Datums & Datum Targets
Transcript of 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
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•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:
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•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:
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•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
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•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
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PLANAR DATUM EXAMPLE
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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
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The irregularities on the datum featurehave been exaggerated to give meaning to the example.
Planar Datum Example
Datum Feature
Part
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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
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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)
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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
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DATUM REFERENCE FRAME
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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
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•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
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•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
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DATUM REFERENCE FRAME
Primary Datum Plane
Part
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•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
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90°
DATUM REFERENCE FRAME
Part
Secondary Datum Plane
Primary Datum Plane
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•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
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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
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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.
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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
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Datum Feature Identification Symbol
A A A
A
A
Examples of Datum Feature
Symbols
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DATUM SELECTION
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•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
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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
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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
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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.
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DATUM PRECEDENCE
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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.
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
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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
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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
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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.
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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.
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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
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Interrelated Datum Reference Frames
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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.
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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
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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
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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
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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
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Cylindrical Datum Features
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•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
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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.)
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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.
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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.
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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.
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Datum Targets
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•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
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•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
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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
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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.
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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.
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Datum Target Point Designation
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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.
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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
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Datum Target Line Designation
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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
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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
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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
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Datum Target Area Designation
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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
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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.
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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.
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Combining Target Symbols
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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
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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
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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
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Datum Target Planar Applications
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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
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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
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Datum Targets Used To Establish A Primary Datum Axis
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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.
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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.
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End of Unit Number Seven