Intro to Geometric Dimensioning and Tolerancing

We are going to review Geometric Dimensioning and Tolerancing  (GD&T) and what it is, why it’s important, and how we use it today. GD&T really began after World War II when the US fabricated parts to specifications, but did not successfully assemble. This created major issues especially since it was during a very critical time. Prior to GD&T, there was conventional tolerating methods that were used, but were not very accurate or reliable. After WWII, a committee investigated many of the defective parts as to why they were not accurately made to assemble if the parts were made to their specifications. They found the parts varied in dimension which caused the parts to be out of spec for assembly. As a result, the committee developed GD&T which allowed for a certain degree of variation of a dimension (tolerance) in order for parts to remain within its specification, properly assemble to mating parts, improve quality, and to essentially reduce cost. This great finding reached other industries other than the government sector, and companies and institutions began adopting the use of tolerances. Since then, new industry standards have been created and are regularly used in today’s manufacturing.

So what is GD&T exactly? Simply put, it is a system that uses symbols to communicate the size, shape, form, location, and orientation of parts and their features. By using this system, parts are properly communicated on how they mate to other parts. This system allows for a uniform interpretation and understanding of manufacturing parts. It properly takes into consideration the fit, form, and function which insures that the parts will assemble every time no matter who manufactures it. Using the system, the design engineer specifies the maximum tolerance the part requires in order to be able to assemble to other parts successfully. This allows for the drawing of a part to communicate not only the size and dimensions of a part’s features, but also communicates the relationship between the features.

Being a metal fabricator, we are very familiar with GD&T since we use it as a foundation of our quality manufacturing. When we fabricate parts, we fabricate them to print. It is a well-known standard in the engineering and manufacturing world to have dimensional prints which need to include tolerances. This properly communicates everything we need to know in order to fabricate the part and what features are critical. Just to show how important GD&T is to our business, we have an entire department dedicated to quality control and quality assurance. Everything is either pass or fail when it comes to meeting tolerances. Not only that, but even the equipment we purchase and the talented staff we hire is based upon the quality of its output in order to fabricate parts within tolerance.

Here is some useful information for sheet metal tolerances:


It is important to understand where industry specifications originate and what makes them so important. When in fact a piece of history has lead to the way manufacturing operates today. Thanks for reading along and stay tuned for our next blog post!

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