New operator, same questions
A new operator is to run a turning operation for the first time. She wants to know which dimensions to check, with which gauge, how often, and what to do if a dimension is outside. The experienced colleague answers from memory, and the answers almost match what the quality department thinks applies. Almost.
The control plan exists so that the answers do not depend on who you ask. It is the shared description of how the process is to be controlled, and the basis for the instructions the operator works to.
What a control plan contains
The AIAG manual for product quality planning and control plan (APQP, 2nd edition, 2008) describes a control plan as a table with one row per characteristic and process step. The most important columns are:
- Process step: which operation it concerns, and which machine, fixture or tool is used.
- Characteristic: which product characteristic (for example a diameter) or process parameter (for example a temperature) is controlled, and whether it is a special characteristic that the customer or your own risk analysis has identified as especially important.
- Requirement: specification and tolerance.
- Measurement method: how the characteristic is measured or assessed, with which gauge.
- Sample size and frequency: how many parts and how often, for example five parts per hour or every part.
- Control method: how the result is used, for example a control chart, a check against a limit, a mistake-proof fixture or a start-up check.
- Reaction plan: what is done when the requirement is not met or the control chart gives a signal.
The manual also distinguishes three phases: control plan for prototype, for pre-launch and for production. In pre-launch, more is often checked and more often, to get to know the process before the checks can be adapted to series production.
From FMEA to control plan
The control plan should not be written on its own. It builds on the process FMEA, where each failure chain has been assessed and controls have been chosen. Two kinds of control are carried over:
- Prevention controls, which reduce the risk of the cause occurring. For example that the tool is changed after a set number of parts, or that the machine setting is checked at start-up.
- Detection controls, which catch the failure if it occurs anyway. For example that a diameter is measured on every part, or that a control chart is followed.
The link works both ways. If the FMEA has rated detection as good because every part is measured, the control plan must say that every part is measured. If the control plan is then changed, for example because the frequency is reduced, the FMEA rating should be reviewed. IATF 16949:2016 (clause 8.5.1.1) requires the control plan to be developed with input from, among other things, the risk analysis and the process flow, and to be reviewed when the product, the process, the measurement or the risk analysis changes. Annex A of the standard lists what a control plan shall contain.
The control plan in production
A control plan is written for the person who plans the quality, not for the person at the machine. So it is translated into inspection instructions: what the operator should measure, in which order, with which gauge, and what is approved. The instruction should be possible to follow without reading the control plan, but it must not say anything different from the control plan.
A few things are worth keeping in mind:
- The measurement method assumes a measurement system that is good enough. A check with a gauge that varies too much in relation to the tolerance gives false confidence. Do a measurement system analysis for the important characteristics.
- The reaction plan must be concrete. “Contact quality” is not enough. It should say what happens to the parts made since the last approved check: block, sort, re-measure. It should also say who decides whether the process may continue.
- The frequency should match the risk and how quickly the process can change. A characteristic affected by tool wear needs checking more often than one determined by a fixture.
Common mistakes
- The control plan and the FMEA do not match, because they are updated by different people at different times.
- The control plan describes how it was intended, not how it is done. An audit often starts by comparing the control plan with what actually happens at the machine.
- The reaction plan only says what is done with the part that was wrong, and not with the parts made since the last approved check.
A living plan
Once the process has run for a while, there is data showing how it behaves. Stable characteristics with a good margin can sometimes be checked less often. Characteristics that often give signals, or that have caused complaints, may need more frequent checks or a better method. Change the control plan, and update the FMEA and the instructions in the same step, so that the three always describe the same thing.
