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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GDT

  • Establish a reference coordinate system.

  • Tell me where ‘perfect’ is.

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


Introduction
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

  • Datum Feature Symbol


Three plane concept
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 concept1
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 concept2
Three-Plane Concept

  • the tertiary datum plane is  to the primary & sec. datums

  • 1 point minimum contact


Three plane concept circular
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 circular1
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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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
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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