Measuring roundness accurately

Accurate roundness measurement is vital in productive manufacturing processes. Metrologist Dr Mike Mills* examines the benefits and limitations of various measurement instruments in use today.

Accurate roundness measurement is vital in productive manufacturing processes. Metrologist Dr Mike Mills* examines the benefits and limitations of various measurement instruments in use today.

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.

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.

The measurement of roundness is a vast topic. In this article I have outlined a number of common approaches to the measurement of roundness, highlighting some of their limitations.

Diameter measurement

Perhaps the first and simplest approach to gauging the roundness of a component is to measure the consistency of its diameter at a number of 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 is able to 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 clearly evident that the coin is not round.

At this stage it is useful to look at the ISO definition of roundness. Roundness is defined in ISO 1101 as the separation of two concentric circles that just enclose the circular section of interest.

It is clear that measurement of diameter as shown above will not yield the roundness of the component in accordance with this definition.

Vee-Block Method

Another method 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 three-point 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.

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. A number of 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. 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.

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

*Mike Mills is Chief Metrologist at Taylor Hobson. For more information, please contact Rosebank Engineering on 03 9721 1300, email sales@rosebank-eng.com.au or visit www.rosebank-eng.com.au.