Try FreeCAD. It's a true solid modeler, like Autodesk Inventor. The solid modelling has roughly the same functionality as Inventor or SolidWorks, but the user interface is far worse. It's so open source. There's a pane which shows you the standard output from Python, in case something happens to print something. (This is mostly useful for people writing extensions to the program in Python, and, in typical open-source style, it's far too prominent.) There are panes in which dialogs appear, but they don't fit properly; they may be higher or wider than the pane. They're all QT-ugly, of course. There's no visibility in the UI of what's important and what isn't. Everything has a large number of properties, most of which are irrelevant unless you're developing. And, of course, the program has a huge number of modal hotkeys.

Basically, someone slapped the UI of an integrated development environment onto a CAD program. That's so open source.

This is a rather good program, too. The underlying solid modeler seems to have all the right stuff. That's the hard part. Then they blew the UI.

3D content generation UIs are very hard. Some of the issues:

- Things may be very cluttered. Selecting can be difficult.

- Some systems allow you to select volumes, faces, edges, or vertices. In tight spots, selection needs smarts.

- For some operations, you need to select a subject and an object. But you may need to pan, zoom, or rotate between those two selections. Exactly when thing get deselected is a big issue.

- Most GUIs are subject-verb. (Select something, then do operation.) That follows Macintosh and Windows practice. This doesn't always work out for CAD, where you may need to select multiple things, multiple kinds of things, deselect mistakes, and perhaps perform other operations while selecting. Bad solutions involve holding down SHIFT or CTRL or two mouse buttons at once.

The thing I don't really understand is why not just opensource it at that point? Keep the rights to the source, but let customers audit, modify, and distribute those modifications to the community. Being able to audit the calculations your software is doing seems like it would bring a lot of confidence to engineers even if a small minority actually look, privacy and FOSS advocates would tolerate it, and they get bug reports and possibly a few free patches.

Businesses and professionals would still pay for valid licenses and enthusiasts get a 'community edition' to get them hooked. It seems like it's a win-win.