A 2D sketch sets the stage for most parametric 3D models.

Sketching, as it pertains to mechanical design software, is somewhat of a misnomer. The commonly accepted meaning of drawing by hand, such as what an artist would make on a sketchpad or an engineer would make on a cocktail napkin, tablecloth, or whatever is handy (we can’t stop ourselves), is not what is meant here. Here we have a more precise version of sketches where lines are straight, circles round and angles right. Sketches in MCAD systems, unlike their hand-drawn counterparts, are neat — by design.

Many solid shapes start as a “sketch” of which defines the profile. The 2D profile is extruded along a straight line to create the 3D shape, a solid. The profile can also be revolved around an axis to create another shape, a solid of revolution. The profile can also follow a spline creating a swept solid.

The profile may have been called a sketch because one can start with 2D geometry that is sans dimensions. You create a circle, for example, without regard for its position and size. This lack of specifics cannot be tolerated for long by anyone designing anything, so MCAD programs allow you to supply specifics immediately upon creation. The circle will have a place where you can override the radius it is drawn with, or a line, its length. You can enter numbers or parameters with a parametric solid modeler. A radius can be entered as 1/4*W, where W is the width. Defining a shape has its advantages as pointed out in What is parametric modeling?

In this way, the profile takes on dimensions and it begins to take on the look of a 2D dimensioned drawing. Don’t worry, the dimensions will be hidden when the solid is created. But there is more to the profile’s precision than meets the eye.

Your MCAD program has been inferring many things about your profile sketch. It has assumed lines are parallel and angles are right angles, for example. It has been applying these inferences, known as constraints, while you have been creating the profile. Most MCAD programs will show you the constraints it has created, but most of the time this is not necessary.

Constraints can be “broken,” i.e., removed. For example, if parallel lines were assumed (making a rectangle) when a wedge shape was desired.

Still, your sketch may not be “fully constrained,” once a requirement before granting entry into the third dimension. In other words, you could not leave 2D, extrude or revolve your sketch, leaving users in a limbo of sorts, trying in vain to track down the missing dimension or constraint in a profile that defied complete definition, a condition that came to be referred to as “constraint hell.”

These days, solid modelers allow you to proceed even if a sketch is not completely constrained.