CAD design rules?

Special Interest Groups Design
#1

I stopped using Fusion360 when AutoDesk started playing feature limitation games with the Educational and Free version. After all, they did the same and then worst with Eagle CAD.
So I started using FreeCAD( http://freecad.org ) and I didn’t find there was much of a relearning curve since I was already new to F360.

But, like F360 there are some bumps and warts to run into and work around. One is attaching sketches to faces, in FreeCAD you don’t want to do that because when you change a geometry related to the face, the internal structure changes and it can effect/lose the reference which the sketch was attached to. It’s known as the Topological Naming Problem( TNP ) and it’s on the list of things to resolve but it’s still there and the fix goes deeply into the code. The work-around is to create a Datum Plane on the surface and attach the sketch to the Datum Plane.

Another FreeCAD user missed this and has struggled with a complex part betting messed up after changing the design and the sketch attachments got lost or wonky and he’s had to rebuild two times and asked for help on why it was happening.

Surprisingly, a current FreeCAD user and former ProE/Creo designer wrote this which looks like a good list of basic geometry design rules:

I used to work professionally in ProE/Creo in a big international organization for many years. We had clear instructions on best practice for the structure and how to build the models.

  1. Always attach sketches to datum planes, never to surfaces.
  2. Never mirror or copy.
  3. In assembling use datum planes as references.
  4. Run “model check” before saving to the server.
  5. All dependencies locked to datum planes.

Despite all this it happened that the models crashed and you had to start all over. Especially when you did late changes.

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#2

Those design rules are valuable. It matters how you create datums (planes, axes, and points). If the references used to create datums are volatile, you still risk problems.

One approach is to employ a skeleton model, a virtual model that includes key datums and drives your parts and assemblies. It’s part of a top down design approach, implying and imposing structure from the beginning.

Sometimes when I don’t have a full enough vision of the end design to properly set up a skeleton, I will “rough” a model using bottom up techniques until I’ve sorted out the design. Then I’ll start over with a well planned top down design and a skeleton.

Some of this depends on the complexity of the design. For a lot of the simpler wood designs, one can reach the endpoint with a basic bottom up approach. Even so, using planes for sketches and boundaries makes for a more robust model.

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#3

The more and more CAD design I do the more I see why people have so much difficulty picking it up for anything more than basic stuff. There’s a need to sit down and think about what you want to do, how to approach it AND how the tools want you to approach it and then there are all the size specifications you have to work out.

I like to do basic drawings on paper while jotting down dimensions and then I build the spreadsheet(FreeCAD thing) for parameterization. I do kick my self sometimes for spending that time up front for even simple-one-off things but I feel it’s good to create a habit and be consistent.
ADHD much?.. LOL

Thanks for the input Tim and again, thanks for taking the time to run a bunch of us through the learning process.