SPC Statistical process control (SPC)
Statistical process control (SPC) · Inspección y ensayos · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “SPC on critical diameter · Cpk ≥1.33” states a statistical process control (spc) requirement.
What controls
Statistical monitoring of a characteristic throughout production, using control charts that separate the natural variation of the process from signals indicating that something has changed, together with capability indices that compare the process spread with the drawing tolerance. It is a process control, not a lot acceptance criterion: a chart in control does not guarantee there are no out-of-tolerance parts, and an in-tolerance part can be a warning signal if it departs from the process pattern.
How to verify
Variables measurement with an instrument of sufficient resolution, taking rational subgroups (consecutive parts within a subgroup, subgroups spaced out over time) per the control plan. Averages and ranges are plotted on the chart, control limits are calculated from the process itself (not from the tolerance) and capability indices are evaluated against the drawing tolerance. The measurement system should be validated first to know how much of the variation comes from the instrument.
Workshop guideline values
- General
- Capability indices of around 1.00–1.33, sufficient for unclassified characteristics.
- Accuracy
- Indices of 1.33 or higher, the usual requirement for key characteristics.
- High precision
- Indices of 1.67 or higher, typical of critical characteristics or regulated sectors.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Confusing control limits with tolerance limits: the former come from the process, the latter from the drawing.
- Using the chart as a lot acceptance criterion instead of as a process control tool.
- Adjusting the machine every time a point deviates, which increases variation instead of reducing it.
How to get it in the workshop / if it comes out of tolerance
Its value lies in acting before making bad parts: a sustained trend on the chart warns of tool wear long before the first out-of-tolerance part appears. It requires variables measurement, with defined subgroups and frequency, and a validated measurement system, because if the instrument contributes too much variation the chart shows noise rather than the process. Reacting to every individual point is counterproductive: constantly adjusting the process increases the spread.
Points outside the control limits: look for an assignable cause (change of tool, material or workholding); do not simply recenter. Continuous trend: wear; compensate on a planned basis. Increasing spread: backlash, vibration, irregular coolant flow. Insufficient capability despite being in control: the process is stable but not suitable for that tolerance, and the solution is to change the process or review the tolerance with engineering.
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