5 Replies Latest reply on Nov 16, 2012 5:26 PM by Charles Culp

    Surface offset problems

    Rolf Anderson

      I am trying to generate an approximately 0.1" thick shell model of a freeform part with many smooth contoured curves. The part has no planar surfaces and no symetry. I have modeled the outside surface of the part as a set of cross section sketches. Each sketch is a closed spline. I can do a surface loft (actually a series of surface lofts, since the number of cross section sketches is quite large), and get a quite good model of the outside surface. The problem is trying to do a surface offset to get the inside surface of the model. In some cases I can offset the outer surface loft just fine. In other cases I cannot offset it because the outer surface has a radius of cuvature of less than .1". In these cases I generate a modified set of outer surface cross section contours with a larger radius of curvature, generate a new loft, and offset that to get the inner surface. But in some cases the surface offsets don't seem to be working properly.

       

      For example, in one case I have generated a surface loft using five sketches as profiles. This loft measures approximately 0.47 inched high. When I do a surface offset, the preview shows the inner surface just fine. The offset generates without error, but the inner offset surface ends up being a VERY thin band measuring as little as .01 iches high! This band is near the bottom of the outer surface loft that I had offset. I can make the outer surface loft taller by including one or more additional profiles, but the generated inner surface offset remains exactly the same. Some lofts offset just fine, but this one will not. What gives?

       

      Maybe someone can tell me a better way to generate my model? Maybe someone can help me figure out why the surface offset does not seem to be generated properly?

        • Re: Surface offset problems
          Jeremy Feist

          I'm not sure what is going on with your offset (a sample model would help or image would help, if that is allowed).

           

          but since you are making a shell from the surface body, have you tried the thicken feature?

          • Re: Surface offset problems
            Charles Culp

            It sounds like you have a very sharp change in curvature near the ends of your surface, that is why it created the band. The software is trying to compensate for you doing an offset that is technically tigher than what it can really do. So it cheats a little by creating the band, but it does a successful offset. Really though, for more help you are going to have to upload an example.

              • Re: Surface offset problems
                Rolf Anderson

                Ok, here is an example. I took my drawing and stripped out everything I could except what was needed for the problem loft that will not offset properly. The drawing ends up with one rebuild error because some reference geometry used to generate one plane was deleted, but you can still see the offset problem. Here is a link to the sample drawing (about 2.2MBytes).

                http://krubow.com/sample/Sample.sldprt

                 

                I didn't see any other way to upload an example. From what I can see, I would expect the offset to fail rather that gererate this obviously wrong result. Offseting surfaces with a small radius of curvature is really problimatic. And Thicken doesn't work any better. I find if I can't offset, I can't thicken either. My shell doesn't need to be exactly 0.1" thick, just approximately. But generating a whole new set of profile sketches for the inner surface seems like a LOT of work.

                  • Re: Surface offset problems
                    Jeremy Feist

                    yeah, a minimum radius of curvature of ~.06" is going to make offsetting .1" difficult and thicken gives a similar result to the offset surface.

                     

                    if you have the design flexibilty, I would try to increase the radius of curvature, otherwise I suspect you will need to add profiles for the inner surface.

                     

                    oh, and to attach files to your replies, you need to use the "advanced editor" - the button is above and to the right of the standard reply editor.

                • Re: Surface offset problems
                  Charles Culp

                  Rolf, Thanks for uploading the model, this explains it.

                   

                  The problem is that, as Jeremy mentioned, your model just has too tight of curvature. You cannot offset the .08 radius .10 inches.

                   

                  The "preview" for offsets of single faces typically always works (unless it is a really awkward face), whether the offset works or not. What you have to do is look at the preview and see where it wrinkles. You can see here in your model that there are two wrinkles, both in the area of tight curvature.

                  Offset-Wrinkle.PNG

                   

                  Your only solution is to create a surface that has a larger radius (above .10). I suggest using the split line tool (http://help.solidworks.com/2013/English/SolidWorks/sldworks/HIDD_DVE_PLINE.htm) to split off the areas that are too tight. Then create a new boundary surface between the areas that are split. Then you can make this a larger radius, and offset that.