Page 61 - MetalForming July 2012

The Science of Forming
02468
Time – Hours
larger drop at lunch time.
As with previous case studies, forming severity was deter-
mined by UTG measurements of thickness at the highest severity location and at a zero-deformation location. During this study, temperature was measured with a laser thermo- gun at UTG-measurement locations on the stamping and the die. The gradual increase in severity during the day shift was due to the temperature increase of the ambient air in the press shop. A different study showed that an increase from 70 F to 84 F was sufficient to cause stamping severity to change from the bottom of the yellow zone (safety margin) to inside the red zone (failure). This increased operating temperature can lead to a reduction in lubricant viscosity, and even lubri- cant breakdown. Die dimensions and shape also are tem- perature-dependent.
Case Study 4
Forming severity climbed to a high level with this difficult stamping, requiring the press operator to make constant
period. The replacement operator, who confesses to being inexperienced with this particular stamping, says that he sim- ply looked up the original die-buyoff settings, made the changes and then left the operation alone.
A number of plants will reset press and dies back to their initial settings (buyoff or die run) before making any other changes to the forming system.
Case Study 5
After approximately 40 hits on a new die, the left edge of a symmetrical stamping began necking and randomly break- ing. Severity data from UTG measurements were acquired at least every five hits from the right and left sides of the stamp- ing. Examining the forming-severity plot reveals three inter- esting features.
Danger Line
Left Edge
Symmetrical Stamping
Right Edge
0 25 50 75 100
Number of Hits
01234
Time – Weeks
adjustments to the press, die or blank.
On Monday morning of the second week of running SDC
evaluations, forming severity was low and stayed low for two weeks. On the Monday morning of the third week, forming severity returned to the high level.
Checking with the personnel office, we find that the nor- mal press operator was on vacation during the low-severity
First, the die initially created nearly equal right/left sever- ity levels that were well away from the danger line. Second, after 25 hits the severity of the right and left side began to dra- matically diverge—the right edge experiencing reduced severity and the left edge increasing in severity. Third, the severity deviations are equal in magnitude and in sync, but completely opposite in direction. This forming pattern sug- gests the blank guides are being intentionally or accidently relocated. Examination showed the guides did not center the blank over the die. Everybody wanted to center the guides, but the severity data were telling a more important story.
The right side of the stamping was being produced cor- rectly, with a very low forming severity. The left guide then was reset to the same relative location as the right guide. Obvi- ously, the blank width had to be made narrower.
Solving this problem not only lowered the forming sever- ity of the entire stamping, but also reduced the blank size and the cost of the stamping. MF
On August 21 and 22 in Detroit, MI, Stuart Keeler and Peter Ulintz will present a Higher Strength Steel seminar. Complete information can be found at www.pma.org.
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Severity
Severity
Severity