Willem wrote:I was not aware of the 2D offset function either. I think it is not logical that this function is not located in the Sketcher and/or the PartDesign workbench, the logical workbenches for 2D functions, but in the Part workbench, which is the ultimate 3D workbench
The Part workbench is a GUI manifestation of the Part module which is the basis for everything in FreeCAD. It's a general purpose workbench that can build all kinds of shapes from the ground up (from vertices to solids). The Part 2D Offset tool not only works on Sketches but on any type of 2D geometry. Therefore it's logical that it be located in the Part workbench.
What would be more suited to the Sketcher would be an offset tool inside the sketch editing mode.
jean.thil wrote:I'd like to find a way that works for most shapes. I have tried some other way but without success.
As far as I can tell, thickness and offset tools are based on similar algorithms. They've proven to be weak and unreliable, even with relatively simple shapes that don't contain B-spline surfaces. It may very well be the OCC kernel itself that's at fault.
So I doubt you'll be able to figure out a 100% working process.