Process capability

Process capability is a statistical measure of how consistently a manufacturing process produces parts within a specified tolerance, typically expressed as an index (such as Cpk) comparing the tolerance width to the process's actual variation. A process can be capable of occasionally producing a part within tolerance while still having poor process capability if its variation is wide relative to that tolerance.

Why a single successful part doesn't demonstrate process capability

A tolerance near the edge of a process's demonstrated capability carries real risk of producing out-of-tolerance parts even without anything going obviously wrong, which is why process capability data, not just a single successful part, is what actually predicts whether a process can reliably hold a given tolerance.

Where process capability is assessed

Not on the drawing; process capability is a property of a specific process running on a specific machine, assessed through statistical data, not stated by the part's requirements.

Common mistakes

Assuming a process is capable of a tolerance because one part measured within it; process capability requires statistical data across multiple parts to assess actual, consistent variation, not a single data point.

Is process capability the same as inspection?

Inspection checks individual parts against requirements; process capability is a statistical assessment of how consistently a process can meet a tolerance across many parts.

What does Cpk measure?

A common process capability index comparing the width of a tolerance to the actual spread and centering of a process's output.

Can a tolerance be technically achievable but not process-capable?

Yes, a process might occasionally hit a tight tolerance while still having poor capability, meaning a meaningful fraction of parts would fall outside it over a longer run.