GD& T Symbols & Definitions Guide

Do you know all of the GD&T Symbols & Definitions? The ASME Y14.5 is an authoritative guideline and is a design language of geometric dimensioning and tolerancing (GD&T). GD&T aims to communicate the design intent in a way that the desired form, fit, function, and interchangeability of feature is conveyed on the drawings without confusion.

The purpose of this standard is to establish requirements of industry and provide a way to communicate drawing objective as per project specifications. The standard establishes symbols, definitions, and rules that are used for defining dimensions and tolerances. It helps further in manufacturing and inspection processes of mechanical parts. It is an international standard that is used throughout the globe.

In this article, we’re going to look at the background and some basics of gd&t symbols and definitions that are important for a engineers and designers to understand:

Background of ASME Y14.5 & GD&T

The ASME (American Society of Mechanical Engineers) has established this standard for dimensioning and tolerancing. The first standard which incorporated completely the GD&T guidelines was ASA Y14.5-1957 (which was adapted/updated from concepts put forth in MIL-STD-8) followed by USASI Y14.5-1966, ANSI Y14.5-1973, ANSI Y14.5M-1982, ASME Y14.5M-1994, ASME Y14.5-2009. The most recent update was in 2018. So, the latest standard for GD&T is that of ASME Y14.5-2018.

The standard, and associated gd&t symbols are fully utilized by companies involved in designing, manufacturing and developing industrial products and parts. No matter what year standard your company is using, the basics are all the same. So, let’s now explore some of the basics of the GD&T standard.

Basic Definitions & Terms

When we talk about the manufacturing process, we need uniformity in drawing specifications and even in interpretation. Guesswork is never recommended in engineered products. GD&T symbols and definitions help convey the designed object and allow for the manufacturing of mechanical parts in a way that improves quality, lowers manufacturing costs, and shortens delivery time.

The standard is divided into logical sections that range from definitions to datum reference frames to the tolerancing each of the fundamental categories of tolerances like Form, orientation, Location, Profile, and Runout.

Let’s now explore each of the categories for a better understanding.

Form

Form tolerances are individual characteristics of individual features, and therefore, are not referenced to datums. A form tolerance is a basic geometry tolerance and it helps define the shape of a feature.

Form tolerances can be stated by four characteristics that are Straightness, Flatness, Circularity, and Cylindricity. Each defines the tolerance zone of geometric form to limit how far an actual surface is permitted to vary from its perfect shape.

Orientation

Orientation tolerance is a set of geometric tolerances that let you define the orientation for the feature relative to a referenced datum. This category of tolerance defines three types of characteristics, i.e. parallelism, perpendicularity, and angularity. These are used to control the “tilt” of features and are always associated with basic angle dimensions that are often used as a refinement of location.

Location

Location tolerance is defined by position, profile, and runout. Position tolerance states the permissible variation in the location of a feature’s axis or center about its true (theoretically exact) position.

Profile

Profile tolerance is a control that defines a tolerance zone within which the surface of a feature must be contained. For specifying profile of a line, the tolerance zone is defined at an ideal cross-section. The profile of a surface tolerance is used to control the entire surface of a feature relative to a true three-dimensional surface, either for a single feature or a group of features, and may or may not be related to datums. Profile is the most powerful of all of the GD&T controls as it may be used to control only form, or form and orientation, or form, orientation, and location, or even size.  In essence, all of the other geometric controls are a “subset” of the profile tolerance.

Runout

Runout tolerance is a geometric tolerance that is specified for the parts designed surfaces of revolution relative to a datum axis. It is used to control a feature within how much variation is allowed when it is rotated 360 degrees about its datum axis. In other words, this tolerance type defines the range in which the feature can “wobble” in comparison to the datum axis.