Geometric Dimensioning and Tolerancing (GD&T)

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GD&T Replaced Conventional Linear Tolerancing

Realizing concepts into useful products is a concern of design engineers, and this can not be accomplished without visual aids. These visual aids include sketches, drawings, models, and any other tool or method considered useful to help the audience and customers to have a comprehensive understanding of future products during design and development, production, and use. Whatever visual aid is used should have the same interpretation in the mind of all proposed users of the design engineer’s work. These drawings play a major role since they contain quantitative and descriptive figures known as dimensions and tolerances to communicate fit, form, and function of the product and it’s components to manufacturing and inspecting people. Geometric Dimensioning and Tolerancing (GD&T) is the symbolic language applied in this prospect.

Back in the “over-the-wall” age, the different parties engaged in product development had their own views and interpretations. The type of linear tolerancing considered state of the art was whatever got their job done easier and faster. In those days product engineer defined dimensions and tolerances with inadequate communication with manufacturing and inspecting people, and these people following after had to come up with these tolerances. Manufacturing engineers were concerned with process capability and ease of assembly, while inspections/quality control people were concerned with capabilities of measurement methods and equipment. In cases of disputes, engineering change orders increased cycle time and product costs.

During the 70s and 80s, competing with Japanese products, along with the oil crises, led western industries to a new business approach based on quality rather than solely on financial results. Quality management systems based on the ISO9000 family of standards and quality awards based on business excellence models such as the Malcolm Baldrige National Quality Award (MBNQA) and the European Foundation for Quality Management (EFQM) Excellence Model were actually based on the Total Quality Management (TQM) “doing the right thing right” approach.

The effect of this approach on engineering design, especially in the fields of mechanical engineering and manufacturing, was the application of Geometric Dimensioning and Tolerancing to replace conventional linear tolerancing. Linear tolerancing was based on limits of size and thus the transmission of orientation or location relationship between the features of a part was not possible.

Geometric and Descriptive Concepts

GD&T, by giving a clear definition of geometric concepts classified as orientation, location, form, profile, and runout, and descriptive concepts classified as modifiers along with appropriate symbols for each concept and feature- and a rational approach to apply them- enabled different parties engaged in a dimensional management process to have a common language and consequently common understanding of fit, form, and function of the product and it’s components.

Orientation concepts are Angularity, Parallelism, and Perpendicularity.

Location concepts are Concentricity, Position, and Symmetry.

Form concepts are Cylindricity, Flatness, and Straightness.

Profile concepts are Profile and Profile of a Line.

Runout concepts are Runout and Total Runout.

Modifying concepts are Maximum Material Condition (MMC), Least Material Condition (LMC), Projected Tolerance Zone, Free State, Tangent Plan, Diameter, Spherical Diameter, Radius, Spherical Radius, Controlled Radius, Reference, Arc length, Statistical Tolerance, and Between.

The basic idea behind GD&T is first determining datum features, features (physical portions of a part like surfaces, holes, or slot) selected as an origin for locating parts on datum simulators (such as machine tool tables, surface plates, chucks, fixtures, and jigs) for manufacturing and measuring purposes. This way the designer should keep in mind the manufacturing process and measuring procedure during the design process.

The second step in GD&T practice is defining nominal distance and orientation (angular relationship) of other features according to determined datum features.

The third step is specifying tolerance zones and/or boundaries and conformance conditions for considered features. Tolerance zones are virtual geometric boundaries corresponded to controlled features shape defined by a) two parallel planes, for width type features like plane or profiled surface features, b) a cylindrical plane or two coaxial cylindrical planes with radius or radial distance equal to specified tolerance, for axes and cylindrical type features, c) a spherical plane with radius equal to specified tolerance, for spherical type features like outer surface of a ball. This way designer should be well aware of manufacturing process capability, i.e. design for manufacturability (DFM).

The fourth and last step is regarding tolerance interactions between parts that are supposed to be assembled in the assembly process. This will lead the designer to take different aspects of assembly like ease of assembly and cost into consideration while defining tolerances, i.e. design for assembly (DFA).

Successful application of GD&T calls for concurrent design and engineering teams consisting of representatives from responsible functions like design, manufacturing (process and tooling), quality, and any other function like purchasing and sale as appropriate.

Detailed explanation of GD&T definition and rules is beyond scope of this article. A reader can find available standards, literature, and internet resources for more information on this subject.

Resources consulted while preparing this article were:

· ASME Y14.5M-1994 Dimensioning and Tolerancing

· Paul J. Drake, Jr., ed. Dimensioning and Tolerancing Handbook. New York: McGraw-Hill, 1999.