A Miniature GD&T Glossary

We have been urged to interrupt our deliberations on GD&T encoding and decoding with a review of the many terms we’ve been bandying about. Try them on, play with them, share them, try to improve their definitions, complain about the missing ones, push us for clarification, entertain your family and friends.

As soon becomes evident, GD&T is a word-rich subject. It must be, because it deals with two rather complex worlds—the perfect imaginary world of GD&T itself, and the imperfect real world of actual parts, and their associated manufacturing, inspection, and assembly processes. People who sit through GD&T courses often suffer severe migraines at the end of the day and return to class haggard from sleepless, geo-dream-infested nights.

If one is to succeed with GD&T—i.e., benefit from its power to refine designs, maximize fault tolerance, guarantee assemblability, minimize manufacturing costs, and turn metrology from a confused set of tribal understandings into a science—clear thinking and clear communication are essential, both of which depend on adequate sets of terms supported by crystalline definitions that full-time players must master. Here we go.

Actual Value: the actual value of a geometric characteristic is the size of the smallest associated tolerance zone that just contains the controlled component of the Considered Feature. Examples: 1) The actual value of the Flatness (the Actual Flatness) of a planar surface is the minimum thickness, orientation, and location-unconstrained, slab-like zone that just contains all the points on its surface. 2) The actual value of the Position (the Actual Position) of a bore is the diameter of the smallest location-constrained cylindrical zone that just contains the entire, bounded axis of the actual feature. Exception: Because Surface Profile tolerance zones naturally straddle the Basic surface of the considered feature, the Actual Value of the Surface Profile is best reported broken down into two components, namely the generally positive in-space Actual Value, and the generally negative in-material Actual Value. See also “Measured Value.”

BASIC Dimension: basic angular or linear dimensions serve to orient and locate tolerance zones, but only those tolerance zones that can be oriented or located. Size and form tolerance zones can be neither oriented nor located. Orientation tolerance zones can only be oriented. Location tolerance zones can be both oriented and located. See also “Nominal Dimension” and “Reference Dimension.”

Bilateral Tolerance: a pair of equal or unequal numerical values that, when added to a nominal dimension, specify permissible upper and lower limits for a variable. All Surface Profile tolerance zones are equal bilateral by default. See also “Unilateral Tolerance.”

Cartesian Coordinate System: a collection of three perfectly straight, mutually perpendicular lines, called axes, that meet in a point, called the origin, that form three perfectly flat, mutually perpendicular planes, called bases planes, and that are outfitted with linear scales. Cartesian coordinate systems have three degrees of rotational freedom—called pitch, yaw, and roll, and three degrees of translational freedom, called Tx, Ty, and Tz. Cartesian coordinate systems provide the framework for specifying the orientations and locations of tolerance zones. Where functionally more appropriate, Cartesian coordinate systems can be converted into cylindrical and spherical coordinate systems. See also: “Datum Reference Frame.”

Considered Feature: a feature currently under consideration, for example being toleranced.

Datums: 1) What are they? Datums are perfect, imaginary reference points, lines, and planes. 2) Where do they come from? Datums are extracted from Datum Feature Simulators and represent the minimum set of a single plane, and/or a single co-planar axis, and/or a single co-axial center point, that together fully characterize the simulator’s orientation and location. 3) What are they for? Datums constrain the rotational and translational degrees of freedom of a starter coordinate system and turn it into a Datum Reference Frame.

Datum Features: 1) What are they? Datum Features are specially labeled, physical surfaces of real objects. 2) What are they for? Datum Features serve to constrain an object’s degrees of rotational and translational freedom during manufacturing, inspection and assembly operations.

Datum Feature Simulators: 1) What are they? Datum Feature Simulators are almost perfect, potentially location- and/or orientation-constrained, inverse Datum Features. 2) What are they for? Datum Feature Simulators serve as a bridge from the imperfect real world of Datum Features to the perfect, imaginary world of Datums and Datum Reference Frames. It is from Datum Feature Simulators that we extract Datums, it is in Datum Feature Simulators that we establish Datum Reference Frames, and it is with Datum Feature Simulators that we transfer Datum Reference Frames to actual parts. Datum Feature Simulators are the physical representations of the True Geometric Counterparts of Datum Features. Examples: Gauge pins, granite surface plates, collets, machinist vises, manufacturing fixtures. See also “True Geometric Counterpart.”

Datum Reference Frames: 1) What are they? Datum Reference Frames are Cartesian coordinate systems established in real parts. 2) What are they for? Datum Reference Frames serve to orient and locate tolerance zones in real parts. 3) How are they established? Datum Reference Frames are established in a six-step process using the Datums extracted from a set of Datum Feature Simulators to constrain a starter coordinate system.

Deviation from Nominal: the numerical difference between the measured and the nominal value of a characteristic. See also “Actual Value” and “Measured Value.”

Deviation from Tolerance: the numerical difference between the measured value and the upper or lower limit of a characteristic that is out of tolerance. See also “Actual Value” and “Measured Value.”

Effective Tolerance: the sum of the specified tolerance and an authorized, Tolerance Zone Size bonus. See also “Specified Tolerance” and “Tolerance Zone Size Bonus.”

Feature: a collection of associated points that form a continuous surface separating solid matter from free space, and are bounded by other similar constructs. Examples: 1) the surface of bore bounded by the opposed planar surfaces of a slab, 2) one surface of a propeller blade.

Feature Component: any physical or conceptual geometric entity associated with a Feature, such as the straight and circular surface lines, the axis and the median line, or all the points on the surface of a cylinder.Feature Control Frame: a rectangular frame consisting of up to three major compartments, the first for specifying a geometry control tool, the second for specifying the shape and size of a tolerance zone along with appropriate tolerance zone size and other modifiers, and the third for listing the datum Features responsible for establishing the required Datum Reference Frame.

Feature of Size (Type I): a collection of associated points that are nominally equidistant 1) from a center point (forming a sphere), 2) from a straight line (forming a cylinder) or 3) from a plane (forming a slab or a slot), and whose associated normal vectors are exclusively oriented toward or away from said point, line or plane.

Feature of Size (Type II): a collection of associated points that are nominally equidistant 1) from a compound curved line, or 2) from a compound curved surface, and whose associated normal vectors are exclusively oriented toward or away from said line or surface.

Geometric Characteristic: a concept characterizing the size, form, orientation or location of a Feature or of a component of a feature. Examples: Diameter, Flatness, Parallelism, or Position. See also “Geometry Control Tool.”

Geometric Entity: any imaginary or real, two or three dimensional geometric construct, thus a point, straight or curved line, plane or curved surface.

Geometry Control Tool: a conceptual tool for imposing limits on the imperfection in the size, form, orientation or location of a feature of a physical object. Examples: Position, Cylindricity, Circular Runout. See also “Geometric Characteristic.”

In-Material Boundary: the boundary of the region inside the surface of a feature, beyond which one can be guaranteed to find material. There are Virtual and Actual in-material boundaries, which can be either unconstrained, orientation-constrained, or orientation- and location-constrained. Virtual in-material Boundaries are also referred to as Virtual Least Material Boundaries. See also “In-Space Boundary.”

In-Space Boundary: the boundary of the region outside the surface of a feature, beyond which one can be guaranteed to find no material. There are Virtual and Actual in-space boundaries, which can be either unconstrained, orientation-constrained or orientation- and location-constrained. Virtual in-space Boundaries are also referred to as Virtual Maximum Material Boundaries. See also “In-Material Boundary.”

(L): the “Least Material Condition” modifier. 1) When placed behind the tolerance value in a Feature Control Frame (L) serves as a Tolerance Zone Size modifier, and expands the tolerance zone by the difference between the Unconstrained in-material Actual Mating Size of the Considered Feature and its LMC size, as the feature departs from LMC toward MMC. 2) When placed behind a Datum Feature label in a Feature Control Frame, (L) serves as a Tolerance Zone Mobility modifier and forecasts potential Datum Reference Frame (L)ability (instablity).

Least Material Boundary: see “In-Material Boundary.”

LMC: “Least Material Condition”, the condition of a feature in which it contains the least material allowed by limits on its size or location.

LMC Envelope: the form-perfect in-space boundary of a feature at its “Least Material Condition”, as imposed by the Envelope Rule in the presence of the Tolerance Zone Size modifier (L).

Lower Tolerance Limit: the sum of a nominal value and its lower tolerance.

(M): the “Maximum Material Condition” modifier. 1) When placed behind the tolerance value in a Feature Control Frame (M) serves as a Tolerance Zone Size modifier, and expands the tolerance zone by the difference between the Unconstrained in-space Actual Mating Size of the Considered Feature and its MMC size, as the feature departs from MMC toward LMC. 2) When placed behind a Datum Feature label in a Feature Control Frame, (M) serves as a Tolerance Zone Mobility modifier and forecasts potential Datum Reference Frame (M)obility.

Material Condition: a measure of the amount of material associated with an object as the size or location of the surface of a specific feature varies between its upper and lower limits known as the Maximum and Least Material Condition.

Maximum Material Boundary: See “In-Space Boundary.”

Measured Value: an approximation of the Actual Value of a geometric characteristic, limited by the uncertainty of a measurement process.

MMC: “Maximum Material Condition”, the condition of a feature in which it contains the most material allowed by limits on its size or location.

MMC Envelope: the form-perfect, in-space boundary of a feature at its “Maximum Material Condition”, as imposed by the Envelope Rule in the presence of the Tolerance Zone Size modifiers (S) or (M).

Nominal Dimension: a numerical value that, together with upper and lower tolerances, specifies the permissible upper and lower limits of a dimension. Toleranced nominal dimensions should only be used in conjunction with size tools, including the Radius and Spherical Radius tools, with the depth tool and with the Dimension Origin tool. See also “Basic Dimension” and “Reference Dimension.”

Reference Dimension: an untoleranced dimension placed inside parentheses that serves as a general reminder of scale. See also “Basic Dimension” and “Nominal Dimension.”

Resultant Condition: an ASME Y14.5M 1994 concept of a variable In-Material Virtual Boundary. See also In-Material Boundary.

RFS: “Regardless of Feature Size”, a statement that the size of a Considered Feature shall have no influence on the size of an associated tolerance zone, or, that the size of a Datum Feature shall have no influence on the mobility of an associated tolerance zone. See also “(S) = the Regardless-of-Feature-Size modifier”

(S): the “Regardless of Feature Size” modifier. 1) When placed behind the tolerance in a Feature Control Frame (S) serves as a Tolerance Zone Size modifier and requires the tolerance value to be (S)tuck at the stated value regardless of departures from its MMC or LMC. 2) When placed behind a Datum Feature label in a Feature Control Frame, (S) serves as a Tolerance Zone Mobility modifier and requires the associated Datum Reference Frame to be (S)table relative to the specified Datum Feature, regardless of its size. Note: The ASME Y14.5M 1994 standard makes the RFS modifier (S) the default in the absence of an explicit (M) or (L) modifier, but still allows its explicit use.

Specified Tolerance: the tolerance specified in a Feature Control Frame. See also “effective tolerance” and “Tolerance Zone Size Bonus.”

Third Angle Projection: a view of the side of a part “folded up” from an adjacent view of the part.

Tolerance: the numerical size of a tolerance zone.

Tolerance Bandwidth: the difference between the upper and lower limits of a variable, thus the “size” of a Tolerance Zone.

Tolerance Zone: a bounded region of space within which a specific component of a feature must lie, and which can be unconstrained or oriented and located by Basic dimensions.

Tolerance Zone Size Bonus: the additional tolerance authorized by a Tolerance Zone Size modifier (M) or (L), and generated by a deviation of the Actual in-space or Actual in-material Mating Size of the Considered Feature from its size and form constrained Maximum Material (M) or Least Material (L) Boundary. See also “Effective Tolerance” and “Specified Tolerance.”

True Geometric Counterpart: The perfect equivalent of a Datum or Considered Feature Simulator, thus the perfect inverse of a size, form, location and/or orientation constrained feature. In the case of a merely size and form constrained feature associated with the Tolerance Zone Size modifiers (S) or (M), there are merely in-space True Geometric Counterparts. In the case of a merely size and form constrained feature associated with the Tolerance Zone Size modifier (L), there are merely in-material True Geometric Counterparts.

Unilateral Tolerance: one of a pair of tolerance values associated with a nominal dimension, of which the other is zero, therefore permitting deviations from nominal in one direction only. Also an explicitly unidirectional tolerance imposed by the Surface Profile tool. See also “Bilateral Tolerance.”

Upper Tolerance Limit: the sum of a nominal value and its upper tolerance.

Future articles
In our next two workshops we’ll return to our investigation of the effect of GD&T on manufacturing and coordinate metrology, and then share some GD&T humor to stitch it all together!

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