Precision vs. Accuracy: What is the Difference Between Them?

by | May 8, 2023 | CNC Machining

Most times, when people say that something is very accurate, or that it has a high degree of precision, they mean the same thing. In everyday speech, precision and accuracy are relatively interchangeable.

However, from a scientific or technical perspective, accuracy and precision actually refer to two different qualities. This article discusses the differences between accuracy and precision, and explains how they apply to CNC machining.

What Is Accuracy?

The scientific definition of accuracy is:

A description of how well a measurement approximates the theoretically correct value of that measurement.

In other words, scientists and engineers use accuracy to show how well a single measurement matches its true, accepted, or target value.

Figure 1. Target Practice Chart (High Accuracy)

Figure 1. Target Practice Chart (High Accuracy)

The classic example used for explaining accuracy and precision is a bullseye target. Accuracy represents how close any measured value is to the center of the bullseye, the desired value.

Any values near the desired or true value are considered relatively accurate, whether they land above, below, or to the sides of the center. The important quality for being accurate is closeness to the correct value.

What is Precision?

The scientific definition of precision is:

A description of how close a collection of measured values match each other.

Precision represents the range of variance in a data set, or the spread between data points.

Figure 2. Target Practice Chart (Low Accuracy-High Precision)

Figure 2. Target Practice Chart (Low Accuracy-High Precision)

In the bullseye example, if multiple measured values are clustered below the center of the bullseye, they are a precise data set. However, because they are not near the center of the bullseye, they aren’t that accurate.

But if the measured values are all focused around the center of the bullseye, we can say they are BOTH accurate and precise.

Precision vs. Accuracy in CNC Machining

Caliper measuring stainless steel flange

While the bullseye example is popular, it can be hard to understand how it relates to fabrication and manufacturing applications. The difference between accuracy and precision in machining relates to how closely a batch of parts matches the CAD model or design blueprint.

Highly accurate CNC machining equipment can produce a part that very closely matches the design model. For example, assume a part design requires a pair of 0.50-inch wide by 5.00-inch long slots machined on its surface, and the slots are 2.0 inches apart.

When a quality specialist examines a single finished part, they measure that the half-inch slots were actually cut to a length of 5.05 inches, and they were only spaced 1.96 inches apart. It is the quality specialist’s job to determine whether the machined part accurately represents the design model. Whether this part is rejected or not will depend on the allowable tolerances built into the design.

If a batch of 100 or 1000 of these machined parts is manufactured and measured, we can assess the precision of the CNC equipment. High-precision CNC machinery will demonstrate a high degree of reproducibility, with repeated measurements showing values consistently close to the true model.

Precision vs. Accuracy in Summary

Inside Gensun Precision machine shop

From a machining perspective, accuracy reflects how closely a fabricated part matches the design model, while precision shows repeatability when creating that part over and over.

Designers want fabricated parts to meet or exceed expectations for machining tolerances. This means finding equipment or vendors that can produce parts with both accuracy and precision.

Gensun offers exactly that. Our reputation for quality and reliability means we work hard to deliver our customers their parts within the design’s specifications on every order. If you need a machine shop to fabricate your next design, our world-class CNC machining services can help you get your parts quickly and at an affordable cost.

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