Precision forming and statistical inspection · Lesson 2 of 4

Precision forming and statistical inspection: materials, standards and design constraints

How material and thickness move a folded angle, AFS's own fabrication limits, why the order and the specification set the tolerance, oil canning, the standards for statistical methods, and how to tell whether a reading can be trusted.

You will be able to: Explain how metal, thickness and machine type move a formed angle, which documents set the acceptance limit, and what makes a measurement trustworthy.

12 min · 9 slides · 10 questions

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Slide 1 of 9

The metal moves the angle

An LVD die-bending patent: folding formulae and tables only take account of the sheet thickness and the V-opening. The angle depends greatly on the material and its composition. The patent says the correction must be established experimentally.

NIST notes that thinner high-strength steels and aluminum alloys show larger and less predictable springback. (Written about automotive stamping.)

Sources

US Patent 5,829,288 Adaptive folding (assignee L.V.D. Co NV; filed 1994, granted 1998) - U.S. Patent and Trademark Office (via Google Patents)
Robustness of the Sheet Metal Springback Cup Test (T. Foecke, T. Gnaeupel-Herold), Metallurgical and Materials Transactions A, vol. 37A, December 2006 - National Institute of Standards and Technology

Glossary

Thickness band: The published minimum and maximum thickness for a gauge; for 24 ga galvanized, Steel Market Update lists 0.0236 to 0.0316 in around a 0.0276 in nominal.
Oil canning: Visible waviness in the flat areas of metal roof and wall panels (MCA), also called elastic buckling or stress wrinkling; the MCA calls it generally an aesthetic issue. On a wide flashing face the term is used by analogy.
Check standard: A stable item, if possible one of the real parts, measured again and again over time to show whether the measurement process is drifting.
Resolution: The ability of a measuring tool to detect and faithfully show small changes; the number of digits on its display does not indicate it.