chap3_datums

Geometric Dimensioning and Tolerancing

Chapter 3, Datums

EML 2023Department of Mechanical and Aerospace Engineering 2

1. Establish a reference coordinate system.

2. Tell me where ‘perfect’ is.

3. 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.

• Datums define 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 bea construction line(or omitted)

should havea 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

Datum features H and L are mated with atheoretical perfect plane contacting thehigh points of the datum feature.

Theory

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

6±0.252X

12.7±0.1

±0.325

Physical

Datum features H and L are mated on the high points of the manufacturing or inspection equipment

Establishing Datums from Primary Plane Surfaces

Physical datum feature simulator(surface plate orprocessing equipment)

WorkpieceDatumfeature

Physical

Establishing Datums from Primary Plane Surfaces

W O R K P IE C E

S U R F A C E

Theoretical datum plane Magnified View

Simulated datum plane

Datum feature simulator

Datum feature

Tool and gage variation

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

M2 SURFACES

6±0.252X

12.7±0.1

±0.325

S 17±0.02

Datum planes established from planar datum features

Datum feature M estab-lishes a plane on the highpoints of the left side of theslot.

Datum feature Kestablishes a plane onthe high points of twosurfaces.

Datum feature H estab-lishes a plane on the highpoints of the bottomsurface.

Datum feature L estab-lishes a plane on the highpoints of the rear surface.

Datum feature N estab-lishes a plane on the highpoints of the left surface.

Datum Features without Size

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

M2 SURFACES

6±0.252X

12.7±0.1

±0.325

S 17±0.02

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

M2 SURFACES

6±0.252X

12.7±0.1

±0.325

S 17±0.02

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

M2 SURFACES

6±0.252X

12.7±0.1

±0.325

S 17±0.02

M-10x1.25

5H7(5/5.012)±0.215.9

25±0.3

M2 SURFACES

6±0.252X

12.7±0.1

±0.325

S 17±0.02

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

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

Datum Features of Size

6±0.252X

12.7±0.1

25±0.1

SR 8.5

E M10x1.25

5H7(5/5.012)±0.215.9

25±0.1

6±0.252X

12.7±0.1

25±0.1

SR 8.5

E M10x1.25

5H7(5/5.012)±0.215.9

25±0.1

6±0.252X

12.7±0.1

25±0.1

SR 8.5

E M10x1.25

5H7(5/5.012)±0.215.9

25±0.1

Sim ulated datum center plane B is

the center plane of the gage block.

PhysicalDatum center plane B is

the center plane of thedatum feature simulator.

Theory

Datum axis C is the axis of the datum feature simulator

TheorySimulated datum axis C is the axis of the gage pin

Physical

6±0.252X

12.7±0.1

25±0.1

SR 8.5

E M10x1.25

5H7(5/5.012)±0.215.9

25±0.1

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

Theory

6±0.252X

12.7±0.1

25±0.1

SR 8.5

E M10x1.25

5H7(5/5.012)±0.215.9

25±0.1

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

Physical

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

Datum feature symbol Bin line with the dimensionline establishes the centerplane of the slot.

Datum feature symbolM on the extension lineestablishes a plane fromthe high points on thesurface.

Datum feature symbol R inline with the dimension line,establishes center plane ofthe width.

Datum featuresymbol K on theextension line,establishes aplane from thehigh points onthe surface.

Datum Feature Symbol Placement is Important

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

M10x1.25

5H7(5/5.012)±0.215.9

25±0.2

12.7±0.1

2X ±0.256

S 17±0.2

Evaluationspecified

dimension , orientation

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

90circle

sizeXX

Xtolerance

conveyX

rocking

chap3_datums

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