Love of loft in 3-D CAD modeling

Like an extrude, revolve, or sweep, a loft is a tool for modeling a 3-D shape. A loft consists of two basic elements—two or more profiles and a set of rules for lofting between the profiles.

Before getting too far into the details of lofts, I am going to offer up a couple of disclaimers. First, if you're not using the same software as I am, some of the operational details and terminology may be a bit different. Second, we're going to cover only a little bit of what lofts can do.

Peek at Peg

Figure 1 shows a cylinder that has a 1-inch diameter and a 1-in. length. Is this an extrude or a loft? If you've been following this column over the past few months, you know that without having access to the model, you really can't answer that question. The part in Figure 1 could have been modeled in any of several ways to achieve the same dimensional result.

This example, however, was created as a loft. Figure 2 reveals the two profile sketches that were required. The sketch with the dimensioned circle was completed first. The next step was to create a reference plane at the proper distance from the plane of the first sketch. That reference plane was used to create a second profile sketch; it is a direct copy of the first sketch.

The final step was to launch the loft tool. The loft tool allows you to select individual sketches and in which order to use them as the profiles. Perhaps the most powerful aspect of the tool is the fact that you can synchronize the profiles using control handles.

In Figure 3a you can see the control handles for this simple peg are arranged—or synchronized—almost vertically from each other. In Figure 3b the upper control handle has been moved along the sketch profile to create a twist in the lofted surface. This results in an exotic hourglass shape instead of a perfect cylinder.

Lofting can be an excellent technique for making all kinds of transitions. As you experiment with this tool, you'll quickly discover that the loft is a very challenging technique for modeling the simple part shown in Figure 1. A quick extrude or revolve would be much easier on the CAD jockey.

So, what use would a designer of manufactured parts have for a loft?

Ductwork

Figure 4 shows an example of a transition from a rectangle to a round. My CAD software will unfold models of sheet metal parts like this as long as the parts can be made on a press brake. If I need to develop parts that are made with conventional stamping dies or coining tooling, I need to purchase add-in software.

I mention this because I'm certain that some sheet metal experts out there will note that Figure 4 isn't really a completely functional ductwork transition piece. As an introduction to lofting techniques, though, Figure 4 will serve us well.

Lofted Bends Turn out Flats

A bit of technical gyration is required in lofting. As an incentive to master this, consider the flat layout shown in Figure 5. This is ready to be sent to a laser for cutting and could also be used by the press brake operator in forming the part; the bend lines and bend angles are shown. It takes practically no effort to generate this kind of flat pattern—if you set up the 3-D model correctly.

For the lofted bend to create the bend lines for us, it needs to have two profiles that have the same number of line segments. Figure 6 shows the pair of profiles. The rectangular end of the transition has eight elements—four lines and four arcs. The "round" end of the transition is made up of the same—just larger radii and shorter line segments.

It is undoubtedly easier to visualize these exotic shapes in a 3-D CAD system as opposed to small, 2-D images in this magazine. If you'd like copies of these models, just let us know.

Next month we'll consider some criteria for selecting the "best" 3-D modeling techniques. How do you know when to use extrude, revolve, sweep, or loft?

Gerald would love to have you send him your comments and questions. You are not alone, and the problems you face often are shared by others. Share the grief, and perhaps we will all share in the joy of finding answers. Please send your questions and comments to dand@thefabricator.com.