Download - Geometric Dimension Ing doc
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GEOMETRIC DIMENSIONING &
TOLERANCING (GD&T)
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What is Geometric Dimensioning and Tolerancing (GD&T)?
A means of dimensioning and tolerancing a drawing with respect to the actual function or relationship of part features which can be
most economically produced
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GD&T provides a universal language for designers, manufactures
and quality inspector and does not leave any scope for ambiguity.
The Designer’s intent is clearly communicated to all.
In absence of tolerance of form, orientation and location defined in
GD&T way, the design is open to subjective interpretation.
GD&T focuses on functional assembly. Cumulative effects of all
types of tolerance are taken into account to work out worst condition.
Components with dimensions departing to the worst possible
extremes within the tolerance zones are also assured of problem-free
functional fitment.
Importance of GD&T
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GD&T defines through datum, the way in which the components
needs to be manufactured, assembled and perform. This ensures a
uniform method of manufacture.
Design based on GD&T principles assures interchangeability across
millions of parts produced at independent locations. In the age of
Global sourcing and Just In Time, components manufactured at
independent locations and assembled at a location half way around
the globe is a common phenomenon.
Importance of GD&T
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GD&T refers to dimensions and tolerances on part features.
According to the ASME Y14.5M standard, feature is the general term
applied to a physical portion of a part such as a surface, shaft, hole or
a slot.
A feature is subject to four different levels of tolerance. Each higher
level tolerance adds a degree of constraint, subjecting the feature
simultaneously to many constraints.
Types & Levels of Tolerances
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Level 1: Control on size
Control on size defines the limits of smallest and largest size of the feature. It
contains the size within the maximum material condition (MMC) and the Least
material Condition (LMC) of the feature.
Level 2: Form control
Form tolerance constrains the ‘shape’ of the feature. Form tolerance are
independent on datum and are always applied without datum features.
Straightness
Flatness
Roundness / Circularity
Cylindricity
Types & Levels of Tolerances
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Level 3 : Orientation Control
Orientation control constrains the orientation between two features. Foam tolerances
can assure assembly only if the features are freely floating and have no restrain on
orientation and location. Orientation tolerance are applied as
Parallelism
Angularity
Perpendicularity
Since orientation tolerance define the relationship between two features, they are
applied with references to datum.
They are applicable to hole and slot boundaries, on axes of features, or on surface
elements.
Types & Levels of Tolerances
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Level 4 : Location Control
The Location control constraint on location of the feature. It is applied in terms
of tolerances on
Position
Concentricity
Symmetry
Types & Levels of Tolerances
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Other types of geometric tolerance that are applied are
Profile Tolerances
These define the bounds of variation of sectional profi les as well as surfaces.
They are applied as
Profile of a line
Profile of a surface
Runout Tolerances
Runout tolerance defines the boundary of surfaces revolving around defined axis.
They are applied either at an individual cross sections or along the axis as
Circular runout
Total runout
Types & Levels of Tolerances
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The traditional way of tolerancing
position results in a rectangular /
square tolerance zone. This
leads to a greater value of
position tolerance in a diagonal
direction than in the orthogonal
direction.
Traditional way of expressing Tolerance
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Defining the position tolerance as a circular tolerance zone resolves the issue indicated in previous slide. As shown, the circular tolerance zone defines a circular locus zone within which the center of the feature will be located. The tolerance of position can be defined for features like holes, shaft and slots. The theoretical location is defined by basic dimensions. The tolerance zone is further defined around this theoretical position.
GD&T way of expressing Tolerance
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The tolerance of size on a feature is defined by its material condition. The Maximum Material Condition (MMC) on a feature occurs at that value of size, when maximum material is left on the work piece. A hole is at its MMC when it is at the lowest limit of tolerance. On the other hand, a shaft is at MMC when at its highest limit of tolerance.
Concept of MMC
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The Least material condition (LMC) on a feature occurs at that value of size at which least material is left on the work piece. A hole is at its LMC when it is at the highest limit of tolerance. On the other hand, a shaft is at LMC when at its lowest value of tolerance.
Concept of LMC
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Virtual Condition is the worst case envelope of boundary that occurs due to the
combination of all tolerances restraining a feature.
Virtual conditions are applicable for all features & assembly.
Virtual condition
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Ø25.8
Virtual Condition
Virtual condition - Shaft
The figure right side shows the
virtual condition for a shaft due to
the tolerance of size and
tolerance of location acting on the
feature.
The shaft is largest at its MMC. In
addition to the MMC size the
location tolerance of the shaft will
create an outer envelope that
defines the bounds within the
feature will be contained.
Image not to scale
Shaft MMC – 25.7 (+) Position tolerance - 0.1 Virtual condition – 25.8
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The worst case envelope (Virtual condition) acts as an outer boundary for
shaft resulted due to all the tolerances reaching their worst case.
Virtual condition - Shaft
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Virtual Condition
Virtual condition - Hole The figure right side shows the
virtual condition for a hole due
to the tolerance of size and
tolerance of location acting on
the feature.
The hole is smallest at its
MMC. In addition to the MMC
size, the location tolerance of
the hole will create an inner
envelope that defines the inner
bounds within the feature will
be contained.
Image not to scale
Hole MMC – 25.3 (-) Position tolerance - 0.1 Virtual condition – 25.2
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The worst case envelope (Virtual condition) acts as an inner boundary for hole
that result in guaranteed clear opening available for a mating feature.
Virtual conditions occur due to tolerances of foam, orientation and location
acting simultaneously with the tolerance of size on a feature. Virtual conditions
at MMC are very imported to design a clearance fit between mating features.
Virtual condition - Hole
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Virtual condition boundary using MMC concept – External Feature
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Virtual condition boundary using MMC concept – Internal Feature
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The variable boundary generated by the collective effects of a size
feature’s specified MMC & LMC, the geometric tolerance for that
material condition.
Resultant condition
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Resultant condition boundary using MMC concept – External Feature
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Resultant condition boundary using MMC concept – Internal Feature
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Tolerance of form, orientation and location are applied as
Regardless of feature Size (RFS)
RFS means that the value of form, orientation & location tolerance are not
dependent on the actual size of the feature.
RFS
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Modifier of MMC or LMC
It means that the value of the tolerance is defined at one of these material
conditions. This value changes as the size of the feature changes. The
additional tolerance that becomes available as a result of change in actual size
is popularly termed as a ‘Bonus Tolerance’.
Modifiers
Modifier of MMC
Modifier of LMC
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If no material condition is specified, then tolerance denotes
“regardless of feature size.”
ASME Y14.5M Rule-1
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Material condition
Shaft diameter
Position tolerance
Virtual condition
Resultant condition
LMC 2.3 0.9 1.42.4 0.8 1.62.5 0.7 1.82.6 0.6 2.0
MMC 2.7 0.5 2.2
3.2
Modifier concept - Virtual & Resultant condition - Shaft
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Material condition
Hole diameter
Position tolerance
Virtual condition
Resultant condition
LMC 2.7 0.9 3.62.6 0.8 3.42.5 0.7 3.22.4 0.6 3.0
MMC 2.3 0.5 2.8
1.8
Modifier concept - Virtual & Resultant condition - Hole
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Virtual condition boundary using LMC concept – External Feature
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Virtual condition boundary using LMC concept – Internal Feature
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Resultant condition boundary using LMC concept – External Feature
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Resultant condition boundary using LMC concept – Internal Feature
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MMC concept - Virtual and Resultant condition
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LMC concept - Virtual and Resultant condition
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The example indicated in previous slide has the combined effect of size and
location for virtual condition.
In assembling features, each of the tolerances (viz. Foam, orientation and
location) is going to place additional restraint on the feature. All restraints
have to be taken into consideration while computing the virtual condition.
ASME Y14.5 Rule-2
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The figure below shows the assembly of features designed to have a clearance fit. The virtual condition of the Hole needs to be of a larger or equal size then that of the mating shaft. Even when both the features are at their MMC, and are shifted in location in diametrically opposite orientation, there will be no surface contact, since MMC size of the shaft is smaller then the MMC size of the mating hole.
Virtual condition for mating parts
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Shaft & Hole design
Shaft – Diameter 86+0.3-0.2
Positional tolerance 0.2 mm w.r.t A & B datums
Design the hole. Derive the normal value of the size of the hole.
Conditions :
1. Zero clearance between hole & shaft.
2. Tolerance of the size of the hole is same as that of the shaft.
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Shaft & Hole design
Steps :
1. Find shaft MMC
2. Find Shaft VC
3. Shaft VC = Hole VC
4. Find Hole MMC
5. Find Hole Nominal
Solution :
Shaft MMC = 86.30
Position tolerance = 0.20
Shaft VC = 86.50
Hole VC = 86.50
Position tolerance = 0.20
Hole MMC = 86.70
Hole nominal = 86.90+0.30-0.20
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Shaft & Hole design
Shaft – Diameter 50+0.3-0.1
Positional tolerance 0.2 mm w.r.t A & B datums
What is the nominal dimension of hole & nominal dimension of flat
Conditions :
1. Zero clearance between hole & shaft.
2. Tolerance of the size of the hole is same as that of the shaft.
15±0.2
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Shaft & Hole design
Shaft Flat MMC = 15.20
Position tolerance = 0.10
Shaft Flat VC = 15.30
Hole Flat VC = 15.30
Position tolerance = 0.10
Hole Flat MMC = 15.40
Flat nominal = 15.60±0.20
Solution :
Shaft MMC = 50.30
Position tolerance = 0.20
Shaft VC = 50.50
Hole VC = 50.50
Position tolerance = 0.20
Hole MMC = 50.70
Hole nominal = 50.80+0.30-0.10
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6 Degree of freedom
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Datums are frames of reference from which the dimensions are measured. They can be treated as starting points from which the dimensions are stated. It is necessary to state datums that are unambiguous and adequate to immobilize the part. The part is immobilized using the primary, secondary and the tertiary datums. The part is located on three points on the primary datums, two points on the secondary datum and one point on the tertiary datum.
Datum – Arresting 6 degrees of freedom
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Features on work pieces are also used as datums. Since the features themselves
are subject to a variation of size, the assumption of the material condition for the
datum has to be clearly started.
Here, the concept of true geometric counterpart becomes re levant. True geometric
counterparts represents the datum of size. The size of the true geometric
counterparts is the size of the feature at its virtual condition.
For a external feature of size the true geometric counterpart is the smallest
circumscribing cylinder that touches the surface. This takes into account, the virtual
condition of the feature.
Similarly for an internal feature the size of the true geometric counterpart is the size
of the largest inscribed cylinder that touches the surface internally.
True Geometric Counterpart
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Primary Internal Datum width - RFS
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Dimension origin is indicated by a small circle at the start of the dimension. As shown in the figure below, arrow heads at both sides of the dimension lie can be interpreted in two different ways. One interpretation assumes the dimension starting point on the smaller arm of the bracket. The other interpretation assumes the dimension starting point on the longer arm. The two interpretations results the tolerance being applied at different locations on the bracket. In absence of explicit of orientation tolerance, the orientation can very within this tolerance of size.
Dimension Origin
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Tolerance Representation
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Feature Control Frames are read from the left to the right.
The first indicator defines whether the frame refers to a location, form, profi le, runout
or an orientation tolerance.
Feature control frames‘Alphabets’ of GD&T
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The second indicator indicates the value of the tolerance, followed by modifier if
applicable. In the above frame, the presence of the modifier symbol suffixing the
tolerance on location indicates that a bonus tolerance is applicable to the tolerance
on location as the size of the feature changes from MMC to LMC.
The last part of the frame denotes the datum stated in the order in which they apply.
The datum symbols are suffixed with modifiers if we are referring to the datums of
size, i.e.. features on the work piece that are subject to variation in size.
It is necessary that only as many datums as are required to uniquely define the
location or orientation should be stated. Stating of redundant datums will lead to
confusion.
Feature control frames‘Alphabets’ of GD&T
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Symbols – Quick reference
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Straightness is a form tolerance that is applied as shown in the figure.
Straightness can be applied to axes as well as surface elements.
Straightness is often expressed over unit length of the element it is
applied on.
Form tolerance - Straightness
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GD&T states that unless explicitly specified, the undulations of form
must be contained within the limits of size tolerances. It assumes a
feature at MMC to be with perfect form, unless the form tolerance is
explicitly stated over and above the tolerance of size.
ASME Y14.5M Rule-3
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Flatness is defined by the separation of two parallel planes at minimum distance that encompass the entire form.
Flatness is often expressed over any unit patch of the surface it is
applied on.
Being a form tolerance, flatness is expressed without any datum reference.
Form tolerance - Flatness
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Roundness is the radial separation of two concentric bounding circles that
encompass the form at a given section.
Roundness is measured at sections perpendicular to the axis of the
feature under measurement.
Form tolerance – Roundness / Circularity
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Cylindricity is the combined effect of roundness, straightness and taper. It is
defined as the radial separation between two coaxial bounding cylinders
that encompass the form of cylindrical features.
Cylindricity is expressed in combination with restriction on roundness, at any
given section of the cylindrical feature.
Form tolerance – Cylindricity
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Perpendicularity is an orientation tolerance that defines the
orthogonal orientation of axes and planes with respect to the
datum feature under consideration.
Perpendicularity is also expressed on axis and is defined by
bounding cylindrical zones rather than planes
Orientation tolerance – Perpendicularity
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Angularity is expressed in a manner similar to perpendicularity, but
at an orientation other than 90 degrees to the datum feature.
Angulariity is also expressed on axis and is defined by bounding
cylindrical zones rather than planes
Orientation tolerance – Angularity
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Parallelism is expressed in a manner similar to angularity, but at an
orientation of 0 degrees to the datum feature.
Parallelism is also expressed on axis and is defined by bounding
cylindrical zones rather than planes
Orientation tolerance – Parallelism
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Concentricity is a location tolerance, and is defined a way similar to the symmetry tolerance. An axis is derived for the datum feature. A cylindrical tolerance zone of the specified value (in this case dia.0.2) is defined around the datum feature axis. Diametrical generators are defined through the surface of the feature being toleranced. For the feature to confirm to the specified concentricity tolerance, all the medians of the diametrical generators need to lie within the cylindrical tolerance zone of the datum feature axis.
Location tolerance – Concentricity
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Symmetry is a location tolerance. As shown in the figure, an axis is derived for the datum reference feature. A tolerance zone of the specified value (here 0.5) is defined around the datum feature axis. Opposite surface generators are drawn along the surface of the feature under the tolerance of symmetry (in this case the slot). For the feature to have the specified symmetry around the datum feature axis, the medians of all the generators defined above need to lie within specified tolerance zone around the datum feature axis.
Location tolerance – Symmetry
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Profile of a line and profile of a surface fall under the category of profile
tolerance.
Profile of a line is defined by the normal direction separation between offsets of the theoretical profile that bound the actual profile.
Profile of a surface is defined by a normal direction separation between
offset surfaces of the theoretical surface that envelope the actual surface.
Profile tolerance
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Circular runout and total runout fall under the category of runout tolerances.
Circular runout is the total indicator movement of a dial gage that probes
the runout of features when rotated about the datum axis. It is expressed at individual sections.
Total runout involves total indicators reading measurement of runout when
the dial is moved along the axis of the feature while probing.
Runout tolerance
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Many times in assembly as shown in the figure, the bottom plate hole is machined without any consideration to the other assembling plate. This results in fouling of fasteners and locators with the other plate. A projected tolerance zone projects the tolerance beyond the surface of the plate and accounts for the assembly into other features.
Projected Tolerance Zone
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Tolerance are often defined as free state tolerances. This implies that the
measurement needs to be done when all the restraining forces on the workpiece that are applied for clamping or machining are removed.
The distortion that occurs due to restraining forces needs to be eliminated
before the tolerance is verified. A free state tolerance is often applied to parts with thin section and parts which can easily be distorted due to external forces.
Free state Tolerance
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Apart from clearly stating the datums, it is also important to state the assumptions about the order in which the datums are to be located. Change in the order of location of datums can change the value of dimensions being measured. The illustration shown below shows the effect of change in the order of location on datums A and B.
Datum precedence
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Thank you for your attention
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Effect of Material Condition and Datum Precedence
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Partial Datum
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Secondary and Tertiary Datum Features - RFS
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Secondary and Tertiary Datum Features at MMC
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Secondary and Tertiary Datum Features at LMC
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LMC applied to Boss and Hole
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LMC applied to Boss and Hole
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LMC applied to Pattern of slots
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LMC applied to Pattern of slots
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Hole pattern identified as Datum
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Thank you for your attention
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Location tolerance on patterns of features is very often defined by composite frames as shown in the figure. The feature is simultaneously governed by both the tolerances. The tolerance in the top segment of the frame is termed as the Pattern Locating Tolerance Zone Framework (PLTZF). It restricts the zone in which the entire pattern of features can be located. The tolerance in the bottom segment of the frame is termed as the Feature Relating Tolerance Zone Framework (FRTZF). This tolerance is tighter and restricts the relationship of individual feature location with each other within the pattern.
Location tolerance on pattern of features
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Relationship of Feature-relating Tolerance zone framework toPattern-locating Tolerance zone framework
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Relationship of Feature-relating Tolerance zone framework toPattern-locating Tolerance zone framework
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Same Positional Tolerance for Holes & Counterbores, same Datum Reference
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Different Positional Tolerance for Holes & Counterbores, same Datum Reference
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Positional Tolerance for Holes & Counterbores, different Datum Reference
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Positional Tolerance for Holes & Counterbores, different Datum Reference
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Positional Tolerancing of Elongated Holes, Boundary concepts
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Positional Tolerancing of Elongated Holes, Boundary concepts
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Positional Tolerancing for Coaxial holes of same size
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Thank you for your attention
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Simultaneous Position & Profile Tolerance
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Purpose
To assist the user in selecting the correct geometric characteristic for a
particular application.
The diagrams encourage the user to think in terms of design intent and
functional requirement
Decision Diagrams for Geometric control
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Design Requirements
Establishing Datums
For Individual or Related FeaturesFor Related FeaturesFor Individual Features
Use Form Controls
Straightness Flatness Circularity Cylindricity
Line Surface
Profile
Position Concentricity Symmetry
Location Orientation
Perpendicularity Angularity Parallelism
Runout
Circular Total
Decision Diagram for Geometric control
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Form
Consider Limits of Size
Surface Elements Axis or Center Plane
Consider Material Conditions
MMC- Specify
RFS - Implied Condition
Flatness Straightness Circularity Cylindricity
Decision Diagram for Geometric control
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AxisCenter plane
Cylindrical Feature Threaded Feature
Projected Tolerance Zone ?
Consider material condition
MMC - Specify LMC - SpecifyRFS
Implied condition
Datums
See Datum selections
Individual Composite
Tolerance
Fixed or Floating
Location
Concentricity Position Symmetry
Decision Diagram for Geometric control
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Consider limits of Size
Projected Tolerance
zone?
Orientation
Consider limits of Location
Feature
Threaded feature Diameter or widthPlane surface
RFS Implied condition
Consider material conditions
MMC Specify LMC Specify
Datums
See Datum selections
Decision Diagram for Geometric control
Perpendicularity Parallelism Angularity
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Runout
Consider limits of size
Datums
See Datum selections
Circular Total
Decision Diagram for Geometric control
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Profile
Consider limits of size
Datums
See Datum selections
RelatedIndividual
Feature
Tolerance Zone
UnilateralInside or Outside
Bilateral Equal or Unequal
Profile of line Profile of surface
Decision Diagram for Geometric control
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Datum selections
Axis Center plane
Consider Material Conditions
RFS Implied conditions
LMCSpecify
MMCSpecify
Datum Feature
Feature of Size Surface
AreSecondary
& Tertiary DatumRequired?
END
NO
YES
Decision Diagram for Geometric control
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Hole pattern located by composite Positional Tolerancing
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Hole pattern located by composite Positional Tolerancing
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Hole pattern located by composite Positional Tolerancing
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Hole pattern located by composite Positional Tolerancing
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Hole pattern located by composite Positional Tolerancing
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Hole pattern located by composite Positional Tolerancing
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