Page 46 - MetalForming-Jan-2018-issue

The Science of Forming By Daniel J. Schaeffler, Ph.D.
See the Future by Exploring the Past
Table 1—Failure Modes and Effects
Failure Mode–
What can go wrong?
Failure Effect–
What happens when something goes wrong?
Size of strip-locator hole too large
Inconsistent die progression from variation in pilot-hole size
Gauge-locator pins worn down
Material flow due to poor gauging led to the blank pulling through the draw beads
Dimensional variation
Lubricant fouling from lack of biocide
Clogged lubrication nozzles leading to sheetmetal splitting due to insufficient lubrication
Clogged lubrication nozzles leading to premature die wear due to insufficient lubrication
Dimensional variation
Incorrect sheetmetal grade brought to press
Uncompensated springback because process was designed for different grade
Finished part characteristics are incorrect
Dimensional variability. Random splits. Surface waves. Any or all of these can cause big produc- tion headaches. Scrambling to fix the problems can lead to a host of other issues. The best time to address prob- lems is before they occur—an ounce of prevention as they say. But how do you know where to focus your up-front efforts? After all, we already are late with tooling buyoff and we need to ship the tools to the home line.
FMEA, or failure modes and effects analysis, can help align your efforts with the issues that can cause the most problems. FMEAs are created before production and rely on your experience with similar parts. Rather than only highlighting the consequences of fail- ure, following this discipline helps pri- oritize the issues that represent the biggest risks to success.
Key to FMEAs is the risk priority num- ber (RPN), which results from multiply- ing the numerical severity, likelihood of occurrence and likelihood of detection ratings. Individually, these ratings can vary from 1 to 10. Failure modes with
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
Tel. 248/66-STEEL
E-mail ds@eqsgroup.com or Danny@learning4m.com
the highest RPNs must be attacked up front. Otherwise, you most likely will deal with them during production.
First, we must identify what we are trying to achieve. Many MetalForming readers seek to produce production quantities of dimensionally accurate, split-free stampings from the full range of acceptable tensile properties asso- ciated with the ordered sheetmetal grade. Doing so requires many process steps. Raw-material receiving and stor- age, die setup, and stamping are just the high-level steps—each of these can be broken down into smaller actions that can interfere with the end goal.
Failure Modes & Failure Effects
What can go wrong, and what hap- pens when something goes wrong? Failures come in all degrees, with some more important than others. Here, we will not make judgments on the levels of importance or their frequency; instead, just focus on what could hap- pen. Failure in this context includes any error or defect. A failure mode describes the way in which a failure occurs. A failure effect represents the immediate consequences of a failure on the operation, function or function-
ality of some item.
Consider just one part of our overall
goal: producing dimensionally accurate parts. In this step we define all of the failure modes arising from incorrect dimensions that could block us from reaching our goal. Each failure mode can bring several failure effects. Some failure modes and effects associated with incorrect dimensions include issues such as those shown in Table 1. Why these failure modes occur and how to address the root causes will be the subject of a future article.
Severity–How Bad Is It?
Severity assesses the seriousness of the effect should failure occur. The highest severity ratings are given to those instances where failure leads to violation of safety or regulatory require- ments, with the highest rating of 10 given for a catastrophic no-warning event. Even if the effect has relatively low values of occurrence and detection, those with high severity ratings should be addressed due to their implications. Table 2 lists possible effects, their sever- ity and their ratings. The actual thresh- olds should be customized for your specific needs and concerns.
44 MetalForming/January 2018
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