What Is Gd&t (geometric Dimensioning And Tolerancing)

GD&T stands for Geometric Dimensioning and Tolerancing and represents a way to define

the size, location, orientation, and form of a part feature. It is essentially a language that is used throughout the mechanical engineering world to communicate the intent of the person designing a product to the engineers in manufacturing who make the part. GD&T is used in manufacturing to help define the features of parts and assemblies and in quality control by specifying controls that can be translated into inspection plans.

Today, there are a variety of standards for GD&T. In the US, most companies use the American society of Mechanical Engineers (ASME) Y14.5-1994 standard. A newer standard for ASME is available, the Y14.5-2009. Internationally there are many standards. ISO is the standard throughout Europe. However, many country specific standards are either based on, or originated from the US ASME standard.

A special set of symbols are used to define relationships between a feature and a measurement reference.

Often referred to as a GD&T callout, the feature control frame communicates the allowable feature variation and the inspection, as well as the design intent of the part.

In a latching claw part, 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.

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

GD&T is used on drawing and on the CAD system models. However, with the increase in automated tools, the ability to begin with GD&T in the models is becoming increasingly. Some software packages exist that are both internal to the design systems, and external to the design system.

by: Sigmetrix

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