatures are in opposite direction.On a smoothly curved surface the directions along the surface of the minimum and maximum curvature will be perpendicular to each other.
     
  2. SukiSolo
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    SukiSolo Senior Member

    Hi David, you are correct you cannot unroll a helix, but that is because effectively you have a coincident point in the 'axis' plane. If you say take a helical surface of less than 360° (twist) and put a cut through the centre ie like a hole punch, you can get the rest out generally. I'm sure I have seen Archimedes screws made like this in steel with welded sections forming the screw. Rhino can develop this type of 'helix' with zero distortion if you get proportons right.

    Anyway, it just highlights how you have to think in the 'node' mode to create developable surfaces. All good fun to test our geometry!
     
  3. DCockey
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    DCockey Senior Member

    The problem with a helical is not "because effectively you have a coincident point in the 'axis' plane." Many perfectly develop surfaces have ruling lines which cross when projected onto certain planes.

    A helix can be approximated in math or in metal by a group of developable surfaces. Any accuracy desired can be achieved by making the individual surfaces sufficiently small. This is what is commonly done when using plywood to build a hull with a non-developable surface. An example is Graham Byrnes' Ocracoke 20 design. http://www.bandbyachtdesigns.com/ocracoke.htm Scroll down to see photos of an Ocracoke 20 under construction.

    A helical surface with a hole in the center is not developable but if small enough it may "close enough". See the attachments for an example. Open the .3dm file and inspect it using the Curvature command. Note that neither principle curvature is zero. This example can be "unrolled" in Rhino using the UnRollSrf command and the area is 0.40% larger when unrolled. Whether that is close enough depends on the material to be used, the fabrication method, and the accuracy required of the resulting surface.
     

    Attached Files:

  4. rwatson
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    rwatson Senior Member

    Thanks David

    FWIW I didnt get any message about percentages - I am using the latest version, updated a few days ago.

    I wonder whats the difference is ?

    One thing I learned, the Parts Number option is a fantastic thing to use.


    PS - oops - yes i did, i was expecting a dialogue box, but it was in the command list, didnt think of that till I played with it again.

    Area is 0.2243 sq feet (0.05 % ) bigger after unrolling
     
    Last edited: Sep 22, 2013
  5. rwatson
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    rwatson Senior Member

    Some areas are worse than others by the look of it.

    Separating the panels, and checking the Zebra affect might help spot the problem areas.

    This example shows some problem areas.

    How did you assemble the shape in the first case Tuggie ?
     

    Attached Files:

  6. rwatson
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    rwatson Senior Member

    Finally, I caught up with all the conversation since the first 'page'. I guess my last two comments are outdated.

    Looking forward though, the problem of creating developable surfaces remains.

    I did a quick experiment from the Level 1 training, where you use Crv2View to create surfaces from plan and elevation curves ( Victory.3dm )

    After I created a panel from that, and unrolled it, it still gave

    "Area is 0.3104 sq feet (0.07 % ) bigger after unrolling"

    So, it seems that the secret is the relationship between the two 'axis' that controls the 'developability'


    Is this why so many people generate the first hull shapes from Freeship ?
     
  7. tugboat

    tugboat Previous Member

    RWatson- it's gotta be. WRT the math-
    1 sq ft = 144 sq inches. so 144 x 0.07 = 10 sq inches- maybe not too much metal but when you start getting it .13% or just above- that's getting into a lot of curvature and extra or decreased metal.
    -I cannot see how you can fair those lines any better unless you spend multiple hours doing it. Although Dcockey is a pretty damn good draftsman. he managed to get them in tolerance. But for an average guy like me- the tolerance is a "check" to make sure I can build the hull -if it was ever to be built by anyone.
    having such low and high tolerances defeats the purpose of the whole program. So yea- hopefully rhino changes the tolerances in some service pack or in the next v.6


    thanks for the check on my surfaces btw-

    I also have tried the Zebra and Gaussian and enviro-map.

    I see so much variation in tugboat designs- but the one ive modeled seems to be one of the only ones and one of the easiest ones for an amateur designer/builder, using single chines.

    :)
     
  8. DCockey
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    DCockey Senior Member

    Lack of "fairness" is not the reason for the change in area when unrolling. The reason is the surface has some compound curvature. And that probably goes back to the how the surface was created.

    Additional fairing will not eliminate the compound curvature and may even increase it.
     
  9. tugboat

    tugboat Previous Member

    yes- I misunderstood- quite correct- those panels then would not be cylindrical either?
    this is where I am lost- when I look at the unrolled surface from the right view- it appears a sort of banana peel shape - so I don't see either conic or cylindrical sections... what I was saying was- as the plate flows towards the bow- its shape changes abruptly. and appears to be developable by looking at it but intuitively the frames seem to suggest compound curvature...
    hope that makes more sense?:)
     
  10. tugboat

    tugboat Previous Member

    this is what throws me- why bigger?? wouldn't it just not unroll? or even be smaller?
     
  11. rwatson
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    rwatson Senior Member

    I have been mulling over Davids wise words, and I am getting it clearer in my head.

    I have attached a file with three panels.

    I used Crv2View to generate 3 'hulls', then Unrolled them.

    The first two simple panels unrolled fine, but when you increase the length of the top of the panel , then the Unroll command starts to complain.

    It appears the big secret is, controlling the ratio of the length of the top and bottom panel edges, to maintain 'rulability'
     

    Attached Files:

    Last edited: Sep 22, 2013
  12. DCockey
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    DCockey Senior Member

    Using Crv2View results in curves. How do you go from curves to surfaces?
     
  13. rwatson
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    rwatson Senior Member

    Oh, yes - then the intermediate step

    "Surface from 2 or 3 edge curves, after highlighting two curves"
     

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  14. DCockey
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    DCockey Senior Member

    Usually that will not produce an "exact" developable surface. Rather the surface will usually have some amount of compound curvature. Depending on the shape of the curves and the application requirements the resulting surface may or may not be close enough to developable.

    I put "exact" in quotes because any method for calculating developable surfaces is not exact in the purely analytical sense. But the algorithms used by Rhino can produce results with accuracy up to 15 or so significant figures.

    Use Loft with the Developable option or DevSrf for an "exact" or close to "exact" developable surface from two arbitrary curves.
     

  15. rwatson
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    rwatson Senior Member

    Thanks David. I used that Loft suggestion to figure out a way of getting a developable hull line.

    I made a 'box', applied slices to get the plan curve, and a curved slice to create an arbitrary first chine.

    Then I DupEdg - ed the intersection of the chine slice and the keel line, and copied the curves out to the side.

    Then, I used Loft to create a develop-able panel, which I Moved back into the hull. The edges of the developable surface then created a revised chine line, that could then be used to create the boat side.

    In this way, its possible to create a whole hull, with guaranteed developable panels.

    Of course, the whole thing is so much easier with Freeship, specify the dimensions, and have the displacement figures done for you.

    This is the first time I ever had a surface get smaller in the Unroll
     

    Attached Files:

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