ASME Y14.5 2018 – Key Terms and Definitions Explained
The ASME Y14.5-2018 standard is one of the most comprehensive and widely used guides for Geometric Dimensioning and Tolerancing (GD&T) in engineering drawings. It provides standardized definitions and symbols to ensure clarity and consistency in design, manufacturing, and inspection processes. This article summarizes and explains the key terms and concepts from the standard to help engineers, designers, and technical professionals better understand and apply GD&T principles effectively.
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Step 1: Key Terms and Definitions Explained
→ 3.1 Angularity: The tilt of a surface or feature at a specific angle from a datum (not necessarily 90°).
→ 3.2 Inner Boundary (IB): Worst-case boundary formed by the smallest feature size and tolerance.
→ 3.3 Least Material Boundary (LMB): Boundary inside the feature when it has the least material.
→ 3.4 Maximum Material Boundary (MMB): Boundary outside the feature when it has the most material.
→ 3.5 Outer Boundary (OB): Opposite of inner boundary; based on largest feature size and tolerance.
→ 3.6 Circularity: Every point on a circle's cross-section must be equally distant from the center.
→ 3.7 Coaxiality: When axes of cylindrical surfaces are aligned.
→ 3.8 Complex Feature: A curved surface or combination of multiple features.
→ 3.9 Constraint: A restriction on motion.
→ 3.10 Continuous Feature (CF): Two or more features treated as one.
→ 3.11 Continuous Feature of Size: Multiple size features acting as a single size feature.
→ 3.12 Coplanarity: Surfaces lying in the same plane.
→ 3.13 Cylindricity: Surface is equally distant from a central axis along its length.
→ 3.14 Datum: A perfect reference (point, line, plane) used for measurements.
→ 3.15 Datum Axis: Axis of a datum reference.
→ 3.16 Datum Center Plane: Middle plane of a datum surface.
→ 3.17 Datum Feature: The actual part feature that establishes a datum.
→ 3.18 Datum Feature Simulator: Tool or device used to simulate a datum during inspection.
→ 3.19 Datum Reference Frame: 3D system with three intersecting planes used for referencing.
→ 3.20 Simulated Datum: A datum derived from a simulator.
→ 3.21 Datum Target: Specific points or areas used to define a datum.
→ 3.22 Derived Median Line: Line connecting center points of cross-sections.
→ 3.23 Derived Median Plane: Plane connecting center points of surface lines.
→ 3.24 Average Diameter: Mean of various diameter measurements.
→ 3.25 Dimension: Numeric value defining part geometry.
→ 3.26 Basic Dimension: Exact value, no tolerance.
→ 3.27 Directly Toleranced Dimension: Dimension with ± or limit tolerance.
→ 3.28 Reference Dimension: Auxiliary dimension for reference only.
→ 3.29 Actual Mating Envelope (AME): Ideal shape that touches the feature's outermost points.
→ 3.30 Actual Minimum Material Envelope: Ideal shape inside a feature touching innermost points.
→ 3.31 Feature: A specific surface or shape on a part.
→ 3.32 Feature Axis: Axis of a cylindrical or revolved shape.
→ 3.33 Feature Center Plane: Center plane of a feature.
→ 3.34 Feature Control Frame: Box showing geometric tolerance and datums.
→ 3.35 Feature of Size: Any feature with size (e.g., hole, shaft).
→ 3.36 FRTZF: A framework managing geometric relations in feature patterns.
→ 3.37 Flatness: Surface lies in a single plane.
→ 3.38 Free State: Part in natural shape without external force.
→ 3.39 Interruption: Break or gap in a feature.
→ 3.40 Least Material Condition (LMC): The condition where feature has the least material.
→ 3.41 Maximum Material Condition (MMC): The condition where feature has the most material.
→ 3.42 Nonuniform Tolerance Zone: Tolerance zone that's not equally spaced.
→ 3.43 Parallelism: Feature is parallel to a datum.
→ 3.44 Pattern: Group of similar features.
→ 3.45 PLTZF: Framework that controls location of feature patterns.
→ 3.46 Perpendicularity: Feature is at 90° to a datum.
→ 3.47 Tangent Plane: A plane touching the highest point of a surface.
→ 3.48 Position: Location of a feature relative to a datum.
→ 3.49 Profile: Outline or shape of a feature.
→ 3.50 RFS: Geometric tolerance applies regardless of feature size.
→ 3.51 RMB: Datum is established regardless of material boundary.
→ 3.52 Represented Line Element: Graphical line showing direction-dependent tolerance.
→ 3.53 Restrained: Part is held or clamped.
→ 3.54 Resultant Condition: Worst-case boundary combining all effects.
→ 3.55 Runout: Total variation while rotating part.
- Circular Runout: Variations in a single circle.
- Total Runout: Variations in the entire surface.
→ 3.56 Simultaneous Requirement: Tolerances apply to all features together.
→ 3.57 Actual Local Size: Measured size at a specific cross-section.
→ 3.58 Limits of Size: Minimum and maximum allowable sizes.
→ 3.59 Nominal Size: General label for identifying size.
→ 3.60 Statistical Tolerancing: Assigning tolerance based on statistical analysis.
→ 3.61 Straightness: Line or surface is perfectly straight.
→ 3.62 Tolerance: Allowed variation in dimension.
→ 3.63 Bilateral Tolerance: Variation allowed in both directions.
- Equal Bilateral: Equal tolerance both ways.
- Unequal Bilateral: Unequal variation in each direction.
→ 3.64 Geometric Tolerance: Tolerance defined by geometric symbols.
→ 3.65 Unilateral Tolerance: Variation allowed only one way.
→ 3.66 True Geometric Counterpart: Ideal reference geometry used for datums.
→ 3.67 True Position: Exact theoretical location.
→ 3.68 True Profile: Perfect shape from basic dimensions or models.
→ 3.69 Uniform Tolerance Zone: Equal offset zone around true profile.
→ 3.70 Virtual Condition (VC): Boundary considering both size and geometric tolerance.
