13.2 Datum-Axis Simulation and Rotating Parts

ASME GDTP Senior Level Exam

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1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Senior Level vs Technologist and Y14.5 Revisions 1.3 How to Use This Guide and Study Strategy 1.4 The Y14.5-2018 Senior Exam and What Changed from 2009

2 Dimensioning and Symbology

2.1 Y14.5 Dimensioning Rules, Limits, and Fits 2.2 Basic Dimensions and Their Role 2.3 Feature Control Frame Structure and Reading 2.4 GD&T Symbols, Modifiers, and Communication 2.5 Screw Threads, Gears, and Splines 2.6 Angular Surfaces, Tapers, Radii, and Special Tolerance Cases

3 Datum Referencing I: Selection and Precedence

3.1 Functional Datum Feature Selection 3.2 Datum Precedence and Degrees of Freedom 3.3 Datum Targets 3.4 Datum Feature Simulators

4 Datum Referencing II: Modifiers and Shift

4.1 Multiple Datum Features and Datum Reference Frames 4.2 RMB, MMB, and LMB on Datum Features 4.3 Datum Shift Behavior 4.4 Planar vs Datum Feature of Size

5 Datum Referencing III: Setup and Calculations

5.1 Inclined and Translated Datum Features 5.2 Inspection Setup and Datum Establishment 5.3 Datum Calculation Walkthrough 5.4 Customized Datum Reference Frames and Rotational Constraint 5.5 Free State, Restrained Condition, and Nonrigid Parts

6 Form Controls

6.1 Flatness and Straightness 6.2 Circularity and Cylindricity 6.3 Rule #1, Form, and Size

7 Orientation Controls

7.1 Parallelism, Perpendicularity, and Angularity 7.2 Orientation Zones and Material Modifiers 7.3 Avoiding Unnecessary Orientation and Over-Control

8 Location I: Position and Boundaries

8.1 Position Tolerancing and True Position 8.2 Virtual and Resultant Condition 8.3 Bonus Tolerance at MMC and LMC

9 Location II: Application and Modifiers

9.1 Selecting MMC, LMC, and RFS by Function 9.2 Fixed and Floating Fastener Formulas 9.3 Projected Tolerance Zones 9.4 Coordinate Conversion and Appendix B Position Calculations

10 Location III: Patterns and Allocation

10.1 Composite Position Tolerancing 10.2 Patterns, Simultaneous Requirements, and Multiple Segments 10.3 Slots, Tabs, and Tolerance Allocation 10.4 Coaxial Feature Controls, Concentricity, and Symmetry

11 Profile I: Fundamentals and Strategy

11.1 Profile of Line and Surface 11.2 Defining the True Profile 11.3 Datum-Referenced Profile Strategy 11.4 Equal and Unequal Disposition

12 Profile II: Scope and Verification

12.1 All-Around, All-Over, and Limited Scope 12.2 Composite Profile and Coplanarity 12.3 Castings, Weldments, Sheet Metal, and CMM Evaluation 12.4 Combined Controls and Profile as a Refinement

13 Runout Controls

13.1 Circular and Total Runout 13.2 Datum-Axis Simulation and Rotating Parts 13.3 Runout vs Position, Profile, and Form

14 Exam Strategy and Calculations

14.1 Virtual/Resultant Condition and Bonus Formulas 14.2 Datum Shift and Worst-Case Boundary Calculations 14.3 Functional Gaging and Verification 14.4 Test-Day Tactics, Common Traps, and Review

13.2 Datum-Axis Simulation and Rotating Parts

Key Takeaways Explain This Topic AI

Runout is evaluated about the datum axis established by the specified datum feature simulator, so the inspection setup must reproduce the drawing-defined datum system.

When two coaxial journals are identified as Y14.5-2009 multiple datum features establishing datum A, both are simulated together to derive one datum axis.

Between-centers, V-block, collet, chuck, and arbor setups are not interchangeable by name alone; the chosen method must realize the applicable simulator and constrain the specified degrees of freedom.

Datum-feature form participates in datum establishment under the standard; fixture, spindle, arbor, seating, and measurement errors are additional metrology errors that must not be misreported as part runout.

A credible result requires calibrated equipment, correct seating, a setup matching the datum reference frame, and sufficient sampling of the controlled surface.

Last updated: August 2026

13.2 Datum-Axis Simulation and Rotating Parts

Quick Answer: The datum axis is not whichever axis the inspector finds convenient. It is established from the datum feature simulator specified by the drawing. When two journals are identified as Y14.5-2009 multiple datum features establishing datum A, both must participate in establishing that single axis.

Drawing Definition Before Fixture Choice

The datum feature is the imperfect feature on the part; the datum is the theoretically exact axis derived through the applicable simulator. Inspection equipment attempts to realize that simulator. The setup must therefore follow the datum reference frame and boundary conditions on the drawing before any indicator reading is interpreted.

For one cylindrical bore datum A, an appropriate precision arbor or expanding simulator may establish A. For two separated, nominally coaxial journals designated together as multiple datum features establishing A, both journals are contacted together so their combined simulation establishes one datum axis. This is not an A|B precedence sequence and is not an ordered A|B datum-precedence sequence.

Common Physical Methods

• Method Typical use Error to manage
• Precision arbor or expanding mandrel Internal cylindrical datum Clearance, arbor form, seating, spindle runout
• Collet or precision chuck External cylindrical datum Gripping distortion, jaw/collet error, seating

Two supports or purpose-built simulator Two journals jointly establishing one axis Support geometry, simultaneous contact, journal sampling

Between centers Center features specified for the setup Center condition, alignment, spindle error

CMM mathematical alignment Accessible measured datum features Sampling density, fitting rule, alignment implementation

A generic V-block or chuck is not automatically the correct datum simulator. Its validity depends on the drawing's datum definition and whether the setup reproduces the required contacts and degrees of freedom.

Part Geometry Versus Measurement Error

Form and size of the specified datum feature legitimately influence the datum established under Y14.5; that is part of the requirement, not automatically a false reject. Separately, fixture runout, worn supports, arbor clearance, dirt, poor seating, indicator error, or an incorrect fitting algorithm can bias the measured result. Those are metrology errors and should be found with calibrated masters, repeat setups, and uncertainty checks.

Diagnostic Sequence

Confirm the datum feature identification, order, and modifiers on the drawing.

Confirm the fixture or mathematical alignment realizes that datum system.

Clean and reseat the part, then repeat the measurement.

Check fixture and spindle error with a calibrated master.

Sample enough of both the datum features and controlled surface for the stated runout control.

The Senior-level distinction is crucial: do not subtract datum-feature form from a result merely because it is inconvenient, and do not accept fixture error as though the drawing allowed it.