This should be avoided, and FreeCAD will notify you if such case occurs. When there are redundant constraints, where one could be removed without allowing the geometry to be moved, it is called over-constrained. When a sketch has the exact number of constraints that prohibits any point of the sketch to be moved, we talk about a fully constrained sketch. For example, you can place a vertical constraint on a line segment to force it to stay vertical, or a position (lock) constraint on an endpoint to prohibit it from moving. These constraints can be applied either on linear segments or on their endpoints or center points, and will force the geometry to adopt certain rules. A Sketch is a 2D object, made of linear segments (lines, arcs of circle or ellipses) and constraints. Part Design objects are fully based on Sketches. Since all the tools from the Sketcher Workbench are also included in the Part Design Workbench, we can stay in Part Design and we will not need to switch back and forth between the two. We will now use exclusively the Sketcher and Part Design tools. In the example below, we will make a piece that is 1.5 times bigger than the original. These are pretty easy to model and print on a 3D printer, and with a bit of patience (3D printing often requires much adjustment and fine-tuning) you can make pieces that are totally compatible and click perfectly into original Lego blocks. The cool thing with Lego pieces is that the dimensions are easy to obtain on the internet, at least for the standard pieces. To illustrate how the PartDesign Workbench works, let's model this well-known piece of Lego: For this reason, in FreeCAD, the Part Design Workbench is the perfect tool to build such pieces, because it will always insure your objects stay solid and buildable. But you always need to have a clear notion of which point is inside the material, and which point is outside, because the 3D printer or the CNC machine needs to know exactly what is filled with material and what is not. Nothing prevents them from being hollow inside, of course. As they will become real, solid objects, this is obvious. When you print objects in 3D, it is of ultimate importance that your objects are solid. Product design is originally a comercial term, but in the 3D world, it often means modeling something with the idea to have it 3D-printed or, more generally, manufactured by a machine, being a 3D printer or a CNC machine.

Freecad extrude part of a sketch