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

You may have across some obscure looking symbols on engineering drawings (that’s “blueprints” for you *cough cough* “seasoned” engineers and technicians) that look something like this:

This ladies and gentlemen is a feature control frame – a GD&T Feature Control Frame. This bad boy does wonders in terms of saving us cash () and locking in the quality we need to ensure that our products function as intended – always.

So what is GD&T?

As designers, design intent is king (future blog on this coming soon), in order for us to communicate the intent of our design and more precisely how accurately our design needs to be manufactured in order for it serve its intended function. A method, or rather, a ‘universal’ language is needed to communicate that intent to everyone who will encounter our designs. Namely, this will be the engineering team (design), the quality team (measurement and inspection), and manufacturing (production and fabrication). When these three departments talk the same language everyone wins. GD&T removes any ambiguity from the equation.

GD&T is used to define how a part feature relates to other part features in the same part or in a mating part; it’s a way to dimension and tolerance with respect to a part’s function – the way it works. In other words, it’s a language for communicating engineering design specifications.

GD&T is a set of rules and symbols used to define the geometric limits of mechanical parts to ensure the design will function as intended in operation. These rules and symbols are set forth by a standard, developed by The American Society of Mechanical Engineers – the ASME Y14.5-1994 (there are editions, such as 2018 being the latest). In order for a design to function as intended, it’s geometric attributes must not exceed certain limits. These limits could be a result of empirical work (lots of trial and error), engineering judgment, or experience. GD&T facilitates both the communication and control of such limits on an engineering drawing (which happens to be a legal contract, so you can safely assume these documents are a tad-bit important).

Function First

The primary goal of GD&T is to communicate and control exactly what is required for proper function of the part. That’s it! Simple, right? Indeed it is, but as with any goal, there are many details that need to come along for the ride. When we talk about control, what is it that we’re “controlling” exactly? – SLOF – Size, location, orientation, and form; these are the four fundamental components to a complete engineering drawing.

Let’s say we are designing an airplane wing – being a primary structural component of an aircraft, this definitely fits the criteria of being one of those mechanical components that we would need to function as intended (preferably with a high degree of confidence).

From the Embraer 170 wing diagram above, we can see that there is an obvious size to the shape, there is also a location to the various features with respect to, typically, a datum identified on the component. E.g., Rib 26 has a y-coordinate of -12196, with respect to the wing reference point (). The wing also has an orientation, in this case that happens to be that the edge at is parallel to the reference line (more exactly, the edge is coincident to the line). Finally, not so apparent on this 2D representation, is that the wing structure has a form. How do we ensure that the wing yields the performance required to deliver on its engineering promises, such as lift and drag characteristics? The shape of the wing needs to be finely tuned in all three-dimensions, a task typically reserved to the engineering design team. The figure below shows a cross-section of this three-dimensional shape, known as an airfoil. As the wing moves through the air, the structure-fluid interaction produces a force known as lift. Lift is a consequence of the high-speed air over the wing versus the low speed under the wing, an attribute of the longer surface distance that air molecules must travel over the wing versus beneath it (known as the Bernoulli principle) – an engineered feature. See figure:

As you might imagine, the aerodynamic characteristics of the wing are quite sensitive to the shape of the airfoil. Therefore, the accuracy in which the wing features are fabricated must be tightly controlled. We can control the shape with a profile of a surface geometric call-out on the engineering drawing.

Now, as you can see in the figure above, no matter how ‘jagged’ the surface is – as long as the highs and lows of the surface do not penetrate the tolerance zone, the component is within specification. Of course, for a wing structure we would not want the surface to be so jagged, but you get the idea. That is, we have now applied a geometric form control to the one of the component’s functional features (the wing surface). Now this – is GD&T in action!

Having met one of the four fundamental control requirements, we would now do the same for the remainder: size (dimensions of the shape), location (locate features with respect to datums), and orientation (orient features as parallel, perpendicular, or off-angle relative to a datum).

Once we have checked off each one of these items on our list, our component’s engineering drawing is fully defined and ready to be sent to the manufacturer.

Learn More

This information is of course only the tip of the ice-berg, and to become proficient at reading and implementing GD&T, like anything else – we must continuously study and practice. You’re encouraged to check out the references below.

References

ASME Y14.5M-1994. (Reaffirmed 2004). Dimensioning and Tolerancing. New York, NY: The American Society of Mechanical Engineers.

Budynas, R. G., & Nisbett, J. K. (2020). Shigley’s Mechanical Engineering Design. New York: McGraw Hill Education.

Bemis, C. (2021, November 13). What is Geometric Dimensioning and Tolerancing (GD&T)? Retrieved from GD&T Basics: https://www.gdandtbasics.com/what-is-gdt/

GD& T Symbols & Definitions Guide. (2021, November 13). Retrieved from gdandt.com: https://gdandt.com/useful-linksresources/gdt-symbols-definitions/