Is the Part in Spec?

Since data is always imperfect, how can we ever know if a measured item is within the specifications?

During the past 20 years it has become fashionable to condemn measurement processes that are less than perfect. Yet the reality is that we must always use imperfect data. Given this fact of life, how can we ever know if a measured item is or is not within the specifications? Put another way, how can we make allowance for measurement error when characterizing product relative to specifications?

To complicate the answer to these questions, we have measurements that have measurement units attached, and we have the uncertainty in those measurements that are expressed in terms of measurement units squared. Much of what has been written about these problems has been flawed. However, rigorous treatments of these issues are available, and in this article I shall present the results of them.

The uncertainty in a measurement

The uncertainty in any measurement may be defined by the standard deviation of repeated measurements of the same item. In practice this quantity may be estimated by having one operator measure a set of items two or three times each, using the same instrument, as follows:

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Comments

Parts in Spec and Measurement Uncertainty

I was hoping to find an article that simplified the confusing world of measurement uncertainty. Instead we now have Dr. Wheeler's uncertainty calculation in addition to the system defined by the ISO Guide to Uncertainty of Measurement (the GUM). While I like Dr. Wheeler's method because it is simpler, I cannot ignore the GUM and its calculations.

I think it would have been better if Dr. Wheeler had mentioned that his system was different than the system that governs calibration and test laboratories that are accredited to ISO 17025.

I hope that the next few years will bring more clarity to this important subject.

Jack Dearing

A nice summary

This is a nice summary of the topics covered in Depth in Wheeler's book. Perhaps a comparison of this technique and the GUM would be a good topic for future columns.

Is the part in spec?

As a tier 1 supplier to the auto industry, I have to insure that any product shipped to my customers (OEM's) meets their specifications. I also have to insure that my suppliers have capable processes and also ship parts to me that meet the specification. When problems occur (and they alsways do!), we are called upon to react quickly to protect the OEM customer. Often this includes temporary 100% inspection while corrective actions are determined and implemented. While costly, this is much cheaper than shutting down a customer assembly plant or incurring a product recall.

If you do not take the measurement uncertainty into account when sorting products from a process that is not capable, then the risk is great that you will accept non-conforming product. This article is spot on, and the actions it recommends should be considered mandatory for containment plans involving sorting parts. It is at our company.

What to do with products rejected for tight tolerance limits?

I asked Dr. D. J. WHEELER for further guidance on products that were rejected within tight tolerance limits.

Dr. DONALD J. WHEELER kindly agreed to give me an explanation. For which I am very grateful to him.

Here are the links provided to me to explain further actions of the article:

1. Where Do Manufacturing Specifications Come From?

2. 100% Inspection and Measurement Error

Respectfully

Sergey Grigoryev