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FreeCAD - Help needed making a hollow shape from a b-spline

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  • FreeCAD - Help needed making a hollow shape from a b-spline

    I am new to FreeCAD (ver 0.19), and to CAD applications generally, and finding climbing the learning curve a bit challenging.
    I have got it into my head that I would like to print (Ender 3 Pro) models of designs for dwellings along the lines of the MARSHA habitat ( https://www.aispacefactory.com/marsha) for Mars.
    Essentially these designs will be a skinny egg shaped structure created from a b-spline and base line. Because I want to do variations on the idea (eventually) measurements are parametric (thanks for the Spreadsheet video Irv, very helpful.)
    My problem is, after using 'Revolution' to create a solid object with the desired exterior shape I can't figure out a way of hollowing it out. (at least not in FreeCAD.) The interior void needs to follow the shape of the exterior to create an even thickness wall. Unfortunately, 'make thick solid' tool doesn't work with b-splines. ( at least it didn't for me). I have tried to 'clone' the shape to use it subtractively after changing its dimensions but it seems the original shape and the clone are linked so that changes in one's dimensions are copied in the other. (Handy for most uses but not this one). I've also looked at copying the Sketch from the original shape into another Body so that I can change the parameters in the copied sketch then use the resulting shape to form the hollow. But there doesn't appear to be a way of copying a sketch. (Simply redrawing it in a new sketch isn't ideal due to the possible complexity of the curves and my non-existent artistic ability)
    Have I missed something blindingly obvious? All suggestions greatly appreciated.
    Thanks

  • #2
    As a thought, instead of just sketching just the outer wall, Sketch an inner wall also as part of the sketch and then do the revolve? Another parameter would be 'wall thickness'.

    Country Bubba

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    • #3
      I hope I understand what you want to achieve.

      In part design simply click on the bottom and select the "thicken" tool.

      First open part design and create b b-spline to create the outer shape on, e.g. x/z axis. Perform a rotational extrude. Basically create a vase like shown in Dr.Vax channel. You are only a few clicks away from making it hollow, when using the "thicken", tool. No need to create inner walls or a second sketch.

      The name "thicken" defines only partly what it does. It is a great tool to hollow any kind of object. You simply select a wall, which will be the opening, define a wall size and your object is a "box". Depending on your measurements you need to select inwards or outwards to keep the dimensions. Basically it takes the outer walls you created and hollows them to the inside, while adding material on the outside (object gets bigger by the wall size) or it goes a wall dimension to the inside and then hollows it out (object size stays the same).

      Sometimes, if your object is bend and twisted adding material version works better.

      The same way can create pipes. All you need to select is two faces (inlet and outlet) and the mode.

      So to sum this up: To create an open box in part design, you just need to create a cube, define dimensions and select the thicken tool, with the face you want to be the opening. Basically two operations. Compared to creating a sketch for the bottom, padding, create a sketch for the walls, padding, this solution is far easier and works for complex shapes, too
      Last edited by Geit; 01-29-2021, 08:19 AM.

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      • #4
        Would this be a place where an 'offset' tool would work?

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        • #5
          Thanks for the suggestions. They are greatly appreciated.

          Country Bubba: The problem with drawing an inner wall is that it is a non standard shape and getting it to be an exact match for the outer wall is a bit beyond my drawing ability. I could draw it roughly but with the same number of points then constrain each point to be an exact distance from its corresponding outer wall point, but that will get very fiddly, very quickly.

          Geit: The 'thicken' tool is what I was hoping to use initially. It just produced a "BRep_API: command not done" error. I am assuming the B-spline shape used to define the shape is just too complex for FreeCAD's 'thicken' system. I did think maybe it was a glitch in ver.0.19 but I had exactly the same result in 0.18 and 0.17. I guess it might also be possibly a glitch in the Linux versions I am using. I'll boot up the old Windows box and test that theory too.

          Ender5r: An 'offset' tool sounds intriguing. I'll have to find it and play with it now to see how it works.

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          • #6
            The offset tool should do what I was saying and give you a consistent "thickness" regardless of your shape. After doing the offset, connect the ends and do the revolve.

            Country Bubba

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            • #7
              Originally posted by Country_Bubba View Post
              The offset tool should do what I was saying and give you a consistent "thickness" regardless of your shape. After doing the offset, connect the ends and do the revolve.
              Indeed.

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