The highly competitive nature of today's global automotive marketplace is forcing automobile manufacturers to either become highly proficient in the design and manufacture of cost-efficient, high-quality automobiles or plan on getting out of the business altogether. In the area of sheet metal usage, this has led to the employment of thinner-gauge metals for weight control, the use of higher-strength steels to maintain part integrity, and the use of corrosion-resistant materials to improve product quality.

At the same time, significant improvements in part quality and production efficiency are being realized in stamping plants through the use of new production monitoring tools and statistical process control techniques. One method for improving both productivity and quality that has proven to be quite successful at Ford's U.S. stamping plants has been the development of a statistical process control program for evaluating the formability of sheet steel. It is based on the use of a forming simulation test known as Limiting Dome Height (LDH).

Using this test, it is possible to statistically determine the consistency and ductility of the steel being applied to a specific part, to evaluate rejected steel for formability, to select appropriate steel for die tryout and when necessary, to verify incoming steel properties and assist in the improvement of stamping line production. This article discusses the Ford program to improve quality control in its sheet metal stamping lines through the use of this statistical process control technique and the LDH test equipment.

Sheet Metal Formability Testing

During most stamping operations, the sheet metal is strained in a total of three different modes, as illustrated in Fig. 1. In the stretch mode, biaxial strain occurs on a positive basis along both the major and minor axes. In the draw mode, strain is positive along the major axis and negative along the minor axis. While in the plane strain mode, strain is positive along the major axis but at or near zero along the minor axis.

Click to Enlarge

Fig. 1-Strain Modes

These strain modes can be analyzed in a stamped part by electrochemically etching a grid-work of circles onto the surface of the sheet metal before it is stamped. By studying the deformation which occurs in a given circle during the stamping process, it is possible to determine the strain state at any location on the part.

A forming limit diagram, such as the one shown in Fig. 2, can subsequently be used to match up the strain modes present in a stamped part with a sheet metal that can be relied on to form the part without failure. A forming failure is typically defined for automotive applications as a split in the part, a reduction in part thickness below acceptable limits or an unacceptable surface wrinkle. Failure is avoided, on the other hand, by reworking the tool, by revising the part design, by switching to a thick or higher grade of steel, or by doing all of the above.

Click to Enlarge

Fig. 2-Forming Limit Diagram

This method of strain and failure analysis-generally referred to as circle grid analysis-can be used for trouble-shooting, feasibility studies and die tryout of a particular stamped part. However, it does not evaluate sheet metal ductility and therefore cannot be used to directly indicate sheet metal problems. Additionally, the amount of time that it takes to carry out the procedure makes it undesirable to use on a routine basis for die tryout or for analyzing production problems.

The optical grid analyzer, discussed in the accompanying sidebar article, greatly enhances and speeds the circle grid analysis process but does not eliminate all of its time-consuming procedures. It has therefore been necessary to develop simpler, simulative tests that correlate with the press performance characteristics of more complex stampings.