Die Science: Step-by-step troubleshooting of stamping dies, Part II

In Part I of this article, which appeared in the January/February 2015 issue, I discussed the first two basic steps in troubleshooting: Identify problems with the part and verify the die and press setup parameters. Following are the remaining seven steps in the process.

Step 3—Reassess and Qualify

This is a pretty basic step. Simply ensure the process parameters are set up properly. If they are, move on. If not, change the parameters until you are satisfied with the setup.

Step 4—Look for Interactivity

Looking at interactivity can help you identify the root cause of the part failure. For instance, if your drawn shell part is splitting, several variables could be contributing to that problem. Drawing pad pressure, lubricant, the sheet metal, and the surface finish of the die are just a few common contributing factors.

For instance, assume the reason for the splitting is overstretching the material in a localized area. So you analyze the drawn shell and notice a great deal of abnormal wrinkling in the scrap area of the part. Does the wrinkling have anything to do with the splitting? Yes. Wrinkles, when pulled into the die, create a great resistance for the metal to flow. When the metal ceases to flow, it must stretch, and overstretching can cause splitting.

Anytime wrinkling is in the binder or draw pad area of a part, it is the result of too much space between the draw pad and the die face. To solve the splitting issue, then, you must find out why the space between the die face and pressure pad is excessive. Whatever the cause, the wrinkling is resulting in splitting.

This is just one example of interactivity. Wrinkles can cause splitting, and splitting can cause wrinkling.

Step 5—Decide the Order of Work

After speculating about the root cause of the part failure, decide the order of work. For example, if splitting is resulting in wrinkling, you need to work to solve the splitting problem. If wrinkling is resulting in splitting, you have to solve the wrinkling problem.

Step 6—Determine Corrective Action Through Experimentation

This is a critical step in the troubleshooting process: the data-collection step. Before permanently altering the dies, you need to be confident in your corrective action. You can conduct several nondestructive experiments to gather data.

For example, if the problem is splitting, you can reduce the blank size in the area where splitting is occurring, which might cause an increase in flow and resolve the problem. Reducing the blank in a given area might result in insufficient material to trim away later, but keep in mind that the objective is to prove that increasing metal flow in a given area will resolve splitting.

There are other ways to increase flow besides changing the blank size. If you are encountering loose metal or wrinkling in a particular area of the part, try putting grit cloth or sandpaper between the blank and metal flow-controlling die surfaces. This simulates the effect of a draw bead. Do not use this technique on a coated tool, however. The key is to experiment with different ideas before you make permanent tool changes.

Step 7—Implement Corrective Action

Once you have a good hypothesis on the appropriate corrective action, you can make permanent changes to the tool. These may include welding, grinding, polishing, increasing or decreasing radii, and shimming.

Step 8—Fine-tune Your Corrective Action

During this step, you might perform a study to prove that your process is robust. It may be as simple as running about 50 parts and checking consistency, or as thorough as conducting a full-blown SPC analysis. It may be conducting a thinning strain analysis or a circle grid analysis.

Step 9—Adjust the Control Plan

If you make any changes to the die setup parameters or to the tooling, document them properly.

The keys to any good troubleshooting process are first to have a comprehensive understanding of all the possible variables involved, and second to do all you can to prove out your theories before making changes in the tool. Acquiring data is essential.