Daniel Schaeffler Daniel Schaeffler

Use Reference Panels When Troubleshooting Forming Problems

October 1, 2018

A part is splitting where no problem previously existed. Changing the cushion pressure to deliver a part that holds water may provide a short-term fix, but it doesn’t help you understand where the problem originated. Another option: Polish or grind the draw beads to allow more metal flow into the cavity. This approach is an even more dangerous Band-Aid, since polishing or grinding makes permanent changes to the tooling. Remember that more metal flow isn’t necessarily a good thing as it may lead to wrinkles.

Powdering or galling could be worsened with increased metal flow on galvanized parts, leading to a physical removal of zinc coating from the steel surface, further restricting sheetmetal flow. Even if you do figure out what to change, to what do you change it? This is where a reference panel helps.

Document, Document, Document

Your manufacturing, engineering, and quality departments all are satisfied with each stamped-metal part at the time of initial part production; otherwise, you would not have bought off on the tooling and process. This makes for an excellent reference point. By documenting the entirety of the conditions used to produce this part, you now have reference settings to dial into the process should problems arise during the part’s production life.

Different aspects of the stamping process can be adjusted within a range and still produce the desired product shape. The entire forming process influences success, so you should capture as many of the variables as possible. Looking at the sheetmetal, this means more than just the tensile properties provided on the material certification—generated from one sample at the end of a mile-long coil likely produced months ago. For the time, effort and resources required for die tryout, the cost of a tensile test from the specific lift used during part development is trivial if it can prevent unnecessary process adjustments made to run what turns out to be out-of-spec. sheetmetal.

The tooling CAD file contains draw-bead information, but it’s accurate only until tool build. The part-development process leads to bead-profile changes, which in turn alters metal flow. Once the hardened tools begin producing acceptable parts, they should be scanned to capture the complete shape of not only the beads, but the punch, die face, draw cavity and binder as well.

Best practice is to not accept tools that reaqire shims to make good parts, but if you must use shims, document their locations and thicknesses. Documenting binder tonnage from each of the four corners, as well as the punch tonnage, helps confirm that, at least at tooling buyoff, loads were evenly distributed and matched simulation predictions. Also, the lubricant type, amount, dilution and distribution interact with the sheetmetal-surface profile and tool-surface profile to impact metal flow. Document this information.

With all of these parameters documented, characterize the produced parts and perform a strain analysis. A contact or noncontact grid-strain analysis is ideal, but a thinning-strain analysis with an ultrasonic thickness gauge will retain some critical information. This analysis should show that all areas exhibit lower strains than the least-formable material that could be supplied to make the part, even after factoring in a safety margin.

Breakdown panels—draw panels retained incrementally off of the bottom of the press stroke in addition to a fully bottomed panel—help show how metal flows to produce the part in question. Keeping a flat blank as well as a panel from binder set allows you to confirm that the blank shape and dimensions have not changed, nor has the initial deformation caused by engaging the beads when setting the blank.

Should troubleshooting be needed during production, look for changes in metal flow as compared to the reference panel. Where differences occur, compare local geometry of the product shape and the draw beads. Skid marks, witness marks and hard spots should match those on the reference panel.

Worth the Effort

Such discipline requires time and effort, as labeling and storing the panels represents no trivial exercise. But compare that against what occurs when production splits begin and cannot be fixed through easy changes. Effective troubleshooting has a cost, but it is less than that of not troubleshooting efficiently. MF

Industry-Related Terms: Draw, Forming, Gauge, Grinding, Point, Polishing, Run, Stroke, Surface, Thickness, Ultrasonic, Blank, CAD, Die
View Glossary of Metalforming Terms


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

Technologies: Materials, Tooling


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