4 Replies Latest reply on Jan 17, 2018 5:50 PM by Robert Weimer

    trouble getting derived sketches to update

    Robert Weimer

      Hi all,


      I am creating a helical gear, and as such, I am lofting a cutting surface and trying to use 3 sketches to create my profile lines.  So there is 1 sketch with all of the dimensions, and 2 derived sketches that are projected to a plame and rotated to be constrained at a "Pierce Point" from another sketch that creates the helical angle of the gear.  Anyway, it works.  I also have created a design table that has several sizes of helical gears that all have the same pressure and helical angle, but different pitch diameters.  So when I changed between the configurations, I get reconstruction errors that are created by the derived sketches only.  In other words the original sketches for the pressure angle and helical angle will accept and choke down the changes, but the derived sketches will not update correctly.

      Then to fix the derived sketches.  I "edit" them, click on the "SketchXpert" and "Diagnose" the situation where it magically comes up and indicates that all is fine, so I hit the green check mark and exit the sketch.  Then the "problems" go away.  I do this for both sketches every time I switch the configuration, or something changes in the original sketch that the derived sketches are created from, or if I change the helical angle sketch that creates the "Pierce Point"s to be constrained to.


      It is really starting to wear on me to do this every time so the question is?


      I just want it to update seemlessly, so is there a technique that I can use because obviously it doesn't like to use the pierce point constraint right off the bat.

        • Re: trouble getting derived sketches to update
          Glenn Schroeder

          Can you post your Part file?   I rarely use derived sketches, so while I may not be able to help someone else will almost certainly be able to, and having the file instead of a description almost always helps, even if it's a good description (and yours is better than usual).

          • Re: trouble getting derived sketches to update
            M. B.

            I use derived sketches often with the parent sketch controlled by design tables defining hundreds of part configurations.  I have never had a problem with the derived sketches updating.

            - remove all relations from the derived sketch(s)

            - open and close the design table to update values

            - switch configurations to see if the derived sketches fail

            My guess is that they will update correctly which would mean that you need to position and orient them with a different relation technique.  The pierce point is probably the culprit.


            Also, why not use a helical sweep with one sketch instead of a loft with three sketches?

              • Re: trouble getting derived sketches to update
                Robert Weimer

                I didn't think to do a helical sweep first, at the time I was all juiced up to try the derived sketch deal, it sounded like something I can employ with some other projects I have going0--=, and when I checked the geometry with the 3D scanned geometry of the actual part, the modeled geometry is within the tolerance (.01mm) to the scanned geometry.  Keep in mind the scanned geometry itself is only +/-.001".  I will look into the helical sweep technique to see if I can recreate the same tolerance to the 3D scan as the loft command.  I agree that the pierce point is the culprit, so that leads me to believe that in the future I should avoid this constraint or think of it as a almost constraint.  It almost works but not quite.