Geometric dimensioning and tolerancing
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Geometric Dimensioning and Tolerancing. Chapter 3, Datums. GDT. Establish a reference coordinate system. Tell me where ‘perfect’ is. Tell me how far off from ‘perfect’ is allowed. Introduction.

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Geometric Dimensioning and Tolerancing

Chapter 3, Datums


GDT

  • Establish a reference coordinate system.

  • Tell me where ‘perfect’ is.

  • Tell me how far off from ‘perfect’ is allowed.


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

  • 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

  • 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


Features with

and without

Size

1.9


Establishing Datums

from Primary

Plane Surfaces

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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


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


Datum Feature Symbol Placement is Important

6.21


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


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