[delhanty]

> I've been using parametric CAD (Pro/E / Creo, Solidworks, and Onshape) for 30+ years.

True - I implemented the first Solidworks Autodimension sketch as a contractor around 2002 based upon earlier work that I'd done for a consultancy client at D-Cubed in the late 1990s. I'm sure it could be improved with AI and a large data set of sketches though.

> Solidworks tries to help with this in the form of cycling through what it thinks are the most likely constraints to remove and then re-solving the sketch. It's OK, but that sort of tool could be better.

I agree it could be better. The behavior with under-constrained sketches depends on D-Cubed's DCM, and I seem to recall they were rather floppy. It seems kind of ridiculous to make users jump through the hoops of making sketches fully constrained once they've added the constraints they care about.

[gerdesj]

I am not an expert but surely an under constrained sketch is not a completed sketch.

Does "fully constrained" mean the simplest set of constraints that yields a shape (volume/hypervolume)? Or something rather more complicated? A simple yes or no, with a pointer to a paper will do!

[rb-2]

To add to delhanty's reply, a "degree of freedom" can be thought of as a dimension of the drawing that can be changed or "stretched" or moved without violating a constraint (this is slightly inaccurate, but it's a good start). In a CAD program, a fully constrained drawing can't be freely stretched or dragged around; the program won't let you and the drawing will feel "rigid".

It's very intuitive if you play around with a CAD program for a bit. There is a free (GPLv3) 2D and 3D CAD program called Solvespace (https://solvespace.com/) that is probably easiest one to obtain and learn. There are detailed tutorials on the website, and you could probably download it and finish the first tutorial in an hour.

[WillAdams]

In addition to Solvespace there is the nascent (but surprisingly polished):

https://dune3d.org/

the Github page of which has the following footnote:

>I ended up directly using solvespace's solver instead of the suggested wrapper code since it didn't expose all of the features I needed. I also had to patch the solver to make it sufficiently fast for the kinds of equations I was generating by symbolically solving equations where applicable. ↩

Which really impressed me because it was the first graphical and interactive 3D program I tried which felt sort of comfortable and understandable (which is why I mostly use OpenSCAD and similar programmatic approaches).

[gcr]

In Fusion 360, you can drag underconstrained parts of the sketch around with the mouse. So the “degree of freedom” is still specified, but by accidental placement of how you drew the sketch. Fully-constrained sketches show a black outline and can’t be changed without explicitly editing a constraint or dimension.

For a quick example, see slide 10 in https://docs.google.com/presentation/d/1TiVoKF7mLlF360Oxy8hN... This is just a quick doc I made to show a friend how to use Fusion 360.

[delhanty]

"fully constrained" means that there are no degrees of freedom left so that there are only a finite number of valid solutions, which are then consequently disconnected in the space of possible solutions.

DCM then chooses one valid solution that it believes is "close" to initial supplied positions/directions/radii etc for the geometry.