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What is Roundness | Methods of Roundness Measurement in Metrology

The measurement of roundness is of critical importance for many applications. There are several different measurement methods from simple calliper measurements to highly accurate dedicated roundness measurement systems. In this article Dr Mike Mills – Chief Metrologist – outlines some of the methods and their limitations.

Roundness is defined in ISO 1101 as the separation of two concentric circles that just enclose the circular section of interest. It is clear measurement of diameter as shown above will not yield the roundness of the component in accordance with this definition.

Why Measure Roundness?

One of the most important fundamental forms for engineering components is the circular cross-section. Circular forms arise in many applications, particularly in bearing surfaces such as rotating shafts and ball bearings. The measurement of out-of-roundness (usually referred to simply as “roundness”) is an extremely important assessment. For example, a rotational bearing whose components are not accurately round will tend to be noisy and is likely to fail prematurely. Accurate roundness measurement is therefore vital to ensure correct function of such parts.

In this article I have outlined several common approaches to the measurement of roundness, highlighting some of their limitations.

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Roundness Measurement Techniques/Methods

Diameter measurement

Perhaps the first and simplest approach to determining the roundness of a component is to measure the consistency of its diameter at several different orientations. This is often done in-process for checking machine set-up and can be adequate for assessing a component where the roundness is a cosmetic, rather than functional, requirement. It can be functionally relevant of course, and a good example of this is the UK fifty pence piece shown in figure 1. One of the requirements for the coin is that it can be used in a coin-operated slot machine. The design as shown works very well in this application as it has a constant diameter. However, it is clear the coin is not round.

Vee-Block Method

Another method for determining roundness that is often used is to place the part in a vee-block and rotate it in contact with a dial gauge or similar indicator. This is essentially a threepoint method rather than the two-point method above. If the part is truly round, with negligible irregularity, the pointer of the gauge will not move.

Errors in the form will cause the dial indicator to show a reading, however the part will also move up and down as the irregularities contact the vee-block. Moreover, in the case of a shaft, the contact with the vee-block is not restricted to the plane being measured. This means that irregularities of the component along its length will affect the dial indicator reading. However, the three-point method is applied, it will always suffer from the limitation that the results may vary according to the vee angle and the spacing of the irregularities.

Co-ordinate Measuring Machine (CMM)

Another way to measure roundness is to use a coordinate measuring machine (CMM). A standard CMM has three accurate, orthogonal axes and is equipped with a touch-trigger probe. The probe is brought into contact with the component being measured and its position is recorded. Several points are taken around the component and these are then combined in a computer to calculate the roundness of the component. Typically, the number of data points is very small because of the time taken to collect them.

Conclusion: As a result, the accuracy of such measurements is compromised.

Rotational Datum Method

The most accurate method for determining roundness of a component is to measure the variation of radius from an accurate rotational datum using a scanning probe (one that remains in contact with the surface and collects a high-density of data points).

A circle can then be fitted to this data and the roundness calculated from knowledge of the component centre. There are many dedicated instruments made for the measurement of roundness. The most common configuration is a system that contains a rotating table onto which the component is mounted. A gauge is mounted on a radial arm, which can be adjusted to bring the gauge into contact with the component. The arm itself is mounted on a column that permits the height of the measurement plane to be adjusted.

The linear axes of such instruments are often motorised and of high form accuracy enabling the instrument to be used to measure other parameters such as flatness, straightness and cylindricity. To view an example of such an instrument, click here.

Conclusion: The advantages of these instruments are that they can measure roundness extremely accurately in a short measurement time.

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