GD&T Basics

What is GD&T?

GD&T stands for geometric dimensioning and tolerancing and represents a way to define the size, location, orientation, and form of a part feature. A special set of symbols are used to define relationships between a feature and a measurement reference. The basic symbols used are:

The GD&T control, along with a tolerance, any modifiers, and a datum reference frame are grouped together in a feature control frame as shown below.


The feature control frame, sometimes referred to as a GD&T callout, communicates not only the allowable feature variation and the inspection references, but also the design intent of the part.GD&T-Basics4

Consider the latching claw part shown here. This part is used as part of a seat latching mechanism. The large planar surface is mounted flush with a planar surface on another part. This surface will be labeled as datum feature A. The large hole is a pivot hole for the claw, representing an important functional feature and is labeled as datum feature B. The small flat plane on the claw tip is the location where the claw makes contact with a striker bar to ensure the seat is secure to the vehicle frame and is labeled as datum feature C. There are several other functional features. These features should be controlled with reference to the mounting features |A|B|C|. All of the remaining features for this part are less important and are controlled with general profile tolerance that can be found on a note.

The current standard is ASME Y14.5–2009 (which replaces ANSI Y14.5M-1994). Some of the key reasons for using GD&T include:

a) Provides a consistent, sound method for communicating design intent
b) Establish a coordinate system (that references the datum features) for inspection and manufacturing
c) Reduces the need to explain complex requirements
d) Facilitates and simplifies gaging requirements
e) Identifies features critical to function of part and assembly
f) Simplifies tolerance analysis

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