Scenario
In the third month of mass production for a new line, a process anomaly occurred. The QA engineer retrieved the Control Plan and found that the control parameters were still set to the trial production settings, with no updates made for process conditions changed after mass production.
The Control Plan existed in name only; in reality, it was an outdated document providing no guidance to the production floor.
What is a Control Plan
A Control Plan is a core document that records "what to control, how to measure, and how to handle anomalies" for each step of the process.
Three versions:
- Prototype Control Plan: During the development phase, to confirm design feasibility.
- Pre-launch Control Plan: For small-batch trial production, to validate the process.
- Production Control Plan: For mass production, serving as the basis for daily control.
Key Fields of a Control Plan
| Field | Description |
|---|---|
| Process Step | Corresponds to the station on the process flow diagram. |
| Control Characteristic | Quality characteristics to be controlled (dimensions, performance, appearance). |
| Specification / Tolerance | Acceptance criteria required by the customer or design. |
| Measurement Method | What measurement tools to use, how to measure. |
| Sample Size and Frequency | How many pieces to measure each time, how often. |
| Control Method | SPC control chart? Visual inspection? Automated sensing? |
| Reaction Plan | When control limits are exceeded, who does what, and within what timeframe. |
Links Between Control Plan and Other Documents
PFMEA → Control Plan
PFMEA identifies high RPN failure modes, and the corresponding Control Plan should have stricter controls:
- High RPN items → Increase measurement frequency.
- Difficult-to-detect failure modes → Switch to 100% measurement or add sensors.
MSA → Control Plan
Every measurement method in the Control Plan must be validated through MSA:
- Gage R&R < 10%: Measurement system is qualified, data can be trusted.
- Gage R&R 10-30%: Conditions are acceptable, but need improvement.
- Gage R&R > 30%: Measurement system is unqualified, control data is meaningless.
SPC → Control Plan
Items marked "SPC" in the Control Plan require corresponding control charts and control limits.
When to Update the Control Plan
The Control Plan must be updated under the following circumstances:
- Design change (adjustment of dimensional specifications).
- Process change (equipment changeover, process parameter adjustment).
- Discovery of new failure modes (customer complaints or internal findings).
- MSA results indicate that the measurement method needs improvement.
- After PFMEA updates.
The Control Plan is a living document, not a one-time document.
Golden Quote
"The Control Plan is the user manual for the process; PFMEA tells you where the dangers are, and the Control Plan tells you how to prevent them. The two documents must be synchronized—if PFMEA is updated and the Control Plan is not, it's like knowing the risk but having no protection."