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5/21/2018 Geometric Dimensioning & Tolerancing
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Geometric Dimensioningand Tolerancing
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ObjectivesObjectives
What is GD&T
How GD&T scores over limit type tolerancing
Symbols and interpretation
Concepts
Datum features Tolerance zones Material condition modifiers Composite tolerancing
Advantages of GD&T
Glossary
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What is Geometrical Dimensioning
and Tolerancing DEFINITION
Geometric Dimensioning and Tolerancing (GD&T) is a universal language of
symbols, used to efficiently and accurately communicate geometryrequirements for features and components. It encourages designers todefine a part based on how it functions (design intent) in the finalproduct.
ASME Y14.5M 1994 is
the accepted geometric
dimensioning and
tolerancing standard.
First glimpse: Part dimensioned using GD&T.
Datum featureDatum feature
Basic dimensionBasic dimension
FeatureFeature
controlcontrolframeframe
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Tolerance can be specified only wherea dimension is defined.
Gives an acceptable range ofvalues of an individual dimension(limits of dimension).
No provision to specify how flat asurface needs to be, or how much ahole can tilt relative to a surface.
Induces problems related toambiguity, guesswork and multipleinterpretation of part drawing. Resultsin deviation from design intent.
Separates the specification of tolerancefrom the dimensioning.
Specifies a geometric region(tolerance zones), such as an areaor a volume, in which the feature mustlie in order to meet the design criteria.
Communicates complex geometricaldescriptions not possible otherwise inlanguage. Allows more flexibility andprecise controls that relatedirectly to the form, fit andf u n c t i o n and not just size of the part,leading to successful end product.
Eliminates guesswork, enables mfg.according to design intent, thusreduces confusion, rejection, reworkand loss of profits.
Geometric
Tolerancing
Limit Type
Tolerancing
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Feature control frame in GD&T Basic sentence in GD&T is put in the form of a feature control frame. It states the requirement for the feature to which it is attached.
Each feature control frame can state only one requirement/message.Only one set up or gage for one FCF.
Parts of a feature control frame
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Feature control frame, Broken down
First compartment-Geometric characteristicsymbol: Specifies the character to
which the tolerance is to beapplied.
E.g. Flatness , angularity,profile , parallelism etc.
Third compartment Datumsystem:
Specifies datums if applicable.
They are significant according totheir precedence in the FCF.
First compartment- contains one of the
14 geometric characteristic symbols.
Third compartment- contains the datum
reference frame
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Feature control frame, Broken down
Second compartment- Symbol tospecify shape of tolerance zone:This symbol precedes the tolerance andspecifies the shape of tolerance zone.
E.g. specifies a cylindrical tolerancezone, specifies a sphericaltolerance zone.
If no symbol is given, the default shapeis parallel planes, or a total wide zone
(like in profile tolerance).
Second compartment -Materialcondition modifiers:Features of size can be provided
bonus tolerances using thesemodifiers.If the feature being controlled is afeature of size, and no modifier isspecified, the default is RFS.
Second compartment- contains actualtolerance, material condition modifier and
other symbols.
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Other symbols in the FCF
The symbols for projectedtolerance zone, free state,
tangent plane, and statisticaltolerance always follow thematerial condition modifier.
New datum feature symbol hasbeen introduced in ANSIY14.5, 1994.
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Concept of Datum System
Datum - A theoretically exact plane, point or axis from which adimensional measurement is made. Datums are points, lines,
planes, cylinders, axes, etc., a, from which the location, or geometricrelationship of other part features may be established or related.
Datum Feature - A datum feature is the actual component featureused (idealized) to establish a datum.
Datum Feature Simulator-- A datum simulator a surface ofadequate precision oriented to the high points of a designateddatum from which the simulated datum is established.
Examples: gage pin, block, and surface of granite block. Diameter
Symbol - the diameter symbol, indicates a circular feature when used onthe field of a drawing or indicates that the tolerance is diametrical whenused in a feature control frame
The inspection equipment (or gage surfaces) used to establish a datumis the simulator.
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Concept of Datum System
Datums are specified in the third compartment of the feature controlframe.
They are theoretically exact features (surfaces idealized to planes, axesetc.) from which dimensional measurements are made.
Feature constrained within
said tolerance with respect todatums A,B
Sample part: Surfaces are idealized to
eliminate ambiguity about from where
dimensions are to be measured
But if both surfaces are idealized simultaneously,
they may not be perpendicular to each other.
Primary Datum
Secondary Datum
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Precedence order in datum planes
Thus, they are specified in an order of precedence governed by the partfunction.
Without precedence order,
either of these two could bethe correct position. Final
position depends on which
side contacts first.
If we use a set of perpendicular datum references, either of the
two positions could be right.
Therefore, the first side to be pressed against one of the edges (in this case,
datum A), will make contact at the two highest points. The part now has onlyone degree of freedom left, it can only slide back and forth along this edge.
Once we butt the perpendicular side of the part with the corresponding straight
edge (datum B), we have a completely constrained position and orientation.
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Concepts of Tolerance Zones
Ensures closeness to real world requirements
Enables specifications (like conical tolerance zones) not otherwise possible
in limit type dimensioning.
Tolerances zones can be defined in GD&T instead of limits of dimension.
Defined according to functional requirement of the part.
These are geometric regions (3D or 2D) in which the feature must lie to be acceptable.
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Symbols in GD&T
GD&T has 14 geometric
characteristic symbols. Various symbols used to
specify tolerance zones
for:
Form
Position
Profile
Orientation Runout
RunoutTotal Runout
RunoutRunout
OrientationPerpendicularity
OrientationParallelism
OrientationAngularity
ProfileProfile of a line
ProfileProfile
PositionSymmetry
PositionPosition
PositionConcentricity
FormStraightness
FormFlatness
FormCylindricity
FormCircularity
GeometryDescriptionSymbol
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Straightness Straightness describes a condition where an element of asurface or an axis is a straight line.
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Flatness
The surface must lie between
two planes 0.25 mm apart.
Flatness is the condition of a surface having all elementsin one plane.
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Circularity
Each circular element of the surface in a plane perpendicular to the axis must lie between two
concentric circles, one having a radius 0.25 mm larger than the other.
Circularity describes the condition on a surface of revolution(cylinder, cone, or sphere) where all points of the surfaceintersected by any plane (1) perpendicular to a common axis
(cylinder, cone), or (2) passing through a common center(sphere) are equidistant from the center.
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Cylindricity
The cylindrical surface must lie between two concentric cylinders one with radius
0.25 mm larger than the other.
Cylindricity describes a condition of a surface ofrevolution in which all points of a surface are equidistantfrom a common axis.
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Parallelism
The surface
must liebetween two
parallel
planes 0.12
mm apart,
which are
parallel to
datum plane
A.
The feature axis must lie within a 0.2 mm dia
cylindrical zone parallel to datum axis A.
Parallelism is the condition of a surface, line, or axis,which is equidistant at all, points from a datum plane oraxis.
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Perpendicularity
The feature axis must lie between two parallel planes 0.2 mm apart, perpendicular to datum axis A.
Perpendicularity is the condition of a surface,axis, or line, which is 90 deg. from a datum planeor a datum axis.
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Angularity
The surface must lie between two parallel planes 0.4 mm apart inclined at an angle of 300 to datum plane A.
Angularity is the condition of a surface, axis, or centerplane, which is at a specified angle (other than 0, 90, 180or 270 deg.) from a datum plane or axis.
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Profile of a surfaceProfile of a surface is the condition permittinga uniform amount of profile variation, eitherunilaterally or bilaterally, on a surface.
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Profile of a line
Profile of a line is used in conjunction with profile of
surface. Profile of a surface defines the shape or location
of a feature while profile of line refines it in one direction.
Each line element of the surface must lie between two
profile boundaries 0.006 mm apart in relation to the
datum reference frame.
Profile of a line is the condition permitting auniform amount of profile variation, eitherunilaterally or bilaterally, along a line element of a
feature.
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Circular runout
At any position, each the circular
element of the surface must be
within the specified runout
tolerance (0.02 mm full indicator
movement) when the part isrotated by 3600, about the datum
axis, the indicator fixed in a
position normal to the true
geometric shape.
Note: circular runout controls the circular elements of the surface,not the complete surface.
Circular runout gives the deviation from thedesired form of a circular element of a partsurface of revolution through one full
rotation (360 deg) of the part on a datumaxis
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Total runout
The entire surface must lie
within the specified runout
tolerance zone (0.02 mm
full indicator movement)
when the part is rotated by3600 about datum axis A,
with the indicator at every
location along the surface in
a position normal to the
true geometric shape without
reset of the indicator.
Total runout is the simultaneous composite control of allelements of a surface at all circular and profile measuringpositions as the part is rotated through 360.
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Concentricity
This controls location, and can have
some effect on the form and orientation of
a feature. The application of
concentricity is complex and rare.
Diametrically opposed dial indicatorsmaybe used to check this.
Concentricity describes a condition in which two or morefeatures (cylinders, cones, spheres, etc.) In anycombination have a common axis.
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Symmetry
Controls opposing points (that form derived
median plane). Same concept as concentricity,
but applied to non-cylindrical features.
Symmetry is a condition in which a feature (or features) issymmetrically disposed about the center plane of a datum feature
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Position
Position tolerancing is used
to locate features of size
(profile is used to locate
features that dont have a
size associated with them).
Defines a zone within
which, the axis, medianplane, or surface of a
feature is permitted to lie.
These tolerance zones can
be cylindrical, conical,rectangular, etc.
Position tolerance (formerly called true position tolerance) defines azone within which the axis or center plane of a feature is permittedto vary from true (theoretically exact) position.
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Positional tolerance for cylindrical zone
Application
Part mounts in assembly on surfaces
shown, holes provide clearance for bolts.
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Position tolerance for rectangular zone
Locates features with a greater tolerance in one
direction than other. Note that the diameter symbol
is not present in the feature control frames indicating
a distance between two parallel planes.
Here the axes of the holes mustlie in a 0.012X0.028
rectangular tolerance zone
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Positional tolerance for spherical zone
The centre point of the spherical
diameter must lie in a spherical
zone of diameter 0.03, basically
located to the DRF.
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Positional tolerance for conical zone
Application
Used to control features such as a deep
drilled hole, closer at one surface than
another.
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Concepts Material condition
modifiers GD&T on holes (and shafts)
provides a powerful method forincreasing inspection yield without
trial and error fitting or binning.
Used when the size of the featureinteracts with its location.
If symbol appears after thetolerance, then the specifiedtolerance holds only at maximummaterial condition.
As feature departs from MMC, the
amount of departure can be addedto the position tolerance.
MMC is commonly used forclearance type applications.
This feature control frame specifies the
positional tolerance zone as a circle ofdiameter .010 at MMC,
centered according to the basic dimensions
given.
The size of the tolerance zone is dependent on
the size of the hole.
MMC of hole = .250
LMC of hole = .255
ToleranceZone diameter
Hole diameter
.015.255 (LMC)
.014.254
.013.253
.012.252
.011.251
.010.250 (MMC)
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Concepts Material condition
modifiers Similarly, if symbol is used, the stated tolerance holds
at least material condition (LMC). As the part departs
from LMC, the amount of departure is added to theposition tolerance.
LMC is commonly used for loose fits.
If no modifier is specified, (or symbol in past practice)then the stated tolerance holds regardless of materialcondition of feature. This is called RFS regardless of
size.
RFS is commonly used for pressed fits.
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Rationale behind bonus tolerances.
Taking an example for MMC
Position tolerance stated at MMC
Obtained tolerance for hole at MMC
Worst case condition.
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Rationale behind bonus tolerances.
Now the centre of the hole can shift further left in the
worst case. The gap is now closed. With a larger hole, thehole position is less stringent, and more parts can be
accepted.
The tolerance zone cantherefore be enlarged by
an equal amount in
diameter.
W/o compromising function, tolerance increased, cost of mfg. reduced.
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Concepts Composite Tolerancing
Can be used with profile
and position tolerance.
The symbol is entered
once, and is applicable to
both horizontal entities.
The upper segment controls
location, orientation, form,and in some cases size.
The lower segment controls
mainly orientation and form.
It does not control location.
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Example Composite Profile Tolerancing.
The upper entry controls location
to the DRF (datum reference
plane)
The lower entry controls,
size/shape and orientation
(perpendicularity) to the specified
datum.
The above specs allow the 0.005
tolzone to float up and down, and
back and forth, and tilt or rotate within
the confines of the 0.030 tolzone. Ithowever, must stay perpendicular to A
Application: Used to provide loose
location but restrictive orientation. Eg.
Pattern of holes to locate nameplate.
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Advantages of GD&T
Functional dimensioning philosophy
Round Tolerance zones. Bonus tolerance by material condition
modifiers.
Datum system for clarity in inspection / fixture
mfg.
Reduces need for drawing notes, providesmore wieldable language for specifications.
Supports Statistical process control (SPC)
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Functional dimensioning philosophy Tolerance and tolerance
zones based on partfunction and requirement.
Allows maximum tolerance
to produce the part.
Functional dimensioningcan often double or triplethe amount of tolerance
on many componentdimensions, whichreduces manufacturingcosts.
Dimensioning matches function.
Dimensioning does not match function.
Fi g . s h o w s b o l t s h o l e s f o r m o u n t i n g a f l a n g e o n t o a p l a t e
( f u n c t i o n ) . W h e n m o u n t i n g t h e f la n g e , t h e p o s i t i o n o f t h e
h o l e s w i t h r e s p e c t t o e ac h o t h e r i s i m p o r t a n t , o r e ls e t h e
f l an g e ( o r p a r t ) w o n t f i t . Fu n c t i o n a l d im e n s io n i n g l e a d s
t o d i m e n s i o n i n g t h e d i s t a n c e b e t w e e n t h e h o l e s , i n s t e a d
o f t h e d i s t a n c e s t o t h e e d g e .
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Case StudyTolerance analysis of gap b/w trunk lidand rear windshield in Indigo SR.
Clearance critical
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Areas affecting tolerance
1
2
Position of holesfor mounting of
hinge on bodySide.
Position ofmounting ofrubber stopper.
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Areas affecting tolerance
5
3
4
Position of holes for
mounting trunk lid onhinge.
Reinforcement
plate connectingtrunklid andhinge.
Hem between innerand outer panel oftrunklid
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Sources / effect of variation no 1
Position of holes for mounting of hinge on body sidehave adjustment of +/- 2mm
2mm
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Sources / effect of variation no 2
0.5mm
Position of mtg. hole for rubber stopper has an adjustment of +/- 1mm
+/-1mm
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Sources / effect of variation no 3
4mm
4mm
Position of slots for mtg. trunk lid on hinge has an adjustment of + 4mm
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Sources / effect of variation no 4
+/- 0.5mm
+/- 0.5
Variation in reinforcement plate connecting trunklid and hinge
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Sources / effect of variation no 5
+/- 0.5mm
+/-0.5
Variation due to hem between inner and outer panel of trunklid
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Analysis of variations
-0.5+0.5Hem of inner trunk lid to outertrunk lid.5.
-3.5+7.5Total
+0.5
+4
+0.5
+2
-0.5Variation due to reinforcementplate.
4.
-0Position of slots for mountingtrunk lid on hinge.
3.
-0.5Position of mounting of rubberstopper.
2.
-2Position of holes for mountingof hinge on body side.
1.
AmountSource of VariationS No.
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Conclusion:
Clearance valuesMax: 13.5mm
Min : 2.5mm
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Recommendation 1:
Not important tocontrol.
Important to control
gap.
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Recommendation 2:Hole to slot edge distance to be controlled in component
Dim to be controlled
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Recommendation 3:
Design position to be at center with +/- 1mm adjustment
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Recommendation 4:
Operator to ensure rearmost position after adjustment, before
tightening bolts
Analysis of variation after
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Analysis of variation after
recommendations.
-0.5+0.5Hem of inner trunk lid to outertrunk lid.
5.
-4.5+1.5Total
+0.5
+1
+0.5
+0
-0.5Variation due to reinforcementplate.
4.
-1Position of slots for mountingtrunk lid on hinge.
3.
-0.5Position of mounting of rubberstopper.
2.
-2Position of holes for mountingof hinge on body side.
1.
AmountSource of VariationS No.
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Round tolerance zones
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Round tolerance zones
Four holes drilled through the block(1), and each holes location relativeto each other and the edges arespecified using a limit tolerance of a
distance and +.005 and -.005. Center of each of the holes must fall
within a square tolerance zone .010x .010 (2).
Actual worst scenario is .014 or +.007 and -.007 (diameter)
(3) Round tolerance zone oversquare tolerance zone for the partgiven in 1. Thus 57% increase inavailable tolerance.
Resulting in more usable parts,
more capable process, reducedmanufacturing costs
1
2 3
Bonus Tolerances using material
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Bonus Tolerances using material
condition modifiers In coordinate tolerancing, the tolerance zone is always fixed in size at all
hole conditions.
GD&T allows tolerance to be increased without compromising function.
Parts that are functional are used, and more tolerance is allowed for
production, resulting in lower operating costs.
Material conditionmodifier for MMC
I n c r e as ed
T o l e r a n c e
z o n e a t
l a r g e s t
h o l e d i a .
T o l e r a n c e
z o n e a t
MMC( s m a l l es t
h o l e d i a )
Example explaining bonus tolerances.
Datum System
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Datum System
Datum - A theoretically exact plane, point oraxis from which a dimensional measurementis made.
Datum Feature - A part feature that
contacts a datum.
Datum Feature Simulator- The inspectionequipment (or gage surfaces) used toestablish a datum.
Datums in GD&T provide areference frame from which thedimensions are measured.
Eliminates ambiguity ininspection.
Datum symbol :
How Datum systems implement
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How Datum systems implement
functional dimensioning Specified Datums and geometric tolerances based
on functional requirements.
Clear communication of design intent.
Leads to successful end product.
Improved wieldability of language
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Improved wieldability of language
Flatness of
surface specified.
Feature control frame
specifies positional tolerance
of hole, bonus tolerance atmax. material condition,
and datum system.
Improved wieldability of language
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Copious notes
required to specify
the same in
conventional
tolerancing.
Improved wieldability of language
Statistical process control
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Statistical process control
Traditionally, quality was achieved by 100% inspection of product, accepting orrejecting based on how well it met its design specifications.
SPC uses statistical tools to observe the performance of the production processand predicts significant deviations that may result in rejected product.
GD&T's Datum system provides the repeatable part measurements that are
necessary for making a meaningful SPC chart. Thus SPC in GD&T helps optimize inspection costs and reduce waste via
rework and scrap.
How SPC works: Under normal conditions,variations in product are near the mean,
following a normal distribution. In special
cases, caused by some error in the
manufacturing procedure, the variations
move away from this distribution. This canbe easily detected and corrected.
Questions.??
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Ques o s .??