Key takeaways
- A control plan names the step, the characteristic, the method, the sample and the reaction to a failure.
- Prototype, pre-launch and production plans are three different documents, not one plan relabelled.
- Every special characteristic needs a control and a reaction plan that an operator can follow.
- A control plan that is not updated after a process change describes a process that no longer exists.
What a control plan does
A control plan describes how a process is controlled. It names the step, the characteristic to be checked, the specification, the method, the sample size, the frequency and what happens when the check fails. It is written for the operator who runs the step and for the auditor who has to see that the control is real.
It is not a quality manual and it is not a list of every dimension on the drawing. It is the short list of controls that keep the process inside the specification. If a characteristic has no control, the plan should say why, or the drawing should not require it.
The columns that carry the control
The order matters. Start with the process step, then the characteristic, then the specification. The evaluation method, the sample and the control method follow from those three. The reaction plan closes the row.
Two more columns are easy to forget: the machine or tool that performs the step, and the person responsible for the check. Without them, the plan describes a process in general rather than the line in front of you.
A useful test is to cover the left half of the row and ask whether the right half still makes sense. If the method or the sample can be understood without the step and the characteristic, the row is too vague to audit.
Three versions across the life cycle
The prototype control plan covers the parts made before production tooling. It is a design aid, so it often checks more dimensions and uses different methods. Its purpose is to learn, not to set the production frequency.
The pre-launch control plan covers the parts made with production tooling but before volume release. It usually adds inspection, containment and a higher frequency while the process is still proving itself. The production control plan is the volume plan. It keeps the controls that are needed and removes the containment that the capability study has made unnecessary.
The three plans should share a structure but not their content. Copying the prototype plan into production leaves the line with the wrong sample sizes and a method that was only meant for a lab.
Special characteristics and classes
A product characteristic is a feature of the part: a diameter, a material property or an appearance requirement. A process characteristic is a setting that controls the part: a temperature, a torque, a time or a pressure. Both belong in the plan when they affect fit, function, safety or regulation.
Special characteristics are the ones the customer designates, plus any the supplier designates for its own process. The symbol is set by the customer, so the plan needs a legend that says what each symbol means. Marking every characteristic as special removes the signal and adds inspection cost without adding control.
Sample size and frequency
The sample and frequency should follow the risk. A new process, a tight tolerance or a history of variation justifies a higher frequency. A stable process with a capable gauge can be checked less often, provided the plan says how stability is confirmed.
Write the frequency in a way the line can follow. Five pieces per shift is clear. Check as needed is not. If the check is continuous, say what that means: every part, or a defined interval. The plan should also say where the result is recorded.
Group similar checks where the line can do them together, but do not merge checks on different failure modes. A dimension and a torque on the same step can share a record sheet. They should not share a reaction plan if one failure needs a different containment.
The reaction plan
The reaction plan is the part of the document that proves the control is real. It should say who stops the process, how the suspect parts are identified and segregated, who is notified, what is recorded and what has to happen before production restarts.
An instruction to notify the supervisor is not a reaction plan. A usable plan names the quarantine area, the person with authority to release the parts and the disposition route. It also says what to do with parts made since the last good check, because that is the quantity most likely to be missed.
Write the reaction plan for the night shift
The person who finds the failure may not be the engineer who wrote the plan. Use plain steps, a named role and a location, and test the plan once before launch.
Keeping the plan current
A control plan changes when the process changes. A new tool, a new material source, a new gauge, a PFMEA action or a customer complaint can all change the controls. The plan should carry a revision and a short change record so the line knows which version applies.
Review the plan at a set interval even when nothing has changed. A plan that has not been read for a year often contains a frequency that the line stopped following, a gauge that has been replaced or a reaction plan with a name that has left the company.
A review checklist
Use the table to check a plan before it is issued. The common mistakes are not exotic. They are the small gaps that let a known failure mode reach the customer.
Ask the operator to explain the row without help. If the explanation matches the written plan, the plan is usable. If it does not, the plan is a document, not a control.
| Column | What it records | Common mistake |
|---|---|---|
| Process step | The operation and its sequence | A step that is missing from the flow diagram |
| Machine or tool | The equipment that makes the feature | A generic cell that names no asset |
| Characteristic | The product or process feature | A dimension with no link to the drawing |
| Specification | The nominal and tolerance | A tolerance that differs from the drawing |
| Evaluation method | The gauge and the method | A gauge that cannot resolve the tolerance |
| Sample and frequency | How many and how often | A frequency the line cannot sustain |
| Control method | How the result is judged and recorded | A check with no record |
| Reaction plan | What happens after a failure | Notify the supervisor and nothing else |
Frequently asked questions
Who writes the control plan?
A cross functional team writes it, usually led by quality or manufacturing engineering. The process owner, the operator and the gauge engineer all have to agree that the method and the frequency are practical. A plan written by one person in an office is often a plan the line ignores.
Does every part need a control plan?
A production control plan is expected for automotive production and is good practice elsewhere. The depth can be proportional to risk. A simple catalogue part may need a short plan, while a safety related part needs a detailed one with special characteristics and containment.
What is the difference between a control plan and a PFMEA?
The PFMEA analyses what can go wrong and how to prevent or detect it. The control plan records the controls that were chosen and how they are run in production. The PFMEA is the reasoning, the control plan is the instruction.
How often should a control plan be reviewed?
At least annually, and whenever the process, tool, material, gauge or customer requirement changes. A new complaint or a capability drop should also trigger a review. Record the review date and the revision.
Sources and further reading
- AIAG Control Plan reference manual
- AIAG APQP, Advanced Product Quality Planning reference manual
- IATF 16949, Quality management system requirements for automotive production
Qumek Quality Desk
Quality team
The quality team responsible for first article inspection, PPAP packs, supplier audits and the records shipped with production parts.
Reviewing a control plan before launch?
Send the flow diagram, the PFMEA and the current plan. The quality desk will check that the three documents describe the same process.
Send the plan