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Daniel Schaeffler Daniel Schaeffler
President

Troubleshooting Sheet Metal Forming Problems, Part 2: The Stamping Process

February 26, 2021


Let’s take a trip back in time. When you first started production, your part was perfect, or at least acceptable in terms of no splits/necks, fitting the fixture, dimensionally in tolerance, and meeting the required characteristics of strength, stiffness, weldability, paintability, etc. To achieve this success, you used the right sheet metal grade and thickness, optimized press settings, applied the correct type and amount of lube, and had a perfect die surface.  

Fast forward to today. Your part may have splits or wrinkles or maybe even both. What happened? Simply put, if nothing has changed, then nothing should have changed. In Part 1 of this series, published in the January-February 2021 issue of MetalForming, we covered the key sheet metal characteristics and properties that can help in your fact-finding mission. Here we present some processing variables to check in order to diagnose a troublesome part.

The root cause of the splits and wrinkles stems from some difference in material flow between then and now. Start the diagnosis by ensuring that the stamping recipe hasn’t changed from when production launched—same binder and ram pressures at the four corners, same lubricant, same strokes per minute, etc. Do not assume that the settings are the same; check the data and records. 

To illustrate the high price of some cost-saving measures, consider a stamping plant manager who ordered lubricant dilution changed to 3:1 rather than the historically acceptable 2:1. Concurrent with this change, press operators noticed an increasing number of splits on a particular part. Rather than reverting to the original conditions used to make good parts, operators merely increased the flow rate of the lube and flooded the part. However, this did not help, as the nozzles remained aimed at the right-hand side of the die where the splitting had become prevalent the year prior, rather than returning them to their nominal positions to spray the entire part―including the left-hand side of the die where the current problem resided.

Once confirming that no production settings, such as that noted above, have changed, take the perfect finished part saved from the start of production (you saved one, right?) and compare it side-by-side with the now-troublesome part.  Look at the on-product radii and look for consistency throughout the part. Check not only the radii dimensions, but also the quality (galling). Some sections may have been touched up to address a concern along the way—either made larger to fix a neck or tightened to smooth a wrinkle. Although these actions may improve local part quality, an often-overlooked step is to blend the modified radius into the initial radius remaining in the adjacent section. If there is a rough transition, this die feature could result in a stress riser as the part forms, which affects how the material flows, and can cause splits or wrinkles.

To extract more information, compare the appearance changes of the draw panels. Examine radii both on and off product—the dimensions should be true on both and be free from galling and scoring. Compare the material outside of the draw bead. If it is flowing the same now as it did before, the amount of material outside of the beads should be identical.  

Look for Changes

Are the bead dimensions the same, or were areas ground down or welded? Check for a smooth transition with the neighboring beads. Material flow also can vary on freshly polished sections while the adjacent section remains untouched, or if the die was tuned for sheet metal at one end of the thickness tolerance and you now are getting material at the other end. Comparing the records from die development, die buyoff, start of production and the current state―with each other and with the part print―may provide some indication of where to look. Differences in score marks and hard spots also are significant. The panel underside in contact with the tool surface may provide clues as to where and how restricted material flow has occurred.

Shops with breakdown panels to compare are that much more ahead in their investigations. The split/wrinkle typically does not just appear at bottom-dead center—most likely it grew sub-critical during the punch stroke.  Look at the current material movement throughout the press stroke alongside what you saved from the acceptable state at the start of production. You should notice areas where material flows differently—these likely will be in the same areas where problems occur on the final part. Knowing when and where the problems start will focus your troubleshooting efforts.

A key point: document as much as possible from the start of production conditions. It will help you to quickly revert to those settings if you drift into problems during the life of the part. MF

Industry-Related Terms: Die, Draw, Fixture, Nominal, Point, Ram, Stroke, Surface, Thickness, Tolerance, Weldability
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials, Quality Control

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