The following is a list of definitions for the most common terms used in GD&T. A more extensive glossary is distributed to each participant in our seminars.
Angularity — The geometric tolerance used to control a surface, axis, or center plane that is designed to be at a specified angle from a given datum.
ASME Y14.5-2018 — The current standard for dimensioning and tolerancing in North America. It replaces the previous 2009 edition, and features many changes, although many are minor changes (but notable is the deletion of concentricity and symmetry). It is available in print book or PDF format.
Basic dimension — A number, traditionally shown within a box, used to describe the theoretically exact size, angle, location, or profile of a feature. Therefore it gets no direct tolerance. It is also used to locate datum targets. On some prints, the basic dimensions are not shown, because they are contained in the “math model,” or the perfect part as defined by the CAD data.
Bonus tolerance — An additional tolerance beyond what is given in the feature control frame. This is allowed if the tolerance number is followed by the MMC (or LMC) modifier. The amount of bonus tolerance is determined from the difference between a feature’s actual size and the MMC (or LMC).
Circularity — A geometric tolerance used to control all points on a surface, at any perpendicular cross-section, to be equidistant from the axis. Sometimes referred to as roundness.
Circular runout — A geometric control that limits that amount that a circular cross-seciton can vary as it rotates around a given datum axis (which must be different from its own axis). Therefore, it controls circularity, concentricity, and orientation. The error is sometimes described as TIR.
Composite feature control frame — Sometimes used for profile or position controls, a feature control frame that displays one symbol centered between two or three tolerance specifications. Depending on the datum references, it can mean something different than a multiple single-segment control.
Concentricity — A geometric tolerance used to control the median points of all diametrically opposed elements of a circular feature, in order to be congruent with a datum axis. It is commonly misused; usually runout or position can suffice. It was deleted in the newest ASME revision (2018).
Cylindricity — A geometric tolerance used to control a surface of revolution (inside or outside diameter) where all points are to be equidistant from a common axis. The cylindricity tolerance will control circularity, straightness, and taper.
Datum — A theoretically exact point, axis, line, or plane derived by contact a datum feature; it then becomes the origin from which geometric characteristics are measured.
Datum feature — The actual feature or surface of the part that is used to establish the datum.
Datum target — A specific point, line, or area on a datum feature that is used to establish a datum (rather than using the full feature to establish the datum).
Feature control frame — The rectangular box that is used to display a geometric tolerance. It is divided into compartments which identify the geometric characteristic symbol, the tolerance value, and any datum references.
Flatness — A geometric tolerance used to control a surface which is designed to have all elements in the same plane. Flatness allows variation within two imaginary planes, separated by the given tolerance amount. With the 2009 standard, it may also be used on a centerplane of a feature of size.
Geometric dimensioning and tolerancing — Affectionately called GD&T, it is a symbolic system of controlling dimensional variation. In addition to providing standardized symbols and rules, it is a design philosophy that ensures that tolerances are based on fit and function.
Least material boundary (LMB) — The limit on a datum feature created by the combined effect of MMC size and any geometric tolerance on that datum feature. It is imposed by the circled L symbol after a datum reference letter.
Least material condition — A number that represents the condition where a feature of size has the least amount of material, while still being within the given size limits. Thus, LMC would be the smallest pin (for an external feature) or the largest hole (for an internal feature).
Maximum material boundary — Often abbreviated MMB, the limit on a datum feature created by the combined effect of MMC size and any geometric tolerance on that datum feature. It is imposed by the circled M symbol after a datum reference letter.
Maximum material condition — Often abbreviated as MMC, the size of a feature of size when it contains the most material allowed (while still being within the acceptable size limits).
Parallelism — The geometric tolerance used to control a surface, axis, or center plane that is designed to be parallel (zero degrees) in relation to a datum.
Perpendicularity — The geometric tolerance used to control a surface, axis, or center plane that is designed to be 90º to a datum.
Position — A geometric tolerance that controls the location of a feature of size. It defines a zone around the perfect location within which the axis or center plane of the feature is permitted to vary.
Profile of a line — A geometric tolerance used to control line elements (taken at any cross-section in the plane of the given view) as they relate to the true profile. It may or may not reference datums.
Profile of a surface — A geometric tolerance used to control a three-dimensional surface as compared to the true profile. It may or may not reference datums.
Projected tolerance zone — A concept sometimes used in GD&T to indicate that a tolerance zone does not exist within the part itself, but is projected above the part surface. This is usually done to ensure that a fastener will assemble through multiple parts without interference.
Regardless of feature size — Abbreviated RFS, it is a term used to indicate that a geometric tolerance is the same, no matter what size the feature is made at. It is the presumed condition, unless the symbol for MMC or LMC is shown.
Regardless of material boundary (RMB) — Abbreviated RMB, it is a term used to indicate that a geometric tolerance is not affected by any variation in the datum feature(s). This requires a simulator to be variable, in order to accommodate the varying conditions on that datum feature. It is the presumed condition, unless the symbol for MMB or LMB is shown.
Resultant condition — A number, similar to virtual condition, that represents the worst-case combination of size and geometric tolerance. However, instead of representing the mating size, this shows how much material may be affected.
Rule #1 — The rule defined in the ASME standard stating that a size dimension (for a feature of size on a rigid part) not only controls the size, but also the form.
Rule #2 —The rule in the ASME standard stating that all geometric tolerances are assumed to be RFS unless the modifier for MMC or LMC is shown. This applies to the tolerance value and any datum references that are features of size.
Runout — The category of geometric tolerancing that controls a surface of revolution as compared to a datum axis other than its own. It can be used to control circularity, “wobble,” taper, and concentricity. The two types of runout are circular runout and total runout.
Shift tolerance — Also called datum shift, this refers to the looseness or “play” that is sometimes allowed on a datum feature. Similar to bonus tolerance, it is the result of using a modifier (usually MMC) after the datum reference in a feature control frame.
Straightness — The geometric tolerance applied to an axis or surface element which is designed to be perfectly straight. It may be applied to a surface (individual elements) or a feature of size (axis or centerplane).
Symmetry — The geometric tolerance used to control the median points of all opposed elements of two or more feature surfaces to be congruent with a datum axis or plane. It is commonly misused; usually position will suffice. It was deleted in the newest ASME revision (2018).
Tolerance zone — The imaginary area or region that the actual surface, axis, or centerplane is allowed to vary within. A tolerance zone can be cylindrical (if a diameter symbol is given) or two parallel planes.
Total runout — A geometric control that limits that amount that a cylindrical surface can vary as it rotates around a given datum axis (which must be different from its own axis). Therefore, it controls circularity, cylindricity, concentricity, and parallelism. It is different from circular runout in that it is three-dimensional, rather than 2-D cross-sectional, controlling all cross-sections of a surface simultaneously.
Virtual condition — A number that represents the worst-case mating size for an individual feature. It represents the combined effect of a feature’s size tolerance and the geometric tolerance.