The Fabricator - August 2009

23 precision matters | by Gerald Davis, Contributing Writer

Flat responsibility in sheet metal modeling

The differences between ‘flatten’ and ‘unfold’

(Editor’s Note: The 3-D CAD software referred to in this column has several sheet metal-specific modeling tools. Other software packages have similar capabilities, but have a different user interface. It will be up to you to make the appropriate translation.)

The most important role of the CAD jockey is to verify that he design can be flattened. Consider the design shown in Figure 1a. Until it is flattened, as shown in Figure 1b, the design appears to be OK.

The generation of the flat pattern for manufacturing is probably best left to the fabricators. To develop an accurate flat pattern, you need to understand the forming tooling as well as the forming characteristics of the workpiece. A flat pattern developed for a folding machine will be different for Gerald Davis is a job shop consultant and chairman of the board of DSM Manufacturing Co.,

www.precisionsheetmetal.com.

CFIGURE 1a This sheet metal design looks OK in 3-D.

CFIGURE 1b The flat pattern reveals an impossible design.

one developed for a press brake. Subtle differences also may exist from one set of press brake tooling to another.

It is unlikely that a designer using 3-D CAD software will know specifically which fabrication process will be used for every production run. The best the CAD jockey can do is provide a design that is easy to adjust to suit a specific manufacturing process.

CFIGURE 2a Best practice: Keep all of the thickness and radius parameters in one place so they are easy to edit.

CFIGURE 2b It’s easy to flatten the part to verify that the design is buildable.

Click to Flatten

If the sheet metal design is correctly modeled, the generation of a flat pattern can be accomplished with a single mouse click. The accuracy of the flat pattern will depend on the math used to predict the stretching of the sheet metal as it is bent. K-factor—a.k.a. neutral bend line—works well for a variety of bend angles. Bend deductions work well for both humans and computers—if the bend angle is taken into consideration. Bend allowances work if everyone understands where the radius starts in the calculation.

The best practice is to model the sheet metal part so that the staff in fabrication conveniently can select the math formula and specify the bend radius, material thickness, and material yield. In Figure 2a, all of the sheet metal parameters can be changed by editing a single entry in the Feature

Manager. The model has been changed from Figure 1 so that it can be flattened as shown in Figure 2b.

Advanced Flatness

Fold and Unfold as well as Flatten are all functions with CAD systems. In general, Flatten is intended to generate a flat pattern for manufacturing. The other tools are intended to make it easier to model some sheet metal features.

Consider a situation where a hole needs to have a bend passing across it. The CAD jockey might use a modeling sequence such as Unfold, Cut, and Fold (see Figure 3a, 3b, and 3c). Sequence awareness is important in feature history-based modeling software.

In the case of Flatten, it prevents bends from being processed into their folded state, thus leaving the part flat.

CFIGURE 3a First, unfold a bend and start sketching the cutout.

CFIGURE 3b Second, complete the cutout.

CFIGURE 3c Third, fold the bend. Note that the cutout properly spans the bend.

CFIGURE 4a Bad practice: Misuse of Flatten might make an OK flat pattern, but ...

CFIGURE 4b When the part is folded, the cutout disappears! Don’t misuse Flatten.

Everything that happens after the Process Bends event is its “child.” ( History-based modeling software assigns a parent-child relationship to features in the model. To be a parent, a feature has to exist before the child, and the child has to have some relationship to the parent, such as a dimension. When something happens to the parent feature, all of the subsequent steps—the children—are affected as well.) So when the part is flattened, Process Bends is suppressed along with everything that happened afterward.

If the CAD jockey selected Flatten and then Cut, Cut would appear in the flat pattern but disappear (be suppressed) in the folded part (see Figure 4a and 4b). That is both frustrating and unrealistic.

With awareness of what the CAD tools are for, this problem is easy to avoid. ■

Gerald Davis uses CAD software to design and develop products for his clients at www. glddesigns.com.

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@the fabricator.com.