A datum, is a point, line or surface of a component to which dimensions are referred and from which measurements are taken during inspection. The datum point or points on a component are in
reality datum features since it is with respect to features that other features relate. Thus, the various features on a component are dimensioned with respect to datum features. The definition of a datum feature will be dependent upon the functional performance of a particular component. In the example shown in Figure 4.2, the datum faces are the flat faces where the underside of the hardened insert contacts the step in the movable jaw. The 15mm dimension to the centre line of the bolt and the 30mm dimension to the top of the movable jaw are dimensioned from the datum surface. The underside of the movable jaw is 10mm from the datum surface but it is in the opposite direction and hence is labelled '-10'.
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