Metrology tools come in all different shapes and sizes and each performs a very specific function to a specific degree of accuracy. If you keep reading you’ll understand why I use the word accuracy in this context, not precision. Just keep in mind that the tool should fit the job.

Depending on what dictionary or thesaurus happens to reside on the shelf next to the textbooks you haven’t opened since college, or the internet (whichever came first), the definitions of these two words can seem at best unclear in their differences, or even interchangeable to a certain extent. On Dictionary.com, the words precision and accuracy are actually used to describe each other. You can’t logically use a word in the definition of itself. (see circular reasoning on Wikipedia)

You may have already encountered this but as you work, grow, and learn in the manufacturing industry, you will find there are differing schools of thought in terms of applying this idea of accuracy and precision. The goal is, or should always be to make perfect parts; every time. But here in the real world, anybody worth his/her paycheck will tell you there is no such thing as a perfect part. A great quote that I’ve always admired was short, easy to remember, and carried a pretty powerful message that applies to more than just machining.

Strive for perfection, but settle for excellence.

Don Shula

That said, a machinist, a QA Inspector, even the business owner needs to be able to quantify and convince themselves and the customer that they are producing good parts and why. Enter stage right – Precision and Accuracy. One way to think about precision is how repeatable the size and location of each feature is from one part to the next. Conversely, accuracy is the amount of deviation from a “perfect” part or nominal dimensions.

While I don’t disagree with this sentiment, I personally attribute precision with the act of machining or manufacturing parts. Being precise requires developing a reliable process that can be tweaked or adjusted as needed for product improvement. Even if the parts are wrong, as long as they are consistent, adjustments can be made to improve quality and throughput. Without that consistency, or repeatability between parts, you can make all the changes you want to your process but you’ll never know what to expect as a result.

Accuracy is more relative to the methods required to fully inspect the part or assembly being manufactured. Generally speaking, inspect the part to the same or higher degree than the part was manufactured to. For example, if a feature is machined and verified to three decimal places (.001”) on the shop floor, then ideally that same feature should be inspected to a resolution of four decimal places (.0001”) to account for rounding errors. A size dimension of a feature could be 3-4 “tenths” (.0003-.0004”) oversized and three-place decimal digital micrometers may still show the part in spec because of rounding.

While these terms are often used interchangeably in today’s industry, they do not necessarily equate to the same concepts of quality assurance and “perfection”. The idea of precision and accuracy should be at the forefront of part design and engineering all the way through to shipping a finished product because the act of manufacturing is cumulative in nature. Every input into the equation will affect the outcome including cost multipliers such as poorly toleranced parts. (Notice I said multipliers, not cost-adders… Food for thought.)