Posted May 25, 2021

What is Geometric Dimensioning and Tolerancing (GD&T)?

What is Geometric Dimensioning and Tolerancing (GD&T)?

A machine shop must correctly position a part’s features and fabricate them within their designated tolerances. For years product designers and manufacturing engineers used coordinate tolerance zones to denote feature placement, though this method fails to communicate actual, precise tolerances. The introduction of a new dimensioning and tolerancing system corrected this problem and standardized the dimensioning of parts for manufacturing. This article will introduce you to the GD&T system and answer the question: “what is geometric dimensioning and tolerancing?”

What is Geometric Dimensioning and Tolerancing

GD&T is a system for specifying the dimensions and tolerances of components manufactured based on the function and intent of the part’s features. GD&T offers an international language designed to communicate this information by symbolic representation throughout the manufacturing chain, including engineering, fabrication, and inspection. GD&T provides the required level of precision for each controlled feature of a part, including a hole or a cutout position.

Geometric dimensioning and tolerancing can trace their origins back to England in WWII. An engineer named Stanley Parker noticed that some of the scrapped parts for naval munitions fit in their assemblies even though they were technically out of tolerance. Upon investigation, he discovered that the specifications of traditional XY coordinate tolerancing did not accurately reflect the tolerance requirements of the fabricated features. So, to reduce the amount of wasted material and effort, he came up with a new method of locating features in manufactured parts that came to be known as “true positioning.” Eventually, this work led to the formation of GD&T—introduced as a military standard in the 1950s.

Traditional coordinate tolerancing will specify the dimensions of a single simple part, but it cannot cover all of the necessary specifications of parts destined for a larger assembly. For instance, a fan blade by itself isn’t complicated but combined with other fan blades, a shaft, and an electric motor will create a more complex fan assembly. Now take that fan assembly and install it in a furnace combined with an air-conditioner, and that simple fan blade is now part of a larger HVAC assembly. Traditional coordinate dimensioning and tolerancing cannot guarantee that the fan blade and the other parts of the heating and cooling system will work together once assembled. On the other hand, geometric dimensioning and tolerancing will specify individual component tolerances designed to enable the entire assembly to work together.

How GD&T is Used to Dimension a Part

Geometric dimensioning and tolerancing, though standardized, still contains a great deal of complexity. Locating a part feature such as a hole offers a simplified starting point for understanding GD&T. Regular coordinate tolerancing would specify the dimensions to the hole and tolerance of plus or minus .005 inches, providing a ten mil square zone to locate the center of the hole. 

The problem is that the corners of this ten mil tolerance zone are 14 mils apart when measured diagonally. Suppose the hole is located precisely -.005 by .005, at the upper left corner of the coordinate tolerance zone. In that case, it will be in tolerance even though the true position lies seven mils away. Alternatively, if the hole is located at -.002 by .006 instead, it will be out of tolerance due to a Y coordinate of .006 even though its total distance from the true position is less than seven mils. In the first example, the distance of the located hole from its true position equals seven mils and lies within the specified tolerance. But in the second example, the distance of the hole from its true position is less than the first but considered out of tolerance when using the coordinate tolerancing system.

To correct this, GD&T omits the coordinate plus or minus tolerances usually printed next to the dimensions and specifies a diameter tolerance in a feature control frame instead. This tolerance specifies a true tolerance zone that, in our example, extends seven mils radially from the true position for a total diameter tolerance of .014 inches. The control frame divides into separate compartments where various symbols and values designate positions, tolerances, and datum planes.

A CAD drawing of a part with dimensions and tolerances.

A feature in a part is located with geometric dimensioning and tolerancing.

The picture above depicts a solid part with a hole in it. The overall size of the part is dimensioned with coordinate tolerancing for reference, but the hole is located by its dimensions only. Instead, the GD&T control frame specifies the hole’s tolerance, where its location has a geometric, or diameter, tolerance zone of .014 inches. This is the equivalent of a traditional coordinate, or rectangular, tolerance zone of .010 inches (plus or minus .005 inches). The diameter zone of the GD&T equals the diagonal dimension of the coordinate square zone.

Using different symbols in the control frame can allow one to communicate many other dimensioning and tolerancing details. Among them are:

  • Form tolerances
  • Profile tolerances
  • Orientation tolerances
  • Location tolerances
  • Material conditions
  • Diameters, radiuses, squares, and centerlines

GD&T provides an encompassing system of measurement and dimensioning, of which this article has only scratched the surface. However, using it can result in quite a few benefits—discussed below.

The Benefits of Using Geometric Dimensioning and Tolerancing

GD&T provides greater precision and more versatility than coordinate dimensioning and tolerancing due to these limitations of the traditional method:

  • The square tolerance zone is more limiting than the diameter tolerance zone of GD&T, causing more manufacturing errors reported.
  • Coordinate tolerancing creates fixed-sized zones instead of allowing flexibility with other zones. The priority function is assembly, and a hole size tolerance should change depending on the positional tolerance and vice versa.
  • The traditional method of tolerancing creates ambiguous inspection instructions. For example, two different inspectors can get different dimensions just by how the part is held and measured, resulting in the rejection of good parts or accepting bad parts.

With GD&T, all fabricated parts will fit correctly and function properly at the final assembly level. Companies will also see savings in time and money by reducing test fits needed of parts to verify manufacturing. Finally, companies will have a proven and reliable method to communicate the intent of your design to manufacturers, inspectors, and the engineers of the next project that is leveraging one’s work.

At Plethora, our engineers and machinists have years of experience working with geometric dimensioning and tolerancing. We can efficiently work from the GD&T data that you send us and assist with any questions about the proper format use of this system. Our goal is to ensure the highest quality in the parts and components that we build, and we will work together with you towards that end. Our online DFM and quoting systems are ready to receive your data so we can begin working with you on your next project. To get started, upload your design files to Quote My Part or call us at 415-726-2256.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: Aerospace