4.3 Datum Shift Behavior

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

4.3 Datum Shift Behavior

Key Takeaways Explain This Topic AI

Datum shift is the allowed movement between a datum feature and its fixed MMB or LMB simulator; it gives location freedom on the datum side, distinct from bonus tolerance on the toleranced feature.

Datum shift applies only when the datum is referenced at MMB or LMB — never at RMB, where the simulator matches the part and there is no clearance to consume.

Shift equals the datum feature's departure from its material boundary (e.g. actual bore diameter minus MMB) and moves the entire DRF, not individual feature zones.

A part may have both bonus on the toleranced feature and shift on the datum; they are additive in effect on acceptance but tracked separately.

Shift does not accumulate arbitrarily across features — the pattern must clear in one setup, so shift is a single translation or rotation of the whole part on the gage.

Last updated: August 2026

Datum Shift Behavior

Datum shift is the allowed movement between a datum feature and its constant-size MMB or LMB simulator. When a datum is referenced at MMB and the actual datum feature is departed from its maximum material boundary (i.e. the hole is larger than MMB, or the pin is smaller than MMB), the part can translate (and sometimes rotate) on the simulator without losing contact. That freedom is "bonus-like" — it adds location freedom — but it lives on the datum side , not on the toleranced feature.

When Datum Shift Applies

Datum shift is available only when the datum is referenced at MMB or LMB . A datum referenced at RMB has no datum shift : the simulator expands or contracts to match the part, so there is no clearance to consume. This is the most common Senior-level trap: candidates apply shift to an RMB datum because they confuse it with bonus tolerance.

The shift amount equals the departure of the datum feature from its material boundary:

For an internal datum at MMB: shift = (actual bore size − MMB). Bore larger than MMB gives radial shift up to half the diametral departure.

For an external datum at MMB: shift = (MMB − actual pin size). Pin smaller than MMB gives radial shift up to half the diametral departure.

For an LMB datum: same logic at LMC.

Bonus vs Shift — Keep Them Separate

Concept Where It Lives Modifier Source of Freedom Direction

Bonus tolerance On the toleranced feature MMC/LMC in the tolerance compartment Feature departs from MMC/LMC Increases the tolerance zone

Datum shift On the datum feature MMB/LMB in the datum compartment Datum feature departs from MMB/LMB Moves the entire DRF relative to the part

A part can have both at once: a position tolerance |POS|Ø0.4 (M)|A(M)| gives bonus on the feature (feature larger than MMC → bigger zone) and shift on datum A (A larger than MMB → DRF can move). They are additive in their effect on acceptance but tracked separately.

Numeric Worked Example

A plate has a datum bore A of Ø25.00 ± 0.10 (MMB = Ø24.90, LMB = Ø25.10) and a four-hole pattern toleranced |POS|Ø0.4 (M)|A(M)| . One part measures: datum bore = Ø25.05, each of the four holes = Ø10.20 (MMC for the holes = Ø10.00, so each hole departs 0.20 from MMC).

Bonus on each hole = actual − MMC = 10.20 − 10.00 = 0.20. The position zone grows from Ø0.40 to Ø0.60.

Datum shift on A = actual A − MMB = 25.05 − 24.90 = 0.15 diametral. The DRF can move radially by 0.075 in any direction relative to the gage pin.

To accept the part, the inspector seats the part on the Ø24.90 gage pin, then translates or rotates the part within the 0.15 diametral shift to bring all four holes into their now-Ø0.60 zones simultaneously. If a setup exists where all four hole zones clear their pins, the part passes. The shift does not add to each hole's zone — it moves the common DRF that locates all four at once.

Traps

Datum shift is not per-feature. It moves the DRF, which locates all features referenced to that DRF simultaneously. You cannot spend 0.15 of shift on hole 1 and another 0.15 on hole 2 independently.

Datum shift does not accumulate arbitrarily. A pattern inspected with shift must still clear in one setup. Shift is a single translation or rotation of the whole part on the gage, not a sum of independent shifts per feature.

Shift is not bonus. If the feature control frame is |POS|Ø0.4|A(M)| (no MMC on the feature), there is no bonus — only shift on A. The feature's zone stays Ø0.40.

RMB datums do not shift. |POS|Ø0.4|A| with A at RMB has zero datum shift regardless of A's actual size, because the simulator expands to match the part.

How Shift Accepts Parts

Because shift moves the DRF, the inspector can use it to find a favorable orientation of the part on the functional gage. This is exactly the assembly behavior the standard is trying to model: a part with a loose bore will shift on its pilot and still assemble. The gage accepts the part if any valid shift orientation exists where all controlled features are simultaneously in tolerance. The inspector does not "add" shift to a feature's tolerance zone; the shift moves the part relative to the gage so that the existing zones line up with the actual features.