Study guide
3.4 Datum Feature Simulators
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
3.4 Datum Feature Simulators
Key Takeaways Explain This Topic AI
A datum feature simulator is the physical manufacturing or inspection equipment that contacts the datum feature; the theoretical datum is derived from the simulator's true geometric counterpart.
The simulator's true geometric counterpart is the theoretically perfect surface (plane, cylinder, two parallel planes) that the real simulator represents when it contacts the datum feature.
For RMB (regardless of material boundary) datums the simulator expands or contracts to make maximum contact with the datum feature; for MMB (maximum material boundary) datums the simulator is fixed at the maximum material size of the datum feature.
The simulator type follows the datum feature geometry: surface plate for a planar datum, expanding mandrel for an internal cylinder (RMB), collet or collapsing gage for an external cylinder (RMB), and a fixed-size pin or ring for an MMB datum.
How the simulator engages the datum feature determines whether datum shift occurs; datum shift is introduced when an MMB datum feature departs from its maximum material condition, and is detailed in Chapter 4.
Last updated: August 2026
Definition: Datum Feature Simulator
A datum feature simulator is the physical equipment — in manufacturing, inspection, or gaging — that contacts the datum feature and from which the datum is derived. The simulator is the bridge between the imperfect real world (the actual part feature with its size and form error) and the theoretical, geometrically perfect datum used as the origin of measurement.
The chain is always: datum feature (real, imperfect) --> datum feature simulator (physical) --> datum (theoretical)
The True Geometric Counterpart
Every datum feature simulator has a true geometric counterpart : the theoretically perfect surface that the real simulator represents when it contacts the datum feature. For a surface plate the true geometric counterpart is a perfect plane; for an expanding mandrel it is a perfect cylinder of the diameter the mandrel has expanded to; for a collet it is a perfect cylinder of the diameter the collet has collapsed to. The datum is the point, axis, or plane of that true geometric counterpart — not of the physical simulator, which has its own form error, and not of the datum feature, which has the part's form error.
RMB vs MMB: How the Simulator Engages
The way the simulator contacts the datum feature depends on the material boundary modifier on the datum reference in the FCF.
Regardless of Material Boundary (RMB)
When a datum is referenced RMB (the default in Y14.5-2009, with no symbol), the datum feature simulator adjusts to make maximum contact with the datum feature:
An internal cylinder (a bore) is engaged by an expanding mandrel that grows until it contacts the bore at maximum material condition of the bore — i.e., the smallest bore within its size tolerance, plus any form error. The datum axis is the axis of the mandrel at full expansion.
An external cylinder (a shaft) is engaged by a collapsing collet or a V-block that closes onto the shaft; the datum is the axis (or, for a V-block, the contact-derived axis).
A planar datum is engaged by a surface plate that contacts the high points of the surface; the datum plane is the plane of the plate.
Because the simulator adjusts, two parts with different actual sizes produce simulators at different actual sizes, but the datum (the axis or plane) is the same theoretically perfect element relative to each part.
Maximum Material Boundary (MMB)
When a datum is referenced MMB (the circled M modifier), the datum feature simulator is fixed at the maximum material boundary of the datum feature — the maximum material size of the feature plus any geometric tolerance applicable to the datum feature. The simulator does not adjust to the actual part:
An internal cylinder at MMB is engaged by a fixed pin at the maximum material size of the bore.
An external cylinder at MMB is engaged by a fixed ring at the maximum material size of the shaft.
Because the simulator is fixed and the part feature may depart from its maximum material condition, there can be clearance between the part and the simulator. That clearance is datum shift — a relative movement between the part and the datum reference frame — and is the subject of Chapter 4. Here it is enough to know that the choice of simulator (adjusting vs fixed) is what makes datum shift possible.
Simulator Types by Datum Feature
- Datum feature Geometry Simulator (RMB) Simulator (MMB)
- Planar surface Plane Surface plate (contacts high points) (MMB rare for planar)
- Internal cylinder (bore) Axis Expanding mandrel Fixed pin at MMB
- External cylinder (shaft) Axis Collapsing collet / V-block Fixed ring at MMB
- Width (two parallel faces) Center plane Expanding parallel-jaw fixture Fixed-width gage at MMB
- Complex / irregular surface Plane / axis Datum targets (point, line, area) Datum targets (MMB rare)
- How the Simulator Establishes the DRF
When a feature is inspected, the part is first placed on the datum feature simulators for the primary datum, then located against the secondary, then against the tertiary, in precedence order. The simulators, contacted in order, lock the six DOF exactly as described in Section 3.2. The DRF is the coordinate system defined by the true geometric counterparts of the engaged simulators — the plane of the primary simulator, the axis or plane of the secondary, and the point or plane of the tertiary.
A subtle but Senior-critical point: the DRF is derived from the simulators, not from the part features . Two parts with the same nominal geometry but different actual sizes (one bore at MMC, one at LMC) produce simulators at different actual sizes (under RMB), but the theoretical datum in each case is the perfect axis of that part's simulator. The measurement is repeatable because the simulator is the reference, not the imperfect feature.
Connecting to Datum Shift (Chapter 4)
When a datum is referenced MMB and the datum feature departs from its maximum material condition, the part can move relative to the fixed simulator. That movement is datum shift and it is an additional tolerance allowed to the features controlled by that DRF. Datum shift is sometimes additive to the geometric tolerance on the controlled feature and sometimes not, depending on the feature's own material condition — a distinction fully developed in Chapter 4. For this chapter, the load-bearing fact is: the simulator's material condition behavior is what creates (or prevents) datum shift. An RMB simulator adjusts and creates no shift; an MMB simulator is fixed and can create shift.
Senior Trap: Confusing Simulator, Feature, and Datum
A frequent Senior-level error is to treat the datum feature, the simulator, and the datum as interchangeable. They are not:
The datum feature is on the part and is imperfect.
The simulator is the equipment and has its own (smaller) error.
The datum is theoretical and perfect, derived from the simulator's true geometric counterpart.
Exam questions that show a bore referenced MMB and ask what establishes the datum are testing exactly this: the datum is the axis of the fixed pin at the bore's maximum material boundary, not the axis of the actual bore. Understanding the simulator chain is the foundation for everything in Datum Referencing II.