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The geometrical layout of profiles is the most important difference between boundary and loft. With loft, you must have a profile at the beginning and end of the feature. With boundary, you can lay out the profile sketch planes like an X. You could also lay the feature out like a T, which would act like a sweep. Using a layout shaped like an F actually combines the functionality of a loft with that of a sweep. So boundary is a very powerful feature, with new options for creating interpolated solid shapes. Face-by-face control, however, still has to come from using surfacing features.

Figure 7.13 shows the boundary PropertyManager and features made with F and X layouts of sketches. These two features represent functionality that doesn't exist with the Loft feature.

FIGURE 7.13

Using different profile arrangements for boundary solid features

Using entities in a loft

For solid lofts, you can select faces, closed loop 2D or 3D sketches, and surface bodies. You can use sketch points as a profile on the end of a loft that comes to a point or rounded end. For surface lofts, you can use open sketches and edges in addition to the entities that are used by solid lofts, but you cannot combine open and closed contours.

Some special functionality becomes available to you if you put all the profiles and guide curves together in a single 3D sketch. In order to select profiles made in this way, you must use the SelectionManager, which is discussed later in this chapter.

The Sketch Tools panel of the Loft PropertyManager enables you to drag sketch entities of any profile made in this way while you are editing or creating the Loft feature, without needing to exit and edit a sketch.

Cross-Reference

I discuss 3D sketches in more detail in Chapter 6.

Comparing lofts and splines

The words loft and spline come from the shipbuilding trade. The word spline is actually defined as the slats of wood that cover the ship, and the spars of the hull very much resemble loft sections. With the splines or slats bending at each spar, it is easy to see how the modern CAD analogy came to be.

Lofts and splines are also governed by similar mathematics. You have seen how the two-point spline and two-profile loft both create a straight-line transition. Next, a third profile is added to the loft and a third point to the spline, which demonstrates how the math that governs splines and lofts is also related to bending in elastic materials. Figure 7.14 shows how lofts and splines react geometrically in the same way that bending a flexible steel rod would react (except that the spline and the loft do not have a fixed length).

FIGURE 7.14

Splines, lofts, and bending

With this bit of background, it is time to move forward and talk about a few of the major aspects of loft features in SolidWorks. It is possible to write a separate book that only discusses modeling lofts and other complex shapes. This has in fact been done. The SolidWorks Surfacing and Complex Shape Modeling Bible (Wiley, 2008) covers a wide range of surfacing topics with detailed examples. In this single chapter, I do not have the space to cover the topic exhaustively, but coverage of the major concepts will be enough to point you in the right direction.

Understanding the need for surfaces

In this chapter, I deal exclusively with solid modeling techniques because they are the baseline that SolidWorks users use most frequently. Surfaces make it easier to discuss complex shape concepts because surfaces are generally created one face at a time, rather than by using the solid modeling method that creates as many faces as necessary to enclose a volume.

From the very beginning, the SolidWorks modeling culture has made things easier for users by taking care of many of the details in the background. This is because solids are built through automated surface techniques. Surface modeling in itself can be tedious work because of all the manual detail that you must add. Solid