ASME GDTP Technologist key terms

A rectangular frame on a drawing containing the geometric characteristic symbol, the tolerance value with any material condition modifier, and the datum references; it states exactly how much variation is permitted for the controlled feature.

A theoretically exact value enclosed in a rectangular frame that carries no tolerance of its own; it is used to locate tolerance zones, datums, and true profiles, with variation controlled by the associated geometric tolerance.

The default Y14.5 rule stating that the surface of a regular feature of size may not extend beyond its actual mating envelope, so perfect form is required at maximum material condition and form error may grow as size departs.

The default condition, shown by the absence of a material condition modifier, in which a geometric tolerance applies equally no matter the feature's actual size; no bonus tolerance is gained.

The region, defined by its shape, size, orientation, and location, within which the toleranced surface, axis, or center plane must lie for the part to be accepted.

A theoretically exact point, axis, or plane derived from a datum feature, from which the location and orientation of tolerance zones and dimensions are established.

An actual feature of the part, such as a face, hole, or slot, used to establish a datum; because real features are imperfect, the datum is derived from a simulator contacting the feature.

Three mutually perpendicular planes established from the primary, secondary, and tertiary datums that lock all six degrees of freedom and act as the origin for dimensions and tolerance zones.

A surface of adequately perfect form, such as a surface plate, expanding mandrel, or chuck, that contacts the datum feature to establish the datum and represents the theoretically exact counterpart.

The first datum listed in a feature control frame; it constrains three degrees of freedom and sets the orientation of the datum reference frame before the secondary and tertiary datums are applied.

The second datum listed; applied after the primary, it constrains two more degrees of freedom, and its simulator must be oriented exactly perpendicular to the primary datum.

The third datum listed in a feature control frame; it constrains the last remaining degree of freedom and completes the datum reference frame, fixing the part fully in space.

Movement of the part relative to the datum feature simulator that is permitted when a datum feature of size is referenced at MMC or LMC, providing extra tolerance beyond the stated value.

A specified point, line, or area on a datum feature, located by basic dimensions, at which the simulator contacts the part; it is used when an entire surface is unsuitable for establishing a datum.

A form tolerance that requires an entire surface to lie between two parallel planes separated by the tolerance value; it is independent of any datum and controls only the surface's own deviation.

A form tolerance limiting how far a line element or an axis may deviate from a straight line; applied to a size feature it controls the derived median line within a cylindrical or parallel-plane zone.

A form tolerance for surfaces of revolution; every cross-section perpendicular to the axis must lie between two concentric circles whose radial distance apart equals the tolerance value.

A form tolerance requiring the whole cylindrical surface to lie between two coaxial cylinders of perfect form separated by the tolerance value, simultaneously controlling roundness, straightness, and taper.

An orientation tolerance controlling how far a surface, axis, or center plane may deviate from an exact 90-degree relationship to a datum, with the tolerance zone oriented exactly perpendicular to that datum.

An orientation tolerance controlling deviation from an exact 0-degree or 180-degree relationship to a datum; the tolerance zone is oriented exactly parallel to the referenced datum.

An orientation tolerance controlling a feature relative to a datum at a basic angle other than 90 degrees, with the tolerance zone oriented at that exact angle and located by basic dimensions.

A location tolerance that establishes a zone, usually a cylinder or two parallel planes, centered on the true position, within which the feature's axis, center plane, or surface must lie.

The theoretically exact location of a feature's axis, center plane, or point, defined by basic dimensions from the datum reference frame; every position tolerance zone is centered on it.

Extra tolerance equal to the amount by which a feature of size departs from its maximum or least material condition, available only when the geometric tolerance is referenced at MMC or LMC.

A location tolerance requiring the median points of diametrically opposed surface elements of a feature of revolution to lie within a cylindrical zone centered on the datum axis; it controls balance, not size or form.

A location tolerance requiring the median points of opposed elements of a non-cylindrical feature to lie within two parallel planes equally disposed about the datum center plane.

A profile tolerance controlling individual line elements of a surface; the zone is two parallel lines offset from the true profile, and datums may be referenced to locate and orient it.

A profile tolerance controlling a three-dimensional surface; the zone is the space between two surfaces offset from the true profile, and datum references may fix its location and orientation.

A profile tolerance zone split equally on both sides of the true profile; unless the drawing shows otherwise, the stated tolerance is divided evenly, half outside and half inside the profile.

A profile tolerance zone placed entirely on one side of the true profile, indicated by unequal distribution values or a phantom line showing which side of the profile the zone occupies.

A runout tolerance checked on individual circular elements; as the part is rotated a full turn, the total indicator reading at each element must not exceed the stated tolerance.

A runout tolerance applied to the whole surface at once; while the part rotates, the indicator traverses the full length and every reading must stay within the stated tolerance, also controlling taper and coaxiality.

Two or more stacked segments in a single feature control frame: the upper segment locates the pattern to datums, while lower segments control feature-to-feature spacing and orientation with tighter tolerances.

The upper segment of a composite feature control frame; it locates the entire pattern of features relative to the full datum reference frame using the larger stated tolerance.

A lower segment of a composite feature control frame; it controls spacing, orientation, and location among features within the pattern using a tighter tolerance and usually fewer datum references.

The constant boundary generated by combining the size of a feature at MMC or LMC with its geometric tolerance; mating features must clear this boundary if the parts are to assemble.

The condition where a feature of size contains the maximum amount of material: the smallest hole or the largest shaft, leaving the least clearance for a mating part.

The condition where a feature of size contains the minimum amount of material: the largest hole or the smallest shaft, leaving the most clearance and the thinnest remaining wall.