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Fact check: The true position of a hole on engineering drawings is defined by 3 datums
Executive Summary
The assertion that “the true position of a hole on engineering drawings is defined by 3 datums” is partly correct but incomplete: Geometric Dimensioning and Tolerancing (GD&T) often uses a three-datum Datum Reference Frame (A, B, C) to fully constrain a feature’s location and orientation, but true position can be specified with fewer datums depending on design intent, feature type, and tolerance zone definitions. Contemporary guidance and forum discussions confirm that a three-datum reference frame is a common practice to constrain all degrees of freedom for a hole’s axis, yet technical explanations emphasize that true position is fundamentally a 3-D tolerance zone around a theoretically exact location and may be applied with one, two, or three datums as appropriate [1] [2] [3] [4].
1. Why engineers often say “three datums” — it’s about constraining all motions
Practitioners frequently state that a hole’s true position is defined by three datums because a three-point Datum Reference Frame (DRF) removes all six rigid-body degrees of freedom and thus fully constrains location and orientation for features of size during measurement and assembly. Forum explanations and technical articles date back several years and reiterate that primary, secondary, and tertiary datums (A, B, C) are used to control translation and rotation of the part so the hole’s axis can be located within a 3-D tolerance zone; the tertiary datum typically addresses the last degree of freedom often described as “clocking” [1] [3]. Recent GD&T explanations echo this practice and present the three-datum DRF as the standard method for ensuring repeatable inspection and assembly references [2] [5].
2. The technical truth: true position is a 3-D tolerance, not inherently “three datums”
Defining true position is primarily about a 3-D tolerance zone around a theoretically exact coordinate defined by basic dimensions or a coordinate system, not inherently about the count of datums. Authoritative summaries explain that true position controls the central elements of a feature of size and that the tolerance zone surrounds the nominal axis or center; how many datums are used is a separate choice tied to how the part is referenced in the drawing and measured [2] [4]. In practice, designers may call out position relative to a single datum when only translation is critical, or two datums when only planar orientation must be fixed; the three-datum approach is common where full spatial control is required, but it is not a universal rule [2] [6].
3. Evidence from forums and articles — consensus with nuance and historical context
Multiple forum threads and GD&T articles spanning 2014–2025 show a consistent consensus: three datums are commonly used, but commentators repeatedly add nuance about when fewer datums suffice. A 2019 forum thread explains the common A, B, C practice for hole axes and orientation control [1]. Articles and more recent resources (2018–2025) clarify the role of the tertiary datum in constraining the final rotational freedom and explore measurement effects when primary or secondary datums deviate, demonstrating a practical and evolving discourse rather than a single rigid rule [3] [2] [5]. These sources are recent enough to reflect current practitioner understanding and inspection realities [4].
4. Practical implications for designers and inspectors — pick the DRF to match intent
The right DRF is a design decision driven by functional requirements: use as many datums as needed to ensure assembly and functional performance, not because a rule demands three. Guidance emphasizes choosing datums that reflect mating interfaces and manufacturing/inspection fixturing so that the tolerance zone corresponds to real-world constraints; misapplication of three datums can over-constrain measurement or create inspection ambiguity if the datums don’t reflect assembly conditions [5] [2]. Inspectors and CAD modelers should confirm that datum features called out on drawings match intended fixturing and that basic dimensions and modifiers correctly place the 3-D true position tolerance relative to the selected DRF [2] [4].
5. Bottom line: accurate statement with required caveats and how to apply it
The concise factual takeaway is: a three-datum DRF is the common method to fully define a hole’s true position, but true position itself is defined as a 3-D tolerance that can reference one, two, or three datums depending on intent. Contemporary sources demonstrate consensus on the utility of A, B, C datums for full constraint while also stressing that designers must choose datums consistent with functional and inspection needs; the tertiary datum’s role in eliminating rotational “clocking” is a recurring practical point in recent analyses [1] [3] [2]. Follow drawing standards and ensure datums map to real fixturing to avoid misinterpretation in manufacture and inspection [4] [5].