Note that the last point-on-point constraint was not absolutely necessary. You can try moving its lines or points, nothing will move anymore. The rectangle will then jump to the origin point, and your sketch will turn green, meaning it is now fully constrained. Finally, select one of the corner points, then the origin point (which is the dot at the crossing of the red and green axes), then add a Coincident Constraint.Select one of the horizontal segments and add a Horizontal Distance Constraint.Select one of the vertical segments and add a Vertical Distance Constraint.You can experiment moving the rectangle around by dragging its lines with the mouse, all the geometry will keep obeying the constraints. You will notice that a couple of constraints have automatically been added to our rectangle: the vertical segments have received a vertical constraint, the horizontal ones a horizontal constraint, and each corner a point-on-point constraint that glues the segments together.You can place the two points anywhere, since their correct location will be set in the next step. Now we can draw a rectangle, by selecting the Rectangle tool and clicking 2 corner points.The sketch will be created and will immediately be switched to edit mode, and the view will be rotated to look at your sketch orthogonally. A dialog will appear asking where you want to lie the sketch, choose the XY plane, which is the "ground" plane. So let's start this by making a rectangular sketch that we will then extrude: Later on we will carve the insides, and add the 8 dots on top of it. Let's start by modeling a cubic shape that will be the base of our Lego brick.The Draft workbench also has a tool that converts Draft objects to Sketches, and vice-versa. When you are done with editing, and leave edit mode, sketches behave like any other FreeCAD object, and can be used as building blocks for all the Part Design tools, but also in other workbenches, such as Part or Arch. Sketches have an edit mode, where their geometry and constraints can be changed. This should be avoided, and FreeCAD will notify you if such a case occurs. When there are redundant constraints, that could be removed without allowing the geometry to be moved, it is called over-constrained. When a sketch has an exact amount of constraints that prohibits any point of the sketch to be moved anymore, 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 to move. 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 PartDesign 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 PartDesign Workbench is the perfect tool to build such pieces, because it will always take care for you that 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 commercial 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, for example a 3D printer or a CNC machine.
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