What Is Geometric Dimensioning & Tolerancing GD&T?

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Geometric Dimensioning and Tolerancing (GD&T) is a language used on mechanical engineering drawings composed of symbols that are used to efficiently and accurately communicate geometry requirements for associated features on components and assemblies. GD&T is, and has been, successfully used for many years in the automotive, aerospace, electronic and the commercial design and manufacturing industries. In today's modern and technically advanced design, engineering and manufacturing world, effective and accurate communication is required to ensure successful end products. Currently, ASME Y14.5-2018 is the accepted geometric dimensioning and tolerancing standard superseding ANSI Y14.5M-1994 used within the USA and ISO 1101 is used outside of the the USA. See "Quick Guide to Differences between ASME & ISO GD&T Standards". Success oriented industries and organizations which, require accurate and common lines of communications between engineering, design, manufacturing and quality should consider geometric dimensioning and tolerancing (GD&T) as their mechanical drawing standard. Some advantages of GD&T (geometric dimensioning and tolerancing) are;

  1. Provides a clear and concise technique for defining a reference coordinate system (datum's) on a component or assembly to be used throughout the manufacturing and inspection processes.
  2. Proper application of geometric dimensioning closely dovetails accepted and logical mechanical design process and design for manufacturing considerations.
  3. Geometric dimensioning dramatically reduces the need for drawing notes to describe complex geometry requirements on a component or assembly by the use of standard symbology that accurately and quickly defines design, manufacturing and inspection requirements.
  4. GD&T concepts such as MMC (maximum material condition) when applied properly will facilitate and simplify the design of cost saving functional check gages, manufacturing fixtures and jigs.
The following ar common reasons one should apply GD&T principles:
  1. Part features are critical to function or interchangeability.
  2. When functional gauging techniques are desired.
  3. When a common reference (origin) or datum is required to ensure communication is consistent between design, manufacturing and inspection.
  4. When a standard interpretation or tolerance is not already implied.
  5. Simplify tolerance analysis.
  6. Replace complex or long geometry requirement description notes with a single geometric symbol.

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