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2.1 Features, Features of Size, & Limits of Size
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
2.1 Features, Features of Size, & Limits of Size
Key Takeaways Explain This Topic AI
A feature is any physical portion of a part (surface, chamfer, pin, hole), but only features with directly opposing elements associated with a directly toleranced dimension qualify as features of size (FOS).
ASME Y14.5-2009 Section 1.3.32 classifies features of size into regular (cylinders, spheres, and pairs of opposed parallel planes) and irregular (directly toleranced or profiled geometries that contain or are contained by an actual mating envelope).
Actual local size measures cross-sectional, two-point thickness with instruments like micrometers or calipers to verify that no individual slice violates the MMC or LMC boundary.
The actual mating envelope (AME) is the boundary of perfect form that best fits the feature: the maximum inscribed cylinder/parallel planes for internal features, and the minimum circumscribed cylinder/parallel planes for external features.
Limits of size establish two independent boundary conditions for regular features of size: cross-sectional limits via actual local size, and the maximum material boundary envelope of perfect form under Rule #1.
Last updated: September 2026
Features, Features of Size, & Limits of Size
Quick Summary: In ASME Y14.5-2009, a feature is any physical portion of a part (such as a planar surface, edge, or fillet), whereas a feature of size (FOS) contains directly opposing points or surfaces associated with a directly toleranced dimension. Y14.5-2009 categorizes features of size into regular features of size (cylinders, spheres, and sets of two opposed parallel planes) and irregular features of size (complex shapes capable of containing or being contained by an actual mating envelope). Verifying a feature of size requires assessing both actual local size (two-point cross-sectional measurements) and the actual mating envelope (AME) (the maximum inscribed or minimum circumscribed boundary of perfect form).
1. Technical Distinction Between Features and Features of Size
In mechanical engineering drawings governed by ASME Y14.5-2009, every physical element of a component is classified under rigorous dimensional taxonomy. The foundational distinction is between a generic feature and a feature of size (FOS) .
Feature (ASME Y14.5-2009, Section 1.3.27): A physical portion of a part, such as a surface, face, pin, tab, hole, or slot, or its representation in drawing views, 3D solid models, or digital product definition data sets. A single planar face, a chamfer, an edge break, a fillet radius, or a tapered draft face is a feature.
Feature of Size (FOS) (ASME Y14.5-2009, Section 1.3.32): One cylindrical or spherical surface, or a set of two opposed parallel surfaces, or a collection of features associated with a directly toleranced dimension, capable of establishing an axis, center line, center plane, or center point.
Why This Distinction Governs GD&T
Not every feature has size. Consider a stepped block with a top horizontal planar face located 50 mm above a bottom base face. The top surface is an individual feature , but taken by itself, it is not a feature of size because it does not possess opposed elements. It has form, orientation, and location relative to datums, but it does not possess an internal or external size dimension.
This distinction is critical on the ASME GDTP Technologist Examination for three operational reasons:
Material Condition Modifiers : Only features of size can be modified at Maximum Material Condition (MMC) , Least Material Condition (LMC) , or evaluated Regardless of Feature Size (RFS) . Modifiers like circled M or circled L can never be applied to single planar surfaces or line elements.
Centerline and Center Plane Generation : Geometric controls applied to a feature of size (such as position or straightness of a derived median line) control the resolved center axis or center plane. Geometric controls applied to a non-size feature control the physical surface itself.
Rule #1 Enforcement : As detailed in Section 2.2, the Envelope Principle (Rule #1) applies strictly to regular features of size. A single flat surface is not subject to Rule #1.
2. Regular vs. Irregular Features of Size
A major structural update in ASME Y14.5-2009 was the formal bifurcation of features of size into regular and irregular categories under Section 1.3.32.
Regular Feature of Size (Section 1.3.32.1)
A regular feature of size is defined as one cylindrical surface, one spherical surface, or a set of two opposed parallel planar surfaces, each of which is associated with a directly toleranced dimension. Classic examples include:
A machined cylindrical shaft, dowel pin, or boss.
A drilled, reamed, or counterbored cylindrical hole.
A precision ground bearing ball or spherical pivot socket.
The width between two opposed, parallel milled slot walls or the thickness of a rectangular tab.
Every regular feature of size is governed automatically by Rule #1 (the Envelope Principle) unless an explicit exception applies.
Irregular Feature of Size (Section 1.3.32.2)
In modern manufacturing, many functional mating features do not conform to pure cylinders, spheres, or pairs of parallel planes. Broached hexagonal sockets, D-shaped shafts, splined hubs, round-ended oblong slots, and complex cast cavities routinely mate with external shafts or internal cores. In earlier standards, designers struggled to reference these geometries as datums at Maximum Material Boundary (MMB) or apply material condition modifiers.
ASME Y14.5-2009 resolved this by codifying two distinct types of irregular features of size :
Type A (Directly Toleranced) : A directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a sphere, cylinder, or pair of parallel planes (e.g., an internal hex socket with a directly toleranced across-flats dimension, an oblong slot with toleranced width and length, or a square shaft).
Type B (Boundary / Profile Established) : A collection of features whose boundary is established by a profile tolerance or other geometric control and can contain or be contained by an actual mating envelope (e.g., an extruded aerodynamic blade profile, a complex cast intake port, or a dovetail way referenced as a datum feature at MMB).
Classification Attribute Regular Feature of Size (1.3.32.1) Irregular Feature of Size (1.3.32.2) Non-FOS Surface Feature (1.3.27)
Geometry Types Cylinder, sphere, or 2 opposed parallel planes Hexagon, square, spline, oblong slot, complex profile Single flat plane, chamfer, fillet, step face
Tolerancing Method Direct limit or plus/minus size dimension Direct tolerance (Type A) or profile boundary (Type B) Basic location, coordinate tolerance, or profile
Rule #1 Application Mandatory default (Envelope Principle) Excluded from Rule #1 by default Never subject to Rule #1
Mating Envelope Type Natural cylinder, sphere, or parallel planes Inscribed or circumscribed envelope fitting geometry No enclosing mating envelope
Datum Referencing RMB, MMB, or LMB RMB, MMB, or LMB RMB (or standard planar simulator)
Generates Axis / Center Yes (axis, center point, or center plane) Yes (axis, center plane, or center of the AME) No (establishes datum plane or tangent contact)
3. Actual Local Size vs. Actual Mating Envelope (AME)
To inspect and verify a manufactured feature of size, quality engineers must evaluate two distinct physical metrics: actual local size and the actual mating envelope .
Actual Local Size (Section 1.3.54)
Actual local size is the measured value of any individual distance at any single cross section of a feature of size. It is a two-point measurement taken perpendicular to the feature's axis or center plane using handheld inspection tools:
External features: Two-point micrometers, snap gages, or vernier calipers.
Internal features: Two-point bore gages, internal micrometers, or telescoping gages.
Actual local size verifies that the part has not necked down, ballooned, or experienced localized cross-sectional thinning or thickening. At every measurable cross section, the two-point reading must lie strictly within the upper and lower limits of size.
Actual Mating Envelope (Section 1.3.25)
The actual mating envelope (AME) is a similar perfect feature counterpart of smallest size that can be circumscribed about the feature so that it just contacts the surface at high points (for external features), or of largest size that can be inscribed within the feature so that it contacts the high points of the interior surface (for internal features).
External Features : The minimum circumscribed cylinder (MCC) or minimum circumscribed pair of parallel planes that fits over the manufactured pin, shaft, or boss.
Internal Features : The maximum inscribed cylinder (MIC) or maximum inscribed pair of parallel planes that expands inside the manufactured hole, bore, or slot.
ASME Y14.5-2009 differentiates between two operational forms of the mating envelope:
Unrelated Actual Mating Envelope (UAME) : Generated without constraint to any datum reference frame. It locates and orients freely in space to achieve the closest possible fit to the feature's surface peaks.
Related Actual Mating Envelope (RAME) : Constrained in orientation and/or location to one or more specified datums. The envelope maintains perfect theoretical orientation or position relative to the datum reference frame while expanding or contracting to contact the feature's extremities.
Why Two-Point Checks Cannot Guarantee Assembly
A central teaching point of ASME GDTP certification is that a feature can pass all two-point actual local size measurements and still fail assembly. Consider a shaft manufactured with a constant-diameter tri-lobed cross section (a Reuleaux triangle profile). If inspected with a two-point micrometer, every cross-sectional diameter measures exactly 20.00 mm. However, because of the three-lobed form error, the minimum circumscribed cylinder (its UAME) measures 20.14 mm. If the shaft's Maximum Material Condition is 20.05 mm, the shaft violates Rule #1 and will physically jam when inserted into a 20.05 mm cylindrical sleeve.
4. Limits of Size Interpretation and Boundary Conditions
The limits of size (Section 2.2) represent the specified maximum and minimum values of a dimension. Under ASME Y14.5-2009, limits of size enforce a two-tier verification requirement on every regular feature of size: Condition 1: LMC Limit ≤ Actual Local Size (Two-Point) ≤ MMC Limit \text{Condition 1: } \text{LMC Limit} \le \text{Actual Local Size (Two-Point)} \le \text{MMC Limit} Condition 1: LMC Limit ≤ Actual Local Size (Two-Point) ≤ MMC Limit Condition 2: Actual Mating Envelope (UAME) must not violate the boundary of perfect form at MMC \text{Condition 2: } \text{Actual Mating Envelope (UAME)} \text{ must not violate the boundary of perfect form at MMC} Condition 2: Actual Mating Envelope (UAME) must not violate the boundary of perfect form at MMC
Step-by-Step Inspection Example
Consider an external dowel pin specified on an engineering drawing as ∅ 16.00 ± 0.10 mm \varnothing 16.00 \pm 0.10\text{ mm} ∅ 16.00 ± 0.10 mm .
- Upper Limit of Size (MMC) : 16.10 mm 16.10\text{ mm} 16.10 mm
- Lower Limit of Size (LMC) : 15.90 mm 15.90\text{ mm} 15.90 mm
- Total Size Tolerance : 16.10 − 15.90 = 0.20 mm 16.10 - 15.90 = 0.20\text{ mm} 16.10 − 15.90 = 0.20 mm
During inspection of a produced pin:
Step 1: Evaluate Actual Local Sizes . The quality technician measures multiple cross sections along the pin's length using a calibrated point micrometer. Readings range from 15.94 mm 15.94\text{ mm} 15.94 mm to 16.08 mm 16.08\text{ mm} 16.08 mm . Because every measurement falls between 15.90 mm 15.90\text{ mm} 15.90 mm and 16.10 mm 16.10\text{ mm} 16.10 mm , Condition 1 is satisfied.
Step 2: Evaluate Actual Mating Envelope (UAME) . The pin is placed inside a precision functional ring gage ground to exactly ∅ 16.10 mm \varnothing 16.10\text{ mm} ∅ 16.10 mm (the envelope of perfect form at MMC). The pin passes through the full-engagement gage without binding. This confirms that even if the pin exhibits slight axial camber (bowing) or circularity error, its minimum circumscribed cylinder does not exceed 16.10 mm 16.10\text{ mm} 16.10 mm . Condition 2 is satisfied.
Step 3: Conformance Determination . The pin satisfies both the local size limits and the envelope requirement, rendering it conforming per ASME Y14.5-2009.
5. Common Exam Traps & Technologist Watchouts
Trap 1: Treating a Single Flat Surface as a Feature of Size. An exam problem may display a block with a top surface toleranced to datums, asking candidates to calculate MMC, LMC, or bonus tolerance. A single planar surface has no opposed elements and no size dimension; it cannot have an MMC, LMC, or bonus tolerance.
Trap 2: Assuming Actual Local Size Proves Envelope Compliance. Two-point micrometer checks verify only actual local size; they cannot detect axis curvature (banana bow) or multi-lobed out-of-roundness. A functional gage, precision spindle, or CMM scan is required to verify the actual mating envelope.
Trap 3: Believing Irregular Features of Size Automatically Require Rule #1. Rule #1 applies strictly to regular features of size. An irregular feature of size (such as an internal hex socket or an oblong slot) does not possess an automatic envelope of perfect form at MMC unless an explicit boundary or profile tolerance is applied.
Trap 4: Conflating UAME with RAME. An Unrelated Actual Mating Envelope floats freely to achieve minimum clearance with the feature. A Related Actual Mating Envelope is locked in orientation or position to datums, which frequently results in a larger external envelope or smaller internal envelope due to squareness or location errors.