Another reason is that Mechanical Desktop does much of the drafting work for you. Even some very complex objects such as helix-shaped springs and threads are created primarily through data you enter in a dialog box. The skills that are important in creating parametric solid models often involve establishing the constraints that control the geometry of models. Constraints have no counterpart in 2D drafting, and to master them you must absorb some new concepts. As shown in Figure 1, properly constrained models are easily modified to accommodate design changes while improperly or inadequately constrained models almost literally come apart during changes.
Constraints are the glue that holds the features of a parametric model together.
The profile, however, is not immediately ready to be transformed into a 3D feature. Its size and shape should first be completed fixed-or constrained. Although nothing prevents you from using unconstrained profiles to make 3D features, such features are not easily modified, and the advantages of parametric modeling are lost. Usually, but not always, both geometric and dimensional constraints are needed to fully constrain a profile. Geometric Constraints Geometric constraints are normally invisible.