GD&T (per ASME Y14.5M-1994) is an international language that is used on engineering drawings to accurately describe the size, form, orientation, and location of part features. It is also a design-dimensioning philosophy that encourages designers to define a part based on how it functions in the final product or assembly.
GD&T is an exact language that enables design engineers to "say what they mean" on a drawing, thus improving product designs and lowering cost. Process engineers and manufacturing use the language to interpret the design intent and to determine the best manufacturing approach. Quality control and inspection use the GD&T language to determine proper set-up and part verification.
By providing company-wide uniformity in the drawing specifications and interpretation, GD&T reduces controversy, guesswork, and assumptions throughout the design, manufacturing and inspection process.
Understanding how to apply and interpret GD&T correctly will help you:
Create clear, concise drawings
Improve product design
Create drawings that reduce controversy, guesswork, and assumptions throughout the manufacturing process
Effectively communicate or interpret design requirements for suppliers and manufacturing
However, because GD&T is such a precise language, it involves a great many symbols and terms. Here is a list of some of the topics involved in geometric dimensioning and tolerancing fundamentals and a short definition of each.
Actual Local Size - The value of any individual distance at any cross section of a feature of size.
Actual Mating Envelope of an External Feature of Size - A similar perfect feature counterpart of the smallest size that can be circumscribed about the feature so that it just contacts the surfaces at the highest points.
Actual Mating Envelope of an Internal Feature of Size - A similar perfect feature counterpart of the largest size that can be inscribed within the feature so that it just contacts the surfaces at their highest points.
All-Around Symbol - A circle placed on the bend of the leader line of a profile control.
Angularity - The condition of a surface, centerplane or axis being exactly at a specified angle
Angularity Control - A geometric tolerance that limits the amount a surface, axis, or centerplane is permitted to vary from its specified angle.
ASME Y14.5M-1994 - The national standard for dimensioning and tolerancing in the United States. ASME stands for American Society of Mechanical Engineers. The Y14.5 is the standard number. "M" is to indicate the standard is metric, and 1994 is the date the standard was officially approved.
Axis Theory - The axis (or centerplane) of a feature of size must be within the tolerance zone.
Basic Dimension - A numerical value used to describe the theoretically exact size, true profile, orientation, or location of a feature or datum target.
Between Symbol - A double ended arrow that indicates the tolerance zone extends to include multiple surfaces.
Bi-Directional Control - Where the location of a hole is controlled to a different tolerance value in two directions.
Bilateral Tolerance - A tolerance that allows the dimension to vary in both the plus and minus directions.
Bonus Tolerance - An additional tolerance for a geometric control. Whenever a geometric tolerance is applied to a feature of size, and it contains an MMC (or LMC) modifier in the tolerance portion of the feature control frame, a bonus tolerance is permissible.
Boundary - The word "BOUNDARY" is placed beneath the feature control frames to invoke a boundary control.
Cartoon Gage - A sketch of a functional gage. A cartoon gage defines the same part limits that a functional gage would, but it does not represent the actual gage construction of a functional gage.
Circularity - A condition where all points of a surface of revolution, at any section perpendicular to a common axis, are equidistant from that axis.
Circularity Control - A geometric tolerance that limits the amount of circularity on a part surface.
Circular Runout - A composite control that affects the form, orientation, and location of circular elements of a part feature relative to a datum axis.
Circular Runout Control - A geometric tolerance that limits the amount of circular runout of a part surface.
Coaxial Datum Features - When coaxial diameters are used to establish a datum axis.
Coaxial Diameters - Two (or more) diameters that are shown on the drawing as being on the same centerline (axis).
Feature Control Frame - A rectangular box that is divided into compartments within which the geometric characteristic symbol, tolerance value, modifiers, and datum references are placed.
Feature of Size - One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.
Feature of Size Dimension - A dimension that is associated with a feature of size.
Fixed Fastener Assembly - Where the fastener is held in place (restrained) into one of the components of the assembly.
Fixed Fastener Formula - H = F + 2T or T = Where: T = position tolerance diameter H = MMC of the clearance hole F = MMC of the fastener
Flatness - The condition of a surface having all of its elements in one plane.
Flatness Control - A geometric tolerance that limits the amount of flatness error a surface is allowed.
Floating Fastener Assembly - Where two (or more) components are held together with fasteners (such as bolts and nuts), and both components have clearance holes for the fasteners.
Floating Fastener Formula - T = H - F Where: T = position tolerance diameter (for each part) H = MMC of the clearance hole F= MMC of the fastener
Functional Dimensioning - A dimensioning philosophy that defines a part based on how it functions in the final product.
Functional Gage - A gage that verifies functional requirements of part features as defined by the geometric tolerances.
Fundamental Dimensioning Rules - A set of general rules defined by ASME for dimensioning and interpreting drawings.
Geometric Characteristic Symbols (14) - The symbols are divided into five categories: form, profile, orientation, location, and runout.
Geometric Dimensioning and Tolerancing (GD&T) - A set of fourteen symbols used in the language of GD&T. It consists of well-defined of symbols, rules, definitions and conventions, used on engineering drawings to accurately describe a part. GD&T is a precise mathematical language that can be used to describe the size, form, orientation, and location of part features. GD&T is also a design philosophy on how to design and dimension parts.
Go Gage - A gage that is intended to fit into (for an internal feature of size) or fit over (for an external feature of size) the feature of size.
Great Myth of GD&T - The misconception that geometric tolerancing raises product costs.
Implied Basic 90° Angles -A 90° basic angle applies where centerlines of features in a pattern (or surfaces shown at right angles on a drawing) are located and defined by basic dimensions, and no angle is specified.
Implied Basic Zero Dimension - Where a centerline or centerplane of a feature of size is shown in line with a datum axis or centerplane, the distance between the centerlines or centerplanes is an implied basic zero.
Implied Datum - An assumed plane, axis, or point from which a dimensional measurement is made.
Inclined Datum Feature - A datum feature that is at an angle other than 90o, relative to the other datum features.
Inner Boundary (IB) - A worst-case boundary generated by the smallest feature of size minus the stated geometric tolerance (and any additional tolerance, if applicable).
International Standards Organization (ISO) - The organization that published an associated series of standards on dimensioning and tolerancing.
Least Material Condition - The condition in which a feature of size contains the least amount of material everywhere within the stated limits of size.
Limit Tolerance - When a dimension has its high and low limits stated. In a limit tolerance, the high value is placed on top, and the low value is placed on the bottom.
Maximum Material Condition - The condition in which a feature of size contains the maximum amount of material everywhere within the stated limits of size.
Median Point - The mid-point of a two-point measurement.
Modifiers (8) - Communicate additional information about the drawing or tolerancing of a part.
Multiple Single-Segment Profile Control - When two or more profile controls are tolerancing a surface relative to different datums.
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Multiple Single-Segment Tolerance of Position Control - When two (or more) single segment tolerance of position callouts are used to define the location, spacing, and orientation of a pattern of feature of size.
No-Go Gage - A gage that is not intended to fit into or over a feature of size. A No-Go gage is made to the LMC limit of the feature of size.
Non-Feature of Size Dimension - A dimension that is not associated with a feature of size.
Outer Boundary (OB) - A worst-case boundary generated by the largest feature of size plus the stated geometric tolerance (and any additional tolerance, if applicable).
Parallelism - The condition that results when a surface, axis or centerplane is exactly parallel to a datum.
Parallelism Control - A geometric tolerance that limits the amount a surface, axis, or centerplane is permitted to vary from being parallel to the datum.
Perpendicularity - The condition that results when a surface, axis, or centerplane is exactly 90° to a datum.
Perpendicularity Control - A geometric tolerance that limits the amount a surface, axis, or centerplane is permitted to vary from being perpendicular to the datum.
Planar Datum - The true geometric counterpart of a planar datum feature.
Planar Feature of Size - A feature of size that contains two features: the two parallel plane surfaces.
Plus-minus Tolerance - The nominal or target value of the dimension is given first, followed by a plus-minus expression of a tolerance.
Primary Datum - The first datum plane that the part contacts in a dimensional measurement.
Profile- The outline of a part feature in a given plane.
Profile Control - A geometric tolerance that specifies a uniform boundary along the true profile that the elements of the surface must lie within.
Profile of a Line Control - A geometric tolerance that limits the amount of error for line elements relative to their true profile.
Profile of a Surface Control - A geometric tolerance that limits the amount of error a surface can have relative to its true profile.
Projected Tolerance Zone - A tolerance zone that is projected above the part surface.
Radius - A straight line extending from the center of an arc or circle to its surface.
Regardless of Feature Size - The term that indicates a geometric tolerance applies at any increment of size of the feature, within its size tolerance.
Rule #1 - Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its form--as well as in its size--are allowed.
Rule #2 - RFS applies, with respect to the individual tolerance, datum reference, or both, where no modifying symbol is specified.
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Unequal Bilateral Tolerance - A tolerance where the allowable variation is from the target value, and the variation is not the same in both directions.
Unilateral Tolerance - A tolerance where the allowable variation from the target value is all in one direction and zero in the other direction.
Variable Gage - A gage capable of providing a numerical reading of a part parameter.
Virtual Condition (VC) - A worst-case boundary generated by the collective effects of a feature of size specified at MMC or at LMC and the geometric tolerance for that material condition.
Virtual Condition Boundary Theory - A theoretical boundary limits the location of the surfaces of a feature of size.
Worst-case Boundary (WCB) - A general term to refer to the extreme boundary of a feature of size that is the worst-case for assembly. Depending upon the part dimensioning, a worst-case boundary can be a virtual condition, inner boundary, or outer boundary.
Zero Tolerance at MMC - A method of tolerancing part features that includes the tolerance geometric value with the feature of size tolerance and states a zero at MMC in the feature control frame.
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