
A Practical SPC Method for High-Capability Processes
By Ray Harkins and Michael J. Vella
Manufacturing professionals who regularly apply Statistical Process Control (SPC) methods, such as X̄ and R charts, know the power of SPC to maintain control of both quality and cost. The standard practice emphasizes attention to variation, with operators trained to take corrective action whenever a process signals an out-of-control condition. Typically, this means adjusting a process input, verifying the correction, and resuming production.
However, not every out-of-control signal requires intervention, especially in processes where inherent variation is extremely small and product quality is consistently high. In some cases, such as tooling wear or small material variations, process center shifts may occur outside of the operator’s ability to correct. Responding to these shifts with standard SPC rules can lead to overcorrection, lost productivity, and increased variation.
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