Supplier process audit: watching the line that makes your part

Audit the part, not the plant

Arrive with the approved process description, the control plan, the drawing and the approval record, and follow your part through the route as it is being made. Compare each step with what was signed off: the machine, the settings, the tooling, the checks and their frequency, the operator instructions, what happens to a part that fails a check. This is a narrow scope deliberately. A broad walk around the factory produces general observations that nobody acts on, whereas a discrepancy on your own part has an owner and a consequence.

Setup and changeover is where most drift enters

Between runs, everything is disturbed: tooling comes off and goes back, settings are re-entered, a different operator may set the job. Watch a setup if the schedule allows, or at least examine the setup record and the first-off approval for recent runs. Ask who is authorised to approve a setup, what happens when the first parts are marginal, and whether adjustments made during a run are recorded. Plants that control setup rigorously and record nothing between setups have a gap that only appears when a batch goes wrong and cannot be explained.

Following a defect backwards through the plant

Pick a real nonconformance from your own receiving records or from the supplier's internal scrap data and trace it end to end: how it was detected, how the batch was contained, how far the containment reached, what analysis was done, what changed as a result, and whether that change is visible in the documents on the line today. This single thread tests detection, containment, analysis and implementation in one exercise, and it exposes the common pattern of corrective actions that were written, closed and never actually reached the shop floor.

What the operators tell you that the documents do not

Ask the person running the machine what the difficult part of the job is, what they do when a dimension starts moving, when they last saw the work instruction, and what they would change. Operators generally answer honestly and their answers reveal whether the documented method matches practice. Where a written instruction contradicts what everyone actually does, the instruction is wrong or the practice is uncontrolled, and either finding is more valuable than another completed checklist. Do this with the supplier's agreement rather than around it. Framed as an attempt to make the job easier, these conversations usually produce more candour than a formal interview would.

Findings that change something

An audit report full of documentation observations produces documentation responses. Prioritise findings by their effect on the part: what could reach you undetected, what could vary batch to batch, what depends on one person. Agree actions with owners and dates, then verify the significant ones on the floor rather than by receiving a photograph. Frequency should follow risk and history, and the supplier should know what would cause the interval to shorten. An audit programme run to a fixed calendar regardless of performance signals that the outcome does not matter.

Frequently asked questions

How often should a supplier's process be audited for a critical part?

Set the interval by consequence and stability rather than by a uniform rule. A part whose failure reaches the end user, made on a process with a history of drift, warrants regular attention. A stable process producing a low-consequence part may need looking at rarely. Events should override the calendar entirely: a process change, a plant relocation, a change of ownership, a new setter on the job, or a cluster of nonconformances each justify a visit whenever they occur.

What is the difference between this and auditing a supplier's quality system?

A system audit examines whether procedures exist and are followed across the organisation, and it is usually conducted against a standard by quality staff or a certification body. A process audit follows one part through one route and compares practice with the approved method. The system audit answers whether the supplier is generally organised; the process audit answers whether the thing you buy is being made the way it was agreed. They find different problems and neither replaces the other.

Can a process audit be done remotely?

Partially. Documents, control records, setup sheets and analysis reports can be reviewed remotely, and a video walk of the line is better than nothing when travel is impractical. What remote review cannot give you is the unstructured evidence: the smell of a coolant that has not been changed, a fixture with a repair nobody mentioned, the pallet of parts in a corner with no label, and the conversation with an operator that a supervisor is not steering.