A 3D model is like a drawing, but instead of being limited to a two-dimensional representation of a part or scene, it contains all the information needed to construct an actual part in three dimensions. Some 3D modeling programs work by expanding on linear drawings, extruding curves in space or stretching surfaces between them. Others let users work directly with solid objects, either as adjustable “primitive” forms (cones, cubes, spheres, etc.) or as amorphous blobs of virtual “clay” material which can be pushed into shape with a set of tools. Some allow 2D to 3D conversions of various sorts, or allow 2D renderings to be derived from 3D scenes, with reflections, refractions, and shadows precisely calculated. Much 3D modeling is directed towards creating animations and imagery, but our focus is on producing models that can be directly transformed into physical objects via CNC milling or additive rapid prototyping processes.