Process Capability and Statistical Quality Control
Variation, control charts, capability indices and measurement systems explained for manufacturing decisions.
Every process varies
Dimensions, chemistry, temperature and defects fluctuate because of material, machine, method, measurement, environment and people. Quality engineering separates common-cause variation from assignable disturbance.
Control charts
Control charts track process behaviour over time using statistically derived limits. A process can be stable yet incapable, or capable in theory yet unstable in operation.
Capability indices
Indices such as Cp and Cpk compare process spread and centring with specification limits. They are meaningful only when the process is stable and the distribution assumptions are reasonable.
Measurement systems
Gauge repeatability, reproducibility, bias and resolution must be understood before judging the process. Poor measurement can create false alarms or hide real variation.
Working rule
Stabilise the process, verify measurement, then assess capability. Inspection cannot compensate for an uncontrolled process.