Five Things You Need to Know

About GD&TJohn-Paul Belanger

Certified Sr. GD&T Professional

July 17, 2014

Agenda

Today, we will be discussing the following topics:

• What is GD&T?

• The true benefits of GD&T usage

• More than learning some symbols

• Dispelling some myths

• Five things to remember!

• Questions/Answers

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What Is GD&T?(One answer: A language)

Basics of GD&T

• Stands for “geometric dimensioning and tolerancing”

• Shown on a print with symbols

• Think of it as a language

• Sometimes creates confusion (not the fault of GD&T!)

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Sample GD&T Drawing

Why Would That Print Be Confusing?

• I am not fluent the GD&T language

• Some of the GD&T may be incorrect / illegal

• The GD&T requirements are not practical for the part’s function

• The GD&T doesn’t tell me how to make the part

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Consider the Following…

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Suppose I’m an art collector, and I want a replica of “Starry Night”

What requirements would I demand of the person making the replica?

Requirements

It depends on what I will use it for, and also on what I am willing to pay!

• If it will only be seen from a distance, then it doesn’t need to have

textured brush strokes; a scan of the real picture will suffice

• If it will be seen up close, I might require textured brush strokes

• The textured strokes will make the replica more expensive

• Regardless, I don’t dictate the procedure, only the final requirements

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The Same with Designing a Product!

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-- Function drives the dimensioning and tolerancing

FUNCTION is what should drive the tolerancing. If anything else is

the main driver, then we may be artificially driving up cost

Think of the simple block that was shown before – suppose the

function is to assemble against another part, and then simply

receive a mating pin…

The First Key Point:It’s all about the function

Without GD&T – Is the Tolerancing Based on Function?

Coordinate Tolerancing

A square tolerance zone

doesn’t make much sense

when we think about the

FUNCTION of the hole.

Sample GD&T Drawing

Sample GD&T Drawing The shape of the tolerance

is now cylindrical

The Second Key Point:It doesn’t make inspection more difficult

How to Inspect This Part

Inspection Methods

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Just based on the GD&T, there are 4 things to be checked:

o Location of the 3 bolts

o Profile of the triangular perimeter

o Flatness of the mating surface

o Location of the large tube

In addition to the regular dimensions (not shown)

If we didn’t use GD&T, would it be easier to inspect?

No!

• Those qualities would still have to be inspected, but they would be

documented by different tolerance expressions (plus/minus, etc.)

• What makes parts difficult to inspect is their functional requirements,

not the language used to express those requirements!

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GD&T Often Makes Inspection Easier

• The GD&T system spells out the datums – those features that the

measurements will be taken from. Traditional tolerances sometimes

resulted in ambiguous datums!

• If it seems like the inspection is made more difficult, it might just be that

the designer has overconstrained some things. Remember, it should

always go back to the functional requirements.

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Ambiguous Datum Sequence

Ambiguous Datum Sequence

Ambiguous Datum Sequence

Ambiguous Datum Sequence

The Third Key Point:It’s not just about memorizing symbols, but overall fluency

Anyone Can Memorize the GD&T Symbols

But fluency in GD&T is the

key! You need to

understand the rules and

concepts that form the

“glue” of GD&T – just like

any other language.

More Than Just Learning Symbols – Another Example

Defined by a Technical Standard: ASME Y14.5

1994 edition 2009 edition

The Fourth Key Point:GD&T can actually reduce part cost

How Can GD&T Make the Part Less Expensive?

• We’ve already seen how GD&T clarifies the datums that dimensions are

taken from

• GD&T allows for many different shapes to a tolerance – more versatile

than the plus/minus system

• By giving the designer more tools in the toolbox, certain tolerances can

be isolated and given larger or smaller values as needed

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And… Another Unique Way that GD&T Reduces Cost

• Using the circled M modifier (“maximum material condition”), a GD&T

callout can actually get more tolerance than what’s shown on the

drawing

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M refers to “maximium

material condition,” which

in this case is the smallest

possible hole (more

material on the part)

Using the “M” ModifierOnly a hole made at “MMC” must be within the 0.4…

Ø17.9Ø18.2

Ø18.5

The Fifth Key Point:GD&T is here to stay

A Long History, and a Long Future

• The GD&T system in its earliest form dates from the late 1940s

• Even small shops need to know this language to stay competitive

• On a personal level, fluency in GD&T is a great skill to have

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GD&T Skills Are Essential for Staying Competitive

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– Even after all this if you don’t think GD&T seems valuable, your

suppliers and customers do!

– It’s been around for more than 50 years, and it is here to stay.

Summary

• Geometric Dimensioning & Tolerancing provides more tools in our design toolbox.

• The result should be better communication between design, manufacturing, and inspection, and reduced costs.

• Because GD&T is a language, it requires knowledge and practice.

Learn More About GD&TWe can provide a GD&T solution that is right for your needs. We have a range of resources available, including:

• Instructors

• Online classes

• Books

• Videos

Check out our upcoming Practical GD&T Workshop

• October 22-23 in Chicago, IL

• Attend virtually or in person!

• Go to toolingu.com/gdtworkshop for more info

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If you would like to learn

more, please contact:

Tooling U-SME

866.706.8665

www.toolingu.com