Page 21 - MetalForming July 2010

Fail
Fail
Safe Safe
Fail
Fail
Major Stretch Major Stretch
AB
1?2 4
Minor Stretch Minor Stretch
3
-3 sigma material. A fatal mistake has been made.
A die is created and spotted to the parameters of the tryout steel. Substitute a thinner sheet into that die and the steel will perform differently. Kiss blocks and draw beads will generate less restraining force on material flow from the binder into the die cavity. Die radii, punch radii, addendum, stamping features and even the global shape of the stamping will provide less restraint over material flow. Substituting a blank with a lower n-value can dramatically change the distribution of stretch. Stretch gradients will become more severe. This local- ization of stretch causes localized thin- ning that can lead to increased spring- back. Even worse, the thinning of the sheet and the reduction of the n-value can cause a synergistic interaction.
The bottom line? There is no practi- cal justification and zero scientific back- ing to take the amount of stretch from a stamping made with one set of prop-
erties and assume that amount of stretch will remain unchanged for all the other possible combinations of properties of incom- ing material. Dots 3 and4inGraphBare possible results.
Can the worst-case
scenario be used cor-
rectly? The only way
is to make a stamping
with sheet having the
-3 sigma properties. Then the mechan- ical properties can be obtained, the FLC generated, and the stretch from the stamping measured with all values reflecting the same event. Obtaining material with the -3 sigma properties would be difficult, if not impossible. An easy solution is to make the stamp- ing in the virtual press shop. The -3 sigma properties are easily entered via the keyboard. The virtual version of
Stretch state (1) from a stamping made with properties used to compute the forming limit curve (A) can not be used (2) for a stamping made with different properties (B).
the die is used to form the stamping. Output from the computer analysis can be automatically displayed similar to Graph A. Now die tryout can proceed in the virtual press shop without building a physical die (or two), requiring hours and hours of grinding and welding. The virtual press shop not only can correct problems caused by the worst- case scenario, but will provide addi- tional time-saving opportunities. MF
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