ADVANCED MECHANICAL DRAFTING

LECTURE #8

Challenge ofMechanical Design

Only discipline where tolerances of 100ths, 1000ths, or 10,000ths

of an inch affect part function

Limitations of Standard Dimensioning Practices

Difficult to control form (shape), orientation, location, etc. by

measuring from other manufactured features

GEOMETRIC DIMENSIONING & TOLERANCING

• Implemented by General Motors in the 1940’s to augment standard dimensioning practices

• A system that defines zones of tolerance within which features must lie

• Dimensions that control zone locations and sizes are different than standard dimensions– BASIC dimensions control locations of tolerance zones

• No tolerance – considered perfect

– FEATURE CONTROL FRAMES control shapes and sizes of zones– DATUM FEATURE SYMBOLS identify reference features

• All three items show in frames (boxes) on the drawing

.006 A B A8.500

EXAMPLE4.00

2.00

THISon the

drawing

COULD LOOK LIKE

4.00

2.00

THISwhen

machined

DIMENSIONS ALONE CANNOT ADEQUATELY CONTROL THE SHAPE OF THE PART

EXAMPLE

1.5031.498

1.5031.498

.005 SQUARE

ZONE

+

+

+

+

+

+

+

+

+

Square position tolerance zones vs round position zones

“+” = actual hole center

Tolerance zone should be round with a diameter

equal to the diagonal distance

across the rectangular zone

THE ROUND ZONE IS 57% LARGER IN AREA THAN

THE SQUARE ZONE !

EXAMPLE

Ø1.500 (1-1/2) THRU

Choose reference features

Locate tolerance zone

Specify shape and size of zone

Note that the part of the dimension outside the frame applies to the feature and the part inside the frame applies to the tolerance zone

DRAWING BASIC DIMENSIONS

1.562

GEOMETRIC SYMBOLS

LOCATION CONTROL

FORMCONTROL

RUNOUTCONTROL

SEE HANDOUTS FOR SYMBOL

CONSTRUCTION

GEOMETRIC SYMBOLSDATUM

REFERENCE REQUIRED

NO DATUM REFERENCE

FEATURE CONTROL FRAMES

g

eom

etri

c ch

arac

teri

stic

tole

ran

ce z

on

e sh

ape

tole

ran

ce z

on

e si

ze

mat

eria

l co

nd

itio

n m

od

ifie

r

pri

mar

y d

atu

m

seco

nd

ary

dat

um

tert

iary

dat

um

REQUIRED IN EVERY FRAME

DEPENDENT ON ZONE TYPE

DRAWING FEATURE CONTROL FRAMES

• Click on tolerance button in dimension toolbar

• Select elements required in frame

• Place frame in drawing

FEATURE CONTROL FRAME EXAMPLES

.005 A

FEATURE CONTROL FRAME EXAMPLES

Ø.005 A B C

FEATURE CONTROL FRAME EXAMPLES

Ø.005 A B C

.011 A

DATUM THEORY

• Any feature (point, line, plane, axis, surface, hole, etc.) can be a datum (measuring reference)

• Considered perfect

• Datum features specified on the drawing are simulated in the inspection process

SIMULATED DATUM

SIMULATED DATUM

SURFACE

SPECIFIED DATUM

FEATURE

SPECIFIED MEASUREMENT

ACTUAL MEASUREMENT

DATUM PRECEDENCE

• Follows order listed in feature control frame

• As many as three datums may be needed to adequately control the location of a tolerance zone

• Levels of precedence:

– Primary datum – SUPPORTS THE PART – 3 points of contact with simulated datum

– Secondary datum – ALIGNS THE PART – 2 points of contact with simulated datum

– Tertiary datum – STOPS THE PART - 1 point of contact with simulated datum

Object with 3 datums

THREE POINTS OF CONTACT

THREE POINTS OF CONTACT

TW

O P

OIN

TS

OF

CO

NT

AC

T

THREE POINTS OF CONTACT

TW

O P

OIN

TS

OF

CO

NT

AC

T

ONE POINT OF CONTACT REQUIRED ON REAR SURFACE

DATUM SELECTION CRITERIA

• Function – how do features affect the way the entire part works

• Relationship to mating part or feature - how do features mate with other part

• Accessibility – can you measure from the feature ?

GSK DESIGN