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Time to read: 14 min

This article was originally published in February, 2017 by Jeremy Hill. The content has been updated.

Geometric Dimensioning and Tolerancing (GD&T or GD and T) is a standardized system engineers and manufacturers use to define and communicate design intent, product geometry, and tolerances to ensure precise control over manufacturing variations. The US military created a proto-GD&T in the 1940s with the MIL-STD-8 publication (NOTE: MIL-STD-8 has since been canceled). GD&T has evolved since then and is now commonly used across the industry. ASME Y14.5 (latest revision is 2018) is now considered the standard guideline for GD&T. 

Index 

This article is a convenient guide for Geometric Dimensioning and Tolerancing (GD&T) and includes:

  1. CNC Machining Tolerances
  2. Datums and Features
  3. Basic Dimensions
  4. GD&T Symbols
  5. Material Condition Modifiers
  6. Feature Control Frames

GD&T Overview

GD&T is a way of describing the dimensions and tolerances that’s different from traditional coordinate measurement plus/minus tolerancing. Fundamentally, engineers design a part with perfect geometry in CAD, but manufactured parts are never perfect. Proper use of GD&T can improve quality and reduce the time and cost of delivery. It accomplishes this by providing a common language to accurately express design intent and focusing on functional interfaces to tolerance a part. 

These are the main benefits of using Geometric Dimensioning and Tolerancing (GD&T):

  • Standardized design language
  • Clear, precise, and consistent communication between customers, suppliers, and production teams
  • Method for calculating the worst-case mating limits
  • Repeatable production and inspection processes
  • Assembly is assured from qualified production parts

By deepening your knowledge of GD&T, you improve communication with your manufacturing and quality control teams, ensuring alignment on top-quality manufactured parts. Fictiv’s global network of highly-vetted manufacturing partners, coupled with our extensive capabilities, further enhances these benefits. Our AI-powered technology platform ensures faster, smarter decisions, while our dedicated team guarantees the highest standards of service and quality, aligning with the precision and efficiency that GD&T brings to custom manufacturing.

There is a lot to learn when it comes to mastering GD&T, so this post will serve as a starting point to help you understand the most important GD&T basics and concepts, including:

  • CNC machining tolerances
  • The Datum Reference Frame 
  • Basic Dimensions
  • Interpreting GD&T Symbols
  • Applying Geometric Symbols to Engineering Drawings
  • Material Condition Modifiers
  • The Feature Control Frame

The alternative to GD&T is the coordinate measurement square tolerancing approach or ‘conventional tolerancing. Square tolerancing uses square tolerance zones formed as a rectangular box around a coordinate, while GD&T uses circular/cylindrical tolerance zones formed around a point — which results in a tolerance zone that is 57% larger with GD&T. Also, coordinate measurement systems do not clearly define inspection requirements or design intent, but GD&T defines both in a clear and concise manner. 

If you want to learn more about this topic, check out our on-demand webinar on Conveying Design Intent with GD&T.