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CNCCookbook Beginner's Guide to GD&T: Introduction

Introduction to GD&T

Geometric Dimensioning and Tolerancing (GD&T) is a set of standardized symbols and conventions that are used to describe parts in a way that makes it easier for customers, manufacturers, and other supply chain participants to successfully communicate. Parts that are manufactured in a shop must meet specific specifications. These specifications are shown on engineering drawings, which are normally produced by CAD software. GD&T is a particular set of conventions used on engineering drawings (often called "prints" from the older "blueprints") that communicate how parts should fit together and how they function. This course will teach you the basics of how to understand GD&T symbols and their use.

GD&T is a standardized international system based on symbols that can be clearly understood by anyone that is familiar with the standard. It is a mathematical language, though it does not require you to be a math expert in order to understand and use the system. GD&T is a significant improvement over traditional older systems for describing dimensions that relate to the fit, form, and function of parts. It is very precise as well as concise relative to these older systems. Each work piece is described in terms of "zones of tolerance" that are relative to the Cartesian coordinate system. As you recall, Cartesian coordinates are just X, Y, and Z measurements from an origin. If you're a CNC'er, you're already quite familiar with them.

A simple drawing with GD&T symbols...

A simple drawing with GD&T symbols. Image Source

If GD&T seems complex, it is only because there is a long list of ways a physical part can deviate from the geometric ideal of what the designer had in mind. A shaft could be slightly bent, slightly tapered, slightly tilted, and slightly offset from where it was intended. When you consider all the different kinds of geometry found in machined parts, it's no wonder that cataloging all the defects in that geometry gets a bit complicated. GD&T actually brings some order to that complexity and makes it systematic.

History and Background of GD&T

GD&T is an international system because the American Society of Mechanical Engineering and the International Organization for Standardization worked together to create it. The actual written standards they produced are ASME Y14.5M and ISO 1101. While the standards are relatively recent, the ideas behind GD&T go back to the 1940's and 1950's.

We should note that while some think of GD&T as defining a standard for inspection of parts, that was not its intent. Rather, it was designed to communicate what the designer intended about the dimensions and tolerances of the part. Such information can be very helpful to the inspector in determine what exactly needs to be inspected to ensure the part meets the designer's original intent.

- Dimensioning describes the nominal or as-intended geometry.

- Tolerancing defines the allowable variation of the form and size of individual features, and the allowable variation in orientation and location between features.

An important goal of GD&T is to ensure that a part defined using GD&T and manufactured within the proscribed limits will fit and function with the largest possible tolerances. Proper use of GD&T can add quality and reduce cost at the same time since having tolerances that are too tight can drive up costs rapidly.

When and Why Use GD&T?

Let's start in a very basic way--GD&T is designed to facilitate communication between entities that need to work together to manufacture something. If you don't have to communicate with anyone, you may not need to know GD&T. CNC Hobbyists may have little use for it, for example. But, if you are a CNC Professional, it will be extremely rare that you work on something where you don't need to communicate. Perhaps you need to make a part someone else designed. Or perhaps your part needs to fit with a part made by someone else entirely. In those cases, GD&T might save you considerable trouble. In modern CNC Shops, it becomes almost a pre-requisite that you know something about it.

Here are some of the key instances that call out for GD&T to be used:

- When part features are critical to function or interchangeability. For example, if you're making a part that has to fit with a part made by someone else, you'd better call out all the features that will govern whether it fits properly.

- When functional gaging teachniques are desirable. Think of functional gages as measuring devices intended to verify a particular feature is within tolerance. For example, a "Go-NoGo" gage.

- When datum references are desirable to ensure consistency between manufacturing and gaging operations. A datum is used to show where a measurement is taken from. For example, CNC'ers are familiar with Part Zero being an important datum that measures where the motion of the CNC machine starts from for a particular part.

- When computers are being used in design and manufacture.

- When standard interpretations and tolerances are not already implied. There is no need to over-specify tolerances that are implied in some other way. For example, we might specify a thread without needing to provide a bunch of GD&T to lay out the dimensions and tolerances of every feature of the thread. Just saying which thread it is tells us most everything we need to know about its dimensions and tolerances.

Advantages and Disadvantages of GD&T

The advantages of GD&T are pretty straightforward:

- It is very succinct and precise. With old-style dimensions and tolerances extensive non-standard notes had to be included to make sure everything was fully specified.

- It is a graphical international language.

- It expresses tolerances in ways that are often beneficial to manufacturing cost. For example, without GD&T, the tolerance on a hole center is often X and Y plus or minus some amount. That is a so-called "Square Tolerance." WIth GD&T, the tolerance is expressed as a round area. This is a much more forgiving tolerance than a Square Tolerance if you think of drawing a circular whose edges just meet the corners of the square. That circle is quite a bit larger than the square.

Most GD&T devotees don't recognize many disadvantages other than that there is a learning curve you will have to get through to learn GD&T. It isn't hard, and fortunately you have already started on a free course.


Some Important Rules and Philosophy Behind GD&T

In order to be rigorous and to meet its goals, GD&T must be applied according to rules and not just willy nilly added to drawings wherever the designer wishes. GD&T does not merely add to traditional dimensions, it is a whole new way of describing the dimensions and tolerances. Here are the basic rules to be followed:


Next Article: GD&T Basic Concepts

GD&T Table of Contents


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