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GD&T 3 - Position

CAD/Fab GD&T - Position 2

Position Control

• Most powerful and versatile control in GD&T

• Capitalizes on MMC bonus tolerances

• Can be used to :

– Locate patterns of holes

– Locate center of feature (e.g. slot)

– Control perpendicularity of holes, etc.

• Easy to check with functional gages

• Insures proper fit of assemblies


Coordinate Dimensioning

1.995

2.005

1.495

1.505

Allowable location for hole center


Coordinate Tolerancing vs. GD&T

X 0.005

Y 0.005.014

57% more area

.010

These points would be

rejected using coordinate

dimensions – but they would

still function correctly

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GD&T Position Dimensioning

DatumsBasic

dimensions

GD&T control frame


BASIC DimensionA numerical value used to describe the theoretically

exact size, profile, orientation or location of a

feature or datum target

Exact (theoretical) value

Box indicates BASIC dimension

Hole size

parameters

The associated tolerance for a BASIC dimension is

given in the GD&T control frame

Implied tolerances do not apply!

Tolerance of location


BASIC Dimensions and Datums

BASIC dimensions must relate to datums

Why Datum D?


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


Tertiary Datum Plane

1 point of contact

0 d.o.f.

(i.e. part is fixed)

Primary Datum Plane

3 points of contact


Secondary Datum Plane

2 points of contact

1 d.o.f.

“3-2-1” Rule

3


How to Measure Holes• Diameter

– Specified by diameter and tolerance (MMC/LMC)

– Measure accurately with micrometer/caliper

– Checked with GO – NO GO gages

• Position

– Specified by GD&T frame & basic dimensions

– Measure accurately with optical comparator

– Checked with functional gages


GO – NO GO Gages

Hole

LMCHole

MMC

GO NO GO Handle

Allowable variation

in hole diameter


GO – NO GO Gages

Gage can enter “good” hole

– but not past “NO GO step


Checking Gear Hole Size

4


GO – NO GO Gages

Go No Go

Go No Go

Gage tolerance –

0.0002” – 0.00002”

$36 $57


26.18 maxHoles at MMC (12.70)

at max distance

25.82 min

Holes at MMC (12.70)

at min distance

Gage pins

dpin = 12.70 – 0.18 = 12.52

When two holes are located at true position at MMC (min. dia.), they

may be gaged with pins 12.52 mm in dia. located 26.00 mm apart

Gaging True Position Holes

Centers of holes must lie

within 0.18 tolerance zone


Pin Gage Dimensions

Pin location specified by

BASIC dimensions

Pin diameter = hole diameter @ MMC –

GD&T position tolerance

i.e. dia = 12.70 – 0.18 = 12.52 mm

Part Drawing

Gage Drawing


Pin Gage

Datum A

Datum B

Datum C

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Position Lab

Snowboard Binding

Perfect part

Reject!


Combined Datums

Combined

datum –

centerline

joining A & B

A B

C

A gear will be

pressed onto

a gear shaft

which runs in

ball bearings

as shown.

Identify

datums on

the drawing

of the shaft to

ensure

proper

operation of

the gear shaft

assembly.


Checking Shaft Runout

1. V-blocks center shaft at each end

3. Dial indicator measures

tip movement accurately

2. Shaft rotated in

V-blocks


Combined Datums

CD

B

A

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