Geometric Dimensioning and Tolerancing: Why ?
Geometric Dimensioning and Tolerancing (GDT) is a method for precisely defining the geometry of mechanical parts. It introduces tools which allow mechanical designers, fabricators, and inspectors to effectively communicate complex geometrical descriptions which are not otherwise able to be described in a defined language.
Figure 1.1
A good example of why GDT is needed is the automobile stamped chassis shown in Figure 1.1. The rear quarter panel must fit snugly in order to allow spot welds and in this competitive business, cosmetic appearance and noise abatement are critical. Without GDT, geometric description of this assembly is difficult if not impossible. Computer modelling of these complex surfaces continues to increase the complexity of interface shapes.
Illustrated in Figure 1.2 is an imprecise sand-cast sewer termination fitting. Its hole pattern must mate with the corresponding pattern of the equally imprecise mating pipe. Flatness is also an issue with these rough-surfaced parts. Inspectors must be able to pass as many parts as possible without sacrificing fit. Tens of thousands of the parts are produced. GDT allows fabricators, inspectors, and assemblers to match covers with equally imprecise pipes.
Figure 1.2
Geometric Dimensioning and Tolerancing is a vast language of which there are many facets. However, what is commonly used is a small subset of the total. This subset is based on concepts which MUST be learned in order to progress further. Without a solid understanding of these fundamentals, one cannot gain a firm grasp of later topics. We will present the most essential (and often misinterpreted) topics in a step-by-step fashion, starting with a simple two-dimensional case. After the 2D case has been understood, the full three-dimensional geometry will be described. We also include common areas of confusion and a reference section, but at this point the primary objective is to explain the fundamentals. Please select "2D DATUMS" from the menu bar to the left to continue.
History of Geometric Dimensioning and Tolerancing
Geometric Dimensioning and Tolerancing symbols have been in use since at least the turn of the century. GDT was especially important during the Second World War in relation to extremely high volume production of Liberty Ships, aircraft, and ground vehicles. The automotive industry, with its high volumes, has also benefited from GDT. The computer industry, in particular mass storage manufacturers, have used GDT extensively to increase their yields of high-volume and low-margin hard disk drives. However, as with most engineering and scientific methodologies, GDT was not rigorously established and documented until later in the twentieth century. The American National Standards Institute publication in 1982 of ANSI Y14.5M-1982 was a turning point in the rigorous, unambiguous standardization of the methodology.
History of Geometric Dimensioning and Tolerancing
Geometric Dimensioning and Tolerancing symbols have been in use since at least the turn of the century. GDT was especially important during the Second World War in relation to extremely high volume production of Liberty Ships, aircraft, and ground vehicles. The automotive industry, with its high volumes, has also benefited from GDT. The computer industry, in particular mass storage manufacturers, have used GDT extensively to increase their yields of high-volume and low-margin hard disk drives. However, as with most engineering and scientific methodologies, GDT was not rigorously established and documented until later in the twentieth century. The American National Standards Institute publication in 1982 of ANSI Y14.5M-1982 was a turning point in the rigorous, unambiguous standardization of the methodology.
GD&T
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.
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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).
Composite Control - Controls the form, location, and orientation of a part feature simultaneously (in a single gage reading).
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Concentricity - The condition where the median points of all diametrically opposed elements of a cylinder (or a surface of revolution) are congruent with the axis of a datum feature.
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Concentricity Control - A geometric tolerance that limits the concentricity error of a part feature.
 Controlled Radius - A radius with no flats or reversals allowed. The symbol for a controlled radius is "CR."
Coordinate Tolerancing - A dimensioning system where a part feature is located (or defined) by means of rectangular dimensions with given tolerances.
Coplanar Datum Features - Two or more datum features that are on the same plane.
Coplanar Surfaces - Two or more surfaces that are on the same plane.
Cylindrical Feature of Size - Contains one feature: the cylindrical surface.
 Cylindricity - A condition of a surface of revolution in which all points of the surface are equidistant from a common axis.
Cylindricity Control - A geometric tolerance that limits the amount of cylindricity error permitted on a part surface.
Datum - A theoretically exact plane, point or axis from which a dimensional measurement is made.
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