Key takeaways
  • A control plan links each process step to the characteristic being controlled, the method, the frequency, and the reaction when it goes out of limits.
  • The reaction plan column is the one that gets skipped and the one that matters when something drifts.
  • It should flow from the FMEA: what the FMEA says is risky is what the control plan controls.
The column people leave blank
The reaction plan

Every control plan states what to measure. Far fewer state what to do when the measurement is bad: who is told, whether the run stops, what happens to the material already made since the last good check. Without that, an out-of-spec reading is a note in a log.

What a control plan contains

  • Process step and the machine or station it happens on.
  • Characteristic being controlled: product dimension, or process parameter such as temperature.
  • Specification and tolerance.
  • Measurement method and the gauge used.
  • Sample size and frequency.
  • Who performs the check.
  • Where the result is recorded.
  • Reaction plan when the result is out of limits.

Where it comes from

The FMEA identifies the failure modes and the controls that address them. The control plan is where those controls become specific: not 'inspect', but 'measure bore diameter with the plug gauge, 3 pieces every 30 minutes, record on the run sheet, and if out of tolerance stop the machine, quarantine back to the last good check, and call the cell leader.'

Frequency without guesswork

  • Base it on how fast the process can drift, and on how much material is at risk between checks.
  • A stable process with a slow drift needs less frequent checking than an unstable one, which is an argument for stabilising the process rather than checking more.
  • After a change, a restart or a tooling swap, check the first piece regardless of the standing frequency.

Keeping it usable

One page per part or process family, at the machine, not in a binder in an office. If the person doing the check cannot see the plan from where they stand, they are working from memory, and memory is what the plan exists to replace.

Phases

Control plans are usually written in three phases: prototype, pre-launch and production. Pre-launch is deliberately tighter, with more frequent checks, because the process is not yet proven. Relaxing to production frequency should be a decision supported by data, not a date on a calendar.