What is geometric dimensioning and tolerancing (GD&T) and why does it matter for manufacturing?


Adam WaraksaMarch 14, 20237 minute read

Across industries, manufacturers are keenly aware of the challenges involved in bringing a complex mechanical device to market. Each part must be manufactured correctly and work in functional harmony with all the other parts in its assembly, and each assembly must do the same across all the systems that make up the machine.

Before 1940, the dimensions of manufactured parts were specified with X-Y areas that did not allow for accurate tolerancing and provided no uniform way to describe how a part is supposed to interact with other parts. In other words, the traditional approach to dimensioning and tolerancing did not guarantee that parts would fit and function well at the assembly level.

Geometric dimensioning and tolerancing, or GD&T, solves this problem. Developed after World War II, it is a comprehensive system for defining the design and manufacturing requirements of parts.

Using symbols and words within 2D drawings and 3D models, GD&T describes the geometry of the part and the tolerances for each feature of the part. GD&T also calls out datum references, which are critical features that provide a common reference point for the geometry of other features. It is these datum references that help ensure the part will work properly in its assembly.

Like any system, GD&T involves special vocabulary, definitions, and rules. There are a few different GD&T standards, including those from the American Society of Mechanical Engineers, or ASME (used in the US) and ISO (used in the rest of the world). It is important that design, engineering, and manufacturing teams all agree on the same standard. When they do, GD&T can provide a number of important advantages.

Amid the broadening adoption of 3D scanning for inspection, it's important that manufacturers look out for 3D metrology software that comes with advanced GD&T tools as this will significantly simplify their process.

The challenges of tolerancing

It’s worth taking a moment to explain why GD&T is necessary, or why tolerancing is such a critical aspect of making functional parts and assemblies.

The reason is simply that no part can be made perfectly. It is physically impossible due to all the variables in the manufacturing process. Any minuscule change to a part’s positional misalignment or tooling or ambient air pressure, or just a human error, will cause deviation from perfection.

What is possible, however, is manufacturing a part that is close enough to perfect that it works as expected in its assembly. Tolerance is the allowable deviation from the specified shape, form, or position.

Tolerances apply to every aspect of the part, but most critically to those elements known as datum references. Tolerances for each feature may also shift based on how the parts fit together. In other words, the tolerances for one part must account for the tolerances of the other parts in the assembly.

Understanding GD&T information

Tolerances for the features of a part — whether it is the diameter of an extruded boss, the position of a bore hole, or the orientation of an interface — can be specified with a high degree of precision using standard GD&T notation. The layout of this notation is called the feature control frame:

GD and TGeomagic Control X, developed by Oqton, comes with extensive GD&T tools

In addition to tolerance (shown above as the numbers after the ± symbol), the feature control frame includes four elements:

1. GD&T symbol or geometric symbol

The GD&T symbol, or geometric symbol, explains the type of tolerance. There are five classes of tolerances: form, profile, orientation, location, and runout. Within each class, there are different symbols that describe subclasses of tolerance:

• Form: straightness, flatness, circularity, cylindricity

• Profile: profile of a line, profile of a surface

• Orientation: perpendicularity, angularity, parallelism

• Location: symmetry, position, concentricity

• Runout: circular runout, total runout

2. Tolerance zone shape and dimensions

The tolerance value represents the total tolerance of the geometric control. For example, if the tolerance is listed as ±0.1 millimetres, or 0.1 equal bilateral tolerance, then the total tolerance value will be .2 millimetres. If the tolerance is diametrical, the diameter symbol (Ø) will be seen before the value.

3. Tolerance zone modifiers

Tolerance zone modifiers are letters that add context to a feature’s description, tolerance, and datum references. There are nine of these, including F (free state), L (least material condition) and M (maximum material condition).

4. Datum references (as needed)

A datum is a virtual plane, line, point, or axis used as a point of reference for defining the geometry of the part. In other words, a certain feature D may have a positional tolerance of .2 millimetres with respect to datum references A, B, and C (e.g., a hole, a bottom edge, and a facing edge).

It should be noted that it is possible to over-specify, or add tolerances that are not actually critical to its fit or function. Keep in mind that the more feature control frames you have, the more difficult it will be to manufacture the part correctly.

What are the benefits of GD&T?

Using GD&T is extremely valuable because it guarantees with 100% certainty that a part will fit and function properly at the assembly level. 

In this way, GD&T saves a great deal of time and money by reducing the number of design-manufacture-test cycles that need to be performed in a variety of common applications, whether you are iterating on a prototype, inspecting a part to assess wear, or trying to determine why a part is having issues during manufacture.

GD&T also helps convey the design intent of the part, which a conventional drawing or model can’t do. Understanding how a part is intended to function in its assembly is an important benefit for manufacturing teams, inspectors, and designers who may inherit designs as part of future projects.

3D scanning and GD&T 

With the emergence of 3D scanning, manufacturers recognized that scan-based inspection has a host of advantages compared to traditional methods. But for 3D inspection software to be really effective, it needs to include GD&T capabilities.

3D scanners3D scanners capture the actual measurements of a part

Scanning quickly captures the most part data in the least amount of time, making it faster and easier to identify issues with the part or see aspects you weren’t already looking for. For example, a scanned part (as opposed one that has been probed in discrete areas) will include all of the edge breaks, fillets, chamfers, and other aspects that were not modeled by the part designer.

With a complete and highly detailed representation of the part, you can use GD&T tools to further analyze the size, form, orientation, and location of features. Control X, developed by Oqton, for example, allows you to measure:

• Linear, angular, radial, elliptical, bore depth, counterbore, countersink, and thickness

• Straightness, flatness, circularity, cylindricity, parallelism, perpendicularity, angularity, position, concentricity, symmetry, line profile, surface profile, runout, and total runout

It also includes automation and interactive reporting to clearly communicate what you learn about the part to other teams. You can share these reports for viewing on multiple desktop and mobile platforms, without the need for special software.

Discover the advantages of GD&T

Find out how Geomagic Control X combines the power of 3D scan-based part inspection with GD&T tools to streamline problem-solving in a wide range of manufacturing applications.

If you're already shopping for a scanner and inspection software, check out our eBook Is 3D Scanning Right for Your Inspection Needs? It will help you choose the perfect hardware and maximize your ROI.

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