5.5 Irregular & Complex Datum Features, Mathematically Defined Surfaces, Restrained Condition & Datum Feature Controls
Key Takeaways
ASME Y14.5-2009 para. 4.9 states that geometric tolerances related to a datum reference frame do not take into account variations in the form, orientation, or location of the datum features, so datum features must be controlled directly by geometric tolerances or indirectly by dimensions such as size.
A datum feature control may reference only higher-precedence datums; controlling datum feature B to datum feature C would create a circular requirement.
Para. 4.13 requires that a compound curve or contoured surface used as a datum feature be defined within a three-dimensional coordinate system, which is the basis for model-based definition of complex datum features.
Para. 4.17 applies MMB, LMB, and RMB to irregular features of size, so a hole pattern referenced as a datum feature at MMB is simulated by fixed pins at its virtual condition and permits datum feature shift.
Para. 4.20 permits a part to be restrained on its datum features to simulate the assembly condition, but the restraint must be specified on the drawing including the magnitude, direction, location, and sequence of the restraining forces.
5.5 Irregular & Complex Datum Features, Mathematically Defined Surfaces, Restrained Condition & Datum Feature Controls
Quick Answer: Four paragraphs handle the datum features that are not simple planes, cylinders, or widths. Para. 4.9 requires that datum features be controlled directly by applying appropriate geometric tolerances, or indirectly by dimensions such as size — because geometric tolerances related to a datum reference frame do not take into account any variations in form, orientation, or location of the datum features themselves. Para. 4.13 covers the mathematically defined surface: a compound curve or contoured surface used as a datum feature shall be defined within a three-dimensional coordinate system. Para. 4.17 extends MMB, LMB, and RMB to irregular features of size. Para. 4.20 provides the restrained condition, the drawing-specified override of the free-state default of rule 1.4(m) for parts that must be clamped to simulate their assembly condition.
Datum Feature Controls (Para. 4.9): The Rule Everyone Forgets
Geometric tolerances related to a datum reference frame do not take into account any variations in form, orientation, or location of the datum features. Datum features shall be controlled directly by applying appropriate geometric tolerances, or indirectly by dimensions such as size.
Read that twice, because it inverts a common intuition. Referencing datum feature A in a feature control frame does not control datum feature A. If the primary datum surface is badly bowed, the tolerance zone on the controlled feature is unaffected — the part simply rocks on the simulator and the datum plane sits on three arbitrary high points.
| How a Datum Feature Gets Controlled | Mechanism | Example |
|---|---|---|
| Directly | A geometric tolerance applied to the datum feature itself | ⏥ 0.05 on datum feature A; ⟂ 0.1 A on datum feature B |
| Indirectly | Limits of size + Rule #1 | A ⌀25.00–25.05 datum bore is form-controlled by Rule #1 |
| Indirectly | A datum target scheme (para. 4.24) | Three target points on a rough casting surface |
The precedence rule inside datum feature controls: a datum feature control may reference only higher-precedence datums. Datum feature B may be controlled
⟂ | 0.1 | A |; it may never be controlled to C. Controlling a higher-precedence datum feature to a lower-precedence one creates a circular requirement.
Complex, Conical, Extruded, and Mathematically Defined Datum Features
Para. 4.11, Establishing Datums, covers datum features well beyond the planar/cylindrical/width trio:
- Conical datum features establish a datum axis and a datum point, constraining five degrees of freedom at RMB — everything except rotation about the axis. A cone is unusually powerful as a primary datum feature, which is why it appears in the customized-frame discussion of para. 4.23.
- Linear extruded datum features (a prismatic contour such as an extrusion cross-section) establish a datum plane and a datum axis.
- Complex datum features — surfaces of non-uniform, non-analytic shape such as a turbine airfoil or an automobile panel — establish a datum reference frame directly, because their shape alone is enough to constrain the part.
- Spherical datum features establish a datum point and constrain three translations, leaving all three rotations free.
Mathematically Defined Surfaces (Para. 4.13)
It is sometimes necessary to identify a compound curve or a contoured surface as a datum feature. A mathematically defined feature shall be defined within a three-dimensional coordinate system.
This is the paragraph behind every model-based turbine blade and body panel drawing. The datum feature exists as a mathematical surface in a defined coordinate system rather than as a set of orthographic dimensions, and the datum feature simulator is the theoretical counterpart of that surface. Where the drawing is a 2D orthographic delineation, this normally means basic dimensions plus a profile tolerance on the datum feature (a direct datum feature control under para. 4.9); where the model is the authority, it means ASME Y14.41 digital product definition.
MMB, LMB, and RMB Applied to Irregular Features of Size (Para. 4.17)
An irregular feature of size (para. 1.3.32.2) is a directly toleranced feature or collection of features that may contain, or may be contained within, an actual mating envelope — a D-shaped hole, an obround boss, a pattern of holes treated as a single feature of size. Para. 4.17 confirms that the same three material boundary concepts apply to them:
| Boundary | Simulator Behaviour on an Irregular FOS | Datum Feature Shift |
|---|---|---|
| RMB | The simulator progresses (expands or contracts) until it makes maximum possible contact with the actual feature | None from size |
| MMB | The simulator is a fixed boundary at the maximum material boundary of the irregular shape | Yes, as the feature departs from MMB |
| LMB | The simulator is a fixed boundary at the least material boundary | Yes, as the feature departs from LMB |
The practical consequence for a pattern used as a datum feature: a bolt-hole pattern referenced as datum feature D at MMB is simulated by a set of fixed pins at the pattern's virtual condition, and the part may shift and rotate on those pins by whatever clearance the actual holes provide.
Restrained Condition (Para. 4.20)
Rule 1.4(m) makes free state the default. Para. 4.20 is the escape hatch:
Unless otherwise specified, all tolerances apply in a free-state condition. In some cases, it may be desirable to restrain a part on its datum features to simulate their assembly condition.
Restraint is not something an inspector may decide to apply. It must be specified on the drawing, and the specification must be complete enough to be repeatable:
- Which datum features are restrained, and in what order.
- The magnitude, direction, and location of the restraining forces — torque values on specified fasteners, clamp locations, or contact force at named datum targets.
- The sequence in which the restraint is applied.
FREE STATE (default, rule 1.4(m)) RESTRAINED (para. 4.20, must be specified)
╭─────╮ ▼ 5 N·m ▼ 5 N·m
╱ ╲ part relaxes and bows ┌──┴────────────────┴──┐
╱ ╲ │ part pulled flat │
═══════════════ simulator ════════════════════════ simulator
Datum plane sits on 3 high points Datum plane simulates the
of the distorted free-state part ASSEMBLED condition
When restraint is the right answer: thin-walled housings, welded frames, large sheet-metal panels, gaskets and seals, and any part whose free-state form is not the form it takes in service. When it is the wrong answer: a rigid machined block, where restraint merely hides real form error.
Companion mechanism: para. 5.5 provides the free-state symbol Ⓕ, which does the opposite job — it says a specific geometric tolerance applies in the free state on a drawing that otherwise specifies restraint. Para. 4.20 and para. 5.5 are the two halves of the non-rigid-part toolkit, and both are cross-referenced by rule 1.4(m) as its exceptions.
A cast bracket references datum feature A, a large as-cast surface, as its primary datum. The drawing applies no geometric tolerance to surface A itself. What does ASME Y14.5-2009 para. 4.9 say about the effect of this omission?
The position tolerances referencing A are void, because a datum feature that carries no geometric tolerance cannot legally be referenced in a feature control frame.
The form of surface A is uncontrolled, because geometric tolerances related to a datum reference frame do not take into account variations in the form, orientation, or location of the datum features; para. 4.9 requires datum features to be controlled directly by geometric tolerances or indirectly by dimensions such as size.
The form of surface A is automatically limited to the smallest position tolerance value referenced to it, since a datum feature can never be less accurate than the features it locates.
The form of surface A is controlled by Rule #1, because Rule #1 applies to every surface identified by a datum feature symbol.
A turbine airfoil drawing designates the aerofoil contour as datum feature A. The contour is a compound curve with no analytic form. What does ASME Y14.5-2009 para. 4.13 require, and how is such a datum feature normally controlled?
The contour must be approximated by a series of tangent circular arcs and toleranced with a controlled radius symbol on each arc.
The contour cannot serve as a datum feature under ASME Y14.5-2009; only planar, cylindrical, spherical, and width features are permitted as datum features.
The contour must be replaced by three datum target areas, because para. 4.13 prohibits the use of a mathematically defined surface as a datum feature simulator.
The mathematically defined feature shall be defined within a three-dimensional coordinate system, and it is normally controlled directly per para. 4.9 by basic dimensions or a defining model together with a profile of a surface tolerance applied to the datum feature.
A thin-walled sheet-metal housing distorts noticeably when unsupported. The design intent is that the mounting flange be evaluated as it sits when bolted into the assembly. What must the drawing do under ASME Y14.5-2009?
Nothing; an inspector may clamp any non-rigid part to its assembly condition, because rule 1.4(m) applies only to rigid parts.
Apply the free-state symbol to every feature control frame on the flange, since the free-state symbol is what invokes an assembly-simulating inspection setup.
Specify the restrained condition per para. 4.20, stating which datum features are restrained and the magnitude, direction, location, and sequence of the restraining forces, because rule 1.4(m) makes free state the default unless otherwise specified.
Convert every flange tolerance to a statistical tolerance under para. 2.17, because distortion of non-rigid parts is a process capability question rather than a datum question.
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