72218577-datums

3Datums

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Explain the purpose of datums Define common terms related to datum features Interpret information provided by datum feature

symbols Describe methods for representing datum

surfaces on drawings Explain how to establish datum reference

frames


Identify the degrees of freedom of a part Explain how to establish datum targets Identify datum target points, areas, and lines Describe methods for representing datum axes

on drawings Describe methods for representing datum

center planes on drawings


Datums

Represent true geometric counterpart of a datum feature

Reference features of an object Establish location and size dimensions


Datum Feature Simulator

Opposite shape of datum feature Two types:

Theoretical datum feature simulator Physical datum feature simulator

Manufacturing examples: Machine tables Surface plates Gauge surfaces Surface tables Specially designed rotation devices


Datum Feature Symbol Do not apply to centerlines, center planes,

or axes Draw using thin lines Use any letter to identify a datum feature

except I, O, or Q Use double letters when number of datums

exceeds number of letters in alphabet


Datum Feature Symbol


Datum Feature

Datum plane

Simulated datum plane

Datum feature

Part

Physical datum feature simulator

Surface of manufacturing or verification equipment


Datum Feature Terms

Actual mating envelopeSimulated datumTangent plane


Datum Feature Symbol Placement for Surfaces


Datum Surface Can be controlled by geometric tolerance Datum plane measurements do not take

variations in datum surface from datum plane into account


Geometric Control of Datum Surface


Datum Reference Frame (DRF) Concept

Select three perpendicular datum features based on their importance to part design

Assign precedence and datum reference order Primary datum Secondary datum Tertiary datum


Datum Reference Frame Concept


Datum Reference Frame Concept: Inspecting the Part

All parts have six degrees of freedomThree degrees of translationThree degrees of rotation

Movement is either translational or rotational


Degrees of Freedom


Establishing Multiple Datum Reference Frames

Datums X, Y, and Z establish one datum reference frame

Datums L and M establish second datum reference frame


Datum Features Specified Individually

Place a note next to datum feature symbols indicating how many datum features to consider separately

Place the note 2X INDIVIDUALLY next to datum feature symbols of two separate datum features identified by same letter


Datum Target Symbols

Identify datum targets Useful on parts with surface or contour

irregularities Connect to datum target point, line, or area

with a leader


Datum Target Symbols


Datum Target Points Establish primary datum plane by locating at least

three points on primary datum surface Establish secondary datum plane by locating at

least two points on related secondary datum surface

Establish tertiary datum plane by locating at least one point on related tertiary datum surface

Dimension using baseline or chain dimensioning Location dimensions originate from datums


Datum Target Points


Datum Target Points

Datum plane established by locating pins at datum target points


Datum Target Areas

Used to establish datums using areas of contact

Locating pins are flat end tooling pins with pin diameter equal to specified size of target area

Top half of the datum target symbol identifies size of target area

Use datum target point at center location when area is too small to accurately or clearly display on drawing


Datum Target Areas


Datum Target Lines

Indicated by a target point on edge view of surface and a phantom line on surface view

Locating pins are cylindrical or knife-edged with datum target line along tangency where pins meet the part

Surface is often placed 90 to the pin to create datum reference frame


Datum Target Lines


Partial Datum Surface


Coplanar Surface Datums

Used to establish coplanar surfaces as one datum plane

Specify number of surfaces with a note such as: "2 SURFACES" below related feature control frame


Coplanar Surface Datums


Datum Axis

Represents two theoretical planes intersecting at 90

Represented in drawings with centerlines Pitch cylinder for screw threads establishes

datum axis unless otherwise specified When not using pitch cylinder for screw threads,

place note such as "MAJOR DIA" or "MINOR DIA" next to datum feature symbol

Simulated datum axis established by inspection equipment


Datum Axis


Datum Feature Symbol Placement for a Datum Axis


Coaxial Datum Features

Separate datum reference letters by a dash in one compartment of the feature control frame


Datum Axis Established with Datum Target Symbols

Primary datum axis established by two sets of three equally spaced targets

Identify datum target points in correlation to adjacent cylindrical datum feature when two cylindrical features of different diameters establish a datum axis


Datum Axis Established with Datum Target Symbols

Cylindrical datum target areas and circular datum target lines can be used to establish datum axis Target area represented by two phantom lines

with section lines between Datum target line represented by phantom line all

around part Establish secondary datum axis by placing

three equally spaced targets on cylindrical surface


Movable Datum Target Symbols and Datum Target Points

When datum targets establish a center point, axis, or center plane on a RMB basis, datum feature simulator movement is normal to true profile


Movable Datum Target Symbols and Datum Target Spheres

Circular phantom line represents spherical datum feature simulator to establish datum axis


Datum Center Plane

Axis and center plane datum feature symbols align with/replace dimension line arrowhead or appear on feature, leader shoulder, dimension line, or feature control frame


Datum Center Plane


Translation Modifier

Added to feature control frame following datum feature reference and any other applicable modifiers

Specifies that basic location of datum feature simulator is unlocked and free to translate within specified geometric tolerance


Translation Modifier


Using Contoured Surface as Datum Feature

3D mathematical coordinate system defines compound surface feature on a part

Datum simulator results from mathematical data Common when design copies contour of an

actual object that has few or no flat surfaces to use as datums


Identify the datum feature, part, simulated datum plane, physical datum feature simulator, and datum plane.

A)Datum featureB) Datum planeC) Part

D) Simulated datum planeE) Physical datum feature simulator


Fill in blanks for Part I A and B and Part II A, B, C, and D. Provide dimensions for Part II B, C, and D.

Part I:A) Datum feature symbolB) Feature control frame

Part II:A) Datum plane AB) 12.2 minimumC) 12.8 minimumD) 0.1 geometric tolerance


A) Basic dimension

Identify items AF.

B) Datum feature symbolC) Target pointD) Datum target symbolE) Datum referenceF) Target numberG) Datum target line


Identify items AC.

A)Datum targetarea

B) Datum target symbol

C) Target area size


A) Datum surface

Identify whether the datum reference symbol represents a datum surface, datum axis, or datum center plane in items AC.

B) Datum center planeC) Datum axis


Actual mating envelope The smallest size that can be contracted about an

external feature or the largest size that can be expanded within an internal feature.

Coaxial datum features Exist when a single datum axis is established by

two datum features that are coaxial. Coplanar surfaces

Two or more surfaces that are on the same plane.


Datum axis The center axis established when a cylindrical

datum feature is used. Datum center plane

The plane that splits a symmetrical feature, such as a slot or tab.

Datum feature An actual feature on a part that is used to

establish the datum.


Datum feature simulator A perfect theoretical boundary or a physical

boundary used to establish a datum or simulated datum from a specified datum feature.

Datum feature symbol A symbol used to identify an object feature

specified as a datum and referred to as a datum feature.

Datum plane The theoretically exact plane established by the

simulated datum of the datum feature.


Datum reference frame (DRF) The "frame" created by three datum features that

are perpendicular to each other and used for layout purposes.

Datum target lines Datum targets indicated by the target point

symbol "X" on the edge view of the surface and by a phantom line on the surface view.

Datum target A point, line, or area of contact on a part used to

establish a datum when it is not possible to use a surface.


Datums A surface, plane, point, or axis considered

theoretically perfect. A datum is assumed to be exact.

Degrees of freedom The number of coordinates it takes to exclusively

control the position of a part.


Normal Term used to describe a feature that is

perpendicular to a plane surface and radial to a curved surface.

Order of precedence The order in which the datums appear in the last

three compartments of the feature control frame. The primary datum is given first, followed by the secondary datum and then the tertiary datum.

Primary datum The primary datum reference established in the

datum reference frame. The primary datum is first in the order of precedence.


Primary datum plane A datum plane established by at least three points

on the primary datum surface. These points are used to provide stability on the primary plane.

Rotational Movement around an axis.

Secondary datum The secondary datum reference established in

the datum reference frame. The secondary datum is second in the order of precedence.


Secondary datum plane A datum plane established by at least two points

on the related secondary datum surface. Two points provide the required stability for the secondary plane.

Simulated datum A point, axis, line, or plane consistent with or

resulting from processing or inspection equipment, such as a surface plate, inspection table, gage surface, or a mandrel.


Simulated datum axis The axis of a perfectly cylindrical inspection

device that contacts the datum feature surface. Simulated datum center plane

The center plane of a perfectly rectangular inspection device that contacts the datum feature surface.

Simulated datum plane The plane derived from the physical datum

feature simulator. It coincides with the datum plane when the datum plane is in contact with the simulated datum plane.


Tangent plane A plane that contacts the high points of the

specified feature surface. Tertiary datum

The tertiary datum reference established in the datum reference frame. The tertiary datum is third in the order of precedence.

Tertiary datum plane A datum plane established by at least one point

on the related tertiary datum surface. One point of contact at the tertiary datum plane is all that is required to complete the datum reference frame and provide stability.


Three degrees of rotation Occurs when a part can rotate about each axis in

the datum reference frame. Three degrees of translation

Occurs when a part can move without rotation in each of the three directions from the mutually perpendicular planes.

Translational Refers to uniform movement without rotation.

Slide 13Slide 3Slide 4DatumsDatum Feature SimulatorDatum Feature SymbolSlide 8Datum FeatureDatum Feature TermsDatum Feature Symbol Placement for SurfacesDatum SurfaceGeometric Control of Datum SurfaceDatum Reference Frame (DRF) ConceptDatum Reference Frame ConceptDatum Reference Frame Concept: Inspecting the PartDegrees of FreedomEstablishing Multiple Datum Reference FramesDatum Features Specified IndividuallyDatum Target SymbolsSlide 21Datum Target PointsSlide 23Slide 24Datum Target AreasSlide 26Datum Target LinesSlide 28Partial Datum SurfaceCoplanar Surface DatumsSlide 31Datum AxisSlide 33Datum Feature Symbol Placement for a Datum AxisCoaxial Datum FeaturesDatum Axis Established with Datum Target SymbolsSlide 37Movable Datum Target Symbols and Datum Target PointsMovable Datum Target Symbols and Datum Target SpheresDatum Center PlaneSlide 41Translation ModifierSlide 43Using Contoured Surface as Datum FeatureSlide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60

72218577-datums

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