Roundness measurement is critical for many applications. It can be measured in several different ways, ranging from simple caliper measurements to high-precision, dedicated roundness measurement systems. In this article, we will introduce you to the definition of roundness, roundness standards, and several common methods of roundness measurement.

How to define roundness?

A circle is defined as a closed curve with a constant radius from a point (the centre of the circle).

Roundness is the degree to which the cross-section of a workpiece approaches the theoretical circle.

Roundness is a value used to quantify the circular error, i.e. the distance from the centre of the circle.

More precisely, roundness can be considered as ‘out-of-roundness’.

Criteria for roundness

There are many different international, national, local and industry standards for circularity, such as the following two ISO standards: ISO 1101:2017 defines circularity as the radial separation of two concentric circles that contain exactly the circularity feature.

Graphic standards: describes circularity and the identification of circularity measurements and analyses.

ISO 12181:2011 (two parts).

Provides definitions of circularity parameters

Introduces different reference circles

Why measure roundness?

The circular cross-section is one of the most important basic forms of industrial components. Round cross sections occur in many applications, especially on bearing surfaces such as rotating shafts and ball bearings. Measurement leads to premature failure. Accurate roundness measurements are therefore essential to ensure the functionality of such parts.

The measurement of out-of-roundness (often referred to simply as ‘roundness’) is an extremely important assessment method. If a rotating bearing component is out of round, it tends to be noisy and is likely to How is roundness measured?

Below you will find several common roundness measurement techniques and methods.

Diameter Measurement (Two-Point Method) Perhaps the first and easiest way to determine the roundness of a part is to measure its diameter in several different directions to see if it is equal. If roundness is a cosmetic rather than a functional requirement, then measuring the diameter is sufficient.

V-Block Measurement (Three-Point Method) Another frequently used method of determining roundness is to place the workpiece to be measured in a V-block and rotate it in contact with a micrometer or similar indicator. This is essentially a three-point method rather than the two-point method described above. If the part is indeed round, the irregularity is negligible and the pointer of the indicator will not move.

Errors in shape will cause the micrometer to show a reading, however the part will also move up and down when the irregularity touches the V-block. In addition, when measuring shafted parts, contact with the V-block is not limited to the plane being measured. This means that irregularities in the part along its radius will affect the micrometer reading. The limitation of the three-point method is that results may vary depending on the V-angle and the spacing of the irregular features.

Coordinate Measuring Machine Method (CMM) Another method of measuring roundness is to use a coordinate measuring machine (CMM). A standard CMM has three precise orthogonal axes and is equipped with a touch sensor. The sensor makes contact with the workpiece being measured and records its position. This method will first collect several points around the workpiece and then combine these points in a computer to calculate the roundness of the workpiece. Typically, the number of sampling points is very small because of the time it takes to collect the data points. As a result, this measurement method is often not accurate enough.

Rotary datum method

The most accurate method of determining the roundness of a workpiece is to measure the change in radius with respect to a precise rotating datum using a probe scanning method, where the probe remains in contact with the surface of the workpiece and a high density of data points are collected. A circle is then fitted from this data and its roundness calculated.

There are a number of instruments specifically designed to measure roundness. The most common configuration is a system that includes a rotary table on which the workpiece is mounted. The sensor is mounted on a radial measuring arm that can be adjusted to bring the sensor into contact with the workpiece. The radial arm itself is mounted on a column that can be adjusted for the height of the measuring plane.

The high form accuracy of the linear axis of these instruments enables them to be used to measure other parameters such as flatness, straightness and cylindricity. The advantage of this type of instrument is that roundness can be measured very accurately in a very short measuring time.