Mechanical Variation & GD&T/GPS Glossary

A

Actual local size: The measured size of a feature at a specific cross-section or location, used when evaluating size and form against GD&T/GPS requirements.
Actual mating envelope: The smallest (for external features) or largest (for internal features) perfect geometry that can be placed over a real feature so that it just makes contact, often used when applying maximum material concepts.
All around symbol: A drawing symbol that indicates a requirement (often a profile tolerance) applies all the way around a part or feature, not just to the portion directly adjacent to the feature control frame.
All over symbol: A drawing symbol that indicates a requirement applies to all surfaces of the part, not just a specific region or side.
Arc length symbol (⌒): A symbol that shows a dimension applies to the length along an arc on a curved outline rather than to a straight-line distance.
ASME GD&T: The geometric dimensioning and tolerancing standard published by ASME (such as ASME Y14.5) that defines rules, symbols, and practices for communicating allowable variation on engineering drawings and 3D models.
Assembly sequence: The defined order in which components are assembled; different sequences can change how variation accumulates and how gaps, flush conditions, and alignments behave.
Angularity (∠): An orientation control with the symbol ∠ that limits how a surface, axis, or center plane is oriented at a specified angle (other than 0°, 90°, or 180°) relative to a datum reference frame.

B

Balance cost and quality: An engineering and manufacturing goal of choosing tolerances that are tight enough to protect functional requirements and quality, while loose enough to keep machining, inspection, and supplier costs under control.
Basic dimension (boxed value): A theoretically exact value, often shown boxed on a drawing, that defines the ideal geometry of a feature; the allowable variation is controlled by the associated geometric tolerance rather than by plus/minus limits.
Bonus tolerance: Additional usable geometric tolerance that becomes available when a feature departs from maximum material condition toward less material, rewarding processes that produce parts away from the tightest fit.
Build variation: All sources of variation that show up during assembly, including part tolerances, fixtures, tooling, and assembly methods, which together influence product performance and yield.

C

Capability index (Cpk): A statistical metric that compares the spread and centering of a process to its specification limits, indicating how consistently the process can produce parts within tolerance.
CETOL 6σ: Sigmetrix 3D tolerance analysis software that models how dimensional and assembly variation stack up across an entire product, helping teams predict performance and make informed decisions about tolerances before releasing designs.
Circularity / roundness (◯): Circularity, with the symbol ◯, is a form control that limits how much any cross-section of a cylindrical or conical surface can deviate from a perfect circle, independent of any datums.
Circular runout (⌰): Circular runout, with the symbol ⌰ in the feature control frame, is a runout control that limits the variation of a surface element as the part is rotated 360 degrees about a datum axis, evaluated at individual circular cross-sections.
Coaxiality: A condition where multiple cylindrical features share the same axis; in modern practice this is usually controlled with a position tolerance referencing a datum axis.
Concentricity (◎): Concentricity, with the symbol ◎, is a location control that limits how closely the median points of a feature of size follow a datum axis. It is a demanding requirement because it is evaluated from derived median points rather than directly from surfaces, so many applications now prefer position or runout.
Conical taper symbol: A notation placed with a dimension, usually as a ratio like 1:10, that indicates the value is a taper ratio expressing diameter change over length for a conical feature.
Continuous feature (CF): The continuous feature symbol CF identifies two or more separate features that must be treated as a single feature for size and geometric controls. When CF is applied, the grouped features share one common tolerance zone and are evaluated as if they form one continuous feature of size, even if there are gaps or steps between them.
Control plan: A structured document that lists key characteristics, measurement methods, sampling plans, and reaction plans used to keep manufacturing and assembly processes within expected variation.
Controlled radius symbol (CR): A radius symbol variant, often shown as CR, used when a contour must form a smooth, fair curve with no sudden reversals, flats, or sharp changes in curvature.
Countersink symbol (⌵): A symbol that indicates a conical enlargement at the opening of a hole, sized and angled to match a flat-head fastener so it can sit flush with or below the surface.
Counterbore symbol (⌴): A symbol that indicates a flat-bottomed cylindrical enlargement at the opening of a hole, used to seat the head of a fastener or create clearance for tooling.
Cylindricity (⌭): Cylindricity, with the symbol ⌭, is a form control that limits how much a cylindrical surface can deviate from a perfect cylinder by controlling circularity and straightness of the surface along the entire length.

D

Datum feature: A physical feature on a part, such as a surface or hole, that is used to establish a datum reference frame for measurement and assembly.
Datum reference frame (DRF): A coordinate system built from one or more datum features that provides a consistent origin and orientation for defining and measuring the location and orientation of features on a part or assembly.
Datum target symbol: A target symbol with a letter and number (for example A1) that identifies a specific point, line, or area on a part used to establish a datum when the full surface is not suitable by itself.
Depth symbol: A symbol placed with a value, often a small arrow or “depth” prefix with a number, to indicate how deep a hole, pocket, or recess extends from the outer surface of a part.
Design for manufacturability (DFM): An approach to product design that considers process capabilities, tooling, and variation early so that parts and assemblies can be built reliably at the desired cost and quality levels.
Dimension origin symbol: A small symbol at one end of a dimension, historically a circle or bullseye, that indicates the origin or starting point from which a linear dimension is measured.
Dimensional inspection plan: A documented plan that defines which dimensions and geometric tolerances will be checked, how they will be measured, and at what frequency.
Dimensional management: The practice of planning, analyzing, and controlling dimensional variation across the product lifecycle so that assemblies meet their functional requirements consistently.
Digital thread: The connected flow of product information from requirements and design through manufacturing, inspection, and service so that each team works from consistent, up-to-date data.
Diameter symbol (⌀): A size symbol ⌀ used to indicate that the associated dimension applies across the full diameter of a cylindrical or circular feature.

E

Envelope principle (GD&T Rule 1): A rule that states the form of a regular feature of size is controlled by its limits of size, so at maximum material condition the entire surface must fit inside a perfect envelope defined by that size.
Envelope requirement symbol (E in a circle): A symbol often represented as E in a circle that is used to state explicitly that the size limits of a feature also control its form relative to a perfect envelope at maximum material condition.
EZtol: Sigmetrix 1D tolerance stack-up analysis tool that runs directly on top of popular CAD systems, helping teams quickly evaluate and adjust linear chains of dimensions in assemblies without leaving their CAD environment.

F

Feature control frame: The rectangular GD&T frame that states which geometric characteristic symbol is being controlled, the tolerance value and modifiers, and any datum reference frame used as a basis for that control.
Feature of size (FOS): A feature that has a directly associated size dimension, such as a hole, pin, slot, or tab, and can be controlled using maximum or least material concepts.
Flatness (⏥): Flatness, with the symbol ⏥, is a form control that limits how much a surface may deviate from a perfect plane, independent of any datum reference frame.
Free-state condition: The natural, unrestrained shape of a part when it is not being held by fixtures or assembly forces, especially important for flexible or thin-walled components.
Free state symbol Ⓕ: A modifier shown as F in a circle that indicates a dimension and its tolerance are to be assessed with the part in an unrestrained free state.
Functional gage: A gage designed to verify that a feature or part satisfies its GD&T/GPS requirements by simulating how it assembles or functions, rather than measuring each dimension separately.
Functional requirement: The specific behavior or performance a part or assembly must achieve in its operating environment, such as fit, alignment, motion, or sealing, that drives how tolerances are applied and evaluated.

G

GD&T (Geometric Dimensioning and Tolerancing): A symbolic language defined by standards such as ASME Y14.5 for specifying the allowable variation of part geometry in a way that relates directly to functional requirements and inspection.
GD&T Advisor: Sigmetrix software that guides users in applying GD&T/GPS inside supported CAD systems, helping ensure that annotations are complete, consistent, and aligned with the governing standards.
GD&T/GPS: A shorthand way to refer to both the ASME GD&T and ISO GPS standards together, acknowledging that organizations may use either or both sets of rules to define and verify product geometry.
Geometric tolerancing: The practice of controlling the shape, orientation, and location of features using geometric tolerances rather than relying only on plus/minus size limits.

H

High-low assembly strategy: An intentional pairing approach where larger features are matched with smaller mating features or vice versa to influence fit, contact patterns, or performance.
Hole pattern: A group of holes or features arranged in a pattern whose relative positions and orientation are often controlled together with GD&T/GPS controls such as position.

I

Independency symbol (I in a circle): A symbol shown as I in a circle that indicates the requirement for perfect form at maximum or least material condition is removed so size and form are controlled independently.
ISO GPS (Geometrical Product Specifications): A family of ISO standards that define how to specify and verify the geometry of parts and assemblies, similar in purpose to ASME GD&T but with different terminology and rules.

K

Key characteristic (KC): A dimension or feature whose variation has a strong impact on functional requirements, safety, performance, or customer satisfaction and therefore receives additional control and monitoring.

L

Least material condition (LMC): The condition of a feature of size where it contains the least amount of material, such as the largest hole or smallest shaft, often used when minimum wall thickness or minimum clearance is crucial.
Least material condition symbol (L in a circle): A modifier shown as L in a circle indicating that a geometric tolerance is related to the least material condition and may increase or decrease as the feature departs from that condition.
Limit dimensioning: A plus/minus dimensioning method that specifies only upper and lower numeric limits instead of a nominal value with a separate tolerance, common in drawings that do not use GD&T/GPS extensively.

M

Maximum material condition (MMC): The condition of a feature of size where it contains the most material, such as the smallest hole or largest shaft, frequently used with GD&T/GPS modifiers that enable bonus tolerance.
Maximum material condition symbol (M in a circle): A modifier shown as M in a circle indicating that a geometric tolerance is applied at maximum material condition and that additional bonus tolerance is allowed as the feature departs from that condition.
Mechanical variation: The differences between nominal design geometry and the actual manufactured parts and assemblies caused by manufacturing processes, fixtures, measurement methods, and operating conditions.
Mechanical variation management (MVM): A systematic approach to understanding, predicting, and controlling mechanical variation across the product lifecycle so that teams can meet functional requirements, reduce rework, and improve yield.
MBD-ready model: A CAD model that has been structured, organized, and annotated so that it can reliably serve as the source for model-based definition, including appropriate PMI and clean, consistent geometry.
Model-based definition (MBD): The practice of using the 3D CAD model as the primary, authoritative source of design specifications by embedding dimensions, GD&T/GPS, notes, and other product information directly on the model.
Model-based enterprise (MBE): An organization that extends MBD beyond engineering so that downstream groups in manufacturing, quality, supply chain, and service consume and act on model-based information throughout the product lifecycle.
Multiple identical features symbol (for example 4X): A notation such as 2X, 3X, or 4X that indicates a single dimension applies to several identical features at once, with the number showing how many features are included.

O

Orientation tolerance: A family of geometric tolerances, such as parallelism, perpendicularity, and angularity, that control how a feature is oriented relative to a datum reference frame without directly fixing its location.

P

Parallelism (∥): Parallelism, with the symbol ∥, is an orientation control that limits how parallel a surface or axis must be relative to a datum plane or datum axis, without directly constraining its location.
Parting line symbol (PL): A symbol or note, often PL, that identifies where the separate halves of a mold or die meet; if it is not specified, the manufacturer typically selects the parting line location.
Perpendicularity (⊥): Perpendicularity, with the symbol ⊥, is an orientation control that limits how close a surface or axis must be to 90 degrees relative to a datum plane or datum axis.
Planarity (flatness of a surface): A geometric tolerance that controls how much a surface can deviate from a perfect plane, independent of any datum reference frame; this is typically implemented as a flatness tolerance.
PMI (Product and Manufacturing Information): Non-graphical information such as dimensions, GD&T/GPS, notes, and surface finish that is associated with 3D geometry in CAD models and used to communicate design specifications to downstream users.
Position tolerance (⌖): Position, with the symbol ⌖, is a location control that defines how far the actual location of a feature of size may deviate from its theoretically exact location relative to a datum reference frame.
Process capability: A measure of how well a process can consistently produce output within specified tolerances, often summarized with indices such as Cp and Cpk.
Process variation: The natural fluctuation that occurs in a manufacturing and assembly process, driven by machines, methods, materials, environment, and human factors.
Product and process knowledge: Information about how a product is designed to function and how it is manufactured, including tolerances, key characteristics, and process capabilities, that organizations aim to capture, reuse, and improve over time.
Profile tolerance: A geometric tolerance that controls how far the surface or outline of a feature can deviate from its ideal geometry, often used to control complex, freeform shapes and sometimes combining form, orientation, and location.
Profile of a line (⌒): Profile of a line, with the symbol ⌒, is a profile control that limits the deviation of a cross-sectional curve from its ideal profile, evaluated in individual sections along a feature.
Profile of a surface (⌓): Profile of a surface, with the symbol ⌓, is a profile control that limits how far an entire surface may deviate from its ideal 3D profile and can control form, orientation, and sometimes location in a single tolerance.
Projected tolerance zone symbol (P in a circle): A modifier represented as P in a circle that indicates the tolerance zone for a feature extends beyond the surface of the part by a specified distance, typically for threaded holes, studs, or pins that affect components some distance away.

R

Radius symbol (R): A size symbol R indicating that the dimension specifies the radius of an arc or rounded feature rather than its diameter.
Regardless of feature size (RFS): The default condition, often implied, in which a geometric tolerance is applied independently of the actual size of the feature, without any bonus tolerance tied to maximum or least material condition.
Repeatability: The degree to which repeated measurements under the same conditions yield the same result, often evaluated as part of a gage repeatability and reproducibility (R&R) study.
Residual variation: Remaining variation in a key measurement after design changes, process improvements, or tolerance adjustments have been applied.
Restrained condition note: A note that indicates a flexible or deformable part is to be restrained in a specific way during inspection so that dimensions and tolerances are evaluated in a defined restrained state instead of a free state.
Runout tolerance: A family of controls, including circular runout and total runout, that limit how much a surface can vary as the part is rotated around a datum axis, capturing combined effects of form, orientation, and sometimes location.
Runout (circular runout, ⌰): Circular runout, with the symbol ⌰, is the base runout control applied to control variation of a surface element at a circular cross-section as the part rotates about a datum axis.

S

Scrap rate: The percentage of parts or assemblies that must be discarded because they do not meet design specifications or functional requirements.
Sensitivity analysis (tolerance analysis): An evaluation that shows how sensitive a key measurement or functional requirement is to variation in each contributing dimension, helping teams identify which tolerances have the greatest impact.
Slope symbol: A notation, often shown as a ratio such as 1:10 with a slope note, that indicates a surface is controlled as a slope value representing change in height over a given length for a flat taper-style feature.
Spherical diameter symbol: A symbol or notation that indicates a diameter dimension applies to a spherical feature, defining a 3D spherical tolerance zone rather than a circular one.
Spherical radius symbol (SR): A symbol SR indicating that a radius dimension applies to a spherical surface rather than a simple arc, defining a spherical tolerance zone between minimum and maximum radii.
Spotface symbol (SF): A symbol or note, often SF, that indicates a shallow counterbore used to create a small, flat seating surface for a fastener head or washer without removing more material than necessary.
Square symbol (□): A symbol □ that indicates a dimension applies to a square cross section or square feature.
Stack-up path: The sequence of dimensions and features that connect one functional surface or feature to another, used when setting up tolerance stack-up analyses.
Statistical process control (SPC): A method of using control charts and statistical techniques to monitor process behavior, detect shifts, and react before output moves outside of tolerance.
Statistical tolerance (modifier): A drawing notation that indicates a group of tolerances is intended to be managed using statistical methods such as SPC so that the combined effect of multiple parts is controlled statistically instead of purely by worst-case limits.
Statistical tolerance analysis: An analysis approach that predicts the distribution of a key measurement by modeling the statistical behavior of individual dimensions, often assuming process distributions rather than relying only on worst-case limits.
Straightness (│): Straightness, often represented by a straight line symbol │, is a form control that limits how much a line element or axis may deviate from a perfect straight line, commonly used for shafts, pins, and edges.
Surface profile tolerance: A profile control applied to entire surfaces, limiting overall form, orientation, and sometimes location of the surface relative to a datum reference frame.
Supplier capability: A supplier ability to hold specified tolerances and deliver acceptable variation over time, typically measured with capability indices and defect rates.
Symmetry (⌯): Symmetry, with the symbol ⌯, is a location control that limits how evenly a feature is distributed about a datum center plane. In many modern designs, position or profile controls are used instead because they are easier to measure and interpret.

T

Tangent plane symbol (T in a circle): A modifier shown as T in a circle that indicates the geometric tolerance applies to a theoretical tangent plane that contacts the highest points of a surface, rather than the entire surface itself.
Time to market: The total time required to move from concept to a launched product; tolerance analysis and mechanical variation management help shorten this time by reducing late-stage redesigns, build issues, and test failures.
Tolerance: The allowed amount of variation from a nominal dimension or geometry that still keeps the part or assembly within its functional requirements and design specifications.
Tolerance analysis: The process of evaluating how dimensional variation across parts, features, and processes affects a key measurement or functional requirement in an assembly.
Tolerance stack-up (tolerance stackup): A method for calculating how multiple dimensions and tolerances combine along a measurement path to affect a crucial gap, alignment, or other key characteristic.
Total runout (double runout symbol): Total runout, shown with a double runout symbol, is a runout control that limits variation of an entire surface as the part is rotated about a datum axis, capturing combined form, orientation, and location effects over the full length or area.
True position (legacy term): A commonly used phrase referring to the theoretically exact location of a feature controlled by a position tolerance; modern standards usually refer simply to position.
1D tolerance analysis: A tolerance stack-up study focused on one linear chain of dimensions, typically along a single axis or measurement path, often used early in design or for simpler mechanisms.
3D tolerance analysis: A more comprehensive tolerance analysis that uses 3D models to account for multiple directions, rotations, and interactions between features throughout an assembly.
Unequally disposed profile symbol: A profile modifier, often shown by specifying unequal distribution of the profile tolerance about the true profile in the callout, that indicates the tolerance zone is not centered around the ideal profile and allows an unequal or unilateral distribution of profile tolerance.
Unilateral tolerance: A tolerance scheme that allows variation in only one direction from the nominal value, such as +0.2 / -0.0, often used when one side of the tolerance band is more important for function.

V

Variation analysis: A general term for studying how dimensional and assembly variation flows through a design and impacts the performance, quality, and yield of the final product.
VariSight: Sigmetrix software that extends mechanical variation management beyond individual analyses, making variation insights more accessible across programs, teams, and the product lifecycle.
Virtual condition: A boundary generated by the combined effect of a feature size at maximum or least material condition and its geometric tolerance, representing the worst acceptable condition that must still assemble or function.

W

Worst-case tolerance analysis: A tolerance analysis method that assumes all contributing dimensions are at their most extreme limits in the worst possible combination, ensuring assembly function but often leading to tighter-than-necessary tolerances.

Y

Yield: The proportion of units produced that meet all design specifications and can be shipped or used without rework, directly influenced by how tolerances and processes are chosen.

Mechanical Variation & GD&T/GPS Glossary

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