Page 30 - MetalForming November 2012

Never Check a Part Again
Here’s a process-control approach to ensuring stamping quality during production.
A Dimensional-Development Process for Stamped Parts
Measure Panels Here
Formability/ Press Allocation
Die Design/ Engineering
Die Construction
Primary Tryout/ Die Shop
Secondary Tryout/ Stamping
Launch/ Stamping
Steady State Production Parts
BY HARI MENON
When a new die comes
into a stamping plant it
typically goes through a
tryout process. During this phase
the primary responsibility of the
stamper is to constantly meas-
ure parts and establish process parameters that ultimately help control the process. You establish process capability and continually evaluate and track process-control parameters.
Perform these functions properly and you’ll never have to measure parts again. Why strive for this? Physically checking parts dimensionally not only is expensive, but often it occurs too late. By the time you determine the existence of a dimen- sional issue and work your way to the process parameter causing the dimensional change, it often is too late and sev- eral out-of-tolerance parts have been produced. Instead, monitoring and controlling the appropriate process param- eters allows a stamper to repeatedly produce accurate parts while avoiding the added costs of dimensional checks.
The caveat, of course, can either be a broken die, a die that requires an engineering change, a customer-requested part alteration or dimensional check, or a process that ventures out of control. In these instances the stamper must physically place parts in a checking fixture and take the necessary measurements.
Lacking such situations, however, and during regular production, stampers should work to define their process- control parameters and manage them within the control limits established early in the game.
Connecting the Virtual and Real World
Consider the typical process flow of stamping-die creation (Fig. 1). Die construction typically occurs in a die shop that
Hari Menon, MBA, is president of Optimized Manufacturing Group llc, specializing in manufacturing and quality con- sulting combining lean manufacturing and Six Sigma meth- ods. He also is the author of the book, TQM in New Product Manufacturing: 248/805-4787; omgroupllc@gmail.com.
Fig. 1
also performs die tryout to ensure that part quality match- es the needs of the assembly operation. This is the critical step in the process, when the parameters necessary to produce the part can be observed, defined and documented for use in the stamping plant. The end game in the die shop: Determine process parameters that will enable the stamping plant to repeatedly produce the agreed-to part.
Die shops use tryout presses to simulate production- press conditions and parameters. However they do not sim- ulate part transfer between presses, nor do they simulate the front-of-line and end-of-line conditions in the actual pro- duction presses. This can cause variability that must be debugged in the stamping plant.
Prior to and during die design, several activities occur, including:
• Defining the blank parameters;
• Defining the production press where the dies will run; • Formability analysis; and
• Processing the dies.
An additional factor during the design phase is comput-
ing the compensation needed for factors such as twist and springback.
After completing the die-design stage, the die-build process shifts from the virtual world to the real world. Then the die shop must validate that the completed dies produce parts that reflect their design intent. In the virtual world, we make best guesses to determine the factors that will determine the ability of the parts to meet dimensional requirements. In the real world, adjustments must be made to compensate for any dimensional deviation from design intent. And, we must validate, during assembly, that geometric dimensioning and tolerancing (GD&T ) requirements are met.
28 MetalForming/November 2012
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