Geometric Dimensioning and Tolerancing

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# Geometric Dimensioning and Tolerancing - PowerPoint PPT Presentation

Geometric Dimensioning and Tolerancing. Chapter 3, Datums. Introduction. Datums are nothing more and nothing less than the physical features of parts from which to make repeatable measurements . These features are considered theoretically exact .

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### Geometric Dimensioning and Tolerancing

Chapter 3, Datums

Introduction
• Datums are nothing more and nothing less than the physical features of parts from which to make repeatable measurements.
• These features are considered theoretically exact.
• Datumsdefine a coordinate system from which to measure everything.
• Datums are usually a plane, a line, or a point.
• We will need to label the datums on our drawing so that the part inspector will know from where to measure.
Datum Feature Symbol
• Datum Feature Symbol
Three-Plane Concept
• define primary, secondary, and tertiary datum planes
• the primary datum is the one that functionally is usually the most critical feature or surface of the part
• the primary datum feature of the part will make contact with the theoretical datum plane in a minimum of three points
Three-Plane Concept
• define primary, secondary, and tertiary datum planes
• the secondary datum plane is  to the primary datum
• secondary datum feature is the one that functionally is the 2nd most critical feature or surface of the part
• 2 point minimum contact
Three-Plane Concept
• define primary, secondary, and tertiary datum planes
• the tertiary datum plane is  to the primary & sec. datums
• 1 point minimum contact
Three-Plane Concept - Circular
• circular parts also require a three-plane concept for repeatable orientation
• the primary datum is usually one flat surface of the part,
• defines a datum axis along the center of cylinder

should be

a construction line

(or omitted)

should have

a center line

Three-Plane Concept - Circular
• datum A defines an axis along the center of the cylinder
Partial Datums
• Occasionally, designs require a datum on a particular surface, but not necessarily the entire surface.

Features with

and without

Size

1.9

Establishing Datums

from Primary

Plane Surfaces

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e

h

i

g

h

p

o

i

n

t

s

o

f

t

h

e

r

e

a

r

s

u

r

f

a

c

e

.

D

a

t

u

m

f

e

a

t

u

r

e

M

e

s

t

a

b

-

l

i

s

h

e

s

a

p

l

a

n

e

o

n

t

h

e

h

i

g

h

p

o

i

n

t

s

o

f

t

h

e

l

e

f

t

s

i

d

e

o

f

t

h

e

s

l

o

t

.

D

a

t

u

m

f

e

a

t

u

r

e

K

e

s

t

a

b

l

i

s

h

e

s

a

p

l

a

n

e

o

n

t

h

e

h

i

g

h

p

o

i

n

t

s

o

f

t

w

o

s

u

r

f

a

c

e

s

.

Datum Features without Size

P

L

5

H

7

(

5

/

5

.

0

1

2

)

1

5

.

9

±

0

.

2

0

.

2

2

5

±

0

.

3

M

-

1

0

x

1

.

2

5

N

M

2

S

U

R

F

A

C

E

S

K

1

2

.

7

±

0

.

1

H

2

5

±

0

.

3

2

X

6

±

0

.

2

5

S

1

7

±

0

.

0

2

D

a

t

u

m

p

l

a

n

e

s

e

s

t

a

b

l

i

s

h

e

d

f

r

o

m

p

l

a

n

a

r

d

a

t

u

m

f

e

a

t

u

r

e

s

6.16

C

R

1

5

.

9

±

0

.

2

5

H

7

(

5

/

5

.

0

1

2

)

2

5

±

0

.

1

M

1

0

x

1

.

2

5

E

F

A

1

2

.

7

±

0

.

1

B

2

5

±

0

.

1

2

X

6

±

0

.

2

5

S

R

8

.

5

0

.

2

M

0

.

4

D

G

Theory

Physical

Simulated datum axis E is the axis of a collet, chuck, CMM etc.

Datum axis E is the axis of the theoretical datum feature simulator

Datum Features of Size

6.18

C

R

1

5

.

9

±

0

.

2

5

H

7

(

5

/

5

.

0

1

2

)

2

5

±

0

.

1

M

1

0

x

1

.

2

5

E

F

A

1

2

.

7

±

0

.

1

B

2

5

±

0

.

1

2

X

6

±

0

.

2

5

S

R

8

.

5

0

.

2

M

0

.

4

D

G

Datum Features of Size

6.19

C

R

1

5

.

9

±

0

.

2

5

H

7

(

5

/

5

.

0

1

2

)

2

5

±

0

.

1

M

1

0

x

1

.

2

5

E

F

A

1

2

.

7

±

0

.

1

B

2

5

±

0

.

1

2

X

6

±

0

.

2

5

S

R

8

.

5

0

.

2

M

0

.

4

D

G

Theory

Physical

D

a

t

u

m

c

e

n

t

e

r

p

l

a

n

e

B

i

s

t

h

e

c

e

n

t

e

r

p

l

a

n

e

o

f

t

h

e

S

i

m

u

l

a

t

e

d

d

a

t

u

m

c

e

n

t

e

r

p

l

a

n

e

B

i

s

d

a

t

u

m

f

e

a

t

u

r

e

s

i

m

u

l

a

t

o

r

.

t

h

e

c

e

n

t

e

r

p

l

a

n

e

o

f

t

h

e

g

a

g

e

b

l

o

c

k

.

Datum Features of Size

6.19

C

R

1

5

.

9

±

0

.

2

5

H

7

(

5

/

5

.

0

1

2

)

2

5

±

0

.

1

M

1

0

x

1

.

2

5

E

F

A

1

2

.

7

±

0

.

1

B

2

5

±

0

.

1

2

X

6

±

0

.

2

5

S

R

8

.

5

0

.

2

M

0

.

4

D

G

Theory

Physical

Datum axis C is the axis of the datum feature simulator

Simulated datum axis C is the axis of the gage pin

Datum Features of Size

6.20

C

R

1

5

.

9

±

0

.

2

5

H

7

(

5

/

5

.

0

1

2

)

2

5

±

0

.

1

M

1

0

x

1

.

2

5

E

F

A

1

2

.

7

±

0

.

1

B

2

5

±

0

.

1

2

X

6

±

0

.

2

5

S

R

8

.

5

0

.

2

M

0

.

4

D

G

Theory

Simulated datum feature D axis and two intersecting planes established from gage

Physical

Datum D axis and two planes are established from a pattern of two holes

Datum Features of Size

6.20

Evaluation

specified

dimension , orientation

planes, axis, center lines, edges, points, areas

no

three

90

circle

X

size

X

X

tolerance

X

X

convey

rocking

X