A Quick Look at Geometric Dimensioning and Tolerancing (GD&T)

[fa icon="calendar"] May 25, 2021 / by Warren McCray

asme-gdandt-book-coverAre you familiar with Geometric Dimensioning and Tolerancing (GD&T)? Maybe you are, but you may lack understanding of how to utilize it or why it might be necessary. In this blog, I will cover what GD&T is, how it can help you develop a consistent part measurement process, and the importance of datums in GD&T for engineers and designers.

Please note that this blog is brief and will not explain everything related to GD&T; we would need more space than this page contains to explain everything about GD&T. Still, it is the first of a series of blogs that will shed some light on the topic and hopefully raise your awareness around GD&T.

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What Is GD&T, and How Will It Help Me?

According to ASME Y14.5-2018, GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the acceptable variation between components. Dimensioning specifications define the nominal, as-modeled, or as-intended geometry.


What is The Purpose of GD&T?

Geometric Dimensioning & Tolerancing (GD&T) is a language representative of engineering drawings to classify deviations and tolerance of part measurements and geometric analysis. GD&T is an efficient way of communicating the measurement conditions and specifications of a part.


Why Are GD&T Standards so Important for Members of Engineering and Design Teams?

The engineering process anticipates some variation, or tolerance, between manufactured parts. In coordinate dimensioning, defining those tolerances and measuring them in complex finished parts and products is difficult. GD&T is a more accurate way of defining tolerances and allows more efficient manufacturing. It allows for greater control of features where more precision is required. 

GD&T uses a round tolerance zone instead of a square tolerance zone increasing the overall accuracy. The Dim Tol Analyst capabilities within SOLIDWORKS 3D CAD can supplement GD&T, allowing GD&T to complement SOLIDWORKS MBD.


What Is This Thing They Call Datums, and Why Are They Important?

Typically, it all starts with a datum. Datums are high precision surfaces or axis from which features are to be measured from. Datum feature symbols enable the inspector or reader to identify and communicate the baseline or surface area of a part from which to measure. Sometimes this is as simple as dimensioning from a flat surface; however, it can get more complex depending on the position and location. Let us consider this...

Parts can have many degrees of freedom, and we need to constrain the degrees of freedom to measure accurately. We must consider some assumptions: 

  1. Theoretically, flat surfaces of datum planes are considered perfectly flat.
  2. Rotational freedom and translational freedom are general ways in which a part can move.
  3. Controlling how much a part or a feature can move in a particular direction is part of the solution.
  4. Limiting this amount of movement makes a feature’s location more precisely located.
  5. Controlling the ID (inside diameter) in comparison to the mating parts OD (outside diameter) ensures the proper fit and reduces assembly errors.
  6. Each will have its unique tolerance for features of size, and one must be compared to the other to achieve the proper fit when assembled—for example, a loose fit vs. a press fit. A loose fit freely slides through one another. A press-fit intentionally interferes with one another and is pressed (forced) together, making it difficult to separate the parts. If the individual tolerances are done incorrectly, one batch may fit together well, and the next batch may not. They are increasing the amount of scrap if you are unable to rework the parts.

Determining what the tolerances should be and what a manufacturer can hold is a delicate balance. Designers and Engineers can call out more precision than needed, increasing cost and manufacturing rejection rates.

Why Is Flatness Important in a Datum Feature?

When measuring from a surface or a datum plane, the surface or datum plane must be flat to measure from it accurately. Any curvature or "hump" in the datum plane will result in inaccurate measurements, so it is vital, when measuring, that the plane is flat. Typically, a flatness callout is required for all datum features so the inspector can measure from a precision surface and not a jagged or inconsistent surface. Therefore, we must understand what flatness is to make sure our datum surface is within the standard.

Flatness is a way to measure the deviation between two perfect, parallel surfaces. For example, if you were driving your car down a rough road, you can hear it and feel the erratic up and down movement made by your tires as they meet the road. When the road is in perfect condition, and the surface is smooth, there is little noise, and the ride is much more enjoyable. A datum surface is similar in that when you measure from it, the surface gage used to measure the difference in height on a surface will glide across the part, and there is almost no deviation on a surface when it is machined to a high degree of precision. For example, .0004" flatness callout is much smoother than .015". In the graphic illustration below, we will exaggerate the surface deviation to illustrate the inconsistency and identify the range or tolerance zone with which a deviation is acceptable. Here we are using a ¼" or .25" tolerance zone. Typically, the more precision you use on a surface you measure from, the more consistent the measurements will be.


This is the orthographic view of the part


Tolerance zone, with exaggerated surface deviation

Now that we understand what flatness means, we can apply this to a datum plane.

The first datum plane is considered the Primary Datum reference plane. If possible, it is typically the most prominent surface that will be stable and the most consistent surface from which to measure. The Primary Datum will constrain 3 degrees of freedom. The second datum plane (secondary datum) is typically smaller and will constrain 2 degrees of freedom. The third (tertiary) datum plane is much smaller and only control one degree of freedom.

first datum plane

First Datum Plane

second datum plane

Second Datum Plane

third datum plane

Third Datum Plane


How Do We Identify Datums?

So how do we identify datums, and how are they placed? Datums can be displayed in a variety of styles; here are a few:


They can be attached directly to a surface or a leader, dimension, or feature control frame. In the 3D space, datums can be called out, but typically they are shown in orthographic views. Their location can also identify the center or an axis.


Datums can be applied to the feature control frame model in 3D Space.



Datums applied to the extension line and the feature control frame within 2D orthographic Views.



Datums applied to the feature control frame and directly to the surface, within a 2D orthographic view, identifying the center axis as the datum



What it means, exaggerated

Datums should not be placed randomly with no relation to constraining the part. Contrary to frequent practice, they do not have to be in alphabetical order either.


Closing Thoughts

In summary, datums aim to inform the inspector of the location from which a feature is measured on a part. Datums help ensure that, regardless of where the part's inspection occurs, the characteristics contained on the part are measured from the same datum plane. By having this consistency, you can ensure a uniform way to inspect the part, reduce errors, and allow for interchanging of parts regardless of where the parts may have been manufactured.

For a complete explanation and more specific instruction, please reference the ASME Y14.5-2018 specification. 

For a more in-depth explanation, with hands-on training, please visit our training index on our website.

Interested in what a GD&T course looks like for your team? Request a quote today!

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Topics: Geometric Dimensioning and Tolerancing

Warren McCray

Written by Warren McCray

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