ASME GDTP Senior Level key terms

A physical portion of a part, such as a surface, hole, slot, or edge. Features are the entities to which tolerances and datum references are applied on a drawing.

A cylindrical surface, spherical surface, or set of parallel opposed surfaces that contains an axis, center plane, or center point. It can be associated with a size dimension and may be regular or irregular.

The largest perfect cylinder or smallest perfect pair of parallel planes that contacts the high points of a feature of size and is oriented to the datum reference frame. It establishes the true geometric counterpart for datum and size evaluation.

A theoretically exact point, axis, line, plane, or combination thereof derived from a datum feature that serves as the origin of measurement for tolerances. Datums are theoretical, not physical.

An actual feature of a part that is identified by a datum feature symbol and used to establish a datum. Its imperfections are simulated by a datum feature simulator during inspection.

A physical or mathematical surface of perfect form, such as a surface plate or expanding mandrel, that contacts the datum feature to establish the datum. It is the practical realization of the theoretical datum.

The three mutually perpendicular planes established by referenced datums, from which basic dimensions locate features and to which orientation and location tolerances are constrained. It defines the coordinate system for inspection.

The order in which datums appear in a feature control frame, indicated by the sequence of datum letters. Primary, secondary, and tertiary datums are constrained in that order during setup.

The six independent motions of a rigid body in space: three translations and three rotations. Datum references constrain these until the part is fully fixed in the datum reference frame.

A theoretically exact dimension, shown in a rectangular frame, that establishes the true profile, location, or orientation of a feature. It carries no tolerance itself; tolerance comes from a feature control frame.

A rectangular box containing the geometric characteristic symbol, tolerance zone description, and datum references. It communicates the type, amount, and constraints of a geometric tolerance applied to a feature.

A tolerance that controls form, orientation, location, or profile of a feature rather than its size. It is stated in a feature control frame and interpreted per the governing standard.

The permissible variation between the maximum and minimum limits of a feature of size, expressed as tolerances or limit dimensions. It controls the actual local size, not form or location.

The default requirement that limits of size also control form. Unless otherwise specified, the surface of a feature of size must not extend beyond a perfect envelope at maximum material size.

The condition where a feature of size contains the maximum amount of material, such as the largest shaft or smallest hole. It is the basis for bonus tolerance when the MMC modifier is specified.

The condition where a feature of size contains the minimum amount of material, such as the smallest shaft or largest hole. It is used where minimum wall thickness or edge distance must be protected.

A condition in which the stated geometric tolerance applies at any actual size of the feature. It is the default when no MMC or LMC modifier appears in the feature control frame.

The additional geometric tolerance gained as a feature of size departs from MMC toward LMC. Its amount equals the departure from MMC and is added to the stated tolerance.

The boundary generated by the collective effects of a feature's size limit and its geometric tolerance at MMC or LMC. It represents the worst-case mating or clearance envelope for the feature.

The boundary generated by the combined effects of a feature's size limit and geometric tolerance at the opposite extreme from virtual condition. It represents the worst-case condition for wall thickness or surface clearance.

A form tolerance that limits how much a surface may deviate from a perfect plane. The tolerance zone is two parallel planes within which all surface points must lie, and no datum is referenced.

A form tolerance controlling how much a line element or axis may deviate from a perfect straight line. Applied to a feature of size, it may use a cylindrical zone and can control the derived median line.

A form tolerance limiting radial variation of a surface of revolution at each cross-section. The tolerance zone is two concentric circles, and no datum reference applies.

A form tolerance that simultaneously controls roundness, straightness, and taper of a cylindrical surface. All points of the surface must fall between two coaxial cylinders separated by the tolerance.

An orientation tolerance requiring a surface, axis, or center plane to be at 90 degrees to a referenced datum. The tolerance zone is oriented exactly perpendicular to the datum.

An orientation tolerance requiring a surface, axis, or center plane to be equidistant from a referenced datum. The tolerance zone is oriented exactly parallel to the datum.

An orientation tolerance requiring a surface, axis, or center plane to be at a specified basic angle to a referenced datum. The tolerance zone is oriented at that exact angle.

A location tolerance that controls the location of a feature of size or its axis relative to basic dimensions and datums. The tolerance zone is typically a cylinder centered on true position.

The theoretically exact location of a feature, established by basic dimensions from the datum reference frame. It is the center of the position tolerance zone.

A position tolerance stated in two or more segments sharing a single feature control frame. The upper segment controls location and orientation; lower segments control orientation and spacing with relaxed datum constraints.

A profile tolerance controlling the location, orientation, and form of a surface relative to true profile. The zone is two parallel boundaries equally disposed about the true profile unless unequally disposed.

A profile tolerance controlling the form and orientation of individual line elements of a surface. Each cross-sectional line must lie within a two-dimensional tolerance zone about the true profile.

The theoretically exact profile of a surface or line defined by basic dimensions or a CAD model. Profile tolerances are applied as a zone about this ideal geometry.

A profile tolerance in which the tolerance zone is unequally distributed about the true profile, specified by a bilateral value with a modifying symbol or a unilateral value. It shifts the zone toward or away from the material.

A runout tolerance controlling the variation of a surface during one full rotation about a datum axis, measured at individual cross-sections. It detects combined eccentricity and form error at each element.

A runout tolerance controlling the variation of an entire surface during rotation about a datum axis, measured along the full length. It simultaneously controls circularity, coaxiality, and taper.

The region within which a feature's surface, axis, or center plane must lie for the geometric tolerance to be satisfied. Its shape and size depend on the characteristic and modifiers used.

A symbol placed after a tolerance or datum reference to change its interpretation, such as MMC, LMC, free state, tangent plane, or projected tolerance zone. It alters how the tolerance applies.

A tolerance zone extended beyond the feature by a specified basic distance, used for threaded or press-fit holes where mating parts assemble. It ensures the axis stays within limits over the assembly length.