GD&T symbols and conventions are crucial for precise communication in mechanical design. They define geometric controls for form, orientation, location, and runout of parts. Understanding these symbols helps engineers ensure proper fit and function of components.

This section covers key GD&T symbols like flatness, circularity, and true position. It also explains important concepts such as feature control frames, datums, and material conditions. These tools allow designers to specify exact requirements for part geometry and assembly.

Form and Orientation Symbols

Flatness and Straightness

• Flatness symbol indicates a surface must lie between two parallel planes separated by the tolerance value
• Measures the overall flatness of a surface, not just a line or section
• Straightness symbol specifies a line element of a surface or axis must lie between two parallel lines separated by the tolerance value
• Controls the straightness of a surface line element or axis, not the entire surface

Circularity and Cylindricity

• Circularity symbol requires that any circular cross section must lie between two concentric circles separated by the tolerance value
• Controls the roundness of a circular feature at any cross section perpendicular to the axis
• Cylindricity symbol specifies that the entire surface of a cylinder must lie between two coaxial cylinders separated by the tolerance value
• Controls the overall form of a cylindrical feature, considering roundness, straightness, and taper

Parallelism, Perpendicularity, and Angularity

• Parallelism symbol requires that a surface or axis must be parallel to a datum plane or axis within the specified tolerance
• Ensures two surfaces or axes are equidistant at all points (parallel slots in a part)
• Perpendicularity symbol specifies that a surface or axis must be perpendicular to a datum plane or axis within the given tolerance
• Controls the orientation of a surface or axis to be 90 degrees to a reference (hole perpendicular to a surface)
• Angularity symbol requires that a surface or axis must be at a specified angle to a datum plane or axis within the tolerance
• Controls the orientation of a surface or axis to be at a specific angle to a reference (angled mounting surface)

Location and Runout Symbols

True Position and Concentricity

• True position symbol specifies that the center, axis, or plane of a feature must lie within a cylindrical or spherical tolerance zone
• Controls the location of a feature relative to datums (pattern of holes on a flange)
• Concentricity symbol requires that the median points of a feature must lie within a cylindrical tolerance zone whose axis coincides with the datum axis
• Controls the coaxiality of a feature relative to a datum axis (shaft and bearing housing)

Runout and Profile

• Runout symbol specifies that the variation of a surface from a datum axis or plane during full rotation must be within the tolerance value
• Controls the combined effects of circularity and coaxiality (runout of a rotating shaft)
• Profile symbol of a line or surface requires that the feature must lie between two equidistant boundaries from a true profile defined by basic dimensions
• Controls the form and location of a feature relative to a true profile (contour of an airfoil)

GD&T Modifiers and Concepts

Feature Control Frame and Datum

• Feature control frame is a rectangular box divided into compartments that contains GD&T symbols, tolerances, and datum references
• Specifies the geometric control applied to a feature and the datums used as references
• Datum is a theoretically exact point, axis, or plane used as a reference for dimensioning and tolerancing
• Establishes a coordinate system and origin for measurements and helps orient and locate features (datum plane on a machined surface)

Maximum and Least Material Condition

• Maximum material condition (MMC) refers to the condition when a feature contains the maximum amount of material within its specified size limit
• Allows for bonus tolerances on mating parts and accommodates manufacturing variations (shaft and hole fit)
• Least material condition (LMC) refers to the condition when a feature contains the minimum amount of material within its specified size limit
• Used when a minimum wall thickness or a maximum clearance between parts is critical (thin-walled casting)