Page 40 - MetalForming June 2015

Tooling by Design
By Peter Ulintz
Methodical Approach to Diagnosing Die Failure
Stamping dies are subjected to an array of stresses, temperatures, chemicals, shock and vibrations. So we should not be surprised when the dies experience all kinds of in- process failures.
When tooling components fail pre-
maturely, the stamper must perform a completefailureanalysisandproper-
ly identify the failure mode(s) in order
to properly repair the tooling and
maintain optimum performance.
Unfortunately, the immediate response for
many failures is to repair or replace the dam-
aged or broken component as quickly as pos-
sible so that production may resume. Under these time- pressured conditions the stamper often fails to perform a properly devised investigation, which usually results in important evidence being discarded or destroyed.
Compile background data— Time spent collecting back- ground data proves vital to the success of any failure analysis. Your technicians should have a thorough under- standing of the entire manufac- turing process and the service histories of the failed compo- nent(s). They should reconstruct, as much as is possible, the entire sequence
ofeventsleadinguptothefailure.
Getting acquainted with the manufacturing
process may require the investigator to obtain die-detail drawings, technical specifications, component process-flow diagrams and all relevant fabrication information. This includes the die materials used, machining methods employed, heattreatment-process parameters and surface- treatment data, including the application methods.
The ability to accurately complete a service history will depend on how detailed the recordkeeping was in the plant prior to the failure. In collecting service histories, environ- mental details prove particularly important. These include normal and abnormal loading, accidental overloads, cyclic loading, in-process temperature variations and die-grinding and sharpening practices.
Visually examine the failed part—Design errors often are contributors to die failure. These errors fall into two groups:
• Those that fail from heattreatment;
• Those that fail in service.
Design faults that cause failures from heattreatment
include the presence of thick sections adjacent to thin sec- tions; sharp corners; blind holes; stamp marks; fillets with radii that are too small; poorly located or designed grooves or notches; abrupt changes in crosssection; and the location of holes that result in thin walls.
Design faults that cause service failures include inade- quate fillets in corners; improper clearance (especially if ther- mal expansion is involved, which is the case with many stamping operations); thin sections; and abrupt changes in crosssection.
Nondestructive testing—These processes allow for the evaluation of surface finish and dimensional specifications without destroying the sample.
Mechanical testing—Tool design and tool-steel selec- tion often receive the most attention from the die shop, while heattreatment is taken for granted. This typically occurs because the heattreat process is least understood
Although the sequence may vary depending on the failure type(s), the pro- cedure for any failure analysis should at least include the following steps:
“Time spent collecting background data proves vital to the success of any failure analysis.”
Collect samples—The
failed die component (bro-
ken, twisted, bent, galled,
chipped, scored, etc.) is the
primary failure sample
because it either is the failure site, or it contains the failure site. Also desirable: a sample part of a similar component which has not failed, preferably one that has run successfully in production. When producing replacement parts in batch lots, the stamper should take a sample from the replacement- parts bin for comparative analysis.
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
38 MetalForming/June 2015
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