Datum

Theory

In engineering drawing and CAD, a datum is an exact theoretical entity (point, straight line, or flat plane) that serves as the measurement origin. Real physical features (surfaces, holes) cannot be perfectly flat or straight: datums represent the idealized reference from which tolerances are measured. The GD&T (Geometric Dimensioning and Tolerancing) system uses letter-coded datum references (A, B, C) to specify which features establish the measurement frame.

Application

In parametric CAD, datums are created as reference planes through which sketches are drawn. The origin planes (XY, YZ, XZ) are the primary datums. Custom datum planes are added at feature offsets, at angles, or tangent to curved surfaces. For 3D printing, good datum selection means the print orientation aligns key functional surfaces with Z-direction strength. In GD&T callouts on engineering drawings for printed parts, datum A is usually the primary mating face.

Common mistakes

Datum selection errors propagate through the entire tolerance chain: if the wrong surface is used as datum A, all tolerances reference an imprecise base. In 3D printing, FDM parts have anisotropic properties; a datum referencing the print bed face (Z=0 plane) is more repeatable than a datum on a top surface affected by Z-height variation. Avoid using datums that shift when the part is re-printed at a different orientation.

Related terms: Sketch Constraint, Parametric Modeling, Draft Angle