5.5 Irregular & Complex Datum Features, Mathematically Defined Surfaces, Restrained Condition & Datum Feature Controls

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.

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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.

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.

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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.

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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:

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.

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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.