20% Maximum Thinning rule should not be a rule
Everyone likes to have a “Rule of Thumb” to use
as a quick and easy guide. To make the best use of these
maxims, it helps to understand where they came from, and what
the limitations are in their use.
When it comes to taking the right steps to ensure
a robust stamping process, a surface strain analysis using a
forming limit diagram is recommended. The forming limit curve
should be generated from the minimum allowable thickness and the
lower mill production limit (or the -3σ value) for the strain
hardening exponent, or n-value. To bypass some of the work
involved in generating this information, some companies have
chosen to use a rule of thumb that calls for a maximum 20%
thickness reduction on a formed part compared with the initial
flat blank thickness. In some cases, this is an acceptable
substitution, but in many cases, using this 20% threshold only
confuses the proper course of action.
More information about how the maximum thinning rule should be
applied can be found in THIS FREE DOWNLOAD.
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