Page 32 - MetalForming November 2012

Never Check a Part Again
Geometric Dimensioning & Tolerancing
Panels produced in the stamping plant typically include GD&T, based on past experience or the expectations of the design engineer, which provides the upper and lower spec- ification limits at interface points. This also may be based on some level of simulation to determine the requirements. This data is used to determine the true nominal dimen- sions of the panel (Fig. 3) and the upper and lower control limits, based on the parameters used for the process-control elements. This can be done with the use of statistical meth- ods based on three or more runs of three parts.
Three or more runs can be defined as data collected after the dies were moved out of the press and removed from the bolster. However, care must be taken to ensure that the vari- ation within each subgroup is not significant, along with vari- ation between subgroups. Calculating the upper and lower control limits without running tests for the variation described above will result in significantly greater shift in nominal and/or the control limits, causing problems down the road.
You’re Happy—How About the Customer?
Once you’ve established process capability and the true means and upper and lower control limits of a stamped panel, the next step is to ensure that the customer (internal or external assembly operation) is satisfied with the newly
established means and the upper and lower control limits. Note: This should have been an ongoing process as mate- rial was submitted to the assembly line during the various builds, along with the three-part analysis. The assembly operation then decides if any of the requested tolerance adjustments cause a build issue. Any dimensional issues in an assembly or subassembly attributed to the stamping requires that the die process be improved to bring the nom- inal closer to the assembly requirements, along with the
upper and lower control limits.
Once the die work is completed, a fresh batch of data must
be submitted to ensure that the process capability is estab- lished to the requirements of the assembly plant. Once the assembly plant is satisfied with the mean shifts and the tol- erance adjustments, process capabilities are deemed estab- lished; from here on out the stamping plant can monitor established process parameters for each of the steps, and run the dies confident in the established process-control process. No more part measuring.
Monitor and React
Ongoing, the stamping plant should develop a plan to col- lect process-control data—which can be automated in the controls—along with triggers to notify engineers should process parameters wander beyond control limits. And, develop a reaction plan should process control be lost. MF
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