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Shop technology and 3-D CAD: Sheet metal modeling

CAD techniques for modeling flanges, inserted bends, converted sheet metal, and flat layouts

Figure 1
These are seven 3-D model variations of the same part. The best one is the one that allows you to get a part fastest to market. A model that is easy for others to edit and understand is also good. In some cases, however, the most important goal is a model that allows for the fastest rebuild time.

Editor's Note: If you would like to download the 3-D CAD files associated with this column, click here.

The versatility of 3-D modeling software allows for many possible solutions to the same problem. To emphasize the nuances among sheet metal modeling techniques, we pre–sent seven variations on the same part, as illustrated in Figure 1.

Here’s a disclaimer: This article does not cover all possible ways to model this example. Let us know as you find others. Further, much of the terminology applies to a specific brand of CAD. The general concepts apply, nonetheless.

First, we consider a conventional modeling technique without regard to sheet metal, and then use a tool to convert that model to include sheet metal behaviors. The reasons for the conversion include the ability to flatten the part, the ability to tune the flat layout calculations to match physical tooling, the ease of adjusting bend radii, and the convenience of having the bends and thickness modeled automatically.

Figure 2a demonstrates a CAD technique using three Boss-Extrudes to model the basic features desired. In Figure 2b, the Convert to Sheet Metal tool has been applied—and part marking added—to complete the part. The only sheet metal-specific skills required are:

  1. To keep the thickness of all extrudes the same.
  2. To set up the Convert tool with matching thickness and appropriate inside bend radius.

Those settings are editable, which is a compelling reason to use the Convert to Sheet Metal tool.

Figure 3a demonstrates modeling the part as it will be manufactured, starting with a flat layout. This requires knowing how to calculate flat layouts. This technique is demonstrated only for comparison and contrast to other—perhaps better—techniques.

The sheet metal CAD tool used in Figure 3a is the Insert Bends tool. Even though the modeled flat does not have bends, the Insert Bends trick makes it possible to add sketched bends to the flat layout.

In Figure 3b the Sketched Bend tool has been used to model the first of two bends. To complete the part, we add a second sketched bend (see Figure 3c). Note that the dimensions for locating the bend line also require skill in sheet metal flat layout calculation. As a CAD technique, this is perhaps the most difficult to edit and understand for sustaining engineers.

Figure 4a illustrates a method that is very similar to that shown in Figure 3a. Start with a flat layout, but this time use the Convert to Sheet Metal tool. In addition to a preference one might have in using one tool over the other, a CAD jockey also should know that the Insert Bends tool rebuilds slightly faster than the Convert to Sheet Metal tool does. (The Insert Bends tool takes 0.34 seconds compared to 0.36 seconds for the Convert to Sheet Metal tool in this model.)

Figure 2a
Use simple Boss-Extrudes—all the same thickness—to model the part without bends.

A Sketched Bend is added in Figure 4b. As with all sketched bends, it helps to have sheet metal skill in locating the bend line. The second bend is added separately (see Figure 4c), and part marking is added to complete the model.

If flat layout is not a design goal, then the modeling technique shown in Figure 5a is pretty straightforward. Sketch a pair of lines, fillet, offset them with end caps to create a closed profile, and then extrude it. An Extrude-Cut is required to prepare for the flange. In Figure 5b the flange is modeled as an extrude, and to make it “look” finished, the part marking is added in Figure 5c. Note, however, that this model does not have the ability to unfold.

The model in Figure 6a was created as sheet metal from the get-go. A pair of lines were sketched as the L, and then a Sheet Metal Base Flange was applied to that sketch to model the bend and thickness. An Edge Flange with bend relief settings completes the sheet metal. Part marking completes the model in Figure 6b.

Figure 7a shows a neat trick with a Miter Flange feature. The model is nearly identical to Figure 6a except that a Miter Flange is used instead of an Edge Flange. The Miter Flange has the ability to offset its start and end. In this example, the end is offset to create the bend relief. Note that Plane 1 shown in Figure 7b was automatically created by the Miter Flange feature.

Making the File Transition

To convert a STEP file, or any other imported 3-D geometry, to sheet metal, the first step is to run Import Diagnostics to clean up the import as shown in Figure 8a. Then in Figure 8b Insert Bends is applied to add sheet metal functionality to the model. This is very similar to the method used in Figure 3a. As with the other techniques demonstrated, part marking is added (see Figure 8c).

Figure 8d shows a flat pattern that was generated from an imported STEP file. Adding bends to STEP files is a reliable method of sending files from design to fabrication. The Figure 8d model also has the best rebuild time—0.31 seconds.

The techniques demonstrated in Figures 6a and 7a are suitable when the design is likely to be edited as well as for final release to fabrication.

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.