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CAD/Fab Intro. to GD&T

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What is Geometric Dimensioning

and Tolerancing (GD&T)?

A means of dimensioning and tolerancing a

drawing with respect to the actual function

or relationship of part features which can be

most economically produced

CAD/Fab Intro. to GD&T

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Why Use GD&T?

• IT SAVES MONEY!

• Provides maximum production tolerances

– “Bonus” tolerances often provided

• Helps achieve design intent

• Easier to measure

• Ensures interchangeability of mating parts

• Reduces guesswork & controversy

• Maximizes quality

CAD/Fab Intro. to GD&T

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99% Quality Means…

• 20,000 lost pieces of mail per hour

• Unsafe drinking water 15 minutes per day

• 5,000 incorrect surgical procedures per week

• Two short or long landings at most major

airports per day

• 200,000 wrong drug prescriptions per year

• No electricity for 7 hours per month

CAD/Fab Intro. to GD&T

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Notes vs. Symbols

The notes above and symbols
provide the same control over
part features and relationships

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CAD/Fab Intro. to GD&T

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Notes vs. Symbols

• A symbol has uniform meaning

• Symbols are an international language

– No translation necessary

• Symbols are more legible

– Drawings are often copied w/ lower quality

• Symbols are compact – notes take more

time & space

.001

A

SURFACE B PERPENDICULAR
TO DATUM A WITHIN .001

=

CAD/Fab Intro. to GD&T

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Feature Control Frame

C

Tertiary Datum

Primary Datum

A

Secondary Datum

B

M

M

Geometric Characteristic Symbol

14 possible characteristics

Tolerance Value

.005

Zone Descriptor

The “alphabet” of GD&T

Material Condition Modifier

M

CAD/Fab Intro. to GD&T

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• Maximum Material Condition (MMC)

– Largest possible external feature (e.g. shaft)
– Smallest possible internal feature (e.g. hole)

Material Condition

Allowable

variation

M

• Least Material Condition (LMC)

– Smallest possible external feature
– Largest possible internal feature

• Regardless of Feature Size (RFS)

– Default condition if not specified

L

CAD/Fab Intro. to GD&T

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Five Categories of Control

Description

Type of

Geometric Characteristic

Symbol

Control

Straightness

Individual

Form

Flatness

Features

Circularity
Cylindricity

Individual

Profile

Profile of a Line

or Related

Profile of a Surface

Angularity

Orientation

Perpendicularity

Parallelism

Related

Position

Features

Location

Concentricity

Symmetry

Runout

Circular Runout

Total Runout

3

CAD/Fab Intro. to GD&T

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As drawn

As manufactured –

Without geometric tolerance control

CAD/Fab Intro. to GD&T

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Table Example

1

30

±

•

Flatness of top controlled within 2 in.

• Tighter control of flatness is desirable

e.g. +/- 1/8 in.

• But that height tolerance is too restrictive

0.25

GD&T flatness

specification

CAD/Fab Intro. to GD&T

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Control shaft cross section with
tight tolerance where necessary

Relax shaft straightness
requirement

Shaft with Linear Ball Bushing

• Good way to control linear motion

• Requires precise size & circularity
of shaft

• This same level of precision for
straightness is neither necessary
nor feasible

CAD/Fab Intro. to GD&T

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Datum

A theoretically exact

point, axis

or

plane

derived from the true geometric counterpart

of a specified datum feature

A datum is the origin from which the location

or geometric characteristics of features of a

part are established

A

Datum feature symbol

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CAD/Fab Intro. to GD&T

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Datum vs. Datum Feature

Ideal

Real

Perfect

Imperfect

Point

Axis

Plane

Surface

Hole

Diameter

Slot

etc.

CAD/Fab Intro. to GD&T

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Surface plate

Ground flat and highly polished – simulates datum plane

Datum Simulators

High precision surfaces and gages are used to

simulate theoretically perfect datums, and

therefore allow accurate measurement

CAD/Fab Intro. to GD&T

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V aligns centers of

circular objects,

regardless of size

Verifying Form

Gages are used to measure feature variation

V-blocks are used with

cylindrical shapes

CAD/Fab Intro. to GD&T

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Controlling Runout

Establishes axis of this
circular feature at Datum A

Specifies the allowable runout of this

surface with respect to Datum A

Datum

feature

Datum

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CAD/Fab Intro. to GD&T

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Verifying Form

Gages are used to measure variation - e.g. “runout”

Motion of tip is displayed to 0.001” precision

Dial indicator

CAD/Fab Intro. to GD&T

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Planar Datum Example

A

Datum plane A is bottom plane of a perfect part

CAD/Fab Intro. to GD&T

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Planar Datum Example

Part

Datum simulator

Simulated datum plane A
Rests on 3 highest points of datum simulator

Neither the part nor the datum

simulator are perfectly smooth

Datum feature

(roughness exaggerated

CAD/Fab Intro. to GD&T

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Planar Datum Example

Datum simulator

Part

•

When the part touches the datum simulator,

they only contact at a few points

Datum plane A

(Theoretical)

•

These points of actual contact establish

theoretical datum plane A

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CAD/Fab Intro. to GD&T

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Orthogonal Datum Planes

Primary Datum Plane

3 points of contact

3 degrees of freedom

Secondary Datum Plane

2 points of contact

1 d.o.f.

Primary Datum Plane

3 points of contact

3 degrees of freedom

Tertiary Datum Plane

1 point of contact

0 d.o.f.

(i.e. part is fixed)

Primary Datum Plane

3 points of contact

3 degrees of freedom

Secondary Datum Plane

2 points of contact

1 d.o.f.

“3-2-1” Rule

CAD/Fab Intro. to GD&T

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CAD/Fab Intro. to GD&T

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Indicating Datum Features

C

A

B

Datum planes

CAD/Fab Intro. to GD&T

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Indicating Datum Features - Axes

A

0.1 M A B C

D

12.00-12.07

In each case, the datum is the axis of

the hole or cylindrical datum feature

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CAD/Fab Intro. to GD&T

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Internal Cylindrical Datum - RFS

Actual part

Datum feature A

A

Drawing

Datum feature simulator

Variable diameter cylinder

• The gage is expanded until it contacts the high points of the hole

• The axis of the gage at that point becomes Datum Axis A

• The material condition is “regardless of feature size” (RFS)

CAD/Fab Intro. to GD&T

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Expanding Mandrel Gage

Diameter changes as sleeve is moved along taper

Axis of mandrel establishes datum axis

CAD/Fab Intro. to GD&T

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Datum Center Plane

(Internal)

Simulated datum A –

parallel planes at

maximum separation

Datum feature

simulator

Datum A

feature

(slot width)

Datum A

center plane

CAD/Fab Intro. to GD&T

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Example