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4.3 Planar, Cylindrical, & Width Datum Feature Establishment

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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

4.3 Planar, Cylindrical, & Width Datum Feature Establishment

Key Takeaways Explain This Topic AI

Different datum feature geometries establish distinct theoretical datums: planar surfaces derive datum planes (3 DOF), cylindrical features derive datum axes (4 DOF), width features derive datum centerplanes (3 DOF), and spherical features derive datum points (3 DOF).

A primary cylindrical datum feature of size constrains 4 degrees of freedom (2 translations and 2 rotations perpendicular to its axis), leaving 1 axial translation and 1 axial rotation (spin) unconstrained.

A width feature of size consists of two opposed parallel surfaces and establishes a theoretical datum centerplane via a simulator of two parallel planes at minimum separation or maximum expansion, constraining 1 translation and 2 rotations.

A spherical datum feature establishes a theoretical datum point at the center of its spherical simulator, constraining all 3 translational degrees of freedom while leaving all 3 rotational degrees of freedom completely unconstrained.

Coaxial datum features referenced as a compound datum [A-B] establish a single common datum axis through two simulators acting simultaneously with equal precedence, unlike sequential callout [A|B] where A takes primary precedence.

Last updated: September 2026

4.3 Planar, Cylindrical, & Width Datum Feature Establishment

Quick Summary: While planar surfaces establish datum planes through the classic 3-2-1 contact system, modern mechanical designs rely extensively on cylindrical features of size (shafts, bores), width features of size (slots, tabs, keyways), spherical features , and coaxial datum features . ASME Y14.5-2009 Section 4 establishes rigorous mathematical and physical criteria for deriving theoretical datums—whether an axis , a centerplane , a plane , or a point —from these varied geometries. Each geometry enforces a unique degree of freedom (DOF) constraint signature . For instance, a primary cylindrical datum feature constrains four degrees of freedom (two translations and two rotations), while a spherical datum constrains three translations but zero rotations. Master technician status requires fluent understanding of these geometric simulator mechanics and the critical distinction between compound coaxial datums (such as [A-B] ) and sequential datum hierarchies ( [A|B] ).

1. Feature Geometry and Datum Classification

Not all datums are planes. Under ASME Y14.5-2009 , the geometric shape of the physical datum feature dictates both the type of datum feature simulator required and the mathematical nature of the resulting datum:

Feature Geometry Physical Feature Example Datum Feature Simulator Derived Theoretical Datum Primary DOFs Constrained

Planar Surface Flat mounting face Perfect plane contacting high points Datum Plane 3 DOF (1 Trans, 2 Rot)

Cylindrical Feature (External) Shaft journal, locating pin Smallest circumscribed cylinder of perfect form Datum Axis 4 DOF (2 Trans, 2 Rot)

Cylindrical Feature (Internal) Reamed hole, bore Largest inscribed cylinder of perfect form Datum Axis 4 DOF (2 Trans, 2 Rot)

Width Feature (External) Keyway tab, tongue Two parallel planes at minimum separation Datum Centerplane 3 DOF (1 Trans, 2 Rot)

Width Feature (Internal) Milled slot, guide groove Two parallel planes at maximum expansion Datum Centerplane 3 DOF (1 Trans, 2 Rot)

Spherical Feature Tooling ball, spherical socket Inscribed or circumscribed sphere of perfect form Datum Point 3 DOF (3 Trans, 0 Rot)

Coaxial Features Dual bearing journals Two coaxial cylinders sharing a single axis Common Datum Axis 4 DOF (2 Trans, 2 Rot)

2. Planar Datum Features & Derived Datum Planes

A planar datum feature consists of a nominally flat surface. When designated as a primary datum feature:

Simulator : A planar surface plate of adequate precision.

Contact : Minimum of 3 non-collinear high points.

Derived Datum : A theoretical datum plane coincident with the simulator.

DOF Constrained (3 DOF) :

Translation perpendicular to the plane (e.g., T z T_z T z ​ ).

Rotations about two orthogonal axes lying in the plane (e.g., R x R_x R x ​ and R y R_y R y ​ , or pitch and roll).

DOF Remaining (3 DOF) : Two translations along the plane ( T x , T y T_x, T_y T x ​ , T y ​ ) and one rotation about the normal axis ( R z R_z R z ​ , or yaw).

3. Cylindrical Datum Features of Size (Shafts & Holes)

Cylinders are the single most ubiquitous features of size in mechanical assemblies. When an external shaft or an internal hole is referenced as a primary datum feature: Cylindrical Datum Feature Simulator Mechanics (Primary)

External Shaft (Circumscribed) Internal Hole (Inscribed)

  • +-----------------------+ +-----------------------+
  • | +-----------------+ | | ~~~~~~~~~~~~~~~~~~~~~ | (Hole Wall)
  • | | ~~~~~~~~~~~~~~~ | | (Shaft) | | +---------------+ | |
  • ========|==|=================|==|======= =====|==|==|===============|==|==|=====
  • Datum | | Axis | | Datum Datum | | | Axis | | | Datum
  • Axis | | | | Axis Axis | | | | | | Axis
  • ========|==|=================|==|======= =====|==|==|===============|==|==|=====
  • | | ~~~~~~~~~~~~~~~ | | | | +---------------+ | |
  • | +-----------------+ | | ~~~~~~~~~~~~~~~~~~~~~ |
  • +-----------------------+ +-----------------------+
  • (Smallest Circumscribed Cylinder) (Largest Inscribed Cylinder)
  • Simulator Mechanics & Derived Axis

External Feature (Shaft) : The simulator is the smallest circumscribed cylinder of perfect form contacting the outermost high points of the shaft.

Internal Feature (Hole) : The simulator is the largest inscribed cylinder of perfect form contacting the innermost high points of the hole.

Derived Datum : The theoretical centerline (axis) of this contracting or expanding simulator cylinder.

Degrees of Freedom Constrained by a Primary Cylinder (4 DOF)

Because a cylinder is symmetric about its axis, it locks four degrees of freedom:

Two Translations : Linear translation along the two axes perpendicular to the cylinder axis (e.g., T x T_x T x ​ and T y T_y T y ​ ).

Two Rotations : Angular rotation/tilt about the two axes perpendicular to the cylinder axis (e.g., R x R_x R x ​ and R y R_y R y ​ , or pitch and yaw).

The Two Remaining Degrees of Freedom (2 DOF)

A primary cylindrical datum leaves exactly two degrees of freedom unconstrained:

One Translation : Sliding along the datum axis ( T z T_z T z ​ , or axial travel).

One Rotation : Spinning around the datum axis ( R z R_z R z ​ , or axial rotation / clocking).

Exam Tip: To fully lock a part when a cylinder is primary, the secondary datum typically stops axial translation (such as a planar shoulder contacting an end-stop), and the tertiary datum stops radial spin (such as a keyway, cross-hole, or dowel pin acting as a clocking feature ).

4. Width Features of Size & Derived Datum Centerplanes

A width feature of size consists of two parallel opposed planar surfaces associated with a toleranced size dimension (e.g., a slot, a tab, a rail, or a keyway). Width Feature of Size: Datum Centerplane

  • +-----------------------------------------------+
  • | Part Material |
  • +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ <- Imperfect Surface 1
  • ================================================= <- Simulator Plane 1
  • - - - - - - - - - - - - - - - - - - - - - - - - <- Derived Datum Centerplane
  • ================================================= <- Simulator Plane 2
  • +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ <- Imperfect Surface 2
  • | Part Material |
  • +-----------------------------------------------+
  • (Simulator: 2 Parallel Planes at Min/Max Separation)
  • Simulator Mechanics & Derived Centerplane

External Width (Tab/Lug) : The simulator consists of two parallel planes of perfect form that contract together to their minimum separation, contacting the outermost high points of both opposed faces simultaneously.

Internal Width (Slot/Groove) : The simulator consists of two parallel planes of perfect form that expand outward to their maximum separation, contacting the innermost high points of both opposed faces.

Derived Datum : A theoretical datum centerplane positioned midway between the two parallel planes of the simulator.

Degrees of Freedom Constrained by a Primary Width (3 DOF)

A primary width feature of size constrains three degrees of freedom:

One Translation : Linear motion perpendicular to the datum centerplane (e.g., T x T_x T x ​ ).

Two Rotations : Angular tilt about the two axes parallel to the centerplane (e.g., R y R_y R y ​ and R z R_z R z ​ ).

Width Feature vs. Planar Surface: Key Differences

While both a single planar surface and a width feature of size constrain 3 degrees of freedom when primary, they are fundamentally different:

A single planar surface is not a feature of size ; it cannot have an MMC or LMC modifier, has no actual mating envelope, and establishes a datum surface plane.

A width feature of size has opposed surfaces with size limits ; it can carry Maximum Material Boundary (MMB) or Least Material Boundary (LMB) modifiers in the FCF, authorizes datum shift, and derives a central datum centerplane.

5. Spherical Datum Features: Establishing a Datum Center Point

When a spherical surface (such as a tooling ball, spherical bearing, or ball-joint pivot) is designated as a datum feature:

Simulator : A sphere of perfect form contacting the high points (circumscribed for an external ball; inscribed for an internal socket).

Derived Datum : A theoretical datum point located at the exact center of the simulator sphere.

Degrees of Freedom Constrained (3 DOF) :

All three linear translations ( T x , T y , T z T_x, T_y, T_z T x ​ , T y ​ , T z ​ ). The center point fixes the component's location in 3D Cartesian space.

Degrees of Freedom Remaining (3 DOF) :

All three rotational degrees of freedom ( R x , R y , R z R_x, R_y, R_z R x ​ , R y ​ , R z ​ ) remain completely free! Because a sphere is perfectly symmetric in every angular direction, contacting a spherical simulator cannot prevent the part from spinning or tilting about the center point.

6. Coaxial Datum Features: Compound [A-B] vs. Sequential [A|B]

Under ASME Y14.5-2009 Section 4.12.2 , when two or more coaxial cylindrical features (such as two bearing journals separated by a shaft body) together establish a single rotational axis, they are specified as a compound datum feature . Compound Datum Feature Axis [A-B]

  • Journal A (Simulator A) Journal B (Simulator B)
  • +-------------+ +-------------+
  • | | | |
  • ========|=============|=============================|=============|========
  • Common | | Shaft Body (Middle) | | Common
  • Datum +-------------+ +-------------+ Datum
  • Axis | | Axis
  • ========|=========================================================|========
  • (Two Coaxial Simulators of Equal Precedence Form Single Common Axis)
  • Compound Callout: [A-B] in a Single Compartment

Notation : Both letters appear separated by a hyphen inside a single compartment (e.g., | ↗ | 0.05 | A-B | ).

Precedence : Datum features A and B have equal precedence . Neither feature is primary over the other.

Simulator : Two coaxial cylindrical simulators of perfect form share a single common axis, expanding or contracting simultaneously to contact the high points of both journals.

Datum Established : A single common datum axis passing through both simulators.

Sequential Callout: [A | B] in Separate Compartments

Notation : Datum letters appear in separate compartments (e.g., | ↗ | 0.05 | A | B | ).

Precedence : Datum A is primary ; Datum B is secondary .

Simulator Mechanics : Simulator A contacts Journal A first, locking 4 degrees of freedom and establishing the primary axis orientation. Simulator B then contacts Journal B, but Simulator B must remain parallel and coaxial to Datum Axis A , acting only to locate or constrain residual degrees of freedom.

Exam Trap Alert: Confusing [A-B] with [A|B] is one of the most common errors on the GDTP Technologist exam. [A-B] represents a single compound datum axis with shared, equal precedence; [A|B] represents a strict primary-to-secondary hierarchy where Datum A dominates orientation!