4.23D solid modeling: extrusion, revolve, sweep, loft & boolean operations
The heart of 3D in AutoCAD is the solids. They are most often created from closed 2D profiles that you turn into a volume: extrusion (giving height to an outline), revolve (rotating a profile around an axis — for revolved parts), sweep (following a path) and loft (connecting several profiles).
Then you combine these volumes with boolean operations: union (merging), subtract (hollowing out, drilling) and intersect (keeping the common volume). That's the logic of solid 3D: start from simple shapes and add/subtract them to build complex parts. This chapter is the true starting point of volumetric modeling.
Section vocabulary
- 3D solid
- Solid volume (as opposed to surfaces/meshes), on which boolean operations can be performed.
- Extrude (EXTRUDE)
- Giving thickness/height to a closed 2D profile to create a solid.
- Revolve (REVOLVE)
- Creating a solid by rotating a profile around an axis (revolved parts).
- Sweep / Loft
- Sweep: running a profile along a path. Loft: connecting several successive profiles.
- Boolean operations
- Union (merge), Subtract (hollow/drill), Intersect (common volume) of solids.
How is a 3D solid most often created?
In practice — Creating and combining solids
- Draw a closed 2D profile (polyline) and extrude it (EXTRUDE) to get a first volume.
- Create a revolved part: a half-profile turned around an axis (REVOLVE).
- Combine two solids with Union, then drill a hole by subtracting a cylinder.
- Try sweeping a profile along a path, and keep the common volume of two solids with Intersect.
Key points to remember
- Solids are often created from closed 2D profiles.
- Extrude (height), revolve (around an axis), sweep (path), loft (profiles).
- Boolean operations: union, subtract (drilling), intersect.
- The logic: start from simple shapes and add/subtract them.
Frequently asked questions
My extrusion doesn't work / creates a surface instead of a solid, why?
Almost always because the profile isn't a closed outline (a closed polyline or a region). If the profile is open or made of separate segments, the extrusion creates a surface, not a solid. Convert the outline into a closed polyline (or a region with REGION) before extruding. A clean, closed profile is the condition of any solid — it's the direct extension of the 2D rigor of level 1.
Solids, surfaces or meshes: what's the difference?
Solids are full volumes (the most used, they support boolean operations): ideal for mechanical parts, furniture, building volumes. Surfaces describe envelopes (complex, curved shapes). Meshes are faceted, suited to organic shapes and sculpting. For everyday technical 3D in AutoCAD, you mostly work with solids.