Overview of Creating Surfaces

Surface modeling provides the ability to edit multiple surfaces as an associative group or in a more free-form way.

In addition to 3D solid and mesh objects, AutoCAD offer two type of surfaces: procedural and NURBS.

Procedural Surfaces can be associative, maintaining relationships with other objects so that they can be manipulated as a group.
NURBS Surfaces - are not associative. Instead, they have control vertices that allow you to sculpt shapes in a more natural way.

Use procedural surfaces to take advantage of associative modeling, and use NURBS surfaces to take advantage of sculpting with control vertices. The illustration below shows a procedural surface on the left, and a NURBS surface on the right.

Choose a Surface Creation Method

Create procedural and NURBS surfaces using the following methods:

Create surfaces from profiles. Create surfaces from profile shapes composed of lines and curves with EXTRUDE, LOFT, PLANESURF, REVOLVE, SURFNETWORK, and SWEEP.
Create surfaces from other surfaces. Blend, patch, extend, fillet, and offset surfaces to create new surfaces (SURFBLEND, SURFPATCH, SURFEXTEND, SURFFILLET and SURFOFFSET).
Convert objects into procedural surfaces. Convert existing solids (including composite objects), surfaces, and meshes into procedural surfaces (CONVTOSURFACE).
Convert procedural surfaces into NURBS surfaces. Some objects cannot be converted directly to NURBS (for example, mesh objects). In that case, convert the object to a procedural surface and then convert it to a NURBS surface. (CONVTONURBS).

Understand Surface Continuity and Bulge Magnitude

Surface continuity and bulge magnitude are properties that are frequently used when creating surfaces. When you create a new surface, you can specify the continuity and bulge magnitude with special grips.

Continuity is a measure of how smoothly two curves or surfaces flow into each other. The type of continuity can be important if you need to export your surfaces to other applications.

Continuity types include the following:

G0 (Position). Measures location only. If the edge of each surface is collinear, the surfaces are positionally continuous (G0) at the edge curves. Note that two surfaces can meet at any angle and still have positional continuity.
G1 (Tangency). Includes both positional and tangential continuity (G0 + G1). With tangentially continuous surfaces, the end tangents match at the common edges. The two surfaces appear to be traveling in the same direction at the join, but they may have very different apparent “speeds” (or rates of change in direction, also called curvature).
G2 (Curvature). Includes positional, tangential, and curvature continuity (G0 + G1+G2). The two surfaces share the same curvature.

Bulge magnitude is a measure of how much surface curves or “bulges” as it flows into another surface. Magnitude can be between 0 and 1 where 0 is flat and 1 curves the most.

Set Surface Properties Before and After Creation

Set defaults that control a variety of surface properties before and after you create the surface objects.

Surface Modeling System Variables. There are a number of system variables that are frequently used and changed during surface creation: SURFACEMODELINGMODE, SURFACEASSOCIATIVITY, SURFACEASSOCIATIVITYDRAG, SURFACEAUTOTRIM, and SUBOBJSELECTIONMODE.
Properties palette. Modifies properties for both the surface objects and their subobjects after they are created. For example, you can change the number of isolines in the U and V directions.