6 Replies Latest reply on Sep 14, 2012 4:53 PM by Qi Lu

    "Reference geometry" better than "3D sketch"?

    Qi Lu



      I would like to ask some question about 3D sketch.


      We can use TAB to select between XY, XZ, and YZ plane on which our cursor position will be positioned. However:


      1. Can we change origin? If we have no way to change the origin, then no matter which sketch tool we choose (line, rectangle, arc, etc.), the first point that is projected into the selected plane upon our mouse-click is would ALWAYS LIE on the current plane. This means we have no ability to place the first point at an off-XYZ-plane location like coordinate (1,2,3), except if we move the drawn entity later.


      2. Even with TAB toggling target projection plane, we are sitll limited to XY, XZ and YZ planes. This means that even if we have used "Reference geometry >> Plane" to create some oblique planes which are not parallel to any of the standard XY, YZ or YZ plane, we cannot make use of them in 3D sketch. Is it true?


      3. I want to know if 3D sketch is a commended functionality or not? As Matt Lombard put in SolidWorks 2011 Parts Bible:


      "Improvements have been made in the past several versions,but 3D sketches still lack some important bits of functionality"


      "As a general caution, keep in mind that solving sketches in 3D is more difficult than it is in 2D. You will see more situations where sketch relations fail, or flip in the wrong direction. Angle dimensions in particular are notorious in 3D sketches for flipping direction if they change and go across the 180-degree mark. When possible, it is advisable to work with fully defined sketches, and also to be careful (and conservative) with sketch relations." 


      Mark's note seems to have some points in common with the two questions I asked above that there are limitations and certain control difficulties when working with 3D sketch. I wonder if 3D sketch is one of the primary tools in SW or not? By using "Reference geometry" we can get much more definite sketches than 3D sketch, so is "Reference geometry" preferred over 3D sketch?




        • Re: "Reference geometry" better than "3D sketch"?
          Alin Vargatu

          1. Not true if you have other geometry already in the model. You can draw entities from existing edges, vertices, faces...

          Also, you can create and use 3DSketch planes to further simplify the work inside 3DSketches:




          2. Again, not 100% true. You can change easily the orientation for the 3DSketch XYZ direction, by pressing CTRL and pre-selecting a face or plane to provide the new orientation




          3. As with any tool or technique, you need to know how to use it. That being said, I like the KISS principle.


          Read this 12 common sense rules for 2D sketching. Some of them, can be easily extended to 3D Sketching.



            • Re: "Reference geometry" better than "3D sketch"?
              Qi Lu





              If I

              1. create a construction line normal to "Right plane" using 3D sketch

              2. Insert a 3D plane to the construction line

              3. Draw a rectangle and a triangle on the plane, add vertical and horizontal relation to them, and fix all their dimensions, and fix one of their verteices. Then by elementary geometry both the triangle and the plane are already completely determined.

              4. However, I found I can still drag/rotate them. And when I an edge of the Δ which has "-" relation rotates, the "-" edges of the rectangle also rorates and {Δ's "-" edge} remain∥to {'s "-" edges}.


              The only conclusion I could draw from this observation is that the 3D plane2 is under defined in the sense that its horizontal and vertical direction is not determined. The Δ's "-" edge is actually defining the horizontal direction of the plane, so when I am dragging it I am actually changing the plane's orientation; since two "-" edges of the should be horizontal with respect to plane 2, they rotate following the Δ's edge.


              Is this interpretation correct? After we have created a plane in, even relations like

                  1. coincident to a fixed point

                  2. coincident to another plane

              have been defined, it can still rotate, so it still remains undefined. Only after we have somehow fixed its horizontal/directional direction would it then become fully defined. Is this correct?






                • Re: "Reference geometry" better than "3D sketch"?
                  Alin Vargatu

                  It is easier if you think of a 3D Sketch plane as simple Sketch entity like a line or an arc or a spline.


                  A 3D Sketch plane has its own relations (again, like any other sketch entity). To prove that, create a 3D Sketch plane by any means and exit back to the 3D space. Now select the plane and you will see whatever relation it has.


                  plane relations.jpg


                  The plane above was defined by 2 relations:


                  1. perpendicular to the line

                  2. coincident to the end point


                  That is all.. There is no Horizontal or Vertical relations assigned to it. It does not have an "orientation" from that point of view. Whatever line you draw on it, can get a local Horizontal or Vertical relation but that is just in regards to the plane. Since the plane is not fixed rotation-wise, the lines are not either.


                  Let's think of an real-life concept. You have a piece of paper on which you draw a vertical and an horizontal line. Now you take the paper and you stick it on a wall with a push-pin. The paper can rotate and the lines will rotate with it in regards to the wall. But the lines are happy because they think that they are still horizontal and vertical on the paper.


                  I guess, everything is relative, my friend.

                    • Re: "Reference geometry" better than "3D sketch"?
                      Qi Lu



                      I think this thread can be concluded. Upon the creation of a plane is should be rotational-free, and entites (particularly line) we drawn on it assigned with relations like horizontal or vertical is only "relative", as you have described. At this moment, the horizontal line in the 3D plane, is like we draw a line on a A4 paper parallel to the shorter edge.


                      However, if we add another fold of reference, for example add a paralle relation between a horizontal line on the 3D plane and another line out of this sketch, this would then fix the horizontal line (in 3D plane)'s direction, and simultaneously fix the rotational "orientation" of the 3D plane. This would like we align the horizontal line drawn to the paper to the vertical bar of the window, hence fix the A4 paper orientation.