Page 92 - MetalForming-Oct-2018-issue

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Use Reference Panels
When Troubleshooting Forming Problems
Apart is splitting where no prob- lem previously existed. Chang- ing 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 prob- lem 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 tool- ing. 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 restrict- ing sheetmetal flow. Even if you do fig- ure 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 satis- fied with each stamped-metal part at
Danny Schaeffler, with
30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
the time of initial part production; oth- erwise, you would not have bought off on the tooling and process. This makes for an excellent reference point. By documenting the entirety of the con- ditions used to produce this part, you now have reference settings to dial into the process should problems arise dur- ing 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 cap- ture as many of the variables as possi- ble. Looking at the sheetmetal, this means more than just the tensile prop- erties provided on the material certi- fication—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 devel- opment 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-devel- opment 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 com- plete 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. Docu- menting 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 dis- tributed and matched simulation pre-
dictions. Also, the lubricant type, amount, dilution and distribution inter- act with the sheetmetal-surface profile and tool-surface profile to impact metal flow. Document this information.
With all of these parameters docu- mented, 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 infor- mation. 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 ref- erence panel. Where differences occur, compare local geometry of the prod- uct 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 pan- els represents no trivial exercise. But compare that against what occurs when production splits begin and can- not be fixed through easy changes. Effective troubleshooting has a cost, but it is less than that of not trou- bleshooting efficiently. MF
90 MetalForming/October 2018
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