4.4 Planar vs Datum Feature of Size

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

4.4 Planar vs Datum Feature of Size

Key Takeaways Explain This Topic AI

A planar datum feature (flat face) yields a plane datum that arrests 3 DOF (one translation plus two rotations) and never produces datum shift because it has no size dimension.

A datum feature of size (cylinder, pin, width) yields an axis or center plane; a cylinder primary arrests 4 DOF and a width primary arrests 2 DOF.

A size datum at RMB self-centers (no shift); the same datum at MMB uses a fixed gage and allows datum shift equal to the departure from MMB.

Choose planar when the part mates on a face (orientation-driven) and size when it mates on a bore or pin (centering-driven); a flange with a pilot commonly uses planar primary plus size secondary at MMB.

Inspection cost rises with variable RMB simulators (mandrels, collets); fixed MMB gages are faster but require the inspector to account for shift.

Last updated: August 2026

Planar vs Datum Feature of Size

Datum features come in two fundamental flavors: planar datum features (a single flat face) and datum features of size (a cylinder, pin, width, or slot — a feature defined by a size dimension). The two behave differently in how they constrain the part, what simulator they use, and whether datum shift is possible. Senior-level selection between them is driven by function and inspection practicality.

Planar Datum Features

A planar datum feature is a flat face. Its datum is a plane contacted by a surface plate (or equivalent). A planar primary datum arrests:

1 translation (normal to the plane)
2 rotations (about the two in-plane axes)

= 3 DOF. The face's orientation (perpendicularity to the next datum) is what locates the part; the face itself does not self-center because it has no size.

Planar datums are repeatable and shift-free . A surface plate contacts the high points of the face; the datum plane is the plate's plane. Because there is no size dimension, there is no MMB or LMB and no datum shift on a planar datum. RMB is effectively the only mode.

Worked Example: Planar Primary

A rectangular cover plate uses the bottom face as primary datum A, a long side as B, and a short side as C. The DRF is three mutually perpendicular planes. A hole pattern is |POS|Ø0.3|A|B|C| . The position zones are located by basic dimensions from the corner formed by A, B, and C. No datum shift is available regardless of the face's flatness — the part is locked to the planes.

Datum Feature of Size

A datum feature of size is a cylinder, pin, or width — a feature with a size dimension that produces a center (axis or center plane). Its datum is the axis or center plane derived from the simulator. The number of DOF it arrests depends on whether it is primary, secondary, or tertiary:

Primary size datum (e.g. a bore as A) : axis arrests 2 translations + 2 rotations = 4 DOF. A tertiary is needed to arrest rotation about the axis.

Secondary size datum (e.g. a pin as B) : arrests 1 translation (radial to the primary) + 1 rotation = 2 DOF, but only after the primary has arrested the others.

Tertiary size datum : arrests the remaining rotation about the axis.

RMB vs MMB on a Size Datum

A size datum can be referenced at RMB or MMB (or LMB) , and this changes behavior fundamentally:

At RMB : the simulator expands or contracts to the actual mating size. The datum is the actual center of the feature. The part self-centers. No datum shift. This is the right choice when the feature mates on its axis and you want the part to center on its own geometry.

At MMB : the simulator is fixed at maximum material (smallest hole / largest pin). The datum is the gage's center , not the part's center. If the part's feature departs from MMB, the part can shift on the gage — datum shift is available. This is the right choice when the part assembles to a fixed mating feature and the shift reflects functional play.

Width (Two Parallel Faces) as a Datum

A width datum is a datum feature of size defined by two parallel faces (a slot or tab). Its datum is the center plane between the faces. At RMB the simulator is two parallel plates that close to actual mating size; the center plane is the datum. At MMB the plates are fixed at MMC separation and shift is available. A width datum arrests:

1 translation (normal to the center plane)
1 rotation (about an axis in the plane)

= 2 DOF, regardless of being primary or higher (the rest of the lock-up comes from the other datums). This is one fewer than a cylinder primary because a width has no axis to arrest two rotations.

Comparison Table

Datum Type Datum Geometry DOF Arrested (as primary) Shift Possible? Typical Simulator
Planar face Plane 3 (1 transl + 2 rot) No Surface plate
Cylinder (RMB) Axis 4 (2 transl + 2 rot) No Expanding mandrel / contracting collet
Cylinder (MMB) Axis of fixed gage 4 Yes (departure from MMB) Fixed pin / fixed ring
Width (RMB) Center plane 2 (1 transl + 1 rot) No Two closing parallel plates
Width (MMB) Center plane of fixed gage 2 Yes (departure from MMB) Two fixed parallel plates
Choosing Between Planar and Size Datums

The choice follows function :

If the part locates by mating on a flat face (a cover bolted to a housing face), use a planar primary. The face sets orientation; the bolts are then located from it.

If the part locates by mating on a bore or pin (a gear on a shaft, a pulley on a hub), use a size primary. The axis is the functional reference; a planar datum would not capture centering.

If both a face and a bore mate (a flange with a pilot), a common pattern is planar primary (face) + size secondary (pilot bore at MMB) : the face orients, the bore locates, and MMB on the bore allows the functional shift of the pilot in the bore.

Inspection Implications

Planar datums are simple: a surface plate and stops. Size datums at RMB need variable simulators (expanding mandrels, collets) that are more expensive and slower. Size datums at MMB use fixed functional gages that are faster and mirror assembly but introduce datum shift that the inspector must account for. A Senior-level drawing choice weighs: functional fidelity (MMB), repeatability (RMB), and inspection cost (planar and simple simulators).

Worked Example: Choosing Planar Primary + MMB Secondary

A pump cover bolts to a housing face (datum A, planar) and pilots on a bore (datum B, Ø20 ± 0.05, MMB = Ø19.95). The bolt pattern is |POS|Ø0.5|A|B(M)| .

A (planar) arrests 3 DOF: Z translation, rotation about X, rotation about Y. No shift.

B (size, MMB) arrests 2 more DOF: X and Y translation. The gage pin is Ø19.95 fixed. If the actual bore measures Ø20.02, datum shift = 0.07 diametral — the cover can shift 0.035 radially on its pilot, exactly as it will in assembly.

The combination orients the cover (face A) and centers it on the pilot (bore B) with functional play — a textbook functional DRF. Replacing B with a planar datum would lose the centering; replacing A with the bore would lose the orientation reference that the bolts need.