Questions about dxf files for aluminum/steel boats

Discussion in 'Boat Design' started by Barry, Sep 10, 2017.

  1. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Metal boat builders who utilize the same basic hull shape for various boats will have a set of cut files, dxf or other, to laser, waterjet or plasma cut the pieces to enable a quick fit up for fabrication.

    Question #1: Is there a set of parameters within boat CAD programs that you can input to ensure that the hull profiles, top, side, back, lines will produce a hull that creates developable surfaces?

    Question #2 Assume that you have created a set of cut files from a CAD program so that the sheets are developable. You need to ensure that the sheets make contact when matching the pieces together, but say at the bow where the bottom of the hull should contact the side, in a planing hull anyway, the thickness of the material where the two adjoining plates come together can easily push the plates apart. Ie the CAD file produces an X-Y two dimensional plane for infinitely thin pieces, but if you were be joining 1/4 inch plate together at acute angles say 45 to 135 degrees, the thickness of the plate could hold the sheets away from each other. As you do not want to be filling 1/4 inch gaps.
    So the question is this, you have a set of developable 2 dimensional cut files, is there ANOTHER PROCESS that must be done to ensure that you will have tight joints or acceptably close joints to permit welding.
     
  2. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    There are several methods to create developable surfaces. One is to create the surfaces as a combination parts of planes, cylinders and cones.

    Another is to start with a pair of edge curves and use a command or plug-in in CAD software which determines the developable surface between parts or all of the input edge curves. This may require extension of the one or both edge curves past the ends of the desired surface. Iteration may be needed.

    A third is "close enough based on experience" with adjustments, if needed, made on the first boat built to the design.
     
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  3. DCockey
    Joined: Oct 2009
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    Location: Midcoast Maine

    DCockey Senior Member

    You can also design the inner and outer surfaces of plates with thickness, and the center (neutral) surface if desired. Good CAD software will have the ability to create offset surfaces with a simple command.
     
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  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    There are several methods to develop non-developable surfaces with sufficient accuracy, geodetic line method, triangulation method, straight base method, ... and the current programs use cones and cylinders adapting various "patches" to the surfaces of the hull. Even in some very complicated cases, 1: 1 scale models of some area of the ship were used, models that were later reduced to scale 1:10, to obtain the templates of the non-developable parts. All these methods have been used for many years in the construction of large ships and have sufficient accuracy.
    Always work with the inside of the hull. This also ensures that the contours of the internal parts, floors, strong plate frames, ... exactly match the surfaces of the hull on which they rest. The external part of the surfaces, in this work, is never used.
    All this work is laborious but it compensates due to the saving that it assumes in assembly workmanship and the precision that is obtained. Even for constructions of only one boat it is worth doing this job.
    By the way, the files can be DXF or many other formats. Current cutting machines understand most of the files generated by major drawing programs. There are also processors that translate drawing files to machine code.
     
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