Figure 13. Here is an example of hole slotted with clinch hardware specified too near the bend. The relief slot at the bend line allows insertion without distortion.

* Angularity. To assure repeatability in bend angles of less than 90° in single-bend vee-die operations it is often necessary to employ special processing and tooling--at additional cost. Use of standard 90° bends wherever possible is preferable. Consistency of angles is affected by variations in material and press repeatability.

* Die Marks. Slight indentations on the outside (die side) of the workpiece (Figure 14) often result from contact with the top edges of the die during forming. These are inherent in the process.

Dimensioning Practices

Practical experience has proven that dimensioning and measuring practices must both be understood and agreed on by all parties to achieve a workable, mutual standard. Formed sheet metal parts present a unique problem in that angular tolerances as well as the flatness conditions interact with single plane dimensions because of the flexibility of sheet metal, especially the thinner gauges. To achieve consistent results when measuring formed parts, a standard has to be established on where and how dimensions are to be taken.

* Form dimensions should be measured immediately adjacent to the bend radius in order not to include any angular and flatness discrepancy. See Figure 15.

Figure 15. Correct measuring practice for form dimensions.

* Feature-to-feature dimensions on formed legs of any length on flexible parts will be assumed to be measured in constrained condition, holding the part fixtured to the prints' angularity specification. See Figure 16. This standard is appropriate for the majority of thin sheet metal parts and results in a functional product. This is always true when the assembled part is, by design held in constrained condition.

* Constraining methods vary from part to part, depending on shape and material condition. For large quantities, a measuring fixture is most practical for speed and repeatability. The relatively high cost is amortized through the high production rate and reliability gained.

The simplest constraining device is dead weight. Where applicable, the weight to be used during the measuring process should be specified, as well as the physical shape. Weight is most often to eliminate a material out-of-flatness condition, sometimes in conjunction with an angular measurement.

As shown in Figure 16, parallel blocks by themselves, or with clamping devices, are probably the most often employed and practical constraints for occasional use, when legs need to be kept at 90° and parallel. In rare cases when restrained measurement is inappropriate, the drawing should reflect this requirement. Such cases normally result in special manufacturing steps such as making notches in the bend more rigid and specific flattening operations at considerable added cost.

Select a single datum close to an end of the part and maintain the same datum in all related drawings (Figure 17). This datum should be a pierced feature in the principal flat surface of the part, selected on the basis of the sequence of bends. Early discussion with the supplier may be useful in selecting datums and dimensioning effectively.

Figure 17. Example of appropriate dimensioning for parts to be formed in a press brake. A single datum (a pierced feature) is selected near the end of the part. The same datum should be retained on all related drawings.In addition to these considerations, using the following guidelines will increase the manufacturability of designs for press brake forming.

For most economical production, dimension the part in a single direction wherever possible. Because of the sequential nature of the forming process, and the fact that dimensional variation is introduced at each bend, dimensioning in a single direction parallels the process and helps to control tolerance accumulation.

It is generally recommended that dimensioning be done from a feature to an edge. Feature-to-feature dimensions in two planes should be avoided. Feature-to-bend dimensions may require special fixtures or gauging.

This also means that tolerances in the title block of a drawing may be unnecessarily restrictive for certain dimensions and angles, while very appropriate for others.

Almost any degree of precision can be achieved if cost is no object. For economical manufacturing, it is necessary to adopt dimensioning practices which consider the characteristics and limitations of the process and highlight truly critical dimensional relationships.

Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!