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How is roundness measured?

Measuring differences in diameter is not sufficient to measure roundness. For example, a 50p has a constant diameter when measured across its centre, but is clearly not round. To measure any component for roundness we require some form of datum.

Roundness is usually assessed by rotational techniques by measuring radial deviations from a rotating datum axis, this axis remains fixed and becomes the main reference for all measurements. There are two common ways of measuring roundness. One method involves rotation of the part while keeping the measuring transducer fixed and the other involves keeping the component fixed while rotating the measuring transducer.

Component Rotation
Figure 1

Fig.1 shows a typical rotating component system such as a Talyrond 265. Here the component is rotated on a highly accurate spindle that provides the reference for the circular datum.

Figure 2

In figure 2, the axis of the component is aligned with the axis of the spindle, using a centring and levelling table. A transducer is then used to measure radial variations of the component with respect to the spindle axis. 
The output of the gauge or transducer consists of three added components: 
i) Instrument error ii) Component set-up error iii) Component form error

By using high precision mechanics and stable electronics, i) is too small to be significant. ii) is minimised firstly by accurate centring and levelling and then the residual error removed by electronic or software means. Form error is the area of interest and once i) and ii) are excluded, this error can be highly magnified and used to derive a measure of the out of roundness. 

b. Rotating Stylus
Figure 3

An alternative method is to rotate the stylus while keeping the component stationary. This is usually performed on small high precision components but is also useful for measuring large, non-circular, for example measurement of a cylinder bore using this method would not require rotation of the complete engine block.

This type of measuring system tends to be more accurate due to continuous loading on the spindle however is limited by the reach of the stylus and spindle.