Transcript 3 - Goodheart

3
Datums
L earning Objectives
• 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.
© Goodheart-Willcox Co., Inc.
Permission granted to reproduce for educational use only.
L earning Objectives
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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.
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Permission granted to reproduce for educational use only.
Datums
• Represent true geometric counterpart of a
datum feature
• Reference features of an object
• Establish location and size dimensions
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Datum Feature Simulator
• Opposite shape of datum feature
• Two types:
– Theoretical datum feature simulator
– Physical datum feature simulator
• Manufacturing examples:
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Machine tables
Surface plates
Gauge surfaces
Surface tables
Specially designed rotation devices
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Permission granted to reproduce for educational use only.
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
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Datum Feature Symbol
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Datum Feature
Datum plane
Part
Datum feature
Simulated datum plane
Physical datum
feature simulator
Surface of manufacturing
or verification equipment
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Datum Feature Terms
• Actual mating envelope
• Simulated datum
• Tangent plane
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Datum Feature Symbol
Placement for Surfaces
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Datum Surface
• Can be controlled by geometric tolerance
• Datum plane measurements do not take
variations in datum surface from datum
plane into account
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Geometric Control of Datum Surface
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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
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Datum Reference Frame Concept
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Datum Reference Frame Concept:
Inspecting the Part
• All parts have six degrees of freedom
– Three degrees of translation
– Three degrees of rotation
• Movement is either translational or rotational
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Degrees of Freedom
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Establishing Multiple
Datum Reference Frames
• Datums X, Y, and
Z establish one
datum reference
frame
• Datums L and M
establish second
datum reference
frame
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Permission granted to reproduce for educational use only.
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
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Permission granted to reproduce for educational use only.
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
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Datum Target Symbols
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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
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Datum Target Points
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Datum Target Points
• Datum plane established by locating pins at
datum target points
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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
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Permission granted to reproduce for educational use only.
Datum Target Areas
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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
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Datum Target Lines
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Partial Datum Surface
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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
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Coplanar Surface Datums
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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
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Datum Axis
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Datum Feature Symbol
Placement for a Datum Axis
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Coaxial Datum Features
• Separate datum reference letters by a dash in
one compartment of the feature control frame
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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
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Permission granted to reproduce for educational use only.
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
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Permission granted to reproduce for educational use only.
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
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Movable Datum Target Symbols
and Datum Target Spheres
• Circular phantom line represents spherical
datum feature simulator to establish datum axis
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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
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Permission granted to reproduce for educational use only.
Datum Center Plane
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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
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Translation Modifier
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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
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Permission granted to reproduce for educational use only.
R eview
Identify the datum feature, part, simulated datum
plane, physical datum feature simulator, and
datum plane.
A. Datum feature
B. Datum plane
C. Part
D. Simulated datum plane
E. Physical datum feature simulator
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Permission granted to reproduce for educational use only.
R eview
Identify Parts A and B.
A. Datum feature symbol
B. Feature control frame
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R eview
Identify parts A and D and provide dimensions for
B, C, and D.
A. Datum plane A
B. 12.2 minimum
C. 12.8 minimum
D. 0.1 geometric tolerance
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Permission granted to reproduce for educational use only.
R eview
Identify items A–G.
A. Basic dimension
B. Datum feature symbol
C. Target point
D. Datum target symbol
E. Datum reference
F. Target number
G. Datum target line
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R eview
Identify items A–C.
A. Datum target area
B. Datum target symbol
C. Target area size
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Permission granted to reproduce for educational use only.
R eview
Identify whether the datum reference symbol
represents a datum surface, datum axis, or datum
center plane in items A–C.
A. Datum surface
B. Datum center plane
C. Datum axis
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Permission granted to reproduce for educational use only.