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Process Capability (Cpk) Improvement

Process Capability

Capability studies and improvement (centering and variation reduction) for key characteristics. Cpk > 1.33 and reduced out-of-spec.

Executive Summary

Key characteristics had low Cpk (process capability index), leading to out-of-spec parts and customer risk. This project conducted capability studies, identified causes of variation (centering vs. spread), and improved the process through tooling, setup, and control. Outcomes: Cpk > 1.33 for critical characteristics and reduced % out of spec.

Cpk > 1.33 for critical characteristics
Reduced % out of spec and scrap
Data-driven setup and control limits

Context & Problem

Some dimensions were barely within spec or had high scrap. The organization needed to quantify capability (Cpk, Cp), determine whether the issue was centering or variation, and implement improvements to achieve and sustain Cpk targets.

Methodology & Frameworks

Capability study with sufficient data from a stable process. Cpk and Cp from mean, standard deviation, and spec limits. Cpk < 1.0 inadequate; 1.0–1.33 marginal; > 1.33 generally acceptable. Cp vs. Cpk indicated centering (low Cpk, OK Cp) or spread (low Cp). Improvement: centering (adjust mean) or spread (reduce variation via tooling, maintenance, method, training). Control: SPC or control plans to sustain capability.

Implementation

Capability studies were run for key characteristics. Where Cpk was low, root cause (centering vs. variation) was identified. Process changes (setup procedure, tooling replacement, fixture improvement) were implemented and verified with a follow-up capability study. Control plans and SPC were used to monitor ongoing capability.

Outcomes

Cpk > 1.33 for critical characteristics after improvement.
Reduced out-of-spec—scrap and rework for those characteristics decreased.
Data-driven control—setup and control limits were based on capability data.

Lessons Learned

Capability studies require a stable process; fix instability first. Distinguishing centering from spread pointed to the right improvement. Re-running capability after changes verified improvement. Sustaining Cpk required ongoing control (SPC, control plan).