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

ASME GDTP Technologist Y14.5-2009

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1 Chapter 1: Scope, Certification Logistics, Fundamental Rules & General Dimensioning

1.1 ASME GDTP Technologist Exam Scope, Prometric Logistics, & Certification Rules 1.2 Fundamental Rules of ASME Y14.5-2009 (Rules a through p) 1.3 Engineering Drawing Standards, Drawing Units, & Basic vs. Toleranced Dimensions 1.4 Dimensioning Features: Diameters, Radii, Chamfers, Countersinks, Counterbores, Spotfaces, Keyseats & Slots 1.5 Application of Dimensions & Location of Features: Lines, Leaders, Reference Dimensions, Rectangular vs. Polar Coordinates

2 Chapter 2: Features, Limits of Size, & Material Condition Principles

2.1 Features, Features of Size, & Limits of Size 2.2 Rule #1 (Taylor Principle / Envelope Rule), Its Exceptions & Overrides 2.3 Material Condition Concepts: MMC, LMC, & RFS Calculations

3 Chapter 3: General Tolerancing & Symbology

3.1 Direct Tolerancing, Limit Dimensioning, & Angular Tolerancing 3.2 The 14 Geometric Characteristic Symbols, Feature Control Frames, & Special Modifiers 3.3 Screw Threads, Gears & Splines, Angular Surfaces, Tapers & Statistical Tolerancing 3.4 Feature Control Frame Placement, Combined Frames, Tolerance Zone Definition & Tabulated Tolerances

4 Chapter 4: Datum Reference Frames & Degrees of Freedom

4.1 Theoretical Datums, Datum Feature Simulators, & Physical Datum Features 4.2 Degrees of Freedom Constraint & The 3-2-1 Datum Precedence System 4.3 Planar, Cylindrical, & Width Datum Feature Establishment

5 Chapter 5: Advanced Datum Applications: Targets, Boundary Modifiers, Customized Frames & Restrained Condition

5.1 Datum Target Points, Lines, Areas, & Movable Datum Simulators 5.2 Material Boundary Modifiers (RMB, MMB, LMB) & Datum Shift Calculations 5.3 Functional Datum Feature Selection, Multiple Datum Reference Frames & Simultaneous vs. Separate Requirements 5.4 Rotational Constraint About a Datum Axis or Point & Customized Datum Reference Frames 5.5 Irregular & Complex Datum Features, Mathematically Defined Surfaces, Restrained Condition & Datum Feature Controls

6 Chapter 6: Form Tolerances

6.1 Flatness & Straightness Controls for Surfaces & Features of Size 6.2 Circularity & Cylindricity Controls, Differences, & Inspection

7 Chapter 7: Orientation Tolerances

7.1 Parallelism Controls for Planar Surfaces & Cylindrical Features of Size 7.2 Perpendicularity & Angularity Tolerances with Basic Angles & Tangent Plane

8 Chapter 8: Location: Position Fundamentals & Calculations

8.1 Position Tolerancing Fundamentals, True Position, & Coordinate Conversions 8.2 Bonus Tolerance & Virtual Condition Calculations for Internal & External Features 8.3 Projected Tolerance Zones (Ⓟ Modifier) & Fixed vs. Floating Fastener Formulas 8.4 Zero Positional and Orientation Tolerancing at MMC and at LMC 8.5 Surface vs. Axis Interpretation of Position & the Boundary Concept for Slots, Tabs & Noncircular Features

9 Chapter 9: Location: Patterns, Composite Controls, Coaxial & Symmetrical Relationships, and Functional Gaging

9.1 Hole & Slot Patterns, Multi-Segment Frames, & Composite Position Tolerancing 9.2 Functional Gaging Principles & Inspection Verification of Position 9.3 Coaxial Holes, Counterbored Holes, Non-Parallel Axes, Spherical Features & Bidirectional Position 9.4 Concentricity & Symmetry Tolerances Under ASME Y14.5-2009 9.5 LMC Applied to Position: Wall Thickness, Breakout Protection & Boundary Calculations

10 Chapter 10: Profile Tolerances

10.1 Profile of a Line & Profile of a Surface Fundamentals & Basic Geometry 10.2 Bilateral, Unilateral, & Unequally Disposed Profile Boundaries (Ⓤ Modifier) 10.3 Composite Profile Tolerances vs. Multiple Single-Segment Profile Controls 10.4 Profile Verification, Coplanarity Controls, & Non-Rigid Parts 10.5 Profile Applications: Conicity, Extruded and Irregular Shapes, and Profile-Controlled Datum Features 10.6 Modifier Rules for Profile & Combined Controls (Profile with Position, Orientation and Runout)

11 Chapter 11: Runout Tolerances & Comparative Control Analysis

11.1 Circular Runout vs. Total Runout Interpretation & Dial Indicator Verification 11.2 Comparative Analysis: Runout vs. Form, Orientation, & Concentricity Controls

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

Key Takeaways Explain This Topic AI

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.

Last updated: September 2026

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.