Datum reference frames in GD&T: Is defining a datum reference frame a necessary step when applying GD&T principles to control relationships among features?

Introduction / Context:
GD&T relies on a common coordinate framework—the datum reference frame (DRF)—to define how a part is oriented and located during inspection and assembly. While some controls do not need datums, most relational controls do.

Given Data / Assumptions:

• Form tolerances (e.g., straightness, flatness) can be datumless.
• Orientation (e.g., perpendicularity), location (e.g., position), and runout typically require referencing a DRF.
• The question addresses applying GD&T principles broadly to control relationships among features.

Concept / Approach:
To control relationships such as “this hole must be positioned relative to that surface,” a DRF is established using primary, secondary, and tertiary datums. These define a reproducible origin and axes for both manufacturing and inspection, ensuring consistent interpretation.

Step-by-Step Solution:
Identify whether the tolerance controls a relationship among features (e.g., position, orientation).If yes, specify appropriate datums to form a DRF.Apply the GD&T callout with datum references to ensure unambiguous inspection.Thus, defining a DRF is necessary in the general application of GD&T to relational controls.

Verification / Alternative check:
Check inspection setups: fixtures emulate the DRF to validate conformance, demonstrating the DRF’s central role.

Why Other Options Are Wrong:
Limiting the need to profile or runout is incorrect; many GD&T controls rely on DRFs. Metric vs. inch drawings are irrelevant.

Common Pitfalls:
Omitting datums on positional tolerances; selecting unstable datum features; failing to define the datum hierarchy (primary, secondary, tertiary).

Final Answer:
Correct