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The Accepted Method to Share Precise Requirements on Drawings

GD&T: The accepted Method for Standardized Measurements and Specification

If I were to tell you that engineers within an organization used a system of specific geometric symbols and measurements that were near universally known, ensuring different departments working on different aspects of the same projects had the same resources as everyone else. What would your reaction be?

I bet the word, DUH!, would be the first thing out of your lips.

"Of course!" you'd say. "Companies must be consistent in their dimensions and specs; otherwise, a project fails."

And I agree with you 100%. It's a simple, common sense idea.

Sometimes, though, common sense leaves us, resulting in project failure. Some of those failures are quiet and not well known. Others? Well, not so much. NASA learned that over 20 years ago, costing the organization $125 million and a hit to its reputation when a simple math mistake when their Mars Climate Orbiter slammed into Mars. What happened? A simple conversion mistake.

During the project, half the project staff used imperial measurements, the other half used metrics. When the spacecraft approached Mars, the engines fired, pushing the craft into orbit. Unfortunately, the probe was within 36 miles of the planet because of the math mixup, leaving the craft too far below the level where it could operate. The result? BOOM!

If only there were standards in place that used specific symbols and dimensions that engineers could recognize regardless of where they were and who they worked for. 

Hold On! There is such a creature:

Geometric Dimensions and Tolerances

Geometric Dimensions and Tolerances (GD&T) is a technique for specifying dimensional specifications with greater accuracy and precision. It’s an internationally recognized standard that uses specific geometric symbols to represent distinct features of a manufactured part. By using GD&T, manufacturers can more easily measure the size and shape of parts during the manufacturing process. These symbols are based on standard dimensions like length, diameter, and angle but are transformed into something more unique to be recognized in any orientation or position.

This article is the first of two parts, covering the background and history of GD&T. Part 2 illustrates six explanations why it's not universally used.

GD&T History

Following the First World War, the Industrial Revolution was in full swing with the mass production of cars, airplanes, and trains. Manufacturing plants were used to build military equipment during the Second World War, and after the war ended, they used these production lines again for civilian vehicles and other goods.

The post-war era saw a shift from centralized factories to smaller plants with more localized production, largely because of technological innovations that made it possible to manufacture smaller batches at a lower cost while still meeting customer demand.

Because of this new paradigm, manufacturing as we know it today came into existence, and Geometric Dimensioning and Tolerancing (GD&T) emerged as an industry standard that continues to evolve.

What is Geometric Dimensioning and Tolerancing?

Geometric Dimensioning and Tolerancing (GD&T) is a standardized language created to communicate the expectations of a product's design. It uses symbols and annotations to depict the critical aspects of a part or assembly while accommodating variability because of manufacturing processes. GD&T is the language of manufacturing specifications. It is a standardized method of communicating design intent and product requirements throughout the supply chain, from design engineers to production operators.

GD&T symbols and requirements describe features such as size, location, orientation, and the allowable variability for those features. The goal of GD&T is to provide all stakeholders with a common language so that there is no misunderstanding about the product requirements. GD&T is not a design standard, but a language applied to any standard. It is the common language used to convey one person's design intent.

The design engineer's design standard may be American, Metric, or something else, but the GD&T symbols are the same, regardless of the standard used.

Why is GD&T important in manufacturing?

When producing goods, the manufacturing industry must adhere to several regulatory standards, such as ISO quality management. To do so, manufacturers typically apply the principles of geometric dimensioning and tolerancing to their design and production processes.

GD&T allows engineers to communicate the critical aspects of their designs while accommodating variability because of manufacturing processes. This means designers and engineers can effectively communicate with manufacturers uniformly.

This standardization in communication is necessary because many manufacturers produce and ship goods globally. Differences in design requirements, engineering conventions, and regulatory standards can cause confusion and delays in the supply chain. GD&T eliminates these issues.


Accuracy refers to a feature or part within the specified tolerance of the desired size. It is an important concept in the manufacturing industry because it is the only way to ensure that parts fit together.

Achieving the desired accuracy can be challenging because many factors affect it, such as the accuracy of the equipment used, the skills of the operator, and the quality of the raw materials.

Some ways to improve accuracy include using better quality raw materials, investing in more precise equipment, and training operators to be more consistent with their work.


Precision refers to how consistent a feature or part is concerning the desired size. While accuracy is important, the real issue is whether multiple parts are consistent with one another.

For example, if one part has a diameter of 1.5 inches and another has a diameter of 1.75 inches, the two parts will not fit together.

Improving precision improved by investing in precise equipment and training operators to be more consistent with their work. It is also important to select raw materials that are consistent in quality.


Conformance refers to how consistently they compare a part or assembly to the design intent.

For example, if two parts are designed to fit together, but one part has a diameter of 1.5 inches while the other has a diameter of 1.75 inches, the two parts are not fully conforming to one another.

Engineers improve conformance by designing parts with appropriate tolerances. For example, if a part needs to be 1.5 inches in diameter, but it is okay to be between 1.425 and 1.575 inches, the part is conforming.

Datum Identification

Datum refers to a specific reference point used in the design of an object. Identifying which datum to use when designing a part is important because it allows designers to account for variation in the manufacturing process, achieved through GD&T symbols.

A few examples of common datum features are geometric features, reference surfaces, and reference axes.

In Sum

A project's success depends on many related processes coming together to complete the finished component. Regardless of tools, training, and parts, one critical function stands out above all others: communication. If each team uses different terminology or measurement standards, that project's evolution—at best—slows. At worst, failure.

Geometric Dimensions and Tolerances provides the consistency needed to ensure drawings and tooling are consistent, making life easier for techs to assemble components consistently and precisely.

Part 2: The Top 6 Reasons Not All Engineers Accept GD&T

Despite its importance and near-universal usage, GD&T isn't accepted by all engineers or firms. Why? In Part 2, we'll explore the top six reasons why GD&T isn't quite universal.



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