advanced mechanical drafting lecture #8. challenge of mechanical design only discipline where...
TRANSCRIPT
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ADVANCED MECHANICAL DRAFTING
LECTURE #8
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Challenge ofMechanical Design
Only discipline where tolerances of 100ths, 1000ths, or 10,000ths
of an inch affect part function
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Limitations of Standard Dimensioning Practices
Difficult to control form (shape), orientation, location, etc. by
measuring from other manufactured features
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GEOMETRIC DIMENSIONING & TOLERANCING
• Implemented by General Motors in the 1940’s to augment standard dimensioning practices
• A system that defines zones of tolerance within which features must lie
• Dimensions that control zone locations and sizes are different than standard dimensions– BASIC dimensions control locations of tolerance zones
• No tolerance – considered perfect
– FEATURE CONTROL FRAMES control shapes and sizes of zones– DATUM FEATURE SYMBOLS identify reference features
• All three items show in frames (boxes) on the drawing
.006 A B A8.500
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EXAMPLE4.00
2.00
THISon the
drawing
COULD LOOK LIKE
4.00
2.00
THISwhen
machined
DIMENSIONS ALONE CANNOT ADEQUATELY CONTROL THE SHAPE OF THE PART
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EXAMPLE
1.5031.498
1.5031.498
.005 SQUARE
ZONE
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+
+
+
+
+
+
+
+
+
Square position tolerance zones vs round position zones
“+” = actual hole center
Tolerance zone should be round with a diameter
equal to the diagonal distance
across the rectangular zone
THE ROUND ZONE IS 57% LARGER IN AREA THAN
THE SQUARE ZONE !
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EXAMPLE
Ø1.500 (1-1/2) THRU
Choose reference features
Locate tolerance zone
Specify shape and size of zone
Note that the part of the dimension outside the frame applies to the feature and the part inside the frame applies to the tolerance zone
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DRAWING BASIC DIMENSIONS
1.562
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GEOMETRIC SYMBOLS
LOCATION CONTROL
FORMCONTROL
RUNOUTCONTROL
SEE HANDOUTS FOR SYMBOL
CONSTRUCTION
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GEOMETRIC SYMBOLSDATUM
REFERENCE REQUIRED
NO DATUM REFERENCE
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FEATURE CONTROL FRAMES
g
eom
etri
c ch
arac
teri
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tole
ran
ce z
on
e sh
ape
tole
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on
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itio
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y d
atu
m
seco
nd
ary
dat
um
tert
iary
dat
um
REQUIRED IN EVERY FRAME
DEPENDENT ON ZONE TYPE
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DRAWING FEATURE CONTROL FRAMES
• Click on tolerance button in dimension toolbar
• Select elements required in frame
• Place frame in drawing
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FEATURE CONTROL FRAME EXAMPLES
.005 A
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FEATURE CONTROL FRAME EXAMPLES
Ø.005 A B C
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FEATURE CONTROL FRAME EXAMPLES
Ø.005 A B C
.011 A
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DATUM THEORY
• Any feature (point, line, plane, axis, surface, hole, etc.) can be a datum (measuring reference)
• Considered perfect
• Datum features specified on the drawing are simulated in the inspection process
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SIMULATED DATUM
SIMULATED DATUM
SURFACE
SPECIFIED DATUM
FEATURE
SPECIFIED MEASUREMENT
ACTUAL MEASUREMENT
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DATUM PRECEDENCE
• Follows order listed in feature control frame
• As many as three datums may be needed to adequately control the location of a tolerance zone
• Levels of precedence:
– Primary datum – SUPPORTS THE PART – 3 points of contact with simulated datum
– Secondary datum – ALIGNS THE PART – 2 points of contact with simulated datum
– Tertiary datum – STOPS THE PART - 1 point of contact with simulated datum
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Object with 3 datums
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THREE POINTS OF CONTACT
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THREE POINTS OF CONTACT
TW
O P
OIN
TS
OF
CO
NT
AC
T
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THREE POINTS OF CONTACT
TW
O P
OIN
TS
OF
CO
NT
AC
T
ONE POINT OF CONTACT REQUIRED ON REAR SURFACE
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DATUM SELECTION CRITERIA
• Function – how do features affect the way the entire part works
• Relationship to mating part or feature - how do features mate with other part
• Accessibility – can you measure from the feature ?
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GSK DESIGN