TABLE OF CONTENTS Geometric dimensioning and tolerancing (GD&T) is a system of specifying certain types of dimensions and tolerances. GD&T is a combination of symbols and characters that supplements conventional dimensions and tolerances. Both systems are used on the same drawing. GD&T provides a clear and concise standard for illustrating tolerances. The symbols are simple and easy to understand and have an exact meaning. The system provides a precise method of specifying tolerances on industrial prints. Since GD&T is an accepted standard that is used throughout the manufacturing world, the symbols have the same meaning in any language. The purpose of this chapter is to provide a brief review of the system as it pertains to basic print reading. A complete description can be obtained from Standard ASME Y14.5, Dimensioning and Tolerancing.
A thorough understanding of GD&T can be obtained from the textbook, Interpretation of Geometric Dimensioning and Tolerancing by Daniel E. Puncochar and published by Industrial Press Inc.
A preliminary example comparing convential tolerancing to GD&T, for a surface that must be round is shown in Fig. 13-1. Another example, for a surface that must be perpendicular, is shown in Fig. 13-2. All dimensions and tolerances that are shown on a print must be maintained. Parts that are made to Fig. 13-3 must conform to the conventional tolerances as well as the GD&T tolerances.
