Introduction to GD&T 301

“Introduction to GD&T” provides a basic introduction to the symbols and vocabulary of geometric dimensioning and tolerancing, or GD&T. GD&T is an international design standard that uses 14 standard geometric tolerances to control the shape of features. GD&T emphasizes the fit, form, and function of a part by comparing the physical features of the part to the imaginary datums specified in the design instructions. Every part feature is described by a series of symbols, which are organized in the feature control frame.

Because GD&T uses tolerance zones that more accurately follow the shape of a feature rather than a square grid and emphasizes the relationship between features, blueprints usually utilize GD&T to describe parts. This means that to fully understand a blueprint, it is necessary to know the GD&T symbols and what they mean. After taking this class, users should be able to better understand the symbols commonly used in a GD&T print.

Class Details

Class Name:
Introduction to GD&T 301
Version:
2.0
Difficulty:
Advanced
Number of Lessons:
22
Related 1.0 Class:
Intro to GD&T 205 (2009)

Class Outline

  • What is GD&T?
  • Datums and Features
  • Datum Order
  • The Feature Control Frame
  • How the Feature Control Frame Works
  • Material Condition Modifiers
  • Modifier Symbols
  • GD&T Symbols
  • Basic and Reference Dimensions
  • GD&T vs. Coordinate Tolerancing
  • Types of Tolerances
  • GD&T
  • Straightness and Flatness
  • Circularity vs. Cylindricity
  • Profile of a Line and Surface
  • Angularity, Perpendicularity, and Parallelism
  • Position
  • Concentricity and Symmetry
  • Circular and Total Runout
  • Circular and Total Runout Inspection
  • Characteristic Categories
  • Advantages of GD&T

Objectives

  • Define GD&T.
  • Distinguish between a datum and a feature.
  • List the contents of the feature control frame.
  • List the material condition modifiers.
  • List the GD&T modifiers not dealing with material condition.
  • Identify basic dimensions and explain their purpose.
  • Distinguish between traditional tolerancing and GD&T.
  • List the major categories of geometric tolerances.
  • Describe the straightness tolerance and the flatness tolerance.
  • Describe the circularity tolerance and the cylindricity tolerance.
  • Describe the profile of a line tolerance and the profile of a surface tolerance.
  • Describe the angularity, perpendicularity, and parallelism tolerances.
  • Describe the position tolerance.
  • Describe the concentricity and symmetry tolerances.
  • Describe the circular runout tolerance and the total runout tolerance.
  • Describe the advantages of GD&T.

Job Roles

Certifications

NIMS
  • CNC Lathe Operations
  • CNC Milling Operations
  • CNC Milling Programming, Setup, & Operation
  • CNC Milling: Operations-FastTrack
  • CNC Milling: Programming, Setup, and Operations-FastTrack
  • CNC Turning Programming, Setup, & Operation
  • CNC Turning: Operations-FastTrack
  • CNC Turning: Programming, Setup, and Operations-FastTrack
  • Drill Press I
  • Drill Press Skills-FastTrack
  • Grinding I
  • Grinding Skills-FastTrack
  • Job Planning, Benchwork, & Layout I
  • Job Planning, Benchwork, and Layout-FastTrack
  • Manual Milling Skills-FastTrack
  • Measurement, Materials, & Safety I
  • Measurement, Materials, and Safety-FastTrack
  • Milling I
  • Turning Operations: Turning Between Centers-FastTrack
  • Turning Operations: Turning Chucking Skills
  • Turning Operations: Turning Chucking Skills-FastTrack

Glossary

Vocabulary Term Definition
American Society of Mechanical Engineers ASME. An organization that publishes technical materials and sets industrial and manufacturing standards. Along with ISO, ASME provides written standarization for GD&T in ASME Y14.5M.
angularity A three-dimensional orientation tolerance that describes the allowable variability in the angular relationship between a surface and a datum. Angularity is a related tolerance.
basic dimension A dimension that is theoretically perfect. A basic dimension has no direct tolerance and is denoted on a GD&T blueprint as a number enclosed in a rectangular box.
circular runout A two-dimensional geometric tolerance that controls the form, orientation, and location of multiple cross sections of a cylindrical part as it rotates. Circular runout is a related tolerance.
circularity A two-dimensional form tolerance that describes the allowable variability in the shape and appearance of a circle. Also known as roundness, circularity is an individual tolerance.
coaxial Sharing a common center between two cylindrical features. Circularity requires any two-dimensional cross section of a feature to remain within two imaginary coaxial circles.
concentricity A three-dimensional locational tolerance that describes the location of opposing points in cylindrical features with respect to a datum reference. Concentricity is a related tolerance.
controlled radius A radius that yields a circle, arc, or sphere with no flat sections or reversals. In GD&T, controlled radius occurs on a blueprint with the symbol CR.
coordinate tolerancing A system for describing the design of a part that compares its features to distances along three linear axes. These axes then create an imaginary rectangular grid on which part features are positioned.
cross section A section of a feature that is formed by an intersecting imaginary plane. Circularity requires any two-dimensional cross section of a feature to remain within two imaginary concentric circles.
cylindricity A three-dimensional form tolerance that describes the allowable variability in the shape and appearance of a cylinder. Cylindricity is a an individual tolerance.
datum A point of reference for machine tools, programs, and fixtures from which measurements are taken. A datum can be a hole, line, or any three-dimensional shape.
datum feature A physical feature that acts as an acceptable substitute for a datum. Datum features relate the various features of the part to each other.
diameter The distance from one edge of a circle to the opposite edge that passes through the center. Round or cylindrical features require diameter measurements.
feature control frame A series of compartments containing symbols and values that describe the geometric tolerance of a feature. The order and purpose of these compartments follow a consistent standard.
flatness A three-dimensional form tolerance that describes the allowable variability in the shape and appearance of a surface that lies in a plane. Flatness is an individual tolerance.
form tolerance A geometric tolerance that limits the amount of error in the shape of a feature. The form tolerances include straightness, flatness, circularity, and cylindricity.
functional gage A gage representing a worst case mating part that provides a simple pass/fail assessment of the inspected part. Functional gages often can quickly inspect several features at once.
geometric dimensioning and tolerancing GD&T. An international standard for communicating instructions about the design and manufacturing of parts. GD&T uses universal symbols and emphasizes the function of the part.
individual tolerance A tolerance that does not require a specified datum. Form tolerances are always individual tolerances, while profile tolerances can sometimes be individual tolerances.
International Organization for Standardization ISO. An organization that establishes documented standards, rules, and guidelines to ensure that products, processes, and services are fit for their purpose. Along with ASME, ISO provides written standarization for GD&T in ISO 1101.
least material condition LMC. The point at which a feature contains the least amount of material within its acceptable size limit. The largest acceptable hole and the smallest acceptable shaft are examples of LMC.
location tolerance A geometric tolerance that limits the location or placement of features. Location tolerances are related tolerances.
material condition modifiers Defines the tolerance of a feature in relation to its acceptable size limits. There are three material condition modifiers in GD&T, maximum material condition, least material condition, and regardless of feature size.
maximum material condition MMC. The point at which a feature contains the greatest amount of material within its acceptable size limit. The smallest acceptable hole and the largest acceptable shaft are examples of MMC.
median point A point that is exactly the same distance between two outer points. Concentricity finds the median point between two points opposite each other forming a diameter on a cylindrical feature.
modifier An element that communicates information about the tolerance of a feature. In GD&T, each modifier has a symbol that distinguishes it with other modfiers on a blueprint.
orientation tolerance A geometric tolerance that limits the direction, or orientation, of a feature in relation to other features. Orientation tolerances are related tolerances.
parallelism A three-dimensional orientation tolerance that describes the equal distance between pairs of points, lines, or planes. Parallelism is a related tolerance.
perpendicularity A three-dimensional orientation tolerance that describes the allowable variability in the 90-degree angular relationship between a surface and a datum. Perpendicularity is a related tolerance.
position tolerance A three-dimensional geometric tolerance that controls how much the location of a feature can deviate from its true position. Position is a related tolerance.
primary datum The datum feature that first situates the part within the datum reference frame. The primary datum is the first feature to contact a fixture or surface during assembly.
profile The outline of the part feature within a given plane. Two-dimensional profiles are toleranced using profile of a line, and three-dimensional profiles are toleranced using profile of a surface.
profile of a line A two-dimensional profile tolerance that describes the allowable variability in the contour of the edge seen in the section view. Profile of a line can be either an individual or a related tolerance.
profile of a surface A three-dimensional profile tolerance that describes the allowable variability in the contour of a surface. Profile of a surface can be either an individual or a related tolerance.
profile tolerance A geometric tolerance that controls the size, location, orientation, and form of a feature. Profile tolerances can be either individual or related.
projected tolerance zone A tolerance zone that extends beyond a feature by a specified distance. Symbolized by a P enclosed in a circle, projected tolerance zones help ensure that mating parts fit during assembly.
radius The distance from a center point to a point on a circle or arc. In GD&T, radius forms a curved feature formed by identifying a uniform distance from a center point to the edge of a circle or arc.
reference dimension A dimension that is provided for informational purposes only. A feature is on a GD&T blueprint for reference, rather than for inspection or production use, if its dimensions are enclosed in parentheses.
regardless of feature size RFS. A modifier indicating that the stated tolerance for a feature applies regardless of its actual size within an acceptable size limit. RFS does not permit bonus tolerance.
related tolerance A tolerance that requires a specified datum. Orientation, location, and runout are always related tolerances.
runout tolerance A geometric tolerance that simultaneously limits the form, location, and orientation of cylindrical parts. Runout tolerances are related tolerances requiring a datum axis.
secondary datum The datum feature that situates the part within the datum reference frame after the primary datum. The secondary datum is the second feature to contact a fixture or surface during assembly.
straightness A two-dimensional form tolerance that describes allowable variability in the shape and appearance of a line in a section view. Straightness is an individual tolerance.
tangent plane A modifier specifying that the tolerance zone applies to a plane defined by the high points on a surface. In GD&T, tangent plane is symbolized by a capital T enclosed in a circle.
tertiary datum The datum feature that situates the part within the datum reference frame after the secondary datum. This plane must be perpendicular to both the primary and secondary planes and is usually the smallest surface of the workpiece.
three-dimensional tolerance A tolerance that controls a shape having a length, width, and depth. Flatness, profile of a surface, and angularity are all examples of three-dimensional tolerances.
tolerance zone An imaginary zone in which a part feature must be completely contained for the part to pass inspection. This zone contains the dimensions between the maximum and minimum limits of a feature's location.
total runout A three-dimensional geometric tolerance that controls the form, orientation, and location of the entire length of a cylindrical part as it rotates. Total runout is a related tolerance.
true position The imaginary perfect position of a feature described by the design specifications. The location of a feature's true position is determined by the positional tolerance.
true profile The perfect, imaginary profile described by the design specifications. The profile tolerances compare the actual profile of a feature to the true profile.
two-dimensional tolerance A tolerance that controls a shape having only a length and width. Straightness, circularity and circular runout are all examples of two-dimensional tolerances.
unequal tolerance zone A tolerance zone used for the profile tolerances that designates if a given tolerance is not equally disposed around the true profile. A profile may be unequal on two sides or one side.