Frequent Quality Checks and False Security of Lot Sampling Statistics

We’re pleased to have Gary Netherton share with us his thoughts on quality control and quality audits. He takes a common sense approach, which I advocate, and stears away from any dogma or rigid approach, which some organizations blindly follow.

Read more about Gary after you read his thoughts on Quality Checks.

What’s the Frequency, Kenneth?  No, I am not singing an old REM tune¦ I am talking about quality.  More specifically, I am talking about dimensional quality checks.  I have a print with several characteristics labeled as critical.  I need to know how often I should be checking these parts.

Sound familiar?  One thing that I’ve learned in my quality journey is the importance of understanding critical dimensions and manufacturing’s obligations to ensure their correctness.  I recently experienced a situation where a manufacturer of a component had a drawing from the design folks that listed 23 dimensions as critical.  He was asking me to talk to the design team about reducing the number of critical dimensions to a more manageable number.

Being familiar with the product, I told him that there were that many dimensions for a reason . . . that the final assembly was very complicated and the component that he was supplying was critical to the final assembly’s functionality.

But my operators only have three to five minutes, maximum, to check dimensions!  How can they check that many dimensions every two hours in three to five minutes?

And so began my journey into confirming and explaining the significance of those critical dimensions.

In general, a dimension is critical because – if it is out-of-spec – the final product will not work in the best case and could violate regulations (Federal, state, or other) or  injure someone in the worst case (including killing them).  In some industries, contractual obligations dictate the frequency of inspection or the type of inspection or SPC that the supplier will use.  In many cases, however, it is strictly up to the supplier to protect the customer – to ensure that everything that he ships to the customer meets specification.

So, how often should we check a critical dimension?  That is up to you.  If, shortly after product launch, you collect a lot of data (which we all should do at product launch) and determine that the a certain dimension is always within specification, perhaps you should check it at shift start and shift end.  If it is a dimension that tends to vary throughout the shift, perhaps you should check it every two hours.

Another consideration is production speed.  If you are only making 10 per shift, then a 100% inspection might not be out of the question.  If you are making 10 per minute, then a sensible sampling plan (SPC anyone?) should suffice.

At the end of the day, the onus is on you to ensure that the customer is protected¦ that is, the customer receives exactly what they are expecting.

Gary Netherton is a multi-certified quality professional with project management experience in leading quality and manufacturing efforts from product and product launch to problem-solving using Six Sigma, PDCA, and other quality tools. His expertise is advanced product quality planning as well as data collection and analysis. He currently reside and work near the Seattle area of Washington.