Sketcher Micro Tutorial - Constraint Practices
|Time to complete|
|Less than 15 minutes|
|Mark Stephen (Quick61)|
|0.15.4671 Release or greater|
Welcome. This tutorial is designed to help the new FreeCAD user become familiar with the techniques and best practices constraining a sketch. This micro tutorial is basically an enhanced copy from a post on the subject in the FreeCAD forum and placed here for more universal and convenient access.
A Quick Explanation
There is a general rule with FreeCAD's sketcher constraints, the fewer amount of dimensional constraints, the better. It is always preferable to use a geometric constraint in place of a dimensional one if possible. This has to do with the internal workings of Sketcher's solver.
As an example, take a square sketch. In the first screenshot, the sketch is fully constrained, but is using only dimensional, (distance), constraints. this is OK, but overly complicated, cluttered, and more math intensive for the solver. While this is not an issue with such a simple example, it can become one with more complex sketches.
A Better Way
The next screenshot is the same square, constrained using some geometric constraints. Using the horizontal and vertical geometric constraints, along with dimensional, (distance), constraints. By just using the horizontal and vertical geometric constraints, you can see the number of required dimensional constraints have been reduced. This is a better constrained sketch than the first, but is still not the optimum constraining scheme for this square.
Now, in this last screenshot, there is only 1 dimensional, (distance), constraint, with the remaining constraints being geometrical. This is the best way to constrain this sketch. While with a simple sketch like this, one way or the other is no big deal, on larger and more complex sketches, sticking to the minimum dimensions rule will prove out to be beneficial to you and your endeavors constructing geometry with sketches. The geometrical constraints used in this sketch are horizontal, vertical, equal, and symmetry.