Improved CAD systems provide automatic generation of simplified and defeatured versions of complex 3D CAD models. The systems receive a 3D model representing a real-world object as an assembly. The model is formed of the assembly, plural components, and features. In response to user command, the systems select one or more components of the plural components. The systems apply a simplification operation to the selected one or more components. The simplification operation derives a simplified geometric entity (e.g., box, cylinder, solid 3D polygonal outline, or solid 3D tight fit outline) by projecting at least one silhouette outline that encloses the selected one or more components. The at least one projected silhouette outline excludes features of the selected one or more components from graphical view. The systems generate a simplified model from the derived simplified geometric entity. The systems may use the generated simplified model to replace components in 3D models.