Machine Design

Turning Sheet-Metal Drawings into 3D Solid Models

Products designed in 2D are prone to fit and position problems. Today’s CAD systems include tools to transform them into more useful 3D models

Kris Kasprzak
Applications engineer
Unigraphics Solutions Inc.
Huntsville, Ala.

Designers working with modern CAD systems enjoy a wealth of software advantages. For instance, new CAD tools help improve a product’s overall fit and finish while shortening design cycles. Digital mock-ups let engineers and clients see finished designs before a punch ever hits metal. The technology is great for new products, but what about older designs? A flat-pattern drawing of a sheet-metal part shows how the latest CAD tools can help revitalize old designs.

The problem with prints
The trouble with designing only in the flat is that downstream tasks get no productivity boost. Stress studies, product visualization, and spotting interferences between assembled parts remain difficult tasks. Converting the design to 3D, however, solves many of these problems. In this example, we’ll generate a 3D model from a 2D flat-pattern for a saw-horse brace.

The example begins by extruding the sketch into a flat solid model. Since a 2D drawing of the flat pattern is available, the battle is half over. After importing the DXF file for the 2D drawing, we build the model from it.

Depending on the CAD system, this operation can be as easy as picking “File Open” and choosing the required drawing. The initial drawing appears complete with bend lines, holes, and the outline of a stiffening rib. A few systems such as Solid Edge from Unigraphic Solutions Inc., can interpret 2D documents to simplify drawing manipulations. The system used in this example accesses layers established within the original system to control and manage element display.

Older 2D systems typically are not based on a parametric modeling foundation. Newer 3D systems use parametrics to control design intent. With this technology, geometry can be constrained to control changes. Constraints describe connections at intersecting geometry. To speed constraint assignments some systems include a tool that automatically applies them.

Determining what to convert first may be confusing. A general rule of thumb says “Build the model as it will be manufactured.” A logical sequence of operations would be to start with the flat pattern, add holes and features, and apply bends.

Following this plan, create the flat pattern from the 2D sketch. Doing so requires importing the drawing and picking on sketch. The system usually highlights the 2D image. Some systems also may automatically apply a default metal thickness. Once complete, users add mounting holes, draw cutout reliefs for the hinge, and create the stiffening rib.

So far, each feature has been created using profiles from the original drawing. For example, the stiffening rib is added by first selecting a dimple command and following prompts for height, size, and location of the rib.

Other methods
An older construction method would add each feature using existing data. However, using library-based features would require an exact profile to avoid editing. An accompanying image, The solid pattern, shows the part and a list of features applied to it. Since most new CAD systems are feature based, the functions in the software carry names that identify shop operations. For instance, menus show functions such as tab, punch, stamp, bend, louver, and dimple. This makes generating the model almost a manufacturing operation.

Users decide how bends are added, but there’s a catch. It’s critical to put the bends in a proper sequence because part features depend on the 2D sketch. For example, folding the innermost bend first makes it impossible to create the outermost bend because that section is no longer part of the 2D sketch.

To overcome this obstacle, some packages can add bends in a suppressed state. The technique keeps the part flat so the order in which bends or other features are added is not critical. Once all features are added the entire model can be folded to generate a 3D model.

Creating the bends simply requires identifying the bend line, and specifying a direction and angle. When a flat pattern does not include bend lines, they can be added either in the 2D drawing or 3D model. Since the part in The first bends is symmetric, a mirroring operation is used to replicate the bends. Notice the steps required to complete the model. It’s constructed using a logical sequence of features that parallel the manufacturing process.

The original flat pattern included bend reliefs but corner reliefs were omitted. Manufacturing this part without corner reliefs would tear the metal and produced a poor quality part. While the relief could be added in the 2D drawing, the task of computing the size and location would be difficult. But a few systems wisely allow adding reliefs after creating flanges. Edge details shows the final bend with the bend and corner-relief options. The user simply chooses the required style of relief and the system takes care of the rest. This changes the flat pattern drawing, but greatly simplifies sheet-metal design.

The complete 3D model presents intelligent data ready for use. The next step with 3D data might include finite-element studies, further interference checking, motion studies, and exploded views.

Revitalizing legacy data with today’s CAD tools can be this easy. Some CAD systems include specific tools to simplify 2D to 3D migration, while others require generating libraries. Regardless of method, the advantages of working with 3D models are too good to ignore.

© 2010 Penton Media, Inc.

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