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Appreciation program in
Geometric Dimensioning and
Tolerancing
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When to use GD&T
When drawing and its interpretation must be same
When features are critical to function or
interchangeability
When it is important to avoid scrapping of
perfectly good parts
When it is important to reduce drawing changes
When functional gauging is required When it is important to increase productivity
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CO ORDINATE TOLERANCINGSYSTEM
Part feature is located (or defined) by means ofrectangulardimensions with given tolerances.
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THREE MAJOR SHORTCOMING OFCOORDINATE DIMENSIONING
1. Square orrectangulartolerance zones.
2. Fixed-size tolerance zones.
3. Ambiguous instruction for inspection.
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1.Square (or illogical )Tolerance Zones.
Diagonally more tolerance (0.707) than vertical andhorizontal direction (0.5)
More logical and functional approach is to allow sametolerance on all sides, creating cylindrical tolerance
zone.
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COMPARISION BETWEEN GD&TAND COORDINATE TOLERANCING.
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Cylindrical vs. RectangularTolerance Zones
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GEOMETRIC DIMENSIONING ANDTOLERANCING.
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0Rectangular Tolerancing
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Geometric Tolerancing
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2.Fixed-Size Tolerance zones
Function ofahole in assembly is , hole location iscritical when the hole is at minimum limit (MMC).
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Co-ordinate tolerancing does not allow
for cylindrical tolerance zones and
tolerance hole that increase with thehole size, lengthy notes have to be
added.
LMC
MMC
10.5
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3.AMBIGUOUS INSTRUCTION FORINSPECTORS
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Different ways to hold the part for inspection,confusion forthe inspectorwhich surface to touch
the gage equipment first, second and third.
Consequence:
Good parts could be rejected or,Bad parts could be accepted.
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3 Benefits of GD & T
A. Cylindrical tolerance zones.
B. Maximum Material Condition.
C. Datums specified in order of precedence.
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Type of dimension Appropriate
use
Pooruse
Size
Chamfer
Radius
Locating part feature
Controlling angularrelationships
Defining the form of
part feature
COORDINATE DIMENSIONING USAGE
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Symbols, Terms, of GD&T
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Symbols of GD & T
Geometric characteristic symbols areaset offourteen Symbols used inthelanguage ofgeometric tolerancing.
The symbols aredivided into fivecategories:
1. Form
2. Profile
3. Orientation4. Location
5. Runout
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FEATURES
A feature is ageneral term appliedtoaphysical portion ofpart, suchas asurface, hole or slots,tabs.
Aneasy way to rememberthis termis to think ofa featureas apartsurface.
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FEATURES
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FEATUREOF SIZE This is one cylindrical or spherical
surface, or set oftwo opposed elements orparallel surfaces associated with sizedimension which has an axis, center lineor center plane contained within it.
Features of size are features, which dohave diameter or thickness.
These may be cylinders, such as shafts
and holes. They may also be slots,rectangular or flat parts, where twoparallel flat surfaces are considered toform a single feature.
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How ma
ny featur
e of sizear
eth
er
e?
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FEATUREOF SIZE NONFEATUREOF SIZE
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EXTERNALAND INTERNALFOS
External FOS are comprised ofpartsurfaces that are external surfaces.
Like shaft diameter or width and
height ofa planner surfaces. Internal FOS is comprised of part
surfaces (or elements) that areinternal part surfaces.
like hole diameter or the width of aslot.
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Example:
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ACTUALMATING ENVELOPE=PERFECT FEATURECOUNTERPART.
The Actual Mating Envelope (AME) ofan external feature of size is a
similar perfect feature counterpart ofthe smallest size that can becircumscribed about the feature so it
just contacts the surfaces at thehighest points with in the tolerancezone.
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Actual Mating Envelope (AME) of an external
FOS
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ACTUALMATING ENVELOPE
=PERFECT FEATURECOUNTERPART
The actual mating envelope (AME) ofan internal feature of size is a similar
perfect feature counterpart of thelargest size that can be inscribedwithin the feature so that it justcontacts the surfaces at their highestpoints with in the tolerance zone.
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Actual Mating Envelope (AME) of an internal FOS
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Actual Mating Envelope (AME) of an internalFOS
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MATERIALCONDITIONS
A geometric tolerance can be specified toapply at the largest size, smallest size oractual size ofa feature of size.
Maximum Material Condition (MMC)Maximum material condition is thecondition in which a feature of sizecontains the maximum amount of material
everywhere within the stated limits ofsize.
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MMC
MMC ofexternal FeatureOf Size
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LEAST MATERIALCONDITION(LMC)
Least material condition is the condition inwhich a feature of size contains the leastamount of material everywhere within thestated limits of size .
LEASTMATERIALCONDITION
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Regardless of feature size(RFS)
Regardless of f eature size is the term thatindicates a geometric tolerance applies atany increment of size ofthe feature withinits size tolerance. NO Bonus tolerance
RFS applied only to size features, such ashole, shafts, pins, etc.; feature which havean axis, centerplane or centerline.
Symbol : S
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Material Condition Usage Each material condition is used for
different functional reasons.
Geometric tolerances are often specifiedto apply atMMC whenthe function ofa
FOS is assembly.
Geometric tolerances are often specifiedto apply atLMC to insureaminimum
distance onapart.
Geometric tolerances are often specifiedto apply at RFS to insure symmetrical
relationships.
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MODIFIERS
Modifiers communicateadditionalinformation aboutthedrawing or
Tolerancing ofapart.
Therearenine common modifiers
used ingeometric tolerancing.
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Eight modifiers
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PROJECTED TOLERANCE ZONE Symbol: P
The projected tolerance zone modifierchanges thelocation ofthe tolerance zone on the part.
It projects the tolerance zone above the part surface.
Height ofthe projected tolerance zone should beequal to the max. thickness ofthe mating part.
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FEATURECONTROLFRAMEWITHA
PROJECTED TOLERENCE ZONE SYMBOL
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Using a Projected Tolerance Zone
A projected tolerance zone is a tolerance zonethat is projected above the part surface.
A projected tolerance zone modifier is specified
as P
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TANGENT PLANEMODIFIER
The tangent plane modifierdenotes that only thetangent plane ofthe toleranced surface needs to bewithin this tolerance zone.
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DIAMETERMODIFIER ( )
Thediameter symbol is usedtwo ways:
inside a feature control frameas a modifierto denotethe shape ofthetolerance zone, oroutside the feature control frameto simplyreplacethe word "diameter.
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Insidethe featurecontrol frame
Outsidethe featurecontrol frame
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ReferenceModifierExample:
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RADIUS MODIFIER (R)
Arcs aredimensioned withradius symbolondrawings.
Aradius is astraight line extending fromthe center ofanarc ora circleto its
surface. The Symbol foraradius is "R.
Whenthe "R" symbol is used, it creates azonedefined by two arcs.
Thepart surface must lie withinthis zone.
Thepart surface may haveflats orreversals withinthetolerance zone.
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Radius modifier
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ControlledRadius (CR)
The symbol fora controlledradius is "CR.
it creates atolerance zonedefined by two
arcs.
Thepart surface must be withinthe
crescent-shapedtolerance zoneand beanarc without flats orreversals.
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CONTROLRADIUS
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DATUMFEATURE SYMBOL
DATUM IDENTIFYING
LETTER
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DATUMFEATURE SYMBOLSONAFEATURE
SURFACE
AN
DAN
EX
TEN
SION
L
INE
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PLACEMENT OFDATUMFEATURE SYMBOLS
ONFEATURES OF SIZE
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PLACEMENTOFDATUMFEATURE SYMBOL IN
CONJUNCTIONWITHAFEATURECONTROLFRAME
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DATUM TARGET SYMBOL
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56BASIC DIMESNSION SYMBOL
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BETWEEN SYMBOL
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COUNTERSINKSYMBOL
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DEPTH SYMBOL
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SYM
BOL
FO
RALL
A
RO
UN
D
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ECON
TROL
F
RAME
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FeatureControl Frame
Geometric tolerances are specifiedonadrawingthroughthe use ofafeature control frame.
Symbol ofGeometric Tol.
Zone ofTolerance
P.D S.D T.D
W or w/o zone Modifier
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FEATURECONTROLFRAME
INCORPORATINGADATUM REFERENCE
SYMBOL
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Feature Control Frame
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ORDEROF PRECEDENCEOFDATUM
REFERENCE
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COMBINED FEATURECONTROLFRAME
AND DATUMFEATURE SYMBOL
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Thank You
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Exercises:
1
.A slot
isa
n . Size.
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Exercises:
1
.A slot
isa
n Int
er
na
l Featur
e of Size.2. The surface ofarectangular plate is a
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2. The surface ofarectangular plate is a feature.
3. The tangent plane modifier is specified for
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3. The tangent plane modifier is specified forparallelism.
4.The projected tolerance modifier is generallyapplied to holes.
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4.The projected tolerance modifier is generallyapplied to tapped holes.
5.. Dimension is a numerical value usedto describe theoretically exact size, true profile,orientation,or location ofa feature.
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5. Basic Dimension is a numerical value used todescribe theoretically exact size, true profile,
orientation,or location ofa feature.6.A . .. is a part feature thatexists on the part & contacts a datum.
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6.A Datum Feature is a part feature that exists onthe part & contacts a datum.
7. .. Dimensions should be used to locatedatum ... Relative to each otherdatums on the part.
7 D Di i h ld b d l
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7. Datum Dimensions should be used to locatedatum Target Points Relative to each other datums
on the part.8. The symbol for Controlled Radius is .
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8. The symbol for Controlled Radius is CR.
9. In ASME Y 14.5M 1998, M stands for
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9. In ASME Y 14.5M 1998, M stands for
Metric.
10. Three major disadvantages of RectangularTolerance zones are:
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10. Three major disadvantages of RectangularTolerance zones are:
1. Square / RectangularTolerance zones
2. Fixed Size Tolerance zones
3. Ambiguous instructions for Inspection.
The End of Chapter2