SolidWorks 2011 Parts Bible - Matt Lombard [263]
Using developable surfaces
Developable surfaces can be flattened without stretching. They are also surfaces that you can extend easily in one or both directions. These include planar, cylindrical, and conical shapes. It is not a coincidence that these are the types of shapes that can be flattened by the Sheet Metal tools.
Using ruled surfaces
Developable surfaces are a special type of a broader range of surface called ruled surfaces. SolidWorks has a special tool for the creation of ruled surfaces that is described in detail in the next section. Ruled surfaces are defined as surfaces on which a straight line can be drawn at every point. A corollary to this is that ruled surfaces may have curvature in only one direction. Ruled surfaces are far less limited than developable surfaces, but are not as easily flattened.
Ruled surfaces are used frequently in plastic parts and plastic mold design where draft and parting surfaces from 3D parting lines are needed.
Defining Gaussian curvature
Gaussian curvature is not referred to directly in SolidWorks software, but you may hear the term used in more general CAD or engineering discussions. It can be defined simply as curvature in two directions. As a result, a sphere would have Gaussian curvature, but a cylinder would not.
Surfacing Tools
Surface feature equivalents are available for most solid features such as extrude, revolve, sweep, loft, fillet, and so on. Some solid features do not have an equivalent, such as the Hole Wizard, shell, and others. Several surface functions do not have solid equivalents, such as trim, Untrim, Extend, Thicken, Offset, Radiate, Ruled, and Fill.
This is not a comprehensive guide to complex shape modeling, but it should serve as an introduction to each feature type and some of the details about how it operates.
Classifying the Extruded Surface
The Extruded Surface works exactly like an extruded solid, except that the ends of the surface are not capped. It includes all the same end conditions, draft, contour selection, sketch rules, and so on that you are already familiar with. Figure 20.3 shows the PropertyManager for the Extruded Surface.
FIGURE 20.3
The Extruded Surface PropertyManager
You can also create extruded surfaces from open sketches, and, in fact, that is probably a more common situation than creating a surface with a closed sketch.
When two non-parallel sketch lines are joined end to end, the result of extruding the sketch is a single surface body that is made of two faces with a hard edge between them. If the sketch lines were disjointed, then the extrude would result in disjoint surface bodies. If the sketch lines were again made end to end, but done in separate sketches, then the resulting surface bodies would be separate bodies; the second body would not be automatically knit to the first one as happens with solid features. This is an important quality of surfaces to keep in mind. If you create surfaces in different features and want them knitted into a single body, then you will have to do that manually.
Starting in SolidWorks 2011, there is a new method you can use to extrude surfaces. If you have a split line on a surface, you can select the surface and start the Extruded Surface command. You then need to select a plane or axis to define the extrusion direction, and specify the depth of the extrusion. Figure 20.4 shows the PropertyManager and result of this new function.
This technique is special because of the additional options: Cap End, Delete original faces, and Knit result. The Cap End option has existed for some time, but the ability to remove the face inside the extrude and to knit it all together is new. It would be nice for these functions to exist with regular sketch-based extrudes and other