If you supply machined or fabricated parts to an automotive customer, sooner or later the supplier quality engineer will ask for your control plan. It is one of the documents an auditor opens first, and one of the easiest to get wrong, because many shops fill it in once for PPAP and never look at it again.

What a control plan is

A control plan is a written summary of how you will control every important product and process characteristic of a part, operation by operation. For each characteristic it tells the operator and inspector what to check, against which tolerance, with which gauge, how often, and what to do if the result is bad.

It is an output of APQP. Quality Magazine notes that control plan requirements originally sat inside the AIAG APQP and Control Plan second edition manual, and that a standalone AIAG Control Plan Reference Manual has been effective since 1 March 2024. The same article says the manual gives guidance and is not prescriptive, so your customer-specific requirements still decide the final format.

The three types: prototype, pre-launch, production

The control plan grows as the part moves from sample to series production:

  • Prototype – built during product design and development. It covers the basic dimensional and material checks needed to confirm the prototype meets design intent.
  • Pre-launch – used for pilot runs before full production, and often includes extra "safe launch" checks such as 100% inspection of critical dimensions. MSI describes this as the version normally submitted with PPAP.
  • Production – the live document for series manufacturing. This is the version auditors review during IATF 16949 audits, according to Six Sigma DSI.

All three use the same columns; what changes is coverage and checking frequency.

The columns, explained for a machine shop

Formats vary between customers, but most control plans carry these columns:

  • Operation number and description – taken straight from your process flow diagram, for example "OP 20 – CNC turning, first setup".
  • Characteristic – the product feature (a diameter, a thread, a surface finish) or a process parameter (spindle speed, coolant concentration, furnace temperature).
  • Special characteristic – the customer's symbol for safety or critical features. These need the strongest controls.
  • Specification and tolerance – exactly as on the drawing, for example 25.00 ± 0.05 mm.
  • Evaluation or measurement technique – the actual gauge: micrometer, plug gauge, height gauge, CMM.
  • Sample size and frequency – for example, 5 parts every 2 hours, or first and last piece per setup.
  • Control method – SPC chart, check sheet, poka-yoke, setup approval.
  • Reaction plan – what the operator does if a part is out: stop, segregate, inform, adjust, re-inspect since the last good check.
A control plan is only as good as its reaction plan. "Inform supervisor" alone does not contain a single bad part.

Link it to your PFMEA and process flow

A control plan should never stand alone. Six Sigma DSI points out that under the AIAG-VDA FMEA approach, failure modes with high or medium action priority need a documented process control, and that control becomes a row in the control plan. The process steps themselves should come from your process flow diagram.

In practice, keep the same operation numbers in all three documents. If PFMEA line 20.3 says "bore undersize due to insert wear", the control plan for OP 20 should show the bore, the bore gauge, the check frequency, and an insert-change rule in the reaction plan.

How to prepare one, step by step

Here is a practical sequence for a typical turned or milled part:

  1. Start from the drawing. List every dimension, note and specification, including material, heat treatment and plating callouts, and mark the special characteristics.
  2. Map characteristics to operations. Each feature should be controlled at the operation that creates it, not only at final inspection.
  3. Choose gauges you actually own. If the plan says CMM and you only have a height gauge, an auditor will find the gap in minutes. Gauges for critical features should have an acceptable MSA study.
  4. Set realistic frequencies. Tighter for special characteristics and new setups; looser only when your capability data supports it.
  5. Write specific reaction plans. Name who decides, how far back to re-check, and where suspect parts go.
  6. Keep it alive. Update it whenever the drawing, material, tooling, gauge or process changes, and after every customer complaint.

Illustrative example: for a 20 mm H7 bore on a housing, the row might read: OP 30 boring, bore Ø20 H7, air plug gauge, first piece plus 1 in 25, SPC X-bar R chart, reaction plan "stop machine, quarantine parts since last good check, change insert, 100% check quarantined lot".

Common mistakes auditors find

  • A plan copied from another part, with features not on this drawing.
  • Frequencies on paper that the shop floor does not follow.
  • Special characteristics missing from the plan.

How SpecReview helps

SpecReview reads your 2D drawing PDF or STEP model and drafts a quality plan and a layout inspection plan, with each characteristic tagged as coming from the spec, the geometry or an assumption. It also builds a PPAP document set, so your control plan starts from the drawing's real characteristics instead of a copied template.