Can the datum be easily measured? : An introduction to the designer's role in the design & interpretation of an engineering drawing : Dimensional : Good practice online modules : Publications : National Physical Laboratory

Can the datum be easily measured?

When designing a component the designer should make sure that the datum surfaces can be easily measured or fixtured.

Datum surfaces should be substantial in size compared with the feature being measured.

If datum features are small CMM measurement errors can play a larger part than if a larger datum was chosen.

size of the datum feature: longest distance being measured

For example...

If the length of a 5 mm diameter, 300 mm long bar is being measured it would be inappropriate to use the 5 mm face as a datum feature for alignment. It would be better to use the axis of the bar. Small measurement errors have less effect over 300 mm than they would over 5 mm.

When setting up a datum for measurement, it is preferable to choose as datum features the surfaces that were used in the manufacturing process to hold the component. This relates the inspection results directly to the manufacturing process.

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