Copyright Goodheart-Willcox Co., Inc. 261 Geometric Dimensioning and Tolerancing UNIT 15 Learning Objectives After studying this unit, you will be able to: ■ ■ Describe the purpose of geometric dimensioning and tolerancing (GD&T). ■ ■■ Define various applications of geometric tolerances. ■■ Identify various symbols used in GD&T. ■■ Define various terms relating to GD&T. ■■ Identify datum features. ■ Interpret the meaning of material condition and material boundary modifiers on a print. ■ ■ Describe the requirements established by form, orientation, position, profile, and runout tolerances. ■ ■ Interpret the meaning of GD&T symbols on a print. Key Terms angularity basic dimension circularity circular runout tolerance cylindricity datum datum feature datum feature simulator datum feature symbol datum target datum target symbol feature feature control frame feature of size flatness form tolerance geometric dimensioning and tolerancing (GD&T) irregular feature of size least material condition (LMC) material boundary modifier material condition modifier maximum material condition (MMC) orientation orientation tolerance parallelism perpendicularity position tolerance profile of a line tolerance profile of a surface tolerance profile tolerance regardless of feature size (RFS) regular feature of size runout runout tolerance straightness tolerance total runout tolerance true position Agreater dvancements in technology have brought about control in the accuracy of machined parts. Still, it is almost impossible to manufacture a perfect part, so it becomes necessary to define the amount of variation permitted on a part. These factors have led to the use of a system known as geometric dimensioning and tolerancing (GD&T). Methods for applying GD&T on part documen- tation are achieved through the use of symbols rec- ommended by the American Society of Mechanical Engineers (ASME). Standard symbols used in GD&T applications are defined in the ASME Y14.5-2018 Dimensioning and Tolerancing standard. GD&T is used in industries where interchangeabil- ity of parts is critical to mass production and tolerance requirements must be precisely controlled. GD&T is used to help clarify production requirements and ver- ify that tolerance requirements have been met. Beyond the precise definition of part requirements, the bene- fits of GD&T include helping reduce drawing changes, increasing productivity, and avoiding the accidental scrapping of functional parts. This unit is a brief introduction to geometric dimen- sioning and tolerancing. Information contained in the unit serves as a partial, summary version of GD&T practices. For more complete and detailed informa- tion, refer to the ASME Y14.5 standard. GD&T Application Applying GD&T involves specifying the allowable variation on a part with respect to the actual function and relationship of part features. Geometric tolerances are applied to the following five areas of concern: form, orientation, location, profile, and runout. ■ ■ Form tolerances control variations in the form of features. Form tolerances control straightness, flatness, circularity (roundness), and cylindricity. ■ ■ Orientation tolerances control angular relationships of features. Orientation tolerances control angularity, perpendicularity, and parallelism. ■ ■ Position tolerances define position (location) requirements for features. A position tolerance controls the allowable location variation of a feature from the true position shown on the drawing.