SolidWorks 2011 Parts Bible - Matt Lombard [107]
Many users struggle when faced with the option to create a loft, boundary, or sweep. Some overlap exists between the three features, but as you gain some experience, it becomes easier to choose between them. Generally, if you can create the cross-section of the feature by manipulating the dimensions of a single sketch, a sweep might be the best feature. If the cross-section changes character or severely changes shape, loft or boundary may be best. If you need a very definite shape at both ends and/or in the middle, loft and boundary are better choices because they enable you to explicitly define the cross-section at any point. However, if the outline is more important than the cross-section, you should choose a sweep. In addition, if the path between ends is important, choose a sweep.
Both types of features are extremely powerful, but the sweep has a tendency to be fussier about details, setup, and rules, while the loft and boundary can be surprisingly flexible. I am not trying to dissuade you from using sweeps, because they are useful in many situations. However, in my own personal modeling, I probably use about ten lofts or boundary features for every sweep. For example, while you would use a loft or combination of loft features to create the outer faces of a complex laundry detergent bottle, you would use the sweep to create a raised border around the label area or the cap thread.
A good example of the interpolated nature of a loft is to put a circle on one plane and a rectangle on an offset plane and then loft them together. This arrangement is shown in Figure 7.10. The transition between shapes is the defining characteristic of a loft, and is the reason for choosing a loft instead of another feature type. Lofts can create both Boss features and Cut features.
FIGURE 7.10
Interpolation inside a loft
Notice how the cross-sectional shape of the loft transitions from the circle to the rectangle. The default setting, shown in Figure 7.10, is for the interpolated transition to happen evenly across the loft, but the distribution of change from one end to the other could be altered, which might result in the transitions shown in Figure 7.11.
FIGURE 7.11
Adding end conditions to a loft alters how the interpolation is distributed.
Both shapes are two-profile lofts. The two-profile loft with default end conditions always creates a straight transition, which is shown in the image to the left. A two-point spline with no end tangency creates a straight line in exactly the same way. By applying end conditions to either or both of the loft profiles, the loft's shape is made more interesting, as seen in the image to the right in Figure 7.11. Again, the same thing happens when applying end tangency conditions to a two-point spline: it goes from being a straight line to being more curvaceous, with continuously variable curvature.
The Loft PropertyManager interface is shown in Figure 7.12.
FIGURE 7.12
The Loft PropertyManager
Comparing the Loft and Boundary features
The Boundary feature is relatively new to SolidWorks. The Boundary surface feature was added first, and was a big hit in surfacing applications, so boundary solid was added later. In my view, the solid feature is not as effective as the surface feature, and solid loft probably offers fewer advantages over boundary than surface loft does. Chapter 20 has more information on the surfacing functionality.
An important difference between boundary and loft is that there are more options for setting up boundary features in terms of the geometrical layout of profiles and guide curves. A second major difference is that there is no such thing in boundary as profiles and guide curves — the two directions are treated equally, and are simply called Direction 1 and Direction 2. In the Loft feature, you don't have as much