Datums & Datum Targets

90
GD&T Datums Table of Contents Return to the Previous Slide Slide 1 Quit Master Table of Contents Glossary 86 57.6 56.6 100 99 9.6 9.4 25.5 25.4 36 34 41.3 41.1 20.00 - 20.13 0.14 A C B M42 X 1.5 - 6g 0.1 M B M 0.06 A 6.6 6.1 9.6 9.4 44.60 44.45 0.08 M A C A 0.02 31.8 31.6 0.1 B 8X 7.9 - 8.1 8X 45° 0.14 M A C M Copyright © 2002 Brigham Young University All Rights Reserved Datums and Datum Targets Unit Engineering Graphics Principles

Transcript of Datums & Datum Targets

Page 1: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 1 QuitMaster Table of ContentsGlossary

86

57.656.6

10099

9.69.4

25.525.4

3634

41.341.1

20.00 - 20.13

0.14 A C

B

M42 X 1.5 - 6g

0.1 M B M

0.06 A

6.66.1

9.69.4

44.6044.45

0.08 M A

C

A

0.02

31.831.6

0.1 B

8X 7.9 - 8.1

8X 45°

0.14 M A C M

Copyright © 2002Brigham Young University

All Rights Reserved

Datums and Datum Targets

UnitEngineering Graphics Principles

Page 2: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 2 QuitMaster Table of ContentsGlossary

•Datums are the perfect geometric representation of feature characteristics, such as points, lines, axes, or planes.In design, manufacture, and inspection, datums provide the origins from which size and location dimensions, and their related tolerances are calculated.

•Datums are established (During Inspection) by bringing designated actual part features, called datum features, such as external and internal surfaces, into contact with precision surfaces on processing equipment or verifying gages that are called datum simulators(surface plates, collets, mandrels, etc.).

•When geometric tolerances are specified in feature control frames, and the specification is a relatedcontrol, such as with orientation, profile, and runout tolerances, datum references are required.

•Geometric tolerances and functional relationships are verified within a datum reference frame, which consists of three mutually perpendicular datum planes (datum simulators). A mechanical part is oriented and immobilized by bringing the datum features on the part into contact with the simulated datums—following a prescribed order of precedence.

INTRODUCTION:

Page 3: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 3 QuitMaster Table of ContentsGlossary

•A datum is a theoretically “perfect” axis, point, line, or plane. Datums are conceptually identical to the true geometric counterpart of specified part features.

•For example, a datum plane would be a perfectly flat plane, with no defined extents for length, depth, or width, no variation in surface texture, and zero thickness. A datum line would be a perfectly straight line of zero thickness, with zero variation in curvature or form.

DEFINITIONS:

Page 4: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 4 QuitMaster Table of ContentsGlossary

•Producing mechanical parts and feature relationships to standards of perfection is beyond human capabilities, so datums are simulated by imperfect (albeit extremely accurate) special tools and precision gages.

•Similarly, measurements can’t be verified from theoreticalentities. Therefore, precision tools and processing equipment—in addition to inspection gages and instrumentation—are used to represent or simulatetheoretical datums. Surface plates, machine tables, mandrels, centering devices, dial indicators, gage blocks, snap gages, ring and plug gages, coordinate measuring machines etc., are all examples of such equipment used to verify the accuracy of dimensional requirements.

THEORETICAL DATUMS andDATUM SIMULATORS

Page 5: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 5 QuitMaster Table of ContentsGlossary

•A datum featureis an actual feature on a part, and may be in the form of a planar surface, threaded protrusion, slot, hole, etc. Once defined, a specified datum feature is brought into contact with a simulated datum (surface plate, machine table, or special gage) in a very specific order of precedence.

•The points of contact between the datum feature and the simulated datum, establish the theoretical datum,and from this association is born the origin of specified dimensions and tolerances.

•The concept of establishingplanar datums is illustrated in the following series of slides. You may want to view them more than once to become comfortable with the basic philosophy.

CONCEPTS TO REMEMBER

Page 6: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 6 QuitMaster Table of ContentsGlossary

PLANAR DATUM EXAMPLE

Page 7: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 7 QuitMaster Table of ContentsGlossary

A

The base of this object has been identified as datum feature A (see datum feature symbol on the bottom surface). We will consider the relationship between the datum feature, and a datum simulator which, when brought together establish datum plane A.

Planar Datum Example

Page 8: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 8 QuitMaster Table of ContentsGlossary

The irregularities on the datum featurehave been exaggerated to give meaning to the example.

Planar Datum Example

Datum Feature

Part

Page 9: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 9 QuitMaster Table of ContentsGlossary

Datum FeatureSimulated Datum

Part

Likewise, the surface shown on the top of the surface plate is imperfect. A straight flat plane extended across the (3) high points of the surface displays the simulated datum plane A, and the imperfections of the simulated datum.

Datum Simulator

Simulated Datum Plane A (Yellow)

Planar Datum Example

Page 10: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 10 QuitMaster Table of ContentsGlossary

INTERPRETATION

Part

Established Datum Plane A(Red Line)

Datum Simulator

Planar Datum Example

As the part is placed on the surface plate, only 3 specific high points of contact between the two imperfect surfaces will be made. The 3 contact points between the two surfaces establish the theoretical datum plane A.

Datum FeatureSimulated Datum

Simulated Datum Plane A (Yellow)

Page 11: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 11 QuitMaster Table of ContentsGlossary

INTERPRETATION

Datum Simulator

Part

Planar Datum Example

The datum plane A is established by the contact points between the datum feature and the datum simulator.

EstablishedDatum Plane A(Red Line)

Datum FeatureSimulated Datum

Page 12: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 12 QuitMaster Table of ContentsGlossary

DATUM REFERENCE FRAME

Page 13: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 13 QuitMaster Table of ContentsGlossary

A datum reference frame consists of three mutually perpendicular planes which provide an origin and direction for verification of dimensional relationships. A datum reference frame is used when more than one datum plane is necessary to insure that the process of verifying measurements is repeatable.

DATUM REFERENCE FRAME

Page 14: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 14 QuitMaster Table of ContentsGlossary

•A datum reference frame is used to verify part dimensions, and to assure that the part has been produced to the specifications called out on the drawing. The process of measurement verification is accomplished by bringing the specified datum features (features that are significant to design function) into contact with appropriate datum simulators (gages) in a prescribed orderand in such a way as to:

– Specifically relate the features on the part to the datum reference frame, as specified on the drawing.

– Restrict linear or rotational motion of the part by maintaining simultaneous contact with the specified datum features on the part.

– Make the verification process repeatable by specifically organizing the order in which the inspection procedure will be executed.

DATUM REFERENCE FRAME

Page 15: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 15 QuitMaster Table of ContentsGlossary

•When making part measurements that are related to a datum reference frame, the part must be brought into contact with the representative or “simulated” datum reference frame by following a specific procedure:

•The primary datum feature is first brought into contact with the primary datum of the datum reference frame. Three points of contact are required.

DATUM REFERENCE FRAME

Page 16: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 16 QuitMaster Table of ContentsGlossary

DATUM REFERENCE FRAME

Primary Datum Plane

Part

Page 17: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 17 QuitMaster Table of ContentsGlossary

•When making part measurements that are related to a datum reference frame, the part must be brought into contact with the representative or “simulated” datum reference frame by following a specific procedure:

•The primary datum feature is first brought into contact with the primary datum of the datum reference frame. Three points of contact are required.

•The secondary datum simulator is next positioned with reference to the datum reference frame (normal to the primary datum) while maintaining acceptable minimum point contact between the datum features and the primaryandsecondary datum plane simulators. Two points of contact are required.

DATUM REFERENCE FRAME

Page 18: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 18 QuitMaster Table of ContentsGlossary

90°

DATUM REFERENCE FRAME

Part

Secondary Datum Plane

Primary Datum Plane

Page 19: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 19 QuitMaster Table of ContentsGlossary

•When making part measurements that are related to a datum reference frame, the part must be brought into contact with the representative or “simulated” datum reference frame by following a specific procedure:

•The primary datum feature is first brought into contact with the primary datum of the datum reference frame. Three points of contact are required.

•The secondary datum simulator is next positioned with reference to the datum reference frame (normal to the primary datum) while maintaining acceptable minimum point contact between the datum features and the primary and secondary datum plane simulators. Two points of contact are required

•Finally, while maintaining minimum point contact between the primary and secondary datum simulators and the respective features on the part, the tertiary datum simulator and feature are brought into contact with each other and oriented in the datum reference frame. Only one point of contact is required.

DATUM REFERENCE FRAME

Page 20: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 20 QuitMaster Table of ContentsGlossary

Tertiary DatumPlane

Primary Datum Plane

Part

Secondary Datum Plane

90°

90°

90°

DATUM REFERENCE FRAME

A minimum of 3 points of contact on the primary datum plane, two points on the secondary datum plane, and one on the tertiary datum plane, must be maintained simultaneously to immobilize the part.

3-2-1 Rule

Page 21: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 21 QuitMaster Table of ContentsGlossary

90°

90°

Direction of measurement

90°

DATUM REFERENCE FRAME

When geometric controls are specified relative to a datum reference plane, verifying measurements must originate from the referenced datum plane.

Page 22: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 22 QuitMaster Table of ContentsGlossary

Datum Axis

DatumAxis

Datum Axis

Datum Planes -- Originof Measurements

90°

90°

90°

DATUM REFERENCE FRAME

Notice that the intersection of any two planes represents an axis of rotation. Also, all three axes and planes intersect at the origin of the datum reference frame.

Origin

Page 23: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 23 QuitMaster Table of ContentsGlossary

Datum Feature Identification Symbol

A A A

A

A

Examples of Datum Feature

Symbols

Page 24: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 24 QuitMaster Table of ContentsGlossary

DATUM SELECTION

Page 25: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 25 QuitMaster Table of ContentsGlossary

•In design, manufacturing, and inspection, datums are established to locate and position a part for checking geometric tolerances relative to specified datum features.

•Datum specification and precedence also determines and controls the location, position, functional relationships, and orientation of parts relative to one another in the assembly.

DATUM FUNCTION AND SELECTION CRITERIA

Page 26: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 26 QuitMaster Table of ContentsGlossary

EXPLANATION:

DRAWING SPECIFICATION

B

A8X 10.0 - 10.5

Datum Control for Orientation and Locationin an Assembly

•Datum features are the surfaces that locate and orient the part to the assembly.• The orientationof this part is deter-mined by surface A.•The location is controlled by surface B. •The mounting holes are controlled relative to the features that locate and orient the part. •Therefore, the holes are dimensioned relative to datums A and B by means of geometric tolerances.•The hole at the bottom (Controlled by a Feature Control Frame) could also be called out as a datum to control the rotation or “clocking” of the part in the assembly.

0.2 M A B M

Page 27: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 27 QuitMaster Table of ContentsGlossary

0.2 M A B M

This is the mating part to the part shown in the previous screen.

Datum Control for Orientation and Location

Page 28: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 28 QuitMaster Table of ContentsGlossary

0.2 M A B MParts 1 and 2 in Assembly

Datum Control for Orientation and Location

Datum FeaturesSelection

The part is clamped against sur-face A, establishing the orienta-tion of the part. Surface A istherefore referenced as the primary datum in dimensioningthe holes. Diameter B positionsthe part and is referenced as the secondary datum in the dimen-sioning of the bolt holes.

Page 29: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 29 QuitMaster Table of ContentsGlossary

DATUM PRECEDENCE

Page 30: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 30 QuitMaster Table of ContentsGlossary

Multiple Datum References—Order of Precedence

•To position a part properly and repeatedly in the datum reference frame(according to design requirements), datum features must be specified in the order of their functional design significance. This arrangement of planar datums is called the order of datum precedence.

•The order of precedence is specified by entering the datum feature reference letters from left to right in the feature control frame. The first datum reference is alwaysthe primary datum, and sometimes a single datum reference is sufficient to fulfill design requirements. However, when multiple datums are required, the primary datum is followed by the secondary and tertiary datum references. Three datums are the maximum for establishing a planar datum reference frame.

Page 31: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 31 QuitMaster Table of ContentsGlossary

•The 3-2-1 Ruleapplies to planar datums. It reflects the order and minimum number of contact points required to establish primary, secondary, and tertiary datums within a datum reference frame.

•First, a primary datum feature is related to the datum reference frame by bringing a minimum of three points of the datum feature in contact with the primary (simulated) datum plane. Next, a minimum of two points on the secondary datum feature are brought into contact with the second (simulated) datum plane. Finally, the process is completed by bringing at least one point of the tertiary datum feature into contact with the third (simulated) datum plane.

Datum Precedence for Multiple Planar Datum ApplicationsThe 3, 2, 1 Rule

Page 32: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 32 QuitMaster Table of ContentsGlossary

20.0

10.048.0

A

B

2X 6.05.6

0 M A B C

30.430.0

0.4

0.2

C

C

The features on this part have been dimensioned with reference to datums A, B, and C. Notice that datum A will, in this case, control the orientationor attitudeof the axes of the holes (the primary datum always controls feature attitude).

Datum Precedence Illustrated

Page 33: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 33 QuitMaster Table of ContentsGlossary

Primary Datum A

0 M A B C

The part is brought into contact with the primary datum plane (a surface plate, most likely). There must be a minimum of threepoints of contact.

The 3-2-1 Rule

Page 34: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 34 QuitMaster Table of ContentsGlossary

90º

Secondary Datum B

Primary Datum A

0 M A B C

The 3-2-1 Rule

A minimum of two points of contact are required between the secondary datum feature and the second datum simulator.

Page 35: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 35 QuitMaster Table of ContentsGlossary

90º

Tertiary Datum C

90º

Secondary Datum B

Primary Datum A

0 M A B C

The 3-2-1 RuleA minimum of onepoint of contact is required between the tertiary datum featureand the third datum simulator.

Page 36: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 36 QuitMaster Table of ContentsGlossary

90º

90º

Tertiary Datum C

90º

Secondary Datum B

Primary Datum A

0 M A B C

Notice the requirement of perfect perpendicularity between the 3 planes that make up the datum reference frame. The part is immobilized when restrained against all three datum plane simulators.

The 3-2-1 Rule

Page 37: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 37 QuitMaster Table of ContentsGlossary

Interrelated Datum Reference Frames

Page 38: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 38 QuitMaster Table of ContentsGlossary

Interrelated Datum Reference Frames

•Depending upon the complexity and functional design requirements of the part, one or more datum reference frames may be required to control and verify finished geometry.

•Any change in the order of datum precedence or material condition, requires different inspection methods, and consequently, establishes different datum reference frames.

•There is no limit to the number of datum reference frames that can be used on an engineering drawing. However, singularalpha characters assigned to datum features may be used only once per drawing.

Page 39: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 39 QuitMaster Table of ContentsGlossary

Any number of feature control frames may be used on a drawing. If a surface at an angle is selected as a datum feature, however, it must be controlled with an angularity geometric tolerance.

Interrelated Datum Reference Frames

B

C

0.1 A B

A

0.2 M A B C

2X 4.0 – 4.5

Page 40: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 40 QuitMaster Table of ContentsGlossary

The two holes in the bottom of the part are controlled with reference to datums A, B, and C. There are also features in the angled surface that reference B and C for location, but a new primary datum will be selected to control the orientation or attitude of the features on the angled plane.

Interrelated Datum Reference Frames

B

C

0.1 A B

A

0.2 M A B C

2X 4.0 – 4.5

Page 41: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 41 QuitMaster Table of ContentsGlossary

D

Datum plane D is established and will control the attitude of the holes on the angled surface.

AB

CE

0.2 M D B C

2.0 – 2.4 1.5

0.2 M A B C

2X 4.0 – 4.5

0.1 A B

Interrelated DatumReference Frames

Page 42: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 42 QuitMaster Table of ContentsGlossary

0.4 M D E M B

D

AB

C

0.2 M A B C

The small hole in the center of the angled surface has been established as datum feature E, and is a major factor in the location of the two associated outside holes on the same surface.

Interrelated DatumReference Frames

2X 4.0 – 4.5

2.0 – 2.4 1.5

2X 3.8 – 4.5

E

0.2 M D B C

0.1 A B

Page 43: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 43 QuitMaster Table of ContentsGlossary

Cylindrical Datum Features

Page 44: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 44 QuitMaster Table of ContentsGlossary

•A cylindrical datum feature is always associated with two theoretical perpendicular planes whose intersection form the datum axis.

•The datum of a cylindrical datum feature is the axis of the true geometric counterpart of the datum feature, which is simulated by a variable cylindrical gage used in the manufacturing or inspection process —expanding mandrels for internal features, and contracting chucks or collets for external features are examples of such tooling.

Cylindrical Datum Features

Page 45: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 45 QuitMaster Table of ContentsGlossary

Establishing Cylindrical Datum Features

Whenever a cylindrical datum feature must be established, the following graphic representation will be present on the drawing:

Two theoretical planes are represented on the drawing by center lines crossing at right angles. The intersection of these planes coincides with the datum axis.

Once established, the datum axis becomes the origin for related dimensions while the second and third planes (center lines) indicate the orientation, or direction for measurements.

(These concepts will be illustrated in the slides that follow.)

Page 46: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 46 QuitMaster Table of ContentsGlossary

C

The horizontal and vertical center lines on the circular view of the object, depict planeswhich intersect at its center. The intersection establishes the axis of the part.

4X 8.4 - 8.6

65.865.4

D

0.2 M C D M

16.5

16.5

16.5 16.5

Page 47: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 47 QuitMaster Table of ContentsGlossary

In instances where rotation of features, relative to the datum axis, have no effect on the function of the part, the rotational orientation of the second and third planes of the datum reference frame is not specified.

In such cases, only two datum features are referenced in the feature control frame—primary and secondary. Planar datum C would control the orientation of the axes of the mounting holes, and datum feature (diameter D) would locate datum axis D.

It would be of no consequence if the part was rotated about its axis when assembled with its mating part.

0.2 M C D M

Cylindrical Datum Features

Page 48: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 48 QuitMaster Table of ContentsGlossary

Primary Datum Plane C

0.2 M C D M

Let’s try to illustrate the principle. Planar datum feature C is the primary datum, and must be established by contact with a minimum of 3 points with the datum simulator.

Cylindrical Datum Features

Page 49: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 49 QuitMaster Table of ContentsGlossary

0.2 M C D M

Cylindrical Datum FeaturesThe first reference plane is established through the centers of two grooves located opposite each other.

Primary Datum Plane C

First Reference Plane

Page 50: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 50 QuitMaster Table of ContentsGlossary

Second Reference Plane

Primary Datum Plane C

0.2 M C D M

The second reference plane is established perpendicular to the first, and passes through the centers of the two grooves located opposite each other.

Cylindrical Datum Features

First Reference Plane

Page 51: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 51 QuitMaster Table of ContentsGlossary

Cylindrical Datum Features

The intersection of the two reference planes establishes the datum axis D. The part is symmetrical, so it would not matter how the part was rotated about its axis, it will

still function as

intended.

Because rotation of

the part is not an issue, only two datums are necessary to control the hole location and orientation.

Second Reference Plane

Datum Axis (SecondaryDatum D)

Second Reference Plane

Primary Datum Plane C

0.2 M C D M

First Reference Plane

Page 52: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 52 QuitMaster Table of ContentsGlossary

4X 6.5 – 6.9

20 20

20

20

22.822.4

C

D

0.2 M C D M

Let’s consider an internal feature. The horizontal and vertical center lines on the circular hole in the center of the object, depict planes which intersect at its center. The intersection establishes the axis from which the mounting holes are located.

Cylindrical Datum Features

Page 53: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 53 QuitMaster Table of ContentsGlossary

Primary Datum Plane C

0.2 M C D M

Illustrating the principle again, the planar datum feature C is the primary datum, and must be established by contact with a minimum of 3 points with the datum simulator.

Cylindrical Datum Features

Page 54: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 54 QuitMaster Table of ContentsGlossary

Primary Datum Plane C

0.2 M C D M

Cylindrical Datum Features

The first reference plane is established through the centers of two holes located opposite each other.

First Reference Plane

Page 55: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 55 QuitMaster Table of ContentsGlossary

Primary Datum Plane C

0.2 M C D M

Cylindrical Datum Features

Second Reference Plane

First Reference Plane

The second reference plane is established perpendicular to the first and passes through the centers of the other two holes located opposite one another.

90º

Page 56: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 56 QuitMaster Table of ContentsGlossary

Primary Datum Plane C

0.2 M C D M

Cylindrical Datum FeaturesDatum Axis (SecondaryDatum D)

The intersection of the two reference planes establishes datum axis D. As stated before, it would not matter how the part was rotated about its axis, it will

still function as

intended.

Because rotation of

the part is not an issue, only two datums are necessary to control the hole location and orientation.

Page 57: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 57 QuitMaster Table of ContentsGlossary

Rotational Orientation

Rotational control of the first and second reference planes about an axis, in a cylindrical datum reference frame, can be established by referencing a third or tertiary datum feature in the feature control frame.

The tertiary datum is a datum axis or center plane aligned with the primary datum axis and one of the two reference planes.

Page 58: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 58 QuitMaster Table of ContentsGlossary

A

0.4 M A B M

C

10.9 - 11.2

0.2 M A

B

2X 4.8-5.0

0.2 M A B M C M

8.07.6

The center plane of the groove on the right side of the object is datum C. It must be perpendicular to datum plane A, and be within the tolerance specified for alignment with the reference plane that establishes datum axis B,.

Rotation Control in Feature Orientation

E

D

Page 59: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 59 QuitMaster Table of ContentsGlossary

Primary Datum Plane A

Rotation Control in Feature Orientation

0.2 M A B M C M

Datum plane A is established by a minimum of three high points on the surface, in contact with simulated datum.

Page 60: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 60 QuitMaster Table of ContentsGlossary

Primary Datum Plane A

Datum Axis B (Secondary Datum) established by intersecting reference planes.

Second Reference Plane

First Reference Plane

Rotation Control in Feature Orientation

0.2 M A B M C M

Datum plane A is established by a minimum of three high points on the surface, in contact with simulated datum.

Page 61: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 61 QuitMaster Table of ContentsGlossary

Primary Datum Plane A

Datum Axis B (Secondary Datum) established by intersecting reference planes.

Second Reference Plane

First Reference Plane

Rotation Control in Feature Orientation

Datum center plane C must be perpendicular to datum plane A, and be aligned with the reference plane that establishes datum axis B.

Datum Center Plane C and

0.2 M A B M C M

Datum plane A is established by a minimum of three high points on the surface, in contact with simulated datum.

Page 62: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 62 QuitMaster Table of ContentsGlossary

Primary Datum Plane A

Datum Axis B (Secondary Datum)

Second Reference Plane

First Reference Plane

Rotation Control in Feature Orientation

Datum center plane C must be perpendicular to datum plane A, and be aligned with the reference plane that establishes datum axis B.

Datum Center Plane C and

Feature controlabout tertiary datum center plane C.

0.2 M A B M C M

Rotational orientation is established by identifying oneof the reference planes as the tertiarydatum in the feature control frame.

Page 63: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 63 QuitMaster Table of ContentsGlossary

Datum Targets

Page 64: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 64 QuitMaster Table of ContentsGlossary

•Some surface types, such as bowed surfaces, non-planar, or uneven surfaces produced by casting or forging, and thin section surfaces subject to warping, cannot, by themselves, be effectively matched with a datum reference frame.

•Datum features must contact datum simulators to establish datums, and due to surface curvature, irregularities or roughness, some surfaces may not be appropriately used as a datum feature without having been equalized or normalized in some acceptable way.

•In such cases, datum targets may be combined to establish a datum reference frame.

DATUM TARGETS

Page 65: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 65 QuitMaster Table of ContentsGlossary

•Datum targets designate specific points, lines, or areasof contact on a part that are used in establishing a datum referenceframe.

•Generally two or more datum targets are required to establish a datum.

Datum Targets

Page 66: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 66 QuitMaster Table of ContentsGlossary

Datum Target Point Datum Target Line Datum Target Areas

There are three different datum targets: points, lines, and areas. You may recall from our discussions on dimensioning and tolerancing that they are drawn with bold (heavy) lines. The symbol for a datum target point is in the form of a large “X”— twice the drawing lettering height. The symbol depicting a datum target line is a bold line configured like a phantom line(sometimes used in conjunction with a datum point, to imply the end view of the line), and datum target areasare outlined with bold phantom lines with crosshatched interiors.

Datum Targets

Page 67: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 67 QuitMaster Table of ContentsGlossary

Datum Target Symbols

•Datum target symbolsare used to establish datum targets on engineering drawings. They are used in conjunction with target points, lines or areas, designated on the part by means of a radial, straight leader line directed at the target.

•For targets on a visible surface, a solid leader is used; for targets on hidden surfaces, leader lines are dashed. The leader lines typically do not have a horizontal shoulder where they originate at the datum target symbol (this characteristic is optional however), and they never end with an arrowhead.

•A datum feature symbolis attached to the appropriate views of surfaces that have been designated as datums by the use of datum targets.

Page 68: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 68 QuitMaster Table of ContentsGlossary

A3

B1

C1

Datum Target Symbols

Datum target symbols are used to identify datum target location, orientation, size, and order of precedence on engineering drawings, and relate each of them to the datum reference frame. Note the use of the solid and dashed leader lines denoting near side and far side applications. Also, notice the optionaluse of horizontal shouldersand the absence of arrowheads on the leader lines and the lack of arrows.

Page 69: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 69 QuitMaster Table of ContentsGlossary

Datum Target Point Designation

Page 70: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 70 QuitMaster Table of ContentsGlossary

C

728

C1

Designating a Datum Target Point

A datum target point is indicated by the target point symbol, and it is dimensionally located on a normalview of the datum feature surface. Basic dimensions are used to locate the datum target point. These basic dimensions, however, are gage dimensions, and toolmaker’s tolerances would be used to locate them as necessary in manufacturing and inspection. There are no tolerances for datum target location dimensions on drawings, and they are not toleranced in feature control frames.

Page 71: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 71 QuitMaster Table of ContentsGlossary

Single point contactat basic location

Spherical tip gage pin

This view interprets the last slide, showing the point contacton the datum feature during manufacturing or inspection cycles.

Verifying a Datum Target Point

Page 72: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 72 QuitMaster Table of ContentsGlossary

Datum Target Line Designation

Page 73: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 73 QuitMaster Table of ContentsGlossary

A datum target line may be indicated by a datum target (phantom) line on the normal view, a target point symbol on an edge view of the surface, or both.

Datum Target Line Designation

Page 74: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 74 QuitMaster Table of ContentsGlossary

A1

A1

A datum target line may be represented by either a datum target point symbol, located on the edge view of the datum feature surface, or by a datum target line located in the normal view of the feature surface, or both. Note that the datum feature symbol is on the edge view of the datum feature, and the target is located with basic dimensions.

Datum Target Line Designation

A

Page 75: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 75 QuitMaster Table of ContentsGlossary

Part

Locating Gage Pin

This image interprets the application of a datum target line application. In theory, the gage pin would make contact along a single line element of the datum feature surface.

Datum Target Line Verification

Page 76: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 76 QuitMaster Table of ContentsGlossary

Datum Target Area Designation

Page 77: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 77 QuitMaster Table of ContentsGlossary

There are times when single point contact may be determined to be insufficient to provide assurances that the datum can be accurately established. If the designer or engineer determines that an areaof contact is necessary for such assurances, a datum target areaof the desired shape is specified. A datum target area is indicated by a bold phantom line outline of the desired target shape, with section lines (crosshatching) filling the interior. Basic dimensions are used to locate and size the datum target area.

Datum Target Areas

Page 78: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 78 QuitMaster Table of ContentsGlossary

10

12

A3

Datum Target Area Designation

Datum target areas of the desired shape are identified on the drawing by outlining the boundary of the area with bold phantom lines, and crosshatching the interior. Basic dimensions are used to locate the datum target area.

Page 79: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 79 QuitMaster Table of ContentsGlossary

10

12

Datum Target Area Designation

The diameter of the circular target area is called out in the upper portion of the symbol, if required.

10

A3

10

A3or

Datum target areas of the desired shape are identified on the drawing by outlining the boundary of the area with bold phantom lines, and crosshatching the interior. Basic dimensions are used to locate the datum target area.

Page 80: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 80 QuitMaster Table of ContentsGlossary

Combining Target Symbols

Page 81: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 81 QuitMaster Table of ContentsGlossary

Combining Datum Target Symbols

Primary datum plane Mhas been established by two datum target pointsand one datum target line. Study this illustration until you understand the placement of datum targets. Remember, tolerances for target placement are not called out on the drawing.

M2

M1

2.625

3.125

M3

M3

M2M1

M

.500

.438

Page 82: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 82 QuitMaster Table of ContentsGlossary

Spherical TipGage pins

(point contact)

Gage pin(line contact)

Two spherical tipped gage pins and one line gage pin would be used to position the part and establish the primary datum.

Combining Datum Target Symbols

Page 83: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 83 QuitMaster Table of ContentsGlossary

This computer-generated image should help you understand the placement and orientation of the gage pins to establish the primary datum.

Combining Datum Target Symbols

Page 84: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 84 QuitMaster Table of ContentsGlossary

Datum Target Planar Applications

Page 85: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 85 QuitMaster Table of ContentsGlossary

10

10 10A2A3

A1

A

Gage pins of the specified diameter (10 mm), and located as shown, using basic dimensions—and tool maker’s tolerances, would be required to establish datum A.

Interpreting Datum Target Area Symbols

Page 86: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 86 QuitMaster Table of ContentsGlossary

Surface Plate

Part

Gage pins (A1, A2, A3, Ø10 mm)

Datum Feature

Datum Plane A

The basic theory behind this approach is that the gage pins will establish a level of accuracy on the specific part feature, that will be sufficient to further establish other features and feature relationships.

Interpreting Datum Target Area Symbols

Surface Plate

Page 87: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 87 QuitMaster Table of ContentsGlossary

Datum Targets Used To Establish A Primary Datum Axis

Page 88: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 88 QuitMaster Table of ContentsGlossary

Datum Targets Used to Establish a Primary Datum Axisof a Single Cylinder

• Two sets of three equally spaced targetsmay be used to establish a datum axis for a singlecylindrical primary datum feature (RFS).

• The two sets of datum targets are spaced as far apart as practicableand dimensioned from the secondary (planar) datum.

• Both sets of targets use the samedatum identifying letter.

Page 89: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 89 QuitMaster Table of ContentsGlossary

B2

B1

B3

B5 B6

B4

B4 B1

B5 B2B6 B3

B

D

Datum Target Points Used on a Single Cylindrical Feature To Establish a Primary Datum Axis

Both sets of equally spaced datum target points use the same datum reference letter to establish datum axis B. Notice that under these conditions, the datum target numbers increase to B6.

Page 90: Datums & Datum Targets

GD&T Datums Table of ContentsReturn to the Previous Slide Slide 90 QuitMaster Table of ContentsGlossary

End of Unit Number Seven