Precision forming and statistical inspection · Lesson 1 of 4

Precision forming and statistical inspection: principles and terminology

Why a folded angle never lands exactly on the programmed value, what AFS sends its folder, and the words statistical inspection uses: specification versus control limits, chance versus assignable causes, SPC versus SQC.

You will be able to: Define springback, nominal, specification limits and control limits, chance and assignable variation, and SPC and SQC, and state the safety boundary for measuring formed parts.

12 min · 10 slides · 10 questions

All lessons in this module

Slide 1 of 10

Why the angle moves

NIST researchers define springback as the elastic shape change of a sheet metal part after forming and removal from the die. A 1992 Amada patent states that the amount of this elastic return is impossible to predict theoretically.

So the angle is measured after forming and corrected. The correction is found by measuring and recorded, never guessed.

A: Lower beam: supports the sheet
B: Upper beam: clamps the sheet on the bend line
C: Folding beam: swings up and contacts the outside of the sheet
D: Folding-beam pivot
E: Sheet (AFS metal)

Springback exaggerated, not to scale, and no value shown: AFS has no measured springback value on record. A generic folder, not a drawing of any maker's machine.

Sources

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
US Patent 5,099,666 Method and device for detecting folding angles of a metal sheet during folding (assignee Amada Co Ltd; filed 1990, granted 1992) - U.S. Patent and Trademark Office (via Google Patents)

Videos for this lesson

What this video shows: A folder maker's tutorial: the sheet is gauged on the bend line, clamped between upper and lower beams, and folded to the programmed angle. A RAS machine, not AFS's folder.

Video: RAS Tutorial: What is metal folding? by RAS Reinhardt Maschinenbau GmbH, on YouTube. AFS checked its description against this lesson.

What this video shows: An ASQ Statistics Division webinar (about an hour): inherent and assignable variation, building control charts, and control limits versus specification limits.

Video: Statistical Process Control Overview and Basic Concepts - What You Need to Know for the CQE Exam by ASQStatsDivision, on YouTube. AFS checked its description against this lesson.

Glossary

Springback: The elastic shape change of a sheet metal part after forming and removal from the tooling: a folded angle opens slightly when released.
Specification limit: An acceptance limit set by the order or the project specification; it says whether the product will function as intended.
Control limit: A line on a control chart computed from the process's own readings, usually 3 standard deviations from the center line, inside which almost all points fall while the process is in control.
Assignable cause: A specific cause of variation worth searching for, as opposed to the chance causes always present in a stable process; a point outside the control limits starts the search.
SPC (statistical process control): Comparing what a process is doing today with what it did before, using control limits computed from its own history.