Datum Reference Frames and Datum Systems
Building a datum reference frame
A datum reference frame (DRF) is a coordinate system established by datum features that constrains the part in space. The primary datum constrains three degrees of freedom, the secondary two, and the tertiary one, for a total of six. Datum features are the real surfaces on the part; the datum is the theoretical plane, axis, or point derived from them. Datum feature simulators (surface plates, mandrels, chucks) establish the datum from the datum feature.
Concepts to command
Know datum precedence and how it affects inspection: the primary datum is contacted first and is typically a flat surface or a feature of size. Understand datum targets (points, lines, areas) for irregular or flexible parts, and datum feature selection based on function. Know the difference between a datum axis established by a cylindrical feature of size (via the actual mating envelope) and one established by two coaxial features.
Traps that appear in questions
- Reversing primary and secondary datum in the feature control frame changes the DRF and therefore the tolerance zone orientation.
- A datum feature that is itself out of form (e.g., a warped surface) still defines the datum through its simulator, not its high points.
- Assuming datum features must be the largest or most convenient surfaces; they must be selected for function.
- Forgetting that datum targets require basic dimensions and are inspected with the specified target simulator.
Rule of thumb
Count degrees of freedom: primary = 3, secondary = 2, tertiary = 1. If a question describes a part that can still rotate or translate after fixturing, the DRF is incomplete.
Worked example
A flat base (primary, 3 DOF), a long side (secondary, 2 DOF), and a short end (tertiary, 1 DOF) fully constrain a rectangular plate. If the side is swapped to primary, the plate is constrained differently and the tolerance zone orientation changes.