What is a Control Plan?

A control plan is the document that says, for every operation in a process, what gets checked, how, how often, and what to do when a check fails.

It is the most operationally important quality document you have, because it is the one that translates planning into instructions somebody on the floor can follow. An auditor walking your line will have it in their hand.

What it contains

Per row — and a row is a characteristic at an operation, not an operation:

• Operation — number and description, matching the process flow exactly.
• Machine or tooling — what performs the operation.
• Characteristic — product or process, and its number.
• Special characteristic class — critical, significant, or none.
• Specification and tolerance — what conformance actually means.
• Evaluation and measurement technique — the method and the gauge.
• Sample size and frequency — how many, how often.
• Control method — how the process is controlled, not just checked.
• Reaction plan — what happens when the result is out of specification.

Product versus process characteristics

A distinction worth getting right. A product characteristic is a feature of the part: bore diameter, surface finish, hardness. A process characteristic is a setting or condition that produces it: spindle speed, coolant concentration, oven temperature.

Controlling the process characteristic is nearly always better. Measuring the bore after cutting tells you what already happened; controlling the tool offset stops it happening. Control plans that consist entirely of product measurements are describing detection, not control.

The reaction plan is not optional

It is the column most often filled with something useless like "inform supervisor". A reaction plan should answer three things: what happens to the suspect product (contain it — how much, back to which point), what happens to the process (stop, adjust, requalify), and who has authority to release production again.

The test is whether an operator on night shift could follow it without phoning anybody.

Its relationship to the process flow and PFMEA

These three documents describe the same process from different angles and they must agree:

• The process flow defines the operations and their sequence.
• The PFMEA analyses what can go wrong at each operation and which controls address it.
• The control plan is those controls written as instructions.

An auditor will check that they agree, and in a great many organisations they do not — because they are three separate files, maintained by the same person under time pressure, and the process changed. This is not a discipline failure; it is a structural one. Three documents built on one operation list cannot drift. Three files can, and will.

Different plans for different phases

Control plans are usually maintained for prototype, pre-launch and production. Prototype covers dimensional measurement and testing during builds. Pre-launch typically calls for increased inspection frequency, because the process is not yet proven. Production reflects normal ongoing control. Moving from pre-launch to production is a decision that should have capability evidence behind it, not just a date.

Three mistakes that make a control plan useless

Inspecting everything. A control plan that checks every dimension at every operation is a plan nobody follows, and unfollowed plans are worse than modest ones, because now the document and reality disagree.

Untraceable revisions. If the plan on the line is not demonstrably the approved revision, the document has no authority. This is what revision control is for.

Frequencies nobody can meet. Five per hour on a characteristic requiring a fifteen-minute CMM cycle is a fiction that will be quietly ignored, and the ignoring will spread to the rest of the plan.