4.4 - Geometric Dimensioning and Tolerance

Geometric Dimensioning and Tolerance

Geometric Dimensioning and Tolerancing (GD&T) is a language of symbols and standards designed and used by engineers and manufacturers to describe a product and facilitate communication between entities working together to produce something. By deepening your knowledge around how to create a well structured GD&T, you will improve communication with your machine shop and ensure everyone involved is speaking the same language.

Review the videos and resources and take notes on each of the symbols and what they mean.

U4.4a - What is GD&T A Simple Explanation.mp4

There are a total of fourteen GD&T characteristics, and the symbols that represent them are shown in the symbol “cheat sheet” below. These symbols are placed in the first compartment of a feature control frame and define the type of tolerance that is to be applied to the feature. The characteristics are grouped together into types of tolerance: form, orientation, location, runout, and location of derived median points.

Feature Control Frame (FCF)

The Feature Control Frame is the notation to add controls to the drawing. The leftmost compartment contains the geometric characteristic. In the example above, it is a location control but it can contain any of the control symbols. The first symbol in the second compartment indicates the shape of the tolerance zone. In this example, it is a diameter as opposed to a linear dimension. The number indicates the allowed tolerance.

Next to the tolerance box, there are separate boxes for each datum feature that the control refers to. Here, the location will be measured related to datum B and C. Next to the tolerance or a datum feature is an optional encircled letter, the feature modifier.

The following possibilities can occur:

M means that the tolerance applies in the Maximum Material Condition (MMC)
L means that the tolerance applies in the Least Material Condition (LMC)
U indicates an unequal bilateral tolerance, i.e. for a 1 mm tolerance it may specify it as minus 0.20 and plus 0.80.
P means that the tolerance is measured in a Projected Tolerance Zone at a specified distance from the datum.
No symbol installs the tolerance regardless of feature size (RFS)

Example Videos

For this example, if the part is not in MMC, a bonus tolerance can be added proportionally to the deviation from MMC. So if a part is at 90% MMC, the tolerance will also loosen by 10%.

U4.4b -GD&T Lecture.mp4

Here is another Lecture on GD&T that does a nice job explaining each of the GD&T features as they pertain to drawings.

U4.4d - GD&T (Part 1 Basic Set-up Procedure).mp4

Here is a video on how to think about determining fits and sizes for a particular design.

U4.4c - Fits and Tolerances.mp4

Lastly, here is a Link to the section of the Machinery Handbook that defines fit specifications.

U4.4e - Machinery Handbook Limits and Fits.pdf

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