Definition
The control limits are calculated by the actual data, not the same as the specifications given by the client.
In detail
Statistical Process Control (SPC) is a set of methods for monitoring process stability through control charts and assessing process capability to meet specifications through capability indices. Control limits on control charts are calculated from process data and are unrelated to customer specification limits, used to distinguish between common cause and special cause variation. In the system, SPC is used for real-time monitoring and continuous improvement, complementing FMEA (preventive design) and MSA (measurement system validation). Its outputs include control charts, capability indices, and abnormal signals, guiding corrective actions.
Example in practice
In the injection molding workshop, to monitor product weight, the operator samples hourly and plots an X-bar chart. When one point exceeds the control limit (the strongest signal of special cause), an alarm is triggered, and the team immediately stops the machine to inspect, finding mold wear. After adjustment, production resumes and the Control Chart is updated. Meanwhile, Cpk is calculated to ensure Process Capability meets customer requirements. The final output is a controlled process record proving the process is stable.