4 Replies Latest reply on Dec 8, 2014 3:21 PM by John Burrill

    Defining a 3d sketch

    Jacob Bonning

      I have been working on a project for my engineering class. I have been modeling a F-117 Nighthawk stealth plane from a small model which I have.  I created a 3d sketch as a sort of skeleton and then created a block around it which I carved up to create the body of the plane.  The problem is my teacher says I need to define this sketch but it is nearly impossible. Is there a way to do this which I don't Know about???  Please help.

        • Re: Defining a 3d sketch
          Christopher Estelow

          All you would have to do is go into each of the sketches and dimension the geometry from the origin.  All of the sketch lines will be blue until they are fully defined at which point they should turn black.

           

          Chris

          • Re: Defining a 3d sketch
            Roland Schwarz

            Defining could be as simple as fixing points and segments so they don't move.

             

            Fully defining sketches when developing concepts is overrated.  A magical thing happens to an unconstrained sketch when a it is closed: NOTHING!  Everything stays where it is until the sketch is opened again.

             

            That said, you may want to break your sketch down into smaller pieces.  First, a main structure, then smaller sections built off of that structure.  Overly-ambitious layout sketches will lead to trouble.

            • Re: Defining a 3d sketch
              Christopher Estelow

              Also wanted to give you a little advice as well......center line/construction lines are your friend when sketching.  Don't be afraid to use them often!!!

               

              Chris

              • Re: Defining a 3d sketch
                John Burrill

                Constraining 3D sketches can be a difficult proposition.  Sometimes, even though nothing moves when you drag it; even though adding any kind of relation will cause the sketch to fail to build, the sketch will still report as under-defined. This isn't like 2D sketching where you can almost always constrain the entire system.  I speculate that the reason for this is gimbal lock.

                If SolidWorks uses Euler rotations to define object constraints then it could be producing solutions that aren't fully constrained but at the same time don't allow the geometry to be rotated any further.  Another thing to consider is that a 3dimenional sketch has 2 dimensional constraints that might not occur to a 3 dimensional solver-like for example if you make a quadrilateral into a parallelogram by making the opposite legs equal length, then you've forced it to be a planar shape and it won't move out of it's plane.  But you haven't identified as being constrained to a plane, So SolidWorks may limit the motion of the vertices when you drag them but regard the shape as under-defined

                When it comes to modelling, using fully defined sketches makes for more stable geometry and faster rebuilds-that's true.

                However, I'm of the opinion that you should never mus-represent what you know in defining a model or sketch.  That leads to confusion when you try to understand your sketches, so placing arbitrary dimensions and relations in a situation where your information is incomplete is not a good plan.

                I'm going to concur with Roland, on a concept design, where you only have partial understanding of the subject, fully defining a sketch is not critical.

                However, you should enlist your instructor's help to complete the assignment the way they directed.  Get used to pressing your colleagues and directors to help you meet their requirements.  No one should be allowed magical expectations of an engineer.