9 Replies Latest reply on Jan 14, 2019 2:58 AM by Thomas Voetmann

    What does 100% really mean for "Untrim Surface"?

    Dan Pihlaja

      OK, here is the scenario:

      I have this little snippet of a surface on a solid body that looks like it was created by a chamfer. This is an imported part and I just wanted to extend the surface so that it crosses my plane that I am working on.

       

      So, I offset the surface 0" to effectively copy the surface.

       

      Then I select "untrim surface" to 100% and select it.

       

      However, this does not extend the surface as far as I want it to.

      So, I thought about it....and I wondered if I could untrim it again.....

       

      I would think that I couldn't...because I previously untrimmed it to 100%.....That is maximum...right?

       

      Sure enough....selecting the surface that I JUST untrimmed and untrimming it again.

      Now, it looks like a half truncated cone.

       

      What if I keep selecting it?

       

      Untrim # 4:

       

      And, apparently, I can keep going, potentially extending the height of the cone infinitely.....

       

       

      So....what does "100%" really mean? How does the program know when to stop?

       

      I looked at the surface areas of the different surfaces and there really isn't a pattern.

      Some of them are almost exactly 4 times the surface area of the previous one...some are almost exactly 6 times.

        • Re: What does 100% really mean for "Untrim Surface"?
          Paul Salvador

          Hello Dan,.. very good question!

           

          I always wondered as well,.. it would seem "relative" to the face/boundary/area?

           

          It would be GREAT if the surface topology was recognized within the feature as either analytical or as nurbs surface and it gave us a choice,.. and, your cone example would have a single option for full 100% untrim within a analytical boundary limit.

            • Re: What does 100% really mean for "Untrim Surface"?
              Dan Pihlaja

              Paul Salvador wrote:

               

              Hello Dan,.. very good question!

               

              I always wondered as well,.. it would seem "relative" to the face/boundary/area?

               

              It would be GREAT if the surface topology was recognized within the feature as either analytical or as nurbs surface and it gave us a choice,.. and, your cone example would have a single option for full 100% untrim within a analytical boundary limit.

              So, how would this analytical boundary limit be determined?  (I don't understand nurbs surfacing enough to be able to comment on that part of it  ).

               

              Would this be something that we would define in the context of the untrim command?  Or would the program automatically put a limit on it?

               

              At first I thought that the "100%" meant that the surface area would be 100% larger....but that is not the case.

               

              In the CATIA Forums, it seems to imply that their program recognizes when untrimming it would result in an infinite surface and then limits it by the bounding box.

                • Re: What does 100% really mean for "Untrim Surface"?
                  Paul Salvador

                  Dan,.. SW would have to have a limit with some of the geometry, the cone wraps to 360 but the base could extend for infinity.. and I would guess it depends on the analytical geometry (plane, cone, sphere, ellipsoid, hyperboloid, paraboloid..) to set  the limit maybe   first set within the work space/area or some ratio of existing area...    and the external limit  would be 1km sq?

              • Re: What does 100% really mean for "Untrim Surface"?
                Roland Schwarz

                That is a great question.

                 

                I was introduced to the concept of untrim in UG(NX). NX remembers the original full untrimmed definition and untrims to that.

                 

                SW is kinda arbitrary. 0 equals 100%, unless it means minimal untrim. 100% = 100% unless it means 200%. There's an elegant randomness to the whole thing.

                • Re: What does 100% really mean for "Untrim Surface"?
                  Matt Lombard

                  For some surfaces, it has a meaning. Like a sphere. 100% is easy to visualize for a sphere. A cone is bounded on one end, but on the other it could be infinite, as with other analytical types. But it doesn't really work that way.

                   

                  You can use Extend where it works better, and sticks to the topological type.

                   

                  100% has more meaning for a bounded general NURBS surface, where the math only goes so far. Extend can extrapolate, but the curvature gets unpredictable..

                   

                  So from these experiments, for analytical types, 100% Untrim could/should be predictable, but isn't. For general spline surfaces, 100% untrim appears to be fully predictable.