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Roundness Measurement on a Helical Surface
Parts such as splined shafts, gears, air bearings and parts with keyways all have interrupted surfaces but still require roundness measurement.  etc. These interruptions cause problems to the user and need to be removed prior to analysis at the measurement stage or during the analysis stage. Other problems that need to be considered are the effects of filtering on interrupted surfaces.

Another error to be aware of is the effect of sharp changes on the profile, if the stylus tip drops down a hole and back out again the chances are that it will cause the stylus to bounce. This will usually manifest itself as a peak at the trailing edge of the hole or interruption on the component.

However care must be taken on interpretation as it is quite possible that the process by which the part was manufactured could also have produced this effect. To reduce this error there are a number of options:

• Reduce the speed of measurement
• Mechanically restrict the gauge deflection (see stop attachments)
• Increase the stylus tip diameter

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