You arrive at a production workstation. The operator performs a measurement. A caliper is lying on the table, the Control Plan is next to it, and the measurement results from the last few hours have been entered into the inspection record.

If the audit ended with checking the documents and records, we could already move on. The problem is that we would have checked very little.

Because this is where the questions begin.

• Is the operator checking the correct characteristic?
• Are they using the measuring device specified in the documentation?
• Does the inspection frequency match the Control Plan?
• And most importantly: what will they do if the result is NOK?

This is often where we discover things that cannot be seen from the records alone.

The Control Plan requires five parts to be measured every two hours. The operator measures only one. The documentation specifies a gauge, but a caliper is being used at the workstation.

The work instruction says that the process must be stopped and potentially affected production must be checked, but the operator says:

“I put the part in the red bin and measure the next one.”

There may also be a different situation. All measured parts are within specification, but the results have been close to the upper tolerance limit for several hours.

Was the inspection performed? Yes.

Do we have a record? Yes.

Is that enough to say that the process is under control? Not necessarily.

That is why, when verifying process quality control, it is useful to follow the entire path:

requirement → PFMEA → Control Plan → work instruction → inspection execution → record → reaction

If one of these elements does not align with the others, sooner or later a problem will occur.

What Does Process Quality Control Mean?

Let’s start with a simple example. We manufacture a component whose one dimension should be: 10.00 ± 0.05 mm

The operator performs a measurement: 10.02 mm

The part is conforming. The question is: What do we know about the process? Actually, very little. We know that one inspected part is within tolerance.

What we do not know is:

• what the previous results looked like,
• whether the process is shifting,
• whether its variation is increasing,
• whether we will still be producing conforming parts one hour from now.

This is where we need to distinguish between two concepts: product inspection and process control.

Product Inspection vs. Process Control

Product inspection primarily answers one question:

Does the inspected part meet the requirement?

We measure a dimension, force, tightening torque or another characteristic and compare the result with the specification. The result is OK or NOK. This is, of course, a simplification because the actual measured value should normally be recorded.

With process control, however, we are also interested in how the results behave over time. Suppose the next five measurements are: 10.00 mm; 10.01 mm; 10.02 mm; 10.03 mm; 10.04 mm All parts are still within specification. But do we really want to wait until we see: 10.06 mm?

Because at that point, we are no longer analysing the process. We already have a nonconforming product. With such a sequence of results, I would start investigating what is happening in the process.

• Is the tool wearing out?
• Is the temperature changing?
• Has someone changed the machine parameters?
• Are we seeing normal process variation, or is a trend developing?

This is where SPC comes into play.

Not because we need another chart to show during an audit, but because we want to detect a change in the process before the first NOK part is produced.

But there is one more thing. Before we start analysing measurement results, we need to know whether we can trust them.

Can We Trust the Inspection Results?

Imagine that three operators measure the same part.

Operator A: 10.01 mm

Operator B: 10.04 mm

Operator C: 9.99 mm

What now? Which result is correct? Is the problem with the product? The measuring device? The measurement method? Or is each operator performing the measurement slightly differently?

This is why process quality control is closely related to MSA.

Simply having a calibrated measuring device does not answer the question of whether the measurement system allows us to make reliable decisions about the product and the process.

These are two different issues.

Process Quality Control Starts Before the Measurement Station

What we see on the shop floor is only the final stage of the entire inspection planning process. The operator takes a part. Performs a measurement. Records the result. But how do we know that this is the characteristic that should actually be checked?

To answer that question, we need to go back a few steps. The Process Flow Diagram shows us how the process is structured. In PFMEA, we analyse what failures may occur, what their effects may be and what controls are used.

In the Control Plan, we define, among other things:

• what is inspected,
• how it is inspected,
• which measuring device is used,
• how frequently the inspection is performed,
• what reaction is required when a problem is detected.

Only then do these requirements reach the production workstation. That is why, during an audit, I would not start by asking:

“Do you perform this inspection every two hours?”

It is much more useful to select one characteristic from the Control Plan and follow it to the workstation.

You can say:

“Please show me how this inspection is performed.”

Then you can verify:

• the method,
• the measuring device,
• how the inspection is actually performed,
• the record,
• the frequency,
• the operator’s reaction.

Only then do we begin to see how process quality control works in practice.

Process Quality Control vs. Process Audit

Process control and process auditing have different purposes. During production, an operator or inspector performs measurements and inspections according to defined requirements. During an audit, we verify whether this method of controlling the process actually works as planned.

That is why simply reviewing the Control Plan and inspection records in the office does not convince me. I select one characteristic.

I go to the production area.

I observe the workstation — sometimes from a distance first.

I ask the operator to perform the inspection.

I look at which measuring device is being used.

I compare this with the documentation.

I review the latest results.

And finally, I ask:

“What will you do if the next result is NOK?”

The answer to this question often shows whether we have a properly functioning control system or simply well-completed records.

5 Things I Would Check During Process Quality Control

You can have an up-to-date Control Plan, measuring devices with valid calibration status and complete records from the last three shifts. That still does not tell us whether the inspection is being performed as planned.

So instead of checking only whether the documents are available, let’s select one characteristic and follow it through the process.

1. Is the Operator Checking the Correct Characteristic?

I start with the Control Plan. I select one characteristic and check where it is supposed to be inspected. Then I go to the workstation and ask the operator:

“Please show me how you check this characteristic.”

It sounds simple, but this is exactly where many problems can be found. The Control Plan requires the diameter to be measured at a specific location, but the operator measures it a few millimetres away. The document requires a gap to be checked, but the operator only visually verifies whether the component has been assembled correctly.

Or the operator performs the measurement correctly but is using an outdated version of the work instruction. That is why I would not stop at asking:

“Do you inspect this characteristic?”

Ask the operator to show you. It takes only a few minutes and provides much more information.

2. Are the OK/NOK Criteria Clear?

The operator now knows what needs to be checked. The next step is to verify whether they know what result is acceptable.

Suppose the drawing specifies: 25.00 ± 0.20 mm

The operator measures: 25.18 mm OK.

The next part: 25.22 mm NOK.

Simple?

Yes — as long as all documents specify the same requirement. The problem starts when the drawing specifies one tolerance, the Control Plan another and the work instruction yet another. Now the operator has to decide which document to trust.

They should never be put in this situation. At the same time, we can also check document revisions:

• Does the Control Plan correspond to the current drawing?
• Has the latest customer requirement change reached the production area?
• Is an outdated work instruction still being used at the workstation?
• Could such seemingly minor inconsistencies eventually result in a customer complaint?

3. Does the Inspection Frequency Match the Control Plan?

The Control Plan says: 5 parts every 2 hours.

We check the records:

08:00; 10:00; 14:00; 16:00

12:00 is missing. So I ask: “What happened at 12:00?” Perhaps production was stopped. Perhaps the inspection was performed, but the result was not recorded. Or perhaps nobody performed the inspection.

I would not immediately assume that we have a nonconformity. First, we need to understand what actually happened. I would also check how the company defines inspection frequency, because “every two hours” can be interpreted in different ways.

• Two hours from the beginning of the shift?
• Two hours from the previous measurement?
• What happens after a changeover?
• What happens after an extended machine stoppage?
• What happens after a tool change?

If the answers to these questions depend on who happens to be working on a particular shift, there is room for different interpretations.

4. Is the Correct Measuring Device Being Used?

The Control Plan specifies a gauge. At the workstation, I see a caliper. So I ask:

“Why is this inspection being performed with a caliper?”

We may then discover that the gauge was damaged two weeks earlier and sent for repair. Production had to continue, so someone decided:

“For now, we’ll measure it with a caliper.”

But what exactly does “for now” mean? Two days? Two weeks? Three months?

If we change the inspection method, we need to verify whether the new method can reliably assess the characteristic in question. The fact that a measuring device can display a result to two decimal places does not, by itself, prove that it is suitable for the measurement.

Among other things, I would check:

• whether the measuring device corresponds to the Control Plan,
• whether it has a valid status,
• whether its measuring range is appropriate,
• whether its resolution is sufficient for the characteristic being evaluated,
• whether the way it is used is consistent with the defined measurement method.

And then I would move on to the next question.

5. Can We Trust the Measurement System?

The measuring device has a valid calibration status. So everything is fine? No. Calibration and MSA answer different questions. Calibration helps confirm the condition and accuracy of a measuring device in relation to a defined reference standard.

MSA allows us to evaluate how the entire measurement system behaves. And the measurement system consists of more than just the measuring device. It also includes:

• the operator,
• the method,
• the manufactured part,
• the measurement conditions.

Suppose three operators measure the same characteristic.

Operator A: 15.01 mm

Operator B: 15.06 mm

Operator C: 14.98 mm

If the tolerance is wide enough, all three results may still be acceptable. But when we are working close to a specification limit, the difference may determine whether a part is accepted or rejected.

That is why I would check whether an appropriate MSA study has been performed for the measurement system and whether the conditions under which it was conducted still correspond to what is actually happening at the workstation.

An MSA study performed several years ago tells us very little if the measuring device, measurement method or part fixturing has since been changed.