7/28/2019 GD&T_3

1/38

1

GEOMETRIC DIMENSIONING AND

TOLERANCING[GD & T]

7/28/2019 GD&T_3

2/38

2

DATUMS

DATUM SYSTEM

Set of symbols and rules that communicate to the drawing user howdimensional measurements are to be made.

DATUM

A datum is a theoretically exact plane, point or axis from which adimensional measurement is made.

A Datum is the true geometric counter part of a datum feature. A true geometric counter part is the theoretical perfect boundary or

best fit tangent plane of a datum feature.

DATUM FEATURE

A datum feature is a part feature that exists on the part andcontacts a datum.

SIMULATED DATUM

A simulated datum is the plane established by the inspectionequipment.

DATUM FEATURE SIMULATOR

A datum feature simulator is the inspection equipment that includesthe gage elements used to establish the simulated datum.

7/28/2019 GD&T_3

3/38

3

DATUM SELECTION

Datum features are selected on the basis of part function andassembly requirements.

Datum features often orient (stabilize) and locate the part in itsassembly.

DEGREES OF FREEDOM

The part is immobilized using primary, secondary & tertiary datums toarrest six degrees of freedom. It should be understand that the

contact of datum surfaces will always be made on high points.

7/28/2019 GD&T_3

4/38

4

In the example shown the primary datum will locate the part withthree contact points, the secondary with two contact points, the

tertiary with one contact point.

Datums Degrees of freedom arrestedPrimary 1 Translational, 2 Rotational

Secondary 1 Translational, 1 Rotational

Teritary 1 Translational

7/28/2019 GD&T_3

5/38

5

DATUM ORDER OF PRECEDENCE

The part is aligned with the datum planes of a reference frame using3-2-1 contact alignment. 3 points of contact align the part to the

primary datum plane, 2 points of contact align the part to the

secondary datum plane, and 1 point of contact aligns the part with the

tertiary datum plane.

7/28/2019 GD&T_3

6/38

6

Fig. The tertiary datum plane is perpendicular to primary and

secondary planes and is located by the tertiary datum feature on apart.

DATUM FEATURE SYMBOLS

7/28/2019 GD&T_3

7/38

7

DATUM REFERENCE FRAME

7/28/2019 GD&T_3

8/38

8

DATUM TARGET SYMBOL

7/28/2019 GD&T_3

9/38

9

MULTIPLE DATUM

FEATURE CONTROL FRAME WITH DATUM FEATURE

ALL ROUND SYMBOL

DATUM FEATURE AT MMC

7/28/2019 GD&T_3

10/38

10

COMPOSITE FEATURE CONTROL FRAME

TWO SINGLE SEGMENTED FEATURE CONTROL FRAME

FEATURE CONTROL FRAME WITH PROJECTED TOLERANCE ZONE

MATERIAL CONDITION SYMBOLS AND MEANING

Maximum material condition Least material condition Regardless of feature size

7/28/2019 GD&T_3

11/38

11

Maximum material conditionMaximum material condition is the condition of a feature where in the

feature contains the most material. It is often thought of as the heaviest

feature. It is the smallest hole or the largest shaft.

Least material conditionLeast material condition is the condition of a feature where in feature

contains the least material. It is often thought of as the lightest feature. It

is the largest hole or the smallest shaft.

7/28/2019 GD&T_3

12/38

12

Regardless of feature sizeThis condition implies that geometric tolerance is to remain the same no

matter what size hole or shaft is produced. RFS concept is used wherebalance is important.

Modifier Commonly used in these

functional applications

Bonus or

datum shift

permissible

Relative cost to

produce &

verify

Assembly Location of non critical

FOS (feature of size)

Yes Lowest

Minimum wall thickness Minimum part distance Minimum m/c stock

YesGreater thanMMC,less than

RFS

To control a symmetricalrelationship

When the effects ofbonus or datum shift will

be detrimental to the

function Centering Alignment

No Highest

7/28/2019 GD&T_3

13/38

13

7/28/2019 GD&T_3

14/38

14

STATISTICAL TOLERANCE

Total Tolerance = Tolerance A + Tolerance B

Tolerance stackups occur as a result of assembling components thathave individual tolerances specified on them. If the tolerances areadded up with worst case calculations the total tolerance can assume

a large value.

The components manufactured by a process under statistical controlshow dimensions that are normally distributed. A normal distribution

has a very small area under its tails. In a process under statistical

control and good process capability, the probability of the components

getting produced at the border of tolerance zone is much smaller than

at center.

Statistical tolerancing may be used for increased individual tolerance.The increased tolerance may reduce manufacturing cost, but shall only

be employed where the appropriate statistical control will be used.

Features identified as statistically tolerance shall be produced withSPC.

7/28/2019 GD&T_3

15/38

15

Features identified as statistically tolerance shall be produced withstatistical process controls, or to the more restrictive arithmetic

limits.

STATISTICAL TOLERANCE WITH GEOMETRIC

CONTROLS

FREE STATE VARIATION

Free State variation is the variation that occurs due to the distortionof the part when the forces applied released by clamping.

This tolerance implies that the measurement needs to be done whenall the restraining forces on the workpiece that are applied for

clamping or machining are removed.

7/28/2019 GD&T_3

16/38

16

A free state tolerance is often applied to the parts with thin sections& parts which can easily be distored due to the external force.

TANGENT PLANE

Tangent plane that contacts the high points of a surface. Tangentplane symbol can be applied to many of the geometric characteristics.The tangent plane symbol is shown below with the parallelism

tolerance.

7/28/2019 GD&T_3

17/38

17

The tangent plane must lie between two parallel planes 0.2 apart whichare parallel to datum plane A. In addition, the surface must be within

the specified limits of size or profile.

PROJECTED TOLERANCE ZONE

When specifying a threaded hole or a hole for a press fit pin, theorientation of the hole determines the orientation of the mating pin.

Although the location and orientation of the hole and the location of

the pin will be controlled by the tolerance zone of the hole, the

orientation of the pin outside the hole cannot be guaranteed, as shown

in Fig. a. The most convenient way to control the orientation of the pin

outside the hole is to project the tolerance zone into the mating part.

The tolerance zone must be projected on the same side and at thegreatest height of the mating part, as shown in Fig.b. The height of

the tolerance zone is equal to or greater than the thickest mating

part or tallest stud or pin after installation. In other words, the

tolerance zone height is specified to be at least as tall as the MMC

thickness of the mating part or the maximum height of the installed

7/28/2019 GD&T_3

18/38

18

stud or pin. The dimension of the tolerance zone height is specified as

a minimum.

7/28/2019 GD&T_3

19/38

19

DATUM TARGETS

Datum targets designate specific points, lines or areas of contact on apart that are used in establishing a datum reference frame.

Because of inherent irregularities, the entire surface of somefeatures cannot be effectively used to establish a datum.

Examples are nonplanar or uneven surfaces produced by casting,forging or molding, weldments etc.

Datum targets may be used to set up the datum reference frame. Inpast these points had a variety of different names such as : set uppoints, principle locating points, tooling points, fixture points etc. All

of these names have been discarded and we now call these points

datum targets.

DATUM TARGET SYMBOLS

The use of solid leader line indicates that the datum target is on thenear (visible) surface.

The use of dashed leader line indicates that the datum target is onthe far (hidden) surface.

7/28/2019 GD&T_3

20/38

20

DATUM TARGET POINTS A datum target point is indicated by the target point symbol,

dimensionally located on a direct view of a surface. Where there is no

direct view, the point location is dimensioned on two adjacent views.

DATUM TARGET LINES

A datum target line is indicated by the target point symbol on an edgeview of the surface, a phantom line on direct view or both. Where the

7/28/2019 GD&T_3

21/38

21

length of datum target line must be controlled, its length and location

are dimensioned.

DATUM TARGET AREAS

Where it is determined that an area or areas of contact is necessaryto assure establishment of the datum, a target area of desired shape

is specified. The datum target area is indicated by section lines inside

a phantom outline of a desired shape, with controlling dimensions

added.

7/28/2019 GD&T_3

22/38

22

7/28/2019 GD&T_3

23/38

23

PRIMARY DATUM PLANE ESTABLISH BY THREE DATUM TARGET AREAS

7/28/2019 GD&T_3

24/38

24

PRIMARY DATUM PLANE ESTABLISH BY TWO DATUM TARGET POINTS

AND ONE DATUM TARGET LINE

COMPOSITE TOLERANCE

Composite tolerance concepts can be applied on position and profile. Here we see composite tolerance with position only.

7/28/2019 GD&T_3

25/38

25

COMPOSITE POSITION TOLERANCING

The PLTZF is located to applicable datums with basic dimensions. Itspecifies the larger positional tolerance for the location of the

pattern of features as a group. Applicable datums are entered in the

upper segment in their order of precedence. The upper segment

locates and orients the features to each other as well as specified

datums.

The FRTZF specifies a smaller position tolerance for the feature tofeature relationship within the pattern. Basic dimensions apply

between the features but do not apply to the datums. The lower entry

is orientation only to the specified datums not location.

If datums are not specified in the lower segment, the FRTZF isallowed to tilt, rotate, and/or shift within the confines of the PLTZF.

If datums are specified, they govern orientation (not location) of the

FRTZF relative to specified datums.

DIFFERENCE BETWEEN A COMPOSITE TOLERANCE AND TWO SINGLE

SEGMENTED FEATURE CONTROL FRAMES

7/28/2019 GD&T_3

26/38

26

COMPOSITE POSITION FRAME

TWO SINGLE SEGMENTED FRAMES

[ NOTE : The difference between the terms location and orientation

should be clear. Location locates features and is associated with basic

linear dimensions. It can also include orientation. Orientation on the

other hand, is not associated with location or with basic linear

dimensions, only basic angles. Orientation is usually thought of as

parallelism, perpendicularity or angularity.]

APPLICATION OF COMPOSITE TOLERANCING

Composite position tolerancing can be used where the locations of theholes are important to each other, but the relation of the holes,

7/28/2019 GD&T_3

27/38

27

relative to datums are not as important. This might be an application

where a switch or bracket attaches with the holes but the location of

the bracket or switch has less importance to the edges.

COMPOSITE POSITION TOLERANCING ONE DATUM FEATURE

7/28/2019 GD&T_3

28/38

28

MEANS THIS

NOTES:

1) The 0.8 Dia. PLTZF is basically located and oriented to the datumreference frame.

2) The 0.25 Dia. FRTZF is basically located and oriented between thefeatures and basically oriented perpendicular to the datum reference

frame. (Plane A)

3) The FRTZF may skew, rotate and/or be displaced within the confinesof PLTZF. The axis of the holes must be in both zones simultaneously.

7/28/2019 GD&T_3

29/38

7/28/2019 GD&T_3

30/38

30

NOTES:

1) The 0.8 Dia. PLTZF is basically located and oriented to the datumreference frame.

2) The 0.25 Dia. Positional zones (FRTZF) are basically located &oriented to each other and basically oriented (perpendicular &

parallel) to the datum reference frame (Plane A & B). Basic dimensions

are unlocked to the datums.

3) The FRTZF may move up & down or left/right within the confines ofPLTZF but it may not skew or rotate. The axis of holes must lie in

both zones simultaneously. The axis of the holes may only rotatewithin the confines of the FRTZF.

POSITION TOLERANCE WITH TWO SINGLE SEGMENTED FRAMES

7/28/2019 GD&T_3

31/38

31

MEANS THIS

NOTES:

1) The 0.8 Dia. Positional zones are basically located & oriented to thedatum reference frame.

2) The 0.25 Dia. Positional zones are basically located & oriented to eachother & basically located & oriented to the DRF. Since this is two

single segmented feature control frames and not composite, the basicdimension applies to datum B.

3) The 0.25 zones, as a group may move left & right within the confinesof the 0.8 zone, but are located up & down to datum B. The 0.25 zones

may not skew or rotate. The axis of the holes must lie in both zones

7/28/2019 GD&T_3

32/38

32

simultaneously and may only rotate within the confines of the 0.25

zones.

VIRTUAL & RESULTANT CONDITION

VIRTUAL CONDITION:

Constant value outer locus & constant value inner locus values arederived & termed virtual condition.

RESULTANT CONDITION:

Worst case inner locus & worst case outer locus values are derived &termed resultant condition.

7/28/2019 GD&T_3

33/38

33

7/28/2019 GD&T_3

34/38

34

7/28/2019 GD&T_3

35/38

35

7/28/2019 GD&T_3

36/38

36

7/28/2019 GD&T_3

37/38

37

7/28/2019 GD&T_3

38/38